Biomechanical evaluation of an atlantoaxial lateral mass fusion cage with C1-C2 pedicle fixation

Early Outcomes of Bone Autografting in the Bilateral Atlantoaxial Joints Applied in Posterior Fusion Surgery

OBJECTIVE

Cable-dragged reduction and cantilever beam internal fixation can provide promising results in the treatment of atlantoaxial dislocation or instability. However, bilateral atlantoaxial joints bone autografting has not been conducted in this technique. We aim to evaluate the safety and effectiveness of bilateral atlantoaxial joints bone autografting in posterior cable-dragged reduction and cantilever-beam internal fixation.

METHODS

In this retrospective study, we included 14 patients with a minimum 24-month follow-up from December 2019 to September 2020. The granular bone harvested from the iliac crest was packed into the bilateral atlantoaxial joints of 14 patients in posterior cable-dragged reduction and cantilever-beam internal fixation. X-ray imaging and cervical computed tomography (CT) were performed during follow-up. The time required for bone fusion was recorded. The clinical outcomes were evaluated using the JOA scores, NDI, and VAS scores. Mann-Whitney U test, the chi-squared test, or the Fisher exact test were used to compare the two groups regarding patient characteristics, clinical outcomes, bone fusion rates, and cervical sagittal alignment.

RESULTS

The operations were successfully performed in all patients without any intraoperative complications. The mean operation time was (169.64 ± 20.91) minutes, and the intraoperative blood loss was (130.71 ± 33.62) mL. All patients received satisfactory reductions and firm bony fusion at the final follow-up. The fusion rates were 64.29% in the atlantoaxial joints and 21.43% in post bone graft area at 3 months postoperatively, and a significant difference was observed (p = 0.022). Besides, the cervical sagittal alignment in all patients was well maintained in the last follow-up compared to preoperatively. Importantly, a complete bony fusion in the atlantoaxial joints was observed in all patients. Moreover, the JOA, NDI, and VAS scores had improved significantly at the last follow-up.

CONCLUSION

Bone autografting of the bilateral atlantoaxial joints is a safe and effective technique to increase bone fusion rates, shorten bone fusion time, and reduce complication rates when the cable-dragged reduction and cantilever beam internal fixation approach is used. Therefore, it is a cost-effective surgical procedure for treating patients with atlantoaxial dislocation or instability.

Study on articular surface morphology of atlantoaxial lateral mass based on differential manifold

OBJECTIVES

To propose a surface reconstruction algorithm based on a differential manifold (a space with local Euclidean space properties), which can be used for processing of clinical images and for modeling of the atlantoaxial joint. To describe the ideal anatomy of the lateral atlantoaxial articular surface by measuring the anatomical data.

METHODS

Computed tomography data of 80 healthy subjects who underwent cervical spine examinations at our institution were collected between October 2019 and June 2022, including 46 males and 34 females, aged 37.8 ± 5.1 years (28-59 years). A differential manifold surface reconstruction algorithm was used to generate the model based on DICOM data derived by Vision PACS system. The lateral mass articular surface was measured and compared in terms of its sagittal diameter, transverse diameter, articular surface area, articular curvature and joint space height.

RESULTS

There was no statistically significant difference between left and right sides of the measured data in normal adults (P > 0.05). The atlantoaxial articular surface sagittal diameter length was (15.83 ± 1.85) and (16.22 ± 1.57) mm on average, respectively. The transverse diameter length of the articular surface was (16.29 ± 2.16) and (16.49 ± 1.84) mm. The lateral articular surface area was (166.53 ± 7.69) and (174.48 ± 6.73) mm2 and the curvature was (164.03 ± 5.27) and (153.23 ± 9.03)°, respectively. The joint space height was 3.05 ± 0.11mm, respectively. There is an irregular articular space in the lateral mass of atlantoaxial, and both upper and lower surfaces of the articular space are concave. A sagittal plane view shows that the inferior articular surface of the atlas is mainly concave above; however, the superior articular surface of the axis is mainly convex above. In the coronal plane, the inferior articular surface of the atlas is mostly concave above, with most concave vertices located in the medial region, and the superior articular surface of the axis is mainly concave below, with most convex vertices located centrally and laterally.

CONCLUSION

A differential manifold algorithm can effectively process atlantoaxial imaging data, fit and control mesh topology, and reconstruct curved surfaces to meet clinical measurement applications with high accuracy and efficiency; the articular surface of the lateral mass of atlantoaxial mass in normal adults has relatively constant sagittal diameter, transverse diameter and area. The distance difference between joint spaces is small, but the shape difference of articular surfaces differs greatly.

