Weight Change and the Risk of Chronic Pain Following Hip and Knee Arthroplasties: A Nationwide Registry-based Cohort Survey Study.
Conditions: Persistent Postsurgical Pain, Arthritis Knee, Arthropathy of Knee, Arthropathy of Hip, Weight Loss, Weight Gain, Obesity
Sponsor: Bispebjerg Hospital

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Weight change and the risk of chronic pain following hip and knee 
arthroplasties: A nationwide registry-based cohort survey study.  
 
Authors/Collaborators: 
Saber M. Saber1,2,3, MD, PhD Fellow; Jens Laigaard1,3, MD, PhD Fellow; Martin Lindberg-Larsen, 
MD, PhD, Associate professor4; Søren Overgaard, MD, DMSc, Professor1,3 
 
Affiliations: 
1. Department of Orthopaedic Surgery and Traumatology, Copenhagen University Hospital, Bispebjerg and 
Frederiksberg, Copenhagen, Denmark; 2. The Parker Institute, Copenhagen University Hospital, Bispebjerg and 
Frederiksberg, Copenhagen, Denmark; 3. University of Copenhagen, Department of Clinical Medicine, Faculty of 
Health and Medical Sciences; 4. Department of Orthopaedic Surgery and Traumatology, Odense Hospital, Odense, 
Denmark.  
 
 
 

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INTRODUCTION  
The treatment of hip and knee osteoarthritis not responding to non-surgical treatments may be surgical 
joint arthroplasty.1 Though arthroplasty is considered an effective and safe treatment, unfavourable 
outcomes have been reported,2 including persistent postoperative pain (PPP). The association between 
body weight and outcomes following total hip arthroplasty (THA) and knee arthroplasty is well documented: 
overweight carries higher risk of bad functional and surgical outcomes.3-9 Also, Preoperative weight 
reduction reduces general health related complications following knee arthroplasty.10 However, only few 
studies have investigated weight change following THA and knee arthroplasty.11-16 Of these, some have 
indicated that weight change influences postsurgical outcomes.  
Identification of factors that are linked to the incidence of PPP could help us to prevent this problematic 
outcome. The objective of this study is to investigate whether weight change is associated with the 
incidence of PPP following THA and knee arthroplasty across non-obese and obese and patients. 
METHODS 
Study design  
The study is a nationwide, register-based cohort survey study. We will report the results following the 
CROSS checklist for standardized reporting of survey studies.17 We will investigate the change of BMI 11-15 
months after THA and knee arthroplasty and incidence of PPP by combining data from national registries 
and surveys.  
Questionnaire 
A 22-question questionnaire will be sent to the eligible candidates (see NCT05900791 and NCT05845177). 
The questionnaire is developed by combining domains from previously validated Patient Reported Outcome 
Measures (PROMs). Five questions are taken from the Western Ontario and McMaster Universities 
Osteoarthritis Index (WOMAC) pain domain; seven questions from the Douleur Neuropathique 4 interview 
(DN4i); a numerical rating scale (NRS) question for pain in the operated joint in the last week; satisfaction; 
a single question for each of the following: willingness to repeat the surgery; pain frequency; pain 
interference with daily activities; other chronic pain conditions; patient-reported use of analgesics; weight; 
height and the permission to contact respondents again later. Patient reported weight and height has been 
recently found valid in a Danish e-survey setting.18 A detailed description of the questionnaires and the 
current validation status of the domains can be found in 2 different papers.19,20 The Danish and an English 
version of the questionnaire will be available in the final publication.  

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Sample characteristics 
Patients with THA and knee arthroplasty who received their arthroplasty 11-15 months prior to the 
intended date of survey administration will be identified based on the NOMESCO (Nordic Medico-
Statistical Committee) Classification of Surgical Procedures from the the Danish National Patient Register 
by using SKS-code. The codes for THA patients will be SKS-code DM16 [hip osteoarthritis] + KNFB20 or 
KNFB30 or KNFB40[Primary Total Hip Arthroplasty]. For total knee arthroplasty (TKA), SKS-code DM17 
[knee osteoarthritis] + KNGB20, KNGB30 or KNGB40 [primary total knee arthroplasty]). For medial 
unicompartmental knee arthroplasty (UKA), SKS-code DM17 [knee osteoarthritis] + KNGB01 or KNGB11 
[primary medial unicompartmental knee arthroplasty]. The baseline characteristics of the sample 
including their BMI will be gathered from the Danish Hip Arthroplasty Register (DHR) and the Danish Knee 
Arthroplasty Register (DKR).  
Survey administration 
The data of this study is pooled from 2 surveys sent to THA and knee arthroplasty patients.19,20 Both surveys 
are distributed via ‘Digital Post’ which is linked to the unique Civil Registration number (CPR) using Research 
Electronic Data Capture (REDCap) software (www.r-project.org). A reminder will be sent 14 days after 
distribution for non-responders. A month after distribution date, a text-message will be sent to non-
responder’s telephone number if it is registered in the electronic patient file.  
Statistical analysis 
We expect to find a sample of around 6400 patients divided between hip and knee arthroplasty. Estimated 
response rates of 70% will leave us with 2240 patients to be analysed in each joint arthroplasty group. With 
an expected 15% of the sample to be weight gainers, we will be able to detect a minimum of 16.6% 
difference in the incidence of the outcome between gainers and non-gainers with a significance level of 0.05 
with a power of 0.80.  
We will report results for THA, TKA and medial UKA separately as we believe these are different patient 
groups. We will categorize patients into weight gainers (≥5% weight gain after arthroplasty), weight losers 
(≥5% weight loss) and those who maintain their weight. Descriptive statistics of the baseline characteristics 
of the three categories with the following variables will be presented in table 1: age, sex, BMI and surgical 
details (type of anaesthesia, operation length, use of local infiltration analgesia (LIA), use of cement, and 
surgical complications).  
The primary outcome is the occurrence of PPP and will be defined as NRS pain score higher than 3 as 
proposed by Moore et. al.21 Secondary outcomes include satisfaction defined as being very satisfied or 

