Researchers compared medial-pivot and ultracongruent prostheses in patients with advanced knee osteoarthritis, tracking muscle status with localized bioelectrical impedance analysis, a technique that measures the electrical properties of tissue directly rather than relying on questionnaires or strength tests alone.
Total knee arthroplasty is the treatment of choice for patients with advanced knee osteoarthritis who do not respond to conservative management, and several prosthetic designs exist, each based on different biomechanical principles. Traditionally, the most widely used designs were cruciate-retaining implants, which preserve the posterior cruciate ligament, and posterior-stabilized implants, which replace its function through a post-cam mechanism. Both have drawbacks: cruciate-retaining designs require a functionally competent and appropriately tensioned ligament, making balancing technically demanding, while posterior-stabilized designs increase prosthetic constraint and polyethylene wear, with potential complications including post-cam dislocation, patellar clunk syndrome, and greater bone resection.
To address these limitations, newer insert designs have emerged. Ultracongruent inserts enhance anteroposterior stability through greater articular conformity without a post-cam mechanism, whereas medial-pivot inserts aim to maintain a stable medial compartment with controlled lateral rollback during flexion. Because these biomechanical differences may produce distinct kinematic patterns and muscle activation demands, the researchers asked whether implant design influences postoperative quadriceps recovery, a question of real consequence given that patients with knee osteoarthritis already exhibit preoperative quadriceps atrophy due to pain, arthrogenic muscle inhibition, and reduced functional loading, and that quadriceps atrophy often worsens after surgery.
The study, conducted between 2022 and 2024, enrolled 40 patients scheduled for primary total knee arthroplasty and randomized them 1:1 to receive either a medial-pivot Evolution implant or an ultracongruent Columbus implant. The final analysis included 30 patients, 15 per group. Patients remained blinded to their implant allocation throughout the study, physical therapists were blinded, and statistical analyses were performed by an analyst blinded to group allocation, although the principal investigator who performed measurements was not blinded, and implant design could potentially be inferred from postoperative radiographs. The trial was not prospectively registered.
The primary outcome was postoperative quadriceps muscle recovery assessed longitudinally by reactance of the rectus femoris, vastus medialis, and vastus lateralis, measured preoperatively and at 6 and 12 months after surgery. In localized bioelectrical impedance analysis, resistance relates to tissue hydration while reactance reflects muscle cell integrity. Measurements followed a standardized tetrapolar protocol with electrodes placed at anatomical landmarks on each muscle, using a phase-sensitive device delivering a constant 245 microampere sinusoidal current at 50 kHz. The contralateral limb served as an internal reference.
The results showed a consistent temporal pattern in both groups. Resistance values remained stable across all time points with no differences between implant designs. Reactance showed a transient decrease at 6 months followed by recovery at 12 months, with values returning to or exceeding baseline in the medial-pivot group and a similar, less pronounced pattern in the ultracongruent group. No intergroup differences were detected at any time point. Analysis of covariance demonstrated that baseline reactance was associated with 12-month reactance values across all three muscle groups, whereas implant design had no significant effect, a finding that persisted in sensitivity analyses adjusted for sex and body mass index.
Radiographic and clinical outcomes told the same story. Postoperative range of motion was similar between groups, at 108.3 degrees in the medial-pivot group versus 105.7 degrees in the ultracongruent group, and adjustment for baseline motion confirmed no effect of implant design. Both designs showed significant improvement in WOMAC and Knee Society Score outcomes over time, with the greatest gains in the first six months, and no intergroup differences at any time point. Radiographic parameters including joint line height, patellar height, posterior condylar offset ratio, and tibial slope showed no significant between-group differences, and mechanical axis alignment improved significantly in both groups.
The authors are careful about what these findings do and do not mean. The study was designed as an exploratory superiority trial and was not powered or designed to establish equivalence or non-inferiority, so the absence of statistically significant differences should not be interpreted as evidence of clinical equivalence between the implants. The sample size calculation was based on surrogate longitudinal changes in reactance rather than an expected between-group difference, 10 of the 40 randomized participants were excluded from the complete-case analysis, and no adjustment for multiple comparisons was applied to exploratory analyses. L-BIA-derived reactance should also be interpreted as a bioelectrical surrogate of muscle status rather than a direct measure of muscle strength, morphology, or functional recovery, and the technique was not validated against such direct measures in this study.
Nevertheless, the study is, to the authors’ knowledge, the first to directly compare medial-pivot and ultracongruent designs while evaluating quadriceps muscle adaptation with localized bioimpedance, offering a tissue-level physiological perspective that conventional clinical scores, which primarily reflect pain and patient-reported function, cannot capture. The finding that preoperative reactance was consistently associated with postoperative reactance suggests that preoperative muscle status may shape recovery trajectories, although the exploratory analyses do not establish a predictive or causal relationship. The authors also note that the nearly identical postoperative tibial slope between groups, 5.2 versus 5.1 degrees, may have limited geometry-related differences in extensor mechanics, an interpretation they label speculative since knee kinematics were not directly assessed.
For patients and surgeons, the practical message is one of reassurance tempered by caution: during the first postoperative year, neither implant design showed an advantage in quadriceps bioelectrical recovery, clinical outcomes, or radiographic parameters, and baseline muscle status emerged as the factor most consistently linked to one-year bioelectrical outcomes. Given the limited sample size and single-centre design with 12 months of follow-up, the authors conclude that these results should not guide implant selection, and that larger, adequately powered comparative studies are needed to determine whether clinically meaningful differences in postoperative muscle recovery exist between medial-pivot and ultracongruent implants.
Subject of Research: Quadriceps muscle recovery after total knee arthroplasty with medial-pivot versus ultracongruent implants, assessed by localized bioimpedance
Article Title: Quadriceps muscle status after medial-pivot versus ultracongruent total knee arthroplasty using localized bioimpedance analysis: a randomized exploratory study
Article References: Pedemonte-Parramón, G., Nescolarde, L., GarcÃa-Oltra, E., López, V., Dzidzishvili, L., & Hernández-Hermoso, J. A. (2026). Quadriceps muscle status after medial-pivot versus ultracongruent total knee arthroplasty using localized bioimpedance analysis: a randomized exploratory study. Arthroplasty, 8(1), Article 68. https://doi.org/10.1186/s42836-026-00429-z
Image Credits: AI Generated
DOI: 10.1186/s42836-026-00429-z
Keywords: total knee arthroplasty, quadriceps muscle, bioimpedance, medial pivot implant, ultracongruent implant, reactance, randomized trial, knee osteoarthritis, WOMAC, Knee Society Score, rehabilitation, implant design
News Source: Ophelia Keating. (October 29, 2026). Bioimpedance Finds No Quadriceps Recovery Gap Between Two Knee Implant Designs. Scienmag.
