A retrospective study of the association between pain intensity and opioid use with length of stay during musculoskeletal inpatient rehabilitation after primary knee and hip arthroplasty

BACKGROUND: The relationship between pain intensity, opioid consumption, and length of stay (LOS) has received little attention in primary, lower extremity joint arthroplasty patients admitted to inpatient musculoskeletal rehabilitation. OBJECTIVE: To assess how initial pain and other clinical factors are associated with rehabilitation LOS. DESIGN: Retrospective chart review. SETTING: Rehabilitation hospital. PARTICIPANTS: One hundred ninety nine patients admitted for inpatient rehabilitation. INTERVENTIONS: Not Applicable. MAIN OUTCOME MEASURES: Pain intensities on the Numeric Rating Scale (NRS) were completed 3 times daily and total daily opioid consumption recorded in terms of morphine equivalents (MEQ). Confounding variables included patient demographics, medical comorbidity burden using the Charlson Comorbidity Index (CCI), and early functional status as measured by the motor subscale from the Functional Independence Measure (FIM). RESULTS: Mean day 3 NRS values of ≥5.2 and total day 3 opioid consumption of >50 MEQ were associated with a prolonged LOS by nearly 3 and 2 days, respectively. Within a multivariate linear regression, age, mean day 3 pain, comorbidity burden, and early motor functional status accounted for 36% of the variability seen in joint replacement rehabilitation LOS. With all other variables remaining constant, for every unit increase in mean day 3 pain and CCI, this amounted to an additional 5% and 4% increase to LOS, whereas each unit increase in admission motor FIM decreased estimated LOS by 3%. CONCLUSION: Mean pain intensity and total opioid consumption on day 3 of inpatient rehabilitation is associated with LOS. For the rehabilitation physician, this is useful information as the earlier identification of patients with poorly controlled pain can lead to directed intervention, better patient care, and significant cost savings.