Normal Variants




Accessory muscles around the ankle are commonly encountered as incidental findings on cross-sectional imaging. Mostly asymptomatic, accessory muscles sometimes mimic mass lesions. They have been implicated as the cause of tarsal tunnel syndrome, impingement of surrounding structures, and chronic pain. Distinguishing these muscles can be challenging, because some travel along a similar path. This article describes these accessory muscles in detail, including their relationships to the aponeurosis of the lower leg. An imaging algorithm is proposed to aid in identification of these muscles, providing a valuable tool in diagnostic accuracy and subsequent patient management.


Key points








  • Accessory muscles are commonly presented as incidental findings on cross-sectional imaging.



  • Although most often asymptomatic, accessory muscles sometimes mimic mass lesions, and have been implicated as the cause of tarsal tunnel syndrome, impingement of surrounding structures, and pain related to muscle ischemia. Unless specifically injured, accessory muscles appear isointense to other muscles on all imaging sequences.



  • Based on the location of the accessory muscles, including their origins, insertions, and relationships to the aponeurosis of the lower leg, an imaging algorithm is proposed to aid in their identification.






Introduction


Anatomic variations of muscles around the ankle consist predominantly of extra or accessory muscles. Generally asymptomatic, these supernumerary muscles are discovered incidentally on imaging studies. Accurate diagnosis of the infrequent symptomatic cases, presenting as mass lesions, tarsal tunnel syndromes, chronic ankle pain, or impingement, is important for management. However, distinguishing between these muscles can be challenging, because some travel along a similar path, and others have either similar origins or similar insertions. This article reviews each accessory muscle in detail, including those located lateral and posteromedial to the ankle. Each muscle’s origin, insertion, imaging features, and clinical presentations, along with its relationship with the aponeurosis of the lower leg, are described ( Table 1 ). In an effort to improve diagnostic accuracy, an imaging algorithm that can be readily applied in daily practice is proposed.



Table 1

Summary of accessory muscles around the ankle





































































Origin Course Insertion Alternative Name Relative to Deep Aponeurosis Notes
Lateral Accessory Muscle
PQ Alternative Name of PQ Based on Insertion The insertion site is highly variable, which gives rise to subtypes and confusing terminology
Retrotrochlear eminence (most common insertion)
Cuboid
Peroneus longus
Peroneocalcaneus externum
Posterior surface of the fibula or peroneus longus or peroneus brevis Travels medial and posterior to the peroneal tendons Peroneocuboideus
Peroneoperoneolongus
Inferior retinaculum
Posteromedial Accessory Muscles
Accessory soleus Fibula, or soleal line of the tibia, or the anterior surface of the soleus muscle Descends posterior to the neurovascular bundle Upper or medial, calcaneal surface Superficial
FDAL Variable origin, can arise from any structures of the posterior compartment Descends with the posterior tibial neurovascular bundle in the tarsal tunnel Quadratus plantae, or the FDL before its division Long accessory of the quadratus plantae
Long accessory of the long flexors
Accessorius of the accessorius of Turner
Second accessorius of Humphrey
Deep Compare with other accessory muscles in or near the tarsal tunnel, such as the TCI or PCI; the FDAL does not insert onto the calcaneus
PCI Lower third of the fibula Travels parallel to but remains lateral to the FHL Base of the sustentaculum Deep Not directly related to the neurovascular bundle, because it travels lateral to the FHL
TCI Medial crest of the tibia Travels posterior to the FHL, within the tarsal tunnel and superficial to the neurovascular bundle Medial calcaneus Deep Has features of both the FDAL and the accessory soleus

Abbreviations: FDAL, flexor digitorum accessorius longus; FDL, flexor digitorum longus; FHL, flexor hallucis longus; PCI, peroneocalcaneus internus; PQ, peroneus quartus; TCI, tibiocalcaneus internus.




Introduction


Anatomic variations of muscles around the ankle consist predominantly of extra or accessory muscles. Generally asymptomatic, these supernumerary muscles are discovered incidentally on imaging studies. Accurate diagnosis of the infrequent symptomatic cases, presenting as mass lesions, tarsal tunnel syndromes, chronic ankle pain, or impingement, is important for management. However, distinguishing between these muscles can be challenging, because some travel along a similar path, and others have either similar origins or similar insertions. This article reviews each accessory muscle in detail, including those located lateral and posteromedial to the ankle. Each muscle’s origin, insertion, imaging features, and clinical presentations, along with its relationship with the aponeurosis of the lower leg, are described ( Table 1 ). In an effort to improve diagnostic accuracy, an imaging algorithm that can be readily applied in daily practice is proposed.



