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 radio-graphs 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.