Solène Prost

Mehdi Boudissa

Stéphane Fuentes

Patrick Tropiano

Jérôme Tonetti

Benjamin Blondel

Minimally invasive triangular lumboiliac and iliosacral fixation of posterior pelvic ring injuries with vertical instability: Technical note

To date, no strong consensus exists on the best way to treat posterior pelvic ring injuries when there is no neurological deficit. Various fixation methods have been described; more recently, constructs that combine lumboiliac and iliosacral fixation have been introduced. This type of fixation is mainly indicated in cases of spinopelvic dissociation with large displacement of fracture fragments in the sagittal plane. However, these techniques are associated with postoperative complications, particularly infections and severe skin complications. This led us to propose a minimally invasive lumboiliac and iliosacral fixation technique for posterior pelvic ring injuries. The procedure is done with the patient prone. It consists of pedicle screw insertion into L4 or L5 and screw fixation of the ilium with fluoroscopy guidance; intra-operative distraction can be done depending on the amount of displacement. An iliosacral screw is then inserted percutaneously to allow reduction in the transverse plane and yield a triangular construct. In the five patients that we have operated using this technique, the mean preoperative vertical displacement was 11.9 ± 6.9 mm (SD) (min 1.3, max 19.7) versus 3.7 ± 3.2 mm (min 0.3, max 6.7) postoperatively and the mean preoperative frontal displacement was 7.5 ± 3.7 mm (min 4.2, max 12.4) versus 2.5 ± 2.0 mm (min 0.3, max 4.3) postoperatively. Minimally invasive iliosacral and lumboiliac fixation is an option for treating posterior pelvic ring fractures free of neurological deficit and especially spinopelvic dissociation.

Introduction

Posterior pelvic ring injuries are rare lesions that mainly occur due to high-energy trauma in younger adults [START_REF] Kleweno | Lumbopelvic fixation for pelvic fractures[END_REF] or because of a fall in older adults with osteoporosis. These injuries are often associated with other fractures or internal organ damage that can be life threatening [START_REF] Ruatti | Which pelvic ring fractures are potentially lethal?[END_REF]. These fractures have been extensively described in the literature; however, there is no consensus as to how to best treat them when no neurological deficit is present.

Pelvic fractures can be classified as outlined by Tile [START_REF] Tile | Acute Pelvic Fractures: I. Causation and Classification[END_REF] to capture the instability caused by the fracture; however posterior pelvic ring injuries also combine U and H fractures of the sacrum along with sacroiliac dislocation. H fractures of the sacrum have been described and classified by Roy-Camille et al. [START_REF] Roy-Camille R | Transverse Fracture of the Upper Sacrum: Suicidal Jumper's Fracture[END_REF] and Denis et al. [START_REF] Denis | Sacral Fractures: An Important Problem Retrospective Analysis of 236 Cases[END_REF]. They are defined by the combination of a transverse perpendicular fracture line in the sacral canal and two vertical fracture lines through the sacral alae. These fractures can create a spinopelvic dis-sociation and generally lead to kyphosis. These posterior injuries can occur simultaneously with anterior injuries such as fracture of the obturator ring or pubic separation, which increases the fracture's instability vertically and rotationally.

Historically, these lesions have been treated conservatively [START_REF] Strange-Vognsen | An unusual type of fracture in the upper sacrum[END_REF], in part due to the lack of appropriate fixation tools and in part because of the difficulty with approaching the fracture site and its increased risk of infection [START_REF] Formby | Operative management of complex lumbosacral dissociations in combat injuries[END_REF]. However, better surgical techniques and the increasingly frequent diagnosis of these lesions thanks to CT scans being done routinely in polytrauma patients [START_REF] Beckmann | Sacral fractures: classification and manage-ment[END_REF] has led to the development of several surgical techniques: percutaneous iliosacral screw fixation [START_REF] Ruatti | Technique for reduction and percutaneous fixation of U-and H-shaped sacral fractures[END_REF], plate fixation or transiliac or transacral bar fixation [START_REF] Boudissa | Posterior transiliac plating in vertically unstable sacral fracture[END_REF][START_REF] Rubash | Comparative mechanical perfor-mances of some new devices for fixation of unstable pelvic ring fractures[END_REF][START_REF] Mehling | Stabilization of fatigue fractures of the dorsal pelvis with a trans-sacral bar. Operative technique and outcome[END_REF] and more recently, constructs that combine lumboiliac and iliosacral fixation [START_REF] Dienstknecht | A Minimally Invasive Stabilizing System for Dorsal Pelvic Ring Injuries[END_REF][START_REF] Schmitz | Clinical application of a minimally invasive cement-augmentable Schanz screw rod system to treat pelvic ring fractures[END_REF][START_REF] Hoffman | Sacroiliac fixation: a new technique after pelvic trauma[END_REF]. The latter fixation methods are mainly indicated for spinopelvic dissociation since they provide a reduction in all three planes and better restore the original pelvic incidence and acetabular orientation. This technique, which was described by Schildhauer et al. [START_REF] Schildhauer | Triangular osteosynthesis of vertically unstable sacrum fractures: a new concept allowing early weight-bearing[END_REF][START_REF] Schildhauer | Triangular Osteosynthesis and Iliosacral Screw Fixation for Unstable Sacral Fractures: A Cadaveric and Biomechanical Evaluation Under Cyclic Loads[END_REF], produces good radiographic results but the skin complication rate is 3% to 13% and the postoperative infection rate is 5% to 10%.

± ± ± ± ±
The development of minimally invasive techniques combining iliosacral screw fixation and a hybrid lumboiliac construct may be relevant for Tile type C, B, A3.3 [START_REF] Tile | Acute Pelvic Fractures: I. Causation and Classification[END_REF], Roy Camille type 1, 2 and 3 [START_REF] Roy-Camille R | Transverse Fracture of the Upper Sacrum: Suicidal Jumper's Fracture[END_REF] and Denis type 1, 2, 3 [START_REF] Denis | Sacral Fractures: An Important Problem Retrospective Analysis of 236 Cases[END_REF] fractures. The aim of the surgery is to restore the balance between pelvic incidence and lumbar parameters by reducing the sacral kyphosis and the vertical and/or rotational displacement of the fractured hemi-pelvis, and to make the orientation of the two acetabula symmetric to prevent gait disturbances and improve the long-term clinical outcomes. The aim of this study was to present a lumboiliac and iliosacral fixation technique that produces a minimally invasive triangular construct for posterior pelvic ring injuries.

Surgical technique

Before the surgery, it is crucial to detect any internal organ injuries. Under general anesthesia, patients are positioned prone on a carbon radiotransparent table with chest and iliac rolls. Once the patient is in position, a preoperative fluoroscopy scan is done to locate the pedicles of the L4, L5 and S1 vertebrae on AP and lateral views, the superior endplate of S1 on AP view by tilting the unit along the patient's pelvic incidence (inlet view described by Pennal et al. [START_REF] Pennal | Pelvic disruption: assessment and classification[END_REF]) and the body of the sacrum in an AP view to identify the sacral holes (outlet view).

The first step consists of conventional percutaneous pedicle screw fixation at one or two levels, typically L4 and/or L5. Determining which vertebra to instrument mainly depends on the degree of lumbar lordosis once the patient is positioned on the table, to make it easier to pass the rod percutaneously. The screw(s) is(are) inserted using the conventional technique (ES2, Stryker, Kalamazoo, Michigan, USA).

