MRI Web Clinic — May 2015

Accessory Muscles of the Knee
John F. Carroll, M.D.

Clinical History:  A 45 year-old male presents with chronic knee pain.  What are the findings?  What is your diagnosis?

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Figure 1:

(1a) Fat suppressed proton density-weighted coronal and a (1b) T1-weighted sagittal image are provided.

 

Findings

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Figure 2:

The coronal fat-suppressed proton density-weighted image through the posterior knee depicts an accessory 3rd head of the gastrocnemius (3), just medial to the lateral head (L). The popliteal vein (arrow) lies between the 3rd head of the gastrocnemius and the medial head (M).

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Figure 3:

The sagittal T1-weighted MR image confirms the accessory 3rd head of the gastrocnemius (3), just distal to its origin from the distal femoral metaphysis.

 

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Figure 4:

(4a,b,c) Sequential axial fat-suppressed proton density-weighted images from superior to inferior in the same patient show the relationship of the popliteal artery (arrow) and vein (arrowhead) to the accessory 3rd head (3) and medial head (M) of the gastrocnemius. Figure 4b shows tapering of the popliteal vein as it courses between the 3rd and medial heads of the gastrocnemius, which can be seen in asymptomatic patients and is not a definitive sign of vascular entrapment. Note that normal signal is present in the popliteal vein, there is no compression of the popliteal artery and a normal popliteal artery flow void is present. The plantaris muscle (P) is also labeled.

 

Diagnosis

Accessory 3rd head of the gastrocnemius muscle.

 

Introduction

Multiple accessory, supernumerary, and anomalous muscles have been described in the medical literature.  Accessory muscles of the knee are typically asymptomatic.  But knowledge of these muscles is important because they can mimic a soft tissue tumor, can compress the popliteal artery and/or popliteal vein, and rarely may cause neural compression.

Magnetic resonance imaging (MRI) is the modality of choice in diagnosing accessory muscles, delineating their relationship to adjacent structures, and differentiating them from soft tissue tumors.  Accessory muscles are isointense to skeletal muscle on all pulse sequences and typically attach by muscular or tendinous insertions.  Accessory muscles around the knee include the 3rd head of the gastrocnemius, the accessory plantaris, the accessory popliteus, and the tensor fasciae suralis muscle.

 

Discussion

3rd head of the gastrocnemius

Normally, the gastrocnemius muscle has 2 heads.  Typically, the medial head is larger and originates above the articular surface, along a depression at the superoposterior margin of the medial femoral condyle, as well as from the femoral margin of the knee joint capsule.  The lateral head of the gastrocnemius originates from a facet on the proximal posterolateral surface of the lateral femoral condyle, an area medial and proximal to that facet, and the adjacent knee joint capsule.  The medial and lateral heads of the gastrocnemius combine to form the Achilles tendon, which with its contribution from the soleus muscle, inserts onto the posterior surface of the calcaneus.

Anomalous origins and accessory slips of the medial or lateral heads of the gastrocnemius may occur.  The accessory 3rd head of the gastrocnemius was first described by Kelch in 1813 and was further detailed by Frey in 1919.1, 2  The estimated frequency of the accessory 3rd head of the gastrocnemius is between 2.0% and 5.5%.2,3,4  The accessory 3rd head of the gastrocnemius is best demonstrated on non-fat saturated sequences and can range in size from a thin threadlike muscle or tendon to a bulky muscle.  It is important to be aware of these variant origins and accessory slips of the gastrocnemius because of their relationship and effects on the popliteal artery and vein.5,6,7  Rarely, the 3rd head of the gastrocnemius can be associated with nerve entrapment.8

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Figure 5:

Accessory 3rd head of the gastrocnemius in a different patient. An axial proton density-weighted image from a 19 year-old female who presents with pain. An accessory 3rd head gastrocnemius (3) originates from the distal femoral metaphysis, slightly medial to midline, coursing anterior and lateral to the popliteal artery (arrow) and popliteal vein (arrow head). The 3rd head of the gastrocnemius in this patient later joins with the lateral head (not shown).

