Biomechanical Effects of Pedicle Screw Configuration and Kyphotic Angulation in a Long-Segment Posterior Construct Utilizing a Synthetic Model

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

INTRODUCTION:
The effects of pedicle screw configuration and kyphotic angulation on the stiffness of long–segment posterior spinal constructs (LSPSC) are unknown. Parameters that can be manipulated to alter construct stiffness include rod stiffness and the number and type of attachment sites. Previous studies using hook–rod LSPSC in the treatment of kyphotic deformities have produced evidence, albeit indirectly, that increasing rod diameter and hence construct stiffness reduces the complications of post–operative loss of correction and pseudarthrosis. Pedicle screws provide more stable anchor points than hooks and may therefore increase construct stiffness and decrease complications. The purpose of this study was to compare the effects of 1) pedicle screw configuration and 2) kyphotic angulation on stiffness 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 spacer, representing T3–T12 were employed for biomechanical testing of LSPSC. Models were instrumented with 5.5–mm titanium rods and one of the following pedicle screw configurations: 1) 8END, 2)12END, 3) 16END, 4) 14ALT, 5) 16ALTo 6 20FULL. Construct stiffness during axial compression was determined.

RESULTS:
The 54°–model construct most closely approximated the loading conditions that would be experienced after operative treatment of a kyphotic deformity. A global significant difference was found among pedicle screw configurations when examining the 54° model (P<0.001). This allowed a statistical ranking and grouping of pedicle screw configurations as follows: 1) 16ALT, 2) 16END, 14ALT, 20FULL, 3) 12END and 4) 8END. Changing the pedicle screw configuration resulted in the following increases in construct stiffness compared to the 8END configuration: 47% for 16ALT, 42% for 16END, 39% for 14ALT and 20FULL and 29% for 12END. Using the same rod diameter and examining all six pedicle screw configurations, there was a 39% decrease in construct stiffness when progressing from straight alignment to 27° of sagittal contour (P<0.001). Progressing from straight alignment to 54° decreased the construct stiffness 63% (P<0.00 1).

CONCLUSIONS:
The 16ALT configuration provides the greatest construct stiffness while allowing segmental correction at each level. Decreasing the kyphotic angulation significantly increases construct stiffness.

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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.