Comparison of biomechanical properties of cervical paravertebral foramen screws, lateral mass screws and pedicle screws

Objective To investigate and compare the biomechanical strength of paravertebral foramen screws (PVFS), lateral mass screws (LMS) and pedicle screws (PS). Methods A total of 30 human cervical spine vertebrae (C3-C6) were harvested from 8 fresh-frozen cadaver specimens whose mean age was 45.3±11.2 years at death. The vertebrae were randomly divided into three groups for specific screws. For each vertebra, one side was randomly chosen for direct pullout strength test (speed 5 mm/s), and the other side for fatigue test (displacement ±1.0 mm, frequency 1 Hz, 500 cycles) and residual pullout strength test. 4.5 mm × 12 mm screws were used for PVFS, 3.5 mm × 14 mm screws for LMS, and 3.5 mm × 24 mm screws for PS. Results The direct pullout strength was 327.10±17.07 N for PVFS, 305.71 ± 11.63 N for LMS, and 635.67 ± 22.82 N for PS. The residual pullout strength was 265.62 ±18.19 N for PVFS, 192.80 ±17.10 N for LMS, and 494.89 ±41.79 N for PS. The residual pullout strength of PVFS, LMS and PS respectively, compared with the direct pullout strength, decreased by 18.8%, 36.93% and 22.15% (tPVFS=7.795, tLMS=17.267, tPS=9.349, P<0.001). The direct pullout strength of PS was higher than that of PVFS and LMS(t=34.245, t=40.741, P< 0.001), as well as PVFS was slightly higher than LMS (t=3.275, P=0.004). The residual pullout strength of PS was the highest, PVFS was the second, and LMS was the smallest (F=314.619, P<0.001). For the fatigue test, the load at the first cycle and the first time when the set position was reached of PVFS were higher than those of LMS (t=3.625, P=0.002; t=5.388, P<0.001) and PS (t=2.575, P=0.019; t=2.680, P=0.015), but there was no difference between those of LMS and PS (t=0.609, P=0.550; t=1.953, P=0.067). The load at the last cycle of PVFS and PS was higher than that of LMS (t=5.341, P<0.001; t=3.439, P=0.003), while there was no difference between PVFS and PS (t=1.606, P=0.126). Conclusion The direct pullout strength of PVFS was slightly higher than that of LMS, and the residual pullout strength was significantly higher than LMS. The property of fatigue resistance of PVFS was similar to PS and obviously better than LMS. In summary, PVFS can be used as an effective substitute for LMS and PS. Key words: Cervical vertebrae; Spinal fusion; Bone screws; Biomechanics