Abstract
Objectives
To investigate whether the association between pain intensity and willingness to undergo surgery is explained by walking difficulties, in patients with knee or hip osteoarthritis (OA).
Methods
This is an observational study using data from the Better management of patients with Osteoarthritis (BOA) register, which collects data from a publicly financed self-management programme for people with OA in Sweden. We included all patients with knee or hip OA who attended the baseline visit between 2008 and 2016. We conducted separate mediation analyses within a counterfactual framework to estimate the mediation effect of walking difficulties (yes/no) on willingness to undergo surgery (yes/no) for each one-point increase in pain (0–10 on a numeric rating scale), adjusted for relevant confounders.
Results
We included 72,131 patients (69% women, mean age 66, mean pain 5.4, 81% had walking difficulties, 27% was willing to undergo surgery). A one-point increase in pain intensity was associated with 1.53 (95% CI: 1.51; 1.55) higher odds of being willing to undergo surgery. Walking difficulties mediated 10%–25% of the effect of one-point increase in pain when pain was <8/10, while at pain ≥8/10 this percentage decreased to 3%.
Conclusions
More than 80% of the BOA patients have mild to moderate pain (<8/10) and walking difficulties can mediate up to a quarter of the total effect of pain on the willingness to undergo surgery in these patients. Trials to evaluate the potential to lower surgery demand by reducing walking difficulties in people with these characteristics are needed.
Keywords: Osteoarthritis, Knee, Hip, Willingness to undergo surgery, Pain
1. Introduction
Osteoarthritis (OA) is among the fastest-growing causes of disability worldwide with public expenditure for its management reaching up to 2.5% of the gross domestic products in the United States (US) and other westernised countries [1,2]. Total joint replacement (TJR) is often necessary and life-changing for people with long-lasting pain. However, the increase of TJR utilisation and the high costs associated with this procedure have raised concerns regarding its long-term sustainability [3,4]. Despite being one of the most common elective surgeries, there are no definitive and shared recommendations for deciding which patients should be considered for TJR, opening for potential bias in referral patterns [[5], [6], [7]]. In this context, a person’s willingness to undergo surgery is one of the strongest identified predictors for future TJR, together with joint symptoms and overall health [8].
The choice of treatment is a complex decision-making process influenced by a wide range of factors among which pain covers a primary role [[8], [9], [10]]. Pain is, in fact, a key symptom of OA and the main determinant of OA associated disability [[11], [12], [13], [14], [15], [16], [17]]. Problems with walking have a major impact on the quality of life of people with OA and are often rated by patients among the most important factors influencing the decision to undergo joint surgery [8]. This suggests that the effect of pain on the willingness to undergo surgery may be partially mediated by the presence of walking difficulties whose effect may vary across pain severity levels. Despite this, evidence on the interplay between pain and perceived walking difficulties is limited. Understanding this relationship in the context of willingness to undergo surgery may provide valuable information to help clinicians to improve OA management potentially minimising the need for TJR.
Thus, this study aims to investigate the possible mediation effect of walking difficulties on the relationship between pain intensity and the willingness to undergo surgery in patients with knee and hip OA.
2. Patients and methods
This is an observational study using register data. The study was approved by the Regional Ethical Review Board in Gothenburg (1059–16). Data from the enrolment visit of the Better management of patients with Osteoarthritis (BOA) register from September 2008 and December 2016 were used for the study [18]. The BOA register is a publicly funded national register, collecting data of people who undergo a structured self-management programme including education and exercise. This intervention is in line with the recommendations by the Swedish National Board of Health and Welfare and international guidelines on OA management [[19], [20], [21], [22], [23]]. Thus, people included in the BOA registers had sought treatment in primary healthcare in Sweden for knee and/or hip pain, and they had been referred for a standardised first-line intervention after a confirmed clinical or radiographic OA diagnosis. The clinical diagnosis was based on clinical history and examination as recommended by the National Board of Health and Welfare in Sweden [24]. Radiographic evidence of OA is not required to be included in the BOA register in accordance with international guidelines [23]. The index joint for the treatment was identified by the physiotherapist based on the patients’ symptoms and medical history. If more than one joint was affected (e.g. both knees or one knee and one hip), the most symptomatic joint was considered as the index joint. A joint with a previous joint replacement could not be considered as index joint. Exclusion criteria were: a reason other than OA for joint problems (e.g., sequel hip fractures, chronic widespread pain, inflammatory joint diseases or cancer), total joint replacement within the past 12 months, other surgery of the knee or hip joint within the past 3 months, other serious diseases that may prevent the person to take part in the programme (e.g. neurological diseases, dementia) and not being able to read or understand Swedish. For this study, we included all participants with knee or hip OA who attended the enrolment visit which took place prior to any treatment or information was delivered. Further details on the register can be found elsewhere [18].
