Measured ambulation and self-reported health status following total joint replacement for the osteoarthritic knee

D. J. Walker, P. S. Heslop, C. Chandler1 and I. M. Pinder

Musculoskeletal Unit, Freeman Hospital, Newcastle-upon-Tyne NE7 7DN and
1 University of Northumbria at Newcastle, Newcastle-upon-Tyne, UK


    Abstract
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Objective. To quantify the increase in ambulation produced by total knee replacement for osteoarthritis (OA) of the knee and to compare this with questionnaire-derived data.

Methods. We compared the measured ambulatory activity and self-reported Nottingham Health Profile (NHP) data of 19 subjects with OA of the knee before and after total knee replacement (TKR).

Results. Subjects were considerably restricted in their measured activity before operation compared with subjects with less severe disease. At 6 months the activity had increased by an average of 79% (P=0.02). The pain scale of the NHP had significantly improved at 3 months, as had the mobility scale. Between 3 and 6 months, however, at the same time the measured activity was increasing, self-reported mobility declined.

Conclusions. Replacement of a knee for OA is an effective way of improving ambulation. At 6 months the average increase amounted to 79%. Self-reported mobility did not correlate with mobility measured objectively.

KEY WORDS: Ambulatory activity (Numact), Nottingham Health Profile, Total knee replacement.


    Introduction
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Total knee replacement (TKR) is the treatment of choice for osteoarthritis (OA) of the knee when the damage is advanced enough for lesser procedures to be ineffective and the symptoms (pain and restricted activity) are severe enough to justify the risks. Evidence of the efficacy of all medical interventions is becoming increasingly important in justifying their use, and total joint replacement is one of the most dramatic and expensive interventions for musculoskeletal disease.

Pain has generally been assessed using analogue scales or questionnaires seeking information on the impact of the pain on activity or sleep [1]. The ability to do things has similarly been assessed with questionnaires which ask the patient what they can do and how easily [2]. Walking has been assessed physically by measuring walking speed over 10 m. We have recently validated the measurement of spontaneous ambulatory activity, using the Numact activity monitor, as an outcome measure for OA of the knee [3]. This has some theoretical advantages over other measures in its objectivity and lack of a language base, and also in terms of quantification, because of the interval and unlimited scale. We were interested to observe how ambulation changed after TKR in terms of both total and peak activity. We also took the opportunity to collect questionnaire data using the Nottingham Health Profile (NHP) [4]. This was an observational study because TKR is so well established that randomization would be judged unethical.


    Methods
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Nineteen subjects, who had been routinely listed for TKR by orthopaedic surgeons at Freeman Hospital, Newcastle, were studied (Table 1Go). Each subject was assessed before operation, within 1 month of surgery and 3 and 6 months after operation. Assessment included a 24-h Numact recording and the NHP, which contains scales for energy, pain, social isolation, sleep, emotional reaction and mobility.


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TABLE 1.  Demographic details

 
Comparative groups consisted of 14 healthy subjects of similar age who did voluntary work in the hospital, who were recorded when they were not working, and 29 subjects with OA of the knee, who had been recruited to studies of NSAIDs previously and whose disease was therefore not judged severe enough to require surgery.

The Numact activity monitor consists of two sensors attached to a data logger. Position sensors assess posture (sit, stand and lie) and an accelerometer assesses trunk accelerations while the subject is standing. This gives a measure of the number and vigour of steps taken and minimizes recording of passive activity. Multiplying the number and vigour of the steps together gives a measure of total ambulatory energy expenditure. This has been validated against observation with energy expenditure, against treadmill activity [5] and for use in OA of the knee [3]. Total ambulatory energy expenditure over 24 h was the primary outcome measure, but data on time spent standing and the number and amplitude of steps were also available.

Because the data were stored in real time it was possible to look at other aspects of activity, and we chose to look at the longest continuous walk (defined as the number of sequential steps without a 2-s break) and the peak 5-min energy (the most energetic 5-min period during the recording). Our reasoning was that the total ambulatory energy expenditure is a measure of participation in life and therefore of handicap. Peak levels of activity will better reflect ability/disability, continuous walking reflecting the ability to walk and peak 5-min energy expenditure reflecting more sustained activity.

Effect size was calculated for the different outcome measures as a way of comparing their sensitivity to change in this population.

Statistical analysis
Activity data from before operation were compared with data obtained 3 and 6 months after operation by the paired t-test. Effect size was calculated for the different outcome measures according to the formula: effect size=mean change/S.D. of pre-intervention results.

This study was approved by the Newcastle and North Tyneside ethics committee.


