Yuri A Mezenov

Stéphanie Bruyere

Nikita K Kulachenkov

Andrei N Yankin

Sergey S Rzhevskiy

Pavel V Alekseevskii

Venera D Gilemkhanova

Semyon V Bachinin

Vyacheslav Dyachuk

Andrei A Krasilin

Julien Zollinger

Thierry Belmonte

Alexandre Nominé

Valentin A Milichko

Probing the dynamics of Cu nanoparticle growth inside metal-organic frameworks upon electron beam irradiation

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Metal-organic frameworks (MOFs) hold a special place in material science and crystal engineering [START_REF] Vleet | In situ, time-resolved, and mechanistic studies of metalorganic framework nucleation and growth[END_REF]. Over the last two decades, such materials have attracted increasing attention of researchers in chemistry, biology and physics [START_REF] Furukawa | The chemistry and applications of metal-organic frameworks[END_REF][START_REF] Lu | Nanoscale metal-Organic frameworks for therapeutic, imaging, and sensing applications[END_REF][START_REF] Mezenov | Metal-organic frameworks in modern physics: highlights and perspectives[END_REF]. Belonging to a class of porous coordination polymers, MOFs consist of metal-containing nodes connected by multi-topic organic linkers. As a result, they are characterized by hierarchical structure with high porosity, chemical versatility and flexibility, which allow them to demonstrate advanced adsorption, catalytic, mechanical and radiative properties [START_REF] Furukawa | The chemistry and applications of metal-organic frameworks[END_REF][START_REF] Lu | Nanoscale metal-Organic frameworks for therapeutic, imaging, and sensing applications[END_REF][START_REF] Mezenov | Metal-organic frameworks in modern physics: highlights and perspectives[END_REF][START_REF] F X Coudert | Nanoscale metamaterials: Meta-MOFs and framework materials with anomalous behavior[END_REF][START_REF] V A Milichko | Van der waals metal-organic framework as an excitonic material for advanced photonics[END_REF][START_REF] Mezenov | Polymer matrix incorporated with ZIF-8 for application in nonlinear optics[END_REF].

The development of synthetic approaches and post-processing methods brings MOFs closer to commercial applications [START_REF] Furukawa | Heterogeneity within order" in metal-organic frameworks[END_REF]. On the one hand, the compatibility of MOFs with chemical vapour deposition technology [START_REF] Stassen | Chemical vapour deposition of zeolitic imidazolate framework thin films[END_REF], plasma synthesis [START_REF] Jiang | Plasma-catalysed reaction Mn+ + L-H → MOFs: facile and tunable construcVon of metal-organic frameworks in dielectric barrier discharge[END_REF], and thin film growth [START_REF] Bétard | Metal-organic framework thin films: from fundamentals to applications[END_REF] supports their integration into microelectronic devices. On the other hand, the post-processing of MOFs by heat [START_REF] Gaillac | Liquid metalorganic frameworks[END_REF], light [START_REF] N K Kulachenkov | Ultrafast melting of metal-organic frameworks for advanced nanophotonics[END_REF][START_REF] Jiang | Nanoscale laser metallurgy and patterning in air using MOFs[END_REF] and electron irradiation [START_REF] Jacobs | Electron beam synthesis of metal and semiconductor nanoparticles using metal-organic frameworks as ordered precursors[END_REF][START_REF] Houk | Silver cluster formation, dynamics, and chemistry in metal-organic frameworks[END_REF][START_REF] Ahlenhoff | Electron beam-induced surface activation of metal-organic framework HKUST-1: unraveling the underlying chemistry[END_REF][START_REF] Ahlenhoff | Electron-induced chemistry of surface-grown coordination polymers with different linker anions[END_REF] highlights their application as optical glasses and active optical elements in nanophotonics. Indeed, such post-processing initiates the deformation of framework backbone and breaking of chemical bonds [START_REF] Canossa | Circumventing wear and tear of adaptive porous materials[END_REF] leading to formation of amorphous glassy compounds [START_REF] Gaillac | Liquid metalorganic frameworks[END_REF], nanocomposites [START_REF] Chen | Location determination of metal nanoparticles relative to a metal-organic framework[END_REF] and nanoparticles (NPs) with different optial properties [START_REF] N K Kulachenkov | Ultrafast melting of metal-organic frameworks for advanced nanophotonics[END_REF][START_REF] Jiang | Nanoscale laser metallurgy and patterning in air using MOFs[END_REF][START_REF] L R Mingabudinova | Laser printing of optically resonant hollow crystalline carbon nanostructures from 1D and 2D metal-organic frameworks[END_REF]. Herein, compared to investigating mechanisms of nucleation and growth of initial MOFs [START_REF] Vleet | In situ, time-resolved, and mechanistic studies of metalorganic framework nucleation and growth[END_REF] and different inorganic NPs [START_REF] Polte | Fundamental growth principle of colloidal metal nanoparticles -a new perspective[END_REF][START_REF] Schottelius | Crystal growth rates in supercooled atomic liquid mixtures[END_REF][START_REF] Li | In situ atomicscale study of particlemediated nucleation and growth in amorphous bismuth to nanocrystal phase trensformation[END_REF], the kinetics of formation of NPs from constituent parts of MOFs remain unexplored.

In this work, we report on exposure of MOFs to focused electron beam (e-beam) of transmission electron microscope (TEM, Fig. 1a) to initiate the nucleation and growth of crystalline metallic NPs with tunable size from ⁓2 nm to ⁓100 nm. The process of the NPs growth, captured in real time, demonstrates the evolution of their size, shape and spatial distribution and can be analyzed by classical kinetic theory taking into account the phase transformation [START_REF] Schottelius | Crystal growth rates in supercooled atomic liquid mixtures[END_REF][START_REF] Li | In situ atomicscale study of particlemediated nucleation and growth in amorphous bismuth to nanocrystal phase trensformation[END_REF].

Results and discussion

As a model compound, we have utilized the commercially available HKUST-1, Basolite® C 300 produced by BASF (Fig. 1b). This is highly porous three-dimensional compound, Cu3BTC2, composed of Cu (II) ions and benzene-1,3,5-tricarboxylate (BTC) linkers [START_REF] Chui | A chemically functionalizable nanoporous material [Cu3(TMA)2(H2O)3]n[END_REF]. HKUST-1 is also a representative class of MOFs that are easy to grow on different surfaces following a layer-by-layer approach [START_REF] Liu | Surface-supported metal-organic framework thin films: fabrication methods, applications, and challenges[END_REF], which makes it promising for applied research. Moreover, the compound has been recently investigated to reveal the mechanism of electron-induced decomposition and the reason for NPs formation [START_REF] Jacobs | Electron beam synthesis of metal and semiconductor nanoparticles using metal-organic frameworks as ordered precursors[END_REF][START_REF] Houk | Silver cluster formation, dynamics, and chemistry in metal-organic frameworks[END_REF][START_REF] Ahlenhoff | Electron beam-induced surface activation of metal-organic framework HKUST-1: unraveling the underlying chemistry[END_REF][START_REF] Ahlenhoff | Electron-induced chemistry of surface-grown coordination polymers with different linker anions[END_REF].

The composition of HKUST-1 and its cubic space group Fm m have been confirmed by Raman spectroscopy and powder X-ray diffraction, respectively, in a recent article of ours [START_REF] N K Kulachenkov | Photochromic free MOF-based near-infrared optical switch[END_REF]. TEM experiments on initiation and simultaneous analysis of the NPs growth have been carried out using MET -ACCEL ARM 200 F operating at 200 kV (emission 15 μA and various current density). For this, the powder of HKUST-1 has been placed on carbon TEM grids. Fig. 1d-k represents distinctive TEM micrographs captured in time with defferent magnification (60 and 80 k). In contrast to previous results [START_REF] Zhu | Unravelling surface and interfacial structures of a metal-organic framework by transmission electron microscopy[END_REF][START_REF] Zhang | Atomic-resolution transmission electron microscopy of electron beam-sensitive crystalline materials[END_REF], demonstrating that MOFs begin to lose their crystallinity when the cumulative electron dose reached 10-20 e-Å-2 with 300 kV-accelerated electron beam, the pores of HKUST-1 are not observable under our conditions. This is due to possible instability of the crystal structure under vacuum and electron irradiation with a higher electron density.

Comprehensive analysis of in situ e-beam initiation of the NPs growth inside the HKUST-1 single crystals and the dynamics of the growth reveal the following: (i) Highly crystalline NPs can grow inside HKUST-1 under focused e-beam (Fig. 2). Energy dispersive Xray (EDX) analysis with a JEOL Centurio, 1 sr detector, shows the exclusive presence of Cu within the NPs after electron irradiation (Fig. 3). This observation is confirmed by the fact that measured interplanar distances (<dhkl> = 0.207 ± 0.004 nm) are in excellent agreement with the (111) interplanar distance of Cu (d111 = 0.2087 nm). Based on the previous results on the effect of e-beam on the chemi cal bounding of MOFs [START_REF] Jacobs | Electron beam synthesis of metal and semiconductor nanoparticles using metal-organic frameworks as ordered precursors[END_REF][START_REF] Houk | Silver cluster formation, dynamics, and chemistry in metal-organic frameworks[END_REF][START_REF] Ahlenhoff | Electron beam-induced surface activation of metal-organic framework HKUST-1: unraveling the underlying chemistry[END_REF][START_REF] Ahlenhoff | Electron-induced chemistry of surface-grown coordination polymers with different linker anions[END_REF] and similar reason for growth of Pt and Au NPs from organometallic precursors [START_REF] Gao | Probing the dynamics of nanoparticle formation from a precursor at atomic resolution[END_REF][START_REF] Yao | Insights into initial kinetic nucleation of gold nanocrystals[END_REF], the possibility of the growth of the NPs from the constituent parts of HKUST-1 is due to breaking of bonds between copper and organic part. Moreover, the growth of pure Cu NPs instead of its oxide form is explained by a lower oxidation potential of Cu and relatively weak Cu COO bond energy, which promote the metallic state [START_REF] Jacobs | Electron beam synthesis of metal and semiconductor nanoparticles using metal-organic frameworks as ordered precursors[END_REF].