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satisfied; willingness to repeat the surgery defined as answering “yes” in willingness question; frequent pain 
defined as experiencing pain constantly, daily or few times a week; use of analgesic; interference with daily 
activities, defined as answering “some”, “much” and “very much”; WOMAC pain score will be reported as 
the mean (standard deviation [SD]). Following outcome dichotomization (except for WOMAC pain), we will 
employ a multivariate logistic regression model to estimate the Odds Ratio (OR) and the corresponding 95% 
Confidence Interval (CI) amongst gainers and losers in relation to those who maintained their weight. For 
WOMAC pain, we will report the mean difference (MD) and the corresponding 95% CI. We will stratify for 
baseline obesity status (non-obese: BMI < 30 kg/m2, Obese: BMI ≥ 30 kg/m2). We will run a non-adjusted 
analysis first, then we will adjust for age, sex and the use of cement. We will do a sensitivity analysis to see 
whether the effect size is different between non-morbidly obese (BMI 30-39 kg/m2) and morbidly obese (BMI 
≥ 40 kg/m2), we will run another sensitivity analysis adjusting for patients having other chronic pain 
condition. Data is handled in the most recent version of R (www.r-project.org). 
Table 1 (Demographics) 
 
THA 
TKA 
Medial UKA 
Demographi
cs 
Weigh
t 
gainer
s 
Weigh
t 
losers 
Weight 
unchange
d 
Weigh
t 
gainer
s 
Weigh
t 
losers 
Weight 
unchange
d 
Weigh
t 
gainer
s 
Weigh
t 
losers 
Weight 
unchange
d 
Total N 
 
 
 
 
 
 
 
 
 
Age (mean) 
 
 
 
 
 
 
 
 
 
Female 
N 
(%) 
 
 
 
 
 
 
 
 
 
BMI (mean) 
 
 
 
 
 
 
 
 
 
General 
Anaesthesia 
N (%)  
 
 
 
 
 
 
 
 
 
Spinal 
Anaesthesia 
N (%) 
 
 
 
 
 
 
 
 
 
Operation 
time (mean) 
 
 
 
 
 
 
 
 
 

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Use 
of 
Cement 
N 
(%) 
 
 
 
 
 
 
 
 
 
Local 
Infiltration of 
analgesia N 
(%) 
 
 
 
 
 
 
 
 
 
Surgical 
complication 
N (%) 
 
 
 
 
 
 
 
 
 
 
Table 2 (Risk of developing PPP) 
 
Weight 
unchanged 
(Reference group) 
Weight gainers 
Weight losers 
 
N PPP: 
N 
(%) 
OR 
(95% 
CI) 
P 
value 
N PPP: 
N 
(%) 
OR 
(95% 
CI) 
Adjusted 
OR (95% 
CI) 
P 
value 
N PPP: 
N 
(%) 
OR 
(95% 
CI) 
Adjusted 
OR (95% 
CI) 
P 
value 
THA 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Non-
Obese 
 
 
1 
 
 
 
 
 
 
 
 
 
 
 
Obese  
 
1 
 
 
 
 
 
 
 
 
 
 
 
TKA 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Non-
Obese 
 
 
1 
 
 
 
 
 
 
 
 
 
 
 
Obese  
 
1 
 
 
 
 
 
 
 
 
 
 
 
Medial 
UKA 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Non-
Obese 
 
 
1 
 
 
 
 
 
 
 
 
 
 
 
Obese  
 
1 
 
 
 
 
 
 
 
 
 
 
 
 
 

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HEALTH RESEARCH ETHICS AND GENERAL CONSIDERATIONS  
Funding 
Section for Biostatistics and Evidence-Based Research, the Parker Institute, Bispebjerg and Frederiksberg 
Hospital is supported by a core grant from the Oak Foundation (OCAY-18-774-OFIL), which had no role in 
study design or writing of this protocol. 
Patient and Public Involvement (PPI) 
This project follows the EULAR recommendations for the inclusion of patient representatives in the 
contemporary scientific process.22 A panel of patients helped in developing the questionnaire. 
Conflict of interest 
All will be disclosed. 
Disclaimers 
The views expressed in the submitted protocol are the authors’ own and not an official position of the 
institution or funder. 
Ethics 
The local institutional review board approved the study, and the Danish Health Data Authority will provide 
contact information for potential respondents. Telephone numbers for non-respondents are found by 
searching the CPR number in the electronic patient files, but without accessing the patients’ health data. 
According to Danish legislation, approval from the national ethics committee is neither required nor 
possible to obtain for survey studies. 
Because we will link survey responses with perioperative data from the Danish Knee Arthroplasty Register, 
responses are not anonymous. However, only the authors will have access to confidential information and 
data will be anonymised as soon as possible. Until then, the data are stored pseudonymised at a logged 
and encrypted drive. 
 
 
 

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