Plantaris Muscle Injuries

Fig. 20.1

Schematic drawing of the posterior view of the lower leg illustrates the plantaris muscle

The long tendon is commonly harvested by orthopedic surgeons as an autograft for ligament and tendon reconstructions, demonstrating that function is maintained despite the absence of the tendon [7]. In cadaveric dissection, the plantaris has been referred to as the “freshman’s nerve”, because its long, slender-white tendon can be mistaken for a nerve [10].

The plantaris has been shown to vary in its origin and insertion [1]. In most cases the muscle is found to originate from the supracondylar ridge of the lateral femoral condyle, coursing medially down the leg to insert on the calcaneus, just medial to the Achilles [7, 11–13].

At the proximal third of the leg, the muscle belly is found between the popliteus muscle anteriorly, and the lateral head of the gastrocnemius muscle posteriorly. The myotendinous junction occurs approximately at the level of the origin of the soleus muscle from the tibia in the proximal portion of the lower leg [7]. The long thin tendon forms part of the medial border of the muscle belly as it courses between the medial head of the gastrocnemius and the soleus muscles in the midportion of the calf [14].

Anatomic studies have demonstrated that the calcaneal insertion may occur independently of the Achilles tendon and this could explain why the plantaris tendon often remains intact when the Achilles tendon ruptures [11]. Neural innervation is provided by the tibial nerve (S1, S2), common to all three muscles of the triceps surae group.

In terms of function, the plantaris is in a position to act with the gastrocnemius as either a flexor of the knee or a plantarflexor of the ankle; However, its motor functions are trivial. It appears to be a highly specialized sensory muscle and is considered to be an organ of proprioceptive function for the more powerful plantarflexors as it contains a high density of muscle spindles [10].

20.3 Pathology

Despite its small size, injuries to the plantaris are relatively common [11] and have been termed “tennis leg”. The pathogenesis of tennis leg, and the existence of an isolated rupture of the plantaris muscle as the cause, has been debated since it was initially described by Powell in 1883 [15]. Since then the term “tennis leg” has been used, but it has been a source of debate in the literature [12, 16–19].

For years, the condition was believed to be a rupture of the plantaris tendon at the medial aspect of the calf. Other authors have implicated a tear of the medial head of the gastrocnemius, soleus, plantaris or a combination thereof [12, 20, 21, 22] and some still question the role of the plantaris tendon in tennis leg [12, 20]; Arner and Lindholm [23] in 1958, Miller [18] in 1977 and Severance and Bassett [12] in 1982, all concluded that isolated rupture of the plantaris was not the cause of this clinical condition. Surgically confirmed cases of ruptures of the plantaris tendon or musculotendinous junction have been reported in conjunction with this clinical diagnosis [6, 19].

A review of the literature demonstrates that injury to the plantaris muscle either on its own, or in combination with injury to the gastrocnemius or soleus, can represent the clinical condition known as tennis leg [24].

Plantaris injuries are not as common as injuries of the gastrocnemius, which is known to be particularly vulnerable due to its superficial location that spans two joints (the knee and the ankle) and its composition of type IIb muscle fibers [17, 25]. A study by Koulouris et al. in 2002 with MRI found that the medial head of the gastrocnemius is the most commonly injured muscle of the calf, closely followed by the soleus [26].

The plantaris is injured most frequently during running or jumping and usually as the result of an eccentric load placed across a forced dorsiflexed ankle with the knee in an extended position [20, 27]. It has been established with MRI, ultrasound and surgical exploration that injuries to the plantaris may in fact occur in isolation [6, 7, 11, 28] or in association with traumatic tears of the anterior cruciate ligament, arcuate ligament complex, and posterolateral corner muscles (lateral head of the gastrocnemius and popliteus) [7]. A posterior compartment syndrome, which requires surgical decompression, is a potential complication of plantaris or medial gastrocnemius ruptures.

20.4 Clinical Presentation

Even though the injury is the result of an indirect mechanism, patients report feeling as though they had been kicked in the back of the leg, and often report a “pop” in the calf [29].

Depending on its severity, the calf pain may cause the athlete to stop playing, or not. The pain usually becomes more severe with rest or on the next day [30]. Swelling may accompany the pain and may extend down to the ankle. Any attempt at active or passive dorsiflexion, and resisted plantarflexion will elicit severe pain [29, 30].

