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Synthetic femurs were used to assess the initial bone-implant interface stability of three total hip systems: Wright Medical Technology's Infinity smooth trochanteric module (S-TM), Infinity porous-coated trochanteric module (PC-TM), and Johnson and Johnson S-ROM with a porous surface. The hips were implanted into synthetic femurs, rigidly fixed, and subjected to internal rotation and cyclic, axial compressive loads. The results showed that all three implants achieved good initial implant stability and would be expected to permit bone ingrowth. The porous-coated implants showed greater initial implant stability with less axial micromotion compared with the smooth implants. This finding suggests that surface texture plays a role in initial stability of uncemented prostheses if the bone behaves similar to the material used in this study.

Five matched pairs of fresh human femurs were used to quantitatively assess the structural integrity of the implanted Wright Medical Technology modular Infinity hip and the Osteonics single-piece Omnifit hip. The results showed that neither bone implant system was able to recreate the femoral anteversion of the intact femur (p < 0.05). Although the micromotion at the bone-implant interface was well within the limits of achieving bone ingrowth, the axial micromotion was greater for the Infinity hip than for the Omnifit hip (p < 0.05), but the rotational micromotion did not show a significant difference between the two bone implant systems (p > 0.5). The anterior proximal femur deformation patterns were similar for both the two bone implant systems and intact femurs. However, the medial proximal femur deformation patterns showed that the Infinity hip-implanted femur was more similar to the intact femur than the Osteonics Omnifit hip-implanted femur.

Bone ingrowth in the porous canine implants does occur; the early clinical results are indeed encouraging and justify further investigation. The quality of ingrowth as measured by depth and extent in the dog was stress-related. Remodelling was superior in the more compliant or elastic acetabular components without an interposed metal shell. No human specimens have been retrieved thus far, but we are encouraged because the results in the canine model with acrylic-fixed implants were comparable to the results we subsequently observed in human THARIES implantation. Those were initially excellent, but with time radiolucencies increased and loosening occurred. The quality of the canine biologic ingrowth suggests optimism for secure and durable fixation. Our patients have continued to improve through the follow-up period. There has not been a single case in which a qualitative increase in pain occurred over the cemented versions after operation. The technical problems of the surgical procedure have been overcome, but it must be performed precisely to achieve the initial inherent stability to assist in biologic ingrowth. The operative procedures in this series have been performed by or under the direct supervision of the senior author. As our competence has grown, we have applied the procedure to younger patients and those who have bone stock deficiency. One of the major advantages of the biologic ingrowth systems is the facility to graft cysts and defects. For humans, the current chamfered cylinder is an extremely bonestock-conservative procedure when combined with reaming of the femoral head to the neck size. Although space is limited in resurfacing, we have been able to achieve a minimum 4-mm bearing thickness of polyethylene. We believe that the compliant mesh chamfered cylinder system provides excellent stability. However, the hemispheric component, although technically more difficult to seat and fix, has definite applications in bonestock deficiency and with further refinement will be useful in primary replacements as well. Nevertheless, because of the uncertain future of any type of implant, especially new, we have encouraged the patients to follow a careful postoperative management program and refrain from heavy activity during the first year. Many are active and some are now participating in athletics, although this is certain to shorten the life of these implants just as was observed in acrylic-fixed devices. The biologic problems of interface corrosion, migration, and metal ion transfer indicate that these implants must be followed for longer periods. Therefore it seems unwise to extend this technology to a wide clinical use at this time.

Long-term experimental data support the hypothesis that biologic bone ingrowth into porous surfaces in a hip-resurfacing model is a viable alternative to stem-type porous implants. Ingrowth of bone was demonstrated in long-term canine arthroplasty. The components were well fixed and stable at the time of sacrifice, although ingrowth occurred to varying degrees. The ingrowth and quality of remodeling was influenced by stress. Dislocations were overcome by a change in surgical technique. Loosening and neck fractures were associated with insufficient stability at operation or to component impingement (prevented by modifications of design). The formation of metal debris-filled cysts is a concern and appeared to be associated with the poor quality of porous sintering and possibly abrasion. The effects of increased surface area of the porous components such as increased corrosion products on biologic compatibility are still not well defined and require further research and longer follow-up examination.

Early and high failure rates for conventional hip replacements in young and active patients are well documented. The hypothesis tested in this study is total hip articular replacement by internal eccentric shells (THARIES) resurfacing hip arthroplasty is a sufficiently durable alternative to conventional total hips in patients younger than 40 years old. Survivorship analysis techniques were applied to 106 THARIES and 98 conventional hip replacements. The primary outcome variable investigated was time to failure, as measured by the need for revision of the prosthesis in the absence of sepsis. Other definitions of prosthesis failure were also investigated. Analysis revealed that even within this young, high-risk population there were subpopulations with different risks for failure. The lowest risk patients comprised all rheumatoid arthritis (RA) and juvenile rheumatoid arthritis (JRA) patients, regardless of age. The intermediate risk patients comprised all non-RA, non-JRA patients older than 30 years of age and the highest-risk patients were all non-RA, non-JRA patients younger than 30 years of age. A comparison of THARIES and conventional prostheses within each of these three subpopulations revealed similar patterns of failure over time, with the conventional total hip replacement having significantly better hip function than the THARIES group only in the highest-risk population, when using a failure definition of revision surgery for aseptic loosening. A broader and more important conclusion was that all acrylic-fixed implants, THARIES or THRs are predicted to undergo early mechanical loosenings in younger-than-30, non-RA, non-JRA patients. In patients younger than 30 years age acrylic fixation of THA is inadvisable.