Introduction Spondylolysis is an acquired stress fracture that occurs in the developing spine as a result of repetitive overload. Although fractures of the pars interarticularis may heal if identified early in the disease process, in a subset of patients the fractures fail to heal. The resultant nonunion may or may not be painful. This condition is ultimately present in up to 5% of older adolescents and adults. Nociceptive fiber ingrowth into the chronically nonunited area may be the mechanism of pain generation in the nonunited condition. Multiple techniques of repair have been described combining techniques of nonunion repair, which include débridement and structural bone grafting with various forms of internal fixation. Relief of pain has been significant in clinical series in which pars defect healing has been achieved by surgical treatment. This investigation will quantify the biomechanical effects of an experimentally induced defect in the pars articularis (spondylolysis) and three different techniques of repair in a cadaveric benchtop model. We hypothesize that a simple, novel method of pars interarticularis repair provides stability comparable to Buck's method of hook-screw-rod methods of pars repair. Eventual implementation with spondylolytic patients may provide increased lumbar spine stability with decreased risk of complications.
Method Twenty-seven fresh-frozen and thawed calf lumbar spines were used for testing. Previous work verified the suitability of a calf spine model as a substitute for human spine construct testing. The second and sixth lumbar vertebrae were potted in polymethylmethacrylate dental cement. Bilateral 5 mm defects in the pars interarticularis were created in the fourth lumbar vertebra using a sagittal saw. Intervertebral rotation and displacement across the defect were measured using linear and rotational extensometers. Each spine underwent testing under the following experimental situations: native spine followed by spine with bilateral pars defects. In addition, three internal fixation techniques were compared: pedicle screw hook rod, arch plate, and a newly designed cable plate construct. Loads were applied using an Instron 5800 material testing machine.
Conclusions Osteotomy of the pars interarticularis increases rotational displacement in a calf spine model. Trends show a rotational stabilizing effect in all three repair methods.
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