(ii) The growth process begins mainly on the MOF's surface, and then propagates throughout the whole volume (SI, video 3). It should be noted that full conversion of MOF to the nanocomposite can be observed after several minutes (Fig. 4a andb). Herein, the time threshold of the detection of small NPs (5 and 11 s for 60 and 80k magnification) in TEM is observed (Fig. 5a andb), which confirms the relatively slow free Cu atom movement process to form a nucleus of 1-3 nm in diameter with other free atoms.

(iii) The rates of the NPs growth (Fig. 1d-k) strongly depend on the parameters of focused e-beam, such as magnification (60 and 80 k at a constant electron energy), determining the electron density. Indeed, an increase of magnification from 60 to 80k leads to a 7-fold increase in rate (Fig. 5a andb), i.e. 9-nm NP can be created in 40 and 5.5 s, respectively.

(iv) Increase in electron density (magnification) also results in the growth of NPs with larger diameter until the saturation of this process is observed (Fig. 5a andb). The maximal diameters of the NPs observed are ~50 and ~144 nm after 2 min e-beam irradiation with 60 and 80 k magnification, respectively.

(v) The evolution of the number of Cu NPs does not follow a monotonic trend over the e-beam exposure time (Fig. 5a andb). This reflects the coexistence of two phenomena: NPs nucleation and growth, that contribute to increasing their number, and NPs coa lescence that decreases their surface energy, which, in turn, decreases the number of the NPs with an increase of their size. The competition between these two phenomena is indeed more noticeable for smaller size of NPs (i.e. beginning of ebeam exposure) which are closer and more mobile than the larger one, and therefore, more subject to coalescence (Fig. 4c andd). For larger NPs, the surface-to-volume energy ratio becomes much more favorable to the volume energy and therefore to the NPs growth. In other terms, a newly formed free Cu atom is much more likely to be adsorbed by large NPs rather than to form a nucleus with other free atoms, and even in this case, the phenomenon of Ostwald ripening would rapidly make the nucleus be absorbed by the larger NPs, probably before the nucleus could even be detected.

(vi) Finally, the shape of the growing NP can also vary due to coalescence (Fig. 4e andf) [START_REF] Li | In situ atomicscale study of particlemediated nucleation and growth in amorphous bismuth to nanocrystal phase trensformation[END_REF]. As a result, spherical NPs with ⁓100 nm in diameter can be observed. In turn, smaller NPs demonstrate welldefined faceted shape that is commonly encountered for crystalline Cu NPs [START_REF] Wang | Copper Nanoparticles and Their Catalytic Properties[END_REF]. Since the larger NPs come from the assembling of several smaller NPs, they are more likely to be polycrystalline and therefore closer to an overall spherical shape. Smaller NPs are more likely to be single crystals and therefore the growth is more likely to follow lower energy directions.

Based on experimental results, we then model the kinetic of phase transformation. The nucleation and growth can be modelled by either the Gualtieri or Johnson-Mehl-Avrami-Kolmogorov (JMAK) models. Van Vleet et al. have demonstrated that the Gualtieri model is more ap propriate for the prediction of MOF nucleation and growth [START_REF] Vleet | In situ, time-resolved, and mechanistic studies of metalorganic framework nucleation and growth[END_REF]. However, in the present case, the phase transformation corresponds to a solid -solid one (MOF → Cu NPs + glassy phase). Moreover, by separaVng the nucleation and the growth process, the Gualtieri model requires input parameters of both steps. In the present case, the determination of nucleation input parameters is challenging due to detecting of the NPs with a minimum diameter of ⁓2 nm (Fig. 5a andb). Hence, we model the growth of Cu NPs using classical growth theory with JMAK equation [START_REF] Jackson | Kinetic Processes[END_REF]:

(1)
Where V NP is the volume of Cu NP formed, V Cu max the maximum volume of Cu that can be formed, K and n the JMAK coefficients [START_REF] Schottelius | Crystal growth rates in supercooled atomic liquid mixtures[END_REF]. The first JMAK coefficient K is expressed by: [START_REF] Furukawa | The chemistry and applications of metal-organic frameworks[END_REF] with dN/dt the nucleation rate (number of new nuclei per unit of volume per unit of time) and the linear growth rate. The later can be estimated from the experimental data as 0.22 nm s -1 and 1.25 nm s -1 for magnification of 60k and 80k, respectively (Figs. 5a andb, andS1). is estimated by the following equation [START_REF] William | Reaction kinetics in processes of nucleation and growth[END_REF]:

(3) where d is the NP size (Fig. 5a andb). The parameter n accounts for the dimensionality of the transformation, the nucleation and the growth type (interface or diffusion limited). If the nucleation rate is nonzero and time-independent, n = m + 1, where m is equal to the dimensionality of the phase transformation for the interface-limited growth and one-half of the dimensionality for diffusionlimited growth [START_REF] Vleet | In situ, time-resolved, and mechanistic studies of metalorganic framework nucleation and growth[END_REF]. Therefore, n has been chosen as equal to 4. This choice is first sustained by relatively small size of the NPs as compared to MOF crystal thickness varying from 0.1-10 μm (atomic force microscopy analysis, Fig. 1c), allowing a three dimensional growth of the crystals. Moreover, the evolution of the growth rate over time is linear (Fig. 5a andb), while parabolic evolution is expected for a diffusion-driven transformation. Rather than a diffusive process, the nucleation and growth of the NPs is attributed to breaking of the chemical bonding by the e-beam, leading to free Cu atoms precipitate into the NPs. This is supported by the fact that some NPs are facetted (Fig. 2d) which tends to show that, while Cu is growing, attachment kinetics is the limiting process. This can be illustrated by the JMAK model predicting a transformation time of 60 s and 25 s for 60 k and 80 k magnification, respectively (Fig. 5c andd). Therefore, it is possible to tune the quantity of Cu NPs before this time. Once the transformation is completed, the exposure time allows one to control the NP number and size thanks to the coalescence phenomenon. Fig. 5c andd also demonstrates the comparison of the experimental data and JMAK modelling of the time evolution of the NPs volume inside MOF, Eq. ( 1). As in case of the complicated process of growth of the crystals [START_REF] Schottelius | Crystal growth rates in supercooled atomic liquid mixtures[END_REF], one can observe a good agreement between the growth theory and the experimental results. This allows us to speculate on relatively classical microscopic-level processes of the crystalline NP growth inside MOF from its constituent parts due to breaking of the bonds between metal ions and organic linkers.

Conclusion

In summary, we experimentally analyzed and modeled the growth of crystalline Cu NPs of tunable size from ⁓2 nm to ⁓100 nm from the constituent parts of MOFs. For this, we utilized focused electron beam to initiate the nucleation and growth of NPs inside HKUST-1. The process of growth, captured in real time using transmission electron microscope, demonstrated the evolution of NP size, shape and spatial distribution. The experimental results are in a good agreement with the classical kinetic theory of the NP growth using JMAK model taking into account a phase transformation. Our results contribute to crystal engineering and developing of functional MOF-based nanocomposites.

eclaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 1 .

 1 Fig. 1. (a) Schematic illustration of exposure of HKUST-1 single crystal to focused electron beam in TEM chamber for initiation and controll the growth of Cu NPs. (b,c) Optical image and atomic force microscopy (AFM) micrograph of HKUST-1 single crystal. Scale bars, 20 μm and 10 μm. Inset (c): AFM cross section of single crystal representing the heigth/length ration as 1/4,6. (d-g) Bright field TEM micrographs illustrating the dynamics of the NP growth inside HKUST-1 under magnification of 60 k. Scale bar, 200 nm. Corresponding video is in SI, video 1. (h-k) Dynamics of the NP growth inside HKUST-1 under magnification of 80 k. Scale bar, 100 nm. Corresponding video is in SI, video 2.

Fig. 2 .

 2 Fig. 2. (a-d) Bright field TEM micrographs of crystalline Cu NPs inside HKUST-1 obtained after 40 s of irradiation with 60 k magnification. (a) Interplanar distances measured < dhkl> = 0.207 ± 0.004 nm. Scale bars, 5 nm (a,c), and 10 nm (b,d).

Fig. 3 .

 3 Fig. 3. (a,b) STEM-EDX map of HKUST-1 before focused e-beam irradiation confirming the homogeneous redistribution of the elements (C, Cu, O). Scale bar, 300 nm. (c,d) STEM-EDX map of HKUST-1 after focused e-beam irradiation. Cu NPs are represented in Red. Scale bar, 250 nm. Overlapping of Cu and O is represented in (b,d) demonstrating homogenity and inhomogeneity of Cu redistribution, respectively.

Fig. 4 .

 4 Fig. 4. (a,b) Bright field TEM micrographs of HKUST-1 after full conversion to the nanocomposite in 2 min upon focused e-beam with magnification of 60 k. Scale bars, 200 nm (a) and 500 nm (b). (c,d) Coalescence of the NPs from (c) in 30 s (d) upon focused e-beam with magnification of 60 k. Scale bar, 40 nm. (e,f) Changes of the NP shape (e) during coalescence in 4 s (f) upon focused e-beam with 80 k magnification. Scale bar, 60 nm.

Fig. 5 .

 5 Fig. 5. (a,b) Analysis of the dynamics of the NP growth (size and the number of NPs) under focused ebeam irradiation with magnification of (a) 60 k and (b) 80 k. Irradiation started at t = 0 s, while the NPs with a minimal size of 2.5 nm have been detected by TEM in 11 and 5 s, respectively. (c,d) Time evolution of the Cu NP volume under e-beam irradiation with magnification of (c) 60 k and (d) 80 k. JMAK model (Eq. (1)) represents the red curve, while experimental data (the volume of growing NPs) are represented in squares. Dashed lines are the error of the JMAK model based on the correct analysis of the volume of MOF and the NPs

Acknowledgements

This work is a part of joint Russian-French project "Hybrid photonics nanodevices" and is supported by The Ministry of Science and Higher Education of the Russian Federation (project No 14.587.21.0050 with unique identification RFMEFI58718×0050). ITMO University is acknowledged for providing access to the unique scientific instruments (AIST-NT TrIOS) for optical experiments.