3. Outcome and exposure and variables
We used pain intensity as the exposure which was measured as the mean pain intensity during the last week in the most affected joint using a numeric rating scale (NRS) ranging from 0 (no pain) to 10 (maximum pain). For descriptive purposes, pain was divided into three categories representing mild pain (pain score <4) moderate pain (pain score ≥4 and < 8), severe pain (pain score ≥8) to show participants’ characteristics at different levels of the exposure.
We used willingness to undergo surgery as our outcome which was assessed by the question “Are your knee/hip symptoms so severe that you wish to undergo surgery? (Yes/No)”. The presence of walking difficulties was assessed by the question “Do you have problems walking as results of your joint problems “which is part of the Charnley score [25]. Based on the definition of walking difficulties used in this study and in the literature, we assumed walking difficulties to be a mediator of the relationship between pain and walking difficulties and pain to affect walking difficulties and not vice versa [12,25,26]. We used direct acyclic graphs (DAGs) to identify possible confounders (Fig. 1). Sociodemographic (age, sex, body mass index [BMI]), level of education, previous joint surgery, health-related quality of life and self-efficacy were identified as confounders [[26], [27], [28], [29], [30], [31]]. Furthermore, we assumed that both pain and walking difficulties impact willingness to undergo surgery, and not vice versa.
Fig. 1.
Direct Acyclic Graph (DAG) describing the relationship between exposure, outcome, mediator and confounders.
The blue nodes represent exposure (pain) and outcome (willingness to undergo surgery). The green node is the mediator. Red nodes represent confounders. The blue solid arrows represent the relationship between exposure and outcome, while the blue dashed arrows represent the mediation effect of walking difficulties on the relationship between pain intensity and walking difficulties. The red arrows represent confounding effects. (BMI: Body mass Index; QOL: Quality of life). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
3.1. Sociodemographic
Participants reported their age, sex and level of education. Bodyweight and height were self-reported from which the body mass index (BMI) was calculated as kg/m2.
3.2. Health-related quality of life
Participants rated their general health status using the visual analogue scale (VAS) included in the EQ5D questionnaire with a score ranging from 0 (poor health) to 100 (excellent health) [32,33].
3.3. Previous surgery
Previous surgery was reported by the physiotherapist based on the patient-reported medical history for both the index and the contralateral joint. Any kind of joint surgery to the hip or the knee were considered and reported in the register (e.g. arthroscopic surgery, anterior cruciate ligament reconstruction, osteotomy, partial and total joint replacement). Joints that received a joint replacement at any time or any arthroscopic surgery <3 months before enrolment cannot be included as index joints in the BOA register. However, participants who received a joint replacement (partial or total, including revisions) in the contralateral joint >12 months before enrolment or that received an arthroscopic intervention in either joint >3 months before enrolment are allowed in the register.
3.4. Education
Self-reported education attainment was recorded in 3 categories: mandatory education (up to 9 years) high school (up to 12 years) and University or higher (>than 12 years).
3.5. Self-efficacy
Self-efficacy was assessed by the Arthritis Self-Efficacy Scale (ASES) which includes 3 subs-scales measuring self-efficacy related to pain, other symptoms and function on a 10 (lowest level of self-efficacy) to 100 (highest level self-efficacy) scale [34]. In BOA, only the scales assessing pain and other-symptoms self-efficacy are included. In the current study, we used only the scale assessing pain self-efficacy.
