Introduction: Currently little is known about factors that impact prognosis following quadriceps muscle strain injury (QMSI) despite persistent prevalence in running and kicking athletes. Elite Australian Football players are particularly susceptible to QMSI, with variable return to play (RTP) time frames (5-82 days) and significant reinjury rates (18%). The aim of this study was to determine whether clinical and radiological findings are associated with time to RTP and re-injury in elite Australian Football players. Methods: 163 QMSI in 127 elite Australian Football players from the Soft Tissue Registry of the AFL were analysed. Clinical data including player characteristics, mechanism of injury, injury type (index or re-injury), and the number of days to RTP were collected. Standardised MRI classification was performed by two independent, blinded radiologists according to the: (1) injury location (primary and secondary muscle injured, position of injury, anatomical location of injury), and, (2) injury severity (connective tissue disruption, severity of disruption, presence of waviness, presence and degree of retraction). Univariate analyses and multivariate Cox regressions were used to determine whether clinical (age, history of previous QMSI, mechanism of injury, dominant leg) and radiological factors were associated with RTP and re-injury. Results: Rectus femoris injuries (n=147, 90.2%) were predominately classified as musculotendinous junction (n=111, 68.1%) or myofascial injuries (n=20, 12.3%). Musculotendinous junction injuries were located at the central (49.5%) posterior (27.9%) and anterior (27.9%) aponeuroses. Injuries in the rectus femoris took greater time to RTP compared to vastii injuries (30.34 +/- 16.7 v 14.5 days +/- 8.4, p<0.05). The anatomical location of injury was associated with prognosis: Rectus femoris injuries in the proximal free tendons (50.4 days +/- 16.2, p<0.05) and the central aponeurosis (33.7 days +/-20.4, p=0.031) showed the longest time to RTP. Injuries with evidence of connective tissue (free tendon or aponeurosis) disruption (34.7 days +/- 19.5, p<0.000), waviness (40.2 +/- 24.9, p=0.007), or retraction (43 days +/- 24.5, p=0.018) took longer to RTP. Multivariate Cox regression demonstrated injuries with a greater severity of aponeurotic disruption (>50% of cross-sectional area) to be associated with a longer RTP (HR 0.39, p=0.021). Older age was the only clinical variable associated with faster RTP (HR 1.06, p=0.024). Re-injuries (n=27) almost exclusively occurred in the rectus femoris (96.3%) and took longer to RTP than index injuries (40.3 days +/- 22.7 v 26.8 days +/- 14.9, p=0.007). No other clinical or MRI factor was associated with re-injury within 2 seasons of primary injury. Discussion: A greater severity of aponeurotic disruption and younger age resulted in a longer time to return to play after QMSI in elite Australian Football players. Rectus femoris injuries with evidence of connective tissue disruption on MRI, and re-injuries result in an extended time to return to play. Impact/Application to the field: An improved understanding of the anatomical location and severity of quadriceps muscle strain injuries, especially the extent of connective tissue injury in the rectus femoris, can assist clinicians determining the expected return to play prognosis. This may help clinicians in the designing and staging of rehabilitation following QMSI and when developing injury prevention programs. Declaration: My co-authors and I acknowledge that we have no conflict of interest of relevance to the submission of this abstract.
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