Talking Music, Making Music: A Comparison Between Rap and Techno 1

Antoine Hennion MINES ParisTech, CSI -Centre de sociologie de l'innovation, CNRS UMR 7185, antoine.hennion@mines-paristech.fr

The Ashgate Popular Musicology Research Companion, ch. 19 pp. 535-555 D. Scott ed., Ashgate, Hampshire, 2009 It's listeners who make music…2

This contribution, being work in progress, gathers together in an open fashion a body of reflections from a seminar given over to a comparison between rap and techno, and I begin with a brief reconsideration of common assumptions. This seminar was given with the object of studying listening, beginning with a series of juxtaposed experiments. 3 The starting idea was simple: to regard music as being of an uncertain affect that depends on what its listener makes of it, and not as being an experience that is merely received. The word 'experiment' can, indeed, create confusion. It does not mean that we subjected guinea-pigs to regulated tests, in order to measure the best possible listener reactions to a given object. 4 It would, of course, be one way of showing what music is. Moving in exactly the opposite direction, however, we have tried to make listeners the experts on their own listening, so that they can teach us what musical experience is for them.

The musical object is not already present, ready to undergo the various listenings that are directed towards it that we would then be able to measure. Even its immediacy is in question. Analyzing the musical moment is not a matter of measuring the way we hear various sounds. This would postulate music as already present, when the question is what makes it appear as such. It is not simply a matter of putting a CD in a CD player, and suddenly there is the music… That is why the cooperation of the listener in the definition of what happens is necessary.

In the first instance, one of our objectives is to benefit from the immense know-how that the listener deploys in seeking out preferences, managing pleasure, evaluating, testing, judging things at once, or delaying diagnoses. We wanted, with the listener's help, to test certain methods of accounting for all this. However, beyond this insider shrewdness, listeners have created a framework for their listening, having built their aural competencies over a long period of time. As much at the personal as at the collective level, ways of listening have a history (it is always necessary to think of listening as a plurality). They are not reducible to the vibration of a tympanum: they have been constructed through the invention of formats and repertoires, systems and arrangements, all of which have been debated by fans of various types of music in passionate discussion among themselves. We also wanted the listener, this person little considered in musicology, to be given back a place in musical analyses. To the extent that, historically, the authorial regime came to occupy the centre ground of artistic creativity, studies of classical music became enthusiastically aligned in conformity with literary production. Instead of treating music as a complete object, they reduced it to the score alone, thus becoming at one and the same time an effect of and driving force behind the new subjugation of music to graphic reason. 5 In passing, they had rid themselves of the listener, who was now handed over to the psychologist or acoustician. Some musicological work has certainly begun to take the listener back into account, 6 but it would still be going too far to speak of the listener's uninterrupted presence in musical analyses. 7 Listening is not, as many musicologists appear to believe, a means of accessing music by the back door. It is both an activity and a presence. The listener who participates in the production of the music he listens to, can be compared with the image of the reader depicted by Proust: 8 a listener (a reader) who is also an actor of the music (of the literature), made up of a sum of attitudes that are not outside the music, but within the music. And this return of the listener calls forth much else besides: music emerges only in a local context, through different places and means of listening; it presupposes bodily drives and is made in different ways; it relies on the existence of a collective. For, it is not a question, vis-à-vis the autonomy of music, of fashioning an equally autonomous musical subject, in order to take this, in turn, as an analytic object cut off from its historic production; no, the listener himself is also an effect of listening reception, in the same way as is the music. 9

Rap and Techno: Music Already Inundated with Discourse…

Starting from the foregoing assumptions, what experiments should be devised for these new kinds of music? Indeed, they pose a particular problem. There are already a great number of social discourses firmly attached to them -youth, revolt, immigration, violence, festivals, drugs, etc. Instead of criticizing and reformulating this critical commentary in our account, we tried to show the way in which rap and techno could present 'themselves'. Rather than pretending to determine the meaning and the contents of the music in line with 5 J. Goody, La raison graphique: La domestication de la pensée sauvage (Paris, 1979). For a musicological critique of the privilege accorded to the score. See Joseph Kerman, Contemplating Music: Challenges to Musicology (Cambridge, MA, 1985), and on the particular difficulties of the scholarly analysis of popular music, see Richard Middleton, Studying Popular Music (Milton Keynes: Open University Press, 1990). 6 Mary Sue Morrow, Concert Life in Haydn's Vienna: Aspects of a Developing Musical and Social Institution (Stuyvesant, NY, 1989); James H. Johnson, Listening in Paris: A Cultural History (Berkeley, 1995); and William Weber, 'Did People Listen in the 18th Century?', Early Music (November 1997): 678-91. 7 As is the case with Tia DeNora's approach: "Music as a Technology of the Self", Poetics, 26 (1999): 1-26;and Music in Everyday Life (Cambridge, 2000). 8 In 'Sur la lecture' ['On Reading'], Preface to Sésame et les Lys [Sesame and Lilies] by John Ruskin, trans. Marcel Proust (Paris, 1987). 9 On reading, which occupies in relation to the book a position very analogous to that of listening for music, we have been inspired by numerous arguments in the work of Roger Chartier; see Lecture et lecteurs dans la France d 'Ancien Régime (Paris: Seuil, 1987); and his edited anthology Pratiques de la lecture (Paris, 1993); see also Hennion, 'L'Ecoute à la question', and Antoine Hennion, 'Listen!', Music and Arts in Action, 1/1 (2008): 39-45; <http://musicandartsinaction.net/index.php/maia/article/view/listen>. such and such a discipline, it was more a practical matter of seeing how various elements combine and co-construct themselves -the discursive and the gestural, the somatic and the musical, the scenic and the ideological (particularly according to issues recurrently raised by those musical genres, such as gender, violence, the media) -and how these 'make' rap or techno. In this way, we were merely extending the seminar's general methodology: each meeting was to begin with situations where the music is played, heard, interpreted and commented on, in order to study the way that the pairing 'music-fan' is animated through the critical activity of 'listeners' themselves, when, starting from past experience that is not shared equally, they discuss between themselves, compare, reconsider their first impressions, return to other experiences, relativize the conditions of the present listening, or correct previous opinions, etc.

This time, the idea behind the experiment was to reap benefit from the ambivalent relations between rap and techno, from their proximity and opposition, and to organize a comparative demonstration: to present these musics, to put them to the listening test, to show their way of functioning, what they hold of interest, the debates that they arouse. And not doing so from the outside, having recourse to musical and social characteristics aimed at objectifying them (musicological descriptions, origins and history, social background, media reports and current success, etc), but, in conformity with the seminar's ground rules, on the basis of how they are listened to and how they are presented by fans, so as to try to understand how and for whom they 'make music'.

One argument maintains that there is an important and arbitrary distance between the musical elements of rap or techno and the social meanings and practices that are grafted onto these musical genres. Another runs contrary to this, claiming that the former is strongly determined by the latter. Only experiment can cast light on the respective merits of these positions, or perhaps reveal a variety of situations and evolutions. This is how the idea originated that we ought to look at both the music that is made and the music that is spoken about: sounds and contexts. By controlling the conditions of the experiment, the registers that are mobilized to present the interest shown in a particular form of a music are left open, and no attempt is made to solve a priori the question of the existence and solidity of the relationship between one and the other. Another way of describing our aims is: to evaluate simultaneously, in reciprocal fashion, music and the criteria of its perception; to resist choosing between music and words, and, instead, to let the musical and social characters of rap and techno emerge together; to discover what the relevant features of these genres are for their followers, to feel what it is they hold, to perceive more easily how they lend themselves to the construction of differences and formulation of judgements, and more generally to understand how they attach their publics to themselves. Lastly, there is an intention to examine the connections, more or less necessary, that these genres establish between the musical products themselves and the comments to which they give rise.

The meeting takes place in a room of the Bibliothèque Nationale de France equipped with a comprehensive sound system, and a technician also present. The two members of the seminar who volunteered to present 'their' music are both free to conceive and organize their presentations how they wish, and make full use of their access to this equipment. Members of the seminar gather their friends to act as a public, and we total about thirty in the room. We first approach rap, then techno, and we then link them together afterwards in a general debate about what has taken place.

Rap: Music or Revolt?

Within the framework given, Pierre, who has put together a presentation on rap (or of rap…), chooses to offer his contribution to the sociology of musical genres by exploring how playing and listening to musical excerpts might change the character of an academic argument: what is it that listening alone is able to render comprehensible? I should like it to be the music that makes us speak rather than the reverse (the sociologist would usually wish to say something concerning the music). I hope that the pieces I've chosen will make the audience react, while I will arrange the recorded titles a bit like a DJ, in accordance with his own reactions…

The idea is to approach as closely as possible the ordinary, commonplace conditions of listening -leaving aside the oddity of this room as an environment for rap. To be strictly accurate, therefore, it is not about putting on a show: there are no vinyl records, sampler, computer, or microphone, only simple CDs and cassettes, the most common supports of rap appreciation (along with radio broadcasts, some of which Pierre had recorded on cassette). Pierre specifies, finally, that he is a researcher, and that the problem is not whether or not he is huge fan of rap: 'I am not trying to convince anyone of the "quality" of the music…'

The first song, 'Touche pas à ma musique' ('Don't Touch My Music'), speaks, itself, about the relationship between words, music and origins -'ma clique' (my crew [gang])and of the relationships of the rapper with the media and the public; it highlights the difficulty of not privileging voice and text in the analysis of this production. Pierre points out that fans act similarly; they read the lyrics in the booklet, and the dedications and other 'messages to Allah', before listening to the CD they have just bought. The photographs, the garish logos and graphics, also influence their listening. The listener would also do well not to disregard the device of self-presentation with which the singers unceasingly make play: scenes of studio activity and what the rapper does in life, obsession with realities, constant references to social background, and continual demands that listeners 'open their eyes' to the world. Another characteristic of rap is to play with this intricate web of enunciations and reminders of the context of these enunciations on the disc itself: the presence of intros, outros, and other interludes which are inserted among the pieces, made up of played scenes, studio outtakes, telephone conversations; these are all means of ensuring the rapper reappears in the context of his production, and are often present also, as Pierre says, 'to reinforce a social layer'. Rap is not detached from the rapper, the production forms part of the product. In a symmetrical way, among fans, the same applies to 'promo' clips, which are always accompanied with abundant notes: they re-forge the link between the music, the rapper, and the social message of rap. The common practice of fans never ceases to show that they themselves privilege these aspects, and they associate them, above all, as values that have come out of what they consider rap. A rapper like MC Solaar, for example, is criticized for the distance he has moved from his social origins, rather than for his music; he is regarded as having become too integrated.

