title: MRI findings and return to play in football: a prospective analysis of 255 hamstring injuries in the UEFA Elite Club Injury Study.
abstract: OBJECTIVE
  The present study evaluated whether the MRI parameters of hamstring injuries in male professional football players correlate with time to return to play (RTP).
  
  
  METHODS
  46 elite European football teams were followed prospectively for hamstring injuries between 2007 and 2014. Club medical staff recorded individual player exposure and time-loss after hamstring injury. MRI parameters were evaluated by two independent radiologists and correlated with the RTP data.
  
  
  RESULTS
  A total of 255 grade 1 and 2 injuries were evaluated in this study. RTP was longer for grade 2 than grade 1 injuries (24±13, 95% CI 21 to 26 days vs 18±15, 95% CI 16 to 20 days; mean difference: 6, 95% CI 2 to 9 days, p=0.004, d=0.39). 84% of injuries affected the biceps femoris (BF) muscle, whereas 12% and 4% affected the semimembranosus (SM) and semitendinosus (ST), respectively. No difference in lay-off time was found for injuries to the three different muscles (BF 20±15 days, SM 18±11 days, ST 23±14 days; p=0.83). The recurrence rate was higher for BF injuries than for SM and ST injuries combined (18% vs 2%, p=0.009). The size of the oedema weakly correlated with time to RTP (r(2)=6-12%). No correlation was found between location of injury and time to RTP. The majority of the intramuscular injuries affected the MT junction (56% in grade 1 and 2 injuries), but no difference in lay-off time was found between the different types of injuries.
  
  
  CONCLUSIONS
  The radiological grade and size of the oedema correlate with time to RTP for both, grade 1 and 2 injuries. No correlations were found between time to RTP and the location and type of injury.
authors.affiliation: Division of Community Medicine, Department of Medical and Health Sciences, Linköping University, Linköping, Sweden Football Research Group, Linköping, Sweden.
authors.forename: Jan
authors.initials: J
authors.lastname: Ekstrand
authors.affiliation: Chelsea and Westminster Hospital, Imperial College, London, UK.
authors.forename: Justin C
authors.initials: JC
authors.lastname: Lee
authors.affiliation: Chelsea and Westminster Hospital, Imperial College, London, UK.
authors.forename: Jeremiah C
authors.initials: JC
authors.lastname: Healy
mesh_terms: D000704:Analysis of Variance; D001265:Athletic Injuries / Q000473:pathology / Q000503:physiopathology / Q000534:rehabilitation; D004487:Edema / Q000209:etiology; D005060:Europe; D000070633:Hamstring Muscles / Q000293:injuries* / Q000502:physiology; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D011446:Prospective Studies; D020127:Recovery of Function / Q000502:physiology; D012008:Recurrence; D000068958:Return to Sport / Q000502:physiology*; D012421:Rupture / Q000209:etiology; D012916:Soccer / Q000293:injuries*
keywords: Cohort study; Elite performance; Muscle injury; Radiology; Soccer