General Session: Biomechanics
Presented by: M. Conti Mica - View Audio/Video Presentation (Members Only)
M. Conti Mica(1), J.I. Romo(2), E. Miliavski(3), D. Baluch(1), L. Voronov(1,4), R. Havey(1,4), B. Wojewnik(1), A. Ghanayem(1), A. Patwardhan(1,4)
(1) Loyola University Chicago, Orthopaedic Surgery and Rehabilitation, Maywood, IL, United States
(2) Loyola University Chicago, Stritch School of Medicine, Maywood, IL, United States
(3) Loyola University Chicago, Chicago, IL, United States
(4) Edward Hines Jr. VA Hospital, Hines, IL, United States
Introduction: Pedicle screws are a common component of thoracolumbar spine surgery. It has been previously demonstrated that up to 80% of the stability achieved with a traditional pedicle screw emanates from the screw-cortical bone interface despite the largely cancellous fixation. With the expanding elderly population, the prevalence of osteoporotic patients requiring spine surgery is increasing. Osteoporosis has been shown to disproportionately affect the trabecular bone. Therefore, boney purchase is compromised with loosening rates as high as 12.9%. Santoni et al proposed an alternative trajectory for screws affording a larger proportion of screw-cortical bone contact. A previous study from our lab confirmed that cortical screws have superior resiliency to cranial-caudal testing in specimens with normal bone density. The purpose of this study was to evaluate the cortical screw compared to traditional pedicle screw fixation in osteopenic/osteoporotic bone.
Methods: Twenty-one fresh frozen cadaveric vertebral levels (T10-L5) harvested from 4 human spine specimens (age range: 73-81 years) with no previous spinal surgery, underwent quantitative computed tomography. Each vertebral body was instrumented with a lateral to medial directed traditional pedicle screw (single lead thread) as well as a medial to lateral directed cortical screw (triple lead thread, both Amedica Corporation, Salt Lake City, UT) (Figure 1). The screw size was determined as it would be clinically. Each specimen was then mounted on a Material Testing System (MTS) where each screw was toggled in a craniocaudal direction with increasing loads every 20 cycles until 2mm of displacement was produced. Immediately following, each screw underwent a uniaxial pullout test to failure. The number of cycles as well as the uniaxial pullout force was then compared between the cortical and traditional screws.
Results: The mean trabecular bone mineral density was 128 mg/cm3 (range: 102-142 mg/cm3) across all specimens. Cortical pedicle screws required a greater number of cycles to reach 2mm of displacement compared to traditional pedicle screws (100 cycles vs. 23 cycles) (p=0.002). A greater amount of force was also required to reach 2 mm displacement in cortical screws versus traditional (299N vs. 209N) (p< 0.001). There was no significant difference in uniaxial pullout force between the two screws (763N cortical vs. 837N traditional) (p=0.23). Four additional specimens with severe osteoporosis (bone mineral density 37.5 - 62 mg/cm3) failed at extremely low cycles (1-2 cycles) regardless of type of screw fixation.
Conclusion: Cortical screws demonstrated superior fixation strength compared to traditional pedicle screws in the osteoporotic spine. Both the number of cycles and load required to reach 2mm of displacement were significantly greater for cortical screw fixation. The difference in pullout force between the previously cycled screws was not statistically significant. Cortical screws appear to have better fixation even in osteopenic specimens. Both cortical and pedicle screw fixation may be inadequate in severely osteoporotic patients.