Axial Screws Research Articles

Percutaneous monoplanar screws versus hybrid fixed axial and polyaxial screws in intermediate screw fixation for traumatic thoracolumbar burst fractures: a case-control study.

To compare the clinical and radiological outcomes of monoplanar screws (MSs) versus hybrid fixed axial and polyaxial screws (HSs) in percutaneous short-segment intermediate screw fixation (PSISF) for traumatic thoracolumbar burst fractures (TTBFs) in patients without neurologic impairment. A consecutive series of 100 patients with single-segment TTBFs and no neurologic impairment who underwent PSISF with 6 monoplanar screws (MS group) or correct were retrospectively enrolled. The demographic data, radiologic evaluation indicators, perioperative indicators and clinical assessment indicators were analysed between the MS group and HS group. The demographic data and perioperative indicators were not significantly different in the two groups (P > 0.05). The postoperative anterior vertebral height ratio (AVHR), kyphosis Cobb angle (KCA), vertebral wedge angle (VWA) and spinal canal encroachment rate (SCER) were significantly improved in both groups (*P < 0.05). The MS group obtained better correction than the HS group in terms of improvement in the AVHR, KCA and VWA after surgery (*P < 0.05). At the last follow-up, the MS group had less correction loss of AVHR, KCA and VWA (*P < 0.05). The MS group presented greater improvement in the SCER at the last follow-up (*P < 0.05). The visual analogue scale (VAS) score and Oswestry Disability Index (ODI) score of all patients were significantly better postoperatively than those preoperatively (*P < 0.05), and the scores collected at each follow-up visit did not differ significantly between the two groups (P > 0.05). In the MS group, no internal fixation failure was observed during the follow-up period, but, in the HS group, two cases of internal fixation failure were observed at the last follow-up (one case of rod loosening and one case of screw breakage). Both MSs and HSs fixation are effective treatments for TTBFs and have comparable clinical outcomes. In contrast, MSs fixation can improve the correction effect, better improve the SCER, and further reduce correction loss as well as reduce the incidence of instrumentation failure. Therefore, MSs fixation might be a better option for treating TTBFs in patients without neurological deficits.

Advances in fixation strength of reorienting rectangular triple pelvic innominate osteotomies – A biomechanical investigation of two screw fixation techniques

BackgroundReorientating pelvic osteotomies are performed to prevent femoral-acetabular impingement or degenerative arthritis. A Toennis-Kalchschmidt triple pelvis innominate osteotomy is used in symptomatic patients. This study aimed to investigate the biomechanical behaviour of two different acetabular screw configurations for triple pelvis innominate osteotomy osteosynthesis. MethodsTwo screw-orientation techniques in rectangular os ilium osteotomy were compared by osteotomising 12 artificial hemipelvises with triple pelvis innominate osteotomy protocol (fragment reorientation: 10.5° inclination and 10.0° anteversion) and randomising them in 2 groups (n = 6) for implantation with three 4.5 mm screws. Bidirectional group had a bidirectional screw orientation and Monoaxial group had a monoaxial direction of all three screws through iliac crest. All specimens were tested under progressively increasing cyclic loading until failure. Group-wise comparisons of acetabular cup medialisation, anteversion and inclination were evaluated via motion tracking at cycles 250, 500, 750, 1000, 1250, 1500, 1750, 2000, 2250, and 2500. Failure was defined as reaching 5° inclination or 5° anteversion. FindingsAcetabular cup medialisation (p ≤ 0.026), anteversion (p ≤ 0.021) and inclination (p ≤ 0.039) all increased significantly during testing in both groups. There were no significant differences for the group-wise comparisons at the cycle points defined in the methods (p ≥ 0.182). No significant differences were detected between groups for cycles to failure and failure load (p = 0.873). InterpretationBidirectional screw alignment does not lead to significant advantages compared to pure monoaxial if all three axial screws are evenly distributed over the osteotomy geometry. The triple pelvis innominate osteotomy is susceptible to changes in anteversion, inclination and medialisation under partial weight-bearing. Cautious rehabilitation protocols are recommended.

