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MRI atlas of accesory muscles in the lower limb
Poster No.:
C-2319
Congress:
ECR 2010
Type:
Educational Exhibit
Topic:
Musculoskeletal
Authors:
A. Salvador, J. Arnaiz, T. Piedra, C. Jimenez, A. García Bolado, J.
Crespo; Santander/ES
Keywords:
Accesory muscles, Anatomy variants, Incidental findings
DOI:
10.1594/ecr2010/C-2319
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Learning objectives
• Describe the anatomy of the accesory muscles in the lower limb.
• Show the MR imaging features of accesory muscles in the lower limb.
• Evaluate the clinical relevance of accesory muscles.
Background
GENERAL FEATURES
•Accesory muscles are rare anatomy variants representing additional muscles with
similar characteristics as the rest of the muscles of the body.
•Accesory muscles are dificult to diagnose because they show no signal alteration on
MRI.
•A great variety of accesory muscles has been described in the literature. Most of them
are asymptomatic and are incidental findings at the new imaging techniques (US, CT,
MRI).
•However they can sometimes be cause of symptoms due to the mass efect to adjacent
structures such as neurovascular ones.
CLINICAL RELEVANCE
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The presence of these muscles is frequently asymptomatic, but sometimes it can cause
symptoms due to overcrowding and mechanical compression of adjacent structures as:
- Fulness
- Swelling
- Post-exercise pain in the region involved
- Nerve or vessels entrapment
DIFFERENTIAL DIAGNOSIS
These muscles may be incidental findings or mimic a soft tissue masses, that have to be
differenciated from soft tissue neoplasms:
- Soft tissue neoplasms: increased signal on T2 and STIR.
- Accesory muscles: isointense to muscle on all pulse sequences.
TREATMENT
Asymptomatic patients do not requiere treatment after confirming the soft tissue mass
to be an accesory muscle.
In symptomatic patients treatment varies from fasciotomy, when exercise-induced
compartimental syndorme occurs, to muscle excision or neurolysis if nerve compression
or claudication are the main symtoms.
MUSCLE DESCRIPTION
The following accesory muscles of the lowe limb are described:
A)ACCESORY MUSCLES OF THE KNEE
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1.Accesory slips of the medial and lateral gastrocnemious muscle.
2.Tensor Fasciae Suralis muscle
3.Accesory popliteus
B)ACCESORY MUSCLES OF THE ANKLE
1.Lateral aspect
1.1 Accesory Peroneal muscles
2.Medial aspect
2.1. Flexor Digitorum Accesory Longus
2.2. PCI (peroneocalcaneous internus)
2.3. Accesory Soleus
2.4. Tibiocalcaneous internus
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Imaging findings OR Procedure details
GENERAL FEATURES OF IMAGING TECHNIQUES
Before the extensive use of MRI, radiography, ultrasonography (US) and computed
tomography (CT) where used to diagnose the accesory muscles.
Conventional Rx
Soft tissue density or soft tissue mass, obliterating the fat planes was commonly seen
(eg. Kager's fatpad obliteration is frequently seen in the accesory soleus)
CT
Soft tissue mass, with similar attenualtion values to the normal muscles.
* These techniques do not provide enough soft tissue contrast between the normal
muscle and soft tissue neoplasms, so it is not possible to differenciate confidently and
accesory muscle from many other bening or malignant entities.
MRI
MR imaging is the modality of choice for accesory muscle diagnosis and to differenciate
this entity from other soft tissue masses.
It is usefull also for preoperative planning, assesing the limits, and the relations with
adjacent structures.
ACCESORY MUSCLES OF THE KNEE
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1. ACCESORY SLIPS OF THE MEDIAL AND LATERAL GASTROCNEMIOUS
MUSCLE
ANATOMY
- The Gastrocnemious muscle has two bellies, wich arise from the posterior aspect of the
femur and join together to form the Achilles tendon.
- The anatomic variation consist of anomalous origin and accesory slips.
COURSE
- Anomalous origin: we identify an insertion in the distal femur located at the intercondilear
notch instead of at the femoral condyle.
- Accesory slips of the medial head: pass between popliteal artery and vein.
- Accesory slips of the lateral head: always lateral to the popliteal artery.
CLINICAL RELEVANCE: All these variants can lead to compression of the Popliteal
artery named PAES (Popliteal Artery Entrapment Syndrome)
IMAGING FINDINGS
CT and MRI
Are useful to identify such accesory slips and the relation with the neurovascular
structures at popliteal fossa.
MR-angiography
Has been used to identify arterial occlusion in patients with symptoms of claudication
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Fig.
References: A. Salvador; Radiology, Hospital Universitario Marqués de Valdecilla,
Santander, SPAIN
2. TENSOR FASCIAE SURALIS MUSCLE
Very rare accesory muscle, causing popliteal soft tissue mass.
