Age and waiting time as predictors of outcome after total hip replacement for osteoarthritis

A.-K. Nilsdotter1,2, and L. S. Lohmander2

1 Department of Orthopedics, Halmstad County Hospital, Halmstad and
2 Department of Orthopedics, Lund University Hospital, Lund, Sweden


    Abstract
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
Objective. To investigate in a prospective study the relationship between age, pre-operative status, waiting time and post-operative outcome in patients assigned for unilateral total hip replacement (THR) due to osteoarthritis (OA).

Method. One hundred and forty-eight patients (mean age 71 yr) with primary OA of the hip were investigated pre-operatively and 3, 6 and 12 months post-operatively with the Medical Outcomes Study 36-Item Short Form Health Survey (SF-36) and the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). For 56 of the patients an additional evaluation was made when they were placed on the waiting list.

Results. One hundred and twenty-four patients fulfilled the study criteria (age 50 yr or over and unilateral THR for OA during the study period). Before surgery there were no differences in the WOMAC or SF-36 subscales (except mental health) between patients aged over and under 72 yr. Post-operatively, the younger patients reached a better score than the older patients. There were no differences in pre-operative status or post-operative outcome between the patients who had been on the waiting list more than and less than 3 months. Most pain relief after hip replacement was obtained by 3 months, while it took at least 1 yr to reach the full benefit in improved function. Ninety per cent of the patients had improved by at least 10 points on a 100-point scale for pain and function at 12 months.

Conclusion. The age of the patients assigned for THR did not determine their pre-operative status. However, younger patients gained more function post-operatively than older patients and reached higher absolute mean SF-36 values, except for pain. An average difference in time on the waiting list of 3 months did not result in a difference in post-operative outcome. At least 1 yr is required for the average OA patient to gain the full benefit of the THR.

KEY WORDS: Osteoarthritis, Total hip replacement, Age, Waiting time, Gender, Outcome.


    Introduction
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
Osteoarthritis (OA) is one of the most common diseases affecting the musculoskeletal system in elderly people and has a great effect on overall health and health-related quality of life [14].

The most effective treatment for patients with severe OA in the hip joint is total hip replacement (THR), which offers the patient improved physical function and pain relief [57]. The surgical techniques of joint replacement have improved continuously over the last decade and have diminished the risks associated with the operation [8].

It is now common to treat patients older than 80 yr of age with joint replacement for OA with successful results [9], provided the patient is in good health [10]. With an increasing average life expectancy, the number of elderly people with hip OA will continue to increase, and it is likely that the demand for hip joint replacement for OA will also increase [3, 11]. However, the consequences of variable waiting list lengths and priority practices for joint replacement for OA with regard to post-operative outcome are unclear.

The purpose of this prospective study was to investigate the relationships between (i) age and pre-operative status in patients with hip OA assigned for unilateral THR (THR), (ii) age and post-operative outcome after THR and (iii) waiting time, pre-operative status and post-operative outcome after THR.


    Patients and methods
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
One hundred and seventy-six patients aged 50 yr and older were assigned consecutively for a THR because of primary OA during February 1997 and October 1998 at the Department of Orthopaedics, Halmstad, Sweden. One hundred and forty-eight of these patients (83 women) with a mean age at the time of surgery of 71 yr (range 51–88 yr) were included in the study. Twenty eight patients (14 women) (mean age 72 yr, range 57–85 yr) were not included during the early recruitment phase due to administrative errors (unrelated to patient-relevant factors). There was no difference in intercurrent diseases or mental health between those not included and those in the study group.

All patients had a primary unilateral THR performed. Eight experienced hip surgeons were involved. The majority (n=109) of the replacements were performed with both components cemented, while in 39 (22 men) the acetabular component was uncemented. The mean age of this subgroup at surgery was 63 yr (range 51–72 yr).

The pre-operative hip radiographs were classified by one radiologist according to Osteoarthritis Research Society International (OARSI) criteria with a radiographic atlas as a guide [12]. OA severity was graded from 0 to 3 according to the degree of joint-space narrowing, where 3 indicates severe OA. Seventy-three per cent of the patients had severe OA and 27% moderate OA.

