Outcomes after hip or knee replacement surgery for osteoarthritis

In Australia, in the financial year 1997/98, Medicare benefits of $13 500 000 were paid for 17 000 hip and knee replacements. This number represents only some of the total surgery performed, as the number of operations on Veterans' Affairs patients and on public patients in public hospitals are not available. Despite joint replacement surgery being one of the most common operations performed in this country, very limited Australian outcomes data have been published.⁴

The Medical Outcomes Study Short-Form 36 (SF-36) has been used extensively to assess the effect on quality of life of several procedures, including total joint replacement,^5,6 and it has been suggested that it should be the focus of preoperative and postoperative outcome evaluation for total hip arthroplasty.⁷ As a generic measure of health-related quality of life with standardised scoring, it enables comparisons between diseases, treatments and published population norms. It measures health on eight 100-point scales: physical function, role physical (ie, role limitations due to physical problems), bodily pain, general health, vitality, social function, role emotional (ie, role limitations due to emotional problems), and mental health. It is one of the most widely used health-related quality-of-life instruments and has been shown to be reliable and valid.

Studies in the US have shown that the SF-36 detects a significant improvement in health-related quality of life in patients undergoing total hip or knee replacement when preoperative scores are compared with postoperative scores.^8-11 The SF-36 also demonstrates a difference in outcome between total hip and total knee replacement patients,^8,10 with patients undergoing total hip replacement achieving significantly better outcomes than the knee replacement patients.

We aimed to compare the health-related quality of life (as measured by the SF-36) of people with osteoarthritis undergoing primary total hip and knee replacement surgery with that of the normal Australian population.² Putting changes in the health-related quality of life of these patients in the context of population norms for the same age group gives us a meaningful measure of the effectiveness of this surgery. It also provides information in a way that allows comparisons with other diseases and treatments, assisting rational choices to be made about the use of the limited healthcare dollar.

Patients with osteoarthritis booked for primary total hip or knee replacement surgery at three Sydney hospitals (St Vincent's public and private hospitals and the Centre for Bone and Joint Diseases, North Ryde) between March 1994 and December 1995 were approached to participate in the study. Recruitment was through regular contact with the orthopaedic surgeons and their practice staff.

Baseline information was collected from patients through a series of questionnaires, including the SF-36. Questionnaires were administered between one week and three months before surgery. After surgery, patients completed the SF-36 questionnaire at the end of each three months for their first postoperative year. Most questionnaires were self-administered, with less than 10% requiring face-to-face interview. This is consistent with the Australian Bureau of Statistics methodology for collecting general population data.² Annual follow-up of these patients by mail is continuing.

This article presents SF-36 information from patients at baseline and 12 months after surgery.

Ethics committee and Medical Board approval was obtained from the St Vincent's campus of the University of New South Wales Medical School, the Centre for Bone and Joint Diseases and the Royal North Shore Hospital. All patients in the study gave written informed consent.

We used one-sample t tests to compare the transformed scores with general population norms derived by the Australian Bureau of Statistics from the 1995 National Health Survey. The t test was chosen on the assumption that the population value was the real value (given that published standard errors were so low) and that we were observing the variance of the study data from the population value.

A P value of 0.05 was considered to be significant and no adjustment was made for multiple comparisons.

At the time of analysis, we had complete information for 151 patients at 12 months' follow-up (59 with hip replacement and 92 knee replacement; 52% female; median age, 72 years). An additional 75 patients (60% female; median age, 74 years) had incomplete follow-up information. Their last recorded SF-36 scores showed no significant differences from scores at the same stage of follow-up among those with complete information (data not shown).

Patients' SF-36 scores before and 12 months after surgery are shown in the Figure, in comparison with scores for the general population.

55-64 years age group: Eight patients undergoing knee replacement were recruited. Six of these patients reported having other illnesses, most commonly cardiovascular disease (reported by five patients).

At 12 months' follow-up, these eight patients showed improvement from baseline according to mean SF-36 scores for physical function, role physical, bodily pain and vitality, but these scores remained significantly lower than the population norms. The power to detect the observed difference to be statistically significant at the 5% level ranged from 83% to 100% for these four comparisons. Their scores on the other four scales (general health, social function, role emotional, mental health) remained lower than population norms, but this difference was not statistically significant (power to detect a significant difference ranged from 7% to 55%).

65-74 years age group: Twenty-eight of the 44 patients (64%) reported having a comorbid illness, with the most commonly reported again being cardiovascular disease (61% of those with comorbidity).

Mean SF-36 scores improved significantly on all scales except general health. The mean general health score was significantly higher than the population norm at 12 months' follow-up, but had been higher to begin with at baseline. Mean physical function, role physical and bodily pain scores remained significantly lower than the population norms at 12 months' follow-up. The power to detect these differences ranged from 85% to 99%. Mean vitality, social function, role emotional and mental health scores improved from baseline to be similar to population norms. The power to detect the significance of these differences ranged from 8% to 36%.

