BACKGROUND Aseptic loosening and osteolysis are rarely associated with cemented 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-substituting 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 identify associated risk factors. RESULTS All sixteen knees (fifteen patients) were rated as good or excellent at one year after the primary replacement, with mean clinical and functional Knee Society scores of 95 and 86 points, respectively. Nine of the fifteen patients who had a revision because of loosening and osteolysis had had 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 transmission of substantial anteroposterior stresses from the posterior-cruciate-substituting mechanism was found. All twelve retrieved knee implants, however, had damage to the lateral and medial side walls of the polyethylene posterior-cruciate-substituting post. Damage to the inferior surface of the 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 metal-cement interfaces, resulting in loosening and osteolysis. A reduction in rotational constraint would be desirable. Patients with bilateral total knee replacement may be at increased risk for this type of loosening.