The intact meniscus and an artificial meniscus prosthesis both act as a shock absorber in the knee joint

Shock absorption is often mentioned as one of the functions of the meniscus, but consensus is lacking. The first objective of this study was to adopt adequate test methods for evaluating shock-absorbing and damping characteristics of the human knee joint and, more specifically, the meniscus. Secondly, the shock-absorbing effect of a novel medial meniscus prosthesis was investigated.

Six cadaveric knee joints were subjected to a droptower experiment, where impact energies of 1.8, 3.5 and 5.3 J were respectively applied on the femur. Outcome measures were the peak force measured under the tibia and loss factor η, which represents the ratio between dissipated and elastically stored energy. Both outcomes were compared between different medial meniscal conditions: Native meniscus, posterior root tear, total meniscectomy and meniscus prosthesis. In addition, all conditions were evaluated during sinusoidal loading at different frequencies (dynamic mechanical analysis; DMA), where a comparable loss factor tan δ was calculated from the phase lag between applied force and measured displacement. Finally, energy loss was directly quantified using the loop area of the hysteresis curves.

Mean tibial peak forces increased after meniscectomy by up to 7%, although the difference was not statistically significant at the lowest impact energy. The meniscus prosthesis reduced the impact force in all cases, to the native level or lower. Impact loss factor η indicated similar effects on knee joint damping but yielded less significant differences. In the DMA, no significant differences in loss factor tan δ were found between the native menisci and the meniscus prosthesis. After meniscectomy, the DMA loss factor was always lowest, although not all differences were statistically significant. Meniscectomy also resulted in a reduction of energy loss compared to the intact condition. The prosthesis was able to increase energy loss, but not fully to the native level.

From a clinical and biomechanical perspective, the meniscus has multiple different functions in the knee joint. This study demonstrated that the native meniscus and the meniscus prosthesis are both shock-absorbers, with the prosthesis performing better at lower impact energies. Meniscectomy reduces damping of the knee joint during cyclic loading, whereas the meniscus prosthesis has the capacity to largely restore the damping characteristics of the knee joint.