75 years and over: Thirty-two of the 40 patients (80%) reported suffering from another illness, most commonly cardiovascular disease (60% of those with comorbidities).

Mean SF-36 scores improved significantly on most scales (particularly role physical, physical function and bodily pain), but not on general health and mental health, which were fairly high before surgery. The mean general health score was significantly higher than the population norm at 12 months' follow-up; the other scores improved to population levels. Given the small difference seen between patient and population scores, large numbers would be required to show statistically significant differences (power ranged from 7% to 33%).

55-64 years age group: Comorbidities were reported by 5 of the 14 patients (36%). All five had cardiovascular disease among other comorbidities.

Mean scores for vitality and social function improved to become significantly higher than the population norms (power to detect significant difference, 90% and 96%, respectively), while the other scores improved to be similar to the population norms, with particular improvement in physical function, role physical and bodily pain.

65-74 years age group: Thirteen of the 28 patients reported suffering from a comorbid illness, most commonly cardiovascular disease (46% of patients with comorbidities).

Mean SF-36 scores improved on all scales except general health, which had a baseline value above the population norm. Mean scores for general health and mental health were significantly higher than the population norms at 12 months' follow-up (power to detect difference, 85% and 75%, respectively) and the other scores improved to be similar to the population norms (power to detect difference ranged from 7% to 58%). The improvement from baseline was particularly evident in role physical, physical function and bodily pain scores.

75 years and over: Comorbidities were reported by 12 of the 17 patients (71%), most commonly cardiovascular disease (reported by 58% of those with comorbidities).

Mean SF-36 scores improved on all scales except general health and mental health. At 12 months' follow-up, mean scores on none of the scales were significantly different from the population norms, possibly due to the small numbers in this group (power to detect a difference less than 30% for most scales). Again, there was improvement to population norms in role physical, physical function and bodily pain.

We found no improvement in general health despite gains in the other scales. This has been shown in other studies,¹¹ and has been suggested to be due to the patients' previous medical history and pre-existing and general health-related conditions. In our study it was apparent that the general health of patients was already higher at baseline than the population norm, suggesting that relatively healthy patients are being selected for total joint replacement.

A US study that compared patients undergoing total hip replacement with age-matched and sex-matched population norms found that age and sex made important differences in SF-36 scores.⁹ Men younger than 65 years had scores lower than the norms in the physical scales, but were comparable in the mental scales, and women scored lower than the norm in all scales. No sex differences were found in our study, but similar age differences were observed, particularly for patients undergoing total knee replacement, among whom the youngest age group had the poorest outcome relative to the age-matched population. We hypothesise that this may be related to the higher population norms for this age group, unrealistic expectations for outcomes among patients, and the presence of existing comorbidities in younger patients requiring knee replacement. One implication of these findings is that older age should not be a barrier to joint replacement surgery, as the outcome is likely to be relatively successful for older patients.

Potential limitations that need to be considered when interpreting these results include the non-randomised cohort design, the power of the analysis given the small sample size of the subgroups, and the overall response rate. However, the inclusion of several groups of surgeons operating at different sites and across the public and private sectors contributed significantly to the generalisability of the results.

This study showed the improvement of SF-36 scores of patients undergoing total hip or knee replacement up to and in some cases beyond population norms. Whether these statistically significant differences are clinically important remains to be seen. Longer follow-up is required to determine whether the dramatic improvement from baseline is maintained. Nonetheless, at one year follow-up, it would appear that total knee or hip joint replacements are successful at restoring health and well-being, and that older age alone should not be a barrier to surgery.

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Department of Medicine, University of New South Wales, Sydney, NSW.
Marita J Cross, BSc(Hons), Research Assistant;
Katherine L Tribe, BSc(Hons), Research Assistant;
Clarissa J M Bachmeier, MD, MMed(ClinEpidem), Research Fellow.

School of Health Services Management, University of New South Wales.
Helen Lapsley, BA, MEc, Senior Lecturer.

Northern Sydney Public Health Unit, Hornsby Ku-ring-gai Hospital, Sydney, NSW.
Alan J M Brnabic, MSc, Statistician.

Department of Orthopaedics, St Vincent's Hospital, Sydney, NSW.
Brett G Courtenay, MB BS, FRACS, Orthopaedic Surgeon.

Faculty of Health Sciences, University of Queensland, Brisbane, Qld.
Peter M Brooks, MB BS, FRACP, FAFPHM, Executive Dean.

No reprints will be available from the authors.
Correspondence: Associate Professor L M March, Department of Rheumatology, Royal North Shore Hospital, St Leonards, NSW 2065.
Email: lmarcATdoh.health.nsw.gov.au