Loosening and osteolysis with the press-fit condylar posterior-cruciate-substituting total knee replacement

Background: Aseptic loosening and osteolysis are rarely associated with cem ented posterior-cruciate-substituting total knee replacements. Consequently , there is a paucity of information on this topic. Methods: After a mean follow-up interval of fifty-six months (range, thirty -seven to eighty-nine months), sixteen (2.9%) of 557 posterior-cruciate-sub stituting primary total knee replacements were revised by a single surgeon because of loosening and osteolysis. Clinical, radiographic, and retrieval analyses were conducted to determine the mechanism of loosening and to iden tify associated risk factors. Results: All sixteen knees (fifteen patients) were rated as good or excelle nt at one year after the primary replacement, with mean clinical and functi onal Knee Society scores of 95 and 86 points, respectively. Nine of the fif teen patients who had a revision because of loosening and osteolysis had ha d a total knee arthroplasty on the contralateral side compared with only 18 % of the patients who did not have a revision (p = 0.026). No evidence of t ransmission of substantial anteroposterior stresses from the posterior-cruc iate-substituting mechanism was found. All twelve retrieved knee implants, however, had damage to the lateral and medial side walls of the polyethylen e posterior-cruciate-substituting post. Damage to the inferior surface of t he polyethylene inserts had a rotational pattern, with the axis of rotation in the medial compartment. Surface damage in a rotational pattern was also present on the superior and inferior surfaces of the titanium tibial base- plates. Conclusions: In the knees in our study, rotational forces were generated by impingement of the side walls of the intercondylar box on the polyethylene post. Such box-post impingement can occur throughout the range of motion. Rotational stresses are transmitted to the modular interfaces and to the me tal-cement interfaces, resulting in loosening and osteolysis. A reduction i n rotational constraint would be desirable. Patients with bilateral total k nee replacement may be at increased risk for this type of loosening.