Karoline Maria Reich

Christian Domitian Huber

Wilburg Reingard Lippnig

Christian Ulm

Georg Watzek

Stefan Tangl
email: stefan.tangl@meduniwien.ac.at

Atrophy of the Residual Alveolar Ridge Following Tooth Loss in a Historical Population

Keywords: jaw atrophy, alveolar ridge resorption, tooth loss, residual ridge, denture adverse effects, dental implants, pathoaetiology

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Introduction

As people grow progressively older in modern societies, health problems associated with advanced age increase dramatically. These include, among others, oral diseases that finally may lead to tooth loss and necessitate prosthetic treatment for the patient to obtain the ability to chew and eat as normally as possible.

Tooth loss may have a variety of causes such as periodontal disease, caries and trauma leading to pulpitis.

Whatever the reason might be, the effect of tooth loss is always the same: The physiologic masticatory forces applied via the roots of teeth to the cancellous alveolar bone no longer persist [START_REF] Devlin | Alveolar Ridge Resorption and Mandibular Atrophy -A Review of the Role of Local and Systemic Factors[END_REF][START_REF] Ulm | Characteristics of the Cancellous Bone of Edentulous Mandibles[END_REF]. According to Wolff's Law [START_REF] Wolff | of Wolff's Das Gesetz der Transformation der Knochen[END_REF] and the Mechanostat Model [START_REF] Frost | Bone's Mechanostat: A 2003 Update[END_REF] disuse and a loss of mechanical stimulation is followed by the reduction of bone mass. This effect was originally demonstrated for limb bones. If these models also hold true for the alveolar process of the jaw and other regions of the skull remains to be investigated in more detail. Also the influence of other factors such as regionally different gene expression (long bones vs. cranial vault) resulting in site-specific osteoblast behavior, osteoclast activity and matrix composition has been discussed in this context recently [START_REF] Rawlinson | Adult rat bones maintain distinct regionalized expression of markers associated with their development[END_REF]. However, loss of teeth leads invariably to atrophy of the residual alveolar ridge being irreversible, chronic, progressive and cumulative [START_REF] Atwood | Reduction of Residual Rides: A Major Oral Disease Entity[END_REF].

The rate of atrophy varies greatly between different individuals [START_REF] Bras | Mandibular Atrophy and Metabolic Bone Loss[END_REF]) and even within one and the same person at different times or in different regions within the jaw [START_REF] Atwood | Reduction of Residual Rides: A Major Oral Disease Entity[END_REF][START_REF] Atwood | Reduction of residual ridges in the partially edentulous patient[END_REF]. However, atrophy is greatest during the first year after tooth loss; the reduction of the residual ridge is a lifelong process [START_REF] Sennerby | Mandibular Bone Resorption in Patients Treated With Tissue-Integrated Prostheses and in Complete Denture-Wearers[END_REF] but the rate of bone loss does decrease [START_REF] Denissen | Anatomic Considerations for Preventive Implantation[END_REF].

Atrophy of the jaw bone can be slowed down, but unfortunately not eliminated, with the help of an appropriate treatment using prostheses balancing the loads to the underlying bone or preferably using implant-supported overdentures [START_REF] Kordatzis | Posterior Mandibular Residual Ridge Resorption in Patients with Conventional Dentures and Implant Overdentures[END_REF] or endosseous implants (Jokstad et al., 2000a;Jokstad et al., 2000b;[START_REF] Bodic | Bone Loss and Teeth -Review[END_REF].

Yet, the influence of fixed or removable dentures is an issue of controversial discussion [START_REF] Devlin | Alveolar Ridge Resorption and Mandibular Atrophy -A Review of the Role of Local and Systemic Factors[END_REF][START_REF] Carlsson | Responses of jawbone to pressure[END_REF]. As complete dentures do not load the alveolus in the same way as the original teeth do, non-physiologic pressure is applied to the bone surface of the affected jaw, which might be a reason for increased resorption. This is particularly plausible for ill-fitting dentures that cause occlusal disharmonies and thus might enhance alveolar bone loss [START_REF] Devlin | Alveolar Ridge Resorption and Mandibular Atrophy -A Review of the Role of Local and Systemic Factors[END_REF]. Hence the quality of the denture as a holistic system is crucial to prevent local overloading of the underlying bone (Xie et al., 1997a). However, reduction of the height of the residual ridge is also observed in patients without denture treatment [START_REF] Devlin | Alveolar Ridge Resorption and Mandibular Atrophy -A Review of the Role of Local and Systemic Factors[END_REF]. Consequently, atrophy of the alveolar bone is not attributable to prosthetic factors alone [START_REF] Atwood | Bone Loss of Edentulous Alveolar Ridges[END_REF]. Anatomic conditions of the jaws, systemic factors such as sex and age, hormonal balance, local inflammations and masticatory habits are supposed to act as co-factors in the development of residual ridge resorption after tooth loss [START_REF] Atwood | Reduction of Residual Rides: A Major Oral Disease Entity[END_REF][START_REF] Gruber | Anatomie und atrophiebedingte Veränderungen der Kieferknochen[END_REF][START_REF] Kingsmill | Post-extraction remodeling of the adult mandible[END_REF].

To gain information about the pathology of atrophy of the jawbone after tooth loss unaltered by the influence of prosthetic procedures, a population without modern dental treatment seems appropriate to serve as a model. For this reason, the skeletal remains of a central European population unearthed from a mediaeval cemetery of the 7th-8th century A.D. was used for this study. As dental treatment was not or only rarely practised in mediaeval populations [START_REF] Watt | Caries Prevalence in the Permanent Dentition of a Mediaeval Population from the South-West of Scottland[END_REF][START_REF] Vodanovic´ | The Frequency and Distribution of Caries in the Mediaeval Population of Bijelo Brdo in Croatia (10th-11th Century)[END_REF], these skeletal remains are very useful for analysing the pure effect of oral diseases such as jaw atrophy on the residual ridge.

The purpose of this study is to characterise the patterns of jaw atrophy after tooth loss in an historical population without modern dentistry using an adapted classification system according to Atwood [START_REF] Atwood | Postextraction changes in the adult mandible as illustrated by microradiographs of midsagittal sections and cephalometric roentgenograms[END_REF] and [START_REF] Cawood | A Classification of the Edentulous Jaws[END_REF]. Understanding the natural history of this disease could help to improve our knowledge about atrophy of the jaw, the final result of tooth loss, and is a prerequisite for the development of new treatment strategies. This study also provides additional new information for the interpretation of living conditions and lifestyles of historical populations [START_REF] Alt | Zahnerkrankungen sind schon tausende von Jahren alt[END_REF][START_REF] Vodanovic´ | The Frequency and Distribution of Caries in the Mediaeval Population of Bijelo Brdo in Croatia (10th-11th Century)[END_REF][START_REF] Meller | Prevalence of oral pathologic findings in an ancient pre-Columbian archeological site in the Atacama Desert[END_REF].

Material and methods

Avarian skeletal remains

A large mediaeval cemetery with skeletal remains of an Avarian population was unearthed during archaeological excavations in the 11th district of Vienna, Austria. This Avarian cemetery of the 7th -8th century A.D. consisted of 705 graves. In total, the skeletal remains of 755 individuals were collected.

263 jaw specimens of this sample met the inclusion criteria of 1) completion of dental development, i.e. presence of permanent dentition and 2) a sufficient state of preservation of the jaw allowing the evaluation of the features described hereinafter.

Estimation of age at death and determination of sex was based on osteological and dental development criteria, [according to the recommendations of Sarospatak [START_REF] Ferembach | Empfehlungen für die Alters-und Geschlechtsdiagnose am Skelett[END_REF]] and had previously been performed

by [START_REF] Großschmidt | Paläopathologische Untersuchnungen an den menschlichen Skeletten des awarenzeitlichen Gräberfeldes Csokorgasse in Wien Simmering[END_REF]. Age at death was determined according to the degree of cranial suture synostosis, surface characteristics of the symphysis of the pubic bone and of the auricular surface of the ilium and the degree of tooth abrasion [START_REF] White | Human Osteology[END_REF]. The degenerative processes evaluated in this study, were not used to establish an age estimate and both factors are therefore independent of one another.

According to these data, the study sample was divided into three age groups of young (21-40 years), middle-aged (41-60 years) and old (61-80 years) individuals. Demographic data concerning age and sex distribution of the analysed population are presented in Table 1.

Methods

Three investigators (WRL, ST, KMR), experienced in the analysis of degenerative alterations of the jaw, discussed and interpreted the variation and expression of signs of atrophy in the sample. The application of the classification systems employed in this study was standardized and calibrated until a consensus was reached.

The practical evaluations and classifications however, were performed solely by one investigator (WRL) to avoid interobserver error. While undertaking these evaluations the investigator was blinded to age and sex of the individuals. The following features were macroscopically diagnosed under bright light.