Questionnaires
Evaluation with the Medical Outcomes Study 36-Item Short Form Health Survey (SF-36) was made at the hospital the day before surgery and 3, 6 and 12 months post-operatively. The SF-36 measures three major health attributes (functional status, well-being and overall health) in eight subscales. These are (1) physical function, (2) role limitations due to physical health (‘role physical’), (3) bodily pain, (4) general health, (5) vitality, (6) social function, (7) role limitations due to emotional health (‘role emotional’) and (8) mental health [13]. The SF-36 scores are calculated on a scale of 0–100 (worst to best). Together, the eight subscales provide a health profile. SF-36 has been translated and validated for Swedish conditions [14].

The Western Ontario and McMaster Universities Osteoarthritis Index version LK 3.0 (WOMAC) was used as the disease-specific instrument. Evaluation with WOMAC was made pre-operatively and 3, 6 and 12 months post-operatively. WOMAC is a self-administered instrument validated for OA in the lower extremities. It consists of 24 items grouped into three categories: pain (five questions), stiffness (two questions) and physical function (17 questions). It is reliable and valid for Swedish conditions [15, 16]. To make comparisons easier with SF-36, WOMAC was transformed to a scale of 0–100 (worst to best) [17, 18].

In order to examine the effect of time on the waiting list for surgery, 56 of the 148 patients (32 women) also received questionnaires (SF-36 and WOMAC) at the time when they were placed on the waiting list for surgery. These patients were selected at random. Their mean age at surgery was 72 yr (range 52–86 yr). This subgroup of patients did not differ in age or sex distribution from the main group. Thirty-three patients in this subgroup had to wait more than 3 months for surgery (mean 5.1 months, range 4–10 months) and 23 patients less than 3 months (mean 2.0 months, range 1–3 months). Three months was chosen as the dividing point on the basis of national Swedish recommendations and the patients' ‘bill of rights’.

Statistics
For comparison of pre-operative and post-operative questionnaire data, Wilcoxon's signed rank test was used. For comparison between two subgroups the Mann–Whitney test was used. No adjustment was made for multiple comparisons.


    Results
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
Of the 148 patients, 24 were excluded during the first follow-up year (13 were operated on the contralateral side, eight refused to participate, two died and one had recurrent dislocations of the prosthesis). Thus, the results of 124 patients (70 women) with a mean age at surgery of 71 yr are presented.

The patients showed improved average scores 12 months after surgery for all subscales of WOMAC (P < 0.0001) (Fig. 1Go) and all subscales of SF-36 except general health (Fig. 2Go) compared with baseline (the day before surgery). The health transition of the patients, based on the second question in SF-36 concerning general health at the time of questionnaire compared with 1 yr previously, showed a significantly better score (P < 0.0001) 12 months after surgery compared with before surgery.



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FIG. 1. Mean values of WOMAC subscales pre-operatively and 12 months post-operatively for patients with OA (n=124). The scale is 0–100, worst to best. •, pre-operative; {circ}, post-operative. Standard deviations are shown in Table 1Go.

 


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FIG. 2. Pre- and post-operative mean SF-36 data for all patients (n=124) pre-operatively. The scale is 0–100, worst to best. •, pre-operative, {circ}, post-operative. pf=physical function; rp=role function physical; bp=bodily pain; gh=general health; vt=vitality; sf=social function; re=role function emotional; mh=mental health. Standard deviations are shown in Table 2Go.

 
Influence of age on pre-operative status and post-operative outcome
Pre-operatively (on the day before surgery) there were no differences in the average values of WOMAC subscales pain, stiffness or physical function between the patients 72 yr of age and younger compared with those older than 72 yr (Table 1Go). Neither were there any differences in the SF-36 subscales, except mental health (P < 0.018), between the two groups at this time (Table 2Go).


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TABLE 1. WOMAC values before and 12 months after THR for OA

 

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TABLE 2. SF-36 values before and 12 months after THR for OA

 
Twelve months after surgery, the patients 72 yr of age or younger had reached a higher average score than those older than 72 yr for the WOMAC subscale physical function (P=0.004) (Table 1Go). Furthermore, the patients 72 yr or younger reached a higher average score than those older than 72 yr for all subscales of SF-36 (P=0.002–0.04) except bodily pain (P < 0.112) (Table 2Go).

