Comparison of Autograft vs. Demineralized Allograft in Anterior Thoracoscopically Instrumented "Scoliosis" Fusions - A Caprine Model**

Peter 0 Newton, MD;
Steven S Lee, MD;
Andrew T Mahar, MS;
Christine L Farnsworth, MS;
Craig Weinstein, MD
* (a – DePuy–Acromed, b – DePuy Acromed)
Children's Hospital, San Diego, CA USA

INTRODUCTION:
Thoracoscopic anterior instrumentation is being developed as a method to correct scoliosis. The source of bone graft for this procedure remains debated (iliac crest, rib, allograft demineralized bone matrix).

PURPOSE:
To assess fusion quality after thoracoscopic multilevel anterior scoliosis–type spinal instrumentation using autogenous iliac crest bone and allogenous demineralized bone matrix gel in a caprine model.

METHODS:
Three groups of six goats each underwent identical anterior thoracic discectomies with four level anterior segmental instrumentation, 4 mm rod, 6 mm screws (MOSS–Miami, DePuy, Inc., Warsaw, IN) utilizing a thoracoscopic approach. Autogenous iliac crest bone graft (ICBG) was placed in the disc spaces in the first group. Specially manufactured caprine demineralized allograft gel (GenSci Corp, Irvine, CA) was used in the second group and no graft material was placed in the third group. After 16 weeks the spines were harvested. Computed tomography (CT) scans were obtained to evaluate bony fusion, and each of the fused segments (hardware removed) were tested on a MTS 858 bi–axial testing machine (Eden Prairie, MN). Torsional loads were applied between ± 2 Nm and total angular range of motion (ROM) was measured. Analysis of variance was used to determine significance among groups (p<0.05).

RESULTS:
Multilevel thoracoscopic anterior instrumentation with a construct like that used to treat scoliosis was technically possible in this model. CT evaluations of bony fusion showed partial or complete bridging bone across 94% of the disc spaces in the ICBG group compared to 28% and 22% for the demineralized allograft gel and no graft groups, respectively (p<0.0001). The ROM between ± 2 Nm of torque was significantly less in the ICBG group (22.6º±8.0º) compared to the demineralized allograft gel (36.1º ± 9.7º) and no bone graft (43.5º ± 9.3º) groups (p<0.001). CONCLUSIONS: At 4 months post–operation the CT scan appearance of a complete fusion was noted only in the ICBG group, with the allograft group demonstrating a fusion rating similar to that in the non–grafted group. The biomechanical testing also suggested greater torsion rigidity in the ICBG group. SIGNIFICANCE: Demineralized allograft gel, as a sole source of bone graft, does not appear to be effective in attaining early solid anterior spinal arthrodesis after thoracoscopic discectomy and instrumentation.

* · 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. For full information, refer to page 3.
** The FDA has not cleared a drug and/or medical device for the use described in this presentation. (i.e., the drug or medical device is being discussed in an “off–label" use).