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Figure 6:

A threadlike accessory 3rd head of the gastrocnemius in a different patient. Sequential axial fat-suppressed proton density-weighted images from superior to inferior in a 15 year-old male with pain, stiffness, and swelling, show a 3rd head gastrocnemius muscle (3) with a threadlike tendinous slip (arrows), extending between the popliteal artery (A) and vein (V). The semimembranosus muscle (asterisks) and lateral head of the gastrocnemius muscle (L) are also labeled.

Popliteal Vascular Entrapment Syndrome

In 1879, Stuart first described entrapment of the popliteal artery, but it was not reported as a clinical entity until 1959, when Hamming described intermittent claudication in a young patient due to an anomalous course of the popliteal artery.9,10  The term popliteal artery entrapment syndrome (PAES) was originally coined by Love in 1965.11  In 1967, Rich described combined popliteal artery and vein entrapment.6  Isolated compression of the popliteal artery or vein can occur, or both vessels can be compromised by these anomalous/accessory muscles.

Compression of the popliteal artery can cause symptoms of claudication and is most frequently seen in young athletic patients (60% < 30 y/o and 15:1 male predilection).12,13   Recurrent arterial compression can cause vascular microtrauma, leading to early atherosclerosis, thrombus formation, aneurysm, distal emboli, and rarely rupture.  Compression of the popliteal vein can result in deep venous thrombosis and pulmonary emboli.5

Multiple subtypes of popliteal vascular entrapment syndrome have been described.11,14,15   Most commonly, the popliteal artery takes an aberrant medial course along a normally positioned medial head gastrocnemius (type I) or around an abnormally positioned muscle (type II).

Table I: Subtypes of Popliteal Vascular Entrapment Syndrome

I Aberrant course of the popliteal artery, extending medial to a normally located medial head gastrocnemius
II Atypical lateral location of the medial head gastrocnemius
III Abnormal medial head gastrocnemius muscle bundle surrounds the popliteal artery
IV Popliteal artery is deep to and entrapped by the popliteus muscle or a fibrous band
V Popliteal artery and vein are both entrapped
VI Popliteal artery is normal in location but entrapped by hypertrophied gastrocnemius muscle

 

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Figure 7:

This series of 3D images demonstrates the anatomic variations that result in popliteal vascular entrapment subtypes I-IV. Illustration by Dr. Michael Stadnick.

Although popliteal vascular entrapment can occur in the neutral position, stress maneuvers can cause or intensify the entrapment. These maneuvers include knee hyperextension and knee extension with the ankle in dorsiflexion or plantar flexion.  On clinical exam, the patient can have normal pulses that decrease or disappear with plantar or dorsiflexion of the ankle.  These maneuvers can also be used to confirm entrapment with MRI/MRA, ultrasound, CTA, and conventional angiography/venography.  Long term PAES causes microtrauma to the vessel, which can result in permanent vascular damage.13  Surgical release, myomectomy, or tendon resection is the ultimate treatment, and early treatment can prevent irreversible vascular damage.13, 16

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Figure 8:

Popliteal artery entrapment syndrome in a 16 year-old male with claudication. (8a) Axial T1-weighted and (8b) axial fat-suppressed T2-weighted MR images show an atypical lateral location of the medial head gastrocnemius (M) with an aberrant course of the popliteal artery (arrow), extending medial to the medial head of the gastrocnemius. Note compression of the popliteal artery between the medial head gastrocnemius (M) and the medial femoral condyle (*). (Images courtesy of Eric A Walker,M.D., Penn State Milton S. Hershey Medical Center).

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Figure 9:

(9a) MR angiogram and (9b) conventional angiogram images in the same patient demonstrate extrinsic compression of the popliteal artery (arrows) in plantar flexion, consistent with popliteal artery entrapment syndrome. This patient underwent surgical correction of his popliteal artery entrapment syndrome. At his 7 month postoperative check-up, the patient had good pulses, a normal duplex ultrasound, and was physically active with no claudication. (Images courtesy of Eric A Walker,M.D., Penn State Milton S. Hershey Medical Center).

