Comparative experience with canine and human cementless acetabular components.

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.