Abstract
Total knee replacement (TKA) is one of the most successful procedures in orthopaedic surgery. Although originally limited to more elderly and less active individuals, the inclusion criteria for TKA have changed, with ever younger, more active and heavier patients receiving TKA. This broadening of indications coincided with the widespread adoption of modular cemented and cementless TKA systems in the 1980's, and soon thereafter wear debris related osteolysis and associated prosthetic loosening became major modes of failure for TKA implants of all designs.
Initially, tibial components were cemented all polyethylene monoblock constructs. Subsequent long-term follow-up studies of some of these implant designs have demonstrated excellent durability in survivorship studies out to twenty years. While aseptic loosening of these all polyethylene tibial components was a leading cause of failure in these implants, major polyethylene wear-related osteolysis around well-fixed implants was rarely (if ever) observed.
Cemented metal-backed nonmodular tibial components were first introduced to allow for improved tibial load distribution and protection of the underlying (often osteoporotic) bone. 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 intraoperative versatility by allowing interchange of various polyethylene thicknesses, and also aided the addition of stems and wedges.
Modular vs. All Polyethylene Tibial Components in Primary TKA: Kremers et al. reviewed 10,601 adult (>18 years) patients with 14,524 condylar type primary TKA procedures performed at our institution between 1/1/1988 and 12/31/2005 and examined factors effecting outcome. The mean age was 68.7 years and 55% were female. 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 polyethylene tibias (HR 0.3, 95% CI: 0.2, 0.5). 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, osteolysis. Among patient characteristics, male gender, younger age, higher BMI were all significantly associated with higher risk of revisions (p<0.008).
In a more recent review from our institution of over 11, 600 primary TKA procedures, Houdek et al. again showed that all polyethylene tibial components had superior survivorship vs. metal backed designs, with a lower risk of revision for loosening, osteolysis or component fracture. Furthermore, results for all polyethylene designs were better for all BMI subgroups except for those <25 BMI where there was no difference. All polyethylene results were also better for all age groups except for those under age <55 where there again was no difference.
Finally, in a recently published meta-analysis of 28 articles containing data on 95,847 primary TKA procedures, all polyethylene tibial components were associated with a lower risk of revision and adverse outcomes. The available current data support the use of all polyethylene tibial designs in TKA in all patients regardless of age and BMI. 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 better survivorship and lower risk of revision than seen with modular metal backed tibial components.