C2 Odontoid Fracture Associated with C1-C2 Rotatory Dislocation: A Retrospective Analysis of 2 Surgical Techniques

BACKGROUND

Odontoid fractures in association with a C1-C2 rotatory luxation reports are seldom found in the literature. The fusion between the lateral mass of C1 and C2 could be of interest to ensure adequate treatment in these particular cases. We report 23 cases where there was coexistence of an odontoid fracture and rotatory subluxation, which were treated surgically using cages between C1 and C2 or just traditional Goel-Harms technique. We evaluated the radiologic fusion rate, reoperation rate, and complications.

METHODS

This was a single-center, retrospective, cohort study of patients with C2 fractures (mixed type and C1-C2 rotatory luxation according to the Fielding classification) who were treated surgically. Radiologic computed tomography scans were used to assess fusion (presence of bridging trabecular bone end plate or pseudoarthrosis) between 6 months and 1.5 years after the surgery.

RESULTS

Twenty-three patients were diagnosed with C2 fractures and C1-C2 rotatory luxation that were treated surgically and were suitable for the analysis; 11 patients underwent C1-C2 fusion with intra-articular cages, and 12 underwent a classical Goel-Harms technique. The fusion rate at the C1-C2 joint was higher in the cages group. Only 12 patients exhibited fusion at the level of the odontoid fracture.

CONCLUSIONS

C2 fractures associated with C1-C2 rotatory dislocation are rare. The fusion rate at the level of the odontoid in these patients appears to be lower than that reported in patients without rotatory dislocation. It may be of special interest to obtain a clear fusion at the C1-C2 joint, where this type of implant seems to offer an advantage.

Posterior Reduction and Intra-Articular Cage Fusion With a C2 Nerve Root Preservation Technique for Treating Posterior Atlantoaxial Dislocation Secondary to Os Odontoideum

BACKGROUND AND OBJECTIVES

Posterior C1-C2 interlaminae compression fusion with iliac bone graft may lead to donor site complications and recurrent C1 posterior dislocation for posterior atlantoaxial dislocation (AAD) secondary to os odontoideum. C1-C2 intra-articular fusion often needs C2 nerve ganglion transection to facilitate exposing and manipulating the facet joint, leading to bleeding from the venous plexus and suboccipital numbness or pain. Therefore, this study was conducted to evaluate the outcomes of posterior C1-C2 intra-articular fusion with a C2 nerve root preservation technique in the treatment of posterior AAD secondary to os odontoideum.

METHODS

Data of the 11 patients who underwent C1-C2 posterior intra-articular fusion because of posterior AAD secondary to os odontoideum were retrospectively reviewed. Posterior reduction was performed using C1 transarch lateral mass screws and C2 pedicle screws. Intra-articular fusion was performed using a polyetheretherketone cage filled with autologous bone from the caudal edge of the C1 posterior arch and cranial edge of the C2 laminar. Outcomes were evaluated by using the Japanese Orthopaedics Association score, Neck Disability Index, and visual analog scale for neck pain. Bone fusion was evaluated by using computed tomography and 3-dimensional reconstruction.

RESULTS

The average follow-up duration was 43.9 ± 9.5 months. All patients achieved good reduction and bone fusion, without transection of the C2 nerve roots. The mean bone fusion time was 4.3 ± 1.1 months. There was no complication related to the surgical approach and instrumentation. Function of the spinal cord manifested by the Japanese Orthopaedics Association score significantly improved ( P < .05). The Neck Disability Index score and visual analog scale for neck pain markedly decreased (all P < .05).

CONCLUSION

Posterior reduction and intra-articular cage fusion with a C2 nerve root preservation technique was a promising treatment of posterior AAD secondary to os odontoideum.

Sagittal Reconstruction of the Atlantoaxial Lateral Mass Complex with an Intra-Articular Cage Fusion Technique for Degenerative Atlantoaxial Instability

OBJECTIVE

To evaluate outcomes of sagittal reconstruction of the atlantoaxial lateral mass complex using a modified intra-articular cage fusion technique for treating degenerative atlantoaxial instability.

METHODS

Data from 15 patients with degenerative atlantoaxial instability were retrospectively reviewed. All patients underwent posterior reduction and intra-articular fusion with a cage filled with local autologous bone. Atlantodental interval values on plain radiography in flexion before and after surgery were recorded. Bone fusion was evaluated on computed tomography reconstruction, and bone fusion time was recorded. Lateral atlantoaxial joint space height before and after surgery was measured on coronal computed tomography reconstruction. Japanese Orthopaedic Association score and visual analog scale score for neck pain before surgery and at final follow-up were compared.

RESULTS

Mean follow-up time was 40.7 ± 13.4 months. All patients achieved good reduction and solid bone fusion at follow-up. Mean fusion time was 4.4 ± 1.1 months. Atlantodental interval decreased from 8.6 ± 1.5 mm preoperatively to 1.9 ± 0.5 mm at final follow-up (P < 0.05). Lateral atlantoaxial joint space height significantly improved from 1.7 ± 0.5 mm preoperatively to 4.7 ± 0.3 mm at final follow-up (P < 0.05). Japanese Orthopaedic Association score significantly improved from 14.9 ± 1.5 preoperatively to 16.7 ± 0.6 at final follow-up (P < 0.05). Visual analog scale score for neck pain markedly decreased from 4.5 ± 1.8 preoperatively to 0.5 ± 0.6 at final follow-up (P < 0.05).

