A biomechanical study of anterior thoracolumbar screw fixation.

STUDY DESIGN: The pullout strength of unicortical and bicortical screws in thoracic and lumbar vertebral bodies was measured as a function of bone mineral density. OBJECTIVES: To determine the influence of bone mineral density and screw insertion technique on the stability of anterior thoracolumbar spine screw fixation. SUMMARY OF BACKGROUND DATA: No previous study has addressed the specific technique of screw insertion or stability of screw fixation in the anterior spine. METHODS: Fifty-one human thoracic vertebral bodies were tested in pullout with 6.55-mm cancellous screws inserted using unicortical and bicortical techniques. RESULTS: Pullout force increased exponentially with increasing bone mineral density for unicortical and bicortical screws. Bicortical screws were significantly stronger in resisting pullout than unicortical screws. CONCLUSION: Advancing an anterior vertebral body screw to engage the second cortex increases resistance to pullout by 25-44%, depending on vertebral bone mineral density. The difference in resistance between unicortical and bicortical techniques was smaller in specimens with low mineral densities.

Factors affecting the pressure-distributing properties of foam mattress overlays.

A universal concern of those in nursing care is the occurrence of decubitus ulcers. To minimize this problem foam overlays are routinely used on hospital beds throughout the country. This study was undertaken to develop laboratory testing procedures which will reliably predict the support characteristics of mattress overlays in clinical service. Initially, test methods based on constant load and constant deformation boundary conditions were developed. These methods were applied to measure the pressure-distributing properties of 10 polyurethane foams fabricated in a wide range of pliancies and densities. Based on these laboratory data, a single index was developed to express the effectiveness of each overlay in distributing interface pressures. This index was found to be influenced by the stiffness (ILD), density, and thickness of each device. These data were then compared with measurements of interface pressure obtained from volunteer subjects who were supported on mattress overlays fabricated from each of the original foams. Correlation of these data with the laboratory results demonstrated that the effectiveness index was an accurate predictor of mattress overlay performance under end-use conditions.