Stem micromotion after femoral impaction grafting using irradiated allograft bone: a time zero in vitro study.

BACKGROUND: A gamma irradiation dose of 15kGy has been shown to adequately sterilise allograft bone, commonly used in femoral impaction bone grafting to treat bone loss at revision hip replacement, without significantly affecting its mechanical properties. The objective of this study was to evaluate whether use of 15kGy irradiated bone affects the initial mechanical stability of the femoral stem prosthesis, as determined by micromotion in a comprehensive testing apparatus, in a clinically relevant time zero in vitro model of revision hip replacement. METHODS: Morselised ovine bone was nonirradiated (control), or irradiated at 15kGy or 60kGy. For each dose, six ovine femurs were implanted with a cemented polished taper stem following femoral impaction bone grafting. Using testing apparatus that reproduces stem loading, stems were cyclically loaded and triaxial micromotion of the stem relative to the bone was measured at the proximal and distal stem regions using non-contact laser transducers and linear variable differential transformers. FINDINGS: There were no significant differences in proximal or distal stem micromotion between groups for all directions (p≤0.80), apart for significantly greater distal stem medial-lateral micromotion in the 60kGy group compared to the 15kGy group (P=0.03), and near-significance in the anterior-posterior direction (P=0.08, power=0.85). INTERPRETATION: Using a clinically relevant model and loading apparatus, irradiation of bone at 15kGy does not affect initial femoral stem stability following femoral impaction bone grafting.

An evaluation of interface contact profiles in two low contact bone plates.

Bone plate design has evolved dramatically in recent years. The Dynamic Compression Plate (DCP) has been superseded by bi- and uni-cortical plates that claim a reduced interface contact between the plate and the underlying bone. It is believed that contact reduction ameliorates the localised ischaemia that develops subsequent to plate application. In this study, the interface characteristics of the Limited Contact-Dynamic Compression Plate (LC-DCP) and the Contour Plus (CP) plating systems have been quantitated using Fuji prescale pressure sensitive film interposed between the plate and the bone. Ten-hole plates were applied to the same aspect of either the humeral, radial or ulnar diaphysis of human cadaveric bone in a reproducible manner. The average pressure, force and interface contact area were calculated using Interactive Data Language (IDL) image analysis software. The CP system was consistently lower, in terms of interface contact, than the LC-DCP in each of the specimen locations tested (P<0.0001). The CP system displayed a 'point-contact' configuration along the interface with high pressures recorded at these points, the significance of which is unknown.

Spatial distribution of hip capsule structural and material properties.

Contemporary computational models potentially allow the practical incorporation of the effects of a joint capsule on both motion and the loads transmitted to the other parts of the joint. However, the required material properties have not been available for this purpose. To determine these properties we took both hip joints from five fresh-frozen, nondiseased cadavers. Following dissection and potting of the hemi-pelvis, distraction of the intact joint was conducted to measure the structural tangent stiffness of the joint capsule. Anatomical insertion points of the hip capsule were then recorded, and a complete capsulectomy was performed. Once excised, the capsule was sectioned into eight, approximately even sectors, and initial geometrical measurements were recorded for material property calculations. Material properties (i.e., structural tangent stiffness, failure load, ultimate strength, tangent modulus) were calculated using the load-displacement and geometric data collected for each of the sectors. This specimen-to-specimen thickness variability reveals significantly lower (p<0.01) average tangent structural stiffness values in the posterior-inferior portion of the capsule. Explorations of hip stability using numerical models can now be enhanced by incorporation of these experimental capsule data.