Introduction: The Oxford Knee Score (OKS) is commonly used in the assessment of outcome for knee arthroplasty. All patients having knee arthroplasty at our institution undergo OKS at both nurse led pre-assessment and admission physiotherapy visit, a period of 10 to 30 days pre-operatively. At both instances, the scoring form is left with the patient and collected at the end of the visit.

Anecdotal evidence from our centre suggested that patients attending for arthroplasty surgery were scoring differently at each visit.

The aim of this study is to establish if there is a significant difference OKS at pre-assessment visit and on admission to the ward.

Statistical Method: A pilot study was carried out. A power calculation revealed a requirement for 44 patients to enter the study. The resultant probability was 90 percent that the study would detect a difference at a two sided 5.0 percent significance level, if the minimum clinical difference is 3 points. This is based on the standard deviation of the difference in the response variables of 6. A clinical difference of 3 is drawn from previous studies investigating the use of the OKS.

44 patients undergoing arthroplasty surgery had their OKS for both visits retrospectively analysed.

The mean of the totals of both visits was analysed and found to conform to normality and hence was further investigated by a paired samples t test.

Comparison of individual scoring revealed a violation of normality and hence was further analysed using a Wilcoxon Signed Ranks Test.

Results: A statistically significant result at the 5% level was observed t= 2.197 (44df), p= 0.03. OKS at pre-assessment was lower than at admission to the ward by 1.1 point. (−2.1 – 0.9 95% CI).

Analysis of the individual scoring at both intervals revealed only three of the pairs achieved statistical significance and in each case, the difference was less than 3 scoring units. No significant difference was seen when time between assessments was analysed.

Conclusion: This study demonstrates that although there is a difference in total scoring using the OKS between two patient episodes prior to arthroplasty, a clinically relevant difference is not detected, and neither is a statistically significant difference detected when all scoring steps are analysed.

This work supports earlier studies that pre-operative assessment using the OKS is robust to variance in the pre-operative scoring window.

All patients undergoing knee arthroplasty at our institution complete Oxford Knee Scoring (OKS) at nurse-led pre-assessment and again at an admission physiotherapy visit on the ward which may be up to 2 months later.

The aim of this study is to establish the extended reproducibility of the OKS by statistical analysis of scores taken at these intervals.

44 patients were required to achieve a 90 % probability to detect a difference at a two-sided 5 % significance level with a minimum clinical difference of 3 points, a cut off used in previous works regarding the use of OKS.

Both the overall population means and the differences between individual questions were analysed by a paired samples t test and a Wilcoxon Signed Ranks Test respectively.

Mean interval between attendance for pre-assessment and admission visit was 16 days (7–60).

A statistically significant result at the 5% level was observed for the t test t= 2.197 (44df), p= 0.03. OKS at pre-assessment was lower than at admission to the ward by 1.1 point. (−2.1 – 0.9 95% CI).

Analysis of difference between individual questions revealed only three of the pairs achieved statistical significance and in each case, the difference was less than 3 scoring units.

This study demonstrates that although there is a difference in total scoring using the OKS between two patient episodes prior to arthroplasty, a clinically relevant difference is not detected, and neither is a statistically significant difference detected when all scoring steps are analysed.

The original validation of the OKS was obtained using test-retest reproducibility over a 24 hour period. This work shows that the OKS is robust to violations in reproducibility at duration much greater than this and for practical purposes is valid if taken at any point during the pre-admission phase of care.