Pierre tries to choose titles from a great variety of styles to counter the current gibe, 'rap, it's always the same' ('Staying Alive' by Wyclef Jean, an American rapper labelled more 'commercial', a production starting out with a well-known 'sample' of the Bee Gees; Wu Tang Clan, a celebrated New York group with a dark and tortured sound; an excerpt from two contrasted albums by Stomy Bugsy; and two deliberately very different songs from the same album by NTM, 10 'La Fièvre' and 'Paris sous les bombes'). The audience comments most of all, in rather crude terms, on the moods that the pieces unleash: harder or more festive, darker or more dancing… The rhythm and tempo sometimes seem more stereotyped, sometimes more unusual or studied. Lastly, the manner of declamation, the accents and use of the voice take us back to what seems to be the most salient element of the genre, speech-song.

In the discussion, which opens up little by little, participants suggest that the rather stable character of the resonant background, once the piece began, invite few remarks, whereas the words call automatically for social commentary. The same goes for rapping: to what extent is it really possible to vary it? Is this not typically the trick, the key feature whose success certainly gave rap its strong image, but also brings the danger, as in the case of punk or pure and hard rockabilly, that such an over-rigid process can be captive to a binary choice: either you obey it (although it is very determining and constraining) and you rap, or you free yourself of it, you twist it, you shift, but almost immediately you do so, you are no longer rapping.

From here, questions slip inexorably towards two topics, in spite of the efforts of Pierre who tries to retain everyone's focus on the precise sound aspects of the songs: on one side, the mode of production, the social context of the rappers, and on the other the content of the words and, more particularly, their social import. The problem, also, is that the object leaves us no choice. The desire to determine it externally, outside its own criteria, does not lead to the revelation of a level that is usually invisible or latent. It just misses the specific space where arrangements, variations, and relevant connections come together, that space to which we are sent back constantly when following up the sayings and doings of actors, musicians as much as consumers or commentators. Pierre is led to explain the way in which rappers work, the origin of the 'samples' and, more generally, the relations that allow the social milieu of rap to function, the DJ market, the origins of the movement in the United States and in France. But he is reticent: all this can be found in the literature that has multiplied on the subject of rap. 11 The debate becomes animated around the greater or lesser degrees of 'gangsterism' found in this milieu and the type of relations that reign there: clans, mafia-like godfathers, influence and threats, machismo, the rule of money and of bragging… In which part they are real, and simply overplaying the myth -in a symbolic inversion of the dominant order, as sociologists might say. What are the practical consequences, in everyday reality, of this aggressive behaviour, even if it is in large part provocation and ostentation?

The room reacts in particular to the violence, racism or sexism of certain words, comparing these features with those of earlier movements such as rock 'n' roll or punk, while trying to decide if they are irony or provocation, and insisting that these are not simply the same things in all cases. Several speakers considered, for example, that the use of Nazi insignia by punks, or the satanic imagery of hard rock, was more a means of displaying excess and of dramatizing their gestures of provocation, whereas the macho violence of rappers seems closer to the reality of their practices. But the positions are not distinctly taken: there is a feeling that it is difficult, where this topic is concerned, not to fall into well-worn debates of this kind, so everyone maintains a highly questioning stance. This method is no less effective for that. It allows two current questions, often hostile to the genre, to be stated and discussed together: does rap reflect (at best) or reinforce (at worst) youth violence? Is it not, at the same time, a commercial and mediatized music, completely integrated, so that its provocative character, far from constituting a revolt and a challenge to 'society', helps it above all else to sell (a question posed in similar terms in connection with the Rolling Stones in the early days of rock 'n' roll)? Moving in another direction, a condescending complacency is identified and criticized, one that would excuse rap of any moral or political responsibility in the name of the social and ethnic origins of its actors: one cannot, without going to extremes, use this as an excuse for regarding an insulting and aggressive vocabulary as merely a matter of choice of style, something neutral or unimportant on the political, human, or moral level, and without effect on individuals or communities.

Techno: Grasping the Sound

Paul, for techno, took a resolutely different approach. For him, showing is doing. He arrived with two friends, one of whom is a DJ, and all their hardware, in particular their metal bags filled with vinyl discs. In front of the two turntables, installed with the help of the technician, they provide a brief history of the origins and the various types of techno. The room quickly stifles its questions about the reality of events or filiations, the alternatives to this account, its mythological character, its grey areas: we understand that there will be no chatting about these. That is not what it is about. This 'history' is that of a young collectivity rewriting itself in real time. It furnishes techno with some worthy ancestors such as Pierre Henry, Tangerine Dream and Kraftwerk, and if this mythology is extended, Schaeffer, Russolo, Stockhausen… It retraces techno's origins to key moments and founding places, already so familiar: Jamaicains in New York, garages in New Jersey, English dance clubs, the French disco DJs that preceded them. It is myth in the strong sense, a performative history: a genre is created in front of us through the story it tells of itself.

In the same way, the succinct presentation of the principles of production and the hardware required ('kick', beat boxes, samplers, MIDI sequencer), followed by a classification of the types of techno presented by their English names and their tempo ('bpm' or beats per minute), leaves us with the impression of a catalogue. Can a genre be adequately defined with reference only to a certain number of drumbeats per minute? From trance, at between 125 and 140 bpm, and tribal techno, one goes up and up, passing by 'acid core' in the neighbourhood of 150-160, then 'gabber' and hard-techno, until we reach 'hard core' at 180-250, but which, we are told proudly, can go up to 300 bpm… We assume that it is not a matter of analysis but, rather, a verbal characterization, and it will be more a question of measuring its appropriateness and effectiveness than engaging in discussion. In fact, it is an introduction to this world, a guide to places, and a lexicon of the names of the various components of these sub-genres. This brief account leads on to a rapid characterization of 'hard core', the genre that Paul and his associate champion, and are now going to illustrate.

They play several discs, as for a mini-set, and show us the principal forms of DJ intervention, how vinyl discs are chosen, the use of headphones and techniques for handling decks in order to superimpose and connect discs, and how they conceive their temporal mode of working as being similar to 'telling a story'. 12 They call some people to the front to have a go at handling the consoles and at coordinating their movements so as to mix sounds and alter speeds ('mixes' and 'scratches'). They discuss with us the tricky moments that the DJ encounters, and whether or not these mistakes are perceived by those in the know, or by us: blank spaces, ruptures, feelings of missed transitions, an overall mood that is awkward to assess.

The opposing strategies adopted by Pierre and Paul are seen in every detail of their two demonstrations: Pierre's warning not to take him for a rapper is answered by Paul's personal image, his shaven head and posture indicating clearly: 'I am one of them!' The analytical effort of the first is answered by the hip vocabulary of the second, which for the uninitiated resembles a code. The effort to objectify and set apart is countered by teaching by example -'see it and do it yourself' -and the imitative seduction that witnessing a passionate interest arouses. Pierre even started debates he wanted to avoid -but these verbal skids clearly reveal rap's constitutional character. Paul creates the desire on our part (at least temporarily) to share in and to live the object that his presence and gestures conjure up before us and among us -at the same time as he forces us to delay any effort to analyze what comes out of this.

And, indeed, as soon as Paul and his friend go into action, the nature of the scene changes. They invested a great deal of themselves in the meeting, and thought meticulously about their discs. They put body and soul into it. The very institutional character of the place acted as a challenge to these underground musicians, who were excited with the idea of dynamiting the interior of such a temple (and, perhaps, the normal procedure of the seminar, too), in 'making the BNF collapse under the kil[owatt]s' (even if, as a concession to our eardrums, they officiate with a tenth of the power of some sound systems). The effect is powerful, and several among us comment on it in various ways: the issue is no longer that of wondering whether or not this music is liked, which would be too much the question of an outsider, but of seeing it actually being made, feeling those moments that are more or less successful, following the variations, recognizing the variables on which the DJ works, the more or less reverberant or dry timbres, the density of texture, the continuity of sound, its ruptures and restitutions, the more or less intense or relaxed moments, etc.

It is an important lesson, one that relates to the communicative effect of practice -the mimetic force of doing compared to saying -but even more so to the multiplicity of possible registers of participation in a musical experiment. Indeed, it is all about the continuity of these positions, rather than a binary opposition between those who would be fans and participants, and those who would be non-fans and distant: we did not become fans of techno, losing at a stroke all critical sense, renouncing any analysis in the pleasure taken. No, thanks to the mediation of a new system, we simply went through a series of intermediate positions together. We allowed ourselves a kind of delight by proxy, which engaged us truly, though only for a limited time and place, in the position of the fan. Even the production of this 'us', with its provisional and fragile status, is an effect of the meeting. As one among us says, 'the principal result of the meeting for me will not be to make me like techno more on a personal level, but to understand that it can be liked'. While it is being played, we are sensitive to the work on 'samples', to this emphasis on over-exploiting a dimension of sound -work that, after all, very much resembles that which, employing more diverse parameters (from the sounds of natural instruments to the voice, from polyphony to rhythmic devices), can be found in many musical practices that pre-date techno. A specific point seems to hold in the technical production of this music: there is no distance between the gesture that makes the music and the music that is made. Admittedly, to a certain extent, sight supports sound this way in any performance: watching executants is helpful when paying attention to their artistic technique. It is said, in fact, that spectators themselves play a little, too, by making imitative gestures with their bodies, such as swaying to the rhythm of the music they hear being played. However, the gap between the gesture made on an instrument and the music that is produced remains important. The visual and sonorous references coincide on secondary parameters (rhythm, intensity…), rarely on melody and harmony, which cannot be 'seen' so directly. There is, by contrast, a striking immediacy between the gestures of the DJ and the sonorous effect produced: the speed shifting of discs, the move from one disc to another, the rise or fall of the volume, all is heard immediately. Techno operates through the manipulation of music already made, and this interval in which the pre-existing material becomes transformed and mixed has, as a curious consequence, an extreme legibility, an easy and instantaneous passage between gesture and sound that helps it to be perceived very clearly. We wonder if the rule that requires DJs to be invisible on techno nights and, instead of being positioned as instrumentalists, to officiate without being seen, is not a means of compensating for this excessive visibility of their actions; the device of having music diffused by the sound system and coming from who knows where, accords with this idea, and is perhaps, in this second form, aimed at giving dancers another interpretation of the music, one suggestive of interior vagueness, of something lacking a place in which to show itself. Within the framework of our meeting, this visual element contributed strongly to the success of the experiment. For music such as this, which merges with the gestures of its own making, to show is to explain: the tactic adopted was perfectly adapted to that end. We even relativize our initial doubts regarding the definition of sub-genres of techno by their tempo alone: in practice, it is striking to see how far this aspect -the work on a throbbing 'boom-boom', and on the very mechanics of the rhythmic repetition of an evolving sound -is, in effect, largely determined by tempo.

