Myotendinous junction (MTJ) is a part of the myotendinous unit, which connects muscle and tendon, and transmits forces between them.
Myotendinous junction consists of interdigitating tendon fibers and terminal myocytes, creating finger-like projections which increase the contact area between a tendon and a muscle, and - as a result - disperse the energy of a contracting muscle, decreasing focal stress.
In a healthy adult, the myotendinous junction is the 'weakest link' of the myotendinous unit and therefore it is its most commonly injured part.
The highest risk of strain occurs in large pennate muscles that cross two joints and produce large tensile forces (e.g. biceps femoris, quadratus femoris, biceps brachii).
Ultrasound and MRI
A grading system of MTJ injuries was created in order to make an accurate diagnosis and prognosis:
- mild strain: feathery interstitial edema and fluid/hemorrhage around the MTJ
- moderate strain: intramuscular hematoma and perifascial fluid/hemorrhage
- severe strain: MTJ tear with laxity/discontinuity of the tendon and muscle ends, sometimes with retraction
In case of an old strain, typical findings of scar tissue, old blood products and atrophy/fatty degeneration of the muscle can be found.
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