Abstract
Introduction
Lateral mass screw fixation has been considered the gold standard in the instrumentation of C1 vertebra since its introduction by Goel and Laheri. The morphometry of pedicle and pedicle screw fixation in C1 vertebra is poorly illustrated in literature. Further biomechanical studies have proved better pullout strength in C1 pedicle screws compared with C1 lateral mass screws. Here we analyze the morphometry of C1 pedicle and lateral mass in Indian population, with comparison of two proposed pedicle screw trajectories.
Materials and Methods 3D CT reconstructed images of C1 vertebra of 100 adult Indian population(50 male and 50 female) were analyzed for 22 parameters(200 pedicles, 4400 measurements). The anatomic parameters include pedicle height(PH), pedicle sagittal angle(PSA), lateral mass height, length, width(LH,LL,LW). Two pedicle screw trajectories, straight(S) and medially angulated(M) were defined and comparative analysis were done using following parameters, entry point midline distance(SEMD,MEMD), axial length(SAL,MAL), pedicle length(SPL,MPL), distance from vertebral foramen(SVD,MVD), angle of freedom from vertebral foramen for screw trajectory(SVAF,MVAF), pedicle screw trajectory width(SPW,MPW), medially angulated pedicle screw transverse angle(PTA), shortest distance from internal carotid artery(ICA) to screw exit point in anterior cortex(SID,MID), angle of freedom from ICA for screw trajectory(SIAF,MIAF).
Results
All 22 parameters were analyzed using SPSS software with mean PH 3.4, PSA 19°, LH 11.3, LL 15.2, LW 13.1, SEMD 18.7, MEMD 21.7, SAL 24.9, MAL 25.4, SPL 9.6, MPL 6.6, SVD 3.3, MVD 4.1,SVAF 12.1°, MVAF 24.5°, SPW 6.6, MPW 8.3, PTA 22.3°, SID 5.8, MID 9.9, SIAF 5.2° and MIAF 20.3°(all linear measurements in mm). PW, PSA, LH, LL, LW, SEMD, MEMD, SAL, MAL, MVD, MVAF, MPW and SID were significantly dependant on sex. There was a statistically significant difference in values of measured parameters between both trajectory S and M. All parameters except MPL are quantitatively greater in trajectory M than S.
Conclusion
Out of 200 pedicles studied the PH of only 89 are ≥ 3.5mm, and 154 are ≥ 3mm. So ¾ of Indian C1 pedicles can accommodate only a 3mm screw. A preoperative CT analysis is necessary in all patients to evaluate the feasibility of C1 pedicle screw and for neurovascular safety. A medially angulated C1 pedicle screw with more lateral entry point is more accommodative and safer in respect to ICA and vertebral artery than the straight screw. Also a bicortical screw purchase is safer in a medially angulated screw than a straight screw, which is comparatively at a higher risk of injuring ICA. Still biomechanical studies have to be done comparing both screw trajectories.