Classification of atrophy stages

Based on the classification system of Nakamato (1968) six separate segments of the jaw were defined as depicted in Figure 1. Each segment of the 263 jaw specimens was assigned to one of the 6 stages of atrophy described by [START_REF] Atwood | Postextraction changes in the adult mandible as illustrated by microradiographs of midsagittal sections and cephalometric roentgenograms[END_REF] and [START_REF] Cawood | A Classification of the Edentulous Jaws[END_REF]. (See Figure 2)

Stage 1 describes the physiological state of preextraction at which the tooth is still in the alveolar socket or the tooth is lost post mortem. Stage 2 is assigned to tooth loss immediately before death. There are slight osseous reactions of new bone formation within the alveolus. The alveolus is still in a good condition and the edges might be sharpened.

In stage 3 the alveolus is completely refilled with newly formed bone. In stage 4 the shape of the alveolar crest alters into a thin and sharp knife-edge; the body of the jaw is still adequate in height and width.

Stage 5: Further resorption leads to a low well-rounded ridge which is flat but already reduced in height and width; the alveolar process is lost.

Stage 6: Continuing excessive atrophy of the residual crest results in a depressed bone level, where even the basal bone shows signs of reduction. [START_REF] Atwood | Reduction of Residual Rides: A Major Oral Disease Entity[END_REF] and [START_REF] Pietrokovski | The Bony Residual Ridge in Man[END_REF] stated that in many individuals the cortical layer cannot close over the former alveolus and adjacent areas sufficiently due to degenerative and resorptive alterations of the alveolar crest. Accordingly, the residual crest exhibits trabecular bone, extending over the whole length of an edentulous ridge. (See Figure 3A) Stage I describes the situation after tooth loss in which the alveolus is totally closed and the crest completely covered by cortical bone.

Exposed trabecular bone at the top of the residual ridge

In stage II, the cortical layer of the alveolar crest is not closed, but discontinuous so that an area of exposed trabecular bone with a width of up to 2mm is identifiable.

In stage III, the cortical layer of the crest is interrupted by even larger areas of exposed trabecular bone of more than 2mm in width.

Bony defects in the region of the former alveolus

Due to poor or incomplete repair of the alveolus after tooth loss, bony defects such as trabecular spots and macroscopic perforations [START_REF] Solar | Die Auswirkungen des Zahnverlustes auf die Anatomie der Maxilla[END_REF] might be observed on the crest of the residual ridge [START_REF] Nakamato | Bony Defects on the Crest of the Residual Alveolar Ridge[END_REF][START_REF] Pietrokovski | Morphologic Characteristics of Bony Edentulous Jaws[END_REF]. (See Figure 3B) As opposed to exposed trabecular bone, the occurrence of this feature is thought to be restricted to the area of the former alveolus [START_REF] Neufeld | Changes in the Trabecular Pattern of the Mandible Following the Loss of Teeth The[END_REF][START_REF] Lammie | The Reduction of the Edentulous Ridges[END_REF]. However, Nakamoto (1968) doubted this limited extension of such defects since he could not detect a relation between the area of former alveolar sockets and the location of defects. To scrutinise this discrepancy and to check whether a discrimination of exposed trabecular bone and defects is expedient and useful, both features are separately assessed in this present study.

In stage I, no defects are present. Stage II characterises a residual alveolar crest with small defects. The cortical layer encircles little trabecular inclusions of e.g. flame-like or channel-like shape.

In stage III, the observed irregularities at the cortical layer are increased in size.

Concave depression of the alveolar crest

In consequence of the loss of several adjacent teeth, the bone tissue in this edentulous space often breaks down.

The wound area in this edentulous part of the jaw is too large, so that bone is not able to regenerate up to the height of the former alveolar crest resulting in a depressed level of the residual ridge. Towards the adjacent tooth (or teeth, if existent) the bone level maintains the original height of the alveolar process. So-called concave depressions can be observed at the distal end of the dentate arch [START_REF] Pietrokovski | The Bony Residual Ridge in Man[END_REF] as well as in an edentulous space between remaining teeth.

A segment is classified as stage I if there is no concavity observed.

Stage II describes an edentulous ridge portion with a concave depression looking like a recess in the residual ridge.

Oral

Exposed mandibular canal

Excessive resorption of the mandibular residual ridge can lead to a reduction of the alveolar crest towards the mandibular canal [START_REF] Ulm | Anatomische Untersuchungen an der atrophen Mandibula. Teil 1. Die Lage des Canalis mandibulae im atrophen Unterkiefer[END_REF]. In the final stage of resorption, the canal containing the mandibular nerve and blood vessels might even lie directly on the surface, solely covered with gingiva in the living individual. In this case, the canal is opened and its borders are well-defined.

In stage I, the mandibular canal is still completely covered with bone. In stage II, the mandibular canal is exposed.

Statistical Analysis

The frequency distribution of ante-mortem tooth loss and the degree of subsequent atrophy and its concomitant features were determined for all segments of the upper and the lower jaw. In case of absence or poor preservation of a segment within an otherwise well-preserved jaw, only segments that could be evaluated accurately were included.

In order to facilitate evaluation of potential trends between anterior and posterior segments, only the worse affected of both posterior segments was selected for analysis. This pooling of data was based on a high correlation of the analysed parameters between the right and the left posterior segments.

The main purpose of the study is to determine at which frequency the different stages of atrophy are present after tooth loss has occurred. Thus, segments without atrophy were specified as "non-affected" and excluded from further calculations. By this means, data about different stages of a certain feature are presented as a percentage of the affected jaw segments only and also as absolute values.

Descriptive analysis, two-sided Fisher's exact tests and cross tabulations were used to assess potential interrelationships between the occurrence and degree of the features.

Results

Frequency and severity of analysed features

Atrophy

Atrophy in at least one segment was present in 120 of the 263 analysed specimens.

The frequency of occurrence of jaw atrophy (including the stage of recent, intravital tooth loss) in this population is presented in Figure 4 for the anterior and posterior segments of the mandible and the maxilla.

Three general trends concerning frequency and distribution of atrophy of the jaw are apparent:

1) Frequency and also severity of atrophy were lowest in the youngest age group (20-40 years) and increased significantly with advancing age. The occurrence of atrophy is highly correlated with age in all segments of the jaw (p≤0.001).

2) The mandible (Md) tended to be worse affected than the maxilla (Mx), independent of the position of the segment and the age group.

3) The posterior segments (post.) of both the mandible and the maxilla exhibited an increased frequency and also more severe stages of atrophy than the anterior segments (ant.) in all age groups.

Differences in the distribution of the 5 stages of atrophy, from postextraction state to a depressed level, in the In the youngest age group (20-40yrs), atrophy was negligibly rare in the anterior segments. In the posterior segments atrophy was slightly more frequent. In the older age groups (41-60yrs and 61-80yrs), a shift from the lower to the higher, more severe atrophy stages was identified. Atrophy stage 4, representing a residual ridge shaped like a knife-edge, was most frequent in anterior segments, particularly of the mandible. By contrast, in posterior segments, atrophy stage 5, characterised by a reduced height of the ridge with a well-rounded crest, and stage 6, a depressed ridge level, were most frequent. Particularly in posterior segments of the mandible, atrophy

showed two peaks, one at stage 2 (postextraction) and one at stage 5 (low well-rounded), whereas the stages in between were very rare.

In brief, the knife-edge shape was the predominant stage of atrophy in anterior segments. Stage 5, the low wellrounded ridge, and stage 6, the depressed ridge, played a less prominent role in anterior segments than in posterior segments, where atrophy appeared to attain more severe stages more rapidly with increasing age.

Mandibulary posterior segments of old individuals seemed to be the most affected of all.

Fisher's exact tests showed a statistically significant relationship between age group and atrophy stage in anterior segments of the mandible (ant. Md: p=0.049) and posterior segments of the maxilla (post. Mx: p=0.034). In the other segments no significant correlation was detected (post. Md: p=0.095; ant. Mx: p=0.974).

Exposed trabecular bone at the top of the residual ridge

This feature was more frequent in the mandible than in the maxilla. Posterior segments were more severely and more often affected than anterior segments. Yet, frequency and extent of exposed trabecular bone increased only up to the middle-aged group and decreased again in the oldest age group. (See Table 2)

Due to the low rate of atrophy in anterior segments of the youngest age group (20-40yrs), the feature of exposed trabecular bone at the crest of the residual ridge was also very rare. In posterior segments, the rate of affection was higher with stage III (area of exposed trabecular bone ≥2mm in width) being the predominant stage in young individuals. In the middle-aged group (41-60yrs), the prevalence of a discontinuous cortical layer increased in all segments, posterior segments again being worse affected by stage III. In old individuals (61-80yrs), the frequency of stage III decreased compared to the middle-aged group. In contrast, stage I, characterised by a closed cortical layer at the crest, became more frequent and even obtained its peak frequency in this age group.

