Discussion

The main function of the gastrocnemius muscle is to plantarflex the ankle. The plantaris muscle is a pencil-sized structure tapering down to a fine tendon that runs beneath the gastrocnemius and soleus muscles to attach to the Achilles tendon or to the medial side of the tubercle of the calcaneus. The function of the muscle is of little importance, and, with rupture of either the muscle or the tendon, the transient disability is due only to the pain of the torn fibers or swelling from the hemorrhage. Most instances of “tennis leg” are now thought to be the result of partial tears of the medial belly of the gastrocnemius muscle or to ruptures of blood vessels within that muscle. Throughout the belly of the muscle, the medial gastrocnemius has several origins of tendinous formation. Most strains or tears occur at this musculotendinous junction. The greater the initial pain and swelling, the longer one can expect the disability to last.

Tendon ruptures typically affect men in their third or fourth decade who are active in sports. The average occurrence of a gastrocnemius muscle tear is in the fourth to sixth decade. A tendon rupture is usually an indicator of intratendinous degenerative changes.

Achilles tendon injury can occur with the identical mechanism of a plantaris tendon or medial gastrocnemius rupture. Because the pain and debility may be similar, clinical differentiation is sometimes difficult.

The diagnosis of Achilles tendon rupture is missed by the initial examiner in up to 25% of patients. One misleading finding is that the patient is able to plantarflex the foot with no resistance, because several other muscles (toe flexors, peroneus) also perform this action. The expected defect in the tendon may also be obscured by edema or hemorrhage. The squeeze test (Thompson or Simmonds test) is usually an infallible sign of complete rupture, but a false negative might arise if the plantaris tendon is left intact. Imaging techniques—including real-time high-resolution ultrasonography and MRI—are now used to aid diagnosis. Ultrasonography usually costs less than MRI; however, when “limited MRI protocols” are available and only a few images of the suspected region of disease are obtained, pricing can be competitive and will give images superior to ultrasonography.

Other entities that can be confused with plantaris tendon and medial gastrocnemius rupture are Baker cyst rupture and deep venous thrombosis. When physical findings are dubious, the history along with Doppler ultrasonography will often help clarify the diagnosis.

Direct injection of steroids and administration of systemic corticosteroids and fluoroquinolone antibiotics are associated with an increase in the risk for Achilles tendon rupture. Other disease processes, such as rheumatoid arthritis, systemic lupus erythematosus, chronic renal failure, hyperuricemia, genetically determined collagen abnormalities, arteriosclerosis, and diabetes mellitus, have been implicated as risk factors for rupture. With age, tendons stiffen from the effects of reduced glycosaminoglycan content and increased collagen concentration. Blood and nutrient supply also are reduced with age.