CONCLUSIONS

Posterior reduction and intra-articular cage fusion with a C2 nerve root preservation technique is effective in treatment of degenerative atlantoaxial instability. Satisfactory reconstruction of the sagittal alignment and the height of atlantoaxial complex can be achieved.

Biomechanical analysis of atlantoaxial intraarticular fusion cages with posterior pedicle screws fixation using finite element method

BACKGROUND

Cadaveric biomechanical studies indicated that atlantoaxial intraarticular fusion cages with posterior pedicle screws fixation could increase the multi-axial rigidity. However, the stress distribution of the fixation construct is still unclear.

METHODS

From computed tomography images, a nonlinear intact three-dimensional C0-2 finite element model was developed and validated. Four finite element models were reconstructed: intact model, unstable model, bilateral atlantoaxial pedicle screws combined bilateral cages model, bilateral atlantoaxial pedicle screws model. The range of motion and maximum von Mises stresses were compared under flexion, extension, lateral bending, and axial rotation.

FINDINGS

Compared with unstable model, both bilateral atlantoaxial pedicle screws combined bilateral cages model and bilateral atlantoaxial pedicle screws model fixation techniques reduced range of motion by >99% in extension, flexion, lateral bending and axial rotation. For bilateral atlantoaxial pedicle screws combined bilateral cages model, the maximum von Mises stress was in the base of the C2 screw head site. In the bilateral atlantoaxial pedicle screws model was stressed at the rod linked C1 and C2 screws. Compared with bilateral atlantoaxial pedicle screws model, bilateral atlantoaxial pedicle screws combined bilateral cages model reduced the maximum von Mises stress on the implants by >90% in extension, flexion, lateral bending and axial rotation.

INTERPRETATION

The finite element model study indicated that, compared with posterior C1-C2 pedicle screws fixation, atlantoaxial intraarticular fusion cages with posterior pedicle screws fixation could not only significantly restore stability to the atlantoaxial junction, but also dramatically reduce the maximum von Mises stress in the C1-C2 pedicle screws.

Safety and Feasibility of DTRAX Cervical Cages in the Atlantoaxial Joint for C1/2 Stabilization

BACKGROUND

Pathological changes in the atlantoaxial joint often lead to instability, pain, and neurological deterioration. One treatment option is the surgical stabilization of the atlantoaxial joint. In other areas of the spine, fusion rates have been improved by the introduction of an interbody cage. Our aim was to use cervical interbody spacers, originally designed to augment fusion across subaxial posterior cervical facets, to optimize the conditions for atlantoaxial fusion.

OBJECTIVE

To evaluate the safety and efficacy of implanting cervical cages in the atlantoaxial joint for C1/2 stabilization.

METHODS

Our retrospective study evaluated patients who had undergone C1/2 cervical fusions by the Harms/Goel technique. This technique was modified by implanting a titanium cervical interbody spacer into the joint space. Mean overall pain, as measured by a 0 to 10 visual analog scale (VAS) and neurological outcomes were measured preoperatively and postoperatively. In addition, radiological outcomes were collected using follow-up imaging.

RESULTS

Nine patients were included in this case series. The mean preoperative VAS for overall pain was 5.0 ± 4.0, which changed to a mean VAS of 2.0 ± 3.0 after an average follow-up period of 41.4 ± 20.4 (P = .043). All patients showed a bony fusion in our case series. None of the radiological imaging during follow-up showed screw loosening, hardware breakage, implant migration, or nonunion.

CONCLUSION

The implantation of cervical titanium cages into the atlantoaxial joint in combination with posterior fixation appears to be a safe and effective method for achieving C1/2 fusion.

[Accuracy analysis and clinical application of the progressive navigation template system to assist atlas-axial pedicle screw placement]

Objective

To investigate the accuracy of progressive three-dimensional navigation template system (abbreviated as progressive template) to assist atlas-axial pedicle screw placement.

Methods

The clinical data of 33 patients with atlas-axial posterior internal fixation surgery between May 2015 and May 2017 were retrospectively analyzed. According to the different methods of auxiliary screw placement, the patients were divided into trial group (19 cases, screw placement assisted by progressive template) and control group (14 cases, screw placement assisted by single navigation template system, abbreviated as initial navigation template). There was no significant difference in gender, age, cause of injury, damage segments, damage types, and preoperative Frankel classification between the two groups ( P>0.05). The operation time and intraoperative blood loss of the two groups were compared. The safety of screw placement was evaluated on postoperative CT by using the method from Kawaguchi et al, the deviation of screw insertion point were calculated, the angular deviation of the nailing on coordinate systems XOZ, XOY, YOZ were calculated according to Peng's method.

