The unloading effect of an extra-articular absorber system on the knee joint medial compartment was compared with high tibial osteotomy (HTO) under physiological conditions in vitro.
Seven fresh-frozen cadaveric knees were used to test isokinetic flexion-extension motions under physiological loading using a biomechanical knee simulator. Tibiofemoral area contact and peak contact pressures were measured using pressure-sensitive film in the untreated medial compartment. Pressures were measured after KineSpring System implantation and HTO (5° and 10° correction angles) performed with an angular-stable internal fixator (Tomofix).
Implantation of the unloading device resulted in significantly decreased medial compartment area contact pressure (Δ0.02 ± 0.01 MPa, p = 0.001) and peak contact pressure (Δ0.3 ± 0.1 MPa, p = 0.001) compared with the first test cycle results in the untreated knee. HTO significantly decreased the pressure (p = 0.001). Compared with the first test cycle, HTO (5° correction angle) decreased the mean contact pressure by Δ0.03 ± 0.01 MPa and peak contact pressure by Δ0.3 ± 0.01 MPa. With a 10° correction angle, HTO decreased contact pressure by Δ0.04 ± 0.02 MPa and peak contact pressure by Δ0.4 ± 0.1 MPa compared with that at the 5° correction angle.
Implantation of an extra-capsular unloading device resulted in a significant unloading effect on the medial compartment comparable to that achieved with HTO at 5° and 10° correction angles. Thus, implantation of an extra-articular, extra-capsular absorber could become the method of choice when treating patients with unicompartmental osteoarthritis that cannot be adequately treated by HTO because of their straight-leg axis.