The Effects of Hook Pattern and Angle of Kyphosis Upon Mechanical Strength and Apical Rod Strain in a Long-Segment Posterior Construct Utilizing a Synthetic Model

Philip J. Belmont, Jr., MD,
David W. Polly, Jr., MD,
William R. Klemme, MD
and Bryan Cunningham, MSc
*· (a – Sofamor Danek, b – Sofamor Danek)
Washington, DC, USA

INTRODUCTION:
The biomechanical effects of hook pattern and kyphotic angulation on the stiffness and apical rod strain of long–segment posterior spinal constructs (LSPSC) are unknown. The management of kyphotic deformities, such as Scheuermann’s kyphosis, with compressive hook–rod instrumentation has been complicated by post– operative loss of correction, hardware failure and pseudarthrosis. Increasing construct stiffness should facilitate maintenance of the correction of the kyphotic deformity and decrease the incidence of rod breakage and pseudarthrosis. Previous studies have provided evidence, albeit indirect, that increasing rod diameter and hence construct stiffness will produce a concomitant decrease in complications. The purpose of this study was to compare the effects of 1) hook pattern in compressive hook–rod spinal instrumentation and 2) kyphotic angulation on stiffness and apical rod strain of LSPSC in the treatment of kyphotic deformities to determine optimal instrumentation strategy.

METHODS:
0°, 27° and 54° synthetic spine models, composed of polypropylene vertebral blocks and isoprene elastomer spacers, representing T3–T12 were employed for biomechanical testing of LSPSC. Models were instrumented with 6.35–mm titanium rods and one of the following hook configurations: 1) 20–hook compression, 2) 16 hook compression, 3) 16–hook claw apex–empty, 4) 16–hook claw apex full or 5) 8–hook claw. Construct stiffness and apical rod strain during axial compression were determined.

RESULTS:
Both the 20– and 16–hook compression patterns provided at least a 45% increase in construct stiffness (P=0.013) and a 22% decrease in apical rod strain (P<0.0001) compared to the claw–hook pattern with the best biomechanical performance. Using the same rod diameter and examining all five hook patterns, there was a 19% decrease in construct stiffness and 27% increase in apical rod strain when progressing from straight alignment to 27° of sagittal contour (P<0.0001). Progressing from straight alignment to 54° decreased the construct stiffness 48% and increased apical rod strain 55% (P<0.0001). Construct stiff–ness was inversely correlated to apical rod strain in all five hook patterns (R2=0.82 to 0.98, P<0.001).

CONCLUSIONS:
Using compressive hook patterns and decreasing the kyphotic deformity significantly increases construct stiffness and decreases rod strain. The stiffness and apical rod strain of LSPSC are strongly inversely correlated.

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* · If noted, the author indicates something of value received. The codes are identified as: a–research or institutional support, b–miscellaneous funding, c–royalties, d–stock options, e–consultant or employee.