For the second step, a small (2 cm) incision is made over the posterior iliac crest; the aponeurosis is opened longitudinally, the posterosuperior iliac spine is resected with rongeurs, then an iliac screw (PassLP, Medicrea, Rieux-la-pape, France) is inserted with fluoroscopy guidance. The fixation is unilateral or bilateral, depending on the type of fracture.

A 6-mm diameter rod precurved into lordosis is then slipped between the lumbar and iliac screw percutaneously. The first locking is done on the lumbar pedicle screws, with in situ percutaneous distraction for Tile type C or B3 fractures with more than 1 cm elevation of the hemi-pelvis on preoperative CT scan or in cases of fracture-induced sacral kyphosis (Roy Camille type > 2); traction is applied intraoperatively on the lower limb by a circulating (nonsterile) surgical assistant. The iliac screws are then locked to the rod after distraction to reduce the hemipelvis under fluoroscopy guidance (Fig. 1).

Closure is done layer by layer by covering the head of the iliac screw with fascia to prevent discomfort related to protuberance of the screw head as best possible.

The final surgical step is done in the same position while focusing on the lateral projection of the S1 body located by fluoroscopy. A skin incision about 1 cm long is made. A 2.8-cm diameter guide wire is inserted with fluoroscopy guidance into the S1 vertebral body as described by Ruatti et al. [START_REF] Ruatti | Technique for reduction and percutaneous fixation of U-and H-shaped sacral fractures[END_REF]. This screw must go beyond the sacral fracture line and reach the center of the sacrum's body. A 7.3-mm diameter, short thread cannulated screw with washer (DePuy Synthes, New Brunswick, New Jersey, USA) is inserted next to compress the transverse fracture of the sacrum ala (Figs. 234).

Preoperative detection of lumbosacral dysplasia is done routinely as stated by Tonetti et al. [START_REF] Tonetti | Management of recent unstable fractures of the pelvic ring. An update Conference supported by the Club Bassin Cotyle. (Pelvis-Acetabulum Club)[END_REF]. In fact, iliosacral screw fixation can only be considered below the final mobile disc.

Postoperatively, partial weight bearing at 30% for 6 weeks is allowed immediately if there are no other injuries, especially in the lower limbs. Unrestricted hip and trunk motion is done as pain allows. A CT scan is always taken immediately postoperative to ensure the implants are in the correct position and the reduction is good in all three planes. The fixation hardware is removed after 1-year postoperative once fracture union has been confirmed on another CT scan.

Along with the complications inherent to this type of fracture, various complications can occur intra-and postoperatively: incorrect screw position, failure of instrumentation due to rod breakage or screw pullout, early or late infections, mainly over the head of the iliac screw. The possibility of these complications must be explored at each follow-up visit.

Results

Our initial, retrospective, single-center cohort (6 months minimum follow-up) consisted of 5 patients who received a minimally invasive triangular construct for a posterior pelvic ring injury. The characteristics of the cohort are summarized in Table 1. All the associated lesions were treated either at the same time or during a subsequent surgery.

The mean preoperative displacement in the frontal plane was 7.5 3.7 mm ( SD) (min 4.2, max 12.4) and in the vertical plane it was 11.9 6.9 mm (min 1.3, max 19.7) (Table 2 ). No patients had postoperative infectious or neurological complications. One patient had urinary bladder overdistension upon removal of his urinary catheter that required implantation of a urethral catheter. The mean postoperative frontal displacement was 2.6 2 mm (min 0.3, max 4.3) and the mean vertical displacement was 3.7 3.2 mm (min 0.3, max 6.7) (Table 2).

Discussion

There is no consensus about how to treat posterior pelvic ring injuries without neurological deficit. Analysis of the fracture using the typical classification systems (Tile, Roy-Camille or Denis) helps to guide which reduction-fixation technique to use. Based on the literature review by Yi and Hak [START_REF] Yi | Traumatic spinopelvic dissociation or U-shaped sacral fracture: A review of the literature[END_REF], the functional prognosis after spinopelvic dissociation is related to the success of the fracture reduction. This reduction must be done as early as possible since the large hematoma that accompanies this type of fracture induces rapid bone union, making it difficult to reduce later on [START_REF] Ruatti | Technique for reduction and percutaneous fixation of U-and H-shaped sacral fractures[END_REF].

Isolated iliosacral screw fixation provides reduction in the transverse plane and compresses a vertical fracture of the sacral ala but does not provide distraction in the vertical plane in case of elevation or rotation of the hemi-pelvis. Doing the initial lumboiliac fixation provides vertical reduction and stabilizes the fracture in the various planes, allowing early return to weightbearing. Reduction using solely closed maneuvers in traction as described by Ruatti et al. [START_REF] Ruatti | Technique for reduction and percutaneous fixation of U-and H-shaped sacral fractures[END_REF] in their article on iliosacral screw fixation does not always provide satisfactory reduction, especially in overweight patients or in cases of kyphotic sacrum fracture. Reduction in the vertical plane is particularly important functionally. In fact, in case of hemipelvis elevation or elevation-rotation, asymmetry in acetabular orientation or indirect leg length difference are not particularly well tolerated functionally. Also, in case of unilateral or bilateral involvement of the L5-S1 joints, lumboiliac fixation helps to prevent destabilization of the lumbosacral hinge upon weightbearing. Furthermore, in case of spinopelvic dissociation, like in the cases of H-fractures of the sacrum, sacral kyphosis induces an alteration to the patient's native pelvic incidence, which has long-term consequences on the sagittal balance and may be the cause of early aging of the last mobile disc. Reduction of this fracture kyphosis must be one of the goals of the surgical treatment [START_REF] Schildhauer | Triangular Osteosynthesis and Iliosacral Screw Fixation for Unstable Sacral Fractures: A Cadaveric and Biomechanical Evaluation Under Cyclic Loads[END_REF][START_REF] Käch | Distraction spondylodesis of the sacrum in "vertical shear lesions" of the pelvis[END_REF].

One of the drawbacks of traditional open lumboiliac fixation is the 16% rate of surgical site infections [START_REF] Okuda | Minimally invasive spinopelvic "crab-shaped fixation" for unstable pelvic ring fractures: technical note and 16 case series[END_REF]. A minimally invasive approach can theoretically help to reduce the infection rate due to cutaneous, muscular and blood sparing, but also because of a reduction in the average operating time [START_REF] Puffer | Increased Total Anesthetic Time Leads to Higher Rates of Surgical Site Infections in Spinal Fusions[END_REF][START_REF] Laghmouche | Minimally invasive treatment of thoracolumbar flexion-distraction fracture[END_REF][START_REF] Kobbe | Minimally invasive stabilisation of posterior pelvic-ring instabilities with a transiliac locked com-pression plate[END_REF]. These parameters are particularly relevant for the treatment of polytrauma patients who may be hemodynamically unstable but also for the treatment of more fragile older patients [START_REF] Dienstknecht | A Minimally Invasive Stabilizing System for Dorsal Pelvic Ring Injuries[END_REF][START_REF] Schmitz | Clinical application of a minimally invasive cement-augmentable Schanz screw rod system to treat pelvic ring fractures[END_REF][START_REF] Rommens | Comprehensive classification of fragility fractures of the pelvic ring: Recommendations for surgical treatment[END_REF][START_REF] Rommens | Minimal Invasive Surgical Treatment of Fragility Fractures of the Pelvis[END_REF]. Thus a transverse construct was proposed by Tempelaere et al. [START_REF] Tempelaere | Percutaneous posterior fixation for unstable pelvic ring fractures[END_REF] and Okuda et al. [START_REF] Okuda | Minimally invasive spinopelvic "crab-shaped fixation" for unstable pelvic ring fractures: technical note and 16 case series[END_REF], combining an L5 pedicle hold to create a crab-shaped lumboiliac construct. Shah et al. [START_REF] Shah | Minimally Invasive Lumbopelvic Fixation for Unstable U-Type Sacral Fractures[END_REF] recently described a minimally invasive lumboiliac fixation with distal fixation at S2. Our technique resembles theirs but with fixation in the posterior wing of the ilium. This allows us to extend the indications to Tile C fractures, Denis zone 1 fractures and sacroiliac dislocation for which S2 fixation is not possible or not needed.

