Evaluation of health outcomes in osteoarthritis patients after total knee replacement: a two-year follow-up

Osteoarthritis (OA), a chronic degenerative disease, is characterized by pain and physical disability, with knee being the most frequently affected joint[1]. OA is among the most prevalent diseases affecting adults and a major contributor to physical disability, morbidity, and utilization of health care resources worldwide[2‐5]. In patients with severe knee OA who have failed conservative treatments (e.g. medications, exercises, and weight loss), total knee replacement (TKR), a surgical option involving replacement of knee joint with artificial components, has been shown to be a highly effective treatment that could result in substantial improvement in physical functioning[6].

It is known that pain, physical functioning, and health-related quality of life (HRQoL) are important outcome measures in OA. Recently there is growing literature that has contributed to the understanding on what could be achieved by TKR[7‐10]. Both disease-specific functional measures such as the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC)[11‐14], the Oxford Knee Score (OKS)[15], and the Knee Society Clinical Rating Scale (KSS)[11, 16], and generic HRQoL instrument such as the SF-36[11, 13, 14, 16‐20] have been used to evaluate the improvement in functioning and quality of life in patients undergone TKR. However, such data are particularly lacking for Asian patients. As prevalence of OA is increasing, TKR is expected to play an important role in reducing pain and improving physical functioning and HRQoL of patients[21]. Thus, there is a pressing need to obtain more empirical evidence on health outcome improvement after TKR in Asian populations.

Therefore, the objective of the present study was to quantify the improvement in health outcomes in Asian patients after TKR.

This was a two-year non-randomized prospective observational study. The institutional review board at the Singapore General Hospital (SGH) had approved this study and patient informed consent forms were collected.

The patients' characteristics are shown in Table 1. At baseline, 298 eligible patients participated in the present study with the mean age of 66.8 years. The majority were female (80.4%) with the mean OA duration of 7.8 years and the mean body mass index (BMI, kg/m2) of 27.9. A total of 176 (follow-up rate: 59.0%) and 111 (follow-up rate: 37%) were followed at six-months and two-years after the surgery, respectively. The reasons for the patients lost to follow up were not known. Nevertheless, the demographic characteristics of the patients at six-months and two-years follow-up were comparable to those of the patients at baseline (Table 1).

The observed mean scores of SF-36 physical functioning, role physical, bodily pain, general health, and role emotional, the OKS, the KSS knee and functioning scores changed significantly over time, while the mean scores of SF-36 social functioning, vitality, and mental health did not change significantly (Table 2).

Table 3 shows the mean changes from the pre-surgery scores predicted by the GEE models. Without the adjustment of demographic characteristics, SF-36 physical functioning score increased by 22.5 at six-months (p < 0.0001) and by 26.7 at two-years (p < 0.0001). Role physical score increased by 32.9 at six-months (p < 0.0001) and 28.7 at two-years (p < 0.0001). Bodily pain score increased by 6.0 at six-months (p = 0.0003), but the change was not significantly at two-years. General health score did not change significantly at six-months and decreased by 4.1 at two-years (p < 0.0001). Role emotional score increased by 15.6 and 12.2 at six-months (p < 0.0001) and two-years (p = 0.0001), respectively. The score increments at six-months were 28.5, 37.5, and 16.2 for the OKS, and the KSS knee and functioning, respectively, while the corresponding increments at two-years were 33.4, 41.3, and 20.9 (all ps < 0.0001).

With the adjustment of age, gender, ethnicity, BMI, and years with OA, the magnitude of predicted changes in these scores were similar to those without the adjustment. Physical functioning score increased by 22.8 at six-months (p < 0.0001) and 27.3 at two-years (p < 0.0001). The corresponding increments were 35.9 (p < 0.0001) and 26.8 (p < 0.0001) for role physical and 15.9 (p < 0.0001) and 12.9 (p = 0.0011) for role emotional. The score increments at six-months were 28.8, 37.0, and 15.8 for the OKS, and the KSS knee and functioning, respectively, while the corresponding increments at two-years were 32.4, 40.4, and 19.4 (all ps < 0.0001).

In this two-year prospective study, statistically significant improvements were observed in the generic SF-36 physical functioning, role physical, and role emotional domains and in the two disease-specific instruments. After the adjustment of covariates including age, gender, ethnicity, BMI, and years with OA, the results were similar. The magnitude of the improvements also exceeded the minimally important difference reported for the SF-36[22]. TKR, as an effective surgery option for severe OA patients, can substantially improve both general physical functioning (as measured by the generic SF-36) and knee-specific physical functioning, and reduce knee-related pain (as measured by the OKS and the KSS). However, no significant improvement in other aspects of health (e.g., mental and social health) or general health has been observed.

The improvement in knee functioning and substantial reduction in knee pain as measured by the OKS and the KSS were consistent with previous studies[13‐17], as was the physical functioning and role physical measured by the SF-36[13, 14, 17‐20]. Surprisingly no significant change in SF-36 bodily pain score at both six-months and two-years was observed. This finding was different from some published studies[9, 10, 13, 14, 17‐20, 22], which reported that SF-36 bodily pain had also been reduced significantly after TKR. Though it is not clear about the true answer to this contrast finding, there are several possible explanations. First is the presence of comorbid back pain in this patient population. SF-36 bodily pain domain was designed for general bodily pain (e.g. back pain) as opposed to knee pain. Veerapen et al., found that back pain was more common than knee joint pain in Asian populations[27] and back pain was reported as a significant factor influencing post-TKR SF-36 bodily pain, vitality, and mental health scores[9]. This might be a possible reason why SF-36 bodily pain had demonstrated minimal improvement after surgery if back pain was a common comorbid condition for this patient population. However, the prevalence of back pain was not captured in the present study. It is thus suggested that the information be collected in future studies. Second is the difference in patient characteristics. The patients enrolled in previous studies were either younger[10] or older[9, 22], and with higher BMI[9, 10, 22]. Bugala-Szpak et al., found that BMI, rather than sex and age, had a significantly influence on post-TKR quality of life scores[17]. A large study is necessary to confirm this finding. Thirdly and importantly, ethnic differences in pain perception between Asian and Western populations might contribute to this discrepancy. Thus caution should be exercised when generalizing the results to other ethnic groups.

Social and mental health as measured by the SF-36 remained unchanged or even a little worse after surgery. Singer et al., suggested that there might be a strong psychological adjustment or adaptation to physical disability in the elderly[28]. Nevertheless, patients' social and mental health was still less satisfactory compared to the same age group of Asian populations[29]. Ayers et al., reported that poorer pre-TKR mental health might have a negative impact on the improvement of post-TKR physical functioning[30]. Escobar et al., also found that pre-TKR mental health was a significant factor predicting post-TKR physical functioning[9]. Some studies have demonstrated that social support might play an important role in moderating the effects of pain, physical disability, and depression in patients with OA[31‐36]. All these evidence may suggest that providing social and mental support to this patient population could be an important way of improving their quality of life in the long term.

The study had higher drop-out rates in following up the patients. A sensitivity analysis was conducted by calculating the mean of the outcome measures at each time point using all available measurements and comparing with those using completers only, and this made very little difference. General health of patients was worse at two-years than that at baseline. General health is also the only significant predictor for the missingness at two-years. This finding was not surprising as more than 80% of the patients were aged over 60 and 40% over 70. Although these patients might be seen in other departments later on, it would be difficult for them to come back to the orthopedic department to complete an additional examination two years after the surgery unless knee OA is getting worse.

In conclusion, both general and knee-specific physical functioning had been significantly improved after TKR, while other health domains remained unchanged after the surgery.