Flexor Femoris Muscle Complex:
Spectrum of Injury

The Flexor Femoris Muscle Complex (FFMC), which is also known as the “hamstring,” flexes the knee, extends the hip and provides medial/lateral stability to the knee.  It is comprised of the biceps femoris, semintendinosus and semimembranosus (1) and is a frequent site of injury (2).  Running, jumping, and kicking commonly result in muscle strain while hurdling, ice-skating, dancing, water-skiing and weight-lifting result in avulsion injuries (3-6). The biceps femoris is the most commonly injured muscle of the FFMC. 

FFMC muscles cross the hip and knee; the biarticular nature leads to increased injury relative to muscles that cross a single joint (7). Correctly identifying and grading FFMC muscular, tendinous and osseous avulsion injuries is critical in effective management (8).  MR is equivalent to ultrasound in detection of hamstring injuries acutely but is more sensitive in follow-up imaging of healing lesions (9). Institution of appropriate treatment regimen often depends upon reliable detection, interpretation and characterization of the injury (8). 

Anatomy:
The semimembranosus, long head of the biceps femoris and the semitendinosus tendons originate at the ischial tuberosity. The semintendinosus and long head of the biceps femoris arise as a common tendon with the semimembranosus origin being superolateral relative to the common tendon (fig 1a&b) (10).    The biceps femoris short head is the only component that does not span two joints.  

Hamstring injuries may be categorized by location (proximal, central or distal). Proximal injuries at the ischial origin are classified as tendinosis, partial tendon tear or complete tendon tear with or without avulsed osseous fragment. Distal injuries are the least frequent and involve distal tendons or insertion of the tendons in a similar fashion (fig 1c). FFMC muscular injuries usually occur at the proximal or distal musculotendinous junctions, but may also occur centrally.

Musculotendinous Injury Grading:
A three tier grading system is used to describe the severity of muscle injury: Grade I - strain, Grade II - partial tear and Grade III - complete tear (11-12).  Grade I injuries are microtears and commonly occur at the proximal musculotendinous junction (5). They are seen on MR as high T2 signal with a feathery appearance on fluid sensitive sequences due to edema and microhemorrhage, (fig 2a). They typically heal well with rest, ice, compression, and elevation (RICE).  Grade II injuries indicate macroscopic fiber disruption has occurred and are seen as focal fluid signal intensity collections, (fig 2b) (13).  They may require an extended time to heal (weeks to months) depending on patient age, overall conditioning, and severity of injury (14).  They usually do not require surgical intervention but may progress with premature return to activity.  Prognosis is a clinical decision and not based on MRI description alone (15).  Grade III injury is a complete tear with disruption of the myotendinous unit, typically with retraction and a gap between the torn ends.   It may necessitate prompt surgical correction depending on the location of injury.  Delayed reattachment is associated with loss of function and increased morbidity.

Rarely, the muscle is injured in a way that a single grade may be sufficient for description; however, a spectrum of damage is more common (Fig 2b).  A thorough description is most beneficial to convey the full extent of damage; the highest grade injury should be described to guide clinical/surgical management. Longitudinal measurement and cross sectional area of the injured region are useful as the size of the lesion correlates directly with prognosis (figure 3a,b). Abnormal muscle comprising greater than 50% of the entire cross-sectional area of the muscle is associated with longer recovery times. The single best prognostic indicator is longitudinal length of the defect on MR (16).

Tendon injuries:
The tendinous elements of the FFMC may demonstrate tendinosis, partial, and/ or complete tear.  The tendons may avulse from their proximal/distal osseous locations, with or without associated avulsed bony fragments. Tendinosis is seen as increased signal and tendinous thickening (fig 4a) while a partial tear is a region of focal fluid signal interrupting the expected course of the tendon (fig 4b).  Complete tear is a total loss of integrity and transection of the tendon with retraction of the tendon and muscle away from the site of injury (Grade 3) (fig 4c).

 Enthesopathic/non-displaced fracture, partial avulsion, or complete avulsion (+/- osseous fragment) may be seen at the ischial tuberosity FFMC origin.  Ischial tuberosity injuries are described by a three-tiered method: enthesopathic/non-displaced fracture, partial avulsion, and avulsion with > 2cm displacement of an osseous fragment.   High signal within the ischial tuberosity on fluid sensitive sequences with tendinosis or partial tear may indicate enthesopathic insertional change or non-displaced fracture (fig 5a).  Contusion, osteomyelitis, or a rare process such as neoplasm may be considered in the differential diagnosis in the appropriate clinical setting (17).  Distal hamstring avulsion injuries at the fibular head or about the proximal tibia occur rarely (18).  Osseous avulsion with fracture fragment distraction of > 2cm is associated with a higher degree of non-union and therefore is often surgically repaired (fig 5b) (19).  High grade muscle, tendon and chronic osseous avulsion injuries may lead to muscular atrophy and manifest as fatty replacement (fig 5c) (17).

Conclusion:
The FFMC spectrum of muscular, tendinous, and osseous injury is effectively evaluated with MR. Hamstring injuries may be systematically described with a series of three-tiered classification systems.  Injuries are generally localized as proximal, central or distal. Muscular injuries are classified as follows: Grade I - microtear, Grade II - macrotear (partial tear) and Grade III - complete tear.  Tendinous injuries are described as tendinosis, partial tear, or complete tear.  Avulsive ischial tuberosity injury may be organized as enthesopathic change/non-displaced fracture, < 2cm to minimal/moderate avulsive displacement, and > 2cm to severe avulsive displacement.  Utilization of these commonly accepted systems provides the radiologist a uniform reporting method that fully describes the clinically relevant spectrum of FFMC injuries.

References
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