The approach that we describe here appears to be adapted to the treatment of injuries without neurological deficit. Like in spinal trauma, this minimally invasive technique can be done early in the management of polytrauma patients [START_REF] Giorgi | Early percutaneous fixation of spinal thoracolumbar fractures in polytrauma patients[END_REF]. In our experience, removing the instrumentation is possible once bone union is confirmed on CT scan. In fact, the instrumentation can cause discomfort because of the thin muscle layer covering the implants, especially the head of the iliac screw and because of the long-term risk of implant fracture since no bone graft is used.

However, our study is limited by the short follow-up and small number of patients reviewed. A longer follow-up of more patients is needed to confirm our results.

Conclusion

Minimally invasive iliosacral and lumboiliac fixation is an option for treating posterior pelvic ring injuries free of neurological deficit and especially spinopelvic dissociation. Beyond the theoretical advantages of a minimally invasive approach, this type of triangular fixation provides good reduction and stabilization of the fracture zones that are subjected to high loads when weightbearing resumes.

Disclosure of interest

Conflicts of interest related to this study: J.

Fig. 1 .

 1 Fig. 1. Insertion of the 6-mm diameter curved rod from the lumbar screw towards the iliac screw.

Fig. 2 .

 2 Fig. 2. Posterior pelvic injury with elevation of the right hemi-pelvis (Tile C); preoperative CT scan (above) and postoperative radiographs showing the minimally invasive lumboiliac and iliosacral fixation (below).

Fig. 3 .

 3 Fig. 3. Posterior pelvic injury with superior S1 fracture on the left side (Tile C); preoperative CT scan (above) and postoperative radiographs showing the minimally invasive lumboiliac and iliosacral fixation (below).

Fig. 4 .

 4 Fig. 4. Postoperative stereoradiographs (AP, lateral views) showing the triangular construct.

Table 1

 1 Demographic data.

		Patient 1	Patient 2	Patient 3	Patient 4	Patient 5	Mean	SD
	Age	27	35	18	50	25	31	12.3
	Sex	W	W	W	M	W
	Other injuries	L1 type C fracture	None	R anterior	Rupture of	T11 fracture
		L both-column fracture		column of	fibrous capsule of
		of the acetabulum, L		acetabulum	testis, pubic
		pneumothorax			separation
	Trauma	Defenestration	MVA	MVA	MVA	Defenestration
	Construct	Unilateral L	Unilateral R	Unilateral R	Unilateral L	Unilateral R
	Classification Tile [3]	C	C	C	C	C
	Roy Camille et al. [4]	Type 1 zone 2	Type 2 zone 2	Type 1 zone 1	Type 2 zone 2	Type 1 zone 2
	Denis et al. [5]	2	2	1	2	2
	Time to surgery (days)	9	2	3	7	2	4.6	3.2
	Operative time (min)	140	175	120	140	110	139	22.5
	SD: standard deviation; W: woman; M: man, MVA: motor vehicle accident; L: left, R: right.

Table 2

 2 Pre-and postoperative radiological data.

	Preoperative

SD: standard deviation.

  Tonetti and B. Blondel are Associate Editors of Orthopaedics & Traumatology: Surgery & Research. Conflicts of interest not related to this study: B Blondel: Consultant for Medicrea, Vexim-Stryker, Implanet, 3M. S. Prost: Travel paid by Zimmer Biomet and Stryker M. Boudissa: Consultant for Surgivisio. S. Fuentes: Consultant for Medicrea, Medtronic, Stryker. J. Tonetti: Consultant for Surgivisio and SpineArt. P. Tropiano: Consultant for LDR-Zimmer, DePuy-Synthes, France Rachis.


Kaissar Farah

Mikael Meyer

Solene Prost

Faisal Albader

Henry Dufour

Benjamin Blondel

Stephane Fuentes

Robotic Assistance for Minimally Invasive Cervical Pedicle Instrumentation: Report on Feasibility and Safety

Keywords: Airo navigation, Cervical spine, Cirq robotic, Minimally invasive Computed tomography CTSF: Cervical transpedicular screw fixation DLP: Dose length product iCT: Intraoperative computed tomography OR: Operating room

BACKGROUND: Accurate screw placement remains challenging, especially in the cervical spine. We present our first experience of minimally invasive posterior cervical and upper thoracic pedicle screw fixation in the lower cervical spine.

■ METHODS: This study reports a case series of patients, undergoing posterior percutaneous pedicle fixation using Cirq robotic assistance coupled to the Airo intraoperative computed tomography scan and Brainlab navigation system. Routine computed tomography was performed on postoperative day 2 to evaluate pedicle screw placement. The effective dose was calculated.

■ RESULTS: Between February 2020 and December 2020, 7 patients (4 men and 3 women) were treated. The mean age was 58.8 years (range, 29e75 years). Fixation was performed with a cannulated PASS OCT Reconstruction System (Medicrea). Overall, 28 screws were placed within cervical and upper thoracic pedicles. According to the Neo and Heary classification, 85.7% were rated as acceptable and 14.3% as poor. The radiation dose received by the patient was 9.1 mSv (range, 7.7e10.6 mSv). The radiation dose received by the surgical staff was 0 mSv. The postoperative course was excellent.

■ CONCLUSIONS: Posterior miniopen fixation using Cirq robotic assistance coupled with an intraoperative computed tomography navigation system is a major innovation that can improve the accuracy of pedicle screw positioning, with acceptable patient radiation and reduced surgical team exposure.

INTRODUCTION

osterior cervical spine fixation is a valuable surgical technique commonly performed to treat various degenerative, neoplastic, infectious, and traumatic conditions affecting the cervical spine. Most common screw fixation techniques include lateral mass, pedicle, intralaminar, and transfacet screws.1 , 2

Cervical transpedicular screw fixation (CTSF) offers more biomechanical stability than do other types of internal fixation, including lateral mass screw.3 However, CTSF placement is technically demanding, and the rate of screw malpositioning was reported as 6.7%e29.1% in conventional fluoroscopy procedures. [START_REF] Abumi | Complications of pedicle screw fixation in reconstructive surgery of the cervical spine[END_REF][START_REF] Hojo | A multicenter study on accuracy and complications of freehand placement of cervical pedicle screws under lateral fluoroscopy in different pathological conditions: CT-based evaluation of more than 1,000 screws[END_REF][START_REF] Ishikawa | Clinical accuracy of threedimensional fluoroscopy-based computer-assisted cervical pedicle screw placement: a retrospective comparative study of conventional versus computerassisted cervical pedicle screw placement[END_REF][START_REF] Neo | The clinical risk of vertebral artery injury from cervical pedicle screws inserted in degenerative vertebrae[END_REF] This procedure can be performed through an open or minimally invasive approach.

