The overall goal of this dissertation is to determine, through a mechanical perspective, how meniscal damage occurs and how subsequent partial meniscectomy can lead to the development of osteoarthritis. Individuals who suffer meniscus injuries suffer both short-term and long-term consequences, from painful joint-locking to premature osteoarthritis. This thesis provides potential mechanisms behind degenerative damage to the posterior horn of the medial meniscus. Once this damage has occurred, this thesis provides an explanation for the changes that occur during walking after partial medial meniscectomy and then suggests a mechanism for why these subjects are at risk for developing premature osteoarthritis.
Patterns of regional variation in transverse plane meniscal displacement during positions representative of normal gait were studied using cadaveric knees in a test bed developed to be compatible with magnetic resonance imaging (MRI). The results demonstrated that during the stance phase of walking, the lateral meniscus appears to displace with the femoral condyle while the medial meniscus, especially at the posterior horn, displaced minimally. The relatively low movement of the medial posterior horn suggests that this region sustains substantial cyclic stress to restrain the normal posterior displacement of the medial femoral condyle at heel strike of each gait cycle. These results help explain the chronic patterns of damage to the posteromedial horn.
Gait analysis of subjects who had undergone partial medial meniscectomy revealed that during stance phase, these subjects had tibias that were externally rotated by 3.2° compared to their healthy contralateral limbs. Their operated limbs also exhibited reduced peak flexion and extension moments compared to their healthy limbs. The altered rotational position found likely causes changes in tibiofemoral contact during walking and could cause the type of degenerative changes found in the articular cartilage following meniscal injury.
A joint level numerical model was developed to look further at the results from the first two studies. This 3D FE model looked at the effects of the previously noted tibial rotational shift and an increased adduction moment on the meniscal strains during a simulated portion of the gait cycle. The increased adduction moment resulted in increased strains of both the anterior and posterior horn of the medial meniscus. The external tibial rotational shift resulted in decreased strains in the posterior medial horn and increased strains in the anterior medial horn. The lateral meniscus exhibited much lower strains than the medial meniscus and minimal changes under the various loading conditions. It appears that the shift towards external tibial rotation seen clinically after partial medial meniscectomy is not likely to cause medial meniscal damage and therefore, the kinematic shift might be a result of the surgery.
Finally, the FE model was then used to look at the effects of varying levels of partial medial meniscectomy on the rotational resistance of the tibia during a portion of the stance phase of gait. Additionally, the stresses and contact pressures in the articular cartilage of the knee were determined. The results showed that the more meniscal tissue that was removed, the greater the loss of rotational stability that was provided by the geometry. Additionally, the location of removal of meniscal tissue was important as the central rim resulted in a greater loss of rotational stability than removal from the anterior or posterior horns. Similarly, the more meniscal tissue that was removed, the higher the increase in contact pressures and stresses within the articular cartilage. Thus, the results indicate that it may simply be the resulting change in meniscal geometry after partial medial meniscectomy that results in the kinematic shift towards external tibial rotation we see in individuals who have undergone partial meniscectomy. The combination of the change in contact location along with the potential increase in cartilage stresses seen post-meniscectomy may place these individuals at higher risk for developing premature osteoarthritis.
Taken together, the results of these studies provide potential mechanisms for the development of damage to the medial meniscus and then the subsequent changes that may lead to the development of premature knee osteoarthritis after partial meniscectomy. Additionally, the insights provided by this work have direct implications into treatments for meniscal injuries in order to prevent degenerative changes in articular cartilage.
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