The motions and moments in the hip and knee in female patients on the waiting list for knee prosthesis surgery with medial (n ¼ 15) or lateral (n ¼ 15) osteoarthritis (OA) were compared with a control group (n ¼ 15). We hypothesized that not only the kinematics and kinetics of the knee but also of the hip would differ between patients the medial and lateral groups. At midstance, patients with lateral OA showed slightly (28) more maximal (peak) adduction ( p ¼ 0.015) of the hip joint and patients with medial OA had 78 more abduction (p < 0.001) than did controls. In patients with lateral OA, the femur was positioned in about 78 more maximum external rotation (p ¼ 0.001), but femur position did not differ between medial OA and controls ( p ! 0.8). There was a tendency to higher internal hip rotation moment in lateral OA compared to controls ( p ¼ 0.021). The maximum values of the internal knee abduction moments were 52% higher in medial OA ( p ¼ 0.005) and 63% lower in lateral OA ( p < 0.001) compared to controls. Cases with medial OA had 98 more, whereas those with lateral OA had 68 less external tibial rotation than controls (medial vs. lateral OA, p ¼ 0.001). We found an association between presence of lateral OA of the knee and the biomechanics of the hip joint. It remains to be evaluated if the changed biomechanics of the hip joint is a reason for development of lateral OA or an observation that is a result of this disease. ß

Our findings suggest that occurrence of medial or lateral OA has a biomechanical background originating from pelvis and hip anatomy.

Background Staging of osteoarthritis (OA) of the knee is commonly based on the Ahlbäck classification. Its value has been questioned, however. We therefore evaluated the reproducibility and validity of this classification of knee osteoarthritis.Patients and methods 48 patients (48 knees) (medial OA: n = 30; lateral OA: n = 8) operated with total knee prostheses were studied. Weight-bearing radiographs were evaluated twice by 4 observers. Presence of bone attrition on radiographs was compared with observations of the resected parts of the distal femur and proximal tibia.Results When the same observer classified the radiographs twice according to Ahlbäck, the repeatability was fair in both medial OA (kappa values = 0.15-0.65) and lateral OA (0.59-0.76), and between different observers it was poor (kappa: 0.1).Comparison between radiographic classification and classification based on visual inspection of the bone pieces removed during arthroplasty revealed an acceptable sensitivity in both medial (67-95%) and lateral OA (43-86%), but the specificity was low (medial: 11-67%; lateral: 25-75%).Interpretation The main problem with the Ahlbäck classification was that a joint space could often be seen radiographically despite the presence of bone attrition on the preparations. According to our study, conventional radiographs do not give sufficient information for correct grading.

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The pattern of knee motion in patients with lateral osteoarthritis may be associated with the pathogenesis of this disease. Based on previous findings of decreased anteroposterior femoral condylar translation in knees with medial knee osteoarthritis, we asked whether a similar pattern of knee kinematics in lateral osteoarthritis could be related to the wear pattern on the tibial plateau. We used sequential radiostereometric images to study kinematics during active weightbearing extension in five knees with lateral osteoarthritis and 11 control knees. At 45 degrees of flexion, the posterior circular center of the lateral femoral condyle displaced posteriorly at 45 degrees and moved anteriorly during extension in both groups. There was also a proximal displacement with extension, but from a more distal position in the osteoarthritis group. We could only partly relate the kinematics of knees with lateral osteoarthritis to the wear pattern on the tibial plateau. Contrary to our hypothesis of normal anterior displacement of the medial and increased posterior displacement of the lateral femoral condyle, we observed no change of anterior-posterior displacements of the lateral condyle.