    Results
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Ambulatory activity and energy
Twenty-four-hour total ambulatory activity before operation [mean and 95% confidence interval (CI)] is shown in Fig. 1Go for the study subjects and the two comparison groups. As expected, the subjects listed for surgery were significantly less active than normal subjects and those recruited to NSAID studies.



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FIG. 1.  Total ambulatory activity in one 24-h period as measured by the Numact Activity monitor. The data show results before operation for patients having TKR and before treatment for those receiving NSAIDs.

 
Figure 2Go shows the total ambulatory energy expenditure before operation and 3 and 6 months after operation. Although the subjects did show some increase at 3 months (amounting to a 29% increase), this showed wide variation and was not significantly different from baseline data. At 6 months the increase in activity amounted to 79% and was significant compared with the baseline (P=0.02, paired t-test).



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FIG. 2.  Total ambulatory activity in one 24-h period in the TKR group before and after operation, as measured with the Numact activity monitor.

 

Other aspects of walking
The average time spent standing during the 24-h period was also assessed and increased from 314 (S.D. 122) min before operation to 336 (138) min at 3 months and 378 (132) min at 6 months. At 6 months, the subjects were standing for 64 min longer during the 24 h of data logging than they had before the operation, a statistically significant difference (P=0.04).

The number of steps taken increased from an average of 10 738 before operation to an average of 12 358 at 3 months and 15 641 at 6 months. The increase of 4900 steps between the preoperation and 6-month tests was statistically significant (P=0.01).

The average amplitude of all the steps recorded during the 24 h data logging period was not significantly different between the measurements before operation (21 units), at 3 months (23 units) and at 6 months (23 units).

The length of the longest continuous walk, defined as the number of repeated steps without a 2-s break, increased from 415 steps before surgery to 534 steps at 6 months. Energy expenditure in the longest continuous walk was significantly greater at 6 months than before operation (P=0.03).

The peak 5-min energy expenditures before operation and 3 and 6 months after operation were in the ratio 18:17:22 (not significant).

Table 2Go shows the NHP data before operation and 3 and 6 months after operation. The improvement in pain was most striking at 3 months. Self-reported mobility had also improved at 3 months, which was before the subjects were actually more active according to the Numact activity monitor. Between 3 and 6 months, self-reported mobility declined whereas actual activity increased considerably. This indicates that reported and actual activities are different things. This may be a function of the improvement in pain. Improvement in energy expenditure was also observed from 3 months on, but social isolation and emotional reaction took longer to improve, presumably requiring a period of adjustment.


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TABLE 2.  NHP data [mean (S.D.)] before and after operation

 
Effect size is a way of quantifying an effect but can also be reversed to test the sensitivity to change of outcome measures used in a particular study. The effect size at 6 months is shown in Table 3Go. The results show that the Numact activity monitor was the most sensitive to change of the measures used.


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TABLE 3.  Effect size calculated for the difference between scores before and 6 months after operation for Numact Activity Monitor data and NHP scales

 


    Discussion
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
This study was performed on a relatively small number of subjects to gain an idea of how ambulatory activity changes, and was not large enough to study the totality of effects of joint replacement. It acts as a pilot study for either larger or more specific studies of joint replacement.

Total knee replacement is one of the most dramatic interventions used for people with arthritis and is so well accepted that randomized study is unethical. For the individual, the benefits need to outweigh the risks, which include the risk of death. Previous studies have shown good results in terms of health-related quality of life for TKR [6], similar to the NHP data shown here. The primary indication for joint replacement is relief of pain, and this relief occurs early, as shown by the NHP data. It is likely that this pain relief accounts for the satisfaction of most patients with this procedure.

Until this study, information on increase in activity was dependent on self-reporting by the patient of how active they thought they were. It is interesting that self-reported mobility improved before the subjects were actually doing more. This may be an effect of the relief of the pain making them feel better able to be mobile even if they are not. It is also interesting that the self-reported mobility dropped off at 6 months even though the subjects were, by then, more active. This suggests that self-reported mobility and actual activity are different. This is illustrated in Fig. 3Go, where these results are contrasted. In reporting their mobility at 6 months, the subjects appear to have discounted the improvement they perceived at 3 months. It may be that the subjects’ self-reporting of many items only related to their more recent experience, e.g. within the last 3 months, and that they could not remember how bad they were before the operation. The gradual improvement in social isolation was as might be expected, the subjects gradually getting back to a more normal life as their activity increased. This would involve the gradual re-establishment of a more active lifestyle and social activities. The improvement in energy and emotional reaction could have been the result of reduced pain.



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FIG. 3.  Differences between self-reported activity and actual activity (measured with the Numact activity monitor) before and 3 and 6 months after TKR.