The fundamental aim of minimally invasive techniques is the reduction of approach-related morbidity secondary to muscle trauma, blood loss, and postoperative neck pain.8 , 9

Navigation techniques can also be used for CTSF. However, no previous reports have described the accuracy of CTSF using Cirq robotic assistance (Brainlab AG, Munich, Germany). in conjunction with intraoperative computed tomography (iCT) scan navigation via a minimally invasive approach.

We present our first experience using this technique for cervical and upper thoracic transpedicular screw fixation.

≤ ≤

METHODS

Patient Selection

We report a retrospective study carried out in our institution. Eligible patients were 18 years old or older, undergoing a complementary second-stage miniopen transpedicular screw instrumentation of the lower cervical and/or upper thoracic spine, for trauma, tumor, spondylodiscitis, or degenerative spinal diseases using intraoperative Airo computed tomography (CT) scan in conjunction with BrainLab curve navigation, Cirq robotic assistance (Brainlab AG, Munich, Germany).

All patients had previously undergone first-stage surgery via an anterior approach to the cervical spine. Anterior cervical discectomy fusion or corpectomy fusion for trauma or oncology or degenerative diseases or anterior debridement fusion for spondylodiscitis was performed.

Patient consent and ethics committee approval were obtained under reference 2019-12-12-007 from the ethics committee of Aix-Marseille University.

Surgical Management and Technique

Under general anesthesia, the patient was placed prone on the Trumpf Medical TruSystem 7500 (Hillrom, Batesville, Indiana, USA), a radiolucent table, with the head maintained in a Mayfield holder. The lower cervical spine was in a neutral position. Required equipment consisted of intraoperative Airo CT in conjunction with BrainLab curve navigation, Cirq robotic assistance (Brainlab) and PASS OCT Reconstruction System (Medicrea, Lyon, France). The posterior neck was shaved, scrubbed, prepared, and draped. The patient reference array was clipped onto the 2-pin fixator, which was attached to the radiolucent Mayfield holder and tightened into position next to the surgical field.

The Cirq was directly mounted to the operating room (OR) table rail on the left side of the patient. The field of the acquisition was determined with a skin marker on the sterile drape and spotted with the laser navigation. Once the scan was completed under apnea to exclude motion artifacts, images were automatically transferred to the BrainLab curve navigation system. Navigation tools were registered. A navigated probe was used to identify the preferred entry point.

A 3-cm left paramedian skin incision was performed. Exposure of the zygapophyseal joints and laminae was achieved at the index level. The Cirq was then aligned and locked firmly in place within the desired screw trajectory. A 1.5-mm diameter navigated drill with image guidance was used through the cannula on the robotic arm to create a pilot hole on the dorsal cortex of the left lateral mass. A K-wire was inserted in the hole to maintain the trajectory for screw placement. Tapping and screw insertion of the PASS OCT Reconstruction System (Medicrea) through the pedicle were performed. The diameter and length of the screw were chosen perioperatively with image guidance help.

The same procedure was repeated for the right side (Figures 1 and2). A new iCT scan was performed to assess the position of the implants. Once good positioning was confirmed, 2 rods were inserted and the system locked firmly. The wounds were then closed (Figure 3).

Radiographic Assessment

Routine postoperative CT scans (slice thickness 1 mm) with axial, coronal, and sagittal reconstructions were obtained on postoperative day 2 or 3. They were interpreted by one of the senior authors (K.F.).

Pedicle screw placement accuracy was evaluated according to the Neo classification system for cervical pedicle screws7 and to the Heary [START_REF] Heary | Thoracic pedicle screws: postoperative computerized tomography scanning assessment[END_REF] classification system for thoracic pedicle screws. We also dichotomized clinical grades as acceptable (grade 1 for Neo classification and II for Heary classification) or poor (grade >2/II).

Quantification of Radiation Exposure

For each iCT-based navigation procedure, we reported: Each procedure was calculated using E/DLP ratio as recommended by the European Guidelines for Multislice Computed Tomography [START_REF] Bongartz | European Guidelines for Multislice Computed Tomography[END_REF] (E k DLP) (k 0.0059 for cervical as the region of the body scanned).

All the operative team stayed outside the OR during imaging acquisition.

RESULTS

Patient Population

Between February 2020 and December 2020, 7 patients (4 men and 3 women) underwent miniopen transpedicular cervical spine screw instrumentation (Table 1).

The mean age was 58.8 years (range, 29e75 years). The mean body mass index was 24.8 kg/m 2 (range, 17.1e38.05 kg/m 2 ).

Five operations were performed for traumatic spine injury (71.4%), 1 for bone metastasis (14.3%), and 1 for infection (14.3%).

The mean surgical time was 115.7 minutes (range, 90e180 minutes). Mean blood loss was 292 mL. 2 and3). In this series, no revision for malpositioned screw was needed. Seventeen screws were placed in the pedicles of the lower cervical spine. According to the Neo classification, 11 were grade 0, 3 grade 1, 2 grade 2, and 1 grade 3. For the thoracic spine, according to the Heary classification, 4 were grade I, 1 grade II, 0 grade III, and 0 grade IV. Of these screws, 87.5% were rated as acceptable and 12.5% as poor.

Radiation Exposure

The mean number of iCT acquisitions for each patient was 3.3 (range, 2e4) (Table 4). Total DLP ranged from 1352.6 to 1803.8 mGy/cm (mean, 1561.2 mGy/cm), which corresponds to 2.77 mSv per single scan. The average total radiation dose for each patient was 9.1mSv (range, 7.7e10.6 mSv). The radiation dose received by the primary surgeon and the rest of the operative team outside the OR, during the imaging acquisition, was considered null.

Postoperative Course

Patients had minimal postoperative pain, easily controlled by class 1 and/or 2 painkillers. Patients were released home on postoperative day 3e4.

No infection, transfusion, or unplanned return to the OR was reported.

DISCUSSION

The accuracy and technical advances in CTSF using the Cirq robotic assistance in conjunction with iCT scan navigation were investigated.