Anatomical study of the technique of the axis laminar screws and development of guide

PurposeTo develop a bidirectional slide guide to assist screw placement in the axial lamina and to preliminarily discuss the accuracy and feasibility of auxiliary screw placement.MethodsCT data from 40 randomly selected patients were imported into the software for modelling, and cross-pinning was used to simulate pinning. According to the different crossing methods of the upper and lower laminar screws, they are divided into two groups. In the software, the position of the needlepoint of each screw is accurately measured, and the needle point is kept unchanged to simulate the movable range of the screw tail under the condition that the body does not penetrate the cortical bone. The data were compared by grouping and gender. Finally, the guide was designed by combining the screw exit point and fine adjustment angle data of all patients with the centripetal principle of the slide rail.ResultsThe needle exit data L1/L2/L3/L4 were 6.44 ± 0.52 mm, 7.05 ± 0.48 mm, 3.55 ± 0.75 mm and 5.09 ± 0.74 mm, respectively, and the fine adjustment angle of the slide rail was 10.51° ± 0.87°. There was no significant difference between the two groups or between men and women (p > 0.05).ConclusionIn this experiment, using the data obtained from the simulation of screw insertion, a two-way slide guide was designed to assist the insertion of axial laminar screws. The guide locks the screw outlet point to position and guides the screw inlet point, which improves the accuracy and safety of screw placement.

The dual Euler-Rodrigues formula in various mathematical forms and their intrinsic relations

This paper examines the variations and derivations of the dual Euler-Rodrigues formula from various mathematical forms, including the matrix in 6 × 6, the dual matrix, Lie group SE(3) of the exponential map of the Lie algebra se(3), and the dual quaternion conjugation, and investigates their intrinsic connections. Based on the dual Euler-Rodrigues formula, the axis, the dual rotation angle, and the new traces are obtained by using the properties of the skew-symmetric matrices. In decomposing the Chasles’ motion, this paper examines two ways of realization of the motion based on the Mozzi-Chasles’ axis. With the equivalent motion, the paper relates the finite displacement screw matrix, the exponential map, and the dual quaternion conjugation to the dual Euler-Rodrigues formula and reveals their connection with the Mozzi-Chasles axis screw, whose parameters are used to construct the Lie algebra, the dual Euler-Rodrigues formula, and the dual quaternion. Further, using the Mozzi-Chasles axis screw, the paper presents a complete geometrical interpretation, including both the translation and rotation, and associates it with the algebraic presentation. By decomposing the equivalent translation induced by the rotation, the paper presents the mapping between the compound translation and the secondary part of the Mozzi-Chasles axis screw. With this map and the compound translation, the paper hence reveals the intrinsic connection between various presentations of rigid body transformations by formulating them into the dual Euler-Rodrigues formula and presents the relations of the exponential map of the Mozzi-Chasles axis screw to the finite displacement screw matrix and the dual Euler-Rodrigues formula, leading to the understanding of the various forms of a rigid body displacement in correspondence to the dual Euler-Rodrigues formula.

Imaging study on the safety of axial pedicle screw placement by the position of the screw trajectory tip on the anteroposterior and lateral radiographs

