MR Imaging of Ankle
MRI is the best imaging modality for diagnosing accessory muscles around the ankle.
The general sequences used in a non-contrast enhanced study include
- T1 weighted images
- T2 weighted images
- STIR
- PD weighted images
Images are taken in all three planes.
Comparison with other modalities
MRI is superior to other modalities like CT or ultrasound by giving better contrast resolution and delineating the course of the muscles which differentiates it from other soft tissue tumors.
Accessory Ankle Muscles
1.
Flexor digitorum accessorius Longus
Origin:
From any of the structures of the posterior compartment like Flexor retinaculum,
the tibia,
the fibula,
the flexor hallucis longus (FHL),
or the soleus.
Insertion:
Either onto the Flexor digitorum longus (FDL) tendon prior to the FDL splitting into its four tendon slips or onto the quadratus plantae muscle.
Course:
Through the tarsal tunnel, posterior to the flexor hallucis longus (FHL) where it abuts the neurovascular bundle (posterior tibial artery and tibial nerve).
MR appearance:
The FDAL is isointense to normal skeletal muscle on all pulse sequences.
Function:
FDAL assists in toe flexion.
2.
Accessory Soleus
Origin:
From the anterior surface of the soleus or from the soleal line of the tibia or fibula.
Insertion:
There are five types of insertions:
i- Muscular insertion in the Achilles tendon
ii- Muscular insertion in the upper surface of the calcaneus
iii- Tendinous insertion in the upper calcaneus
iv- Muscular insertion in the medial surface of the calcaneus
v- Tendinous insertion into the medial aspect of the calcaneus.
Course:
Superficial to the tarsal tunnel and flexor retinaculum,
it descends anterior or anteromedial to the Achilles tendon.
MR appearance:
AS has its own fascia which distinguishes it from the normal soleus.
The AS is isointense to normal skeletal muscle on all pulse sequences.
Clinical Symptoms:
Tarsal tunnel syndrome,
likely due to the extrinsic compression.
3.
Accessory peroneal Muscles; Peroneus Quartus:
Multiple accessory peroneal muscles have been described in the literature,
including
i- Peroneus Tertius (prevalence 83–95%)
ii- Peroneus Accessorius,
iii- Peroneocalcaneus Externum,
iv- Peroneus Digiti Minimi (prevalence 15.5-34%)
v- Peroneus Quartus (PQ) (prevalence 10-26%)
Peroneus Quartus refers to several or even all of the accessory peroneal muscles.
Origin: from the Peroneus brevis,
peroneus longus or the posterior surface of fibula
Insertion:
There are variable insertions:
i- At the retro trochlear eminence of the calcaneus
ii- Peroneal tubercle of the calcaneus
iii- Inferior peroneal retinaculum,
cuboid, or the peroneus longus.
Course: Medial and posterior to the peroneal Tendons.
4.
Peroneocalcaneus Internus:
Origin:
From the inner part of the lower third of the fibula.
Insertion:
Onto a small tubercle on the medial calcaneus below the sustentaculum tali.
Course:
It courses posterior and lateral to flexor hallucis longus and anterior to the soleus.
It is separated from the peroneal muscles by the fascia laterally.
5.
Tibiocalcaneus Internus:
Origin:
From the medial crest of the tibia.
Insertion:
Onto the medial surface of the calcaneus,
below the sustentaculum tali.
Course:
It runs deep to the flexor retinaculum and behind the neurovascular bundle.