4. Analysis
We reported characteristics of the sample including frequencies and percentages for categorical variables and means with relevant standard deviation (SD) for continuous variables (Table 1). We used linear logistic regression to estimate the exposure-outcome association as well as the mediator-outcome and the exposure-mediator association. We conducted a mediation parametric analysis with a counterfactual framework using the Stata command medeff, which is designed for causal mediation analysis [35]. In this approach, the total effect of pain intensity on the willingness to undergo surgery is divided into a direct and indirect effect to produce a percentage of the total effect that is mediated (i.e. indirect effect/total effect) by walking difficulties [36]. The modelling is based on a counterfactual framework, which means that two levels of exposure are compared. In this study, the pain intensity was used as exposure. We have thus performed the mediation analysis for each 1-point increment in pain estimating the mediation effect of a 1-unit incremental increase in pain score. This approach also enabled us to take into account that the mediation effect can vary across pain levels. All the analyses, were adjusted by age, sex, body mass index (BMI), level of education, previous joint surgery, health-related quality of life and self-efficacy. We also repeated stratifying the sample by joint and by sex due to the possible influence of these variables on the willingness to undergo surgery [28,36,37]. For all mediation analyses, 1000 simulations were used to compute standard errors and 95% confidence intervals for the total, direct, and indirect effects.
Table 1.
Participants demographic for the whole sample and stratified by levels of exposure.
| All participants N = 72,131 | Mild pain (Pain <4) N = 13,936 | Moderate Pain (Pain ≥4; <8) N = 48,203 | Severe Pain (Pain ≥8) N = 9369 | |
|---|---|---|---|---|
| Age, mean (SD) | 65.9 (9.6) | 66.8 (9.5) | 66.0 (9.5) | 64.2 (9.9) |
| Sex | ||||
| Men, n (%) | 22,480 (31) | 4689 (34) | 15,135 (31) | 2576 (27) |
| Women, n (%) | 49,312 (69) | 9247 (66) | 33,068 (69) | 6793 (73) |
| BMI, mean (SD) | 28.0 (5.0) | 26.9 (4.4) | 28.1 (4.9) | 29.3 (5.5) |
| Pain NRS, mean (SD) | 5.4 (2.0) | – | – | – |
| Education | ||||
| Mandatory education (up to 9 years), n (%) | 24,625 (34) | 4271 (31) | 16,916 (35) | 3316 (36) |
| highschool (up to 12 years), n (%) | 26,716 (37) | 4645 (33) | 18,117 (38) | 3871 (42) |
| University or higher (>than 12 years), n (%) | 20,165 (28) | 4973 (36) | 12,999 (27) | 2128 (23) |
| Joint | ||||
| Hip, n (%) | 22,716 (31) | 4003 (29) | 15,372 (32) | 3166 (34) |
| Knee, n (%) | 49,415 (69) | 9933 (71) | 32,831 (68) | 6203 (66) |
| Previous surgery contralateral joint | ||||
| No, n (%) | 64,177 (89) | 12,664 (91) | 42,758 (89) | 8199 (88) |
| Yes, n (%) | 7546 (11) | 1201 (9) | 5168 (11) | 1116 (12) |
| Previous surgery index joint | ||||
| No, n (%) | 62,592 (87) | 12,215 (88) | 41,830 (87) | 8007 (86) |
| Yes, n (%) | 9351 (13) | 1685 (12) | 6253 (13) | 1336 (14) |
| ASES PainScore, mean (SD) | 62.2 (19.1) | 71.6 (17.7) | 61.4 (18.1) | 52.4 (20.4) |
| EQ5D VAS, mean (SD) | 65.8 (19.5) | 76.0 (17.1) | 65.3 (17.9) | 54.0 (22.6) |
| Walking Difficulties | ||||
| No, n (%) | 13,288 (19) | 5943 (43) | 6814 (14) | 481 (5) |
| Yes, n (%) | 58,014 (81) | 7880 (57) | 41,087 (86) | 8835 (95) |
| Willingness to undergo surgery | ||||
| No, n (%) | 51,846 (73) | 12,925 (93) | 34,723 (73) | 4041 (44) |
| Yes, n (%) | 19,074 (27) | 943 (7) | 12,911 (27) | 5158 (56) |
EQ-5D: EuroQol 5 dimensions, BMI: Body Mass Index, ASES: Arthritis Self-Efficacy Scale.
5. Results
A total of 72,131 people with knee or hip OA were included in BOA between September 2008 and December 2016 of which 19,074 (27%) were willing to undergo surgery for their joint problems (knee OA patients: 25%, hip OA patients: 31%) (Table 1) (Supplementary material 1A).
