The soleus muscle: MRI, anatomic and histologic findings in cadavers with clinical correlation of strain injury distribution.

OBJECTIVE: The purpose of this study was to describe the normal anatomy of the soleus muscle using magnetic resonance (MR) imaging, anatomic dissection and histologic correlation in cadavers. The second objective of this study was to analyse the morphometry of the soleus muscle in normal volunteers. The final objective was to undertake a retrospective review of soleal strain injuries confirmed with MR imaging, with correlation made between the cadaveric anatomic findings and the MR imaging features.
MATERIALS AND METHODS: Eleven fresh cadaveric legs were studied using a high resolution 3.0 T (T) MR imaging scanner to obtain images in the axial, coronal and sagittal planes. After imaging, six specimens were dissected and evaluated by histological analysis, with the remaining five specimens then frozen and cut into axial sections. The corresponding levels on the MR examination were then compared with the levels of anatomic sectioning. MR imaging was also used to examine the soleus muscle in both legs of 20 healthy volunteers. Finally, 55 clinical cases of soleus muscle strains diagnosed between October 2006 and January 2011 that had also previously undergone MR imaging were re-evaluated. The location of strain injury was reviewed and correlated with the anatomic information that had been revealed in the anatomic component of our study.
RESULTS: Dissection of the soleus muscle revealed two proximal intramuscular aponeuroses (medial and lateral) that are formed as a direct continuation of the surrounding epimysium. From an anatomic, functional and pathologic perspective, these aponeuroses are considered in this study as intramuscular tendons, however they have been not previously described as such. These tendons penetrate deep into the muscle belly, from which the proximal muscle fibres of the soleus arise. Inferiorly, these muscle fibres insert onto a long distal central tendon that becomes confluent with the overlying distal tendon of gastrocnemius to form the Achilles tendon. Significant differences between the length of the central tendon on the right side (31.35 cm) and the left side (30.36 cm) were observed (p = .002), as well as the length of insertion of this tendon onto the Achilles tendon on the right side (7.19 cm) compared with the left (7.94 cm) (p = .02). The retrospective analysis identified five sites within the soleus where strains were distributed: musculotendinous junction sites (proximal medial strains accounting for 25.5% of all injuries, proximal lateral strains accounting for 12.7% and distal central tendon strains accounting for 18.2%) and myofascial sites (anterior strains accounting for 21.8% of all injuries and posterior strains accounting for 21.8%). Strains of the proximal medial musculotendinous junction were the most common of soleal muscle injuries, comprising 56.4% of all cases.
CONCLUSION: Current information on the detailed anatomy of the soleus muscle in the anatomic and radiological literature is lacking. Knowledge of this anatomy accounts for the distribution of sports-induced injuries within the soleus muscle-tendon unit and therefore assists in the accurate identification of these injuries, with possible prognostic benefit.