Regardless of their completed level of schooling, patients improve in HRQoL and have a high satisfaction after THR. After TKR, we found that patients with higher completed levels of schooling had a larger improvement in role-physical functioning, general health and the Physical Component Summary scale and a smaller improvement in mental health and the Mental Component Summary scale, although the found differences in the SF36 subscales were smaller than recently published within-group MCIDs at two-years follow-up.
All other dimensions of HRQoL and patient satisfaction showed no differences between the completed levels of schooling, thereby failing to refute our hypothesis.
Strengths of our study include the rigorous efforts to minimise confounding and the generalisability of our study population, due to the multi-center setting and the similarity of the demographics of our study population to those of large-scaled national joint registries.
Weaknesses of the study include the low participation rate and the variation in follow-up period after joint replacement. Although participation rates of 100% are feasible in small-scaled studies with hard endpoints,
participation rates in epidemiological studies have been steadily declining in the last 30 years.
Even sharper declines have been reported in the past few years.
Unfortunately, the participation rate of this study follows this general trend, and therefore we cannot exclude the presence of self-selection bias. In order to limit the extent of this bias, we have sent multiple reminders and have called all patients who did not answer our reminders and who did not return the questionnaire. As incentives, we have included an appealing information brochure in which the primary goals of the follow-up study were explained and a study pen as a small gift. Additionally, patients were urged to participate by their treating physician. However, the participation rate alone does not determine the extent of bias present in any particular study.
The difference between participants and nonparticipants is far more important.
As the found differences in demographics were small, it is unlikely that the study results will be severely biased.
The follow-up period after joint replacement varies between 1.5 and 6 years in this study ( and ). Theoretically, this broad range could influence our findings. In order to exclude this variable, all patients should have been followed for the exact same amount of time. In our data, we found no clear evidence of a relationship between the improvement in HRQoL after joint replacement and the follow-up period (See Appendix S1
for scatter plots of the improvement in HRQoL as a function of the follow-up period length, stratified per completed levels of schooling and Appendix S3
for scatter plots of the NRSS after surgery as a function of the follow-up period length, stratified per completed levels of schooling). In order to account for this range, we stratified our analysis per quartile of follow-up period. Stratifying for an additional variable inevitably leads to a loss of power, thereby increasing the probability of a type 2-error. In our analysis, this loss of power was negligible, as unstratified analyses showed similar results, supporting our conclusions (data not shown).
Follow-up Period in Years for THR and TKR patients.
Although a residual effect of follow-up length within each stratum cannot be excluded, we do not think this is very plausible, as recent evidence suggests that the improvement in HRQoL after completion of the initial rehabilitation-period is sustained up to 7 years after joint replacement surgery.
The minimum follow-up period is well beyond the length of the expected rehabilitation-period, suggested by a recently published systematic review.
Two other studies have investigated the relation between SEP and patient-reported outcomes after THR or TKR.
Allen Butler et. al. have studied this relation in a randomised controlled trial, which compared two THR designs.
In this study, the effect of SEP was studied on a multitude of outcome measures, including the WOMAC, Short Form-12 (SF12) and degree of patient satisfaction. An association was found between lower levels of education and a degree of satisfaction which was “less than very satisfied”. Unfortunately, the authors have only reported their significant findings; differences in WOMAC or SF12 between social classes are not reported. Additionally, only p-values are reported instead of mean differences or relative risks, precluding any judgment on the clinical relevance of their findings. Finally, it is unclear for which factors any associations were adjusted, as the authors applied forward stepwise logistic regression modeling, without mentioning which variables were included in the final model. Davis et. al. have measured WOMAC scores before surgery and at 3, 12 and 24 months after TKR.
Whilst comparing WOMAC scores at each time point between patients of different income categories, patients with more disadvantaged SEP had worse preoperative WOMAC scores and similar postoperative WOMAC scores as patients with less disadvantaged SEP. These findings imply a larger improvement in disease-specific quality of life in patients with more disadvantaged SEP than in patients with less disadvantaged SEP. However, not all patients were measured at each time point. A cross-sectional comparison at each time point precludes judgment on the actual within-patient improvement in disease-specific quality of life. Due to methodological shortcomings of both other studies which investigated the relation between SEP and patient-reported outcomes after joint replacement, no meaningful comparison of results can be made.
Our findings have large implications for policymakers, as a more advantaged SEP is associated with greater use of health services in general.
A recent systematic review and numerous studies indicate that this also holds for THR
in post-industrialised countries. Additionally, the need for joint replacement appears to be higher in patients with more disadvantaged SEP,
thereby increasing the inequity in access to joint replacement. Under-treatment of patients with more disadvantaged SEP cannot be justified, given the similar improvement in HRQoL and postoperative level of satisfaction with surgery between the examined groups of completed level of schooling.
A number of factors might explain the found differences in improvement in HRQoL after between THR and TKR patients per completed level of schooling groups. Biomechanical factors might play a role. The hip joint is a relatively simple ball and socket joint, which is adequately mimicked by a THR. The adequate mimicry of the biomechanics is reflected in a highly consistent improvement in HRQoL, regardless of completed level of schooling. The biomechanical aspects of the knee joint are more difficult to imitate, as the knee is a pivotal hinge joint with 6 degrees of freedom. These degrees of freedom are generally not restored after TKR, which is substantiated in kinematic and kinetic studies.
However, more complex biomechanics might explain a less consistent improvement in HRQoL in TKR patients, but does not explain differences between patient groups with different completed levels of schooling.
Differences between THR and TKR patients might be part of the explanation. Better general health, physical, emotional and social function, motivation and self-efficacy and lower levels of pain before surgery and during the rehabilitation period are associated with improved short- and medium-term outcomes.
In our study population, differences in the preoperative health status between completed level of schooling groups are more pronounced in TKR patients than in THR patients ( and ). Finally, differences in rehabilitational options could play an important role. TKR patients require more rehabilitation than THR patients in order to achieve optimal results.
TKR patients with higher completed Level of Schooling might have better access to physical therapy or other rehabilitational facilities, and therefore gain more in role-physical functioning and general health than less advantaged patients. This effect might be exacerbated by the higher prevalence of obesity and co-morbidity in TKR patients compared to THR patients. Unfortunately, we do not have any information on the rehabilitational regime of our THR and TKR patients, leaving this hypothesis to be addressed in future research.
Quality of Life before Knee Replacement: A Comparison Between Patients with different Completed Levels of Schooling.