Clinical Results and Failure Mechanisms of a Nonmodular Constrained Knee Without Stem Extensions

Abstract

In the setting of persistent knee instability despite appropriate ligament balancing for primary total knee arthroplasty, most surgeons advocate the use of an implant with increased articular constraint. These implants are commonly supplemented with stem extensions to improve stress transfer and decrease the risk of aseptic loosening. However, disadvantages exist with the use of stem extensions, including increased cost, intramedullary invasion, and diaphyseal pain. The objectives of this study were to (1) compare the clinical results as assessed by the Knee Society, Hospital for Special Surgery, and SF-12 scores, (2) determine the incidence of failure as defined by the need for a revision procedure, and (3) to analyze the causes or modes of failure of a nonmodular constrained condylar knee without the use of stem extensions versus a conventional, posterior-stabilized design. From 2002 to 2007, 190 TKAs were implanted using a primary, nonmodular constrained (NMC) prosthesis without stem extensions. During the same time period, clinical data were available for 140 TKAs implanted using a standard, posterior-stabilized (PS) design. Preoperative demographic data was reviewed, in addition to the rate and reason for revision in each cohort. Clinical data included HSS, Knee Society, and SF-12 scores at the latest follow-up, and the results of the NMC and PS cohorts were statistically compared using a Student's two-tailed t test. The mean age of patients in the NMC cohort was 72.3 ± 10.2years, and the mean length of follow-up was 7.3 ± 2.1years. The mean age of the PS cohort was 67.1 ± 8.7years, with a mean follow-up of 6.1 ± 2.2years. No statistically significant differences in the HSS, Knee Society, or SF-12 scores were appreciated between the two cohorts. The revision rate in the NMC cohort was 4.2% compared to 4.3% for the PS cohort. The most common cause of failure in the NMC cohort was femoral component loosening, all of which occurred when Palacos cement was used for fixation. NMC components (55.6%) implanted with Palacos cement failed due to femoral component loosening. In contrast, all PS components requiring revision were revised for persistent instability. At mid-term follow-up, NMC prostheses without stem extensions have excellent clinical results and are a viable option for patients with ligamentous instability. The use of Palacos cement in this scenario was associated with a high rate of femoral component loosening, possibly due to the decreased intrusion depth of Palacos when compared to Simplex cement.