Objective Unlike several other fields of healthcare, little is known about the size of ‘therapist effects’ on patient outcomes following rehabilitation for musculoskeletal conditions. We aimed to estimate the proportion of variance in patient outcomes from a structured rehabilitation program explained by therapist effects.

Methods For our observational cohort study we accessed data from the national multicentre Good Life with osteoArthritis in Denmark (GLA:D) osteoarthritis management program. Analyses included 23,021 consecutive eligible adults with hip or knee osteoarthritis (mean (SD) age 65.0 (9.8) years, 71% female) treated by 657 therapists between October 2014 and February 2019. The primary outcome was ≥30% reduction in pain intensity on 0-100 VAS at 3 months. Therapist effects were estimated as the variance partition coefficient (intra-class correlation coefficient (ICC)) from two-level random intercept logistic regression models before and after adjusting for patient-level case-mix factors and therapist-level characteristics (number of patients treated, days since therapist certification). Analyses were repeated for a range of secondary outcomes using multiply imputed data and complete-case analysis.

Results 52% of patients reported a ≥30% reduction in pain intensity on 0-100 VAS at 3 months. In the null model the ICC was 0.007 (95%CI: 0.005, 0.009), which changed little after adjusting for patient- and therapist-level covariates. Upper confidence limits for ICC estimates across all secondary outcomes in multiply imputed and complete case analyses were less than 0.03.

Conclusions In a nationally implemented osteoarthritis management program delivered by trained healthcare professionals, therapist effects made a minimal contribution to variation in patient outcomes.

What is already known on this topic ‘Therapist effects’ - defined as the effect of a given therapist on patient outcomes as compared to another therapist - have been observed in several fields of healthcare and have important consequences for selection, training, and service improvement. In musculoskeletal rehabilitation five previous studies suggest that 1-12% of variation in patient-reported outcomes may be attributable to therapist effects, but these estimates were based on relatively small datasets resulting in substantial uncertainty.

What this study adds Our cohort study analysed registry data from 2014-2019 on 23,021 patients and 647 trained therapists from the nationally implemented GLA:D structured osteoarthritis management program in Denmark. We found that therapist effects accounted for less than 3% of total variation in patient-reported pain and quality of life outcomes 3 months after beginning the program

How this study might affect research, practice, or policy Our findings suggest that contextual factors that relate to therapist effects – therapist characteristics or therapist-patient interaction and alliance - make a minimal contribution to variation in patient outcomes from this structured, group-based rehabilitation intervention. Any contextual effects must be attributable to alternative sources, e.g. patient expectations, intervention setting.

EMR is the copyright holder of Knee injury and Osteoarthritis Outcome Score (KOOS) and several other patient-reported outcome measures, and co-founder of the Good Life with Osteoarthritis in Denmark (GLA:D), a not-for profit initiative to implement clinical guidelines in primary care hosted by University of Southern Denmark. STS has received personal fees from Munksgaard and TrustMe-Ed, outside the submitted work, and is co-founder of GLA:D. PEO, DA, JP, DY, DTG, GMP have no competing interests to declare.

This study was funded by the Danish Physiotherapy Association, the Danish Rheumatism Association, the Physiotherapy Practice Foundation, Region Zealand (Exercise First), EU Horizon 2020 (No 945377 (ESCAPE)), Innovative Health Initiative Joint Undertaking (IHI JU) (No. 101219324 (PROBE)), Danish Regions (No R232-A5132), The Department of Sports Science and Clinical Biomechanics, Faculty of Health Sciences, University of Southern Denmark, NSR Hospitals, Denmark (No A1683), School of Health and Social Care, Sheffield Hallam University, and the EPSRC South Yorkshire Digital Health Hub (EP/X03075X/1).

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

Yes

The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

The Danish Data Protection Agency waived ethical approval for this work.

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I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

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Email: pobasohannigerpoly.edu.ng

Email: j.palmer1shu.ac.uk

Email: D.Aldersonshu.ac.uk

Email: d.yukeele.ac.uk

Email: dgronnehealth.sdu.dk

Email: erooshealth.sdu.dk

Email: stskouhealth.sdu.dk

Email: g.peatshu.ac.uk

The data used in this study cannot be shared publicly because of potentially identifiable or sensitive information. Data may be accessed upon reasonable request by contacting the GLA:D administration.