The effect of kyphosis on the mechanical strength of a long-segment posterior construct using a synthetic model

Study Design. This experimental study used synthetic spine models to compar e the effect of the angle of kyphosis, rod diameter, and hook number on the biomechanical stiffness of a long-segment posterior spinal construct. Objective. To examine the biomechanical effects of incremental kyphosis on variously instrumented long-segment posterior spinal constructs. Summary of Background Data. Euler's formula for loading of curved long colu mns would suggest that kyphosis has a profound impact on the biomechanical behavior of long-segment posterior spinal constructs. The effects of sagitt al contour on the mechanical properties of long-segment posterior spinal co nstructs have not been well documented. Methods. Kyphotic and straight synthetic spine models were used to test lon g-segment posterior instrumentation constructs biomechanically while varyin g rod diameter and the number of hook sites. The synthetic spines, composed of polypropylene vertebral blocks and isoprene elastomer intervertebral sp acers, were fabricated with either 0 degrees, 27 degrees, or 53 degrees of sagittal contour. The models were instrumented with 5.5- or 6.35-mm titaniu m rods, and with either 8 or 12 hooks. The models were loaded from 0 to 300 N in a cyclical ramp fashion using an MTS 858 Bionix testing device testin g device. Construct stiffness (force and displacement) during axial compres sion was determined. Results. Straight model: Changing the hook number from 8 to 12 caused a 32% increase in construct stiffness with the 5.5-mm rod. Changing the rod diam eter from 5.5 to 6.35 mm caused a 36% increase in construct stiffness with the 8-hook pattern. Changing both the rods and hooks caused the stiffness t o increase 44%. 27 degrees Model: Changing the hook number from 8 to 12 cau sed a 20% increase in construct stiffness with the 6.5-mm rod. Changing the rod diameter from 5.5 to 6.35 mm caused a 29% increase in construct stiffn ess with the 12-hook pattern. Changing both the rods and hooks caused the c onstruct stiffness to increase 26%. 53 degrees Model: Changing the hook num ber from 8 to 12 caused a 14% increase in construct stiffness with the 6.35 -mm rod. Changing the rod diameter from 5.5 to 6.35 mm caused a 17% (P < 0. 0005) increase in construct stiffness with the 12-hook-pattern. Changing bo th rods and hooks caused the stiff ness to increase 21%. Summary data on an gular kyphosis: Using the same rod diameter and the same number of hooks, a nd progressing from a straight alignment to 27 degrees of sagittal contour decreased construct stiffness 32%. Going from straight alignment to 53 degr ees decreased the stiffness 59.6%. All reported values were statistically s ignificant (P < 0.0005). Conclusions. The biomechanical stiffness of the straight spine was sensitiv e to both an increase in hook fixation sites and an increase in rod diamete r. The kyphotic spines, however, were more sensitive to variations in rod d iameter. Although with increasing kyphosis, the optimum instrumentation str ategy will maximize both rod diameter and the number of hook sites, instrum ented kyphotic spines remain biomechanically "disadvantaged" as compared wi th nonkyphotic instrumented spines.