Dr Tom Entwisle
MRI demonstrates a grade III strain of the lateral intramuscular aponeurosis of the right soleus muscle in a 30 year old AFL footballer. There is disruption and retraction of the aponeurosis with the distal portion demonstrating a wavy irregular appearance. Myofibril disruption is seen at the junction with the aponeurosis over 11 cm. Blood fluid tracks into the torn myofibrils and along the deep surface of the muscle toward the skin.
The soleus is a multi-pennate muscle, designed for power, with a predominance of slow-twitch fibres and hence postural control and walking. The medial and lateral heads arise from the posterior aspect of the proximal half of the tibia and fibula respectively, with a fibrous arch connecting the two. The medial and lateral aponeuroses arise from the anterior / deep aspect of the medial and lateral epimysial surfaces and penetrate into the respective muscle bellies and head toward the midline as they fade out inferiorly. Condensations of the anterior and posterior epimysium form deep / anterior and superficial / posterior aponeuroses. The central tendon (CT) originates from the upper aspect of the anterior aponeurosis and penetrates the central bulk of the muscle as it descends in a sagittal plane. Distally it blends with the posterior aponeurosis and central anterior aspect of the Achilles tendon (AT). It can also variably form the medial or lateral portions of the Achilles.
The combined action of the soleus–gastrocnemius is not only to plantar-flex, but also to act as an agonist for the posterior cruciate ligament and antagonist of the anterior cruciate ligament. Soleus alone translates the tibia posteriorly and thus, an anterior cruciate ligament agonist. Further to the locomotive role, the CMC (in particular the soleus) functions as a venous pump for the lower limb.
Strains on MRI can be broadly divided into peripheral epimysial / myofascial surface strains and musculotendinous junction strains. Grade I strains demonstrate a hyperintense feathery appearance as the oedema and haemorrhage tracks between the muscle fibres. There is no architectural distortion but mild swelling may be seen. Grade II strains demonstrate hyperintense retraction of muscle fibres from the epimysium or tendon. The intervening haematoma demonstrates variable signal characteristics depending on age. Grade II strains demonstrate complete epimysial or tendinous disruption and discontinuity of the adjacent muscle fibres. The muscle gap fills with blood-fluid which can be seen to track within and around the muscle.
The classical clinical history of a calf strain is that of a sharp tearing or popping sensation during physical activity, usually with ankle dorsiflexion and knee extension. Clinically, there is a reduced plantarflexion strength, swelling and/or ecchymosis. Acute compartment syndrome is a rare complication.
Traditionally, gastrocnemius strains were considered more common than soleus strains and was attributed to the higher force generation due to it spanning two joints and having a higher proportion of fast-twitch muscle fibres. This results in an increase in passive stretching and rapid eccentric contraction which is when muscle fibres are most susceptible to injury. The previously reported lower incidence of strains within the soleus was attributed to the slow twitch fibre predominance and mono-articular course.
Koulouris et al however reported a similar incidence of gastrocnemius and soleus strains in a cohort study of 59 patients in 2007. A higher than expected incidence of simultaneous strain in both muscles was also reported and attributed to their single musculotendinous unit function via the common Achilles tendon insertion. Soleus strains in this situation may be secondary to a preceding gastrocnemius strain.
Soleus muscle strains are more difficult to diagnose and may be present subacutely, possibly due to the greater slow twitch fibres and deep location. It has also been suggested that soleus strains have been underreported due to ultrasound, the traditional imaging modality, having a reduced sensitivity for the detection of tears in this muscle due to it’s deep location.
- Koulouris G, Ting AY, Jhamb A, Connell D, Kavanagh EC. Magnetic resonance imaging findings of injuries to the calf muscle complex. Skeletal radiology. 2007;36(10):921-7.
- Balius R, Alomar X, Rodas G, Miguel-Perez M, Pedret C, Dobado MC, et al. The soleus muscle: MRI, anatomic and histologic findings in cadavers with clinical correlation of strain injury distribution. Skeletal radiology. 2013;42(4):521-30.