Intrinsic and extrinsic factors associated with recurrent hamstring injuries include inadequate warm-up, muscle fatigue, inadequate preseason training, muscle strength imbalances, decreased flexibility, increasing age, and history of prior injury . Some suggest that an injured muscle may heal with scarring resulting in suboptimal muscle length that predisposes recurrent injury .
Because of the high risk of recurrent injury and variable convalescence period, imaging may have a prognostic role in evaluating hamstring injuries, particular for the elite athlete where strategic and financial stakes can be high.
The first MR study that described findings with poor prognosis of muscle injury evaluated 14 patients and found that muscle rupture and retraction, hemorrhage, ganglion-like fluid collections, and greater than 50% cross-sectional involvement were associated with convalescent periods of more than 6 weeks .
A more recent study of 30 MRI-proven hamstring injuries in Australian Rules football players showed high correlation with volume of involvement (range 0.04 cm3 to 175.6 cm3, median 16.8 cm3) and maximum cross-sectional percentage (8 to 100, median 46%) with time lost from competition (13 to 48 days, median 27) . Linear fluid signal representing the length inter- and intramuscular fluid and edema showed strong correlation but was not statistically significant in this study. More injuries occurred distally (19 versus 11 defined as above or below origin of biceps femoris short head) but there was no correlation with location of injury and missed competition (Fig. 6).
One important concept to reiterate is the myotendinous junction was involved in 28 of 30 cases with 24 of these cases involving the intramuscular tendon of the muscle and only 4 cases involving the conventional proximal or distal myotendinous junctions. Five cases involved the intramuscular tendon and then extended to the conventional myotendinous junction.
In terms of predicting recurrence and length of convalescence another study imaged 31 Australian Rules footballers with clinical grade 1 injury. Forty-five percent had a negative MR exam and returned to full team training in 6.6 days versus 20.2 days for the MR-positive group. In this study the length of the injury had a stronger correlation coefficient than the cross-sectional areas with the rehabilitation interval unlike the previously mentioned study. Six of 17 MR-positive cases developed recurrent strains with no correlation between length or cross-sectional area as a predictor for recurrence .
Verrall and colleagues  also compared the clinical finding of posterior thigh injury with MRI findings of hamstring strain. Again, not all clinically suspected hamstring injuries had findings on MR for muscle strain. Of the 83 patients imaged, 68 (82%) had typical hyperintense signal on fluid-sensitive images as interpreted by a musculoskeletal radiologist, compatible muscle strain, whereas 12 (14%) had no signal change at all. The remaining three patients had MR evidence of muscle injury outside of the hamstring muscle complex (lower gluteus maximus, vastus lateralis, and adductor magnus). Those with MRI-detectable signal changes had more pain (5/10 versus 2/10), were
more likely to have acute onset, and missed more days from practice (27 versus 16 days) as compared with the group without MR findings of muscle strain. The authors hypothesized that those without MR findings have a referred pain syndrome or neuromeningeal cause of posterior thigh pain. Thus, MR helps accurately define the extent and location of injury and helps define causes of referred pain and types of injury that might heal more quickly.
One should consider the possibility that some of the above-mentioned findings may be sport specific and influenced by type of sport and culture. For example the prior studies were based on injuries to Australian Rules football players. The professional American football player experience is somewhat different as one study showed that the majority of cases result in no loss from game competition . The reported 13-year National Football League (NFL) experience from 1985 to 1998 found 431 hamstring injuries with 324 first-degree type and 107 second and third-degree type injuries. The first-degree type injuries had no loss of practice or game time. Some of the more advanced cases with a focal palpable abnormality (58 cases) at the expected location of the proximal myotendinous junction underwent intramuscular steroid injection within 72 hours. Average time loss until full practice was 7.6 days and the average training room treatment time was 24 days. All patients returned to prior level of performance and only eight players missed one game and one player missed two games. Those with more serious injuries were not treated with injections.
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