The effect of BMI and other factors on post-operative length of stay: A multivariable regression analysis of 860 patients undergoing total knee arthroplasty.

Background: There is a trend towards minimising length of stay (LOS) after total knee arthroplasty (TKA), as greater LOS is associated with poorer outcomes and higher costs. Patient factors known to influence LOS post-TKA include age and ASA grade. Evidence regarding the effect of body mass index (BMI) in particular is conflicting, with some studies finding that increased BMI predicts increased LOS, while others have found no relationship. Few previous studies, which have mostly been conducted outside the UK, have examined the effect of living alone or socioeconomic deprivation, which may be confounders.

Methods: We conducted a retrospective cohort study of 1031 consecutive primary TKAs performed between 1 April 2021 and 31 December 2021 in a single high-volume arthroplasty centre. A multivariable negative binomial regression model was performed for the 860 patients with complete data, using pre-operative (BMI, age, gender, ASA grade, smoking, ethnicity, socioeconomic deprivation, living arrangement, EQ5D quality of life score, and indication for surgery) and peri-operative variables (surgeon, surgical approach, tourniquet use, a.m./p.m operation, operation side, duration, and day of the week).

Results: Mean LOS was 2.6 days. BMI and socioeconomic deprivation had no effect on LOS (P > 0.05). Increased LOS was associated with living alone, lower EQ5D, age and ASA grade (all P < 0.001), p.m. operation (P < 0.01), female gender and duration of surgery (P < 0.05).

Conclusion: BMI and socioeconomic status were not correlated with LOS after TKA. Living alone, which has not been previously reported and lower pre-operative EQ5D status were significant risk factors, which merit consideration in pre-operative planning and counselling.