A Biomechanical and Evolutionary Perspective of the Menisci Indicate They Are Not Shock Absorbers

Human bipedalism entails high-magnitude, short-duration impact transient forces that occur every heel strike. These transient forces, or 'shock', are attenuated as they propagate up the body, and many structures have been proposed as 'shock absorbers' within the body. The medial and lateral menisci of the knee are a pair of fibrocartilaginous structures that have often been cited as shock absorbers. However, despite the frequency with which this claim is made in scientific literature, medical education and clinical practice, there is little primary data that supports the notion that the menisci function as shock absorbers. Here we compile evolutionary and biomechanical data to critique this oft-cited notion. First, we show that the origin of this idea draws on a limited number of studies, each with caveats that make that evidence for a shock absorptive function dubious. We then review evolutionary and comparative evidence to show that human menisci are unremarkable in morphology compared with most other tetrapods. Humans possess two, crescentic-shaped menisci that are similar in morphology to most mammals and many tetrapods. This suggests that the null hypothesis of meniscal function is one of similarity among tetrapods. Furthermore, we show that among tetrapods, shock during locomotion is uncommon; humans stand out as one of the only tetrapods that regularly experience high levels of shock during locomotion. Thus, a shock-absorption function does not explain the origin of menisci, nor are human menisci specialized in any way that would explain this unique function during human gait. We further assess the mechanical material properties of menisci and show that the menisci overwhelmingly behave like springs and are poorly suited for energy absorption. We compiled data on phase angle and loss tangent, two properties used to assess energy absorption or release. Loss tangents of the menisci are typically between 0.1-0.2, close to an ideal spring (0.0) as opposed to an energy absorbing damper (>1.0). Similarly, phase angles were generally <10°, more similar to an idealized spring value of 0°, as opposed to a pure damper at 90°. Thus, when deformed menisci release most of this energy as opposed to shock absorbers, which dissipate energy. Finally, we integrated data on material properties of the meniscus and loading data from previous studies to estimate actual meniscal energy absorption in comparison to other known energy-absorbing structures in the body. The heel pad, a well-known contributor to absorbing energy from heel strike, absorbed 50-69% (0.70 +/- 0.11 J) of impact loading when using realistic loading conditions in vitro. All knee structures combined absorbed 0.12 +/- 0.01 J of impact energy, only 6-17% of the energy absorbed by the heel pad alone. Available loading data suggest that energy absorption of the menisci alone likely account for only ~4% of all energy absorption at the knee. Thus, our estimates of the total shock absorption capacity of the menisci is essentially negligible. These findings indicate that the menisci did not originally develop for shock-absorption in tetrapods and did not develop a specialized shock absorbing function in humans.