The plantaris muscle is a variably developed accessary plantar-flexion muscle of the calf [1]. The tendon is long and thin, often referred to as the "fool's nerve". Until recently, this tendon was suspected to frequently rupture and result in the clinical syndrome of "tennis leg" [2-5]. Clinical symptoms of plantaris rupture are considered to be less severe than a rupture of other calf muscles [2,5]. Currently, many sports medicine experts believe that plantaris rupture is a myth as no surgical cases have ever been proven and that "tennis leg" is caused by partial rupture of medial head of gastrocnemius muscle [3,4]. We report two cases of plantaris rupture imaging and discuss anatomical features of the diagnosis.

CASE REPORTS

Case 1- A 52 year old man complained of right calf swelling and pain which had worsened over the previous week. There was no history of trauma or unusual exertion. There was obvious swelling but no erythema of the calf and a positive Homan's sign. Plain films and a right leg venogram were normal. Laboratory examination and CT of the abdomen were unremarkable. MR of the calf showed a "mass" between the soleus muscle and medial head of gastrocnemius (Fig. 1). This was consistent with a retracted plantaris muscle belly from rupture of the plantaris tendon. No signal abnormality to indicate edema or hemorrhage was seen on T2-weighted images (not illustrated). The patients' clinical symptoms resolved over a few weeks.

Case 2- A 40 year old man noted sudden pain in his right calf while seated in an airplane. The pain was worsened with walking. Swelling also developed over the next few days. The primary physician referred the patient for an ultrasound to rule out deep vein thrombosis. Compression sonography of the venous system was normal. On exam, the patient was noted to have a focal area of firm swelling in the upper calf. Imaging with comparison to the contralateral side revealed the presence of an "extra" ovoid structure similar in echogenicity to muscle which appeared to be sandwiched between the soleus and gastrocnemius muscles, which is believed to represent a contracted and enlarged plantaris muscle which had ruptured at its' tendon (Fig. 2). The patient was asymptomatic after ten days of rest.

ANATOMY

The plantaris muscle arises from the posterosuperior aspect of the lateral femoral condyle, next to lateral head of gastrocnemius. It is the vestigial component of the triceps surae complex as it co-functions with soleus and gastrocnemius muscles in the superficial posterior compartment of the calf. In the calf, this muscle belly is between the medial head gastocnemius and soleus muscles, after having coursed obliquely posterior to the knee (Fig. 3). The long thin tendon then inserts into the medial aspect of the Achilles tendon [1]. This tendon is primarily known to the orthopedic sugeon as being useful for grafts, in which case it has been noted to be present in 93% of people [6].

The authors surveyed twenty cadavers during dissection in the anatomy course at University of Miami School of Medicine to determine variability of the muscle development. We determined that the size of the muscle belly varied from one by seven cm to three by thirteen cm with an average of 1.5 by ten cm. Size of plantaris generally correlated with the overall degree of muscular development and tapered abruptly at the musculotendinous junction in most cases. The tendon measured one by three mm and merged with Achilles tendon ten to fifteen cm above the calcaneous. On retracting the plantaris tendon failure occurred in three of four cases near junction of plantaris and Achilles tendons. Fibers of plantaris muscle were often difficult to distinguish from lateral head of gastrocnemius near its insertion.

Based on these observations, our cases of plantaris rupture were in persons who had larger than average muscles. Because the tendon varied less than the muscle, we postulate that patients with larger muscles would be more prone to tendon failure and more likely to develop compressive symptoms in that event. Because of small size, obliquity of course, and poor separation from proximal gastocnemius, the plantaris muscle is difficult to identify on routine MRI of knee or calf. We have imaged the plantaris tendon as a distinct structure in cases of complete rupture of the Achilles tendon, which also might explain retained ability to plantar flex the foot, in those cases (Fig. 4).

DISCUSSION

In our two cases the classic history of a sudden snapping sensation was absent, leading to clinical considerations of the diagnosis of deep venous thrombosis. Our contracted plantaris muscle was considerably larger than the typical pencil-size which has been described, perhaps accounting for both the propensity to rupture and the rather impressive symptoms, which led to hospitalization in one of our cases. Muscular strains and ruptures in the calf are among the more frequent sites these injuries [5]. Often the site of injury is at the musculotendinous junction of a muscle that spans at least two joints. A ruptured vessel which may accompany direct muscular injury is usually more disabling than than rupture of tendon alone [2]. In cases of medial gastrocnemius muscle injury hemorrhage is usually seen, and has been severe enough to result in development of compartment syndrome [3,4]. The clinical diagnosis of the specific injury is inexact. Adams states in his orthopedic book that "a ruptured tendo calcaneous is often overlooked, the symptoms being wrongly ascribed to a strain or to a ruptured plantaris muscle" [7].

MRI has revolutionized musculoskeletal diagnosis [8,9]. Muscular hematoma has specific signal characteristics on MR which enables a precise diagnosis. Muscle contusion, partial tear, and complete tear can be differentiated [9]. Tendinous injury is less likely to be associated with bleeding, but discontinuity, enlargement, and abnormal fluid might be seen, particularly with larger tendons such as Achilles. The plantaris tendon is avascular and not likely to be imaged in normal persons. The diagnosis of ruptured plantaris tendon relies on presence of a "mass" which is isointense to muscle between the gastrocnemius and soleus muscles.

Although MRI is not routinely recommended for evaluation of musculotendinous injuries to the calf, it can be helpful in problem solving and indicate whether surgical intervention is required as in the case of compartment syndrome or Achilles rupture [3,8,9]. Occasionally bone or soft tissue neoplasms will masquerade as sports injuries and prompt imaging will prevent delayed diagnosis [10].

Ultrasound is less expensive but more operator dependant than MRI. Tendinous and muscle injuries are evident by alteration in the normal linear echogenic septations between different tissues [11]. Fluid collections present as areas of diminished echoes or non-linear septations. By comparison with an asymptomatic extremity, subtle abnormalities can also be noted. Thus we were able to note the presence of a contracted plantaris muscle in case 2. Whereas ultrasound is more frequently used in the initial evaluation of suspected deep venous thrombosis and musculosketal injuries can mimic this diagnosis, ultrasound might be considered a comprehensive modality for evaluation of leg or calf pain.

In summary, we have presented two imaging instances of a ruptured plantaris muscle, one with ultrasound and the other with MRI. Both had atypical histories and were initially considered to have deep venous thrombosis. These cases are especially important in that the "nonscientific anachronism" of plantaris rupture is resurected [4]. Therefore, imaging can provide a specific diagnosis in clinically problematic cases, especially when the radiologist is familiar with the entity of plantaris rupture, a task which this report has attempted.

REFERENCES

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