[The influence of the elasticity module of the femoral shaft and neck of a total hip prosthesis on the distribution of stress in the femur ]

Bone stock in revision femoral arthroplasty: a new evaluation

PURPOSE

The purpose of this study was to finalize a method allowing a qualitative and numerical evaluation of the bone stock and to confirm its reproducibility, to verify the relationship between the secondary bone stock value and the functional results, and to determine the main factors influencing the value of the bone stock.

METHODS

A clinical and radiological evaluation was performed in a group of 150 revisions of total hip replacements according to a new method taking into account cortical bone thickness, bone density and bone defects.

RESULTS

Interobserver reproducibility was evaluated at an average of 0.6 and intra-observer reproducibility was considered good at 0.8. Between the initial bone stock and at the last follow-up, no significant difference was noticed. For secondary bone stock considered as "very good or good", the gain was +38.1 points versus +29.9 points for patients evaluated as "average or poor" (p < 0.0001). Between the initial bone stock assessment and at last follow-up, a significant relation was found in numerical values for the global type of primary fixation and in the presence of osteopenia (p < 0.0001).

CONCLUSIONS

Deficient secondary bone stock can result in less favourable functional results. The numerical scores confirm the importance of strategic choices during surgery in order to manage bone stock preservation.

A Biomechanical Evaluation of Orthopaedic Implants for Hip Fractures by Finite Element Analysis and In-Vitro Tests

The aim of this study was to test the hypothesis that a reinforced gamma nail for the fixation of subtrochanteric fractures would experience less stress during loading compared with a common gamma nail. The issue of whether the use of the stronger implant would result in more stress shielding in the surrounding bone was also addressed.

A finite element analysis (FEA) of a synthetic bone was employed to calculate the stress distribution in implant and bone for two fracture types (AO 31-A3.1 and AO 31-A3.3). The FEA was validated by mechanical tests on six synthetic femurs. To test the hypothesis in vitro, mechanical tests on six pairs of fresh-frozen human femurs were conducted. The femurs were supplied with a common or a reinforced gamma nail in a cross-over study design. Strains were measured on the nail shaft to quantify the loading of the nail.

The FEA resulted in 3—51 per cent lower stresses for the reinforced gamma nail. No increase in stress shielding could be observed. In the in-vitro tests, the reinforced gamma nail experienced less strain during loading ( p < 0.016). The study demonstrated the benefit of a reinforced gamma nail in subtrochanteric fractures. It experienced less stress but did not result in more stress shielding.

Effect of implant material properties on the performance of a hip joint replacement

A composite material implant prosthesis for hip replacement has been developed. The design of the prosthesis substructure was based on investigation of the stress and strain fields that were developed in the human femur at the proximal end when a prosthesis stem had been inserted into it. The prosthesis stem structure was of unidirectional fibrous composite material core (graphite fibres in polysulfone matrix), wrapped with four layers of the same material but orientated at different angles. The orthotropic moduli of the outer layer are very close to the moduli of a human cortical bone in the vertical and circumferential directions. The moduli increased gradually from the outer layer to the inner core. A three-dimensional finite element model of the prosthesis and the bone has been constructed and loaded with the range of forces that might appear upon operation. The behaviour of the composite prosthesis and the femur was then compared with the intact femur and three other types of prosthesis materials, namely stainless steel, titanium, an isoelastic material and a hypothetical one with moduli identical to the cortical bone. The titanium has modulus of elasticity that is only half of the stainless steel. It was found that the composite prosthesis gave the best performance for most of the categories that were examined.

Ultrasonically determined elasticity and cortical density in canine femora after hip arthroplasty

Effects of canine hip replacement (with a porous-coated femoral component) on the material properties of surrounding cortical bone were evaluated. The hypotheses were: (1) after four months of implantation, mechanical properties of the cortex would change, and (2) a collared implant would be associated with smaller changes than a collarless design. Unilateral total hip arthroplasty was performed in 15 mixed-breed dogs. Nine received a collared and six received a collarless femoral component. Four months after implantation, longitudinal ultrasonic wave propagation velocities and bone mineral densities (from dual energy X-ray absorptiometry) were measured in harvested femora and used to calculate the axial elastic constitutive coefficients for the cortex surrounding the implants. Results showed no difference in bone elasticity or bone density between collared and collarless designs. Significant velocity decreases from control values (p < 0.0001) were noted in all implanted femora at four months. Bone mineral densities also displayed decreased values after four months of implantation (p < 0.0145). Elastic coefficients were consistently less after four months of implantation when compared to control values (p < 0.0001). This alteration in material properties would affect load transfer into the implanted femur via the increased disparity between implant and bone stiffnesses regardless of the component design. Significant differences in the elastic coefficients between implanted and control femora support hypothesis 1. However, no group differences were found between collared and collarless implantations; thus, the study does not support hypothesis 2.