There are many different sizes and shapes of muscle. Some are long like the biceps femoris with tendon insertions on bone at both ends and cross two joints (biarticular). Others are short with a single tendon insertion spanning a single
*Corresponding author. Department of Radiology, University of Pittsburgh Medical Center,
200 Lothrop Street, Pittsburgh, PA 15213.
E-mail address: [email protected] (D.R. Armfield).
joint like the popliteus. Some have long muscle bellies (sartorius) whereas others have long tendons (plantaris). Some have muscle fibers aligned with tendons in a colinear fashion (unipennate) whereas others have muscle fibers inserting at an angle on an intramuscular tendon (bipennate), which increases muscle fiber density and ultimately contractile forces (ie, hamstrings, rectus femoris).
What they have in common is a propensity for injury at the interface of two different materials namely muscle and tendon typically referred to as the my-otendinous junction [3,4]. The term musculotendinous junction has also been used synonymously. It is important to note that the typical connotation of a my-otendinous junction is that of a specific focal point at either the proximal or distal aspect of a muscle before tapering to the tendon insertion. However the hamstring and quadriceps tendons have large intramuscular or central tendons and injury often occurs along this interface [5,6].
Most sports-related muscle injuries involve strains, contusions, and uncommonly lacerations. Muscle strains or tears often affect muscles with primarily fast-twitch type-2 muscle fibers, span two joints (biarticular), and undergo eccentric contraction . As mentioned earlier, strain injuries typically occur at the myotendinous junction. However, strains have also been described involving the periphery of a muscle (instead of the myotendinous junction) extending to the epimysium seen with ultrasound and MRI [8,9]. Based on the patient's age and the underlying condition of the tendon itself, injury can occur anywhere along the muscle-tendon-bone chain . For example those with degenerated tendon because of aging or chronic use may develop a tear of the tendon itself. Those with strong tendons may experience an avulsion of the tendon from the bone or myotendinous strain. In skeletally immature patients, an apophyseal avulsion may occur, as this junction biomechanically represents the weakest interface.
Strains are often diagnosed clinically on a three-point scale: 1 = mild, 2 = partial tear, 3 = complete . Mild injuries have no discernable loss of strength or motion restriction. Partial tears demonstrate some loss of strength and motion that is not complete, unlike type 3 injuries . Strain injury is associated with inflammation, edema, and sometimes hemorrhage with proliferation of inflammatory cells and fibroblastic activity in the first 24 to 48 hours . Histological animal models of muscle stretch injury have shown that my-otendinous injury results in inflammation, bleeding, and muscle fiber necrosis initially. This destructive phase is followed by a concomitant repair and remodeling phase involving recruitment of progenitor cells, scar formation, and remodeling of organized tissue .
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