Irrespective of age, the patients showed a rapid average improvement in the SF-36 subscale of bodily pain after surgery, reaching a plateau at 3 months (Fig. 3Go). In contrast, both age groups showed a more gradual but continuous average improvement with regard to the SF-36 subscale of role physical after hip replacement.



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FIG. 3. Pre- and post-operative mean values and 95% confidence intervals for patients aged <=72 yr (n=65) and >72 yr (n=59) for the SF-36 subscales of role function physical and bodily pain. {blacktriangleup}, role function physical <=72 yr; • role function physical >72 yr; {triangleup}, bodily pain <=72 yr; {circ}, bodily pain >72 yr.

 

Influence of gender on pre-operative status and post-operative outcome
There were no gender-related differences in pre-operative SF-36 and WOMAC scores except for pain; women had a worse score than men for both questionnaires (SF-36, P=0.014; WOMAC, P=0.044). Post-operatively, there were no differences between the two groups (SF-36, P=0.3–0.9; WOMAC, P=0.5–0.9; data not shown).

Influence of pre-operative status on post-operative outcome
The patients with the best pre-operative scores (highest quartile) showed a greater average improvement 1 yr after THR (expressed as score points) than the patients with the worst pre-operative scores (lowest quartile) (P < 0.0001). As a consequence, patients with the lowest pre-operative scores for the WOMAC subscales of physical function and pain still performed worse 1 yr after surgery than the patients with the highest pre-operative scores (P=0.001 and P=0.041 respectively). The same was true for the pain subscale in SF-36 (P=0.05) but not for the physical function subscale (P=0.1) (data not shown).

Individual change in pain and function
We calculated the individual change in pain and function, measured by WOMAC, as the difference between the pre-operative and 1 yr post-operative scores. The cut-off level for a clinically significant improvement was set at 10 units on the 100-unit scale [19]. Ten per cent of the patients had improved by less than 10 score units 1 yr after surgery (one patient for pain only, three patients for function only and three patients for both pain and function) (Fig. 4Go). Four patients improved by less than 20 score units for pain only and seven by less than 20 units for function only, while five improved by less than 20 units for both pain and function.



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FIG. 4. Individual change in WOMAC pain (left) and function (right) after THR, calculated as the difference between pre-operative score and 12 month post-operative score. Each line represents one patient.

 

Influence of waiting time on post-operative outcome
The SF-36 and WOMAC results at the time when the patients were placed on the waiting list for surgery did not differ between the patients who had to wait more than 3 months for surgery and those who had to wait less than 3 months (P=0.137–0.907) (Tables 3Go and 4Go). Furthermore, there were no differences in pre-operative score on the day before surgery or in post-operative outcome between these two groups. In both groups there was a significant improvement in the SF-36 general health subscale and the WOMAC pain subscale between the two pre-operative examinations of these patients.


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TABLE 3. WOMAC values before and 12 months after THR for OA according to waiting time

 

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TABLE 4. SF-36 values before and 12 months after THR for OA according to waiting time

 


    Discussion
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
THR for OA is a successful intervention in terms of improvement of quality of life and cost-effectiveness [2022]. A considerable body of research has identified and improved procedure- and implant-related factors important for the outcome [8, 23, 24]. However, our understanding of the patient-related factors that influence in particular the patient-relevant outcome is limited [25].

The results of this study show that patients receiving THR for OA and who are older than 72 yr experience a degree of improvement (expressed in score points) similar to that experienced by those younger than 72 yr at surgery, except for physical function. However, those older than 72 yr at surgery do not reach the same absolute level, in statistical terms, of SF-36 health status after THR as those younger than 72 yr. To express this in a different way, it is never too late to do a THR for OA, but younger patients may stand to gain more than older patients with hip OA. These results corroborate a previous study [26] and may in part be explained by the larger number of comorbidities in the older age group.

