Analysis of the kinematics of different hip simulators used to study wear of candidate materials for the articulation of total hip arthroplasties.

We developed an analytical technique to determine the paths traced by specific points on the femoral head against the acetabulum in the human hip joint during gait. The purpose of the study was to apply this technique to the mechanical hip simulators chosen to conduct wear tests on polymeric acetabular liners used in total hip replacements. These simulators differ from one another in the type of motion produced, apart from other variables such as type of lubricant and head position. Due to the variation in the kinematics between the machines, the paths traced by the points on the femoral head against the acetabular liner ranged from simple linear traces to figure-8 loops and quasi-elliptical paths during a single simulator cycle. The distances traveled by these points during the same period also varied appreciably among the different hip simulator designs. These results are important when combined with other studies that have shown that kinematics can play an important role in the outcome of in vitro wear experiments. The kinematic differences quantified in this study can partially explain the substantial differences in wear data reported from different simulator designs and also underscore the usefulness of the technique described in this study in judging the results from different hip simulator experiments.

Factors influencing stability at the interface between a porous surface and cancellous bone: a finite element analysis of a canine in vivo micromotion experiment.

Several factors contribute to the success of stable bony ingrowth into the porous coated surfaces of orthopaedic implants used in hip arthroplasty. Despite having good bony apposition, bony ingrowth might not occur if the relative motion between bone and implant is large. Therefore, determining the limiting micromotion value that inhibits stable bony ingrowth is important. From a previous canine in vivo micromotion study performed at our laboratory, this limiting value was found to be 20 microns. Initially, cementless orthopaedic implants are stabilized only by frictional forces at the bone-implant interface. Therefore, other parameters such as the coefficient of friction and the compressive force normal to the interface should be considered as important factors which stabilize the interface along with micromotion. The purpose of this analytical study was to elucidate how the stability at the bone-implant interface is influenced by various factors, namely, motion of the implant, the coefficient of friction, the degree of pres fit, and the modulus of the surrounding cancellous bone in determining the stability of the bone-implant interface. Nonlinear and linear finite element models which simulated the immediate postsurgical condition and the end point of the canine in vivo micromotion experiment, respectively, were used to this end. From the results of the finite element models it was possible to identify the displacement magnitude for which the implant slipped relative to the bone as the motion of the implant was increased incrementally. This was done for combinations of the coefficient of friction, press fit, and Young's modulus of cancellous bone. This was used as an indicator of the limiting implant motion value beyond which bony ingrowth will be inhibited. The stress distribution in the surrounding cancellous bone bed was also obtained from the results of the finite element analyses for different press-fit conditions. The results of the study indicated that under slight press-fit conditions, the implant slipped relative to bone for implant motions as low as 20 microns. For higher degrees of press fit and reasonable values for the coefficient of friction, no slip occurred for implant motions as much as 100 microns. Although higher degrees of press fit were theoretically conducive to better implant stability, the concomitant high stresses in the adjacent cancellous bone will tend to compromise the integrity of the press fit. This was also evident when the results of an analytical model with a lower degree of press fit correlated well with those of the canine in vivo experiment in which a higher press fit was used, suggesting a possibility of achieving a less than desired press fit during the process of implantation. Through this study the importance of factors other than implant motion was emphasized. The results of the study suggest that the limiting value of implant motion that inhibits bone ingrowth might vary with the degree of press fit for reasonable coefficients of friction.

Differences in stiffness of the interface between a cementless porous implant and cancellous bone in vivo in dogs due to varying amounts of implant motion.

To determine the mechanical properties of the interface between the tissue ingrowth into porous coatings and the implant, porous-coated cylindrical implants were inserted into the distal femur in 20 mature dogs and oscillated in vivo 8 hours per day for 6 weeks at fixed amounts of micromotion (0, 20, 40 and 150 microns). Applied torques and resulting displacements were recorded. The torsional resistance per unit angular displacement (TR/AD), reflecting the stiffness of the bone-porous coating interface, was 0.88 +/- 0.25 N-M/deg immediately after implantation in the 20-micron displacement group. It increased with time after surgery, reaching a maximum of 1.25 +/- 0.60 N-M/deg at 6 weeks. The TR/AD was lower initially (0.77 +/- 0.43 N-M/deg) in the 40-micron group and gradually decreased with time after surgery, reaching a maximum of 0.54 +/- 0.13 N-M/deg at 6 weeks. The TR/AD was even lower (0.24 +/- 0.10 N-M/deg) in the 150-micron group initially and remained the same (0.16 +/- 0.09 N-M/deg) with time after surgery. Histologic evaluation showed bone ingrowth in continuity with the surrounding bone in the 20-micron group consistent with the high stiffness values at sacrifice. In contrast, a mixture of fibrocallus and bone were found at the bone-porous coating interface in the 40-micron group, consistent with the intermediate stiffness values. In contrast, despite the fact that bone was found in the depth of the porous coating in the dogs in the 150-micron group, the low stiffness values were a reflection of fibrous tissue formation at the interface in that group, because of the large motion disrupting bony ingrowth at the bone-porous coating interface. By monitoring the torsional resistance per unit of angular displacement dynamically in vivo, it was possible to evaluate the mechanical properties of the bone-porous coating interface as tissue ingrowth proceeded. Twenty microns of oscillating displacement was compatible with stable bone ingrowth with high interface stiffness, whereas 40 and 150 microns of motion was not.

Loci of movement of selected points on the femoral head during normal gait. Three-dimensional computer simulation.

Wear of ultrahigh-molecular-weight polyethylene and the subsequent lytic response to the particulate wear debris are the dominant problems in total joint arthroplasty surgery. Wear testing apparatus can play a vital role in the in vitro evaluation of the many factors involved in wear, such as head size, surface roughness, materials for the head, and new materials for the socket. Wear of ultrahigh-molecular-weight polyethylene may be influenced by the wear path. For the related polymer, high-density polyethylene, the wear path is critical to wear magnitude. What is the actual path taken by a single point (or by multiple representative points) on the femoral head of a total hip arthroplasty as it passes through the gait cycle? The goal of this computer simulation study was to trace the paths of specific points on the femoral head as they moved against the polyethylene cup during a single cycle of normal gait to illustrate the motions occurring at the intraarticular surface of the hip joint. This study also yielded unusual data on the "distance traversed" by these points during a single gait cycle. It was found that there was not one path, but rather there were many, and the paths varied widely in both shape and length depending on the location on the femoral head. Moreover, the differences in excursion and direction at different sites during the loaded phase were great. In addition, distances traveled by different points on the femoral head of any given size varied by a factor greater than 2. Most of the points traced quasielliptical paths. This automatically means that the paths of neighboring points cross each other, creating multidirectional shear forces on the acetabular cup surface which may be important in the localization and extent of wear. The plots of traces of the points derived from this study can serve as benchmarks for the ability of hip simulators to reproduce the actual distances and paths of travel of individual points on the femoral head.