Objective: To propose a method to judge the safety of axial pedicle screw placement based on the position of the tip of the screw trajectory on the anteroposterior and lateral X-ray radiographs. Methods: The cervical CT data of 40 patients admitted to the Second Affiliated Hospital of Wenzhou Medical University from December 2020 to December 2021 were selected, including 24 males and 16 females, with a mean age of (47.6±13.2) years. Based on the three-dimensional model reconstruction of Mimics software and its function of X-ray, the transmission of the axial pedicle screw and its anteroposterior and lateral films was simulated. The position of the tip of the simulated screw trajectory was divided into 5 regions (regions Ⅰ-Ⅴ) from the inside to the outside on the anteroposterior virtual radiographs, and the upper and lower regions (regions a, b) on the lateral virtual radiographs. By adjusting the direction of the screw, the tip of the screw was located in the corresponding 10 regions (80 screws in each area) on the virtual projections of the anteroposterior and lateral virtual radiographs respectively, and its accuracy was analyzed by CT to determine whether each screw penetrated the medial wall of the pedicle or vertebral artery foramen. The anteroposterior and lateral X-rays and postoperative CT data of 34 patients who underwent axial pedicle screw placement (67 axial pedicle screws were placed in total) from January 2014 to December 2021 were collected, including 18 males and 16 females, with a mean age of (45.8±14.1) years. The position of the tip of the screw trajectory on the anteroposterior and lateral films was divided in the same way. The number of screws in the corresponding 10 positions was counted, and CT analysis was used to determine whether each screw penetrated the medial wall of the axial pedicle or the vertebral artery foreman. Results: The results of the imaging simulation screw placement study showed that the perforation rate of the vertebral artery foramen in region Ⅳ and Ⅴ was 75.0% (120/160) and 100% (160/160), respectively, while the perforation rate of the medial wall of the axial pedicle in the region Ⅰ was 85.6%(137/160). The failure rate in regions Ⅱ and Ⅲ was relatively lower, and the performance of simulated screws located in the region a was better than those in region b. The perforation rates of the medial wall in regions (a-Ⅱ) and (a-Ⅲ) was 7.5% (6/80) and 0 (0/80), respectively, and the perforation rates of the vertebral foramen was 0 (0/80) and 21.3% (17/80), respectively. The retrospective imaging study also showed a higher rate of placement failure in regions Ⅰ, Ⅳ and Ⅴ, and relatively lower in regions Ⅱ and Ⅲ. There were total of 15 screws in region a-Ⅱ and a-Ⅲ, and no destruction of the medial wall of the axial pedicle and the vertebral artery foreman occurred there. Conclusions: Regions a-Ⅱ and a-Ⅲ are the "safety areas" of the tip of the pedicle screw trajectory in the axial vertebra. By analyzing the tip of the pedicle screw trajectory on the anteroposterior and lateral radiographs, the operator can determine the reasonable trajectory of axial pedicle screw placement, prevent the injury of the cervical spinal cord and vertebral artery, and reduce the risk of operation.

The Classification of Axial Deformity in Patients with Basilar Invagination.

To summarize the variation types of the axis in patients with basilar invagination (BI), then propose a classification scheme of the axis deformity. From December 2013 to September 2020, 92 patients (male 42, female 50) who were diagnosed with BI were studied retrospectively. Based on the imaging data of CT, the width and height of the axis pedicle and the sagittal diameter of the lateral mass were measured in each patient. According to the development of axis pedicle and lateral mass, the types of axis variation were summarized, and then the classification scheme of axis deformity was put forward. All cases were analyzed and axis deformities were divided into four types. Type I: the axis is basically normal (53 cases, 57.6%). Type II: axis lateral mass is dysplasia (eight cases, 8.7%), which includes two subtypes: type IIA, the axis unilateral lateral mass is dysplasia (three cases); type IIB, the axis bilateral lateral masses are all dysplasia (five cases). Type III: axis pedicle is dysplasia (11 cases, 12%), which is subdivided into two subtypes: type IIIA, the axis unilateral pedicle is dysplasia (six cases); type IIIB, the axis bilateral pedicles are all dysplasia (five cases). Type IV: axis pedicle and lateral mass are all dysplasia (20 cases, 21.7%), this type contains the following four subtypes: type IVA, the unilateral axis pedicle and unilateral lateral mass (contralateral or ipsilateral) are all hypoplasia (four cases); type IVB, the unilateral axis pedicle and bilateral lateral masses are all hypoplasia (five cases); type IVC, the bilateral axis pedicles and unilateral lateral mass are all dysplasia (seven cases); type IVD, the bilateral axis pedicles and bilateral lateral masses are all dysplasia (four cases). The left and right abnormal lateral mass sagittal diameter (Type II) was (7.23 ± 1.39) mm and (5.96 ± 1.37) mm, respectively, the left and right abnormal pedicle width (Type III) was (2.61 ± 1.01) mm and (3.23 ± 0.66) mm, respectively, left and right abnormal pedicle height (Type III) was (5.43 ± 2.19) mm and (4.92 ± 1.76) mm, respectively. Moreover, the classification scheme has good repeatability and credibility. The classification about axis deformity could provide personalized guidance for axis screw placement in the BI and other upper cervical surgery, and axis screw placement errors would be effectively avoided.

