Classification of osteoarthritis of the hip is fraught with difficulty Although different patterns of disease are recognised, there is no accepted classification or grading system. We aim to develop a classification system that reflects both the radiological changes, and the local disease process within the joint.

After ethical permission and consent tissue was taken from 20 patients undergoing primary hip replacement surgery. Intra-operative tissue samples were taken from each patient and the steady state gene expression of several cytokines (TNF-α, IL1-β, IFN-γ, IL-6, RANKL and OPG) measured quantitatively using Taqman RT-PCR. Relative expression was calculated for each sample using standard curves and normalised to 18S expression. The technique was consistent with high correlations for repeated measures from the same tissue type (κ=0.99) and from different tissue types in the same joint (κ=0.92). Intra-observer (κ=0.93) and inter-observer (κ=0.89) reliability for the technique were also found to be high.

Preoperative radiographs were scored by two independent observers and joint space narrowing, cysts, osteophytes and sclerosis noted in each of the DeLee-Charnley zones on the femoral and acetabular side. Based on these scores patients were then classified to either lytic or sclerotic type and subclassified into either hypertrophic or atrophic.

Subgroup analysis of cytokine expression by radiographic type was performed. There were statically significant differences in expression of macrophage stimulating cytokines (IL-1γ and OPG) in the lytic group as compared to the sclerotic group (p< 0.05). Conversely, the sclerotic group expressed significantly higher levels of IL-6. Individuals with atrophic subtype demonstrated significantly higher levels of IL-1β and IL-6, but lower levels of IFN-γ

Our results demonstrate greatly differing patterns of disease within osteoarthritic hip joints. These changes are reflected in radiographic appearances of osteoarthritis. Our proposed classification system can be used grade and classify osteoarthritis in a manner that reflects the disease process.