CTSF has more advantages than fixation through the spinous process, lamina, and anterior intervertebral plate, but because of anatomic complexity and possible severe complications, the application of CTSF has been limited. [START_REF] Kandziora | Posterior stabilization of subaxial cervical spine trauma: indications and techniques[END_REF] CTSF is an established technique for spinal stabilization and deformity correction. Its biomechanical advantages include increased bony purchase in the pedicle, and spanning of all 3 columns of the vertebra. It is a more effective technique in restoring segmental lordosis and global cervical lordosis compared with lateral mass fixation. [START_REF] Dickerman | Cervical pedicle screws vs. lateral mass screws: uniplanar fatigue analysis and residual pullout strengths[END_REF][START_REF] Duff | Does pedicle screw fixation of the subaxial cervical spine provide adequate stabilization in a multilevel vertebral body fracture model? An in vitro biomechanical study[END_REF][START_REF] Hey | Accuracy of freehand pedicle screws versus lateral mass screws in the subaxial cervical spine[END_REF][START_REF] Kotani | Biomechanical analysis of cervical stabilization systems. An assessment of transpedicular screw fixation in the cervical spine[END_REF] Abumi et al. [START_REF] Abumi | Pedicle screw fixation for nontraumatic lesions of the cervical spine[END_REF] were the first to introduce this technique in 1994, and there have been many other reports since. [START_REF] Perna | Pedicle screw insertion techniques: an update and review of the literature[END_REF][START_REF] Yukawa | Placement and complications of cervical pedicle screws in 144 cervical trauma patients using pedicle axis view techniques by fluoroscope[END_REF] The most devastating complication associated with this technique is vertebral artery injury and its potentially severe consequences. [START_REF] Yukawa | Cervical pedicle screw fixation in 100 cases of unstable cervical injuries: pedicle axis views obtained using fluoroscopy[END_REF] Other complications such as nerve root injury can also occur. [START_REF] Tokioka | Minimally invasive cervical pedicle screw fixation (MICEPS) via a posterolateral approach[END_REF] Recently, intraoperative imaging, navigation systems, and robotic guidance have been developed to assist surgeons in correct screw placement in thoracolumbar [START_REF] Devito | Clinical acceptance and accuracy assessment of spinal implants guided with SpineAssist surgical robot: retrospective study[END_REF][START_REF] Farah | Prospective comparative study in spine surgery between Oarm and Airo systems: efficacy and radiation exposure[END_REF][START_REF] Kantelhardt | Perioperative course and accuracy of screw positioning in conventional, open robotic-guided and percutaneous roboticguided, pedicle screw placement[END_REF][START_REF] Pechlivanis | Percutaneous placement of pedicle screws in the lumbar spine using a bone mounted miniature robotic system: first experiences and accuracy of screw placement[END_REF][START_REF] Tian | Robot-assisted posterior C1-2 transarticular screw fixation for atlantoaxial instability: a case report[END_REF] and upper cervical spine surgery. [START_REF] Farah | Cirq robotic assistance for minimally invasive C1-C2 posterior instrumentation: report on feasibility and safety [e-pub ahead of print[END_REF][START_REF] Meyer | Minimally invasive percutaneous C1-C2 fixation using an intraoperative three-dimensional imaging-based navigation system for management of odontoid fractures[END_REF] Multiple manufacturers have introduced robotic platforms for use in spine surgery. The current generation of these spinal robots are all designed to assist the surgeon in the placement of screws. In general, these robotic platforms share a similar workflow. In contrast with that of the larger floormounted robotic platforms, which are designed to maxi-mize rigidity of the end effector of the robotic arm to minimize motion away from the intended screw trajectory, the Cirq robotic assistance is smaller and lighter (11 kg). [START_REF] Khalsa | Commentary: Cirq(R) robotic assistance for minimally invasive C1-C2 posterior instrumentation: report on feasibility and safety [epub ahead of print[END_REF] The use of navigation for CTSF was reported with good results; accuracy was up to 95% [START_REF] Ito | Efficacy of computerassisted pedicle screw insertion for cervical instability in RA patients[END_REF][START_REF] Kotani | Improved accuracy of computer-assisted cervical pedicle screw insertion[END_REF] with navigation versus 70%e90% with traditional methods. [START_REF] Reinhold | Cervical pedicle screw placement: feasibility and accuracy of two new insertion techniques based on morphometric data[END_REF][START_REF] Takahata | A novel technique of cervical pedicle screw placement with a pilot screw under the guidance of intraoperative 3D imaging from C-arm cone-beam CT without navigation for safe and accurate insertion[END_REF] Reports of severe complications were rare. The use of this technique is attractive, especially in traumatology, because in some cases, the articular is fractured and lateral mass instrumentation is more difficult. In this case, performing a 2-level CTSF is less compromising for cervical spine motion (flexion-extension) compared with 3 or 4 levels of lateral mass fixation. Limitations Some technical limitations are to be noted in our series. On the one hand, sources of errors were discussed as resulting from the mobility of the lower cervical spine (especially in patients with trauma), registration inaccuracies, and accidental displacement of reference frames. We believe that iCT-based navigation systems should be used when possible when performing posterior fixation in the lower cervical and upper thoracic spine. All sources of potential error of accuracy must be identified and adjusted before the screw placement stage.

On the other hand, we were limited technically, because we could not perform >4 levels per acquisition because of the length and mobility of the robotic arm. Also, we were limited when placing screws on the right side because it was barely possible to reach a higher convergence angle for screw positioning, because of the robotic length and limitation in mobility. In 2 cases, we had to undrape the patient and position the robotic arm on the right side to perform adequate CTSF on the latter side. This situation can explain the less acceptable positioning precision on the right side.

The system that we used for the CTSF seemed adequate because we could easily place the rods within the screw heads. Nevertheless, an adequate system with a cannulated screw should be developed to improve implant placement.

Robotic assistance is a major innovation in spine surgery. [START_REF] Joseph | Current applications of robotics in spine surgery: a systematic review of the literature[END_REF] In our experience, it reduced muscle trauma and blood loss and allowed an acceptable rate of screw positioning.

Patients undergoing this type of surgery may require shorter hospitalization and encounter fewer medical complications during their stay because of the accurate minimally invasive approach.

We report the first case series of mainly CTSF with Cirq robotic assistance in conjunction with iCT scan navigation. We are at the beginning of the learning curve for this technique.

The overall clinical acceptance rate was good (85.7%). This rate was mainly caused by familiarization with this technology. Nevertheless, no screw repositioning was necessary, specially the one grade 3 in C7.

More cases are needed to assess efficacy, screw placement accuracy, and fusion. Although this technology is designed to assist the surgeon in the placement of screws, a dedicated screw and rod system should be developed to facilitate the surgical workflow.

CONCLUSIONS

Posterior miniopen fixation using Cirq robotic assistance coupled with iCT navigation system is a major innovation that can improve the accuracy of pedicle screw positioning in the lower cervical and upper thoracic spine. The procedure is associated with acceptable patient radiation and reduced surgical team exposure. Future studies are needed to assess reproducibility, pedicle screw positioning, radiation exposure, and cost-effectiveness.

CRediT AUTHORSHIP CONTRIBUTION STATEMENT

Figure 1 .

 1 Figure 1. Perioperative screenshots for (A) C5 right pedicle showing real-time three-dimensional coronal, axial, and sagittal navigation views and projection of the entry point. Note the C5 left pedicle K-wire already

  mass index (kg/m 2 ), mean (range) 24.8 (17.1e38.05) Surgical time (minutes), mean (range) 115.7 (90e180) Number of instrumented vertebra 16

Figure 3 .

 3 Figure 3. Two 2-cm paramedian vertical wounds.

Figure 2 .

