Associations between pre-operative radiographic changes and outcomes after total knee joint replacement for osteoarthritis

Abstract

Purpose: Knee joint replacement (KJR) for OA is a successful intervention, but about 20% of KJR patients obtain little or no improvement. The reasons for this are unclear. The purpose of this study was to investigate whether pre-operative radiographic changes predict pain and functional outcomes. Methods: Data from a prospective single-centre cohort study of patients undergoing TJR between Jan 1st 2006 and Dec 31st 2007 were analysed (n = 525). Pre-operative data collection included demographics (age, sex, BMI, self-reported co-morbidities), the International Knee Society Score (IKSS), and the Short Form Health Survey (SF-12). Pre-operative (within 6 months of surgery) AP standing and lateral radiographs were read by a single observer using the Kellgren and Laurence (K&L) and Altman atlases, and the scoring repeated on a random 10% sample to assess rater reliability. Post-operative data collection at 12 and 24 months included the IKSS and SF-12. Linear and logistic regression analyses were undertaken to assess the relationships between baseline radiographic features and pre and post-operative pain and function. Results: Of the 525 cases 24 were excluded as they had undergone uni-compartmental replacements, and 23 radiographs were rejected, leaving 478 TJR cases for inclusion. 5 and 24 were unable to complete the 12 or 24 month data respectively due to death, loss to follow-up or refusal. The amount of missing data was well below 5%. The mean age of the subjects was 70.8 years (+/- 8.3), 69% were female and the mean BMI was 32.2. Pain scores improved from a mean of 4.0(+/-7.3) to 34.9 at 12 months and 34.8 (+/- 15.9) at 24 months; however about 30% of patients still complained of moderate or severe pain in the operated knee at each time point. Function scores improved from a mean of 37.5 (+/- 18.1) pre-operatively to 58.8 and 55.5 (+/- 26.8) at one and two years respectively. Inter-rater reliability scores for radiographic features were satisfactory, ranging from a kappa of 0.80 (lateral joint space narrowing) to 0.65 (lateral osteophyte score). The majority had OA K&L grade 3 (57 with mild (3a) and 200 with severe (3b) JSN) or 4 (87 without (4a) and 119 with (4b) bone attrition). The commonest compartmental distributions were medial and patella-femoral (244), or medial only (104). There were no significant associations between pre-operative radiographic scores and pre-operative pain or function, but radiographic severity was associated with outcome. Those with lower K&L grades and less compartmental involvement were less likely to have obtained improvements in pain at either 12 or 24 months (e.g. the odds ratio for ongoing pain in the operated joint at 12 months comparing those with K&L grades <3 to those with grade 4b was 5.23 (CI 1.5 to 17.6), p=0.008). The relationships between pre-operative radiographs and function were similar but less strong. Outcome did not appear to differ for those with predominant medial, lateral or patella-femoral joint involvement. Multivariate analyses showed that other significant, independent determinants of improvement in pain and function included age, mental function score of the SF-12 and pre-operative pain and function (both pain and function), as well as gender and BMI (function only). Conclusions: The data suggest that we should exercise caution when considering surgery for patients with severe pain but mild radiographic changes, and conversely, that those with severe joint damage, irrespective of which compartments are involved, are more likely to respond well to surgery. A possible explanation for this is that in those with severe pain and mild joint damage the pain is driven more by central sensitisation than by any nociceptive input from the joint.