Results

All patients completed the operation successfully; the operation time and intraoperative blood loss in the trial group were significantly less than those in the control group ( t=-2.360, P=0.022; t=-3.006, P=0.004). All patients were followed up 12-40 months (mean, 25.3 months). There was no significant vascular injury or nerve injury aggravation. Postoperative immediate X-ray film and CT showed the dislocation was corrected. Postoperative immediate CT showed that all 76 screws were of grade 0 in the trial group, and the safety of screw placement was 100%; 51 screws were of grade 0, 3 of gradeⅠ, and 2 of gradeⅡ in the control group, and the safety of screw placement was 91.1%; there was significant difference in safety of screw placement between the two groups ( χ2=7.050, P=0.030). The screw insertion point deviation and angular deviation of the nailing on XOY and YOZ planes in the trial group were significantly less than those in the control group ( P<0.05). There was no significant difference in angular deviation of the nailing on XOZ between the two groups ( t=1.060, P=0.290).

Conclusion

Compared with the initial navigation template, the progressive navigation template assisting atlas-axial pedicle screw placement to treat atlas-axial fracture with dislocation, can reduce operation time and intraoperative blood loss, improve the safety of screw placement, and match the preoperative design more accurately.

Minimally Invasive Microscope-Assisted Stand-Alone Transarticular Screw Fixation without Gallie Supplementation in the Management of Mobile Atlantoaxial Instability

Study Design

Retrospective study.

Purpose

To evaluate the clinico-radiological efficacy of stand-alone minimally invasive transarticular screw (MIS-TAS) fixation without supplemental Gallie fixation in the management of mobile C1–C2 instability.

Overview of Literature

Data evaluating the efficacy and feasibility of MIS-TAS in the literature is scanty.

Methods

Patients with mobile atlantoaxial instability and >2 years follow-up were included and managed by stand-alone TAS fixation using the Magerl technique and morselized allograft without additional fixation. Patient demographics and intra-operative parameters were noted. Clinical parameters (Visual Analog Scale [VAS] and Oswestry Disability Index [ODI]), neurology (modified Japanese Orthopaedic Association [mJOA]), and radiological factors (anterior atlanto-dens interval and space available for cord) were evaluated pre and postoperatively. Computed tomography (CT) was performed in patients who did not show interspinous fusion on X-ray at 1 year, to verify intra-articular fusion. Statistical analysis was performed using IBM SPSS ver. 20.0 (IBM Corp., Armonk, NY, USA); the Student t-test and analysis of variance were used to assess statistical significance (p <0.05).

Results

A total of 82 consecutive cases (three males, one female; mean age, 36.26±5.78 years) were evaluated. In total, 163 TASs were placed. Significant improvement was noticed in clinical (mean preoperative VAS=7.2±2.19, postoperative VAS=3.3±1.12; mean preoperative ODI=78.3±4.83, postoperative ODI=34.05±3.26) and neurological features (mean preoperative mJOA=14.73±2.68, postoperative mJOA=17.5±2.21). Radiological evidence of fusion was noted in 97.5% cases at final follow-up. Seventeen patients were found to have no interspinous fusions upon X-rays, but CT revealed facet fusion in all patients except in two. Inadvertent vertebral artery injury was noted in three cases.

Conclusions

Stand-alone TAS fixation with morselized allograft provides excellent radiological and clinical outcomes. The addition of a supplementary tension band and structural graft are not essential. This provides the opportunity to avoid the complications associated with graft harvesting and wiring.

Biomechanical comparison of a novel C1 posterior U-construct with four other techniques in a C1-C2 fixation model

Background

Compare the biomechanical stability of a novel "U" posterior cervical fixation construct to four other posterior cervical atlantoaxial fixation constructs.

Methods

Eight fresh frozen human cadaver spines were tested after a simulated odontoid fracture, and following stabilization with each construct.

Results

All constructs significantly decreased flexion-extension and axial rotation compared to the destabilized spine. The U construct provided significantly more axial stability than the Brooks wire technique.

Conclusion

The novel U construct demonstrated comparable biomechanical stability to the existing constructs in all three planes of motion with the exception of axial rotation, in which it was inferior to TAS.

Biomechanical evaluation of C1 lateral mass and C2 translaminar bicortical screws in atlantoaxial fixation: an in vitro human cadaveric study

BACKGROUND CONTEXT

Atlantoaxial fixation with C1 lateral mass-C2 translaminar bicortical (C1LM-C2TB) screws has been reported to afford good stability with the least risk of injury to vertebral artery. However, no comparative in vitro studies have been conducted to evaluate the biomechanical stability of this method.