A significant correlation between age group and the occurrence of different stages of exposed trabecular bone was identified for the posterior segments of the mandible (post. Md: p=0.011) and the maxilla (post. Mx: p=0.010), but not for anterior segments of either jaw (ant. Md: p=0.375; ant. Mx: p=1.000).

Bony defects at the crest of the residual ridge

Bony defects occurred with a higher frequency in the mandible than in the maxilla. Posterior segments were more frequently affected than anterior segments. Similarly to atrophy, frequency increased with advanced age and reached a plateau in the middle-aged (41-60yrs) and old group (61-80yrs). Indeed, also the frequency of stage I (properly healed alveolus covered by a continuous cortical layer) was increased in old individuals. (See Table 2)

In detail, bony defects in the region of the former alveolus were very rare in anterior segments of young adults (20-40yrs), which is the result of low affection by atrophy. In posterior segments, the frequency of poorly healed alveoli was higher, showing a pronounced occurrence of small defects (stage II) in the posterior segment of mandible.

Concave depression of the alveolar crest

With advanced age, the frequency of concave depressions increased. In the mandible, concave depressions occurred more frequently than in the maxilla. Posterior segments were affected with a higher frequency than anterior segments. (See Table 3)

In anterior segments of young adults (20-40yrs), concave depressions were almost non-existent. In the middleaged group (41-60yrs), this feature occurred with a frequency of 14.3% in the anterior mandible and 22.2% in the anterior maxilla. In the oldest age group (61-8yrs), the anterior segments of the lower and the upper jaws were conversely affected with an increased frequency of 31.8% in mandibular segments and a decrease to 14.3% in maxillary segments.

In posterior segments, the frequency of concave depressions was increased compared to anterior segments in young and middle-aged individuals; the latter representing the most affected age group. Posterior segments of old individuals (61-8yrs) showed a decline in frequency.

This tendency was not pronounced enough to show a significant relationship between the occurrence of concave depressions and age in any segment (ant. Md: p=0.544, post. Md: p=0.542; ant. Mx: p=0.300, post. Mx: p=0.431).

Exposed mandibular canal

An exposed mandibular canal (stage II) was a very rare feature. With increasing age its frequency rose slightly.

In the middle-aged group (41-60yrs) 3 (6.7%) and in old individuals 7 (16.7%) posterior segments were affected by such signs of excessive resorption.

Correlation of atrophy and concomitant features

Exposed trabecular bone & atrophy

In both the anterior and the posterior segments of the mandible, a statistically significant correlation between atrophy classes and the extent of exposed trabecular bone was identified (ant. Md: p=0.004; post. Md: p<0.001).

The size of exposed trabecular bone areas seemed to decrease with the progress of atrophy in the mandible. (See

Table 4)

A continuous cortical layer covering the former alveolus without exposed trabecular bone at the surface (stage I) was the predominant feature in anterior mandibular segments classified as atrophy class 4 (knife-edge ridge). In posterior segments, this feature increased with the progress of atrophy and reached the highest frequency in depressed residual ridge (atrophy stage 6).

Exposed trabecular bone of up to 2mm in width (stage II) was most common in posterior segments classified as atrophy stage 5 and 6. Exposed trabecular bone of more than 2mm in widths (stage III) was most frequently observed in segments classified as atrophy stage 2 postextraction. At this atrophy stage, tooth loss did not date back very long; the alveolus already showed first signs of regeneration but the cortical layer had not closed completely over the trabecular area.

In the maxilla, there was no significant relationship between the dimensions of atrophy and exposed trabecular bone (ant. Mx: p=0.310; post. Mx: p=0.655). A statistically significant correlation between atrophy stages and the occurrence of bony defects at the crest of the residual ridge was found in anterior segments of the maxilla (ant. Mx: p=0.046) and the posterior segments of both jaws (post. Md: p<0.001, post. Mx: p<0.001).

Bony defects at the crest & atrophy

No defects were most frequently found in postextraction segments (atrophy stage 2) with recent tooth loss where the alveolus is not refilled with bone and thus has no covering cortical layer. In segments with first signs of atrophy (high well-rounded ridge, stage 3), cortical layers lacking defects were non-existent.

In highly atrophic segments with a low well-rounded (stage 5) or a depressed ridge shape (stage 6), size of bony defects was increased compared to earlier stages of atrophy. Thus, the occurrence of perforations of the cortical layer with trabecular spots at the area of the former alveolus increased with the progress of atrophy. Also cortical layers without defects increased slightly with higher atrophy stages.

Concave depression & atrophy

The presence of concave depressions of the crest of the residual ridge and the extent of atrophy were significantly correlated in all segments of the jaw (ant. Md: p=0.022; post. Md: p<0.001; ant. Mx: p=0.043; post.

Mx: p<0.001).

Discussion

Atrophy of the jawbone following tooth loss is a very common oral disease particularly in elderly people.

Affected patients suffer not only from alterations of the anatomic structure of their jaw and loss of function but also from changes of the facial shape and psychological problems [START_REF] Atwood | Reduction of Residual Rides: A Major Oral Disease Entity[END_REF]) such as a reduced quality of life (Jokstad et al., 2000a;Jokstad et al., 2000b), feelings of shame, insecurity and decreased attractiveness [START_REF] Trulsson | Edentulousness and Oral Rehabilitation: Experiences from the Patients' perspective[END_REF]. Thus, the major tasks of prosthetic or implant-supported treatment are, on the one hand, to restore the function including mastication (Şahin et al, 2002) and speech, and on the other hand, to preserve the appearance and well-being of the patient (Jokstad et al., 2000a;Jokstad et al., 2000b;[START_REF] Sutton | Changes in Facial Form Relative to Progressive Atrophy of the Edentulous Jaws[END_REF][START_REF] Heydecke | The impact of conventional and implant supported prostheses on social and sexual activities in edentulous adults: Results from a randomized trial 2 months after treatment[END_REF][START_REF] Pan | Sex differences in denture satisfaction[END_REF][START_REF] Fenlon | An investigation of factors influencing patients' satisfaction with new complete dentures using structural equation modelling[END_REF].

Despite these undoubtedly positive results of prosthetic treatment, there are diverging opinions as to whether or not there are severe side effects. Several studies have shown that in patients wearing dentures the reduction of the residual ridge was worse than in patients without dentures [START_REF] Thoma | Progressive Atrophie des Unterkiefers und ihre Behandlung[END_REF][START_REF] Campbell | A comparative study of the resorption of the alveolar ridges in denture-wearers and nondenture-wearers[END_REF][START_REF] Carlsson | Changes in height of the alveolar process in edentulous segments. II. A longitudinal clinical and radiographic study over 5 years of full upper denture patients with residual lower anteriors[END_REF]. As an explanatory concept, pressure atrophy is particularly plausible for ill-fitting dentures [START_REF] Carlsson | Changes in height of the alveolar process in edentulous segments. II. A longitudinal clinical and radiographic study over 5 years of full upper denture patients with residual lower anteriors[END_REF]Xie et al., 1997a). By contrast, atrophy was also described in patients without prosthetic treatment [START_REF] Devlin | Alveolar Ridge Resorption and Mandibular Atrophy -A Review of the Role of Local and Systemic Factors[END_REF][START_REF] Thoma | Progressive Atrophie des Unterkiefers und ihre Behandlung[END_REF] supporting the concept of disuse atrophy.

Atwood [START_REF] Atwood | Reduction of Residual Rides: A Major Oral Disease Entity[END_REF][START_REF] Atwood | Reduction of residual ridges in the partially edentulous patient[END_REF] assumed that three major groups of co-factors seem to influence the series of events from wound healing of the affected alveolus to resorption of the alveolar process. These are (1) anatomic structure of the jaw, the (2) biology of the individual (age, sex, hormonal balance etc.) and ( 3) mechanical factors such as force applied to the alveolar bone and the influence of prosthetic procedures.

Anatomy and biology of the patient are predetermined. Hence, one factor left to influence the inevitable progress of atrophy is prosthetics. Clinicians can select from a wide variety of prosthetic and surgical treatment options including traditional removable prosthesis, implant supported dentures, or a combination of both. However, the outcome of the chosen therapy is hardly predictable [START_REF] Carlsson | Changes in height of the alveolar process in edentulous segments. II. A longitudinal clinical and radiographic study over 5 years of full upper denture patients with residual lower anteriors[END_REF]. For that reason, and to find the best individual solution for every patient, it is necessary to increase our knowledge about the preserving or destructive effect of prosthetic procedures. Due to the complex pathoaetiology of atrophy [START_REF] Carlsson | Responses of jawbone to pressure[END_REF], the true, unbiased influence of prosthetic treatment is difficult to isolate from the broad spectrum of co-factors.

