The kinematics and stability of single‐radius versus multi‐radius femoral components related to Mid‐range instability after TKA

Abstract

There continues to be some dissatisfaction with the function of total knee arthroplasties (TKA). ''Mid-range instability'' has been linked to multi-radius femoral components allowing transient ligament slackness and instability during knee flexion. Singleradius designs have been introduced to avoid this. We compared the kinematics and stability of eight natural knees versus multi-radius and single-radius TKAs in vitro. The loading conditions imposed across the range of active knee extension were anterior-posterior drawer forces, internal-external rotation torques, and varus-valgus moments. Significant differences were not found between the biomechanical behavior of the two TKAs. Both were significantly different from the natural knee in allowing greater anterior drawer laxity near extension, probably caused by excision of the anterior cruciate ligament, but no difference occurred beyond 308 flexion. No differences were found for any of the other degrees-of-freedom of movement. A geometric analysis suggested that the multi-radius design may tense the MCL more than the single-radius in mid-flexion, contrary to expectation. These kinematic and stability tests did not find mid-range instability of the knees, and so they could not demonstrate enhanced mid-range stability of the single-radius TKA over the older multi-radius implant. This suggests that mid-range instability may relate to unrecognized ligament laxity during surgery, rather than being inherent to a specific feature of implant design. ß

The influence of polyethylene bearing thickness on the tibiofemoral kinematics of a bicruciate retaining total knee arthroplasty

“…Deviations in the bearing thickness from the balanced configuration resulted in increased alterations to knee kinematics. Interestingly, previous studies have noted similar differences in knee kinematics following arthroplasty for both single-radius and multi-radius TKA designs [23,24]. The hypothesis that a varus constellation as introduced by the insert would result in more normal kinematics was not proven to be correct.…”

“…In theory, preservation of the ACL with BCR TKA should result in tibiofemoral kinematics that closely mimic the native knee, however, there are relatively few studies documenting this [8,10,24]. Moreover, the chosen thickness of the polyethylene bearing can have important implications on implant wear, alignment, knee kinematics, and ligament tensioning, among other factors, for both total and unicondylar knee arthroplasty [16,30,31].…”

Clinical biomechanics of instability related to total knee arthroplasty

“…For example a clinically-orientated approach may define instability 212 subjectively as the patient"s complaint of the knee "giving-way", or more objectively from 213 measured increases in tibiofemoral joint laxity when a normal knee is modified by injury or 214 arthroplasty (Stoddard et al, 2013). An engineer, however, may repeat the same path of 215 motion after modification and record the decrease in required displacing force as an 216 objective measure of stability (Rudy et al, 1996).…”

“…Performing a varus/ valgus stress test at 0° and 90° flexion may give a 296 "feeling" of laxity at these angles, but does not address the balance at angles between these 297 limits (D'Lima et al, 2011). This is problematic, as an implanted TKA may experience "mid-298 range instability" if the soft tissue is not tense within 0-90° flexion (Stoddard et al, 2013; 299…”

The superficial medial collateral ligament is the primary medial restraint to knee laxity after cruciate-retaining or posterior-stabilised total knee arthroplasty: effects of implant type and partial release

“…The tibia was aligned centrally in the bone pot using a jig with a 82 pointer that located the centre of the tibia as between the tips of the tibial spines [2]. To access 83 the tibial plateau, a midline incision was made to the skin and subcutaneous fat layer, followed 84 by a medial parapatellar arthrotomy [28]. The arthrotomy was opened and resutured at each stage of the experiment.…”