Modelling the fibrous tissue layer in cemented hip replacements: experimental and finite element methods.

The long-term fixation of cemented femoral components may be jeopardised by the presence of a fibrous tissue layer at the bone-cement interface. This study used both experimental and finite element (FE) methods to investigate the load transfer characteristics of two types of cemented hip replacements (Lubinus SPII and Müller-Curved) with a fibrous tissue layer. The experimental part investigated six stems of each type, where these were implanted in composite femurs with a specially selected silicone elastomer modelling the soft interfacial layer. Two fibrous tissue conditions were examined: a layer covering the full cement mantle, representing a revision condition; and a layer covering the proximal portion of the cement mantle, representing a non-revised implant with partial debonding and fibrous tissue formation. The FE method was used to model the full fibrous tissue layer condition, for both implants. The layer was modelled as a homogeneous, linearly isotropic material. A cross-comparison was performed of the experimental and FE findings. Agreement between experimental and FE models was verified to be within 15%. Varying the stiffness parameter of the FE soft tissue layer had little influence on the cortical bone strains, though had considerable effect on the cement strains. Stress shielding occurred for both stems under both fibrous tissue conditions, with the greatest reduction around the calcar. However, the cortical bone strains were generally larger than those for the equivalent well-fixed stems. The fibrous tissue layer was not found to increase the general strain pattern of the cement mantle, though localised regions of high stress were detected.

Experimental investigation of bone remodelling using composite femurs.

OBJECTIVE: To determine the load transfer patterns of femurs in the intact, immediate post-operative and long-term (remodelled) post-operative implanted conditions for Lubinus SPII and Müller-Curved cemented hip prostheses, and to examine to what extent remodelling may influence the long-term outcome. DESIGN: Experimental and finite element (FE) methods were applied to composite femurs under loads representing the heel-strike phase of gait, determining cortical bone and cement strains for the different femur conditions. BACKGROUND: The authors previously developed protocols to measure bone and cement strains, and to produce remodelled femur specimens, yet these have not been applied together to compare strain patterns of different femur conditions. The Lubinus SPII is clinically more successful than the Müller-Curved stem, with failure mainly due to aseptic loosening. METHODS: Cortical bone strains were determined in intact femurs. Six femurs each were implanted with the two stem types and cortical bone and cement strains were measured. Bone remodelling was recreated using a validated CAD-CAM procedure to remove a layer of proximal cortical bone, replicating a typical scenario found in stable clinical retrievals. Strains were remeasured. FE methods were used to compliment the experiments. RESULTS: Stress shielding was reduced with remodelling, though bone strains did not return to their intact values, particularly around the calcar. Cement strains increased with remodelling. Differences occurred between the two stems; the Müller-Curved produced a more severe strain transition. CONCLUSIONS: Procedures were successfully combined together to investigate in vitro the performance of two cemented stems, in immediate and long-term post-operative conditions. The increase of cement strains with remodelling is a potential indicator for in vivo cement failure. RELEVANCE: The consequences of femoral bone remodelling on the long-term success of joint replacements are not well understood, where remodelling may lead to increased bone and cement stresses.

A CAD-CAM methodology to produce bone-remodelled composite femurs for preclinical investigations.

Femoral bone remodelling, following total hip arthroplasty, is a clinically observed phenomenon attributed to the changed stress environment of the postoperative implanted hip. While this process cannot be avoided, there is concern as to its consequences on the long-term survival of hip joint replacements. Previous methods of studying remodelling, such as clinical or animal-based studies, or finite element analyses, have their limitations. The aim of this study is to develop experimental specimens incorporating bone resorption features typical of clinically successful implants. This work describes the use of computer aided design/manufacturing methods (CAD-CAM) to produce these specimens, based on modifying commercially available composite femurs. The procedures are investigated and verified for two different designs of cemented prostheses (Lubinus SPII and Muller Curved). Quantitative clinical data is used to define the remodelled geometry of a CAD model of the femur for each stem design. Composite femur specimens are machined using a three-axis milling machine, where each specimen can be accurately positioned using a custom-designed jig and a digitizer system. The accuracy of the process is assessed by analysing the deviation of the digitized premachined and postmachined surfaces of each specimen in relation to the CAD model. The results demonstrate that the procedure can be used for developing in vitro specimens with bone resorption features. These specimens are proposed as a useful tool for performing preclinical trials, such as load transfer or longevity/stability testing, with the advantage of modelling a long-term clinical situation, rather than solely analysing implanted femurs in an immediate postoperative state.

The bicycle spoke injury: an avoidable accident?

A prospective study of the bicycle spoke injury over a 1-year period included a total of 71 spoke injuries. Of these, 67 injuries occurred on an adult bicycle and four occurred on a child's bicycle. All children sustained ankle and foot injuries, which consisted of contusion and superficial abrasion (N = 45), skin loss (N = 10), skin laceration (N = 4), and undisplaced fractures (N = 12). A biomechanical study was conducted to investigate the use of a protective cover over the wheel to prevent the foot from slipping between the spokes. Wind resistance studies showed that a cover with a mesh size of 10 mm hexagonal could prevent this and at the same time stop the cover from acting as a sail if a flat cover without holes was used instead. The mesh cover, however, will prevent the toes from entering between the spokes but will not prevent the foot from becoming jammed between the wheel and the fork. To prevent this, a plastic shield to bridge the gap between the fork and the horizontal upright has been designed. With these modifications, the bicycle spoke injury can become an injury of the past.