Consequently, there is a growing demand for a purpose-made study design, someway comparable to a knockout experiment in mice. In a knockout study, the effect of a specific gene of unknown function is analysed by 10 deactivating the gene. Thus, differences from the normal condition can be examined and the probable function of the gene can be inferred. In this present study, the influence of prosthetic treatment was "knocked out" by analysing an historical population that did not practice any kind of modern dentistry. By this means the natural course of disease could be studied excluding the mechanical factor of prosthetics.

Our results proved that atrophy of the jaw after tooth loss definitely does occur in a population without prosthetic treatment, which is in strong agreement with the concept of a multifactorial disease [START_REF] Atwood | Reduction of Residual Rides: A Major Oral Disease Entity[END_REF].

Furthermore, we could show that even without prosthetics the residual ridge underwent similar changes in height and shape as described by [START_REF] Atwood | Postextraction changes in the adult mandible as illustrated by microradiographs of midsagittal sections and cephalometric roentgenograms[END_REF] and [START_REF] Cawood | A Classification of the Edentulous Jaws[END_REF]. As expected, old individuals were more frequently and worse affected than younger ones. This seems self-evident since tooth loss usually dates back further in old people's lives. Hence, atrophic processes had less time to proceed in individuals that died at an earlier age. In addition, with increasing age the loss of several teeth accumulates -a situation that might promote the development of advanced atrophy stages. The prevalence of postextraction segments increased in middle-aged individuals, possibly indicating that this age group is very susceptible to tooth loss. Notable differences in rate and degree of atrophy were observed between the mandible and the maxilla as well as between anterior and posterior segments in all age groups, which might be the result of differences in anatomy.

Posterior segments showed a higher frequency and degree of atrophy than anterior segments, which is in concordance with the fact that molars tend to be lost at an earlier age than anterior teeth. Hence, atrophy might develop in the posterior part of the jaw first and progress further [START_REF] Ulm | Characteristic features of trabecular bone in edentulous mandibles[END_REF]. Interestingly, in maxillary posterior segments the residual ridge seemed to skip atrophy stages 3 to 5 resulting rather directly in a depressed bone level (stage 6) in both older age groups. Accordingly, stages 3 to 5 seem to be shorter processes, so that the jaw does not remain long in these stages. By contrast, in the mandible the transition of stage 5 to stage 6

(depressed) took place with some time lag in the oldest group. This might indicate that in the initial phases of atrophy mandible offers more resistance to vertical resorption, probably due to the greater thickness of its cortical bone, than the maxilla. However, subsequently the mandible was more frequently and worse affected than the maxilla. Several studies on denture-wearing patients have attributed the higher atrophy rate in the lower jaw to the smaller surface and the unfavourable shape of the mandibular ridge on which mechanical loads are applied to in edentulous patients [START_REF] Tallgren | The Continuing Reduction of the Residual Alveolar Ridges in Complete Denture Wearers: A Mixed-Longitudinal Study Covering 25 years[END_REF][START_REF] Tallgren | Roentgen Cephalometric Analysis of the Ridge Resorption and Changes in Jaw and Occlusal Relationships in Immediate Complete Denture Wearers[END_REF][START_REF] Bodic | Bone Loss and Teeth -Review[END_REF]. By contrast, the maxilla is thought to resist loads of dentures more successfully, because of the larger supporting surface of the hard plate [START_REF] Tallgren | The Continuing Reduction of the Residual Alveolar Ridges in Complete Denture Wearers: A Mixed-Longitudinal Study Covering 25 years[END_REF]. In the present study, prosthetics cannot be held responsible for the higher degree of resorption of the lower jaw. That leads us back again to other factors that might be involved. It seems plausible that forces through chewing and swallowing are applied to the residual ridge also in non-denture wearers. Alterations in frequency, intensity and direction of such forces might lead to an imbalance of the neuromuscular stability [START_REF] Budtz-Jörgensen | Restoration of the partially edentulous mouth -a comparison of overdentures, removable partial dentures, fixed partial dentures and implant treatment[END_REF][START_REF] Kingsmill | Post-extraction remodeling of the adult mandible[END_REF] and thus to an impaired activation of osteoblasts and osteoclasts, probably resulting in a discrepancy of resorption [START_REF] Atwood | Some clinical factors related to rate of resorption of residual ridges[END_REF]. As recently shown by [START_REF] Rawlinson | Adult rat bones maintain distinct regionalized expression of markers associated with their development[END_REF] also differences in gene expression of developmentally and/or functionally distinct skeletal sites play a role in their response to mechanical stimuli. The described differences in gene expression profiles between the skull vault and limb bones were attributed to cell (osteoblast) populations of distinct embryonic lineages. This developmental, "positional identity" of bone cells has been shown to determine cell activity and matrix characteristics probably leading to differences in their susceptibility to osteoporosis and adaptation to mechanical strain.

In anterior segments of either jaw, a knife-edge shape of the residual ridge was predominant. This stage 4 is particularly problematic for implantology since the width of the crest is insufficient for the insertion of endosseous implants. Heights of 6mm and 10mm as well as widths of 6mm and 5mm are regarded as minimum dimensions for successful implantation in the maxilla and the mandible, respectively [START_REF] Neukam | Compromised jawbone quantity and its influence on oral implant placement[END_REF]. and6), which are predominant in posterior segments, do not meet the standard criteria for implantation and require pre-treatmentcomplex surgical procedures, such as the removal of the thin bone structure [START_REF] Nishimura | Knife-edge Residual Ridges: A Clinical Report[END_REF] or/and alveolar ridge augmentation by bone graft techniques [START_REF] Ulm | The Edentulous Maxillary Alveolar Process in the Region of the Maxillary Sinus -A Study of Physical Dimensions[END_REF][START_REF] Coulthard | Interventions for replacing missing teeth: preprosthetic surgery versus dental implants[END_REF][START_REF] Bodic | Bone Loss and Teeth -Review[END_REF]. Even in the most recent systematic reviews on the subject [START_REF] Esposito | The efficacy of various bone augmentation 16 procedures for dental implants: a Cochrane systematic review of randomized controlled clinical trials[END_REF][START_REF] Aghaloo | Which hard tissue augmentation techniques are the most successful in furnishing bony support for implant placement?[END_REF][START_REF] Rocchietta | Clinical outcomes of vertical bone augmentation to enable dental implant placement: a systematic review[END_REF], there are no indications which frequencies of cases that necessitate bone augmentation procedures a surgeon has to expect. According to the results of the present study, implantologists might figure that up to 75% of elderly patients require some kind of surgical pre-treatment (depending on the region of the jaw). A comparison with modern populations is hardly possible since data on prevalence are very rare and information about the sample is mostly lacking. [START_REF] Denissen | Anatomic Considerations for Preventive Implantation[END_REF] analysed a sample of 61 middle-aged patients (40-65 years of age) treated with prostheses. 51.6% of these dried edentulous mandibles showed a knifeedge shape (atrophy stage 4) compareable to a prevalence of 50% in ant. Md. segments of middle-aged individuals of this present study. [START_REF]A radiographic study on the prevalence of knife-edge residual alveolar ridge at proposed dental implant sites[END_REF] found a similar prevalence of 46.5% in an inhomogeneous sample of 30 17-67 year old patients. [START_REF] Pietrokovski | Morphologic Characteristics of Bony Edentulous Jaws[END_REF] studied a sample of 123 human edentulous dried skulls of unknown age, sex and treatment and described 38% of these maxillary and mandibulary ridges to be knife-edged.

Another aspect of clinical relevance is the bone quality of the cortical layer covering the crest of the residual ridge. In fact, a large amount of continuous cortical bone provides good primary stability of the implant and hence decreases the risk of implant failure [START_REF] Rabel | Clinical study on the Primary Stability of Two Dental Implant Systems with Resonance Frequency Analysis[END_REF]. Alterations in cortical layer, such as areas of exposed trabecular bone and osseous defects, might have negative implications for the insertion of endosseous implants [START_REF] Pietrokovski | Morphologic Characteristics of Bony Edentulous Jaws[END_REF]. A closed cortical layer offers more resistance to the bur used than does a crest with macroscopic openings and exposed trabeculae. Thus, there is a risk of overdosing the force at the transition from the outer surface to the trabecular compartment when drilling a hole for the implant [START_REF] Pietrokovski | The Bony Residual Ridge in Man[END_REF].

According to our observations, the differentiation based on the localisation at the crest [START_REF] Atwood | Reduction of Residual Rides: A Major Oral Disease Entity[END_REF][START_REF] Pietrokovski | The Bony Residual Ridge in Man[END_REF][START_REF] Neufeld | Changes in the Trabecular Pattern of the Mandible Following the Loss of Teeth The[END_REF][START_REF] Lammie | The Reduction of the Edentulous Ridges[END_REF] was not unequivocal, so that in many cases trabecular spots were classified as both exposed trabecular bone and bony defects. Yet, the results demonstrated that exposed trabecular bone and defects at the crest do occur in absence of prosthetic treatment. Also, an exposed mandibular canal was detected in a few individuals as a result of excessive atrophy. In the analysed population, the prevalence of an exposed mandibular canal was 6.7% for 41-60 year-old individuals and 16.7% for individuals over 61 years of age. Xie et al. (1997b) reported an even higher prevalence of 27% in a sample of 128 76-86 year old patients. The exact localisation of the mandibular canal is essential to know for an implantologist when planning and inserting endosseous implants in the atrophic mandible [START_REF] Ulm | Location of the mandibular canal within the atrophic mandible[END_REF][START_REF] Lindh | Measurements of distances related to the mandibular canal in radiographs[END_REF], since an injury would implicate neurosensory damage or even loss of sensation [START_REF] Ulm | Location of the mandibular canal within the atrophic mandible[END_REF].

