Marked by cartilage and bone degradation, progressive knee osteoarthritis (OA) is believed to result in part from a combination of several local mechanical factors. Lack of joint stability, poor joint alignment and injuries to the disks of meniscal tissue that cushion the knee all affect load distribution and the toll to the vulnerable OA knee. However, in studies of knee OA progression, either meniscal damage or malalignment has been considered, but not both together, and no studies have included joint laxity.
Unlike radiographs, magnetic resonance imaging (MRI) can distinguish articular cartilage from meniscal tissue and detect the impact of specific factors on OA progression. Almost all studies of knee OA progression with MRI outcomes define cartilage loss with qualitative cartilage assessments. For a clearer picture of the role of various mechanical factors in knee OA, researchers at Northwestern University and Salzburg Univeristy applied quantitative measures of cartilage lossdecreases in cartilage volume and thickness and an increase in bare bone areaas well as cartilage integrity score. Their results, presented in the June 2008 issue of Arthritis & Rheumatism (www.interscience.wiley.com/journal/arthritis), indicate three factors that independently predict cartilage loss with a direct impact on knee OA: medial meniscal damage, lateral meniscal damage, and varus, or bow-legged, malalignment of the knee joint. Notably, quantitative cartilage loss outcome measures were more sensitive in revealing these relationships than previously applied qualitative approaches.
The study's participants, 153 women and men with radiographic evidence of knee OA, were recruited from local senior citizens groups and the registry of the Beuhler Center on Aging at Northwestern University. The mean age was 66 years and the mean BMI was 30. None of the subjects had a history of rheumatoid arthritis, gout, joint infection, or meniscectomy. A total of 251 osteoarthritic knees were thoroughly scanned with MRI and rigorously studied. Meniscal damage and meniscal extrusion were graded using the Whole-Organ MRI Score (WORMS). Varus-valgus alignment and medial-lateral laxity were also measured. Focusing separately on medial and lateral segments of tibial and weightbearing femur cartilage of the knee joint, cartilage volume, percentage of subchondral bone covered with cartilage, exposed subchondral bone area, and the average thickness of cartilage were measured using specialized software. Cartilage integrity was also scored. Two years later, the entire process was repeated. Odds ratios were determined for each of the four mechanical factors using logistic regression analysis, adjusting for age, sex, BMI, and the other factors.
Medial meniscal damage significantly increased the likelihood of cartilage volume loss, cartilage thickness decrease, and denuded bone increase in both the medial tibial and the medial weightbearing femoral segments. Similarly, lateral meniscal damage predicted quantitative cartilage loss in both the lateral tibial and the lateral weightbearing femoral segments. Varus malalignment was strongly associated with cartilage loss from each medial surface; valgus (knock-knee) malalignment was not associated with lateral surface loss. Meniscal extrusion and joint laxity had inconsistent effects. Using the qualitative cartilage assessment, however, no significant relationship with outcome was detected for either meniscal damage or malalignment.
Dr. Leena Sharma, the study's leading author and spokesperson, commented "It is important to note that local factors, such as those examined in this study, may participate in vicious circles with the worsening of knee OA. Whenever along the OA disease timeline a local impairment develops, it may contribute to subsequent OA progression and cartilage loss, especially given the vulnerable milieu of the already damaged OA knee. Ultimately, strategies that interrupt these vicious circles may be especially powerful."
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