| BackgroundPosterior atlantoaxial arthrodesis is a widely accepted procedure for the treatment of upper cervical spine instability. It requires placement of a bone graft in a properly prepared environment that includes decorticated bony surfaces, compressive forces between the graft and native bone, and limited motion.Multiple posterior fixation techniques have been described for surgical stabilization of the upper cervical spine. In all of these techniques, an intact posterior atlantal arch usually is necessary to secure a bone graft across C1 to C2 for fusion. However, in certain patients, such as those with congenital anomalies or bone loss from prior surgeries, the posterior arch of C1 is deficient, which makes it difficult to use the posterior arch of C1 for placement of the graft. In this situation, an alternative fusion technique is necessary for the treatment of atlantoaxial instability.The recent popularity of the use of interbody fusion cages fused to the spine suggests that atlantoaxial lateral mass fusion with the use of a cage could be a theoretical alternative treatment for atlantoaxial stabilization when previous surgeries have failed or when regional anatomic variation makes posterior atlantoaxial fusion impossible. If successful, this approach would allow rigid C1–C2 fixation without the need to extend the instrumentation and fusion to the occiput.ObjectivesTo preserve the normal occiput-C1 motion, which can be an important component of the patient's flexion and extension motion and is especially important in the initiation of swallowing, a fusion cage was designed that could be used to achieve lateral mass fusion after insertion into the interspace of the atlantoaxial. By a series of studies, we attempt to achieve the following goals: 1. Provide anatomic parameters of the atlantoaxial lateral mass related to the cage design.2. To design an atlantoaxial lateral mass fusion cage that could be used to achieve lateral mass fusion after insertion into the interspace of the atlantoaxial and preserve the the normal occiput-C1 motion.3. Evaluation the biomechanical properties of an atlantoaxial lateral mass fusion cage combined with C1–C2 pedicle screw fixation and compare them to the properties of other posterior rigid fixation techniques.4. To construct three-dimensional (3D) finite element model (FEM) of C1+C2+Cage and compare the biomechanical stability and rang of motion (ROM) with normal upper cervical spine model, AAD model and C1+C2 fixation. The stress distribution of cage was also investigated.5. To give data support for further revision of cage.Materials and Methods1. Thirty five dry C1 and C2 cervical vertebrae and 46 CT scans and 3D reconstruction of the normal cervical spine were obtained for anatomic measurements of the atlantoaxial lateral mass. The parameters measured were as follows: transverse diameter of lateral mass of atlas (TDLMat); longitudinal diameter of lateral mass of atlas (LDLMat); interval of atlantoaxial lateral mass (IALM); transverse diameter of lateral mass of axis (TDLMax); and longitudinal diameter of lateral mass of axis (LDLMax).2. To design and produce an atlantoaxial lateral mass fusion cage based on the anatomic study.3. Six fresh-frozen human cadaveric cervical spines were used in the biomechanical study. Specimens were tested in their intact condition, after destabilization via transverse-alar-apical ligament disruption, and after implantation of three fixation constructs: (1) transarticular screws combined with Gallie wires (Magerl+G), (2) C1–C2 pedicle screws (C1+C2), and (3) atlantoaxial lateral mass fusion cage combined with C1–C2 pedicle screws (C1+C2+Cage). Pure moment loading up to 1.5 Nm in flexion-extension, right-left lateral bending, and right-left axial rotation was applied to the occiput, and relative intervertebral rotations were determined using stereophotogrammetry. Range of motion (ROM) for the intact, destabilized, and three fixation scenarios were determined. Comparison of data was performed using one-way analysis of variance (ANOVA) for independent samples followed by Fisher's least significant difference PLSD post hoc test for multiple comparisons.4. To construction the FEM of normal upper cervical spine, CT scans of cervical spine from C0 to C5 of a healthy man of 31 years old were obtained. The Geomagic8.0 software system and Abaqus6.9 software were applied to model the spinal segment and the material properties were deferent according to literatures. At the basis of the normal upper cervical spine FEM, AAD model was constructed by removal of transverse ligament of C1. Then, C1+C2 fixation model and C1+C2+Cage fixation model were built base on the AAD model.4. The ROM of C1+C2+Cage model and the stress distribution of the lateral mass of axis were recorded, and compare the ROM with normal upper cervical spine model, AAD model and C1+C2 fixation.Results1. No significant difference was found between the left and right sides. The mean IALM for all specimens was 3.0±0.5 mm, (range 2.1–4.8 mm). A total of 93% of atlantoaxial lateral masses had an interval≥2.5 mm, 52% had an interval≥3.0 mm, and 16% had an interval≥3.5 mm. The mean transverse and longitudinal diameter of the lateral mass were, respectively, 16.27±1.36 mm and 16.96±1.41 mm for C1, and 16.32±1.33 mm and 17.69±1.37 mm for C2.2. The anatomic data indicated that the following parameters were feasible for cage design: 3.5 mm height, 11 mm length, and 8 mm width. The cage was made of titanium (Ti-6Al-4V), and included a hollow rectangular base section, and a nose section that extended integrally from the base section.3. The average ROMs (n = 6) of intact specimens were 17.78°±3.78°(mean±one standard error) in combined flexion extension, 9.56°±1.20°in combined lateral bending, and 44.19°±4.34°in combined axial rotation. Destabilization via transverse-alar-apical ligament disruption significantly increased intervertebral ROM in flexion-extension, lateral bending, and axial rotation by 72% (30.69°±4.91°, P﹤0.01), 86% (17.18°±1.53°, P﹤0.01), and 30% (57.30°±11.01°, P﹤0.01), respectively, as compared with the intact condition. All fixation techniques significantly decreased motion in all testing modes when compared to the intact and destabilized spines (P﹤0.01). In flexion/extension, the C1+C2+Cage fixation allowed the highest atlantoaxial motion (2.94°±0.19°), and the Magerl+G fixation produced the least motion (2.25°±0.33°). In lateral bending, the C1+C2 fixation allowed the highest atlantoaxial motion (2.05°±0.30°), and the C1+C2+Cage fixation produced the least motion (1.80°±0.31°). Finally, In axial rotation, the C1+C2 fixation allowed the highest atlantoaxial motion (3.91°±1.39°), and the Magerl+G fixation produced the least motion (1.13°±0.31°). Differences in ROM among the three fixation scenarios were not statistically significant for any motion. 4. The normal upper cervical spine FEM was built and was validated according to the biomechanics in vitro. The other FEMs were also successful constructed. The C1+C2+Cage fixation has allowed the least atlantoaxial motion in axial rotation. Stress concentration mostly at the base section of the cage.ConclusionA fusion cage was designed based on anatomic measurements of the atlantoaxial lateral masses. A cadaver model was used to evaluate and compare the stability of the fusion cage combined with the C1+C2 pedicle screw technique with that of the Magerl+G and C1+C2 pedicle screw methods. No statistically significant differences were found among the three stabilization methods. The C1+C2+Cage technique was able to provide other fusion spots for atlantoaxial stabilizatixon with similar stability to methods currently used. Thus, it may be a viable alternative for atlantoaxial stabilization when the posterior arch of the atlas is absent or removed for decompression and a Gallie fixation is impossible. |
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