Sensitivity of periprosthetic stress-shielding to load and the bone density-modulus relationship in subject-specific finite element models.

Subject-specific finite element (FE) computer models of the proximal femur in hip replacement could potentially predict stress-shielding and subsequent bone loss in individual patients. Before such predictions can be made, it is important first to determine if between subject differences in stress-shielding are sensitive to poorly defined parameters such as the load and the bone material properties. In this study we investigate if subject-specific FE models provide consistent stress-shielding patterns in the bone, independent of the choice of the loading conditions and the bone density-modulus relationship used in the computer model. FE models of two right canine femurs with and without implants were constructed based on contiguous computed tomography (CT) scans so that subject-specific estimates of stress-shielding could be calculated. Four different loading conditions and two bone density-modulus relationships were tested. Stress-shielding was defined as the decrease of strain energy per gram bone mass in the femur with the implant in place relative to the intact femur. The analyses showed that for the four loading conditions and two bone density-modulus relationships the difference in stress-shielding between the two subjects was essentially constant (1% variation) when the same loading condition and density-modulus relationship was used for both subjects. The severity of stress-shielding within a subject was sensitive to these input parameters, varying up to 20% in specific regions with a change in loading conditions and up to 10% for a change in the assumed density-modulus relationship. We conclude that although the choice of input parameters can substantially affect stress-shielding in an individual, this choice had virtually no effect on the relative differences in femoral periprosthetic stress-shielding between individuals. Thus, while care should be taken in the interpretation of the absolute value of stress-shielding calculated with these type of models, subject-specific FE models may be useful for explaining the variation in bone adaptation responsiveness between different subjects in experimental or clinical studies.

Functional adaptation and ingrowth of bone vary as a function of hip implant stiffness.

The purpose of the present study was to test the hypothesis that cortical bone loss, trabecular bone density and the amount of bone ingrowth vary as a function of stem stiffness in a canine cementless hip replacement model. The study was motivated by the problem of cortical bone atrophy in the proximal femur following cementless total hip replacement. Two stem stiffnesses were used and both designs were identical in external geometry and porous coating placement. The high stiffness stem caused approximately 26% cortical bone stress-shielding and the low stiffness stem caused approximately 7.5% stress-shielding, as assessed by beam theory. Each group included nine adult, male canines who received unilateral arthroplasties for a period of six months. The animals with the low stiffness stems tended to lose less proximal cortical bone than the animals with high stiffness stems (4% +/- 9 as opposed to 11% +/- 14), but the difference was not statistically significant (p = 0.251). However, the patterns of bone ingrowth into the implant and change in medullary bone density adjacent to the implant were fundamentally different as a function of stem stiffness (p < 0.01). Most importantly, while the high stiffness group had peaks in these variables at the distal end of the stem, the low stiffness group had peak values proximally. These different patterns of functional adaptation are consistent with the idea that reduced stem stiffness enhances proximal load transfer.

Maintenance of proximal cortical bone with use of a less stiff femoral component in hemiarthroplasty of the hip without cement. An investigation in a canine model at six months and two years.

A canine model of hemiarthroplasty of the hip was used to determine if the use of a less stiff femoral stem can reduce the amount of bone loss induced by stress-shielding. Two types of stem were used: the stiffer stems were made of a titanium alloy, and the less stiff stems were composed of a cobalt-chromium-alloy core with an outer polymer layer. The stems were identical in shape, and both types were circumferentially coated along their entire length (except for the distal five millimeters) with commercially pure titanium fiber metal. Ten dogs with each type of stem were followed for six months, and twelve dogs with each type of stem were followed for two years. Loss of cortical bone from the proximal part of the femur was associated with both types of stem, but typically 50 per cent less bone was lost with the less stiff implants. Most of the cortical loss occurred at the subperiosteal surface. The amount of medullary bone adjacent to the proximal and distal aspects of both types of stem increased; the less stiff stems were associated with a greater increase in the proximal region, and the stiffer stems were associated with a greater increase in the distal region. Similarly, there were peaks in the amount of bone growth into the proximal and distal portions of both types of stem, with a greater peak in proximal bone growth into the less stiff stems and a greater peak in distal bone growth into the stiffer stems.

Initial in vitro stability of the tibial component in a canine model of cementless total knee replacement.

The tibial component of a canine cementless total knee replacement model was used to determine the degree to which pegs and screws contributed to the initial in vitro stability of the device. Three implant designs were investigated: (1) a four-peg implant in which cortical bone screws passed through the pegs, (2) the four-peg implant without adjuvant screw fixation, and (3) a flat implant with screws placed in the same positions as in the first design. For measuring the interface motion, the tibial component and proximal tibia were modeled as rigid bodies and an experimental method was developed which permitted all six degrees of freedom of the motion between these two objects to be determined. In tests performed to validate this methodological approach, the potential confounding influences of tibial deformation and differential amounts of tibial deformation with the use of screws or pegs were shown to be minimal, supporting the use of the rigid-body method. In general, the areas of greatest motion were at the periphery of the bone-implant interface, regardless of whether or not screws or pegs were used. The components secured with screws had up to five-fold reductions in interface motion compared to components which had pegs but lacked screw fixation. Components with pegs and screws and components with screws only had the same amount of interface motion. Thus, in the presence of screw fixation, the addition of pegs did not increase the stability of the tibial component.