No abstract

Five hundred ninety-eight consecutive primary low contact stress total knee replacements were done in 502 patients between 1985 and 1990. Clinical review was available for 495 knees (406 patients), 228 knees with meniscal-bearing prostheses and 267 knees with rotating-platform prostheses. The average followup was 12 years (range, 10-15 years). The average postoperative knee and functional scores were 87 points and 75 points, respectively. The average postoperative range of motion was 110 degrees. Fifty-six knees (11%) required revision for excessive wear of the tibial insert (41), dislocation (10), patellar polyethylene breakage (one), component loosening (one patellar, one tibial), and infection (two). During revision, osteolysis (20 knees), patellar polyethylene failure (33), and femoral component fracture (one) were seen. The overall survivorship was 88.1% at 15 years using Kaplan-Meier analysis. The survival rate was 83% for the meniscal-bearing prostheses and 92.1% for the rotating-platform prostheses. The Low Contact Stress mobile-bearing knee prosthesis has no superiority over that of fixed-bearing knees, especially for the meniscal-bearing design in prevention of polyethylene failure or revision. Based on the results of this study, the use of the LCS meniscal-bearing prosthesis does not appear to be justified.

Osteolysis induced by ultrahigh molecular weight polyethylene wear debris has been recognized as the major cause of long-term failure in total joint arthroplasties. In a previous study, the prevalence of intraoperatively identified osteolysis during primary revision surgery was much higher in mobile bearing knee replacements (47%) than in fixed bearing knee replacements (1 3%). We postulated that mobile bearing knee implants tend to produce smaller sized particles. In our current study, we compared the particle size and morphology of polyethylene wear debris between failed mobile bearing and fixed bearing knees. Tissue specimens from interfacial and lytic regions were extracted during revision surgery of 10 mobile bearing knees (all of the low contact stress (LCS) design) and 17 fixed bearing knees (10 of the porous-coated anatomic (PCA) and 7 of the Miller/Galante design). Polyethylene particles were isolated from the tissue specimens and examined using both scanning electron microscopy and light-scattering analyses. The LCS mobile bearing knees produced smaller particulate debris (mean equivalent spherical diameter: 0.58 tiin in LCS, 1.17 pm in PCA and 5.23 pm in MIG) and inore granular debris (mean value: 93% in LCS, 77%) in PCA and 15%) in MIG).

Polyethylene wear of bearing components is the most common long-term complication in total knee arthroplasty. One would anticipate differing kinematics would generate different wear patterns (including wear type, degree, and symmetry) on the articulating surface of mobile-bearing and fixed-bearing inserts. Because mobile-bearing designs facilitate movement of the insert relative to the tray when the knee rotates, we hypothesized mobile-bearing designs would reduce the incidence of rotational asymmetric wear. We examined 51 worn tibial inserts, including 15 from mobile-bearing rotating-platform posterior-cruciate-sacrificing dished prostheses and 36 from fixed-bearing posterior-cruciate-retaining flat prostheses, which were retrieved at revision surgery with an average implantation time of 115 months. We divided wear types into low-grade wear (burnishing, abrasion, and cold flow) and high-grade wear (scratching, pitting, metal embedding, and delamination) to assess wear degree of polyethylene. To assess symmetry of wear, the insert surface was divided into medial and lateral sides and each side was further divided into three equal zones along the anteroposterior direction. Low-grade wear was more common in mobile-bearing knees, whereas high-grade wear was more common in fixed-bearing knees. We identified no internal/external rotational asymmetric wear or anteroposterior asymmetric wear in mobile-bearing knees.