 
For the first time, we have measured subjects’ actual activity before and after TKR. The overall increase in ambulatory activity amounted to 79% at 6 months, but this increase was not apparent at 3 months. The pain relief was therefore not immediately translated into activity. This may have been for a number of reasons, including the gradual healing of soft tissues and the gradually expanding horizons of a less disabled person. Some of the improvement in activity would have come from other interventions associated with the surgery, such as physiotherapy.

The increase of 79% may be compared with an increase of 65% in RA patients with steroid injection of the knee [7] and a 50% increase in RA patients treated with NSAIDs [8]. It will be interesting to see if the improvement in activity continues with time, and if so for how long. Ultimately, the failure of the joint would be expected to decrease activity, but studies suggest that 85% of replaced joints survive at 13 yr [9].

The findings in relation to other aspects of activity are interesting. The overall time spent standing rose significantly after the operation, and the subjects stood for an extra 64 min. It is logical that, with less pain on weight-bearing, subjects will be able to stand longer, but again this was only apparent at 6 months. The number of steps had risen by an average of 4900 at 6 months. The average amplitude of step, however, did not increase significantly after operation. While one would expect the replaced joint to allow more vigorous walking than previously, this effect may be diluted by more prolonged standing with low-amplitude steps. This study suggests that the increased ability to stand for longer is the main influence allowing greater activity.

Measures of peak energy expenditure produced different results. The sustained activity of 5-min energy was not increased, but the longest walk was significantly increased. This suggests that the quality of walking was improved, and it may take longer for this to translate into a more active lifestyle.

The measurement of ambulatory activity is therefore complementary to the use of multidimensional health status measures. These data are more onerous to collect as operator time and equipment are needed. However, it is possible to perform studies on smaller numbers of subjects because of the sensitivity to change. What subjects actually do is clearly important and measuring it should be performed in studies aimed at improving lower limb function. For practical reasons, it may be sensible to collect it only on a sample of subjects in studies such as pharmaceutical studies, in which a large sample size is important when safety data are being obtained.

It would be possible to explore peri-operative procedures and their effect on the timing of recovery using measured ambulatory activity. For example, what effect does preoperative physiotherapy have on recovery?

Conclusion
Total knee replacement for OA knee produces a significant improvement in ambulation, and the improvement is greater than that given by lesser interventions. Self-reported mobility does not relate well to this and should not be relied upon.

Competing interests
DJW and CC were involved in the development of the Numact activity monitor.


    Acknowledgments
 
This study was supported by a grant from Action Research.


    Notes
 
Correspondence to: D. J. Walker. Back


    References
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 

  1. Bellamy N, Buchanan WW, Goldsmith CH, Campbell J, Stitt LW. Validation study of WOMAC: a health status instrument for measuring clinically important patient relevant outcomes to antirheumatic drug therapy in patients with osteoarthritis of the hip or knee. J Rheumatol1988;15:1833–40.[ISI][Medline]
  2. Fries JF, Spitz PW, Young DY. The dimensions of health outcomes: the Health Assessment Questionnaire, disability and pain scales. J Rheumatol1982;9:789–93.[ISI][Medline]
  3. Walker DJ, Heslop PS, Kay LJ, Chandler C. Spontaneous ambulatory activity as a quantifiable outcome measure for osteoarthritis of the knee. Br J Rheumatol1998;37:969–71.[ISI][Medline]
  4. Hunt SM, McKenna SP, Williams J. Reliability of a population survey tool for measuring perceived health problems: a study of patients with osteo-arthritis. J Epidemiol Community Health1981;35:297–300.[Abstract]
  5. Walker DJ, Heslop PS, Plummer CJ, Essex T, Chandler C. A continuous patient activity monitor: validation and relation to disability. Physiol Meas1997;18:49–59.[ISI][Medline]
  6. Hawker G, Wright J, Coyte P et al. Health related quality of life after knee replacement. Results of the Knee Replacement Patient Outcomes Research Team study. J Bone Joint Surg Am1998;80:163–73.[Abstract/Free Full Text]
  7. Kay LJ, Heslop PS, Chandler C, Walker DJ. Objective measurement of patient activity before and after steroid injection of the knee in rheumatoid arthritis. Br J Rheumatol1996;35(Supp1.):208.
  8. Walker DJ, Heslop PS, Chandler C. Ambulatory activity as an objective and quantifiable measure of non-steroidal therapy. J Rheumatol1998;25:768–70.[ISI][Medline]
  9. Emmerson KE, Moran CG, Pinder IM. Survivorship analysis of the kinematic stabiliser total knee replacement: a ten to thirteen year follow-up. J Bone Joint Surg Br1996;78:441–5.[Medline]
Submitted 1 November 2000; Accepted 12 February 2002





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