C2 translaminar screw fixation in pediatric occipitocervical fusion

PurposeRigid occipitocervical (O-C) instrumentation can reduce the anterior pathology and has a high fusion rate in children with craniovertebral instability. Typically, axis (C2) screw fixation utilizes C1–C2 transarticular screws or C2 pars screws. However, anatomic variation may preclude these screw types due to the size of fixation elements or by placing the vertebral artery at risk for injury. Pediatric C2 translaminar screw fixation has low risk of vertebral artery injury and may be used when the anatomy is otherwise unsuitable for C1–C2 transarticular screws or C2 pars screws.MethodsWe retrospectively reviewed a neurosurgical database at UCSF Benioff Children’s Hospital Oakland for patients who had undergone a cervical spinal fusion that utilized translaminar screws for occipitocervical instrumentation between 2002 and 2020. We then reviewed the operative records to determine the parameters of C2 screw fixations performed. Demographic and all other relevant clinical data were then recorded.ResultsTwenty-five patients ranging from 2 to 18 years of age underwent O-C fusion, with a total of 43 translaminar screws at C2 placed. Twenty-three patients were fused (92%) after initial surgery with a mean follow-up of 43 months. Two patients, both with Down syndrome, had a nonunion. Another 2 patients had a superficial wound dehiscence that required wound revision. One patient died of unknown cause 7 months after surgery. One patient developed an adjacent-level kyphosis.ConclusionWhen performing occipitocervical instrumentation in the pediatric population, C2 translaminar screw fixation is an effective option to other methods of C2 screw fixation dependent on anatomic feasibility.

Retropharyngeal Reduction Plate for Atlantoaxial Dislocation: A Cadaveric Test and Morphometric Trajectory Analysis.

ObjectiveTo evaluate the placement feasibility and safety of the newly designed retropharyngeal reduction plate by cadaveric test and to perform morphometric trajectory analysis.MethodsThe five cadaveric specimens with intact atlantoaxial joint were enrolled in this study. They were used for simulating the placement process and evaluating the placement feasibility of the retropharyngeal reduction plate. The atlantoaxial dislocation (AAD) of five cadaveric specimens were obtained by proper external force after dissecting ligaments. The retropharyngeal reduction plate was placed on atlantoaxial joint of cadaveric specimens. The X‐ray and three‐dimensional (3D) spiral CT were used for evaluating the placement safety of retropharyngeal reduction plate. The DICOM data was obtained after 3D spiral CT scanning for the morphometric trajectory analysis.ResultsThe reduction plates were successfully placed on the atlantoaxial joint of five cadaveric specimens through the retropharyngeal approach, respectively. The X‐ray and 3D spiral CT showed the accurate screw implantation and satisfying plate placement. The length of the left/right atlas screw trajectory (L/RAT) was, respectively, 1.73 ± 0.01 cm (LAT) and 1.71 ± 0.02 cm (RAT). The length of odontoid screw trajectory (OST) was 1.38 ± 0.02 cm. The length of the left/right axis screw trajectory (L/RAXT) was, respectively, 1.67 ± 0.02 cm (LAXT) and 1.67 ± 0.01 cm (RAXT). There was no statistical significance between left side and right side in terms of AT and AXT (P > 0.05). The angles of atlas screw trajectory angle (ASTA), axis screw trajectory angle (AXSTA), and odontoid screw trajectory angle (OSTA) were 38.04° ± 2.03°, 56.92° ± 2.66°, and 34.78° ± 2.87°, respectively.ConclusionThe cadaveric test showed that the retropharyngeal reduction plate is feasible to place on the atlantoaxial joint, which is also a safe treatment choice for atlantoaxial dislocation. The meticulous preoperative planning of screw trajectory based on individual differences was also vital to using this technique.

