Introduction
Hip osteoarthritis (OA) is a degenerative joint condition that is characterized by changes in joint cartilage, synovium, bone, tendons, and muscle.1,2 Symptoms experienced by patients with hip OA are pain, reduced physical function, and joint stiffness.2 Being moderately physically active and maintaining muscle strength is thought to be primary prevention of hip OA.3 Leg extensor muscle power is markedly hampered in individuals with hip OA which is associated with reductions in physical function.4,5 Compared to muscle strength, muscle power is more closely related to physical function, which is true among patients with hip OA and in older adults in general.6,7 It has been shown that in patients with hip OA who exercise, those who experience greater improvement in muscle power also attain larger improvements in all measured aspects of physical function.8
Higher body mass index (BMI) among patients with hip OA is correlated with an increased risk of hip arthroplasty,9 and increased risk of poor physical function and activities of daily living (ADL) scores after hip arthroplasty,10 which could be attributed to the observation that patients with hip OA and a BMI over 25 experience higher levels of pain, lower quality of life (QoL), and more impaired physical function than those with a BMI under 25.11 Moreover, individuals with a higher BMI are often less physically active.12 A literature review found that physical activity reduces pain, improves physical function, and increases health-related QoL for individuals with hip or knee OA.13
To better understand the impact of hip OA and inform individually tailored treatment, it is important to investigate factors that may influence pain, physical function, and QoL in patients whose symptoms are not yet severe enough to require surgery. Identifying these factors may help to delay surgery, given its associated risks and the variability in patient outcomes.14 Furthermore, understanding the characteristics of patients, who are referred but not deemed eligible for surgical treatment, may help optimize the patient pathway.
The primary purpose was to explore associations between patient characteristics and key outcomes of pain, physical function, and QoL among individuals with hip OA. The secondary purpose was to investigate differences in pain, physical function, and QoL between patients recruited from hospitals and physiotherapy clinics in Denmark and to explore whether the associations between patient characteristics and pain, physical function and QoL varied by recruitment setting.
The primary hypothesis was that higher BMI, older age, longer symptom duration, female sex, and hospital recruitment are negatively associated with the subscales Pain, QoL, and ADL of the Hip disability and Osteoarthritis Outcome Score (HOOS) as well as the 30-second Chair Stand Test (30s-CST). Conversely, higher muscle power and the use of analgesics are positively associated with these outcomes. Moreover, the secondary hypothesis was that patients with hip OA recruited from hospitals have worse scores on HOOS pain, QoL, and ADL subscales as well as the 30s-CST compared to patients recruited from physiotherapy clinics.
Materials and Methods
Study Design
For this explorative study, baseline data from the Hip Booster Trial, a multicentre cluster-randomized controlled trial were utilized.8 The study was approved by the Central Denmark Region Committee on Health Research Ethics (Journal No. 1-10-72-267-20) and registered at ClinicalTrials.gov (NCT04714047) and the Danish Data Protection Agency (Journal No. 1-16-02-11-21) before inclusion of patients.8
Participants
The study population in The Hip Booster Trial consisted of 160 patients with clinically diagnosed hip OA (no information on radiographic OA severity) recruited from physiotherapy clinics (n=112) and hospitals (n=48) across three regions in Denmark. Recruitment was initialized in January 2021 and was completed in April 2023.8 Inclusion criteria were: (1) Diagnosis of OA of the hip joint according to the National Institute for Health and Care Excellence criteria3; (2) An episode of pain during activity of at least 3 out of 10 on a Numerical Rating Scale (NRS) in the index hip within the previous two weeks; (3) Age ≥45 years; (4) ≤ 30 minutes of hip joint morning stiffness; (5) No lower extremity surgery within the previous six months; (6) No comorbidity markedly affecting hip function; (7) Adequacy in written and spoken Danish, and (8) Not being a candidate for total hip arthroplasty. Exclusion criteria were: (1) BMI > 40; (2) Pregnancy; (3) progressive resistance training (PRT) or neuromuscular exercise (NEMEX) for the lower extremities exceeding 12 sessions over the prior six months or six sessions over the prior three months, and (4) Planned vacation of more than 14 days without the possibility of extending the intervention accordingly.
Data collection
The following characteristics regarding the independent variables were collected at baseline: body weight (kg), height (cm), age (years), sex (male/female), symptom duration (<2 years/>2 years), physical activity level (high/low according to The Swedish National Board of Health and Welfare questionnaire),15 recruitment site (hospital/physiotherapy clinic), current use of analgesics (yes/no), and educational level (low/high). Participants engaging in over 150 minutes of moderate activity or more than 60 minutes of vigorous activity per week were categorized as having a high physical activity level, while those below this threshold were classified as having a low physical activity level. Education was divided into two categories, high education defined as having completed high school. Leg extensor muscle power (LEP), performed as a forceful extension of hip and knee joint, was measured unilaterally using the Nottingham Power Rig.16 The participants performed at least five trials with each leg and the test with the greatest power output for the affected leg was used in this analysis.17
The dependent variables were assessed at baseline including the subscales Pain, ADL function, and QoL from the HOOS questionnaire. The HOOS questionnaire consists of 40 questions divided into five subscales. For each subscale, the patient can score a minimum of 0 (worst) and a maximum of 100 (best).18 Physical function was measured using the 30s-CST, which assesses the number of times the patient can rise from a chair to a fully upright position within 30 seconds.19
Statistical analysis
The distribution of continuous variables was assessed using histograms and QQ plots.