In this study, some limitations were encountered. Since only individuals with a sufficient state of preservation were included, the sample might be biased to some extent [START_REF] Meinl | Caries frequency and distribution in an early medieval Avar population from Austria[END_REF]. Due to the lack of information about the time point of tooth loss and hence of the duration being (partly) edentulous, no conclusions about the rate of atrophy (i.e. the amount of bone being resorbed) per time unit could be drawn. The observed features within the jawbone mirror only the conditions at time of death. The role of osteoporosis in the development and progression of jaw atrophy is still controversially discussed [START_REF] Hildebolt | Osteoporosis and oral bone loss[END_REF][START_REF] Bollen | Number of teeth and residual alveolar ridge height in subjects with a history of self-reported osteoporotic fractures[END_REF][START_REF] Felton | Edentulism and comorbid factors[END_REF][START_REF] Yüzügüllü | Radiomorphometric indices and their relation to alveolar bone loss in completely edentulous Turkish patients: A retrospective study[END_REF]. However, due to the large sample size of 263 jaw specimens it was economically not possible to study systemic bone disorders such as osteopenia and osteoporosis by means of x-rays or DEXA. Several studies reported that osteoporosis might promote ridge resorption [START_REF] Bays | Systemic bone disease in patients with mandibular atrophy[END_REF][START_REF] Kribbs | Oral findings in osteoporosis. Part II: Relationship between residual ridge and alveolar bone resorption and generalized skeletal osteopenia[END_REF][START_REF] Hirai | Osteoporosis and reduction of residual ridge in edentulous patients[END_REF] Klemetti et al. 1993a;Klemetti et al. 1993b;[START_REF] Bollen | Number of teeth and residual alveolar ridge height in subjects with a history of self-reported osteoporotic fractures[END_REF]. [START_REF] Devlin | A study to assess the relative influence of age and edentulousness upon mandibular bone mineral density in female subjects[END_REF] state that age-related bone loss of the jaw resulting in cortical porosity and a reduced BMD and BMC [START_REF] Hildebolt | Osteoporosis and oral bone loss[END_REF] has to be clearly distinguished from external alterations of the mandibular alveolar process due to tooth loss. Moreover, a recent review of [START_REF] Slagter | Osteoporosis and edentulous jaws[END_REF] reported no more than a minimal evidence for a relation between osteoporosis and jaw atrophy. Future studies evaluating radiographic data of atrophic jaws would be of great value to shed light onto this issue. So far, most published studies dealing with atrophy after tooth loss did not meet modern scientific standards, mainly being written in a descriptive style, and lacking information on the sample. Moreover, they had been limited to the assessment of single features of this complex oral disease. Here we present a study analysing concomitant features described in literature and questioning/challenging the consistency and applicability of the classification system. The strength of this current study lies in the analysis of the natural history of atrophy in a large sample of a mediaeval population excluding the influence of prosthetic treatment.

Results demonstrated that atrophy evidently did occur in the absence of prosthetics and followed the pattern of resorption described for modern populations [START_REF] Atwood | Postextraction changes in the adult mandible as illustrated by microradiographs of midsagittal sections and cephalometric roentgenograms[END_REF][START_REF] Cawood | A Classification of the Edentulous Jaws[END_REF]. Concomitant features, i.e. exposed trabecular bone, osseous defects, concave depression and exposed mandibular nerve, were present and often correlated with the occurrence of advanced atrophy stages. A differentiation of exposed trabecular bone and bony defects at the crest of the ridge appears not to be useful since they were hardly distinguishable and their developmental mechanisms might overlap to some extent. Yet, their presence might be of clinical relevance for the planning and realisation of dental implants and prostheses. Above all, our findings present basic information about the pathology of jaw atrophy and provide a deeper insight into the structural characteristics of this common oral disease. Further research in this field is required to study the effect and relative weights of other factors involved in this multifactorial process.

Conclusions

1. Atrophy of the jawbone following tooth loss is a multifactorial disease that is not attributable to prosthetics alone, since its occurrence is observed in a population without modern prosthetic treatment. 2. The residual ridge undergoes a series of changes in shape and height following the pattern of resorption described for modern populations. The severity of these alterations is associated with the age of the individual and the region within the jaw. 3. Atrophy is frequently related to structural alterations of the covering cortical layer which all together might have clinical relevance for planning and realisation of dental implants and prostheses. [START_REF] Atwood | Postextraction changes in the adult mandible as illustrated by microradiographs of midsagittal sections and cephalometric roentgenograms[END_REF] and [START_REF] Cawood | A Classification of the Edentulous Jaws[END_REF]

Legends to Illustrations

  Atrophy of the Residual Alveolar Ridge Following Tooth Loss in a Historical Population

Figure 1 :

 1 Figure 1: Segments of the jaw according to the classification system of Nakamato (1968). Mandible: SEG 1 (anterior), SEG 2 / SEG 3 (posterior, right/left). Red arrowheads indicate mental foramina, separating anterior from posterior segments; Maxilla: SEG 4 (anterior): between the two canines, SEG 5 / SEG 6 (posterior, right/left): between the 1 st premolar and the tuberosity at the distal end of the alveolar arch. Red arrowheads indicate borderline between canines and 1 st premolars.

Figure 2 :

 2 Figure 2: Classification system of six atrophy stages in the maxilla (A) and the mandible (B) according to Atwood (1963) and Cawood and Howell (1988). Atrophy stage 1: preextraction, stage 2: postextraction, stage 3: high well-rounded ridge, stage 4: knife-edge shaped ridge, stage 5: low well-rounded ridge, stage 6: depressed bone level.

Figure 3 :

 3 Figure 3: Photographs of two mandibular (right) posterior segments illustrating the differences between the features exposed trabecular bone and bony defects. (A) Area of exposed trabecular bone at the crest of the residual ridge (small black arrowheads). (B) Bony defects at the crest of the residual ridge (black arrowheads).

Figure 4 :

 4 Figure 4: Prevalence of atrophy (stages 2 to 6) and of the non-affected state before tooth loss (stage 1) in anterior and posterior segments of the mandible and the maxilla. Jaw segments: le.=left, ri=right, ant.=anterior, post.=posterior, Md.=mandible, Mx.=maxilla.

Figure 5 :Figure 1 :Figure 2 :

 512 Figure 5: Frequency distribution of the atrophy stages 2 to 6 (postextraction to depressed bone level) in the jaw segments of young (A), middle-aged (B) and old individuals (C). Due to the low frequency of atrophy in young individuals data were presented as absolute values instead of percents. By this means, the total increase of affected segments is evident. Atrophy stages: 2=postextraction, 3=high well-rounded, 4=knife-edge, 5=low wellrounded, 6=depressed. Jaw segments: ant.=anterior, post.=posterior, Md.=mandible, Mx.=maxilla.

Figure 3 :Figure 4 :Figure 5 :

 345 Figure 2: Classification system of six atrophy stages in the maxilla (A) and the mandible (B) according to[START_REF] Atwood | Postextraction changes in the adult mandible as illustrated by microradiographs of midsagittal sections and cephalometric roentgenograms[END_REF] and[START_REF] Cawood | A Classification of the Edentulous Jaws[END_REF]. Atrophy stage 1: preextraction, stage 2: postextraction, stage 3: high well-rounded ridge, stage 4: knife-edge shaped ridge, stage 5: low well-rounded ridge, stage 6: depressed bone level. 85x76mm (600 x 600 DPI)

  The frequency and severity of atrophy as well as four clinical

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aspects of bone quality and resorption are examined with respect to age within different segments of the jaw.

  The shape of the original alveolus is no

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longer identifiable and the top of the alveolar process finally becomes well-rounded due to first signs of resorption. However, there is no notable reduction in height.

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analysed segments of all age groups are shown in Figure

5

. Percentages of atrophy stages were calculated from affected segments only (atrophy stages 2 to 6), hereinafter referred to as 100%.

  The middle-aged group showed a notable increase of stage I in anterior segments and of stage III (large trabecular spots at the crest) in posterior segments. In old individuals stage III was mainly identified in In the other segments, the frequency of stage III decreased in favour of an increase of stage I, when compared to the middle-aged group. Bony defects did not show a statistically significant correlation with age in any jaw segment(ant. Md: p=0.699, post. Md: p=0.120; ant. Mx: p=0.312, post. Mx: p=0.218).