Biomechanical analysis of spinal pedicle screws under static compression and tensile bending

Spinal implants or instrumentation are commonly used for the re-alignment and treatment of spinal injuries. Pedicle screws are the type of bone screw designed for implementation into vertebral pedicle during the treatment of the disorders. Numerous variations in design of the pedicle screw have been developed to increase its load carrying capacity and thus resist the forces acting on the vertebral column. The main purpose of this study is to investigate the strengths of the commonly used pedicle screws i.e. mono axial and poly axial pedicle screws under bending conditions. For this purpose, static compression bending and static tension bending tests were carried out according to the ASTM standards and the behaviour of the pedicle screw implant was investigated. Furthermore, FE model was developed as identical as the experimentation setup and numerical simulation was carried out to study the fracture behaviour of the pedicle subassembly under static compression and tensile bending. This combined biomechanical and numerical analysis opens up the ways to improve the strength and load carrying capacity of the pedicle screw.

Posterior atlantoaxial fixation with new subfacetal axis screw trajectory avoiding vertebral artery with customized variable screw placement plate and screws to enhance biomechanics of fixation.

Fixation for atlantoaxial dislocation is a challenging issue, and posterior C1 lateral mass and C2 pars–pedicle screw plate–rod construct is the standard of care for atlantoaxial instability. However, vertebral artery injury remains a potential complication. Recent literature has focused on intraoperative navigation, the O-arm, 3D printing, and recently use of robots for perfecting the trajectory and screw position to avoid disastrous injury to the vertebral artery and enhance the rigidity of fixation. These technological advances increase the costs of the surgery and are available only in select centers in the developed world.Review of the axis bone anatomy and study of the stress lines caused by weight transmission reveal that the bone below the articular surface of the superior facet is consistently dense as it lies along the line of weight transmission A new trajectory for the axis screw 3–5 mm below the midpoint of the facet joint and directed downward and medially avoids the course of the vertebral artery and holds the axis rigidly. Divergent screw constructs are biomechanically stronger. Variable screw placement (VSP) plates with long shaft screws permit manipulation of the vertebrae and realignment of the facets to the correct reduced position with fixation in the compression mode.The video can be found here: https://youtu.be/E1msiKjM-aA

Posterior fusion of the craniocervical junction in the pediatric spine: Wright's translaminar C2 screw technique provides for more safety and effectiveness.

Posterior fusion of the craniocervical junction (CCJ) has always been challenging in children with rare congenital diseases and malformations. At our institution, the introduction of the translaminar C2 screw technique led to a significant improvement in the quality of treatment. Retrospective analysis of a pediatric cohort at a single institution whounderwent CCJposterior fusion between 2007 and 2018. Patients were divided into group 1 (other posterior fusion techniques, n = 12) and group 2 (translaminar axis screw placement, n = 19). Diagnosis, sex, age at surgery, surgical technique, immobilization, revisions, fusion, reduction, and complications were assessed. Follow-up ranged from 12 to 145months (mean 50.7). The initial fusion rate detected at 3months by CT differed significantly (66, 7% in group 1 vs. 100% in group 2, p = 0.018). Full reduction of C1/C2 malalignments was achieved in 41, 6% of group 1 versus 84, 2% of group 2 (p = 0.007). Immobilization was applied in 83, 3% of group 1 versus 26, 3% of group 2 (p = 0.0032). Ten complications were treated conservatively, and 15 events required revision surgery (80% in group 1 vs. 20% in group 2). Eight complications were related to immobilization. The implementation of the translaminar C2 technique resulted in significantly more safety and efficiency regarding pediatric posterior fusion CCJ surgery at our institution, with significantly higher rates of rigid fixation, full reduction, and fusion, and significantly lower rates of complications and immobilization. These slides can be retrieved under Electronic Supplementary Material.

Application of electric drill drive technique in atlantoaxial posterior screw trajectory preparation