PURPOSE

This study aimed to compare in vitro biomechanics of fixation with C1LM-C2TB with fixation with C1 lateral mass-C2 translaminar unicortical screws (C1LM-C2TU) and with C1 lateral mass-C2 pedicle screws (C1LM-C2PS).

STUDY DESIGN

This is an in vitro biomechanical study.

METHODS

Fifteen fresh-frozen human cadaveric cervical spines (C1-C3) were tested after destabilization by transverse-alar-apical ligament disruption. Instrumentation was performed with three fixation constructs: C1LM-C2PS, C1LM-C2TU, and C1LM-C2TB. Flexion, extension, lateral bending, and axial rotation were tested. Range of motion and neutral zone pre-fatigue and post-fatigue values were measured.

RESULTS

No significant differences were observed in flexion-extension among the three groups. However, C1LM-C2TB fixation was superior to C1LM-C2TU fixation in lateral bending and axial rotation.

CONCLUSION

C2 translaminar bicortical screws are biomechanically superior to C2TU screws for fixation of the atlantoaxial complex, and it is equivalent to C2PS fixation. C2 translaminar bicortical screws or C2PS should be preferred over C2TU screws.

Machined cervical interfacet allograft spacers for the management of atlantoaxial instability

Background:

The use of cervical interfacet spacers (CISs) to augment stability and provide solid arthrodesis at the atlantoaxial joint has not been studied in detail. The aim of this work is to report the outcomes with the use of machined allograft CISs at C1-2.

Methods:

A retrospective review of 19 patients who underwent an atlantoaxial fusion with the use of CISs was performed. All patients had instability documented with flexion and extension lateral radiographs. This instability was due to trauma, degenerative stenosis, symptomatic C1-2 arthropathy, and os odontoideum. Clinical and radiological outcomes were assessed. Fusion was determined based on a lack of hardware failure, absence of motion on flexion and extension plain X-ray films, and presence of bridging trabecular bone which was most often demonstrated by a computed tomography.

Results:

The mean age was 69.1 ± 12.9 years. Eight patients had traumatic fractures, six patients had degenerative stenosis, two patients had C2 neuralgia due to C1-2 arthropathy, two patients had C1-2 ligamentous subluxation, and one patient had an unstable os odontoideum. The occiput or subaxial spine was included in the arthrodesis in 10 patients. Rib autograft was utilized in most patients. No patient had postoperative neurological worsening, malposition of hardware, or vertebral artery injury and there were no mortalities. The fusion rate was 95%. The mean follow-up was 12.1 ± 5.5 months.

Conclusions:

CIS is a promising adjuvant for the treatment of atlantoaxial instability.

Biomechanical study of novel unilateral C1 posterior arch screws and C2 laminar screws combined with an ipsilateral crossed C1-C2 pedicle screw-rod fixation for atlantoaxial instability

INTRODUCTION

Current surgical methods to treat atlantoaxial instability pose potential risks to the surrounding blood vessels and nerves of operative approach. Therefore, more secure and highly effective methods are expected. This study sought to assess the biomechanical efficacy of a novel unilateral double screw-rod fixation system by comparing with traditional and emerging fixation methods in cadaveric models.

MATERIALS AND METHODS

Ligamentous cervical spines (C0-C7) from ten fresh cadaveric specimens were used to complete range of motion (ROM) test in their intact condition (control group), destabilization, and stabilization after different fixations, including unilateral C1-C2 pedicle screws (PS) with a screw-rod system (Group A), bilateral C1-C2 PS with screw-rod systems (Group B), unilateral C1 posterior arch screws (PAS) and C2 laminar screws (LS) combined with an ipsilateral paralleled C1-C2 PS-rod (Group C), and unilateral C1 PAS and C2 LS combined with an ipsilateral crossed C1-C2 PS-rod (Group D). After that, pullout strength test was performed between PS and PAS using ten isolated atlas vertebras.

RESULTS

All fixation groups reduced flexibility in all directions compared with both control group and destabilization group. Furthermore, comparisons among different fixation groups showed that bilateral C1-C2 PS-rod (Group B), unilateral C1 PAS + C2 LS combined with an ipsilateral paralleled C1-C2 PS-rod (Group C) and unilateral C1 PAS + C2 LS combined with an ipsilateral crossed C1-C2 PS-rod (Group D) could provide a better stability, respectively, in all directions than unilateral C1-C2 PS-rod (Group A). However, no statistical significance was observed among Groups B, C, and D. Data from pullout strength test showed that both C1 PS (585 ± 53 N) and PAS (463 ± 49 N) could provide high fixed strength, although PS was better (P = 0.009).

