In this article, we report unusual findings of multiple accessory limb muscles (ALMs) in a healthy 31-yr-old man who served as an educational model for musculoskeletal ultrasonography (US). During the educational program for US of the knee, one ALM, the right tensor fascia suralis, was observed. The model was then systematically scanned with US (18- or 14-MHz linear probe Toshiba; Aplio 500) for the known ALMs of the upper and lower limbs described by Vanhoenacker et al.1 in their comprehensive article published in 2018.
The following ALMs were identified in his lower limbs (Video 1):
- Tensor fascia suralis (prevalence: rare; exact prevalence unknown2) was present bilaterally, originating from the semitendinosus muscle belly and inserting on the medial head of the gastrocnemius and surrounding fascia. The muscle was visible without US as an elongated mass spanning the popliteal fossa during resisted knee flexion.
- Flexor digitorum accessorius longus (prevalence, 2%–8%3,4) was present bilaterally, originating from the flexor digitorum longus muscle belly and inserting on the quadratus plantae muscle. When the subject was asked to actively flex his toes, movement of the quadratus plantae could be observed because of this insertion. The flexor digitorum accessorius longus was not detectable on physical examination.
- Peroneus tertius (prevalence: very frequent, range: 40%–93%3,5) was present bilaterally, originating from the mid-distal fibular diaphysis and lateral aspect of the extensor digitorum longus muscle and inserting at the base of the fifth metatarsal. The peroneus tertius tendon was visible without US with active ankle dorsiflexion and eversion.
No ALMs were found on US of the upper limbs.
Accessory limb muscles can be mistaken for mass lesions. Therefore, knowledge of ALMs can be helpful to diagnose and treat patients accurately. As US has a good capacity to reveal muscular structures, it is a useful tool for evaluating the presence of ALMs.
Most previous studies on the presence of multiple ALMs have found one particular ALM bilaterally. To our knowledge, only two case studies reported various types of ALMs in a single individual.6,7 In 2017, Bale et al.7 described a case of bilateral tensor fascia suralis and unilateral flexor digitorum accessorius longus that was found incidentally in a male cadaver used as a dissection model. A genetic component might be involved, as one previous study of an asymptomatic individual with digastric flexor carpi ulnaris, gastrocnemius tertius, and supernumerary fibularis longus revealed that 9 of 13 of his family members available for scanning also had at least one of these three accessory muscles.6
Thus, our study is the first to describe a combination of bilateral ALMs including tensor fascia suralis, flexor digitorum accessorius longus, and peroneus tertius in an individual. In addition, we provided clinicians information on US findings of ALMs in the lower extremity to help them make an accurate diagnosis and treat musculoskeletal disorders appropriately.
1. Vanhoenacker FM, Desimpel J, Mespreuve M, et al.: Accessory muscles of the extremities. Semin Musculoskelet Radiol
2. Kim KH, Shim JC, Lee GJ, et al.: MR imaging and ultrasonographic findings of tensor fasciae suralis muscle: a case report. J Korean Soc Radiol
3. Borne J, Fantino O, Besse J, et al.: MR imaging of anatomical variants of ligaments, muscles and tendons at the ankle [in French]. J Radiol
4. Sookur PA, Naraghi AM, Bleakney RR, et al.: Accessory muscles: anatomy, symptoms, and radiologic evaluation. Radiographics
5. Martinoli C, Perez MM, Padua L, et al.: Muscle variants of the upper and lower limb (with anatomical correlation). Semin Musculoskelet Radiol
6. Ata AM, Kara M, Aydin G, et al.: Ultrasound imaging for muscle variations: digastric flexor carpi ulnaris, gastrocnemius tertius, and supernumerary fibularis longus in an asymptomatic family. Am J Phys Med Rehabil
7. Bale LS, Herrin SO: Bilateral tensor fasciae suralis muscles in a cadaver with unilateral accessory flexor digitorum longus muscle. Case Rep Med