Objective To investigate the application effect of electric drill drive in the preparation of atlanto-axial posterior screw trajectory. Methods A retrospective case-control study was conducted to analyze the clinical data of 29 patients with atlanto-axial fracture and dislocation admitted to the Lanzhou University Second Hospital from February 2016 to March 2019. There were 19 male and 10 females, aged from 10 to 72 years, with an average of 44.6 years. All patients received posterior atlantoaxial screw fixation. Electric drill drive was adopted in 14 patients to prepare the screw trajectory (electric drill group), and bare hand technique was used in 15 patients (bare hand group). In the electric drill group, 28 atlantal lateral screws and 28 axial pedicle screws were implanted; in the free hand group, 30 atlantal lateral screws and 30 axial pedicle screws were implanted. The trajectory preparation time and intraoperative blood loss between the two groups were compared. After the operation, the X-ray films and 3D CT were taken to observe the lateral mass of atlas as well as the position, loosening or rupture, and bone healing of axial pedicle screws, and complications. Frankel grading and visual analogue scale (VAS) at the last follow-up were compared between the two groups. Results All patients were followed up for 3 to 36 months, with an average of 21 months. The electric drill group took significantly shorter time in preparing both atlantal and axial pedicle screw trajectories than the bare hand group [(8.8±4.2)minutes ∶(16.5±9.7)minutes; (4.1±1.2)minutes∶ (6.1±3.2)minutes] (P 0.05). In the electric drill group, the penetration rate of atlantal screws was 0, the complete position rate was 96% (27/28), and the good rate of screw placement was 100% (28/28), superior to 23% (7/30), 73% (22/30) and 73%(22/30) in the bare hand group (P 0.05). There were no complications such as cerebrospinal fluid leakage, spinal cord injury, cerebellar infarction, loosening and fracture of internal fixation, and nonunion of bone. At the last follow-up, Frankel grading and VAS of the two groups were improved to different degrees (P 0.05). Conclusion For atlantoaxial fracture and dislocation, preparation of atlantoaxial internal fixation screw trajectories by electric power can significantly shorten the operation time and improve the accuracy of atlas screw placement. Key words: Atlanto-axial joint; Fracture fixation, internal; Electric drill

Short-term effectiveness of axis laminar screws for reducible atlantoaxial dislocation

To investigate reliability and short-term effectiveness of axis laminar screws for reducible atlantoaxial dislocation (RAAD). A clinical data of 41 patients with RAAD who were admitted between February 2013 and February 2018 and met the inclusion criteria was retrospectively analyzed. The atlases in all patients were fixated by lateral mass screws, and the axes were fixed by laminar screws in 13 cases (LS group) and by pedicle screws in 28 cases (PS group). There was no significant difference in gender, age, and preoperative Japanese Orthopedic Association (JOA) score between the two groups ( P>0.05). The effectiveness was estimated by post-operative JOA score; and the accuracy of the axis screw, atlantoaxial bone graft fusion, and the fixation stability were examined by X-ray film and CT. All incisions healed by first intention. All patients were followed up 12-17 months (mean, 13.8 months) in LS group and 12-20 months (mean 14.1 months) in PS group, and the difference in follow-up time was not significant ( Z=-0.704, P=0.482). At last follow-up, JOA scores were 13.9±1.6 in LS group and 14.3±1.8 in PS group, which significantly improved when compared with the pre-operative scores in the two groups ( t=-9.033, P=0.000; t=-15.835, P=0.000); while no significant difference was found between the two groups ( t=-0.630, P=0.532). Twenty-five screws of 26 screws in LS group and 54 screws of 56 screws in PS group were implanted accurately, with no significant difference in the accuracy of the axis screw between the two groups ( Z=-0.061, P=0.951). All patients obtained atlantoaxial bone graft fusion, except 1 case in PS group. There was no significant difference in the atlantoaxial bone graft fusion between the two groups ( Z=-0.681, P=0.496). For RAAD, Axis laminar screws can maintain the atlantoaxial primary stability and had a good short-term effectiveness. So, it could be an alternative and reliable technique for axis screw.

Tuber-to-Anterior Process Angle (TAPA): A cadaveric study and surgical technique for placing axial calcaneal screws

We describe results of a cadaveric study and an accompanying surgical technique which simplifies posterior-to-anterior axial screw placement into the calcaneus, often utilized during fixation of displaced intra-articular calcaneus fractures or calcaneal osteotomies. By defining the Tuber-to-Anterior Process Angle (TAPA), this technique facilitates axial screw placement, thereby decreasing reliance on intraoperative fluoroscopy and reducing operative time.