COURSE
-In most cases it arises from the distal semitendinous muscle.
-It is located superficially in the popliteal fossa, between semitendinous and
semimembranous medially and biceps femori laterally.
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-Inserts either into the posterior fascia of the leg, medial head of the gastrocnemious or
Achilles tendon.
Fig.
References: A. Salvador; Radiology, Hospital Universitario Marqués de Valdecilla,
Santander, SPAIN
3. ACCESORY POPLITEUS
COURSE
-Arises from the lateral femoral condyle.
-Courses obliquely through the popliteal fossa, between the vessels and the posterior
knee capsula, parallel to the normal popliteus muscle.
-Inserts into the posterior.
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-medial tibia, above the popliteal line.
CLINICAL RELEVANCE
It results sometimes in compressive symptoms.
ACCESORY MUSCLES OF THE ANKLE
LATERAL ESPECT
ACCESORY PERONEAL MUSCLES
4. PERONEUS TERTIUS
A third peroneal tendon is senn in 83-95% of cases in cadaveric studies.
COURSE
-It arises from the anterior surface of the distal fibula and extensor digitorum longus
muscle
-Passes deep to the inferior extensor retinaculum
-Inserts into the base and dorsal surface of 5th metatarsal.
5. PERONEUS QUARTUS
GENERAL FEATURES
•It is the most frequently reported accesory peroneal muscle.
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•This name is used for a great variety of accesory peroneal muscles including peroneus
quartus, peroneus accesorius, peroneocalcaneus externum and peroneus digiti minimi.
•They are frequently bilateral and more common in men.
COURSE
-Peroneus quartus arises at the distal lateral aspect of the fibula.
-Passes medial and posterior to peroneal tendons.
-Inserts more commonly in the calcaneus, called peroneocalcaneus externum, inserting
at:
•Peroneal tubercle of the calcaneus
•Retrotrochlear eminence posterior to peroneal tubercle.
CLINICAL RELEVANCE
Commonly asymptomatic, but it can cause lateral ankle pain, inestability, tenosynovitis,
subluxation or longitudinal tears of peroneal tendons.
MRI
-On MRI axial images peroneus quartus is identified posteromedial or medial to peroneus
brevis and separated from it by a fat plane.
-It can be misdiagnosed as a tear of peroneal tendons, but in the proximal images a
muscular belly can be seen
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Fig.
References: A. Salvador; Radiology, Hospital Universitario Marqués de Valdecilla,
Santander, SPAIN
MEDIAL ASPECT
6. FLEXOR DIGITORUM ACCESORIUS LONGUS (FDAL)
Second in prevalence after peroneus quartus muscle.
COURSE
-It arises from any structure of the posterior compartiment, including flexor retinaculum,
the tibia, the fibula, flexor hallucis longus, and the soleus.
-The tendon courses through the tarsal tunnel, where remains muscular, intimately
related to the posterior tibial artery and tibial nerve (Different from accesory soleus which
is located outside the tunnel)
-It inserts onto the quadratus plantae muscle or the flexor digitorum longus tendon.
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CLINICAL RELEVANCE
Because of the relation between the tendon and the neurovascular structures in the tarsal
tunnel it is related to tarsal tunnel syndrome and tenosynovitis of FHL tendon.
MRI
On MRI axial images we can identify muscle fibers into the tarsal tunnel, superficial to
neurovascular bundle.
Fig.
References: A. Salvador; Radiology, Hospital Universitario Marqués de Valdecilla,
Santander, SPAIN
7. PERONEOCALCANEUS INTERNUS (PCI)
It is probably the least common of all accesory muscles of the ankle.
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COURSE
-Arises from the inner part of the lower third of the fibula.
-It courses posterior and lateral to FHL and pass inferior to sustentaculum tali. Important
to differenciate from an accesory soleus wich courses superficial to flexor retinaculum;
lateral to FHL, displacing it medially
-Inserts into the medial surface of the calcaneus, below the sustentaculum tali.
CLINICAL RELEVANCE
-Usually asymptomatic, due to the absence of direct contact of this muscle to the
neurovascular bundle.
- Sometimes, it can displace the flexor hallucis longus medially compressing the
neurovascular bundle and causing tarsal tunnel syndrome, or mechanical attrition and
tenosynovitis.
MRI
On MRI it can be difficult to differenciate from FDAL
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Fig.
References: A. Salvador; Radiology, Hospital Universitario Marqués de Valdecilla,
Santander, SPAIN
DIFFERENTIAL DIAGNOSIS
Between FDAL and PCI, another accesory muscle located deep to the flexor retinaculum
(depicted in Fig. 5)
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Fig.
