WHAT INFLUENCES FORGOTTEN JOINT SCORE? WHAT IS ITS CORRELATION TO NEW KNEE SOCIETY SCORE?

Introduction

Patient reported outcome measures (PROMs) are recognized as crucial in evaluating the outcome of total knee arthroplasty (TKA). New Knee Society Score (NKSS), introduced in 2011, is reported to be an effective, such outcome measure. Forgotten Joint score (FJS), introduced in 2012, has been validated but has only a few studies in literature reporting upon it. In a normal population without arthritis, the FJS is reported to be between 50–95, a higher score representing better status. Our aim was to determine 1) the FJS at 1-year post TKA, distributing patients in 2 groups of FJS less than/more than 50; assessing its ceiling and floor effect 2) the influence of age, sex, BMI, diabetes, thyroid, type of deformity, pre/post-operative flexion and 3) to compare and correlate FJS with NKSS and its sub-scores - Objective knee score(OKS) and Subjective knee score(SKS).

Methods

We enrolled 181 patients (222 knees), who had primary TKA performed by the same surgeon at Lilavati Hospital & Research Centre, Mumbai, between June 2016 to February 2017. NKSS was administered to each patient preoperatively. At 1 year, they were prospectively called for review and NKSS and FJS were administered. 151 patients attended the review clinic and 17 patients completed the forms with the help of their physiotherapist and sent them via email. 13 patients who could not do either, were excluded from the study. Thus, we had 168 patients (207 knees) whose complete data was analyzed. Of 168 patients, 37 were males and 131 were females, with an average age of 67 years (37–85). Patients were divided into two groups based on their FJS score - Group A (FJS<50) and Group B (FJS≥50). The demographics and NKSS in both groups were compared. The study was approved by our Institutional Review Board.

Statistical analysis was done using SPSS software. Raw data statistics for FJS was determined and unpaired t-test used to compare all parameters in Groups A and B. Correlation of NKSS to FJS was analyzed using Pearson's correlation test.

Results

1. 1)

   FJS at 1 year: The median FJS at 1 year was 68.8 (IQR 41.7, mean 68, SD 25.3, range 0–100). It exhibited a 14% ceiling and 0.5% floor effect. There were 49 (24%) TKAs in Group A and 158 (76%) TKAs in Group B.

2. 2)

   Comparison of parameters of age, sex, BMI, diabetes, thyroid disorder, type and severity of deformity and pre/post-operative flexion between the two groups showed no difference (unpaired t-test p>0.05) for each parameter, with the numbers available.

3. 3)

    

   1. In Group A, the median values of NKSS, OKS and SKS were 174, 94 and 87 respectively as compared to the corresponding median values of 198, 98 and 100 in Group B. The difference in their corresponding values was seen to be significant (p<0.005).

   2. For both groups, the change in scores from preoperative to postoperative values was significant for NKSS (median of 73 vs 69, p=0.003) & SKS (median of 39 vs 30, p=0.006) but not for OKS (median of 47 vs 46, p=0.655).

   3. Correlation of the FJS to NKSS at 1 year was seen to be significant (p<0.005), the strength of correlation was found to be moderate (r=0.43). Each sub-score also showed significant correlation (p<0.005), which was weak to moderate (r=0.32 to 0.43).

Conclusion

Mean FJS at 1-year post TKA was 68 which compares well with a mean of 72 reported in the normal population without arthritis. It exhibited 14% ceiling and 0.5% floor effects. FJS was not influenced by age, sex, BMI, co-morbidities, type/extent of deformity or pre/post-operative flexion range. Patients with higher FJS also had higher NKSS and higher OKS/SKS values but change in scores was significant only for NKSS & SKS. We observed a moderately positive correlation of FJS with NKSS at 1 year.