Introduction: As the number of adult spinal deformity surgeries increase, a more optimal understanding of instrumentation loads that may lead to failure is needed. Very little is known regarding the distribution of rod strains and screw loading in longer construct models.
Methods: Seven cadaveric L1-S specimens were instrumented L2-S with bilateral pedicle screws and rods. Strain gages were placed along the posterior rods midway between screw-heads. Screws were instrumented with strain gages circumferentially (90° apart) around the neck and calibrated via bending. Specimens were dynamically tested to 7.5 Nm in flexion (FL), extension (EX), lateral bending (RLB/LLB), and axial rotation (LAR/RAR), and to 400N compression. Data were compared using a one-way ANOVA with Holm-Sidak post hoc analysis (p < 0.05).
Results: During FL mean rod strain magnitudes were significantly higher at mid L4-5 (197µe) and L3-L4 (185µe) levels versus L2-3 (9µe) and L5-S (62µe) (p≤.003). The highest screw bending moments occurred caudally with L5 (0.36Nm) significantly greater than L2 (0.04Nm) and L3 (0.02Nm) (p <.05), and S (0.33Nm) greater than L3 (p=.048). In EX, L3-4 (176µe) and L4-5 (215µe) rod strains were greater than L2-3 (6µe) (p≤.015). In LLB highest rod strains occurred at L2-3 (153µe) which was significantly higher than all levels (L3-4: 77µe, L4-5: 72µe, L5-S: 46µe) (p≤.037). Screw bending at L5 (0.26Nm) was greater than L2 (0.07Nm), L3 (0.04Nm), and L4 (0.09Nm) (p≤.04). RLB rod strains mirrored LLB. In LAR screw bending at L5 (0.66Nm) was greater than L2 (0.19Nm) and L3 (0.18Nm) (p=.031). In RAR screw bending was highest at L5 (0.72Nm) which was greater than that for L3 (0.11Nm) (p≤.016). The highest screw load occurred during compression at L5 (1.2Nm) which was greater than L2 (0.03Nm), L3 (0.04Nm) and S (0.32Nm) (p≤.023).
Conclusion : There were clear trends in the distribution of strains which were also highly dependent on loading. Rod strains were highest mid region in flexion-extension and at the upper most level in lateral bending. Screw bending was higher in the caudal levels with L5 typically exhibiting the greatest load.
How to Improve Patient Care: Understanding stress distributions can help to guide clinical decision making to optimize construct strength and longevity.
