A functional classification of spinal instrumentation.

BACKGROUND CONTEXT: Spinal instrumentation systems have evolved far beyond the original Harrington design, with increasing complexity and capabilities. This review attempts to de-emphasize the "product names," as systems are often generically and inappropriately referred to as "Harrington rods," by introducing a functional classification for current systems. PURPOSE: The aim of this paper is to review the intended design purpose of current spinal instrumentation systems for more accurate interpretations of radiographs and complications. STUDY DESIGN: The principles involved in each type of instrumentation system are described, followed by the characteristic components and the normal appearance radiographically, with examples of complications, which should be recognized. METHODS: The historical contribution of specific systems in the developing art of internal fixation of the spine is provided as a background to understand the biomechanical forces imparted by spinal hardware. Recognizing the functional intent of spinal constructs will enhance descriptions of those images. RESULTS: Spinal instrumentation has been categorized into five functional types: Distraction and compression, segmental stabilization, coupled or derotation systems, translational or pedicle screw systems, and anterior instrumentation. CONCLUSIONS: Analysis of radiographs from a functional viewpoint can enhance the descriptive interpretation and specifically allow assessment of the success or of the presence of complications, which are crucial to the analysis of instrumentation effectiveness.

Assessor cognitive processes in an operational assessment center.

The purpose of this study was (a) to provide additional tests of C. E. Lance, Newbolt, et al.'s (2000) situational specificity (vs. method bias) interpretation of exercise effects on assessment center postexercise dimension ratings and (b) to provide competitive tests of salient dimension versus general impression models of assessor within-exercise evaluations of candidate performance. Results strongly support the situational specificity hypothesis and the general impression model of assessor cognitive processes in which assessors first form overall evaluations of candidate performance that then drive more specific dimensional ratings.

Rhabdomyolysis in lumbar spine surgery: a case report.

STUDY DESIGN: The case of a complication sustained by an obese patient undergoing a lumbar spine fusion is reported. OBJECTIVES: To minimize further complications by reporting on this case complicated by rhabdomyolysis in a lumbar fusion, and to include a review of the literature. SUMMARY OF BACKGROUND DATA: Rhabdomyolysis has been described as a clinical syndrome initially characterized by darker urine, muscle pain, and weakness from prolonged muscle compression, as in intoxicated patients or after a seizure. Rhabdomyolysis and myoglobinuria have also been reported in the prolonged positions of some surgical procedures. METHODS: A 40-year-old man developed progressively incapacitating lumbar pain above a previous fusion from L4 to S1, with a severe decline in his functional capacities and a large weight gain. Revision surgery removing the previous instrumentation and performing an instrumented fusion from L2 to L4 was complicated by rhabdomyolysis. RESULTS: After an unremarkable surgery, this patient was hemodynamically unstable when transferred from the operating table to the supine position and required aggressive treatment of severe rhabdomyolysis, which has not previously been reported in this setting. CONCLUSION: Patients undergoing lumbar spine surgery require decompression of the abdomen, and in this case, a Jackson table was used. Unfortunately, his abdominal girth, even on a Jackson table, was not fully decompressed and resulted in this complication. Further consideration should be given to modifications of positioning for morbidly obese patients.

Piriformis syndrome.

Seven patients underwent release of the piriformis from the femur. Patients with residual symptoms after conservative treatment had dramatic relief of sciatica and 70% resumed customary work after surgery. Minimum follow-up was 31 months (average: 51 months). Early diagnosis can avoid prolonging ineffective empiric treatment and disability with satisfactory results achieved in most patients by conservative treatment and relief of sciatica in selected surgical cases.

Characterization of a developing lumbar arthrodesis in a sheep model with quantitative instability.

BACKGROUND CONTEXT: Mechanical forces have been considered responsible for stress shielding an arthrodesis, but the biology of a developing lumbar fusion has not been well characterized. PURPOSE: A large animal model was used to test the hypothesis that mechanical forces modify the biological processes involved in a developing bony fusion. STUDY DESIGN: Lumbar fusion was performed in an ovine model using custom instrumentation that permitted a controlled degree of anterior-posterior translation after surgery. Fusion sites were evaluated by radiography, microradiography, histology and histomorphometry at time points that corresponded with predicted early and later stages of bone healing. METHODS: Fourteen skeletally mature ewes underwent lumbar spinal fusion under general anesthesia. In the control (stable) group, the spine was rigidly fixed with a cage anteriorly and pedicle screws posteriorly. In the experimental (unstable) group, the spine was destabilized by an annulectomy (with no anterior implant) and custom pedicle screws that allowed 2 mm of anterior-posterior translation. Animals were euthanized 6 and 12 weeks after surgery. RESULTS: Radiographs confirmed that the fusion mass had not fully consolidated at either time point. Microradiographs revealed a trend toward increased bone formation at 6 weeks in the stable case as compared with the unstable, but by 12 weeks, this trend had reversed (p=.03). Intramembranous bone formation was the primary mechanism of healing near the transverse process in animals with both stable and unstable fixation. In the area between the two transverse processes, new bone formation occurred primarily through endochondral ossification. At 12 weeks, the stable case had significantly more cartilage formed (p=.023) but less newly formed bone (p=.07) as compared with the quantitatively unstable. CONCLUSIONS: This clinically realistic animal model allowed characterization of the biology of the developing arthrodesis before fusion. Under stable or unstable conditions, endochondral ossification was the predominant mechanism of new bone formation within the intertransverse process region. This finding, which contrasts with previous reports from small animal models of spine fusion, may reflect a difference in biology that results from the increased size of the intertransverse space in sheep as compared with small animals. Interestingly, mechanical instability increased the formation of new bone within this region, but not at the transverse process. Endochondral ossification therefore appears to respond to mechanical factors in the fusion site. The ovine model shows promise as an alternative to the rabbit model and may provide a more stringent test for potential new surgical and nonsurgical strategies for spine fusion.