Sociology 'of' Techno, Sociology 'of' Rap

The concluding discussion relates to social milieu, the sociality of techno fans, the market forms of this music: the disc is its original material (the DJ's vinyl) at the same time as it is one of the end products; but a good share of techno reality breaks free of the recording in much the same way as it does from orthodox visual media -that said, and more, it is also listened to at home and is sold in the form of commercial recordings. Finally, if a part of the activity is semi-clandestine and displays an underground and anti-commercial character, the other part could not be more overt, installed, as it is, in the world of nightclubs, and subject to the strong internal divisions carved out in a predictable manner among enthusiasts for this dance music and followers of rave parties.

From here, of course, the debate could bounce back and the comparison with rap recommence. It seems that the sociology of techno is played out elsewhere, in a form somewhat removed from the classical opposition between a 'pure' origin and the commercial success to which rap aspires, a form that more closely resembles that of avantgarde or 'underground' movements: the development of a common taste and practice gradually produces the new identity of a group, then around it, if this is successful, a 'generation' arises, produced and expressed in that same gesture. It is on location, while going to the clubs, the dance nights, and the rave parties, that, in line with ethnological practice (and in the manner achieved by works on rock by Frith or Hebdige thirty years earlier) one will begin to 'feel' one's way towards a sociology of techno, beginning from its capacity to engender a collective.

In the case of rap, there is constant reference to life style, to 'social problems' (misery and injustice, affairs and scandals…), to imaginary universe of the media rappers are part of (animated drawings, gangster or kung-fu films, TV news…), or to the record market and television networks. This is a long way from the usual split between musicians who take a purely musical view of their production and sociologists who come along to reveal what truly determines it (despite the resistance that producers and fans are supposed to marshal against such revelations): here, instead, it is the producers who ensure that sociology is at the forefront in registering the manufacture and interpretation of their music. This is what makes the sociology of rap problematic: the comments made by specialists give the impression of merely repeating -in a heavier fashion and with too much seriousness -the analyses of rappers themselves. The sociologist, being used to dealing with the aesthetic opinions and blithely anti-sociological comments of fans, is caught on the back foot by an explicitly sociological discourse. His discipline has become a central resource of the actors he observes, and his own analyses compete with theirs: the sociology 'of' rap is, at the same time, something that rap itself puts to work, and what sociologists make of it.

Finally, in this context, and in a symmetrical way, the question of the sociologist's own tastes weighs once more upon him: 'Hey you! Aren't you a bit of a fan, too? What drives you to write on the sociology of this music?' It is not that being reflexive implies that you can only study what you like, and that yesterday's prohibition should become today's imperative (the sociologist engaging his tastes in his work, instead of making an abstraction of his 'subjectivity' and adopting a pose of benevolent neutrality), but an account of how the analyst relates musically to the music that is being analyzed ought to be added to the analysis as another level of reflexivity, one bearing on the tastes and reactions of the sociologist, and showing how he is affected by what he hears in the capacity of listener or fan.

Conclusion: Towards Reflexivity, Reflexivity and More…

When compared, the two methods of presentation can be seen as putting to the test the tacit assumptions that each presenter held about the music they were dealing with: the first envisaged it as an object captured on tape, ready to be dissected; the second thought that the only way to put the music across was to produce it live, trusting in the gestures it made and the sonic events it propelled into being. It was a choice crowned with success, at least in the local circumstances provided by our comparative exercise. The fact that Paul hardly speaks does not mean that his activity does not 'speak'. It speaks because, even without words (or almost without), he shows how the activity relates to itself in the course of its realization. Through his hardware, his gestures, his attentiveness to our reactions, his effort to make the progressive and hesitant production of a sound world emerge before us and within us, Paul presents his music to us by offering us a grasp of something the noninitiated can seize for themselves. In response to his work, we do not take a distant and objective stance but, rather, our bodies are engaged in the experiment, the body being one of the experiment's variables in two meanings of the term: a changing element, not fixed or given, and one with reactions to be measured. 'Seek out the reflexive, the reflexive and more!'

The method we had imagined -music to be heard, music to be seen -was completely redundant in the case of techno and, in contrast, incoherent when it came to rap. Rap is full of demands, of setting in context, of provocation, and these social registers call for a response or reaction that quite naturally maps itself around the same issues. The last and most important level of reflexivity is revealed here: 13 music is not an inert object, but rather an animated project that already proposes its own interpretations. It calls for certain reactions, and often constructs and evolves itself as a genre as a result of those reactions. It is for that reason that a genre's history becomes part of the genre itself; it participates in the collective work to isolate relevant features and to characterize what needs to be done. This it achieves even if it means some features become solidified, either musically, as over-rigid formulas (following the tendency of music that is easy to identify but difficult to revitalize, such as the tango and rockabilly mentioned earlier), or socially, as is the danger with rap, resulting in a 'posture' of revolt or a quickly stereotyped social gesture, even if the desire was to be seen as a protester.

These elements can be linked to musical features, but also to much else besides: to ways of doing things or being together, to practices or ritualized signs, to age groups and various social or ethnic identities. It is not for us to sort them out. A very particular analytical interest concerning these present-day musical styles comes precisely from the coconstruction of these features among themselves. Their contemporary character enables us to assist in this process 'live', before music history solidifies them into musical genres. Analysis consists partly of recognizing these criteria, rather than imposing criteria from without.

It is thus that, ultimately, this double meeting invited us to participate in a lesson on reflexivity. There is no middle-of-the-road position in this matter. The sociologist may feel able to take his leave, discrediting the spontaneous sociologies of the actors he has observed, and emphasizing the distance between his own critical and scientific analyses and their partial and interested accounts, but we would argue that he does not actually 'leave' at all. On the contrary, he dives right into the problem: he accepts the need to take issue with the totality of accounts given about a certain type of music, his own included. This is not to say that he abandons all critical direction when situating himself among and playing with this mass of lay opinion, but, instead, that he goes along with all the little differences of perception, which may be described as the variable series of effects and adjustments that the friction of reflexivities involves. This ranges from his own questions, as a sociologist, about what is occurring before him and in him (as much sensory as cognitive), to the words of actors who, far from lacking categories in which to think their practice, do not differentiate between musical activity in itself and the activity of drawing a framework around it. 14 14 It is clear that the concept owes much to ethnomethodology; see Harold Garfinkel, Studies in Ethnomethodology (Englewood Cliffs, 1967). For classical presentations of what is at stake in reflexivity, see James Clifford et George E. Marcus (eds), Writing Culture: The Poetics and Politics of Ethnography (Berkeley, 1986); Steve Woolgar, Knowledge and Reflexivity (London, 1988(London, ) (1988)); Malcolm Ashmore, The Reflexive Thesis: Wrighting [sic] Sociology of Scientific Knowledge (Chicago, 1989);and Ulich Beck et al., Reflexive Modernization (Cambridge, 1994).

11

  Among the best-informed French texts are the following, rather arbitrarily selected from a now abundant field: George Lapassade and Philippe Rousselot, Le Rap ou la fureur de dire (Paris, 1990); S.B.G. Desse, Freestyle (Paris, 1993); Hugues Bazin, La Culture hip hop (Paris, 1995); O. Cachin, L'Offensive rap (Paris, 1996); José-Louis Bocquet and Philippe Pierre-Adolphe, Rap ta France (Paris, 1997); Manuel Boucher, Rap expression des lascars: significations et enjeux du rap dans la société française (Paris, 1998); Christian Béthune, Le Rap: une esthétique hors la loi (Paris, 1999); E. Grynzpan, Bruyante Techno (Nantes, 1999); S. H. Fernando Jr., The New Beats: culture, musique et attitudes du hip hop (Paris, 2000); and 'Territoires du hip hop', Art Press, Hors Série (December 2000).

Translated from the French original by Derek B. Scott.

To paraphrase Marcel Duchamp's famous 'Ce sont les regardeurs qui font les tableaux' (it's the viewers who make paintings).

I am only the compiler of this essay; it would be more correct to speak of the collective work of participants of the CSI-School of Mines/CNRS seminar 'Aimer la musique. Musicologie du goût, sociologie de la musique, histoire de l'amateur', which I directed with J.-M. Fauquet (Antoine Hennion, 'L'Ecoute à la question', Revue de musicologie, 88/1[2002]: 95-149). I thank in particular Morgan Jouvenet for the part on rap. An early French version of the present text appeared as'Musiques, présentez-vous!', French Cultural Studies, 16/2 (2005): 121-34.

It is not a question, either, of revealing that the social determinations of people's tastes are personal and subjective, in accordance with the familiar critical posture of the sociologist (Pierre Bourdieu, Distinction: A Social Critique of the Judgment of Taste[London, 1984]), nor of imagining a wild typology of listeners born of our own aesthetic conceptions, following the example of what Theodor Adorno does with as little scruple as talent ('Types d'attitude musicale', Introduction à la sociologie de la musique[Genève, 1994], pp. 7-25).

'Nique ta mère' ('fuck your mother'), one of the first famous rap groups in France.

Morgan Jouvenet, '"Emportés par le mix": les DJ et le travail de l'émotion', Terrain, 37 (September 2001): 45-60.

Anoine Hennion, 'Those Things That Hold Us Together: Taste and Sociology', Cultural Sociology, 1/1, ed. D. Inglis, R.Wagner-Pacifici (2007): 97-114.