CONCLUSION

The surgical technique of unilateral C1 PAS + C2 LS combined with a ipsilateral crossed C1-C2 PS-rod fixation could provide a better stability than the traditional unilateral PS-rod fixation and a same stability as bilateral PS-rod fixation, but with less risk of neurovascular injury. Therefore, this new technique may provide novel insight for an alternative of atlantoaxial instability treatment.

Gallie technique versus atlantoaxial screw-rod constructs in the treatment of atlantoaxial sagittal instability: a retrospective study of 49 patients

Background

The objectives of this study are to investigate the clinical curative effect of Gallie technique and atlantoaxial screw-rod constructs (SRC) on atlantoaxial sagittal instability and determine the indication of Gallie technique.

Methods

Data of 49 patients with atlantoaxial sagittal instability from February 2008 to May 2015 were analyzed retrospectively. The visual analog scale (VAS) score and the neck disability index (NDI) were used to evaluate the curative effect. Postoperative radiological outcomes were used to evaluate the stability of atlantoaxial joint and bone fusion. Perioperative parameters such as blood loss, operation time, radiographic exposure times, and hospital expense were also recorded and analyzed.

Results

Forty-nine patients (36 men and 13 women) were included in this study. The mean age was 41.4 ± 8.9 (range from 19 to 64). All patients were followed up for 24–67 months. Among these patients, 25 of these patients underwent Gallie surgery and 24 underwent SRC surgery. The pain in the occipitocervical area of all the patients has been relieved. NDI scores and VAS scores were lower in Gallie group than in SRC group in early postoperative period. The proportion of the patients who achieved good bone fusion within 3 months after operation was 88.0% (22/25) in the Gallie group and 100% (24/24) in the SRC group. The Gallie group is lower than the SRC group in blood loss, operation time, radiographic exposure times, and hospital expense. Statistical difference was observed between the two groups.

Conclusions

For patients with atlantoaxial instability who has (1) the atlantodental interval (ADI) which is bigger than 5 mm on lateral flexion-extension X-ray, or Anderson-D’Alonzo type II odontoid fracture, (2) no asymmetry between odontoid process and lateral mass on open-mouth anterior-posterior X-ray, and (3) no dislocation of lateral mass joint on the CT 3D reconstruction, Gallie technique can be chosen as a safe and effective method if atlantoaxial reduction can be achieved preoperatively. Compared with SRC, Gallie technique can relieve the pain in the occipitocervical area earlier and it can shorten operation time and reduce intraoperative bleeding, radiographic exposure times, and hospital expense effectively. However, for patients with irreducible atlantoaxial dislocation, the Gallie technique should be used with caution.

Biomechanical Comparison of Modified TARP Technique Versus Modified Goel Technique for the Treatment of Basilar Invagination: A Finite Element Analysis

STUDY DESIGN

A finite element analysis.

OBJECTIVE

The aim of this study was to determine the biomechanical differences between atlantoaxial fusion cage combined with transoral atlantoaxial reduction plate fixation (TARP + Cage, modified TARP technique) and that combined with C1 lateral mass screw and C2 pedicle screw fixation (C1LS + C2PS + Cage, modified Goel technique) in the treatment of basilar invagination (BI) by finite element analysis.

SUMMARY OF BACKGROUND DATA

Clinical studies have shown that transoral anterior atlantoaxial release followed by TARP fixation can achieve reduction, decompression, fixation, and fusion of C1-C2 through a transoral-only approach. Although cage has been used to reduce the BI through posterior approach, there are no studies referred to the cage combined with TARP for C1-C2 fusion.

METHODS

A finite element model was used to investigate and compare the stability between TARP + Cage fixation and C1LS + C2PS + Cage fixation in the treatment of BI. Vertical load of 40  N was applied on the C0, to simulate head weight, and 1.5  Nm torque was applied to the C0 to simulate flexion, extension, lateral bending, and rotation.

RESULTS

In comparison with the C1LS + C2PS + Cage model, the TARP + Cage model reduced the ROM by 44.7%, 30.0%, and 10.5% in extension, lateral bending, and axial rotation, while the TARP + Cage model increased the ROM by 30.0% in flexion, and the TARP + Cage model also led to lower screw stress in all motions with one exception (anterior C2PS stress in extension).

CONCLUSION

Our results indicate that the TARP + Cage fixation may offer higher stability to C1LS + C2PS + Cage in extension, lateral bending, and axial rotation but lower stability in flexion. Compared with modified Goel technique, the modified TARP technique not only has the capability of transferring the load and distributing the stress but also can provide neural decompression, stabilization and fusion, and restore C1-C2 normal fusion angle.

LEVEL OF EVIDENCE

N/A.

New Posterior Atlantoaxial Restricted Non-Fusion Fixation for Atlantoaxial Instability: A Biomechanical Study

BACKGROUND

Loss of axial rotation and lateral bending after atlantoaxial fusion reduces a patient's quality of life. Therefore, effective, nonfusion fixation alternatives are needed for atlantoaxial instability.

