The effects of hook pattern and kyphotic angulation on mechanical strength and apical rod strain in a long-segment posterior construct using a synthetic model

Study Design. Synthetic spine models were used to compare the effects of hook pattern and kyphotic angulation on stiffness and rod strain in long-segment posterior spinal constructs. Objectives. To examine the biomechanical effects of hook patterns and kyphotic angulation on long-segment posterior spinal constructs. Summary of Background Data. Kyphotic deformities managed by increasing rod diameter and hence construct stiffness have shown decreased postoperative loss of correction and hardware complications. The biomechanical effects of hook pattern and kyphosis are unknown. Methods. Spine models of 0°, 27°, and 54° sagittal contour, composed of polypropylene vertebral blocks and isoprene elastomer intervertebral spacers, representing T3-T12, were used for biomechanical testing of long-segment posterior spinal constructs. Models were instrumented with 6.35-mm titanium rods and one of the following hook configurations: 20-hook compression, 16-hook compression, 16-hook claw apex-empty, 16-hook claw apex-full, or 8-hook claw. Construct stiffness and rod strain during axial compression were determined. Results. The compression-hook patterns provided at least a 45% increase in construct stiffness (P = 0.013) and a 22% decrease in rod strain (P< 0.0001) compared with those obtained with the claw-hook pattern with the best biomechanical performance. When analyzing all five hook patterns, there was a 19% decrease in construct stiffness and 27% increase in rod strain when progressing from straight alignment to 27° of sagittal contour (P < 0.0001). Progressing from straight alignment to 54° decreased construct stiffness by 48% and increased rod strain by 55% (P < 0.0001). Construct stiffness was inversely correlated to rod strain in all five hook patterns (R² = 0.820.98, P < 0.001). Conclusions. Using compressive-hook patterns and decreasing the kyphotic deformity significantly increases construct stiffness and decreases rod strain.