Hugues Albini Lomami

Camille Damour

Giuseppe Rosi

Anne-Sophie Poudrel

Arnaud Dubory

Charles-Henri Flouzat-Lachaniette

Guillaume Haiat

Ex vivo estimation of cementless femoral stem stability using an instrumented hammer

Keywords: Hip arthroplasty, Cementless femoral stem, Primary stability, Impact hammer In preparation for Clinical Biomechanics

Background

The success of cementless hip arthroplasty depends on the primary stability of the femoral stem. It remains difficult to assess the optimal number of impacts to guarantee the femoral stem stability while avoiding bone fracture. The aim of this study is to validate a method using a hammer instrumented with a force sensor to monitor the insertion of femoral stem in bovine femoral samples.

Methods

Different cementless femoral stem were impacted into five bovine femur samples, leading to 99 configurations. Three methods were used to quantify the insertion endpoint: the impact hammer, video motion tracking and the surgeon proprioception. For each configuration, the number of impacts performed by the surgeon until he felt a correct insertion was noted Nsurg. The insertion depth E was measured through video motion tracking, and the impact number Nvid corresponding to the end of the insertion was estimated. Two indicators, noted I and D, were determined from the analysis of the time variation of the force, and the impact number Nd corresponding to a threshold reached in D variation was estimated.

Findings

The pullout force of the femoral stem was significantly correlated with I (R²=0.81). The values of Nsurg, Nvid and Nd were similar for all configurations.

Interpretation

The results validate the use of the impact hammer to assess the primary stability of the femoral stem and the moment when the surgeon should stop the impaction procedure for an optimal insertion, which could lead to the development of a decision support system.

Introduction

Cementless hip arthroplasty is now commonly performed in clinical practice to replace defective hip joints. Prior to the femoral stem (FS) insertion into the femoral shaft, the femur is reamed with a rasp that is slightly smaller than the size of the FS following a press-fit procedure, in order to obtain an adapted primary implant stability.

The FS primary stability into the host bone is an important determinant of the surgical success and the biomechanical properties of the bone-implant interface (BII) plays a crucial role in osseointegration phenomena that determines secondary stability. Micromotion at the BII should be minimized in order to avoid fibrous tissue formation that may lead to aseptic loosening. However, excessive stresses at the BII should be avoided since it may lead to bone necrosis and/or to femoral fracture.

In order to obtain an optimal stability of the FS and to avoid the aforementioned problems, the surgeon must adapt the number and the energy of the impacts realized to insert the FS into the femoral shaft, which is done using an orthopedic hammer. On the one hand, the FS should be completely inserted into the femoral shaft in order to maximize the bone-implant contact. On the other hand, impacts with too much energy may lead to intraoperative and/or post-operative peri-prosthetic femoral fracture. Surgeons use their proprioception to determine when impacts on the FS should be stopped, by listening to the sound of the hammer impact on the ancillary [START_REF] Sakai | Hammering Sound Frequency Analysis and Prevention of Intraoperative Periprosthetic Fractures during Total Hip Arthroplasty[END_REF][START_REF] Morohashi | Acoustic pattern evaluation during cementless hip arthroplasty surgery may be a new method for predicting complications[END_REF]. However, such criterion remains qualitative and highly depends on the surgeon's perception and experience.

A few quantitative methods aiming at assessing the insertion of the FS into the host bone have been reported in the literature. One of them consists in analyzing micromotions at the BII using linear variable differential transducers (LVDTs). This method was used to compare the FS stability in bone mimicking phantoms using different kinds of stem fixation [START_REF] Baleani | Initial Stability of a Cementless Acetabular Cup Design: Experimental Investigation on the Effect of Adding Fins to the Rim of the Cup[END_REF], for resurfacing hip implants with different loading conditions [START_REF] Cristofolini | Device to measure intra-operatively the primary stability of cementless hip stems[END_REF], to study the effects of implant undersizing [START_REF] Fottner | Influence of undersized cementless hip stems on primary stability and strain distribution[END_REF], and by considering unidirectional micromotion [START_REF] Leuridan | A biomechanical testing system to determine micromotion between hip implant and femur accounting for deformation of the hip implant: Assessment of the influence of rigid body assumptions on micromotions measurements[END_REF]. Other studies investigated the behavior of FS with different sizes or shapes in human femur [START_REF] Bieger | Biomechanics of a short stem: In vitro primary stability and stress shielding of a conservative cementless hip stem[END_REF] and in composite bone [START_REF] Schmidutz | Influence of different sizes of composite femora on the biomechanical behavior of cementless hip prosthesis[END_REF]. Nonetheless, due to complex experimental settings, it remains difficult to employ LVDTs in clinical practice.

Another technique consists in using vibrational analysis to follow the FS insertion [START_REF] Denis | Chapter 20 -Vibration Analysis of the Biomechanical Stability of Total Hip Replacements[END_REF]. This method has been validated in artificial bones [START_REF] Pastrav | Detection of the insertion end point of cementless hip prostheses using the comparison between successive frequency response functions[END_REF] [START_REF] Pastrav | In vivo evaluation of a vibration analysis technique for the per-operative monitoring of the fixation of hip prostheses[END_REF] and in cadaveric specimens [START_REF] Varini | Assessment of implant stability of cementless hip prostheses through the frequency response function of the stem-bone system[END_REF]. Although such approach is promising, much work remains to be done for clinical implementation.

Micro-computed tomography (µCT) has been used to quantify the micromotions at the BII [START_REF] Gortchacow | Simultaneous and multisite measure of micromotion, subsidence and gap to evaluate femoral stem stability[END_REF]. This technique was studied to assess FS primary stability in cadaveric femurs [START_REF] Malfroy Camine | Effect of a collar on subsidence and local micromotion of cementless femoral stems: in vitro comparative study based on micro-computerised tomography[END_REF]. However, metallic implants cause artefacts, thus limiting the quantitative investigation of the BII [START_REF] Park | Correlation between microtomography and histomorphometry for assessment of implant osseointegration[END_REF]. The use of synchrotron radiation µCT has been suggested [START_REF] Müller | Comparison of Microfocus-and Synchrotron X-ray Tomography for the Analysis of Osteointegration around Ti6Al4V Implants[END_REF][START_REF] Neldam | Synchrotron μCT imaging of bone, titanium implants and bone substitutes -A systematic review of the literature[END_REF], but this technique cannot be employed in clinical practice.

A method based on the analysis of the variation of the force applied between an instrumented hammer and the ancillary during impacts was developed by our group primarily in the context of the assessment of the acetabular cup (AC) implant stability. This approach was first assessed using mass drops, which validated the possibility of monitoring AC cups insertion in bovine bone samples by analyzing the behavior of the contact duration [START_REF] Mathieu | Biomechanical determinants of the stability of dental implants: Influence of the bone-implant interface properties[END_REF][START_REF] Mathieu | Variation of the impact duration during the in vitro insertion of acetabular cup implants[END_REF]. Another indicator based on the impact momentum was used to assess the AC implant primary stability [START_REF] Michel | In Vitro Evaluation of the Acetabular Cup Primary Stability by Impact Analysis[END_REF]. The method was validated ex vivo (Michel et al., 2016a) and in anatomical subjects (Michel et al., 2016b). An alternative hammer instrumented by strain sensors was also developed to assess the AC implant stability [START_REF] Tijou | Ex Vivo Evaluation of Cementless Acetabular Cup Stability Using Impact Analyses with a Hammer Instrumented with Strain Sensors[END_REF]. In order to better understand the phenomena related to the AC insertion, static [START_REF] Nguyen | Influence of anisotropic bone properties on the biomechanical behavior of the acetabular cup implant: a multiscale finite element study[END_REF] and dynamic (in the time [START_REF] Michel | Finite element model of the impaction of a press-fitted acetabular cup[END_REF] and frequency (Henyš et al., 2018) domains) finite element models (FEM) were developed. Another group has used a comparable approach using a force cell instrumented hammer to measure an impaction energy on AC implants (Henyš and Čapek, 2018).

More recently, this same technique using an instrumented hammer was applied to follow the FS insertion in bone mimicking phantoms [START_REF] Tijou | Monitoring cementless femoral stem insertion by impact analyses: An in vitro study[END_REF]. Another indicator D based on the time history of the force recorded by the hammer during successive impacts was used to estimate the FS insertion endpoint. However, the in vitro study implied using bone mimicking phantoms, which have different material properties compared to bone tissue.

The aim of the present study is to validate a method based on the analysis of the variation of the force as a function of time obtained during impacts realized between an instrumented hammer and a FS inserted in bovine femoral samples. The signal processing technique aims at quantifying the insertion endpoint of the FS using three methods: i) the instrumented hammer, ii) video motion tracking and iii) the surgeon proprioception. To do so, the instrumented hammer was used to insert different sizes of FS in five bovine femurs. A system using optical markers was used to follow the FS insertion with video motion tracking (VMT). The orthopedic surgeon determined the FS insertion endpoint using his proprioception. Eventually, the impact momentum was determined and compared with the FS pullout force.

Materials and methods

Implants and specimens

Cementless FS (CERAFIT R-MIS) manufactured by Ceraver (Roissy, France) were used. These implants were made of titanium alloy (TiAl6Al4V) and coated by hydroxyapatite to enhance osseointegration phenomena.

Seven sizes were tested, from size 7 to 13, with their corresponding rasps also manufactured by Ceraver (Roissy, France) used prior to FS insertion. An ancillary was screwed into the FS so as to obtain a rigid bilateral fixation between the two parts.

Five bovine femurs were collected from the local butcher's shop and were prepared similarly to [START_REF] Tijou | Monitoring cementless femoral stem insertion by impact analyses: An in vitro study[END_REF]. The sample was cleaned from all residual soft tissues. Each sample was cut at the boundary between the diaphysis and the distal epiphysis, lower than the middle of the femur, so that the biggest implant could be fully inserted in trabecular bone. The distal end of the sample was then embedded in a fast-hardening resin (SmoothCast 300 polymer, Smooth-On, Easton, USA). Eventually, an osteotomy was performed on the femoral neck according to the clinical procedure. Figure 1 shows an image of the sample.