Mismatch between rod bending and actual post-operative lordosis in short lumbar arthrodesis with poly axial screws

Restoring sagittal balance is a common goal in modern spine surgery (Le Huec and Roussouly 2011). To do so, the adequate lordosis need to be restored between the fused vertebrae. In degenerative po...

Mismatch between rod bending and actual post-operative lordosis in lumbar arthrodesis with poly axial screws

BackgroundThe question of rod bending is essential during posterior lumbar fusion. The role of posterior instrumentation during spinal surgery remains to be defined. Despite an appropriate bending, a mismatch between rod lordosis and lumbar lordosis can occur. There is no study on the link between rod bending and lordosis. The purpose of this study was to evaluate parameters that explain the mismatch between lumbar lordosis and rod bending in lumbar surgery using polyaxial screws. HypothesisRadiological parameters explain the mismatch between the rod and the lordosis. MethodsThis study was monocentric, retrospective, descriptive and analytic. All patients with posterior L3L5 fusion in an university-affiliated hospital in 2017 were included. Patients with past surgical history of anterior fusion on the levels L3L5, Coronal malalignment with a Cobb angle superior to 5°, the use of dynamic fixation systems were excluded. We measured on immediate post-operative standing profile x-ray: pelvic incidency, lumbar lordosis, lordosis of the instrumented segment, the distance between posterior wall and rod (EcarT) which reflect how homogeneously the screws are put in depth, the angle between screw and rod (thetaMA), the angle between screw and superior endplate (lambdaMA), the rod lordosis. Univariate and multivariate analysis were conducted to see if there was a link between all those parameters and the mismatch: vertebral lordosis-rod lordosis. ResultsA total of 74 patients were included, mean age was 67. Eighteen were 360° fusion and 56 were postero-lateral fusions. There was no statistical association between demographic data, pelvic parameters, use of interbody devices and the mismatch. There was a statistical association between thetaMA, lambdaMA, EcarT and the mismatch (P<0,0001). A multivariate linear regression model was developed to create a new index: Mismatch analysis index. ConclusionOur study is the first on the link between rod bending and lumbar lordosis. Three radiologic factors are involved in not obtaining the planned lordosis in short lumbar fusion with polyaxial screws. Two factors depend on the way the surgeon positions screw parallel to the superior vertebral endplate(lambdaMA), and with a homogeneous depth (EcarT). And the last factor: ThetaMA is depending on the surgical technique (compression on screws, osteotomies, monoaxial screws, use of interbody devices). Level of evidenceIV.

Axis screws: results and complications of a large case series.

To present the surgical results of patients who underwent axis screw instrumentation, discussing surgical nuances and complications of the techniques used. Retrospective case-series evaluation of patients who underwent spinal surgery with axis instrumentation using screws. Sixty-five patients were included in this study. The most common cause of mechanical instability was spinal cord trauma involving the axis (36 patients - 55.4%), followed by congenital craniocervical malformation (12 patients - 18.5%). Thirty-seven (57%) patients required concomitant C1 fusion. Bilateral axis fixation was performed in almost all cases. Twenty-three patients (35.4%) underwent bilateral laminar screws fixation; pars screws were used in twenty-two patients (33.8%), and pedicular screws were used isolated in only three patients (4.6%). In fourteen patients (21.5%), we performed a hybrid construction. There was no neurological worsening nor vertebral artery injury in this series. Axis screw instrumentation proved to be a safe and efficient method for cervical stabilization. Laminar and pars screws were the most commonly used.

Minimally invasive treatment of calcaneal fractures via the sinus tarsi approach based on a 3D printing technique.

The sinus tarsi approach can be used for a limited exposure of the calcaneal fracture site. The reduction of the posterior articular surface, the shape of the calcaneus, the precise placement of the sustentacular screw (SS), the posterior articular surface screw of the calcaneal (PASS), and the long axis screw of the calcaneal (LAS) are still challenging. To that end, we proposed a minimally invasive technique for the treatment of calcaneal fractures via the sinus tarsi approach in combination with a three-dimensional (3D) printing technique. First, a 3D reconstructed model of the bilateral calcanei was obtained according to the computed tomography (CT) scan data and was used to simulate the placement of screws and acquire the screw trajectory parameters. Next, using 3D printing, a model of the calcaneus was printed, and the minimally invasive steel plate was pre-shaped to fit the lateral wall of the model. Finally, a total of 25 patients underwent this procedure. The results showed significant accuracy improvement in terms of the SS, PASS and LAS placement and in terms of the parameters including Bohler's angle, Gissane's angle, and the calcaneal width. In this work, the technique of the personalized minimally invasive treatment of calcaneal fractures improved the accuracy of screw placement (SP) and the reduction rate of posterior articular surface, improved the shape of the calcaneus, and increased the precision of the minimally invasive treatment of calcaneal fractures via the sinus tarsi approach.

