Purpose: This study evaluated the effects of continuous passive motion (CPM) on accelerated flexion after total knee arthroplasty (TKA) and whether CPM application measures (i.e. initial angle and daily increment) are associated with functional outcomes. Methods: A retrospective investigation was conducted at the rehabilitation centre of a university-based teaching hospital. Patients who received CPM therapy immediately after TKA surgery were categorized into rapid-, normal-, and slow-progress groups according to their response to CPM during their acute inpatient stay. Knee pain, passive knee flexion, and knee function—measured using the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC)—were assessed preoperatively at discharge and at 3- and 6-month outpatient follow-up visits. Results: A total of 354 patients were followed for 6 months after inpatient-stay discharge. The patients in the rapid-progress group (n = 119) exhibited significantly greater knee flexions than those in the slow-progress group did (n = 103) at the 3-month follow-up [mean difference (MD) = 10.3°, 95 % confidence interval (CI) 4.3°–16.3°, p <0.001] and 6-month follow-up (MD = 10.9°, 95 % CI 6.3°–15.6°, p <0.001). Significant WOMAC score differences between the rapid- and slow-progress groups were observed at the 3-month follow-up (MD = 7.2, 95 % CI 5.4–9.1, p <0.001) and 6-month follow-up (MD = 16.1, 95 % CI 13.4–18.7, p <0.001). CPM initial angles and rapid progress significantly predicted short- and long-term outcomes in knee flexion and WOMAC scores (p <0.001). Conclusion: When CPM is used, early application with initial high flexion and rapid progress benefits knee function up to 6 months after TKA. Level of evidence: II.
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