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Differences in patient characteristics and external validity of randomized clinical trials on pain management following total hip and knee arthroplasty: a systematic review
  1. Casper Pedersen1,
  2. Hannibal Troensegaard2,
  3. Jens Laigaard1,
  4. Seda Koyuncu1,
  5. Henrik Morville Schrøder3,
  6. Søren Overgaard4,
  7. Ole Mathiesen1,5 and
  8. Anders Peder Højer Karlsen1
  1. 1Centre for Anaesthesiological Research, Department of Anaesthesiology, Zealand University Hospital Koge, Koge, Denmark
  2. 2Department of Anaesthesia, Bispebjerg Hospital, Kobenhavn, Denmark
  3. 3Department of Orthopaedic Surgery, Nastved Hospital, Nastved, Sjaelland, Denmark
  4. 4Department of Orthopaedic Surgery, Odense University Hospital, Odense, Denmark
  5. 5Department of Clinical Medicine, University of Copenhagen, Kobenhavn, Denmark
  1. Correspondence to Dr Casper Pedersen, Centre for Anaesthesiological Research, Department of Anaesthesiology, Zealand University Hospital Koge, 4600 Køge, Denmark; casperpedersen17{at}


Background The external validity of randomized controlled trials (RCTs) is critical for the relevance of trial results in a clinical setting. We aimed to assess the external validity of RCTs investigating postoperative pain treatment after total hip and knee arthroplasty (THA and TKA) by comparing patient characteristics in these trials with a clinical cohort. Further, we assessed the use of exclusion criteria of the included RCTs.

Methods We searched PubMed, Embase, and Cochrane Central Register of Controlled Trials for relevant RCTs up to June 2019. Data on patient characteristics from this research population were compared with an unselected clinical cohort from the Danish Hip and Knee Arthroplasty Registries in the period 2005–2019. Trends in patient characteristics and the use of exclusion criteria were assessed with control charts.

Results In total, 550 RCTs with 48 962 participants were included in the research cohort. The clinical cohort included 101 439 THA patients and 90 505 TKA patients. Patient characteristics (age, body mass index (BMI), American Society of Anesthesiologists (ASA) score and sex distribution) in the research cohort resembled those of the clinical cohort. Age, BMI and ASA scores did not change over time in the research cohort. In the clinical cohort, age increased among both THA and TKA patients, and BMI and ASA scores increased among TKA patients. Most commonly used exclusion criteria in the RCTs were high ASA score (62%), older age (45%), obesity (32%) and chronic opioid use (41%). Exclusion of chronic opioid users and individuals with obesity increased over time.

Conclusion Patient characteristics in research trials investigating postoperative pain management after THA and TKA currently resemble those of a clinical cohort. However, individuals in the clinical cohort are getting older, and TKA patients more obese with increasing ASA scores. Concomitantly, RCTs increase the tendency to exclude patients with older age, obesity, chronic pain and/or opioid use. This trending discrepancy can hinder the generalizability of future research results, and therefore increased focus on pragmatic trials resembling real-world conditions are needed.

PROSPERO registration number CRD42019125691

  • pain medicine
  • acute pain
  • postoperative pain

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  • Contributors APHK, OM, and CP conceived the presented idea; conducted tables, figures and appendices; wrote the paper. APHK, SK, OM, HMS, SO, and CP designed the analysis and data collection. HT, JL, and CP extracted data. JL and APHK conducted SPC analyses. HMS, SO, OM, and APHK supervised and revised the project and its full content.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.

  • Patient consent for publication Not required.

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Data availability statement Data are available upon reasonable request. We do not have permission to share data from the clinical cohorts (Danish Hip Register and Danish Knee Register). The research database is currently under expansion for a parallel project on Minimal Clinical Important Differences (MCID). If data should specifically be requested, we will gladly provide it.