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  and tooth loss[START_REF] Nicopoulou-Karayianni | Tooth loss and osteoporosis: the OSTEODENT Study[END_REF] whilst others failed to demonstrate a correlation

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	Tables
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		o p y o p y o p y o p y o p y o p y
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Table 1 : Age and sex distribution of the analysed historical sample consisting of 263 individuals.

 1

			female		male	total (both sexes)
	Age group	N	%	N	%	N	%
	20-40 years	70	57.9 %	69	48.6%	139	52.9%
	41-60 years	22	18.2%	58	40.8%	80	30.4%
	61-80 years	29	24.0%	15	10.6%	44	16.7%
	all age groups	121	46.0%	142	54.0%	263	100%

Table 2 : Frequency distribution of the features exposed trabecular bone and bony defects stages I to III in the jaw segments of young (20-40yrs), middle-aged (41-60yrs) and old individuals (61-80yrs).

 2

				Mandible		Maxilla				Mandible		Maxilla
		Trab. bone	anterior N %	posterior N %	anterior N %	posterior N %	Defects	anterior N %	posterior N %	anterior N %	posterior N %
	20-40 years	I II III Total	0 1 1 2	0% 50% 50% 100%	5 4 10 52.6% 26.3% 21.1% 19 100%	0 1 1 2	0% 50% 50% 100%	4 0 10 14	28.6% 0% 71.4% 100%	I II III Total	1 2 0 3	33.3% 66.7% 0% 100%	3 13 68.4% 15.8% 3 15.8% 19 100%	0 1 0 1	0% 100% 0% 100%	7 3 4 14	50% 21.4% 28.6% 100%
	41-60 years	I II III Total	3 7 4 14	21.4% 50% 28.6% 100%	10 22.2% 8 17.8% 27 60% 45 100%	2 2 5 9	22.2% 22.2% 55.6% 100%	1 1 27 29	3.4% 3.4% 33.1% 100%	I II III Total	7 7 0 14	50% 50% 0% 100%	11 24.4% 17 37.8% 17 37.8% 45 100%	6 3 0 9	66.7% 33.3% 0% 100%	11 4 14 29	37.9% 13.8% 48.3% 100%
	61-80 years	I II III Total	7 13 2 22 100.0% 42 31.8% 23 54.8% 59.1% 8 19% 9.1% 11 26.2% 100%	4 3 6 13	30.8% 23.1% 46.2% 100%	6 4 14 24	25% 16.7% 58.3% 100%	I II III Total	9 10 3 22 100.0% 42 40.9% 15 35.7% 45.5% 16 38.1% 13.6% 11 26.2% 100%	11 3 0 14	78.6% 21.4% 0% 100%	4 6 13 23	17.4% 26.1% 56.5% 100%

Trabecular bone: Stage I=closed cortical layer, stage II=exposed trabecular bone ≤ 2mm in width, stage III= exposed trabecular bone ≥ 2mm in width. Defects: Stage I=no defects, stage II=small defects, stage III= large defects.

Table 3 : Frequency distribution of the features concave depression stages I and II in the jaw segments of young (20-40yrs), middle-aged (41-60yrs) and old individuals (61-80yrs).

 3

				Mandible		Maxilla
		Concavity	anterior N %	posterior N %	anterior N %	posterior N %
	20-40 yrs	I II Total	2 0 2	100% 0% 100%	9 10 52.6% 47.4% 19 100%	0 1 1	0% 100% 100%	8 6 14	57.1% 42.9% 100%
	41-60 yrs	I II Total	12 2 14	85.7% 14.3% 100%	15 33.3% 30 66.7% 45 100%	7 2 9	77.8% 22.2% 100%	11 18 29	37.9% 62.1% 100%
	61-80 yrs	I II Total	15 7 22 100.0% 42 68.2% 15 35.7% 31.8% 27 64.3% 100%	12 2 14	85.7% 14.3% 100%	12 11 23	52.2% 47.8% 100%
		Stage I=no concave depression, stage II=concave depression existent.

Table 4 : Cross tabulation demonstrating the interrelationship between atrophy stages and the occurrence of different degrees of exposed trabecular bone at the crest of the residual ridge.

 4

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			Mandible						Mandible
			anterior						posterior
				Atrophy stages					Atrophy stages
			2	3	4	5	6	Total		2	3	4	5	6	Total
		I	1	0	9	0	0	10	I	3	1	5	10	19	38
	trabecular bone	II III	10% 0 0% 4	0% 3 14.3% 61.9% 90% 13 0 3	0% 5 23.8% 0	0% 0 0% 0	100% 21 100% 7	II III	7.9% 1 5% 18	2.6% 1 5% 6	13.2% 26.3% 3 10 15% 50% 3 18	50% 5 25% 3	100% 20 100% 48
			57.1%	0%	42.9%	0%	0%	100%		37.5% 12.5%	6.3%	37.5%	6.3%	100%
		Total	5	3	25	5	0	38	Total	22	8	11	38	27	106
		%	13.2%	7.9%	65.8%	13.2%	0%	100%	%	20.8% 7.5%	10.4% 35.8%	25.5%	100%
			Maxilla						Maxilla
			anterior						posterior
				Atrophy stages					Atrophy stages
			2	3	4	5	6	Total		2	3	4	5	6	Total
		I	1	3	1	1	0	6	I	4	0	0	1	6	11
	trabecular bone	II III	16.7% 50.0% 16.7% 0 3 2 .0% 50.0% 33.3% 6 3 1	16.7% 1 16.7% 1	0% 0 0% 0	100% 6 100% 11	II III	36.4% 0 0% 15	0% 0 0% 1	0% 0 0% 2	9.1% 0 0% 9	54.5% 5 100% 23	100% 5 100% 50
			54.5% 27.3%	9.1%	9.1%	0%	100%		30%	2%	4%	18%	46%	100%
		Total	7	9	4	3	0	23	Total	19	1	2	10	34	66
		%	30.4% 39.1% 17.4%	13%	0%	100%	%	28.8% 1.5%	3%	15.2%	51.5%	100%
		Atrophy stages: 2=postextraction, 3=high well-rounded, 4=knife-edge, 5=low well-rounded, 6=depressed.

Trabecular bone: Stage I=closed cortical layer, stage II=exposed trabecular bone ≤ 2mm in width, stage III= exposed trabecular bone ≥ 2mm in width.

Acknowledgements

The authors thank Martina Traindl-Prohazka of the Department of Anthropology, University of Vienna, for providing the skeletal remains, stored in the Department of Anthropology, University of Vienna, Austria. We express gratitude to Karl Großschmidt for providing his data on sex and age determination of the study sample.

Thanks are also due to Robert Hütter of the Department of Oral Surgery, Medical University of Vienna, Austria for his technical support in taking photographs and creating figures.

Conflict of interest

None declared.

Authors Contributions

KMR was the primary author of the manuscript, was involved in the development of standardised and calibrated evaluation criteria, analysed and interpreted the results, reviewed literature and submitted the manuscript. CDH performed the statistical analyses and was involved in the interpretation of the results. WRL participated in study design, establishment of methods and performed practical evaluation and classification, being blinded to individuals' age and sex. CU and GW supervised the progress of the project, reviewed the manuscript critically and helped to finalise the manuscript. GW also provided funding for the study. ST designed and planned the study and monitored the progress of the project. He was also involved in the development of standardised and calibrated of evaluation criteria and in data interpretation and helped to prepare the paper.

All authors have contributed to, seen and approved the manuscript.


MD Harald Willschke

MD Anette-Marie Machata

MD Thomas Benkoe

MD Stephan C Kettner

MD Lydia Brenner

MD Peter Marhofer
email: peter.marhofer@meduniwien.ac.at

Management of hypertrophic pylorus stenosis with ultrasound guided single shot epidural anaesthesia -A retrospective analysis of 20 cases

Keywords: Hypertrophic pylorus stenosis, thoracic epidural anaesthesia, ultrasound

Aim: To retrospectively describe the performance of ultrasound guided thoracic epidural anaesthesia under sedation for anaesthesia management of open pyloromyotomy.

Background: Anaesthesia management for hypertrophic pylorus stenosis is usually performed under general anaesthesia with tracheal intubation. Only a few publications describe avoidance of tracheal intubation in infants by using spinal or caudal anaesthesia. The present retrospective analysis describes the performance of ultrasound guided thoracic epidural anaesthesia under sedation for anaesthetic management of open pyloromyotomy.

Methods: Twenty consecutive infants scheduled for pyloromyotomy according to the Weber-Ramstedt technique were retrospectively analysed. After sedation with nalbuphine and propofol, an ultrasound guided single shot thoracic epidural anaesthesia was performed with 0.75 ml . kg -1 ropivacaine 0.475%. Insufficient blockade was defined as increase of HR > 15% from initial value and / or any movements at skin incision. In those cases we were prepared for rapid sequence intubation according to the departmental standard.