Table 1

Summary of accessory muscles around the ankle





































































Origin Course Insertion Alternative Name Relative to Deep Aponeurosis Notes
Lateral Accessory Muscle
PQ Alternative Name of PQ Based on Insertion The insertion site is highly variable, which gives rise to subtypes and confusing terminology
Retrotrochlear eminence (most common insertion)
Cuboid
Peroneus longus
Peroneocalcaneus externum
Posterior surface of the fibula or peroneus longus or peroneus brevis Travels medial and posterior to the peroneal tendons Peroneocuboideus
Peroneoperoneolongus
Inferior retinaculum
Posteromedial Accessory Muscles
Accessory soleus Fibula, or soleal line of the tibia, or the anterior surface of the soleus muscle Descends posterior to the neurovascular bundle Upper or medial, calcaneal surface Superficial
FDAL Variable origin, can arise from any structures of the posterior compartment Descends with the posterior tibial neurovascular bundle in the tarsal tunnel Quadratus plantae, or the FDL before its division Long accessory of the quadratus plantae
Long accessory of the long flexors
Accessorius of the accessorius of Turner
Second accessorius of Humphrey
Deep Compare with other accessory muscles in or near the tarsal tunnel, such as the TCI or PCI; the FDAL does not insert onto the calcaneus
PCI Lower third of the fibula Travels parallel to but remains lateral to the FHL Base of the sustentaculum Deep Not directly related to the neurovascular bundle, because it travels lateral to the FHL
TCI Medial crest of the tibia Travels posterior to the FHL, within the tarsal tunnel and superficial to the neurovascular bundle Medial calcaneus Deep Has features of both the FDAL and the accessory soleus

Abbreviations: FDAL, flexor digitorum accessorius longus; FDL, flexor digitorum longus; FHL, flexor hallucis longus; PCI, peroneocalcaneus internus; PQ, peroneus quartus; TCI, tibiocalcaneus internus.




Imaging of accessory muscles


Although the accessory muscles of the ankle can be detected on all cross-sectional imaging modalities, most published reports are of magnetic resonance (MR) imaging studies because of its superior soft tissue contrast. However, MR imaging does have limitations. First, the origins of these muscles may not be included in the field of view of a routine ankle examination. Second, if insertions are small, they can be difficult to separate from surrounding soft tissues. Lateral radiography captures only the accessory soleus and is therefore of limited use.




Accessory muscles in the lateral ankle


Peroneus Quartus


The peroneus quartus (PQ) ( Fig. 1 ) refers to the fourth peroneal muscle, after peroneus longus, peroneus brevis, and peroneus tertius. It represents a group of accessory peroneal muscles found only in humans. Its absence in other species and its function in stabilizing hind foot pronation led Hecker to postulate that these muscles are evolutionary developments to accommodate the bipedal posture.




Fig. 1


PQ. Axial intermediate-weighted image of a 16-year-old girl with medial pain and instability following remote trauma, from above the ankle joint ( A ), at ankle joint ( B ), distal to ankle joint line ( C ) and beyond malleolar tips ( D ). In the lower leg, the PQ ( yellow arrow ) arises from the peroneus brevis muscle ( blue arrowhead ). The PQ descends with the peroneal tendons ( light blue arrow ). At the tip of the lateral malleolus, the PQ is posterior to the peroneal tendons. It then inserts onto the retrotrochlear eminence ( pink curved arrow in D ).


The reported prevalence of the PQ ranges from 5.2% to 22%. A systematic review pooling 46 studies reported 16% prevalence. The most common subtype is the peroneocalcaneus externum, found in 79% and 91% of all PQ.


Origin


The origin is variable and includes the posterior surface of the fibula or an origin from the peroneus longus or peroneus brevis muscle. Origin from the peroneus brevis origin is the most common based on both cadaveric studies and imaging.


Insertion


The insertion site is highly variable, which gives rise to subtypes and confusing terminology ( Fig. 2 ). Hecker’s classification, based on the various insertions, is listed in Table 2 .




Fig. 2


Peroneoperoneolongus, a peroneus quartus variant that inserts onto the peroneus longus tendon. Axial intermediate-weighted image of a 47-year-old man with chronic ankle pain following injury, from ankle joint line ( A ) to cuboid fossa ( C ). At the lateral malleolus ( A ), the peroneoperoneolongus muscle ( yellow arrow ) travels medial and posterior to the peroneal tendons. It becomes tendinous ( yellow arrow in B ) before inserting onto the peroneus longus (PL). At the cuboid fossa ( C ), the tiny tendinous slip is no longer seen. Blue arrowhead in ( A ) and ( B ) indicates the peroneus brevis. Yellow arrow indicates peroneoperoneolongus muscle and tendon.


Table 2

Insertion sites of PQ



















Insertion Site Alternative Name of PQ Based on Insertion
Retrotrochlear eminence a Peroneocalcaneus externum
Cuboid Peroneocuboideus
Peroneus longus Peroneoperoneolongus
Inferior retinaculum

a Most common insertion site.



The most common insertion of the PQ is at the retrotrochlear eminence, one of 2 bony protuberances arising from the lateral cortex of the calcaneus (see Fig. 1 ). The retrotrochlear eminence lies posterior to the peroneal tubercle, which separates the 2 peroneal tendons.


Imaging Features


The tendon is described mostly in MR and infrequently in ultrasonography (US) studies. The accessory muscle is seen as a separate structure traveling medial and posterior to the peroneal tendons (see Fig. 2 ). Its origin may be difficult to separate from the peroneal muscles. It becomes tendinous with variable insertions (see Fig. 2 and Table 2 ).


Clinical Features


Although generally asymptomatic, the PQ muscles may contribute to crowding in the peroneal tunnel ( Fig. 3 ), thereby predisposing patients to peroneal tendinopathy, peroneal brevis dislocation, synovitis, pain, and snapping. The use of PQ for repair of the superior peroneal retinaculum has also been reported.


Sep 18, 2017 | Posted by in MAGNETIC RESONANCE IMAGING | Comments Off on Normal Variants
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