 2 Figure 2. Intraoperative photograph showing the Cirq positioned in a way to use the 1.5-mm-diameter navigated drill with image guidance to create a pilot hole on the dorsal cortex of the right C5 pedicle before introducing the K-wire. Note the other K-wire already positioned within the C5 left pedicle.

  dose per single scan (mSv) 2.77 Radiation dose received by the primary surgeon performing the 0 imaging acquisition (mSv)

  Kaissar Farah: Conceptualization, Methodology, Writing -original draft, Writing -review & editing. Mikael Meyer: Conceptualization, Writing -original draft, Writing -review & editing. Solene Prost: Writing -original draft, Writing -review & editing. Faisal Albader: Writing -original draft, Writing -review & editing. Henry Dufour: Writing -review & editing. Benjamin Blondel: Writingreview & editing, Supervision. Stephane Fuentes: Conceptualization, Methodology, Writing -review & editing, Supervision.

Table 1 .

 1 Patient Demographics and Surgery Data Accuracy of Pedicle Screw Placement Revision of malpositioned screws was based either on intraoperative judgment by the surgeon or on neurotoxic screw placement (Tables

Table 3 .

 3 Clinical Grading for Cervical and Thoracic Spine Instrumentation

	Neo/Heary (%)

Table 4 .

 4 Radiation Exposure Data in the Group

Table 2 .

 2 Neo and Heary Classification for Cervical and Thoracic Pedicle Screw Placement

6 3 1


Solène Prost

Benjamin Blondel

Vincent Pomero

Guillaume Authier

Christophe Boulay

Jean Luc Jouve

Sebastien Pesenti

Description of spine motion during gait in normal adolescents and young adults

Keywords: Gait cycle, Trunk motion, Dynamic sagittal balance, Spinal junction

Introduction Standing radiographs allow analysis of spinal segments and their relative positioning. However, it requires a specific positioning influencing spinal alignment. Knowledge of trunk movements when walking is therefore an essential step to evaluate dynamic sagittal balance. Our objective was to define spinal junction kinematics and their correlations during gait in a healthy population. Method This is a prospective, single-center study. Between 2015 and 2017, 25 healthy volunteers were included. The measurements were taken in a motion analysis laboratory. Several kinematic parameters were studied, including spinal junction movements in the three planes and dynamic sagittal vertical axis (Dyn-SVA). Pearson correlation coefficients were calculated to determine the interactions between the spinal junctions. Results In the sagittal plane, the average amplitude of variation of the dyn-SVA was 25.5 cm (SD = 8.9). The average range of motion of the lumbosacral and thoracolumbar junction was approximately 3°, they operated in anti-phase during the gait cycle and were strongly correlated (r = -0.5069, p = 0.01). In the transverse plane, the anti-rotation of the upper body relative to the pelvis was mainly ensured by the opposite movements of the lumbosacral and thoracolumbar junction (r = 0.5689, p = 0.003). In the frontal plane, the lateral inclination in the lumbar region was made in the opposite direction from the pelvis toward the carrying member. Conclusion Although there is substantial inter-subject variability, our study characterized the angular movements in the three planes of the different spinal junctions, of the pelvis and the lower limbs during a gait cycle in a healthy population.

Introduction

Since the last decade, the analysis of spinopelvic interactions has been reported to be crucial for the understanding of spine sagittal alignment [START_REF] Huec | Sagittal balance and spinepelvis relation: a French speciality?[END_REF]. Thereby, in daily practice, full spine radiographs including the pelvis have become the gold standard for analysis of the so-called lumbopelvic complex [START_REF] Diebo | Sagittal alignment of the spine: what do you need to know?[END_REF][START_REF] Dubousset | A new 2D and 3D imaging approach to musculoskeletal physiology and pathology with low-dose radiation and the standing position: the EOS system[END_REF][START_REF] Hasegawa | Normative values of spino-pelvic sagittal alignment, balance, age, and health-related quality of life in a cohort of healthy adult subjects[END_REF] providing information for either pathological condition or preoperative planning. Based on this, many spinopelvic parameters have been described and after age adjustment, serve as realignment goals in spinal deformity patients [START_REF] Lafage | Defining spino-pelvic alignment thresholds: should operative goals in adult spinal deformity surgery account for age?[END_REF][START_REF] Moal | Clinical improvement through surgery for adult spinal deformity: what can be expected and who is likely to benefit most?[END_REF]. Recent literature has stressed the importance of fulfilling these goals to improve functional results in spine surgery [START_REF] Lee | Early complications and outcomes in adult spinal deformity surgery: an NSQIP study based on 5803 patients[END_REF][START_REF] Smith | Change in classification grade by the SRS-Schwab adult spinal deformity classification predicts impact on health-related quality of life measures: prospective analysis of operative and nonoperative treatment[END_REF][START_REF] Smith | Change in classification grade by the SRS-Schwab adult spinal deformity classification predicts impact on healthrelated quality of life measures: prospective analysis of operative and nonoperative treatment[END_REF].

However, conditions in which radiographs are taken may be misleading. Indeed, standard radiographs are performed in a constrained position and a restrained environment. In these conditions, radiographic spinal alignment may differ from daily life. In 2020, it was shown that in adolescents with idiopathic scoliosis, dynamic sagittal alignment was not related to X-ray parameters, questioning the validity of sagittal alignment assessment based on radiographic measurements [START_REF] Pesenti | Does static trunk motion analysis reflect its true position during daily activities in adolescent with idiopathic scoliosis?[END_REF]. On the other hand, technologies such as gait analysis provide data about dynamic spinal posture and could therefore give valuable information about sagittal alignment [START_REF] Blonde | Postural spinal balance defined by net intersegmental moments: results of a biomechanical approach and experimental errors measurement[END_REF].

Gait cycle is a sequence of stance and swing phases during which balance is voluntarily compromised, to provoke forward projection of the body [START_REF] Sutherland | The evolution of clinical gait analysis: part II kinematics[END_REF]. Ducroquet et al. stressed the importance of pelvic rotation during gait and introduced the "pelvic step", described as a forward rotation of the pelvis in the transverse plane, allowing an increased step length of the ipsilateral leg during the swing phase [START_REF] Charnley | La Marche et les Boiteries. Étude des Marches Normales et Pathologiques. (A Study of Normal and Abnormal Gait[END_REF]. Simultaneously, the upper part of the trunk initiates an opposite rotation, to prevent complete rotation of the pelvis [START_REF] Waters | Translational motion of the head and trunk during normal walking[END_REF][START_REF] Crosbie | Patterns of spinal motion during walking[END_REF][START_REF] Crosbie | Age, gender and speed effects on spinal kinematics during walking[END_REF]. Although the rotational motion of the trunk and pelvis is well-known, the behavior of spinal segments linking the upper trunk to the pelvis remains unknown, as well as their behavior in sagittal and coronal planes.

A specific gait analysis protocol was recently described with the ability to describe a segmental analysis of the spine [START_REF] Blondel | Sagittal spine posture assessment: feasibility of a protocol based on intersegmental moments[END_REF][START_REF] Pesenti | Correlations linking static quantitative gait analysis parameters to radiographic parameters in adolescent idiopathic scoliosis[END_REF]. This protocol demonstrated significant correlations with usual radiographic parameters in adolescent idiopathic scoliosis patients [START_REF] Mulroy | Use of cluster analysis for gait pattern classification of patients in the early and late recovery phases following stroke[END_REF]. Knowledge of segmental spinal motion appears crucial, to be able to understand various spinal conditions and properly adapt physical therapy or surgical treatment. Therefore, this study aimed to describe kinematic mechanisms within the spine in asymptomatic volunteers during walking, using gait analysis.

