This study investigated the difference in proximal tibial cortical strain distribution using a fixed or mobile bearing design for TKA. Eight fresh frozen human cadaver tibias were used. The strain magnitude and distribution on the anterior cortex of the proximal tibia during axial and rotational loading of the knee were measured with a quantitative full-field strain measurement technique (Electronic Speckle Pattern Interferometry). First, strain distributions of the intact knee were acquired. Subsequently, strain distributions after implantation of conventional and mobile bearing PCL retaining total knee implants (Scorpio®) were measured Under each loading condition, the minimum principal strain was greater in magnitude as compared to the maximum principal strain. Under 1,500 N axial loading, the resulting minimum principal strain magnitude and orientation was nearly identical between the mobile bearing configuration(500 ± 287m;e;), and the fixed bearing configuration (500 ± 286m;e;). In response to 10° internal rotation, this strain increased to 782 ± 371m;e; and 1000± 389m;e; for the mobile and fixed tibial component, respectively. In 10° external rotation, minimal principal strain decreased to 421 ± 233m;e; for the mobile bearing, but increased to 632 ± 293m;e; for the fixed bearing. These differences between mobile and fixed bearing scenarios were highly statistically significant. For this in-vitro study under exact controlled loading conditions the mobile bearing design induced less strain in the proximal tibia than the fixed bearing tibial component. The difference in strain levels may be of importance for bone remodeling and osseointegration.
Menisci contribute to load distribution, damping and stabilization of the knee. Meniscal tears are a common injury in the young and active population during combined axial loading and twisting of the knee. The in situ effect of combined axial loading and knee rotation on hoop strain in the medial meniscus of human cadaveric specimens was examined. Four fresh-frozen human cadaver knees were rigidly potted in base fixtures. Muscle tissue was removed, and the joint capsule and ligamentous structures were preserved. Through two arthrotomies, strain sensors (DVRTs) were placed in the peripheral border into the mid-substance of the medial meniscus. These DVRTs captured circumferential hoop strains e;AM, e;PM in the anteromedial and posteromedial medial meniscus. Each specimen was mounted in a knee loading simulator, driven by a biaxialmaterial test system and were axial loaded with 1,4 kN. While maintaining axial load, ± 10° tibial rotation (IR, ER) was subsequently applied at 1°/s. Tests were conducted for knees flexion between 10° and 60° in 10° intervals. Strain reports e;AM and e;PM were highly similar for any given test. Therefore, they were averaged to express meniscal hoopstrain as e;AVG. At 30° flexion, 1.4 kN axial load yielded e;AVG =0.9%±0.4%. ER resulted in a significant strain increase (2.1%±0.8%) (p=0.003). IR caused a decrease (0.2%±0.7). At 60° knee flexion, 10° ER induced significantly less strain (1.3%±0.9%) as compared to the 10° flexed knee(2.8%±1.3). For knee flexion from 10° to 50°, combined ER and axial loading-caused significantly higher strain as compared to axial loading alone. This study documents for the first time strain in the medial meniscus under combined axial and torsional loading. The finding that meniscal strain can increase over two-fold during 10°external rotation has implications for injury biomechanics and meniscal repair strategies.