The association of each independent variable (BMI, age, sex, symptom duration, physical activity level, recruitment site, use of analgesics and muscle power) with the outcomes (HOOS Pain, QoL, and ADL, and the 30s-CST) was estimated using multiple linear regression, with model assumptions assessed by diagnostic plots of the residuals.
The differences in HOOS Pain, QoL, and ADL, and the 30s-CST between the two recruitment groups were estimated using linear regression models, adjusting for recruitment site. Interaction terms were included to evaluate potential differences in the effect of independent variables between recruitment sites. To evaluate the clinical significance of the estimates obtained, the change in the independent variable required to achieve a minimal important difference (MID) in the dependent variables, HOOS Pain (7.7 points), HOOS QoL (8.4 point), HOOS ADL function (8.6 points), and 30s-CST (0.5 reps) was calculated.8 The MID was previously established in patients with hip OA in the randomized controlled trial The Hip Booster Trial by using an anchor-based approach after three months of exercise, defined as the difference in mean change in outcome between participants who reported having experienced a “small but not important change” in global perceived effect (GPE) from the mean score for those reporting an “important change” in GPE.8
Data are presented with 95% confidence intervals (95% CI) if data was normally distributed. Consistent with contemporary statistical guidelines, dichotomous interpretation of p-values and statistical significance were not applied.20,21Analyses were performed using R, version 4.3.2 (R Foundation for Statistical Computing).
Results
The randomized controlled trial The Hip Booster Trial included 160 participants, of whom 159 had observations for all included variables, making them eligible for inclusion in this exploratory study. Patient characteristics are presented in Table 1.
Associations between patient characteristics and pain, physical punction, and QoL
HOOS Pain
Older age, higher education, greater muscle power, and lower physical activity were associated with less reported hip pain. In contrast, individuals with higher body weight, those recruited from a hospital, and those who used analgesics reported greater hip pain (Table 2). Sex and symptom duration were not associated with HOOS Pain.
HOOS QoL
Older age and greater muscle power were associated with better QoL, whereas analgesic use was associated with worse QoL (Table 2). Female sex, BMI, symptom duration, physical activity level, recruitment site, and educational level were not associated with HOOS QoL.
HOOS ADL
Female sex, low physical activity, greater muscle power, and higher educational attainment were associated with better ADL, whereas higher BMI, hospital recruitment, and analgesic use were associated with lower ADL (Table 2). Age and symptom duration were not associated with HOOS ADL.
30s-CST
Female sex, higher muscle power, and higher educational attainment were associated with better performance on the 30s-CST, whereas higher BMI, hospital recruitment, and analgesic use were associated with poorer performance on the 30s-CST (Table 2). Age, symptom duration, and physical activity level were not associated with the 30s-CST.
Differences in characteristics and associations depending on recruitment site
Participants recruited from hospitals had lower HOOS Pain and HOOS ADL scores and slightly lower mean scores for HOOS QoL and the 30s-CST (Table 3).
Associations were dependent on recruitment site when considering the point estimates and the 95% CI describing interactions between recruitment site and associations between patient characteristics and pain, physical function, and QoL (Appendix A, Tables 1-4). Higher education was associated with less pain, better QoL, and better ADL function among patients recruited from a hospital, but not among those recruited from a clinic (Appendix A, Table 1). Moreover, low physical activity levels were associated with better ADL function among patients recruited from hospitals but not among those recruited from physiotherapy clinics.
Discussion
In this study population of patients with hip OA, there were several associations between patient characteristics and measures of pain, physical function, and QoL. However, only a few of these associations reached the MID previously estimated in this cohort and the 95 % CI overlapped the threshold of no clinically relevant difference.8 As such, the associations identified should be interpreted with caution due to substantial statistical uncertainty and the exploratory nature of this study.
Muscle power
Higher leg extensor muscle power was associated with better scores in all investigated patient-reported measures but stronger associated to objectively measured physical function, as assessed by the 30s-CST. For the 30s-CST, a difference in muscle power of only 24.7 watts would be associated with a MID, while a much larger difference is needed for a MID in any of the patient-reported measures. This seemingly stronger association with the 30s-CST may be attributed to the shared characteristic of LEP and 30s-CST being objective performance-based measures of physical function, or that muscle power is a very important characteristic for maintaining the ability to perform ADL activities.7 The important role of muscle power for physical function is consistent with findings from previous studies on muscle power and hip OA.6,8 Higher leg extensor muscle power also seems to be associated with lower pain levels and higher physical function in patients with knee OA.22
Use of analgesics
Contrary to our hypothesis, patients who reported using analgesics had higher pain levels, lower QoL, and worse physical function (Table 2). Furthermore, a larger amount of the patients from hospitals were using analgesics (Table 1) and they reported significantly worse outcomes on HOOS Pain (Table 3).
