Abstract
Total knee replacement (TKR) is one of the most successful procedures in orthopaedic surgery. Although originally limited to more elderly and less active individuals, the inclusion criteria for TKR have changed, with ever younger, more active and heavier patients receiving TKR.
Initially, tibial components were cemented all-polyethylene monoblock constructs. Subsequent long-term follow-up studies of these implants have demonstrated excellent durability in survivorship studies out to 20 years. Aseptic loosening of the tibial component was one of the main causes of failure in these implants. Polyethylene wear with osteolysis around well-fixed implants was rarely (if ever) observed. Cemented metal-backed nonmodular tibial components were subsequently introduced to allow for improved tibial load distribution and to protect osteoporotic bone. Long-term studies have established that many one-piece nonmodular tibial components have maintained excellent durability.
Eventually, modularity between the polyethylene tibial component and the metal-backed tray was introduced in the mid-80s mainly to facilitate screw fixation for cementless implants. These designs also provided intra-operative versatility by allowing interchange of various polyethylene thicknesses, and also aided the addition of stems and wedges. Other advantages included the reduction of inventory, and the potential for isolated tibial polyethylene exchanges.
Since the late 1980's, the phenomena of polyethylene wear and osteolysis have been observed much more frequently when compared with earlier eras. The reasons for this increased prevalence of synovitis, progressive osteolysis, and severe polyethylene wear remain unclear, but there is no question that it was associated with the widespread use of both cementless and cemented modular tibial designs.
Mayo Data: Modular versus All Polyethylene Tibial Components in Primary TKA: The study population included 10,601 adult (>18 years) patients with 14,524 primary TKR procedures performed at our institution between 1/1/1988 and 12/31/2005. Mean age was 68.7 years and 55% were female. Overall revision rates and revisions for loosening, wear/osteolysis were compared across different designs using Cox proportional hazards regression models adjusting for age, sex, calendar year and body mass index (BMI). Over an average 9 years follow-up, a total of 865 revisions, including 252 tibia revisions were performed, corresponding to overall survival of 89% (Confidence intervals (CI): 88%, 90%) at 15 years. In comparison to metal modular designs, risk of tibial revision was significantly lower with all-poly tibias (HR 0.3, 95% CI: 0.2, 0.5). Overall, posterior cruciate-retaining (CR) designs performed better than the posterior-stabilised (PS) designs (p=0.002). With any revision as the endpoint, there were no significant differences across the 18 designs examined. Similarly, there were no significant differences across the 18 designs when we considered revisions for aseptic loosening, wear, and osteolysis. Among patient characteristics, male gender, younger age, higher BMI were all significantly associated with higher risk of revisions (p<0.008).
Available data support the use of nonmodular tibial designs in TKR in order to prevent or reduce the chance of backside wear, third body particles from resulting metallic debris and associated polyethylene induced osteolysis. In all patients, (not just older individuals) use of an all polyethylene tibial component is an attractive and more cost effective alternative, and is associated with the best survivorship and lowest risk of revision.
