Total knee arthroplasty as we know it today, for the vast majority of patients, is really a resurfacing of the knee. Approximately a centimeter of bone is shaved off all of the surfaces of each bone and replaced with metal and/or plastic. Patients with deformity, bone loss, and/or ligamentous instability of the knee present substantial challenges that cannot be managed with a resurfacing replacement. These patients require the implant to not only resurface the joint but also accommodate the missing support. Hence, constrained prostheses more typical of a revision surgical procedure are necessary. It seems logical that these more constrained knee prostheses would be unlikely to obtain the longevity that patients with standard total knee prostheses enjoy. However, data to verify such predictions have not been available, even from sources such as the Swedish Knee Arthroplasty Register1.
The study by Martin et al. provides data to help to clarify these issues. This study compared the results of more constrained prostheses in patients who underwent primary arthroplasty, namely varus and valgus constrained and rotating-hinge prostheses, with the results of standard primary total knee replacements. It was a long-term, retrospective study in a large group of patients who underwent primary total knee arthroplasty. Compared with patients who underwent routine primary total knee arthroplasty, the patients who required constrained implants were younger and had lower body mass indexes, and there was a greater proportion of female patients. These variables, aside from constraint, would be expected to negatively impact the survival of the prostheses. The reoperation-free survival was lowest for the patients with rotating-hinge prostheses, highest for the patients with routine total knee replacements, and somewhere in between for the patients with varus and valgus constrained implants. Wound complications caused reoperation more frequently in the two complex total knee arthroplasty groups, as might be expected in more deformed knees. The rate of revision at 10 and 20 years for wear and osteolysis problems was higher (but not significantly higher) for the two complex total knee arthroplasty groups compared with the routine primary total knee arthroplasty group. These slightly higher rates could possibly be attributed to age and body mass index differences and suggest that design differences are not likely the cause of the higher revision rates.
Overall, the complex primary total knee replacements survived surprisingly well at 10 and 20 years. This is reassuring to the surgeon presented with a deformed or unstable knee requiring knee replacement. The surgeon can provide the patient with a relatively optimistic outlook.
The Swedish Knee Arthroplasty Register has demonstrated a revision rate of about 1% per year, or a 10% cumulative rate at 10 years, and is showing some improvement in survival without revision in recent years. The present study by Martin et al. covered 20 years and had a similar failure rate for all-cause reoperation of 12% at 10 years for routine primary knee replacement. The reoperation rate for the varus and valgus constrained prostheses was about twice as great, or 2% per year, but the rotating-hinge prostheses required reoperation at about 5% per year for the first 10 years. All-cause revision rates were much less for all implant types at both 10 and 20 years, so that even a rotating-hinge implant has a 40% chance of functioning at 20 years without revision. It is apparent that the more constrained total knee arthroplasty designs should be reserved for the patient who needs the additional constraint.
This means that a rotating-hinge or varus and valgus constrained implant is more likely to have failed at 10 and 20 years than a routine total knee replacement. The complex total knee arthroplasty glass is half empty. However, the patient is likely to have much improved function in the replaced knee with the constrained prosthesis for the time that it lasts. The complex total knee arthroplasty glass is half full.
↵* The author indicated that no external funding was received for any aspect of this work. The Disclosure of Potential Conflicts of Interest form is provided with the online version of the article.
- Copyright © 2016 by The Journal of Bone and Joint Surgery, Incorporated