 

Popliteal artery aneurysm

Aneurysms are classified as true aneurysms when all arterial wall layers are involved and false, or pseudoaneurysms, when there is trauma or infection, causing a defect in the arterial wall. True aneurysms of the popliteal artery are fusiform in shape and are the most common peripheral arterial aneurysm.  Approximately 50-70% of popliteal artery aneurysms (PAA) are bilateral, and 30-50% of patients with PAA also have coexisting abdominal aortic aneurysms.13 PAA can manifest as rest pain, claudication, or ischemia with associated thrombosis or embolization, and occasionally rupture.

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Figure 10:

Popliteal artery aneurysm and associated 3rd head of the gastrocnemius. (10a) Sequential axial fat-suppressed proton density, (10b) coronal fat-suppressed proton density, and (10c) sequential sagittal T1-weighted MR images in a 62 year-old male with pain, 3 weeks status post knee injury, demonstrate a partially thrombosed saccular popliteal artery aneurysm (arrowheads) and an adjacent accessory 3rd head of the gastrocnemius (3). The 3rd head of the gastrocnemius courses between the popliteal artery (A) and vein (V), and later joins with the medial head (M). The semimembranosus muscle is also labeled (asterisk).

In cases of popliteal artery aneurysm, it is important to notify the surgeon of a coexistent 3rd head of the gastrocnemius, as the accessory muscle should be resected at the time of arterial repair.  Failure to resect an anomalous/accessory muscle at the time of arterial repair can result in extrinsic compression of the vascular graft.16

 

Accessory Plantaris

In 2009, Herzog performed a retrospective study of 1,000 consecutive MRI studies of the knee and found accessory plantaris muscles (APM) in 6.3% of patients.17  There was no statistically significant difference in the prevalence of the APM in males versus females.  The APM most frequently merged with the origin of the normal plantaris muscle, but one merged with the origin of the lateral head of the gastrocnemius.  The APM inserted onto the iliotibial tract, iliotibial band, or lateral patellar retinaculum.

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Figure 11:

Accessory Plantaris Muscle. (11a) axial fat-suppressed proton density, (11b) coronal fat-suppressed proton density, and (11c) sagittal T1-weighted MR images in a 30 year-old female demonstrate an accessory plantaris muscle (arrow) inserting on the iliotibial band (arrowheads).

 

Accessory popliteus

In 2004, Duc was the first to report the accessory popliteus muscle and postulated that it could be associated with compressive neurovascular symptoms.18  The accessory popliteus has a common origin with the lateral head of the gastrocnemius, passes anterior to the popliteal vessels, and inserts on the posteromedial knee joint capsule.19

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Figure 12:

With the medial and lateral heads of the gastrocnemius muscle partially removed, this 3D representation demonstrates the accessory popliteus muscle (red arrow) and its course paralleling the popliteus muscle (white arrow). Illustration by Dr. Michael Stadnick.

 

Tensor fascia suralis muscle 

This rare muscle can arise from any of the hamstring muscles, most commonly from the distal semitendinosus, and inserts into the medial head of the gastrocnemius, the posterior fascia of the leg, or via a thin tendon onto the superficial aspect of the Achilles.20

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Figure 13:

Tensor fascia suralis muscle.  (13a) Sequential axial fat-suppressed proton density and (13b) sagittal T1-weighted images in a 36 year-old male reveal an accessory tensor fascia suralis muscle (arrowheads), posterior to the semimembranosus muscle (*) and the medial head of the gastrocnemius muscle and lateral to the semitendinosus tendon (arrow).

 

 

Conclusion

Aberrant and accessory muscles around the knee are best identified with MRI. These muscles are frequently asymptomatic, but can mimic soft tissue tumors. More important is their relationship and effect on adjacent structures. An accessory 3rd head of the gastrocnemius can cause popliteal artery entrapment syndrome (PAES), resulting in claudication in young patients.  Repetitive microvascular trauma can cause early atherosclerosis, fibrosis, stenosis, thrombosis, aneurysm, and emboli, and occasionally result in aneurysm rupture.  Early detection and surgical repair of PAES can alleviate symptoms of claudication, and prevent long-term sequelae of repetitive vascular microtrauma.  When a popliteal artery aneurysm is caused by one of these muscular variants, it is important to resect the anomalous muscle at the time of arterial repair to avoid postoperative compression of the repaired vessel.