Methods

Stu d y d esig n

This is a prospective single-center study of asymptomatic volunteers. Inclusion criteria were as follows: no history of chronic back pain, major trauma, orthopedic condition or previous orthopedic surgery. To limit the risk of biased analysis, subjects must have been between 12 and 30 years old to be included as mature trunk parameters are acquired in adolescents from the age of 12 [START_REF] Pesenti | Spinal alignment evolution with age: a prospective gait analysis study[END_REF] and degenerative changes of the spine may occur from the age of 30 [START_REF] Oh | Whole spine disc degeneration survey according to the ages and sex using pfirrmann disc degeneration grades[END_REF]. This study was approved by our local ethics committee, and before inclusion, informed consent was collected from every participant.

G ait p rotocol

Gait analysis was performed in a laboratory equipped with 6 high-res infrared cameras. Subjects were prepared with a 36-marker protocol derived from the Helen Hayes and Plug-in-Gait protocol, enhanced with four additional markers along the spine [START_REF] Blondel | Sagittal spine posture assessment: feasibility of a protocol based on intersegmental moments[END_REF]. In total, six markers were used for spine description (C7, T7, T9, T12, L3, S1). Subjects were asked to walk barefoot, at a self-selected speed along a 9-m walkway. An average number of four trials were performed and the best one was kept for analysis.

Evalu ation p aram eters

Several parameters were measured in the three planes of the space (Fig. 1).

In the coronal plane, shoulder-line and pelvis obliquity were collected. Coronal vertical axis (Dyn-CVA) was defined as the horizontal distance between the S1 marker and a line dropped from the C7 marker, in the coronal plane.

In the sagittal plane, we measured dynamic thoracic kyphosis (dyn-TK) defined as the angle between the C7-T7 and the T9-T12 lines; the dynamic lumbar lordosis (dyn-LL), defined as the angle between the T12-L3 and the L3-S1 lines; pelvic tilt (PT); and the sagittal vertical axis (dyn-SVA), defined as the horizontal distance between the S1 marker and a line dropped from the C7 marker, in the sagittal plane.

Finally, in the transverse plane, shoulder-line and pelvis rotations (SL-rotation and P-rotation, respectively) were measured, as well as the acromion-pelvis angle (APA), defined as the angle between the bi-acromial line and the line passing through the anterior superior iliac spines.

Additionally, spinal parameters were measured at the cervicothoracic (CTh), thoracolumbar (ThL) and lumbosacral (LS) junctions in the three planes of the space: coronal (lateral bending), sagittal (flexion-extension) and transverse (torsion). CTh was the relative motion of the Vertex-C7 line with regard to the C7-T12 line; ThL was the relative motion of the C7-T12 line with regard to the T12-S1 line, and LS was the relative motion of the T12-S1 line with regard to the pelvis.

Statistical an alysis

Results are presented as means (±standard deviation). By convention, data were acquired during the right gait cycle and calculated at every percentage of the gait cycle. Negative values indicate rotation toward the left side in the coronal and transverse planes and toward the back in the sagittal plane. In a secondary analysis, the average value of each parameter during a whole gait cycle was calculated, as well as the range of motion (ROM), defined as the difference between the minimal and maximal value of a parameter during the gait cycle. Interactions between spinal segments were investigated using Pearson correlation tests. Threshold for statistical significance was set à 5% (i.e., p-value < 0.05).

Results

In clu d ed p op u lation

At the end of the inclusion period, 25 asymptomatic subjects were included (15 females and 10 males), with a mean age of 16.1 years (from 12.6 to 29.9 SD = 5.7) and an average body mass index of 20.0 kg/m 2 (from 15.8 to 27.8 SD = 3.3). In average, walking speed was 1.1 m/s (± 0.1), step length was 0.6 m (± 0.1) and stride length was 1.2 m (± 0.2).

G ait resu lts

In the sagittal plane, the mean dyn-SVA value was 59.3 cm (± 27.6), corresponding to a forward tilt of the trunk during walking, with a ROM of 25.5 cm (± 8.9) (Table 1). The flexion-extension motion of spinal junctions is presented in Fig. 2 and Table 1. On average, the thoracolumbar junction was in flexion, whereas cervicothoracic and lumbosacral junctions were in extension.

In the transverse plane, mean shoulder-line and pelvis rotations were -0.9° (± 2.4°) and 0.5° (± 2.6°), respectively. Acromion-pelvis angle was 0.4° (± 4.7°). These results read as a symmetric rotation of pelvis and shoulder-line, with as much rotation to the left and the right during a full gait cycle. During the right stance phase (0-60% of the gait cycle), the pelvis was rotated toward the left at initial contact. Rotation toward the right was then initiated to allow bodyweight transfer on the right lower limb. It finally led to an anterior excursion of the center of gravity, in order to engage forward propulsion of the body. Meanwhile, the upper body rotated in the opposite direction (Fig. 3). Torsion between the upper and lower body was related to the rotation of the lumbosacral junction. Indeed, we observed opposite rotation of lumbosacral junction with regard to the pelvis, with the largest ROM at this level (8.3° at the lumbosacral junction, 3.3° at the cervicothoracic junction and 3.9° at the Data are expressed in degrees (°). Dyn-APA dynamic acromion-pelvis angle, ROM range of motion thoracolumbar junction) (Fig. 4). Rotations of the different spinal junctions are presented in Table 2.

In the frontal plane, pelvis lateral inflection peak occurred at 15% and 65% of the gait cycle. The lumbar spine responded with opposite inflection to keep the trunk vertical in the frontal plane (Fig. 5). Inflection of the different spinal junctions is presented in Table 3.

R elation s b etw een sp in al ju n ction s

Correlations linking the relative motion of the spinal junctions are presented in Table 4. In the transverse plane, motion at the different spinal levels was not significantly correlated, corresponding to independent moves of spinal junctions during the gait cycle. On the other hand, the lumbosacral extension was significantly correlated to thoracolumbar flexion (r = -0.5069, p = 0.01). Finally, there were significant correlations between relative motions of spinal junctions in the coronal plane (Fig. 6).

Discussion

The pelvic step was introduced by Ducroquet et al. in 1956 and describes pelvis rotation in the transverse plane [START_REF] Jean | Walking and limping; a study of normal and pathological walking[END_REF]. This concept shed light on the pelvis, being the fundamental link for coordination between lower limbs and the trunk during gait. Since then, modern tools have been developed that allowed for accurate description and quantification of this phenomenon, as well as the relative motion of individual segments [START_REF] Blondel | Sagittal spine posture assessment: feasibility of a protocol based on intersegmental moments[END_REF][START_REF] Ceccato | Comparison of trunk activity during gait initiation and walking in humans[END_REF][START_REF] Bruijn | Coordination of leg swing, thorax rotations, and pelvis rotations during gait: the organisation of total body angular momentum[END_REF]. To the best of our knowledge, our work is the first one to go deeper into the segmental analysis of the trunk by describing the motion at different spinal junctions during gait.