Results: All pyloromyotomies could be performed under single shot thoracic epidural anaesthesia and sedation. One case of moderate oxygen desaturation was treated with intermittent ventilation via face mask.

Conclusions: Thoracic epidural anaesthesia under sedation for pyloromyotomy has been a useful technique in this retrospective series of infants suffering from hypertrophic pylorus stenosis. In 1/20 infants short term assisted ventilation via face mask was required.

Undisturbed surgery was possible in all cases.

Introduction

Hypertrophic pyloric stenosis (HPS) is a frequent disease in infants with an incidence of 0.9-5.1 per 1000 cases [START_REF] Nielsen | Infantile hypertrophic pyloric stenosis. Decreasing incidence[END_REF][START_REF] O'donoghue | The increasing incidence of infantile hypertrophic pyloric stenosis[END_REF][START_REF] Applegate | The epidemiology of infantile hypertrophic pyloric stenosis in New York State, 1983 to 1990[END_REF][START_REF] Wang | Prevalence of infantile hypertrophic pyloric stenosis in Texas, 1999-2002[END_REF]. The main symptoms of HPS are progressively worsening "projectile" vomiting, poor feeding and dehydration caused by a gastric outlet obstruction due to a hypertrophic pylorus. The average age and weight of infants with HPC is 5 weeks and 4 kg, respectively [START_REF] Bissonnette | Pyloric stenosis[END_REF].

Anaesthesia management for HPS is usually performed under general anaesthesia with tracheal intubation. Tracheal intubation puts these infants at risk of regurgitation, with the potential of aspiration of gastric contents, and rapid sequence intubation is indicated. Beside the special character of anaesthesia induction in children with HPS, rapid sequence intubation in infants should be always considered as high risk procedure. Despite preoperative correction of acid-base balance and hypovolemia, prolonged mechanical ventilation might be required due to remaining metabolic alkalosis and a subsequent delayed equilibrium of the cerebrospinal fluid with the systemic circulation [START_REF] Fuzaylov | Delayed awakening from general anesthesia in a hypovolemic infant[END_REF][START_REF] Frei | Hypertrophe Pylorusstenose[END_REF].

Only a few publications describe avoidance of tracheal intubation and mechanical ventilation in infants undergoing surgery for treatment of HPS by using spinal [START_REF] Kachko | Spinal anesthesia in neonates and infants -a single-center experience of 505 cases[END_REF][START_REF] Somri | The effectiveness and safety of spinal anaesthesia in the pyloromyotomy procedure[END_REF] or caudal anaesthesia [START_REF] Aguirreurreta | Caudal epidural anesthesia in pyloromyotomy in infants: our experience[END_REF][START_REF] Moyao-Garcia | Caudal block with 4 mg x kg-1 (1.6 ml x kg-1) of bupivacaine 0.25% in children undergoing surgical correction of congenital pyloric stenosis[END_REF]. The major drawback of high spinal anaesthesia is the unpredictable cranial subarachnoidal spread of local anaesthesia with subsequent respiratory failure. Caudal anaesthesia on the other hand might be insufficient for pyloromyotomy with a skin incision above the umbilicus.

Methods

We included 20 consecutive infants with HPS in this retrospective analysis. Parent's informed consent included an exact description of the anaesthesia procedure (aspiration of gastric juice via a naso-gastric tube, sedation, ultrasound guided single shot epidural anaesthesia) and possible need for rapid sequence induction. After initial diagnosis of HPS by clinical status, ultrasound and blood gas analysis (BGA), 10 mL . kg -1. h -1 Elo-Paed balanced plus glucose 1% (Fresenius Kabi Inc., Graz, Austria) was administered via a peripheral venous access until a HCO 3 of ≤ 28 mmol . l -1 and a BE of < +2 was achieved.

Pre-epidural preparation

The epidural puncture site between the T10 and T11 vertebral levels was prepared with EMLA cream 30 min prior epidural puncture and infants were premedicated with midazolam 1 mg . kg -1 via the rectal route. After transfer to the operation room, children were placed on a forced-air warming device (Bair Hugger warming blanket, Arizant Inc., Eden Prairie, MN, USA). Standard monitoring included ECG, SpO 2 and non-invasive blood pressure. Sedation was induced with nalbuphine 0.1 mg . kg -1 and a loading dose of propofol 1.0-2.0 mg . kg -1 , administered over 30 seconds. If necessary, supplemental doses of propofol 0.5 mg . kg -1 were administered until adequate sedation was achieved. Sedation was considered adequate, when the patient slept, arousable only with significant physical stimulation. This type of sedation was previously published by Machata et al. and Brenner et al. in 500 and 512 cases, respectively [START_REF] Brenner | Caudal anaesthesia under sedation: a prospective analysis of 512 infants and children[END_REF][START_REF] Machata | Propofol-based sedation regimen for infants and children undergoing ambulatory magnetic resonance imaging[END_REF]. Gentle aspiration of gastric juice via a naso-gastric tube was performed before initiation of the sedation procedure. The maintenance of spontaneous respiration was continuously verified by an end-tidal CO 2 line placed inside a face mask. Via this face mask oxygen / air (FiO 2 50%) was administered. Infants received 10 mL . kg -1. h -1 Elo-Paed balanced plus glucose 1%.

Epidural puncture

The single shot epidural anaesthesia was performed under sterile conditions in left lateral position between the T10 and T11 vertebral spaces. The neuraxial structures were directly visualized with a sterile covered 38 mm 13-6 MHz linear ultrasound probe and a transportable ultrasound machine (M-Turbo, SonoSite Inc., Bothell, WA, USA) from paramedian. Once the dura mater and the epidural space were identified, the puncture was performed with a 20G, 50 mm Tuohy needle and an 8 ml loss-of-resistance (LOR) syringe (BBraun Inc., Melsungen, Germany) via a median approach (Figure 1) using ropivacaine 0.475%. A total volume of 0.75 ml . kg -1 ropivacaine 0.475% (= 3.56 mg . kg -1 ) was administered under ultrasound observation of the spread of local anaesthetic (Figure 2). After performance of the epidural blockade the children were turned in supine position.

Surgical procedure

Fifteen minutes after performance of the block, skin incision was performed via a right lateral horizontal approach, according to the Weber-Ramstedt technique [START_REF] Pfeifer | Results of the Weber-Ramstedt operation in infantile pyloristenosis[END_REF]. After pyloromyotomy, saline was administered in the surgical wound, and a moderate volume of air was insufflated via the gastric tube to exclude accidental perforation of the pylorus.

Emergency management

All equipment for advanced airway management was prepared in cases of respiratory failure.

Respiratory failure was defined as the development of paradox ventilation, disappearance of end-tidal CO 2 curve and / or decrease in SpO 2 < 92%. The following sequential airway management was initiated to re-establish adequate oxygenation: Definitions for bradycardia and hypotension were a decrease in heart rate and MAP > 25% from initial values and treated with atropine 0.01 mg . kg -1 and a fluid bolus of 10 ml . kg -1 , respectively.

Insufficient blockade was defined as increase of HR > 15% from initial value and / or any movements at skin incision. In those cases we were prepared for rapid sequence intubation according to the departmental standard.

Postoperative management

After transfer to the recovery room, pain status of the children was monitored via OPS score, in which objective behavioural variables (crying, facial expression, position of torso and legs, motor restlessness) are assessed. Each pain variable is scored on a three-point scale (0 = none, 1 = moderate, 2 = severe) to give a maximum cumulative score of 10. The scores were evaluated after admission in the recovery room and every 30 min during the first 2 postoperative hours. If the OPS score was 6 in two subsequent measurements, the child received acetaminophen 40 mg . kg -1 rectally. Due to the retrospective nature of this study no OPS scores could be evaluated on the ward.

Postoperative nutrition was performed by ad libitum feeding [START_REF] Garza | Ad libitum feeding decreases hospital stay for neonates after pyloromyotomy[END_REF]. The epidural puncture site was examined 24 h postoperatively to detect local infection, according to the departmental standard.

Results

We analyzed the first 20 consecutive infants undergoing pyloromyotomy according to Weber-Ramstedt with single shot epidural anaesthesia. Pertinent patient data are illustrated in Table 1. The relevant blood gas values after admission in the hospital and before surgery are illustrated in Table 2.

Anaesthesia management via sedation (details are described in Table 2) and ultrasound guided single shot thoracic epidural blockade was successful in all infants. Thus, no rapid sequence intubation as described in the methods section and reversal of neuromuscular blockade was required.

As expected, median (range) decrease in heart rate after administration of 0.75 ml . kg -1 ropivacaine 0.475% was 18% (5-30%) and remained stable on post-epidural lower level during the entire surgical procedure (Figure 3). Oxygen saturation remained stable between 97 and 100% in all cases throughout the entire anaesthesia and surgical procedure, except in one case where SpO 2 decreased to 92% 10 min after epidural anaesthesia. Treatment of this short episode of decrease in SpO 2 was performed by assisted positive pressure ventilation via face mask.