Hip replacement for OA is above all surgery for relieving pain and improving physical function [27]. The younger and older groups of patients experience a similar degree of pain relief. The age of the patient appears to be more important for the improvement in physical function than for the improvement in pain. In the present study, women had significantly worse pre-operative pain scores than men, corroborating a previous study [28]. However, this is in contrast to other studies [5, 29], which found no pre-operative differences in any of the subscales. One year after hip replacement there were, in the present study, no differences between the sexes in outcome.

An important finding of our study is that pain relief is experienced very soon after surgery (Fig. 3Go), while adaptation to the new health status, as expressed by the SF-36 role function subscale takes at least 1 yr (Fig. 3Go). These observations are of clinical relevance and of use in the planning of future follow-up studies of hip replacement.

The patients with the lowest pre-operative scores did not reach the same level post-operatively as the patients with the highest pre-operative scores. This contrasts with an earlier study [9], but is in concordance with the results of Fortin et al. [30], who suggested surgery earlier in the natural history of the disease rather than later.

Ninety per cent of the patients improved their WOMAC pain or function score by at least 10 units on a 100-unit scale during the first year after surgery (Fig. 4Go). In the present analysis of patient-relevant outcome after THR for OA, we excluded 13 patients who received an operation on the contralateral hip for OA during the follow-up year, and one patient who had recurrent dislocations of the operated hip. These exclusions were motivated by our wish to focus the study on (i) the outcome of unilateral hip replacement for OA and (ii) patient-related factors rather than implant-related factors. Finally, eight patients declined to participate in the study.

In the comparatively small subgroup of 56 patients with a follow-up of 1 yr, we found no effect of waiting time on the post-operative outcome. The interpretation of this is limited by the relatively small difference in waiting time between the two groups. However, our results corroborate those of a previous study [31]. As in that study, the waiting time was unrelated to the pre-operative severity of pain and disability and did not appear to have a negative impact on outcome.

The observed improvement in general health and pain during the waiting time for surgery could be considered as an example of regression to the mean, in that patients might be expected to be at their worst at the time when the decision is taken to put them on the waiting list for hip surgery.

The involvement of eight surgeons raises the possibility that variation in patient-relevant outcome could have been greater than in a single-surgeon study. On the other hand, it increases the likelihood that the results of this follow-up study of THR for OA performed at a county hospital are more broadly representative.

In 39 of the replacements, the acetabular component was uncemented. There was no difference in the outcome between this group and the group in which it was cemented when the data were adjusted for age (A.-K. Nilsdotter, H. P. Roos and L. S. Lohmander, unpublished results).

With regard to the waiting-list priority, the patients' own perceptions of their overall symptom burden and their ability to tolerate delayed pain relief should be considered along with information derived from clinical judgement [32]. The use of patient-related questionnaires like SF-36 and WOMAC may assist the surgeon in placing the patient in the queue for surgery [31, 33].

The present prospective study showed no difference in pre-operative status between patients under or over 72 yr of age or with regard to time on the waiting list. These results are supported by a similar prospective study published while this manuscript was in preparation [34]. Although both age groups improved post-operatively, the younger age group reached higher absolute score values. In a previous study [5] we showed that patients assigned for THR and with moderate radiographic OA changes had the same outcome as those with advanced radiographic OA changes. These results, taken together, suggest that it might be beneficial to operate earlier in the course of disease than is currently done in Sweden, particularly in the light of the improving survival time of hip implants [8]. The ‘life years with disability’ [3] lost while waiting for a THR are not regained after surgery.


    Acknowledgments
 
Financial support was obtained from the Scientific Council, Province of Halland, Council for Medical Health Research in South Sweden, Swedish Medical Research Council, Swedish Rheumatism Association, Lund University Hospital and Medical Faculty, King Gustaf V 80-year Birthday Fund and the Thelma Zoéga Foundation. The constructive critique of the anonymous reviewers considerably improved the manuscript.


    Notes
 
Correspondence to: A.-K. Nilsdotter, Department of Orthopedics, Lund University Hospital, S-221 85 Lund, Sweden. Back


    References
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 

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Submitted 13 February 2001; Accepted 26 April 2002