Experimental set-up for impacts

Femoral stem insertion

A schematic representation of the experimental set-up is shown in Fig. 1. The resin block was clamped inside a vise in order to avoid any displacement perpendicular to the femur axis. The 1.3 kg hammer used to insert the FS inside the sample was the same as in our previous study [START_REF] Tijou | Monitoring cementless femoral stem insertion by impact analyses: An in vitro study[END_REF]. A dynamic piezoelectric force sensor (208C05, PCB Piezoelectronics, Depew, New York, USA) with a measurement range up to 22 kN in compression was screwed in the center of the hammer impacting face. s(t) was defined as the variation of the force (given in kN) applied to the ancillary and recorded by the force sensor as a function of time t (given in ms). A data acquisition module (NI 9234, National Instruments, Austin, TX, USA) with a sampling frequency of 51.2 kHz and a resolution of 24 bits was linked to a LabView interface (National Instruments, Austin, TX, USA) to record s(t) for a duration of 13 ms.

Video motion tracking

Video tracking was carried out to follow the insertion of the FS into the femoral shaft during the impaction procedure. A camera (Powershot SX410 IS, Canon, Tokyo, Japan) was used, with a frame rate of 24 images per second and a resolution of 1280x720 pixels. Optical markers were glued on the FS, the ancillary and the femur, as shown in Fig. 1. Their relative displacement was calculated with the software Tracker (Cabrillo College, Aptos, CA, USA). The following distances were defined by analyzing the image after a given impact #i by using the markers on the FS and the ancillary (which is constant throughout the experiment) as a scale to convert pixels into centimeters. 𝑑(𝑖) (given in centimeter) was defined by the distance between the markers located on the bone sample and on the ancillary after impact #i. Therefore, 𝑑(0) corresponds to the initial distance between these same markers (before impaction). The indicator E (given in centimeter) was defined by the relative motion of the FS compared to the femoral bone compared to the initial position following:

𝐸(𝑖) = 𝑑(0) -𝑑(𝑖) (in cm) (1)

Femoral stem stability assessment

After each test, the FS was pulled out from the bone sample in the direction of the axis of the stem, using a tensile testing machine (DY25, Adamel Lhomargy, Roissy en Brie, France). The resin block was fixed and a constant displacement rate of 5 mm/min was applied to a hook screwed to the FS at the ancillary location and attached to the machine. The pullout force F, given in daN, defined as the maximum value of the measured force, was determined for each test and corresponds to the FS primary stability.

Signal processing

A dedicated signal processing method was implemented in order to analyze the signals s(t), as illustrated in Fig.

2

. Two different indicators were calculated for each impact.

The first indicator, noted D and given in ms, was defined as the time difference between the time of the second and the first local maxima of the signal s(t), and was calculated as follows:

𝐷 = 𝑓 2 (𝑠) -𝑓 1 (𝑠) (in ms) (2)
where the functions f1 and f2 are applied to a signal s(t) and were defined by the time of the maximum value of the first and second peak of the signal s(t), respectively.

The second indicator, noted I and given in kN, corresponds to the impact momentum, and was defined similarly to our previous works [START_REF] Tijou | Monitoring cementless femoral stem insertion by impact analyses: An in vitro study[END_REF] as:

𝐼 = 1 𝑡 2 -𝑡 1 ∫ 𝑠(𝑡) 𝑑𝑡, 𝑓 1 (𝑠)+𝑡 2 𝑓 1 (𝑠)+𝑡 1 (in kN) (3)
where t1= 0.08 ms and t2= 0.35 ms delimit the integration bounds. The choice of the time window will be discussed in section 4.

Experimental protocol

Two series of experiments were performed by a trained orthopedic surgeon. In the first series (similar to what was done previously in [START_REF] Tijou | Monitoring cementless femoral stem insertion by impact analyses: An in vitro study[END_REF], see Fig. 3), all FS were inserted in a cavity that was reamed with the size of the rasp corresponding to the FS, while in the second series (see Fig. 4), which aimed at obtaining relatively unstable configurations, the size of the FS was smaller than the rasp size used to ream the cavity. Four bovine samples were considered for the first series, while only one bovine sample was used for the second series.

In the first series of experiments, a cavity was initially created in each bovine bone with the smallest available rasp (size n=7). Then the impaction procedure was performed following three steps. First, the FS -ancillary system was impacted with the instrumented hammer until the surgeon considered that the FS was stable. The corresponding number of hits was noted Nsurg. Secondly, 12 additional impacts with a first-peak amplitude comprised between 1 and 12 kN were carried out in order to assess whether the implant could be further inserted into the femoral shaft, and to determine the average value of the indicator I obtained for stable conditions of the FS. The choice of the limit values of the first peak amplitude (1 -12 kN) and the number of additional impacts (12) will be discussed in section 4. For each impact, the indicators D and E were determined as described above. Eventually, the implant was pulled out axially and the pullout force F was measured.

The aforementioned procedure was repeated four times with the same implant as far as i) the sample was not fractured and ii) the surgeons felt that it was possible to obtain a good FS stability. Then, the surgeon checked if the femur was fractured. If so, the tests were stopped for the sample. If not, a bigger size of rasp and the corresponding FS was used.

The protocol followed for the second series is described in Fig. 4. In order to obtain looser fixations, the size m of the femoral stem must always be inferior to the size n of the reamer. We started the procedure with a reamer of size n=11 and a femoral stem of size m=n-1. For the same cavity created by a reamer size n, we carried out the femoral stem insertion 4 times. Then, we checked the level of instability by measuring the pullout force.

 First, if the pullout force obtained is superior to 65 N after the fourth test, the configuration was not considered as unstable with the femoral stem size m=n-1 and we updated the size m of the rasp to m=m-1 until m=n-3.

 Second, if the pullout force obtained is inferior to 65N, the configuration was considered unstable and another configuration was carried on with a n=n+1 reamer size starting with a m=n-1 femoral stem size.

The procedure was stopped either when we reached n = 13 (because we did not have a reamer size greater than 13) or when the sample was fractured. The three-step impaction procedure was identical to the one applied in the first series of experiments. Each procedure was repeated four times.

Post-processing and data analysis

Following the experimental protocol described in the last subsection, three different analyses were performed for each insertion procedure in order to estimate the required number of impacts to reach the insertion endpoint, corresponding to the end of the migration phase of the FS into the femur.

The first method consists in following the variation of the indicator D as a function of the impact number i noted D(i). For each impaction procedure, the parameter Nd was defined as the number of the first impact for which:

𝐷(𝑖) ≤ 𝐷 𝑡ℎ , (4)
where Dth is a threshold chosen equal to 0.47 ms. The choice of this parameter will be discussed in section 4.

The second method uses the variation of E as a function of the impact number i, noted E(i). For each impaction procedure, the parameter Nvid was defined as the lowest impact number that respects:

𝐸(𝑁 𝑣𝑖𝑑 ) ≥ 𝐸 𝑚 -𝛿 × 𝐸 𝑠𝑑 (5)
where Em and Esd are respectively the average and the standard deviation of the values of E for the last ten impacts and 𝛿 is a parameter empirically chosen equal to 7. Therefore, the parameter Nvid corresponds to the impact number where the value of E becomes relatively close to its endpoint value given by Em. The choice of 𝛿 value will be discussed in section 4.

The third method corresponds to the one used in the clinic and is based on the surgeon proprioception, as described in section 2.2.5. When the surgeon felt that the insertion endpoint of the FS was reached, the parameter Nsurg was noted.

To summarize, Nd, Nvid, Nsurg correspond to three values of insertion endpoint obtained for each insertion procedure using the instrumented hammer, the video motion tracking and the surgeon proprioception, respectively. In order to determine the accuracy of the three aforementioned approaches, it was necessary to quantify the difference obtained between the different parameters values Nd, Nvid, and Nsurg. To do so:

 𝑀 𝑑 = 𝑁 𝑑 -𝑁 𝑠𝑢𝑟𝑔 was defined as the difference between the insertion endpoint obtained with the instrumented hammer and the surgeon proprioception,  𝑀 𝑣𝑖𝑑 = 𝑁 𝑣𝑖𝑑 -𝑁 𝑠𝑢𝑟𝑔 was defined as the difference between the insertion endpoint obtained with the video motion tracking and the surgeon proprioception,

 𝑀 𝑐 = 𝑁 𝑣𝑖𝑑 -𝑁 𝑑 was defined as the difference between the insertion endpoint obtained with the video motion tracking and the instrumented hammer.

Subsequently, for each impaction procedure, the average and standard deviation values of the indicator I (noted Im and Isd, respectively) were calculated. To obtain comparable results, only the six signals having their first-peak amplitude closest to 4500 N were selected. For each configuration, the standard deviation was calculated as the mean value of two batches of three signals, following the procedure used in our previous study [START_REF] Tijou | Monitoring cementless femoral stem insertion by impact analyses: An in vitro study[END_REF]. Choices of the number of signals and the central first peak amplitude will be discussed in section 4.

Results

Stable configurations (first series)

Table 1 shows the number of impaction procedures considered for each bovine bone sample for the first series of experiments. For sample #1 and implant size equal to 8 and for sample #3 and implant size equal to 8, only three configurations were considered because of a loss of stability obtained after three configurations, which made it difficult to exploit the fourth configuration. This loss of FS stability may be due to a modification of bone properties due to repeated impacts. For all other samples and implant sizes for both series of experiments, four configurations were considered.

Unstable configurations (second series)

Table 2 shows the number of impaction procedures considered for each bovine bone sample for the second series of experiments. Four configurations were considered for all implant and reamer sizes.

Combining stable and unstable configurations

When pooling the first and second series of experiments, the total number of configurations was equal to 99. 3. They are all in the same range of variation.

The distribution of the values of

The relationship between the indicator Im and the pullout force F is shown in Fig. 9 together with a linear regression analysis. A significant correlation is obtained between Im and F and the determination coefficient R² is equal to 0.81. The errorbars correspond to the standard deviation Isd obtained for each configuration.

Discussion

The main purpose of the present study was to determine whether an instrumented hammer could be used to follow the femoral stem insertion and determine its endpoint in bovine bone samples. This approach has previously been studied by our group with artificial bone samples [START_REF] Tijou | Monitoring cementless femoral stem insertion by impact analyses: An in vitro study[END_REF] with the same hammer and the originality of the present study is to consider bone tissue that has different material properties compared to bone mimicking phantoms. The decrease of D as a function of the number of impacts can be explained to the increase of the bone-implant contact ratio during the insertion of the FS, resulting in an increase of the stiffness of the bone-implant system that may in turn explain the increase of its resonance frequency. Note that other authors [START_REF] Oberst | Vibro-acoustic and nonlinear analysis of cadavric femoral bone impaction in cavity preparations[END_REF]) (Henyš et al., 2018) (Henyš and Čapek, 2018) also found a similar evolution of the resonance frequency during the insertion, with a comparable approach, using force signal tracking.