C2 Translaminar Screw Fixation in Children.

Axis (C2) screw fixation has been shown to be effective in treating disorders that necessitate cervical stabilization. Although translaminar C2 screws have demonstrated clinical efficacy in adults, this technique has not yet been thoroughly investigated in children. This study describes the indications, technique, and results of translaminar C2 screw fixation in a case series of pediatric cervical spine disorders. We searched the orthopaedic database at our institution for patients who had undergone a cervical spinal fusion that encompassed C2 between 2007 and 2017. Operative records were reviewed to determine if C2 screw fixation was performed and, if so, the type of C2 screw fixation. Clinical data with regard to patient age at surgery, diagnosis, procedure details, intraoperative complications, and postoperative complications were recorded. Preoperative and postoperative computer tomographic scans were reviewed to determine laminar measurements and containment, respectively. In total, 39 C2 translaminar screws were placed in 23 patients that met our inclusion criteria. The mean age was 12.6 years (range, 5.2 to 17.8 y) with a mean of 2 levels fused (range, 1 to 6). Diagnoses included 7 patients with instability related to skeletal dysplasia, 6 os odontoideum, 4 congenital deformities, 3 basilar invaginations, 2 cervical spine tumors, and 1 fracture. Indications for C2 translaminar screws included 14 cases with distorted anatomy favoring C2 translaminar screws, 6 cases without explicit reasoning for translaminar screw usage in the patient records, and 3 cases with intraoperative vertebral artery injury (1 sacrificed secondary to tumor load and 2 others injured during exposure because of anomalous anatomy). The vertebral artery injuries were not due to placement of any instrumentation. There were no screw-related intraoperative or postoperative complications and no neurological injuries. All patients demonstrated clinical union or healing on follow-up with no episodes of nonunion. Translaminar C2 screw fixation can be reliably used in the pediatric population. Our series contained no screw-related complications, no neurological injuries, and all patients demonstrated clinical union or healing. Level IV-Case series.

Three-dimensional finite element analysis of biomechanical properties of a posterior atlanto-axial dynamic fixation system

Objective To evaluate the biomechanical features of a posterior atlanto-axial dynamic fixation system by finite element method. Methods A 25-age-year old healthy male volunteer underwent computed tomography (CT) scan at C0-C2 (slice width=1 mm). Three-dimensional finite element model of atlanto-axial was established by Mimics, Freeform and Ansys. Then, three-dimensional finite element model of atlanto-axial traditional fixtion and dynamic fixation were established. The range of motion (ROM) and load transfer under different direction were evaluated biomechanically in flexion-extension lateral bending and axial rotation load. Results The ROM in flexion-extension lateral bending and axial rotation of dynamic fixation were 8.9°, 8.2°, 2.8° and 10.4°, respectively, was better than traditional fixation in all direction, the ROM increased 21.9%, 10.8%, 12.0% and 126.9%, respectively. Stress in flexion-extension lateral bending and axial rotation of dynamic fixation were 1 410, 1 410, 1 410 and 1 520 mPa, respectively, were average three times bigger than traditional fixation and found concentrating around the bottom of atlanto, axial screws. Stress of vertebral in flexion-extension was bigger than traditional fixation. Stress of rod was bigger in traditional fixation. Conclusion Anterior transarticular screws show excellent biomechanical, meanwhile, maintain the atlanto-axial range of motion. Stress was found concentrating around the bottom of atlanto, axial screws, which predispose the instrument at failure. Key words: Atlanto-axial; Dynamic fixation system; Biomechanics; Finite element analysis