Deep venous thrombosis and a ruptured Baker cyst should be kept in mind in the differential clinical considerations when evaluating a patient with a probable plantaris injury [11, 14, 20].

20.5 Imaging

Magnetic resonance imaging (MRI) and ultrasound can be used in the diagnostic evaluation of patients with the clinical diagnosis of posterior lower leg pain. Ultrasound is less expensive, but operator dependent. However, ultrasound is more frequently used in the initial evaluation of suspected deep venous thrombosis and should be considered in the differential consideration of a patient with posterior calf pain. Imaging will help the clinician confirm the clinical diagnosis and exclude other etiologies (Figs. 20.2 and 20.3).

Fig. 20.2

(a) Longitudinal ultrasound obtained over the medial popliteal space shows a Baker cyst (BC). Note the pointed appearance (arrow) of the inferior part of the cyst due to recent rupture. (b) Transverse ultrasound obtained over the middle part of the medial head of the gastrocnemius muscle (MH) shows a fluid accumulation (arrows) located inside the subcutaneous tissue of the medial calf

Fig. 20.3

(a) Transverse ultrasound obtained over the medial part of the soleus muscle shows an intramuscular mass (arrows) of mixed echogenicity that was a hemangioma. Corresponding axial T1- (b) and fat-suppressed contrast-enhanced T1-weighted (c) MRI confirm the ultrasound findings. MH medial head of the gastrocnemius muscle

On ultrasound, the muscle belly of the plantaris muscle is initially located in the proximal calf region using a transverse scan. It is seen as a triangular structure having the soleus muscle as its base and the medial and lateral bellies of the gastrocnemius as it sides [14].

While scanning with the transducer in a transverse plane it is possible to follow the muscle from its most proximal attachment on the lateral femoral condyle, down to the myotendinous junction at the level of the fibular head. Since the tendon forms the medial border of the belly, it will be seen in the transverse plane as a subtle thickening at the medial corner of the triangular muscle belly. The tendon is generally not visualized well in the transverse plane, distal to the myotendinous junction [14].

The fiber orientation of the plantaris muscle belly and tendon is best visualized by rotating the transducer longitudinally along the long axis of the muscle. In most cases, the fibrillar pattern of both the muscle and tendon is well depicted [14] (Fig. 20.4).

Fig. 20.4

(a) Transverse and (b) longitudinal ultrasound images demonstrate black arrowheads represent the anterior aponeurosis of the medial head and the white arrowheads depict the normal plantaris tendon. GMH medial head of the gastrocnemius muscle

As noted earlier, tennis leg can be a tear of the medial head of the gastrocnemius (Fig. 20.5), soleus, plantaris or a combination thereof (Fig. 20.6). Imaging will help define the extent of injury and potentially exclude more serious conditions such as deep venous thrombosis [11, 29] (Fig. 20.7). Tears of the plantaris most often occur at the level of the myotendinous junction, although there have been reports of isolated midsubstance tears [35] (Fig. 20.8).

Fig. 20.5

Longitudinal ultrasound obtained over the medial head of the gastrocnemius muscle (MH) shows a myotendinous avulsion (arrow) of the inferior part of the medial head of the gastrocnemius muscle. Note interruption of the fibroadipose septa (white arrowhead) as well as of the distal aponeurosis (black arrowheads) of the medial head. No local hematoma can be detected. The proximal part of the medial head and the soleus muscle are normal

Fig. 20.6

(a) Axial fat-suppressed T2-weighted MRI demonstrates fluid between the soleus muscle and the medial head of the gastrocnemius muscle. Irregular low signal intensity within the fluid collection represents the remnant of the ruptured plantaris tendon (long white arrow). Feathery edema involving both medial and lateral heads of the gastrocnemius is compatible with strain (short white arrows). (b) Coronal fat-suppressed T2-weighted MRI again demonstrates fluid between the soleus muscle and the medial head of the gastrocnemius muscle and irregular low signal intensity within the fluid collection that represents the remnant of the ruptured plantaris tendon (arrow). (c) Longitudinal ultrasound demonstrates the same

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