References: A. Salvador; Radiology, Hospital Universitario Marqués de Valdecilla,
Santander, SPAIN
TREATMENT
From surgical excision of the muscle and release of the tarsal tunnel to neurolysis of the
posterior tibial nerve.
8. ACCESORY SOLEUS
COURSE
-Arises from the anterior surface of the soleus, or from the fibula and soleal line of the tibia.
-Courses anterior to Achilles tendon.
-It has five different types of insertion: Achilles tendon, muscular insertion in the upper
surface of the calcaneus, tendinous insertion in the upper calcaneus, muscular insertion
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in the medial surface of the calcaneus or tendinous insertion into the medial aspect of
the calcaneus.
-It's supplied by the posterior tibial artery and innervated by the posterior tibial nerve.
CLINICAL RELEVANCE
-More commonly seen in men in the 2nd or 3rd decades of life, more frequent in athletes.
-Soft-tissue mass in the medial aspect of the ankle.
-Ankle pain due to a compartimental syndrome because of high intrafascial pressure or
decrease of blood supply from posterior tibial artery.
-Compression of posterior tibial nerve.
-Although it courses outside the tarsal tunnel, tarsal tunnel syndrome can exist in cases
of medial calcaneus insertion
DIFFERENTIAL DIAGNOSIS
-Accesory soleus:
•Muscle mass separated from the normal soleus and sorrounded by its own fascia.
•Insertion onto the medial cortex of the calcaneous.
-Normal soleus:
•Inserts onto the gastrocnemious tendon, forming the Achiles tendon.
-Low-lying soleus muscle can mimic an accesory soleus
•Normal muscle fibers extending to within an inch of the insertion into the calcaneal
tuberosity.
IMAGING FINDINGS
Conventional Rx
Obliteration of Kager fat by a soft-tissue mass.
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CT and MRI
Muscle belly anterior to Achilles tendon and superficial to the flexor retinaculum.
On MRI, in cases of compartyimental syndrome abnormal signal intensity can be seen.
Fig.
References: A. Salvador; Radiology, Hospital Universitario Marqués de Valdecilla,
Santander, SPAIN
9.TIBIOCALCANEUS INTERNUS
Rare accesory muscle, located deep to the flexor retinaculum in the posterior
compartiment of the lower leg.
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COURSE
-Arises from the medial crest of the tibia.
-Courses deep to the flexor retinaculum and posterior to the neurovasculare structures.
-Inserts onto the medial surface of the calcaneus, below the sustentaculum tali.
CLINICAL RELEVATION
-Frequently asymptomatic.
-It can cause tarsal tunnel syndrome due to the location within the tarsal tunnel.
Images for this section:
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Fig. 1
Fig. 2
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Fig. 3
Fig. 4
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Fig. 5
Fig. 6
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Fig. 7
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Conclusion
•The knowledge of the anatomy of the accesory muscles in the lower limb is necessary
for a precise diagnosis on MRI.
•We should be familiar with the radiologic appearence of these muscles in order to avoid
misdiagnosis, specially excluding the posibility of a neoplasm.
•The mayority of the acessory muscles are asymptomatic and represent incidental
findings at imaging techniques, but they can cause symptoms sometimes.
•Careful evaluation for acessory muscles may help identifying these muscles as a
potential cause symptoms.
•MRI is the modality of choice in the evaluation of suspected accesory muscles.
•MRI allows preoperative planning in symptomatic cases, evaluating the limits and
relationship between the surrounding structures.
Personal Information
References
•Accesory muscles: anatomy, symptoms, and radiologic evaluation. Paul A. Sookur,
MRCP , Ali M. Naraghi, FRCR, Robert R. Bleakney, FRCPC, Rosy Jalan, FRCR, Otto
Chan, FRCR, Lawrence M. White, MD. RadioGraphics 2008; 28:481-499
•MR imaging of accessory muscles around the ankle. Cheung YY, Rosenberg ZS. MRI
Clinics of North America 2001; 9(3):465-473.
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•Peroneus quartus muscle: MR Imaging features. Yvonne Y. Cheung, MD. Zehava S.
Rosenberg, MD. Rajeev Ramsinghani, MD. Javier Beltran, MD. Melvin H. Jahss, MD.
Radiology 1997; 202: 745-750.
•The peroneus quartus muscle. Anatomy and clinical relevance. J. Zammit, D. Shing.
The journal of bone and joint surgery (Br). Vol 85-B. No 8. November 2003.
•Posterior impingement of the ankle caused by anomalous muscles. A report of four
cases. Alistair best, FRCS ED (TR&ORTH), Eric Giza, MD, James Linklater, Franzcr,
and Martin Sullivan FRACS. The journal of bone and joint surgery. Vol 87-A. Number 9.
September 2005.
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