“The shock absorbing truth about meniscus” by Andrews et al. (2011) initiated a controversial discussion about the assumption, that the meniscus acts as a shock absorber inside the knee joint. Although there is evidence in literature indicating the shock absorbing potential of meniscus tissue, there is no publication objectively corroborating this assumption/hypothesis. Therefore, the aim of this biomechanical in-vitro investigation was to enlighten the question whether the meniscus acts as a shock absorber in the knee joint or not.
The soft tissue of fourteen porcine knee joints were removed leaving the capsuloligamentous structures intact. The joints were consecutively mounted in 45° neutral knee flexion in a validated drop tower setup. Six joints were exposed to an impact load of 3.54J. Then the loss factor (??), which is used to interpret the damping behavior of the meniscus within the knee joint under impact loads was calculated. Then, the setup was modified in a way to allow for application of sinusoidal loads by integrating the setup frame into a fatigue testing machine. Subsequently, the eight remaining knee joints were exposed to a dynamic mechanical analysis (DMA) applying 10 frequencies from 0.1Hz to 10Hz at a static load of 1210N and a superimposed sinusoidal load of 910N (2.12xBW). The force (F) and deformation (l) were continuously recorded at a sampling rate of 1kHz and the damping factor tand was calculated. For both experiments four different meniscus states (intact, medial posterior root avulsion, medial meniscectomy, complete bilateral meniscectomy) were used to investigate the shock absorbing contribution of the meniscus. Non-parametric analyses were used to determine statistical differences, while p=0.05 was considered significant.
During the drop tower experiments the intact state possessed a loss factor of ?? = 0.1. Except for the root avulsion state (-15%, p=.12) the loss factor decreased significantly (p<.046) by a maximum of -68% for the total meniscectomy state (p=.028) when compared to the intact state. Sinusoidal DMA testing revealed, that knees with an intact meniscus had the highest damping factors, ranging from .10 to .15. Any surgical manipulation lowered the damping factor: Medial meniscectomy resulted in a reduction of 24%, while the resection of both menisci lowered tand by 18% compared to the intact state.
The results of this biomechanical study shows that the damping factor, and thus the shock absorbing ability of a knee joint is lower the more meniscal tissue is resected. Therefore, we can conclude that the meniscus significantly contributes to the shock absorption of the knee joint not only during impact loads but also during sinusoidal loads representing activities of daily living. The findings may have an impact on the rehabilitation of young, meniscectomized patients who want to return to sports. Consequently, such patients are exposed to critical loads at the articular cartilage, especially when performing sports with recurring impact loads transmitted through the knee joint surfaces.