A comparison of parallel and diverging screw angles in the stability of locked plate constructs.

We investigated the static and cyclical strength of parallel and angulated locking plate screws using rigid polyurethane foam (0.32 g/cm(3)) and bovine cancellous bone blocks. Custom-made stainless steel plates with two conically threaded screw holes with different angulations (parallel, 10° and 20° divergent) and 5 mm self-tapping locking screws underwent pull-out and cyclical pull and bending tests. The bovine cancellous blocks were only subjected to static pull-out testing. We also performed finite element analysis for the static pull-out test of the parallel and 20° configurations. In both the foam model and the bovine cancellous bone we found the significantly highest pull-out force for the parallel constructs. In the finite element analysis there was a 47% more damage in the 20° divergent constructs than in the parallel configuration. Under cyclical loading, the mean number of cycles to failure was significantly higher for the parallel group, followed by the 10° and 20° divergent configurations. In our laboratory setting we clearly showed the biomechanical disadvantage of a diverging locking screw angle under static and cyclical loading.

[Comparison of regional distribution of cancellous bone in osteoporotic and non-osteoporotic distal radii]. / Vergleich der lokalen Spongiosadichte osteoporotischer und nichtosteoporotischer distaler Radii.

BACKGROUND: In cases of distal radius fractures information about the regional distribution of cancellous bone is essential for adequate fixation of osteosynthesis material. MATERIALS AND METHODS: Using quantitative computed tomography (qCT) measurements 12 distal radii were divided into 2 groups (good and bad bone quality). High-resolution CT scans were obtained at 13 locations (1-40 mm distant from radial styloid processes). The resulting cross-sections were divided into 4 quadrants and the cancellous bone density was assessed in each of them. RESULTS: In both groups regional cancellous bone density was highest immediately beneath the joint surface and decreased towards the metaphyseal region. Comparing the radial with the ulnar quadrants significant differences were found between distances of 4 mm and 12 mm from the joint surface in both groups. The comparison of the palmar and the dorsal quadrants resulted in an almost identical cancellous bone distribution. CONCLUSION: In osteoporotic distal radii cancellous bone density decrease occurs homogeneously in all regions and is least pronounced beneath the joint surface. Comparing the regional cancellous bone density (radial versus ulnar and palmar versus dorsal) significant differences were mainly found near the joint surface in both groups.