OBJECTIVE

To evaluate the initial stability and function of posterior atlantoaxial restricted nonfusion fixation (PAARNF), a new protocol, using cadaveric cervical spines compared with the intact state, destabilization, and posterior C1-C2 rod fixation.

METHODS

Cervical areas C0 through C3 were used from 6 cadaveric spines to test flexion-extension, lateral bending, and axial rotation range of motion (ROM). With the use of a machine, 1.5-Nm torque at a rate of 0.1 Nm/s was used and held for 10 seconds. The specimens were loaded 3 times, and data were collected in the third cycle and tested in the following sequence: (1) intact, (2) destabilization (using a type II odontoid fracture model), (3) destabilization with PAARNF (PAARNF group), and (4) rod implantation (rod group). The order of tests for the PAARNF and rod groups was randomly assigned.

RESULTS

The average flexion-extension ROM in the PAARNF group was 7.44 ± 2.05°, which was significantly less than in the intact (P = .00) and destabilization (P = .00) groups but not significantly different from that of the rod group (P = .07). The average lateral bending ROM (10.59 ± 2.33°; P = .00) and axial rotation ROM (38.79 ± 13.41°; P = .00) of the PAARNF group were significantly greater than in the rod group. However, the values of the PAARNF group showed no significant differences compared with those of the intact group.

CONCLUSION

PAARNF restricted atlantoaxial flexion-extension but preserved axial rotation and lateral bending at the atlantoaxial joint in a type II odontoid fracture model. However, it should not be used clinically until further studies have been performed to test the long-term effects of this procedure.

Biomechanical comparison of a novel transoral atlantoaxial anchored cage with established fixation technique - a finite element analysis

BACKGROUND

The transoral atlantoaxial reduction plate (TARP) fixation has been introduced to achieve reduction, decompression, fixation and fusion of C1-C2 through a transoral-only approach. However, it may also be associated with potential disadvantages, including dysphagia and load shielding of the bone graft. To prevent potential disadvantages related to TARP fixation, a novel transoral atlantoaxial fusion cage with integrated plate (Cage + Plate) device for stabilization of the C1-C2 segment is designed. The aims of the present study were to compare the biomechanical differences between Cage + Plate device and Cage + TARP device for the treatment of basilar invagination (BI) with irreducible atlantoaxial dislocation (IAAD).

METHODS

A detailed, nonlinear finite element model (FEM) of the intact upper cervical spine had been developed and validated. Then a FEM of an unstable BI model treated with Cage + Plate fixation, was compared to that with Cage + TARP fixation. All models were subjected to vertical load with pure moments in flexion, extension, lateral bending and axial rotation. Range of motion (ROM) of C1-C2 segment and maximum von Mises Stress of the C2 endplate and bone graft were quantified for the two devices.

RESULTS

Both devices significantly reduced ROM compared with the intact state. In comparison with the Cage + Plate model, the Cage + TARP model reduced the ROM by 82.5 %, 46.2 %, 10.0 % and 74.3 % in flexion, extension, lateral bending, and axial rotation. The Cage + Plate model showed a higher increase stresses on C2 endplate and bone graft than the Cage + TARP model in all motions.

CONCLUSIONS

Our results indicate that the novel Cage + Plate device may provide lower biomechanical stability than the Cage + TARP device in flexion, extension, and axial rotation, however, it may reduce stress shielding of the bone graft for successful fusion and minimize the risk of postoperative dysphagia. Clinical trials are now required to validate the reproducibility and advantages of our findings using this anchored cage for the treatment of BI with IAAD.

Biomechanical analysis of screw constructs for atlantoaxial fixation in cadavers: a systematic review and meta-analysis

OBJECT

The unique and complex biomechanics of the atlantoaxial junction make the treatment of C1–2 instability a challenge. Several screw-based constructs have been developed for atlantoaxial fixation. The biomechanical properties of these constructs have been assessed in numerous cadaver studies. The purpose of this study was to systematically review the literature on the biomechanical stability achieved using various C1–2 screw constructs and to perform a meta-analysis of the available data.

METHODS

A systematic search of PubMed through July 1, 2013, was conducted using the following key words and Boolean operators: “atlanto [all fields]” AND “axial [all fields]” OR “C1–C2” AND “biomechanic.” Cadaveric studies on atlantoaxial fixation using screw constructs were included. Data were collected on instability models, fixation techniques, and range of motion (ROM). Forest plots were constructed to summarize the data and compare the biomechanical stability achieved.