As expected, pain and walking difficulties are related. Among those with mild pain, 57% (n = 7880) had walking difficulties. This percentage increases to 85% (n = 41,087) among those with moderate pain, and 95% (n = 8835) among those with severe pain. Each point increase in pain intensity was associated with a 1.53 (95% CI: 1.51; 1.55) increase in the odds of being willing to undergo surgery and with a 1.47 (95%CI: 1.45; 1.49) increase in the odds of having walking difficulties. Walking difficulties was associated with a three-fold (OR 2.86; 95% CI 2.73; 3.20) increase in the odds of being willing to undergo surgery compared to patients without walking difficulties.
The mediation analysis suggested that perceived walking difficulties were an important mediator of the relationship between pain intensity and willingness to undergo surgery but that the percentage of the total effect mediated decreased with increasing pain (Fig. 2). In persons with mild pain intensity (<4/10), one-fourth of the effect of a one-point increase in pain on the willingness to undergo surgery was mediated by the presence walking difficulties, while at severe pain levels (>7, defined as severe pain) this percentage decreased to 3%. Analysis stratified by sex and by index joint yield similar results with higher mediation at lower levels of pain (Fig. 3).
Fig. 2.
Percentage of the total effect of one-unit pain increment on the willingness to undergo surgery mediated by walking difficulties
Mediation effect of walking difficulties on the relation between pain intensity and willingness to undergo surgery across pain intensity levels. In the mediation analysis, one-unit difference in pain level is considered (i.e. score 1 compared to 0, 2 compared to 1, etc). Analysis are adjusted by age, sex, body mass index (BMI), level of education, previous joint surgery, health-related quality of life and self-efficacy.
Fig. 3.
Percentage of the total effect of one-unit pain increment on the willingness to undergo surgery mediated by walking difficulties and stratified by (A) joint and (B) sex
Analysis are adjusted by age, sex, body mass index (BMI), level of education, previous joint surgery, health-related quality of life and self-efficacy.
6. Discussion
Pain is one of the main symptoms driving health-seeking behaviours. In this large register-based cohort study, we estimated that one-point increase in pain intensity is associated with 1.5 higher odds of a person being willing to undergo surgery. Walking difficulties is an important mediator in this relationship and can mediate up to 25% of this association. The mediation effect was more pronounced at low levels of pain intensity and showed a gradual but constant reduction with the increase in pain intensity. Conversely, the mediation effect of walking difficulties was scarcely influenced by sex or by the affected joint (knee or hip) and the observed differences appear to be of scarce clinical importance.
Previous studies reported perceived disease severity as one of the main factors linked to a person’s willingness to undergo surgery [8,9]. Thus, when only mild pain is present, perceived walking difficulties may have a more predominant role in determining the perception of disease severity while it may have a less prominent role when severe pain is present. Walking difficulties may impair the ability of a person to carry out activities of daily living or to participate in social life potentially reducing a person’s quality of life driving the treatment choice even when only mild pain is present [10].
Conversely, perceived walking difficulties appear to have little or no impact on the willingness to undergo surgery in patients with severe pain. This result may be interpreted as the pain intensity having a greater impact on the person’s willingness to undergo surgery. However, at high levels of pain (≥8/10), nearly the totality (95%) of the participants reported walking difficulties making the effect of these two factors on the outcome indistinguishable thus explaining the low mediated percentage.
Currently, willingness to undergo surgery is the strongest predictor of future joint replacement despite severe pain, disability and non-response to first-line interventions are recommended criteria to deem surgery eligibility [30,38]. Based on these criteria, previous studies reported that approximately a third of total knee arthroplasties may be considered inappropriate with >70% due to the presence of mild pain [39,40]. The relatively large mediation effect of walking difficulties in OA patients may partially explain the willingness to undergo surgery in persons with the presence of mild symptoms and the large number of surgeries deemed inappropriate. In light of these results, perceived walking difficulties may significantly influence the demand for joint surgery in OA.
Despite relatively few people with mild (7%) and moderate pain (27%) were willing to undergo surgery, intervention addressing perceived walking difficulty and desire for surgery may have important clinical implications. TJR is one of the largest expenditure for many health care system in the western world while people with mild to moderate pain account for the largest share of inappropriate surgeries performed [40]. In this context, even a small reduction in the share of people willing to undergo surgery may have a large economical and societal impact.
