The influence of multimorbidity on the management of patients with hip osteoarthritis
Keywords: Multimorbidity
Osteoarthritis
Function
Total hip replacement
Complication

Background & Aims
Osteoarthritis of the hip is a major cause of pain and disability worldwide. There is no successful disease modifying therapy, and treatment primarily consists of pain management with total hip replacement for advanced symptomatic disease. The success of total hip replacement may be evaluated through of a number of complementary outcomes, including the risks of complications, the need for further surgery, and the improvement in pain, function and quality of life.
Multimorbidity is associated with increased risks and reduced benefits of a range of elective surgical procedures. Evidence on the influence of multimorbidity on the outcomes of total hip replacement specifically, however, remains limited. Patients with hip osteoarthritis and concurrent multimorbidity, and their clinicians, must decide whether to proceed with total hip replacement without personalised information available to them on the likely risks and benefits of surgery. This lack of information may lead to worse health outcomes, with the uncertainty increasing decisional anxiety and regret. It is unknown how such patients are currently managed.
The aim of this thesis was to investigate the influence of multimorbidity on the rate of patients with hip osteoarthritis receiving total hip replacement and, in those patients receiving surgery, the influence of multimorbidity on the outcomes of surgery.
Methods & Results
Following a validation of diagnostic codes for hip osteoarthritis, a cohort study was performed using data from the Clinical Practice Research Datalink (CPRD) to investigate the influence of multimorbidity on whether patients with hip osteoarthritis and concurrent multimorbidity receive total hip replacement in the UK. Multimorbidity was measured in five different ways: (1) Charlson Comorbidity Index, (2) a count of chronic diseases, (3) a count of medications prescribed in the previous 12 months, (4) a count of contact episodes with primary care in the previous 12 months, and (5) the Electronic Frailty Index. Overall, patients with increased multimorbidity were one third to two thirds less likely to undergo surgery. With the same cohort, the influence of multimorbidity on the outcomes of total hip replacement recorded in the Hospital Episode Statistics Admitted Patient Care (HES APC) and Patient Reported Outcomes Measures (HES PROMs) databases was then investigated. Patients with multimorbidity were at a slightly increased risk of complications and hospital readmission, and had an increased length of stay, between 1-3 times greater than patients without multimorbidity. However, the difference in benefits of surgery by having multimorbidity, in terms of pain, function and quality of life, was less than the established minimum clinically important differences of the PROMs investigated.
Given weaknesses in data quality on revision and other reoperations in HES APC, a further cohort study using data from two arthroplasty registries (Geneva Arthroplasty Registry and Swedish Hip Arthroplasty Registry) was performed, with multimorbidity measured with the American Society of Anesthesiologists (ASA) Classification System. Patients with ASA class III-IV had a three times higher risk of revision surgery and two times higher risk of other reoperation compared with ASA class I within three months of total hip replacement.
Comparisons between arthroplasty registries are performed to understand of variations in practice and outcomes. For comparisons of outcomes to be valid, controlling for the differences between the populations is required. In turn, the controlling relies on a consistent approach to the scoring of the variable in question. A study was performed that investigated the between-registry variation in multimorbidity in patients undergoing total hip replacement recorded in one regional (Kaiser Permanente, USA) and six national (Australia, Finland, Netherlands, New Zealand, Norway and Sweden) arthroplasty registries. Large variations in the distribution of ASA class were observed between the participating registries. The broad consistency in mortality rates by ASA class suggests there was consistency in assignment of ASA class in the different countries.
Conclusions
The increase in risks of total hip replacement for patients with multimorbidity appears to be modest and there is no clinically meaningful reduction in benefit. It is notable therefore that patients with multimorbidity are one third to two thirds less likely to undergo total hip replacement. It is possible that the substantial difference in the rate of total hip replacement by multimorbidity reflects previous inaccurate estimations of the risks and benefits of the surgery.
The variation in distribution of ASA class of patients undergoing total hip replacement in different countries, and the consistency in assignment of ASA class observed, underlines the importance of controlling for ASA class in future comparisons between arthroplasty registries.