All OPS scores remained < 5 and therefore no child received additional systemic pain therapy in the recovery room. The examination of the epidural puncture site 24 hours postoperatively was uneventful in all cases.

Discussion

This consecutive case series describes a novel anaesthesia management for pyloromyotomy in infants suffering from HPS. Ultrasound guided single shot thoracic epidural anaesthesia under sedation and spontaneous respiration has been a useful technique in this retrospective series of infants for pyloromyotomy. In this particular retrospective study of 20 cases, airway manipulation and mechanical ventilation could be avoided.

Pyloromyotomy is usually performed under general anaesthesia, thus requiring tracheal intubation and rapid sequence induction of general anaesthesia [START_REF] Bissonnette | Pyloric stenosis[END_REF]. The use of regional anaesthesia techniques depends on the exact site of surgery. As most surgical techniques for pyloromyotomy require a supraumbilical skin incision, spinal and caudal blockade seem inadequate. Kachko et al. suggest spinal anaesthesia only for low abdominal procedures (8), whereas Somri et al. and Jetzek-Zader et al. describe high spinal blockade with bupivacaine 0.5% (0.8 mg . kg -1 and 1.3 mg . kg -1 , respectively) as possible regional anaesthetic technique and as an alternative to general anaesthesia for pyloromyotomy [START_REF] Somri | The effectiveness and safety of spinal anaesthesia in the pyloromyotomy procedure[END_REF][START_REF] Jetzek-Zader | High spinal anaesthesia in a formerly preterm infant undergoing pyloromyotomy[END_REF]. Moyao-Garcia et al.

suggest caudal blockade with bupivacaine 0.25% and a volume of 1.6 ml for pyloromyotomy and describe a success rate of 96% [START_REF] Moyao-Garcia | Caudal block with 4 mg x kg-1 (1.6 ml x kg-1) of bupivacaine 0.25% in children undergoing surgical correction of congenital pyloric stenosis[END_REF]. In spite of all these encouraging reports, our clinical experience was based on the observation that caudal blockade is insufficient for Weber-Ramstedt repair of HPS due to a required cranial analgesic level between T4 and 6.

Preliminary and unpublished data show that only in the minority of cases a spread above T12 can be achieved via the caudal approach, even with a volume of local anaesthetic of 1.5 mg . kg -1 . On the other hand, spinal anaesthesia for pyloromyotomy may cause uncontrolled high blockade and subsequent respiratory insufficiency.

During the past 10 years our study group acquired a substantial experience in the area of central [START_REF] Willschke | Epidural catheter placement in neonates: sonoanatomy and feasibility of ultrasonographic guidance in term and preterm neonates[END_REF][START_REF] Willschke | Epidural catheter placement in children: comparing a novel approach using ultrasound guidance and a standard loss-of-resistance technique[END_REF][START_REF] Marhofer | Pilot study of neuraxial imaging by ultrasound in infants and children[END_REF]) regional anaesthetic techniques with ultrasound guidance. In the light of the findings above and our own significant experience with epidural anaesthesia in infants, we considered ultrasound guided single shot thoracic epidural blockade with sedation as a possible alternative to other techniques. The analysis of our first consecutive cases showed that thoracic epidural blockade under ultrasound guidance is a useful anaesthesia method for Weber-Ramstedt pyloromyotomy. Once the epidural space is identified via a combination of LOR and direct visualization, the spread of local anaesthetic can be directly observed.

We administered from the first case ropivacaine 0.475% (1:1 mixture of ropivacaine 0.75 % and 0.2 %) with a volume of 0.75 ml . kg -1 and observed an adequate cranial spread of local anaesthetic with no alterations in ventilation. This volume and concentration may be considered as relatively large, but pharmacokinetics of epidural ropivacaine in infants is insufficiently described. Anyway, systemic resorption of ropivacaine from the epidural space seems to be slower as compared with bupivacaine, thus increasing the safety of epidural ropivacaine in infants [START_REF] Karmakar | Ropivacaine undergoes slower systemic absorption from the caudal epidural space in children than bupivacaine[END_REF]. Due to the fact that no data existed regarding optimal volume and concentration of local anaesthetic for this particular indication, we have been prepared for alterations in volume and concentration of local anaesthetic. Fortunately the initial choice of volume and concentration of local anaesthetic was effective and seems to be safe regarding spontaneous ventilation. Anyway, further studies should investigate lower concentrations of epidural ropivacaine for treatment of HPS.

It is important to highlight that the described technique requires particular training and handskills. Anyway, paediatric anaesthesia nowadays is a highly specialized profession and therefore children, independent of the severity of their disease, should be treated only by dedicated paediatric anaesthetists and surgeons [START_REF] Allan | Determinants of good outcome in pyloric stenosis[END_REF]. In experienced hands, ultrasound guided thoracic epidural anaesthesia may be considered as safe technique. Careful miscellaneous management (sedation under maintenance of spontaneous respiration, suctioning of the stomach, etc.) is equally important for the safe and successful management of these cases. A major advantage of the reported technique is the fact that it can be used for all surgical open approaches. Despite not investigated in this retrospective analysis, paraumbilical procedures can be also treated with epidural single shot blockade. Another possible advantage is the operation time saving effect of pure regional anaesthesia based methods as compared with general anaesthesia. Kachko et al. reported about the time saving effects of spinal anaesthesia for HPS [START_REF] Kachko | Impact of spinal anesthesia for open pyloromyotomy on operating room time[END_REF]. Despite we did not exactly evaluate the anaesthesia control time in our retrospective analyses, is seems to be obvious that the avoidance of emerge from general anaesthesia is directly associated with faster procedural times.

The clear limitation of this report is that it is just a descriptive consecutive case series. However, from our point of view descriptive reports are useful and sufficient to describe anaesthetic techniques for particular surgical procedures. Another limitation is that we were )

  Thoracic epidural anaesthesia might be an option for anaesthetic management of HPS. Until today, a lot of practitioners have concerns against thoracic epidural punctures in infants due to safety reasons. Recently our study group has developed a technique to directly observe the spread of local anaesthetic inside the epidural space in neonates and infants by ultrasound (12, Consequently, we used this technique for single shot thoracic epidural punctures in infants undergoing pyloromyotomy, and analysed the first 20 consecutive cases in a retrospective manner.

  via face mask with inspiratory pressure < 10 mmHg -rapid sequence intubation according to the departmental standard (propofol 8.0 mg.kg - 1 , rocuronium 0.6 mg . kg -1 ).

  is only possible when open surgical procedures are performed. Nowadays laparoscopic procedures become more and more popular, which is also the case for pyloromyotomy. Two recent meta-analyses are available comparing open versus laparoscopic pyloromyotomy. Sola et al. identified 6 studies with 303 patients sufficient to be included in a meta-analysis and found slight advantages for laparoscopic procedures in terms of shorter time to full feeding, shorter postoperative length of stay and a reduced rate of total complications (22). Conversely, Hall et al. published a meta-analysis based on 8 studies and 595 patients and found fewer complications and a higher efficacy when open pyloromyotomy was performed (23). None of these publications consider the implications of anaesthesia management on morbidity. Anyway, from todays point of view laparoscopic pyloromyotomy shows no clear advantages compared with the open technique. Moreover, avoidance of tracheal intubation, subsequent ventilation and possible postoperative respiratory depression when thoracic epidural anaesthesia is performed may serve as another argument for open pyloromyotomy.

  evaluate OPS scores on the ward and therefore no statement regarding pain and behaviour after transfer from the recovery room can be provided.In summary, thoracic epidural anaesthesia under sedation for pyloromyotomy has been a useful technique in this retrospective series of infants suffering from HPS. We did not observe any haemodynamic or respiratory complications in this consecutive series of 20 infants undergoing Weber-Ramstedt pyloromyotomy. Undisturbed surgery was possible in all cases.

Figure 1 Figure 2 3

 123 Figure 1

Table 1 . Pertinent patient data. Values are median (min-max) except where indicated otherwise

 1

	Tables
	Patient data
	Gender (m/f)	17/3
	Age (months)	1.7 (1.5 -4.0)
	Weight (g)	3895 (1800 -5000)
	Duration of surgery (min)	25 (10 -40)

Table 2 . Relevant blood gas values, sedation details and volume of local anaesthetic for epidural single shot blockade. Values are mean (min-max or SD)

 2

			Relevant blood gas values
	pH at admission	7.52 (7.41 -7.67)
	HCO 3	-(mmol . l -1 ) at admission	30.9 (21.1 -44.2)
	pH preoperative	7.40 (7.36 -7.49)
	HCO 3	-(mmol . l -1 ) preoperative	25.3 (21.6 -28.0)