RESULTS

Fifteen articles met the inclusion criteria. An average (± SD) of 7.4 ± 1.8 cadaveric specimens were used in each study (range 5–12). The most common injury models were odontoidectomy (53.3%) and cervical ligament transection (26.7%). The most common spinal motion segments potted for motion analysis were occiput–C4 (46.7%) and occiput–C3 (33.3%). Four screw constructs (C1 lateral mass–C2 pedicle screw [C1LM–C2PS], C1–2 transarticular screw [C1–C2TA], C1 lateral mass–C2 translaminar screw [C1LM-C2TL], and C1 lateral mass–C2 pars screw [C1LM–C2 pars]) were assessed for biomechanical stability in axial rotation, flexion/extension, and lateral bending, for a total of 12 analyses. The C1LM–C2TL construct did not achieve significant lateral bending stabilization (p = 0.70). All the other analyses showed significant stabilization (p < 0.001 for each analysis). Significant heterogeneity was found among the reported stabilities achieved in the analyses (p < 0.001; I2 > 80% for all significant analyses). The C1LM–C2 pars construct achieved significantly less axial rotation stability (average ROM 36.27° [95% CI 34.22°–38.33°]) than the 3 other constructs (p < 0.001; C1LM–C2PS average ROM 49.26° [95% CI 47.66°–50.87°], C1–C2TA average ROM 47.63° [95% CI 45.22°–50.04°], and C1LM–C2TL average ROM 53.26° [95% CI 49.91°–56.61°]) and significantly more flexion/extension stability (average ROM 13.45° [95% CI 10.53°–16.37°]) than the 3 other constructs (p < 0.001; C1LM–C2PS average ROM 9.02° [95% CI 8.25°–9.80°], C1–C2TA average ROM 7.39° [95% CI 5.60°–9.17°], and C1LM–C2TL average ROM 7.81° [95% CI 6.93°–8.69°]). The C1–C2TA (average ROM 5.49° [95% CI 3.89°–7.09°]) and C1LM–C2 pars (average ROM 4.21° [95% CI 2.19°–6.24°]) constructs achieved significantly more lateral bending stability than the other constructs (p < 0.001; C1LM–C2PS average ROM 1.51° [95% CI 1.23°–1.78°]; C1LM–C2TL average ROM −0.07° [95% CI −0.44° to 0.29°]).

CONCLUSIONS

Meta-analysis of the existing literature showed that all constructs provided significant stabilization in all axes of rotation, except for the C1LM–C2TL construct in lateral bending. There were significant differences in stabilization achieved in each axis of motion by the various screw constructs. These results underline the various strengths and weaknesses in biomechanical stabilization of different screw constructs. There was significant heterogeneity in the data reported across the studies. Standardized spinal motion segment configuration and injury models may provide more consistent and reliable results.

Biomechanical investigation of a novel integrated device for intra-articular stabilization of the C1-C2 (atlantoaxial) joint

BACKGROUND CONTEXT

The anatomy of the atlantoaxial joint makes stabilization at this level challenging. Current techniques that use transarticular screw fixation (Magerl) or segmental screw fixation (Harms) give dramatically improved stability but risk damage to the vertebral artery. A novel integrated device was designed and developed to obtain intra-articular stabilization via primary interference fixation within the C1-C2 lateral mass articulation.

PURPOSE

To assess the atlantoaxial stability achieved with a novel integrated device when compared with the intact, destabilized, and stabilized state using the Harms technique.

STUDY DESIGN

A biomechanical study of implants in human cadaveric cervical spines.

METHODS

Six human cadaveric specimens were used. Biomechanical testing was performed with moment control in flexion-extension, lateral bending, and axial rotation. Range of motion (ROM) was measured in the intact state, after both destabilization by creation of a Type II odontoid peg fracture and sequential stabilization using the integrated device and the Harms technique.

RESULTS

Mean flexion-extension ROM of the intact specimens at C1-C2 was 14.1°±2.9°. Destabilization increased the ROM to 31.6°±4.6°. Instrumentation with the Harms technique reduced flexion-extension motion to 4.0°±1.4° (p<.01). The integrated device reduced flexion-extension motion to 3.6°±1.8° (p<.01). In lateral bending, the respective mean angular motions were 1.8°±1.1°, 14.1°±5.8°, 1.4°±0.7°, and 0.4°±0.3° for the intact destabilized Harms technique and integrated device. For axial rotation, the respective mean values were 67.3°±13.8°, 74.2°±16.1°, 1.4°±0.7° and 0.9°±0.7°. Both the Harms technique and integrated device significantly reduced motion compared with the destabilized spine in flexion-extension, lateral bending, and axial rotation (p<.05). Direct comparison of the Harms technique and the integrated device revealed no significant difference (p>.10).

CONCLUSIONS

The integrated device resulted in interference fixation at the C1-C2 lateral mass joints with comparable stability to the Harms technique. Perceived advantages with the integrated device include avoidance of fixation below the C2 lateral mass where the vertebral artery is susceptible to injury, and access to the C1 screw entry point through the blade of the integrated device avoiding extended dissection superior to the C2 nerve root and its surrounding venous plexus.