The results show that for the first series of experiments corresponding to stable configurations, the values of the pull-out force is comprised between around 100 and 250 daN, which is lower but of the same order of magnitude than values obtained with cadaveric specimen [START_REF] Barr | Effect of Simulated Early Weight Bearing on Micromotion and Pullout Strength of Uncemented Distal Femoral Stems[END_REF][START_REF] Grechenig | A novel locking screw hip stem to achieve immediate stability in total hip arthroplasty: A biomechanical study[END_REF]. The lower values of pull-out force obtained herein may be explained by the fact that i) the geometrical shape of bovine femoral shaft is not adapted to the shape of the FS and ii) the fact that repeated insertion may lead to modifications of the bone properties. In the present study, we considered unstable situations (the second series of experiments) in addition to the relatively stable ones in order to obtain a wide range of FS stability.

Figure 9 shows a significant correlation between the indicator Im and the pullout force F, for 99 configurations.

The determination coefficient equal R²=0.81 is in agreement with the results obtained in vitro (R²=0.67). The present study confirms that the previous results are still valid when considering bone tissue instead of polyurethane foam.

Figure 8Figure 8 shows a good agreement between the three methods developed to assess the number of impacts required to reach the FS insertion endpoint. Again, these results are consistent with the previous in vitro results [START_REF] Tijou | Monitoring cementless femoral stem insertion by impact analyses: An in vitro study[END_REF]. However, there remain some discrepancies between the methods, as shown in 8 and Table 3. The average value of Mc, equal to -0.19, shows a good agreement between the results obtained with VMT (Nvid) and the instrumented hammer (Nd). However, the average values of Mvid (-1.17) and Mc (-0.98) indicate that the surgeon evaluation of the insertion endpoint is assessed in average one impact after the results obtained with VMT and the instrumented hammer. Several factors can explain these differences. First, the surgeon proprioception is mainly qualitative and depends on the surgeon's tactile feelings, hearing and vision. Second, errors may occur using VMT due to i) changes of angular position, ii) possible macroscopic 3D movements and plastic bone deformation, iii) errors based of the image processing, such as the precision of markers tracking, which can be observed in the fluctuation of E indicator even after reaching stability (Fig. 6).

In this study, several parameters were chosen empirically. First, the number of additional impacts after Nsurg was chosen equal to twelve, resulting from a compromise between i) a sufficiently high number of impacts to Fourth, the value of δ was chosen empirically to find a compromise between a sufficiently high value to account for the constant increase of E after Nsurg (due to plastic deformation of the bone phantom) and a sufficiently low value in order not to underestimate the value of Em. Fifth, the values of t1 and t2 used to determine the indicator I result from an optimization study and maximize the determination coefficient of the linear regression between Im and F. Varying the value of t1 between 0.07 ms and 0.12 ms and the value of t2 between 0.30 ms and 0.35 ms did not alter significantly the results, with a difference of less than 1% in R² value. Furthermore, the number of impacts considered to determine Im (equal to six) was chosen sufficiently high to obtain a consistent value of Im and standard deviation. These selected impacts corresponded to the impacts with first peak amplitude closest to the overall median value equal to 4.5 kN.

This study presents several limitations. First, the biomechanical properties of the pelvic bone and of bovine femoral bone are different, as shown in Table 4 that compares results obtained in different studies (Michel et al., 2016b) [START_REF] Chiba | In vivo structural analysis of subchondral trabecular bone in osteoarthritis of the hip using multi-detector row CT[END_REF]. Although the variation range of bone volume over total volume ratio (BV/TV), trabecular spacing (Tb.Sp) and structure model index (SMI) are similar, there are significant differences for trabecular thickness (Tb.Th) and trabecular number (Tb.N). Furthermore, bovine femur has a much superior thickness than human femur and thus a different quantity of surrounding bone tissue, which can lead to discrepancies. These differences justify the need to compare our results in different bone models: artificial and bovine bone samples and in cadaveric specimen in the future. Note that we did not use human cadaveric samples because the ethical committee requires to validate the method in bovine bone samples before using cadaveric samples.

Second, the multiple use of the bovine specimen is not representative of clinical situations and could lead to changes of bone properties. In particular, the same FS was inserted consecutively four times in the same bovine sample without modifying the cavity, which constitutes a limitation because the bovine sample may deform inelastically and therefore may lead to a modification of the FS seating. However, we checked that the averaged relative variation of the pull-out force during the four impaction procedures realized with the same sample was equal to 6 daN, which is significantly lower than the value of the pull-out force. Moreover, a modification of the FS seating would not affect the reliability of our approach because it would correspond to a modification of the pull-out force that is likely to be detected by the instrumented hammer.

Third, our study only focused on one type of FS (CERAFIT R-MIS) and ancillary and further studies should be performed to evaluate the method for implants manufactured by others.

Conclusion

This study is the first ex vivo validation of the use of an instrumented hammer to assess the insertion endpoint of the FS as well as its primary stability. The results are in agreement with a previous in vitro study. The time variation analysis of the force applied by the hammer on the ancillary during the impacts allows the assessment of the number of impacts required for obtaining a satisfactory press-fit condition into the host femoral bone.

Further work will focus on the adaptation of this method to clinical conditions, in order to develop a medical device that could help the surgeons estimating the FS stability, with minimal changes of the surgical protocol.

Figure 5

 5 Figure5shows different examples of force signals s(t) obtained for six impacts of a given configuration (Sample #4, Implant size #9, cavity #9, test #3) corresponding to the first series of experiments. The respective number of each impact is indicated above each corresponding second peak of the signal. The value of D(i) first decreases with i and then stays constant after the fifth impact. Note that the shape of the signal corresponding to the insertion endpoint (n=5) changes compared to the signals obtained earlier in the insertion protocol.

  The variation of the two parameters D and E as a function of the impact number i obtained for the implant size equal to 7 and reamer size equal to 7 is shown in Fig.7. The dark vertical line represent the value obtained for Nsurg and Nd and the clear vertical line represent the value obtained for Nvid which is equal to 7. For this configuration, Nsurg= Nd =8 and Nvid=7, so Mvid=-1; Md=0;

  Md, Mvid and Mc, corresponding to the difference between the different methods developed for the estimation of the FS insertion endpoint is shown in Fig. 8. The average and standard deviation values of Md, Mvid, and Mc obtained for the 99 configurations are shown in Table

Figures 6

 6 Figures 6 and 7 indicate that the indicator D, corresponding to the time difference between the first and the second peak of the signals, decreases as a function of the number of impacts, until it reaches a constant value.

  obtain a reproducibility of the indicator I and a convergence in the variation of the indicators D and E and ii) a sufficiently low number to minimize fracture risk. Second, the upper limit of the range of variation of the maximum force of the impacts performed after Nsurg [1-12 kN] was chosen sufficiently low to minimize fracture risk. Note that this range of variation is similar to the usual range applied by the surgeon during the insertion, before Nsurg. Third, the choice of the threshold Dth = 0.47 ms was made after analyzing the overall values of D as a function of the first peak amplitudes for impact after Nsurg+1. Figure10shows the variation of the values of D obtained for the impacts performed after Nsurg. The dotted line corresponds to the threshold Dth chosen equal to 0.47 ms. No significant variation of D was observed as a function of the amplitude of the first peak. Moreover, the value of D was higher than Dth for only 4 impacts out of 1822, which validates the approach and explains the choice of Dth.

Figures captions Figure 1 :

 captions1 Figures captions

Figure 2 :

 2 Figure 2 : Illustration of a signal s(t) corresponding to the variation of the force as a function of time obtained for a given impact obtained with an implant size of 7 and a reamer size of 7. The quantities I, f1(s)+t1, f1(s)+t2, f1(s), f2(s), s(t) and D are indicated. The value of I corresponds to average value of s(t) between f1(s)+t1 and, f1(s)+t2.

Figure 3 :

 3 Figure 3 : Schematic representation of the protocol followed for each bovine sample of the first series of experiments corresponding to relatively stable femoral stem

Figure 6 :

 6 Figure 6 : Variation of D (gray line) and E (black line) indicators as a function of the impact number for the same configuration as the one corresponding to Figure 5.The horizontal black dashed line represents the penetration equal to 𝑬 𝒎 -𝜹 × 𝑬 𝒔𝒕𝒅 and the horizontal gray dashed line represents the threshold 𝑫 𝒕𝒉 = 𝟎. 𝟒𝟕 𝒎𝒔. The vertical line represent the values obtained for Nd, Nvid and Nsurg which are all equal to 5.

Figure 7 :

 7 Figure 7: Variation of D (gray line) and E (black line) indicators as a function of the impact number i for the implant size equal to 7 and reamer size equal to 7. The horizontal black dashed line represents the penetration equal to 𝐸 𝑚 -𝛿 × 𝐸 𝑠𝑡𝑑 and the horizontal gray dashed line represents the threshold 𝐷 𝑡ℎ = 0.47 𝑚𝑠. The vertical black dashed-dotted line represents the value obtained for Nsurg and Nd which is equal to 8 and the vertical gray dashed line represent the value obtained for Nvid, which is equal to 7.

Figure 8 :

 8 Figure 8 : Distribution of the values obtained for Md, Mvid and Mc

Figure 9 :

 9 Figure 9 : Relationship between the indicator Im obtained using the instrumented hammer and the pullout force F (R²=0.81 and p=1.49 ×10 -36 ). The horizontal error bars correspond to the stand deviation values Istd;. The black and grey points correspond to the first and second series of experiments, respectively.

Figure 10 :

 10 Figure 10 : Variation of the indicator D for all impacts performed after Nsurg+1 for all configurations as a function of the amplitude of the first peak. The horizontal dotted grey line represents the threshold Dth= 0.47 ms chosen to determine when the femoral stem is fully inserted (Nd)

Acknowledgment This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement No 682001, project ERC Consolidator Grant 2015 BoneImplant).