 

References

  1. Kelch WG.  Abweichung des Biceps Femoris.  Beitrage zur pathologischen Anatomie.  Berlin: C Salfeld; 1813. p. 8. s42, art 36.
  2. Frey H. Musculus gastrocnemius tertius. Gegenbaurs Morph Jahrb 1919; 50: 517-530.
  3.  Koplas MC, Grooff P, Piraino D, Recht M. Third head of the gastrocnemius: an MR imaging study based on 1,039 consecutive knee examinations. Skeletal Radiology 2009; 38:349-354.
  4.  Iwai T, Sato S, Yamada T, Muraoka Y, Sakurazawa K, Kinoshita H, Inoue Y, Endo M, Yoshida T, Suziki S. Popliteal vein entrapment caused by the third head of the gastrocnemius. British Journal of Surgery 1987; 74: 1006-1008.
  5.  Liu PT, Moyer AC, Huettl EA, Fowl RJ, Stone WM. Popliteal vascular entrapment syndrome caused by a rare anomalous slip of the lateral head of the gastrocnemius muscle.  Skeletal Radiology 2005; 34:359-363.
  6.  Rich NM, Hughes CW.  Popliteal artery and vein entrapment. Am J Surg 1967; 113:696-698.
  7.  Connell J. Popliteal vein entrapment. Br J Surg 1978; 65: 351.
  8.  Guru A, Kumar N, Shetty SD, Nayak S. Presence of third head of gastrocnemius and entrapment of the sural nerve.  International Journal of Anatomical Variations 2013; 6:194-196
  9.  Stuart T. Note on a variation in the course of the popliteal artery. J Anat Physiol 1879; 13:162-165.
  10.  Hamming JJ.  Intermittent claudication at an early age, due to an anomalous course of the popliteal artery.  Angiology 1959; 10: 369-371.
  11. Love JW, Whelan TJ. Popliteal artery entrapment syndrome.  Am J Surg 1965; 109:620-624.
  12.  Fowl RJ, Kempczinski RF. Popliteal artery entrapment. In: Rutherford RB, ed. Vascular surgery. 5th ed. Philadelphia, PA: Saunders, 2000; 1087-1093
  13. Wright LB, Matchett WJ, Cruz CP, James CA, Culp WC, Eidt JF, McCowan TC. Popliteal Artery Disease: Diagnosis and Treatment.  Radiographics 2004; 24:467-479
  14. Rich NM, Collins GJ, McDonald PT, Kozloff L, Clagett GP, Collins JT.  Popliteal vascular entrapment: its increasing interest.  Arch Surg 1979; 114: 1377-1384
  15. Karaman B, Battal B, Akgun V, Hamcan S, Bozkurt Y, Bozlar U.  Popliteal artery entrapment syndrome with thrombosed popliteal aneurysm: multidetector computed tomography angiography findings of a case.  Clinical Imaging 2012; 36: 850-853
  16.  Macedo TA, Johnson CM, Hallett JW, Breen JF.  Popliteal Artery Entrapment Syndrome: Role of Imaging in the Diagnosis.  AJR 2003; 181:1259-1265
  17.  Herzog RJ. Accessory Plantaris Muscle: Anatomy and Prevalence. HSSJ 2011;7: 52-56
  18.  Duc SR, Wentz KU, Kach KP, Zollikofer CL.  First report of an accessory popliteal muscle: detection with MRI.  Skeletal Radiology 2004; 33:429-431
  19.  Sookur PA, Naraghi AM, Bleakney RR, Jalan R, Chan O, White LM.  Accessory Muscles: Anatomy, Symptoms, and Radiologic Evaluation.  Radiographics 2008; 28:481-499
  20.  Chason DP, Schultz SM, Fleckenstein. Tensor Fasciae Suralis: Depiction on MR images. AJR 1995; 165:1220-1221