First-line interventions, which include exercise as the core treatment and focus on the self-management of OA symptoms, have been shown to be associated with a reduction in willingness to undergo surgery in people with hip or knee OA [41]. These treatments have also been reported to be effective in improving physical function which may partially explain their ability to influence a person’s willingness to undergo surgery [[42], [43]]. Thus, steering away from a pain-centred and passive approaches focusing more on function and perceived walking difficulties may be worthwhile [[44], [45], [46]]. Walking is one of the fundamental expression of movement, which is a complex construct that goes beyond the physical capability of performing an action [47,48]. Movement is, in fact, conceptualised on a continuum that incorporates physical and pathological aspects with social and psychological considerations [49]. Thus, movement can be described both as a shift in position and as an emotional expression which is connected with a person’s body image and self-confidence [48]. In this context, surgery may not answer the patient needs especially when perceived walking difficulties coexist with mild pain. In these cases, addressing the existing difference between desired and current movement potential, using a biopsychosocial model for movement, is recommended and may significantly improve the quality of life while potentially reducing the number of inappropriate surgery.
Hip OA patients have been reported to have more severe pain and willingness to undergo surgery when starting an exercise-based self-management programme [36,50,51]. In our sample, 31% of the hip OA patients vs 25% of those with knee OA were willing to undergo surgery. Despite this, the role of walking difficulty in determining willingness to undergo surgery appears to be similar regardless of the affected joint. Similarly, little or no difference in the mediation effect of walking difficulties was found between men and women. Differences in the rate of surgery consideration between knee and hip OA patients may thus be due to differences in disease severity or other unmeasured factors.
Some limitations need to be discussed. This study includes a sample of OA patients who sought first-line treatments for their joint problem. Generalisation of the results should, therefore, be made with care. Walking difficulties in this study were self-reported. Despite objective measurements may provide a quantitative measure of the effect of physical limitations in the process leading to considering joint surgery, a person perception of walking impairments may better reflect the perceived disease severity and may, therefore, better relate to the desire to undergo surgery. In addition, factors other than pain may contribute to walking difficulties. Accounting for these factors may help to disentagle further the relationship between walking difficulties and willingness to undergo surgery. Despite adjusting for numerous confounders identified through the use of a DAG, we recognise the possibility that residual confounders may exist. Of note, previous experience with exercise or physical therapy for their OA problems may influence the relationship under investigation. However, we did not have sufficient information on previous exercise or physical therapy for OA to be able to take it into account in our analysis. Moreover, the possibility to establish causality in the relationship between pain, walking difficulties and willingness to undergo surgery is limited considering that all the variables were measured at a single time-point. However, the directionality of this relationship may be hypothesised based on clinical evidence supporting our assumption of a link between pain, walking difficulties and willingness to undergo surgery. Finally, the question assessing willingness to undergo surgery was not specific for TJR and could comprise other surgical procedure for OA (e.g. osteotomy). However, TJR is the most common and most requested surgical intervention for people with OA and therefore we expect it to account for the largest part of people willing to undergo surgery somewhat minimising the potential bias. Moreover, interpretation of the results needs to take into account the fact that other factors not included in the study (e.g. previous injuries, treatment expectations) and not accounted for in the analysis may influence willingness to undergo surgery and confound the studied mediation. Similarly, different measures of pain (e.g. NRS measuring average pain in the last months) may yield different results.
7. Conclusions
Preoperative decision making is a complex process which often relies on the patients’ willingness to undergo surgery. Our study indicates that walking difficulties mediate up to a quarter of the total effect of pain on the willingness to undergo surgery in patients with mild to moderate pain while decreasing with increasing pain severity. Considering the high ratio of TJR considered as inappropriate due to only mild symptoms being present, and to the strong influence that a person’s willingness to undergo surgery has on the referral path leading to the operation, addressing walking difficulties may be important in treatments aiming to reduce the demand for surgery due to OA.
Source(s) of support
Greta och Johan Kocks stiftelserF 2019/302. AFA Insurance, Sweden (160,176).
Declaration of competing interest
This manuscript describes original work and is not under consideration by any other journal. All authors approved the manuscript and this submission. Thérése Jönsson is part of the steering committee of the BOA registry, for which she does not receive any compensation. Leif Dahlberg is the Co-founder and Chief Medical Officer of Joint Academy, a company which provides web-based non-surgical interventions for patients with hip and knee osteoarthritis. All other authors report no competing interests.
Acknowledgements
We would like to thank all participating patients and PTs reporting data to the BOA registry, and others involved in the BOA programme.
Footnotes
Supplementary data to this article can be found online at https://doi.org/10.1016/j.ocarto.2021.100157.
Appendix A. Supplementary data
The following is the Supplementary data to this article:
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