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

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.

The John Charnley Award. Wear is a function of use, not time.

Polyethylene wear (linear penetration) in 37 hip replacements was assessed from digital images using a validated two-dimensional, edge detection-based computer algorithm. Patient activity was assessed with a pedometer, a step activity monitor and a simple visual analog scale. Joint use was related to wear at the 90% confidence level. Without three recognized outliers, wear was highly correlated to use. The visual analog scale activity rating was significantly related to wear for the 24 hip replacements with standard polyethylene. Univariate regression analysis indicated that male gender, height, weight (which were both highly correlated to male gender) and hip center of rotation were significantly correlated to wear. Multivariate regression analysis indicated that male gender, femoral off-set, and Hylamer were significantly correlated to wear. Based on the wear and activity data from the 24 hip replacements with standard polyethylene, the average volumetric wear rate per million cycles with a 70 kg patient weight was 30 mm3. This unique in vivo result can be considered a target wear rate for standard polyethylene in hip simulator studies.