title: Hamstring muscle injuries in professional football: the correlation of MRI findings with return to play.
abstract: BACKGROUND
  Hamstring injury is the single most common injury in professional football. MRI is commonly used to confirm the diagnosis and provide a prognosis of lay-off time.
  
  
  OBJECTIVE
  To evaluate the use of MRI as a prognostic tool for lay-off after hamstring injuries in professional football players and to study the association between MRI findings and injury circumstances.
  
  
  METHODS
  Prospective cohort study where 23 European professional teams, were followed between 2007 and 2011. Team medical staffs recorded individual player exposure and time-loss injuries. Radiological grading was performed using a modified Peetrons classification into four grades where grades 2 and 3 represent fibre disruption.
  
  
  RESULTS
  In total, 516 hamstring injuries occurred and 58% of these were examined by MRI. Thirteen per cent were grade 0 injuries, 57% grade 1, 27% of grade 2 and 3% of grade 3. Grade 0 and 1 injuries accounted for 56% (2141/3830 days) of the total lay-off. The lay-off time differed between all four radiological grades of injury (8±3, 17±10, 22±11 and 73±60 days, p<0.0001). Eighty-three per cent of injuries affected the biceps femoris while 11% and 5% occurred to the semimembranosus and semitendinosus, respectively. Re-injuries (N=34/207) constituted 16% of injuries. All re-injuries occurred to the biceps femoris.
  
  
  CONCLUSIONS
  MRI can be helpful in verifying the diagnosis of a hamstring injury and to prognosticate lay-off time. Radiological grading is associated with lay-off times after injury. Seventy per cent of hamstring injuries seen in professional football are of radiological grade 0 or 1, meaning no signs of fibre disruption on MRI, but still cause the majority of absence days.
authors.affiliation: Department of Medical and Health Sciences, Linköping University, Linkoping, Sweden. jan.ekstrand@telia.com
authors.forename: Jan
authors.initials: J
authors.lastname: Ekstrand
authors.forename: Jeremiah C
authors.initials: JC
authors.lastname: Healy
authors.forename: Markus
authors.initials: M
authors.lastname: Waldén
authors.forename: Justin C
authors.initials: JC
authors.lastname: Lee
authors.forename: Bryan
authors.initials: B
authors.lastname: English
authors.forename: Martin
authors.initials: M
authors.lastname: Hägglund
mesh_terms: D000041:Absenteeism; D001265:Athletic Injuries / Q000209:etiology / Q000473:pathology / Q000534:rehabilitation; D012090:Cumulative Trauma Disorders / Q000209:etiology / Q000473:pathology / Q000534:rehabilitation; D004292:Dominance, Cerebral / Q000502:physiology; D006801:Humans; D007869:Leg Injuries / Q000209:etiology / Q000473:pathology* / Q000534:rehabilitation; D008279:Magnetic Resonance Imaging; D008297:Male; D018482:Muscle, Skeletal / Q000293:injuries*; D011446:Prospective Studies; D020127:Recovery of Function / Q000502:physiology; D012008:Recurrence; D012420:Running / Q000293:injuries; D012916:Soccer / Q000293:injuries*; D013848:Thigh / Q000293:injuries