Osteoarthritis (OA) is a major health care problem affecting an estimated 26.9 million US adults in 2005 over age 25 . It is an important and growing reason for disability and lost work productivity [1-4]. Damage to fibro-cartilaginous structures about the knee and the hip (the meniscus and the hip labrum) is commonly associated with osteoarthritis development. Although the mechanical consequences of meniscal and labral tears are believed to directly lead to OA development, the role of these fibro-cartilaginous structures in joint mechanics and cartilage protection is not well understood. In this dissertation, I aimed to explore the mechanical function of the meniscus and acetabular labrum in knee and hip joints, and to understand the biomechanical effects of damage to these structures.
In order to understand the biomechanical consequences of meniscectomy, I developed a novel technique, described in Chapter 3, to accurately measure articular cartilage deformation under realistic cyclic loading using an MRI scanner. This new method provided 3-dimensional articular cartilage images in statically loaded sheep knees with high accuracy and reproducibility. The effect of meniscectomy on articular cartilage contact area and nominal strain was examined in statically loaded sheep knee models. Meniscectomy resulted in a contact area reduction of 60 % which caused concentrated deformation of the central articular cartilage. Nominal strain in meniscectomized articular cartilage was markedly increased centrally but decreased peripherally. This pattern of change in articular cartilage nominal strain is consistent with the areas of central cartilage fibrillation and peripheral osteophyte formation that are commonly observed in in vivo osteoarthritic knees following meniscectomy.
Time-dependent articular cartilage deformation, described in Chapter 4, during and after cyclic loading was also significantly altered in meniscectomized sheep knees. A sequence of MR images taken every 2.5 min of a sheep knee undergoing realistic cyclic loading at physiologic levels was able to provide 'semi-real-time' data of articular cartilage geometry at different time points during cyclic loading and after cessation of loading. Articular cartilage in meniscectomized joints experienced greater maximum strains at the center of the load bearing region and reached steady-state deformation more rapidly. After the cessation of cyclic loading, articular cartilage in meniscectomized knees exhibited markedly prolonged recovery time. With the combination of greater strain and delayed recovery, articular cartilage in meniscectomized joints was found to be exposed to abnormal loading conditions for a longer period time than in intact joints. The prolonged deformation of the matrix and prolonged dehydration of the compressed articular matrix seen after meniscectomy may be detrimental to the articular cartilage and could be a signal leading towards osteoarthritis.
Localized changes in articular cartilage chemical and mechanical properties in meniscectomized knees were found to be related to the altered articular cartilage strain patterns following meniscectomy. This is described in Chapter 5. Imaging and cartilage modeling were developed to provide 3D cartilage nominal strain maps of sheep knees that were subjected to a physiologic magnitude of cyclic loading. Localized changes in articular cartilage nominal strain following meniscectomy were compared with the changes in chemical composition and mechanical properties that had been previously described in the articular cartilage of intact and meniscectomized in vivo sheep. A strong quadratic relationship was found between the changes in shear modulus and nominal strain in meniscectomized articular cartilage while a linear relationship was observed between GAG content and nominal strain. The results suggested that articular cartilage mechanical integrity was damaged in both abnormally high and low strain environments, but their underlying mechanisms of cartilage degeneration in these two strain environments are likely to be quite different.
Finally, in Chapter 6, I investigated the biomechanical role of the acetabular labrum. Previous studies have shown that the acetabular labrum acts as a seal for the hip joint. Since synovial fluid has been shown to be a critical element for articular cartilage lubrication, I hypothesized that damage to the acetabular labrum would result in impaired sealing of the hip joint and thus increased friction in the hip joint. Changes in joint friction were tested by measuring resistance to rotation (RTR) of hip joints following partial and complete resection of acetabular labrum. RTR was measured to be significantly increased in partially labrectomized hips at physiologic magnitude of axial loadings (2~3 times body weight). It may be that labrum maintains a low friction environment in the hip joint by sealing the joint from fluid exudation.
In conclusion, damage to the fibro-cartilaginous meniscus and acetabular labrum were shown to introduce mechanical alterations in knee and hip joints. Redistribution of joint loading in the knee following meniscectomy was found to be closely related to in vivo cartilage degeneration patterns, and increased joint friction in the hip following injury to the acetabular labrum could also be a risk factor for cartilage degradation. New technical methods developed in this dissertation have suggested new concepts regarding the role of fibro-cartilaginous tissues in articular cartilage protection, and has provided insights into the possible mechanisms of articular cartilage degradation after damage to the knee meniscus and hip labrum.
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