In the sagittal plane, an increase of dyn-SVA corresponds to a trunk anterior tilt to promote forward propulsion of the body. Anterior tilting leads to a temporary imbalance of the trunk. Thereafter, the trunk tilts back in a less anterior position during the swing phase, to replace the center of gravity over the pelvis. In our study, the lumbosacral extension was significantly correlated to thoracolumbar flexion. The lumbosacral junction was in extension during gait as the compensation for anterior pelvic tilt. Therefore, thoracolumbar flexion was necessary to provoke anterior tilting of the trunk. On the other hand, the cervicothoracic junction was in extension but presented a great variability among our population (SD = 13.7°), reflecting the larger mobility of this segment. Higher mobility of the cervicothoracic junction allows subjects to keep a horizontal gaze when walking, which is crucial for gait stability.

In the transverse plane, our subjects presented symmetric rotations of pelvis and shoulder-line. These two segments presented opposite rotations. Considering the right gait cycle, while the pelvis rotated toward the left side to increase step length, the shoulder-line rotated in the opposite direction, as shown in Fig. 3. Our results are consistent with previous literature, this phenomenon is known as the anti-phase between trunk and pelvis [START_REF] Crosbie | Patterns of spinal motion during walking[END_REF]. Within the spine, our results suggest that trunk torsion mainly occurs at the lumbosacral level as a response to the pelvic rotation, while thoracolumbar and cervicothoracic junctions did not show great motion in the transverse plane. Once again, coordination between trunk and pelvis appears as a crucial factor for gait stability. Indeed, several authors have reported alterations of the trunk-pelvis coordination in some spinal conditions, such as chronic low-back pain or idiopathic scoliosis [START_REF] Seay | Low back pain status affects pelvis-trunk coordination and variability during walking and running[END_REF][START_REF] Lamoth | Pelvis-thorax coordination in the transverse plane during gait[END_REF][START_REF] Selles | Disorders in trunk rotation during walking in patients with low back pain: a dynamical systems approach[END_REF][START_REF] Pesenti | Curve location influences spinal balance in coronal and sagittal planes but not transversal trunk motion in adolescents with idiopathic scoliosis: a prospective observational study[END_REF]. Therefore, motion analysis opens the door to physical therapy in such patients, aiming to improve transverse plane trunk motion but further research is necessary to confirm the exact implications of such disorders. We found that within the spine, the lumbosacral junction presented the largest mobility, especially in the transverse and frontal plane (Tables 2 and3). This finding in a healthy population raises serious concerns about rotational behavior of the spine in patients undergoing extended spinal fusion including the lumbosacral junction. Indeed, torsion of the trunk at this level will therefore not be possible anymore and will probably occur at a more proximal level, leading to decreased step length. In 2014, Wu et al. [START_REF] Wu | Effects of experimentally increased trunk stiffness on thorax and pelvis rotations during walking[END_REF] studied the rotational behavior of the trunk with artificially increased stiffness. They reported consistent results, with a decrease of trunk-pelvis coordination associated with compensatory increased pelvis-leg phase, finally leading to decreased step length. Upper trunk and upper limbs rotation being mandatory during gait, it is likely that lumbosacral junction stiffness will lead to compensatory increased rotation at the thoracolumbar and cervicothoracic junctions, as the head must remain straight to assure gait stability.

In the frontal plane, the motion of spinal junctions was significantly correlated at every level. While thoracolumbar and lumbosacral junctions moved in the same direction, there was an opposite inclination of the cervicothoracic junction. This contra-rotation of the cervicothoracic junction appears as a compensatory mechanism, to keep a horizontal gaze.

Conclusion

In conclusion, our work is the first one to describe motion within the spine during normal gait. Our results allow a better understanding of biomechanical behavior of the spine during daily-life activities and could help to understand pathological mechanisms in patients with spinal conditions or after spinal fusion.

Fig. 1

 1 Fig. 1 Marker protocol and gait parameters. a: Thoracic cobb, b: dynamic thoracic kyphosis, c: dynamic lumbar lordosis, d: dynamic pelvic tilt, e: dynamic CVA, f: dynamic SVA, f: shoulder-line rotation, g: pelvis rotation, h: acromion-pelvis angle

Fig. 2

 2 Fig. 2 Spinal junctions motion in the sagittal plane during a full gait cycle

Fig. 3 Fig. 4

 34 Fig. 3 Relative motion of shoulder-line and pelvis in the transverse plane during a full gait cycle

Fig. 5

 5 Fig. 5 Spinal and pelvis motion in the frontal plane during a full gait cycle

Fig. 6

 6 Fig. 6 Correlation analysis of motion at the different spinal junctions in frontal and sagittal planes

Table 1

 1 Results of gait parameters in the sagittal plane

	Parameter	Mean	ROM
	Dyn-SVA	59.3 (± 27.6)	25.5 (± 8.9)
	Dyn-TK	28.7 (± 5.6)	4.5 (± 2.6)
	Dyn-LL	15.2 (± 6.7)	3.3 (± 2.1)
	Dyn-PT	10.9 (± 3.9)	3.0 (± 1.1)
	Cervicothoracic flexion-extension	-6.4 (± 13.7)	6.3 (± 3.2)
	Thoracolumbar flexion-extension	-17.8 (± 6.6)	3.1 (± 1.8)
	Lumbosacral flexion-extension	15.6 (± 6.3)	3.3 (± 1.6)
	Except for Dyn-SVA, data are expressed in degrees (°). Dyn-SVA
	dynamic sagittal vertical axis (expressed in centimeters), Dyn-TK
	dynamic thoracic kyphosis, Dyn-LL dynamic lumbar lordosis, Dyn-

PT dynamic pelvic tilt, ROM range of motion

Table 2

 2 Results of gait parameters in the transversal plane

	Parameter	Mean	ROM
	Dyn-APA	-0.4 (± 4.7)	14.1 (± 1.4)
	Shoulder-line rotation	-0.9 (± 2.4)	7.1 (± 0.7)
	Pelvis rotation	0.5 (± 2.6)	7.6 (± 1.6)
	Cervicothoracic torsion	-0.3 (± 0.9)	3.3 (± 2.0)
	Thoracolumbar torsion	-1.0 (± 1.1)	3.9 (0.7)
	Lumbosacral torsion	-0.9 (± 2.3)	8.3 (± 1.4)

Table 3

 3 Results of gait parameters in the frontal plane

	Parameter	Mean	ROM
	Dyn-CVA	-0.1 (± 0.1)	3.0 (± 0.0)
	Shoulder-line obliquity	0.1 (± 2.8)	4.9 (± 2.3)
	Pelvis obliquity	-0.2 (± 2.3)	8.1 (± 1.4)
	Dyn-Thoracic Cobb	-0.1 (± 2.8)	8.5 (± 1.5)
	Dyn-Lumbar Cobb	0.3 (± 1.6)	5.6 (± 1.0)
	Cervicothoracic inflection	-0.6 (± 1.0)	2.7 (± 1.2)
	Thoracolumbar inflection	0.4 (± 0.4)	1.2 (± 0.3)
	Lumbosacral inflection	0.2 (± 4.3)	12.4 (± 1.6)
	Except for Dyn-CVA, data are expressed in degrees (°)Dyn-CVA
	dynamic coronal vertical axis (expressed in centimeters), ROM range
	of motion

Table 4

 4 Correlations linking motion of the spinal junctions

	Correlation coefficient	p	Linear regression
	(r)