Pain as an Exploratory Marker of Rehabilitation Engagement After ACL Reconstruction: Combined Ligament Injuries and Digital Disengagement in a Sensor-Based Monitoring Cohort

Background/Objectives: To analyse postoperative pain trajectories after anterior cruciate ligament (ACL) reconstruction using data from a digital rehabilitation system, and to determine (i) whether combined ligament injuries are perceived as more painful than isolated ACL tears, (ii) which patient characteristics are associated with clinically relevant pain (visual analogue scale [VAS] > 5), and (iii) whether higher early pain is associated with later discontinuation of digital monitoring. Methods: This retrospective cohort study used routine data recorded by a validated sensor-based home rehabilitation system in patients after ACL reconstruction. This approach has previously been used to analyse functional recovery trajectories. All patients with ACL-related indications who performed at least one postoperative test were included and classified into four groups: isolated ACL rupture, ACL + meniscus, ACL + collateral ligament, and ACL + collateral ligament + meniscus. Pain during exercises and tests was recorded on a 0–10 VAS. High pain was defined as VAS > 5. Group comparison between indication types, anthropometric and activity-related variables and the proportion of high-pain events were performed using chi-square tests. Early pain (first postoperative month) was analysed in relation to the presence of later tests (≥3 months) to explore associations with discontinuation of digital monitoring. Results: Combined ligament injuries showed a significantly higher proportion of high-pain events during rehabilitation compared with isolated ACL ruptures (5.8% vs. 2.4%, overall p < 0.001). In particular, combined ligament injuries with ACL + collateral ligament rupture were associated with a greater share of VAS > 5 ratings in the early rehabilitation phases. No relevant association was observed between sex or BMI category and the occurrence of high pain, while age group showed an overall association without a consistent directional pattern. Sport activity level showed a strong relationship with high pain (p < 0.001). Early pain demonstrated a small but statistically significant negative correlation with later test participation (r = −0.15, approximately 2% of variance, p = 0.0076); however, this association was attenuated and no longer statistically significant when analysed using mixed-effects models accounting for within-patient clustering, indicating that patients with higher early pain tended to discontinue digital monitoring. Conclusions: Digital routine data after ACL reconstruction suggest that (i) combined ACL–collateral ligament injuries are perceived as more painful than isolated ACL tears, (ii) high postoperative pain is more closely related to activity level and injury pattern than to sex or BMI, while age group shows an overall but non-directional association, and (iii) higher early pain shows a weak bivariate correlation with digital disengagement that was not confirmed in mixed-effects models. Pain is therefore an exploratory marker warranting further investigation, rather than a confirmed independent predictor of adherence in app-based rehabilitation.