More than half of the analgesics taken among participants was Acetaminophen8 which is conditionally recommended against by OARSI due to lack of efficacy compared to alternatives such as NSAIDS.2 The prevalent use of a less efficacious drug, Acetaminophen, could be one possible explanation of why those who take analgesics also reported worse HOOS Pain levels. However, it is also highly plausible that those who have worse HOOS Pain levels may simply require more pain relief due to greater symptom severity and opt for oral analgesics. The results from this exploratory study indicate that the use of analgesics may not be adequate for relieving symptoms of hip OA. This is consistent with the general consensus that the first-line treatment of hip OA consists of exercise, patient education, and a weight-loss intervention if needed.23
Physical activity level
Surprisingly and contrary to findings in other studies and treatment recommendations for patients with hip OA,13,24,25 the findings in this study indicate an association between low physical activity level and better outcomes for patient-reported pain and QoL (Table 2). There also seemed to be an association between low physical activity level and patient-reported physical function, but only for the patients recruited from hospitals (Appendix A, Table 3). These associations could be spurious findings, as the confidence intervals are wide and include the point of no association. Moreover, it is likely that the highly active participants report higher levels of pain and worse QoL because they feel more severely affected by their disability in the activities that they wish to engage in, compared to more sedentary participants.
Physical function dependent on recruitment site
Patients recruited from hospitals had lower patient-reported and objectively measured physical function compared to patients recruited from clinics (Table 3). The point estimates indicate that these differences are not, or just barely clinically relevant differences and the lower limits of the 95% CI are well below the MID.8 This difference was expected to be larger, as patients recruited from hospitals are referred to evaluation of eligibility for surgery. These findings suggest that patients in the Danish healthcare system, who are referred to and attend consultation for the end-stage treatment for osteoarthritis, only have slightly worse impairments and pain than patients attending physiotherapy for conservative care. However, the participants recruited from the hospitals for this trial population were those who declined or were not found eligible for surgery by the orthopaedic surgeon, which could explain the lack of substantial differences between the subgroups.
Clinical implications
This study found that greater muscle power was associated with better results across all outcome measures, reinforcing its role in managing hip OA.6,8 As such, there seems to be some evidence for evaluating and targeting muscle power in clinical practice. The results also indicate a relationship between analgesics and the severity of symptoms and impairments. The stronger association observed between analgesic use and PROMs, compared with muscle power, may reflect confounding by indication rather than a true causal effect. The substantial use of inefficient analgesics in patients with hip OA indicates a need for optimizing guidance in the use of analgesics in clinical practice.
Patient characteristics and outcomes did not differ substantially between patients recruited from physiotherapy clinics and hospitals. This suggests a need to optimize the referral process. Currently, general practitioners refer patients they consider suitable for surgery. However, physiotherapists, who possess more specialized expertise in musculoskeletal conditions, could play a more active role in the referral process in primary care or collaborate with general practitioners to help reduce inappropriate referrals, highlighting a potential area for future research.
Limitations
As this is an explorative study examining a large number of outcomes, the risk of committing a type-1 error is substantial. Furthermore, the sample size calculation is based on the original study by Kjeldsen et al.17 which increases the uncertainty of the statistical power of observations. As a result of this and consistent with contemporary statistical guidelines, a dichotomous interpretation of p-values and statistical significance were not applied.20 Moreover, physical activity level and the outcomes of interest are patient-reported, which may lead to uncertainties regarding the classification. The categorization of ‘Symptom duration’, ‘Education’, and ‘Physical activity level’ is very broad, which may lead to additional uncertainty. As anchor-based MIDs are inherently sample-dependent and may vary according to baseline severity, patient expectations, and contextual factors, the MID applied in the present study should be interpreted as an approximate benchmark of clinical relevance rather than a universal constant. Lastly, the exploratory nature of this study entails that it is not possible to make causal inferences. Therefore, the results from this study should be considered as inspiration for further research rather than recommendations for clinical practice.
Conclusion
In patients with hip osteoarthritis, the findings of the current study suggest that muscle power was positively associated with HOOS Pain, QoL, and ADL, and the 30s-CST, while the use of analgesics was negatively associated with these outcomes. Patients recruited from hospitals seem to have slightly worse HOOS Pain and ADL scores than patients recruited from physiotherapy clinics. Associations between patient characteristics and HOOS Pain, QoL, and ADL, and the 30s-CST were not dependent on recruitment site.
Disclosure of interest
The authors report there are no financial or non-financial competing interests to declare.