ESSR 2019 / P-0069
Unusual neurovascular compressive lesions around the knee: A pictorial review of key anatomy and imaging findings
Congress: ESSR 2019
Poster No.: P-0069
Type: Educational Poster
Keywords: Musculoskeletal joint, Musculoskeletal soft tissue, Anatomy, MR, CT, Ultrasound, Imaging sequences, Education, Cysts, Trauma, Pathology
Authors: K. Paramesparan, A. Shah, W. J. Rennie; Leicester/UK

Imaging findings OR Procedure Details

Relevant Anatomy:

Common peroneal nerve anatomy


The common peroneal nerve (CPN) is the lateral division of the sciatic nerve tracking from the posterolateral side of the knee around the fibular head down towards the anterolateral side of the lower leg Fig. 1 . On MRI the course of the peroneal nerve and important landmarks are best appreciated on the axial T1W imaging Fig. 2 . They appear as bundles of fascicles surrounded within a thin sheath.  Once the CPN has branched off the sciatic nerve, it twists around the biceps femoris muscle and passes through the peroneal tunnel. The peroneal tunnel is created between the insertion of the peronus longus muscle and the fibula. Once exiting the tunnel, the CPN trifurcates into a recurrent articular, superficial and deep branch. There is a close relationship between the CPN and medial aspect of the distal biceps femoris muscle. The main two branches of the CPN are the superficial and deep peroneal nerves. These innervate the lateral and anterior compartments below the knee respectively. The superficial peroneal branch has both motor and sensory innervation to the peroneus longus and brevis muscles Fig. 3 . The deep peroneal branch provides motor innervation to the tibialis anterior, extensor hallucis longus and extensor digitorum muscles. On knee MRI imaging one can appreciate the tibialis anterior and extensor digitorium muscles in the most inferior axial slices.

Fig. 1: Diagrammatic representation of the normal anatomy of the common peroneal nerve (CPN) in the sagittal oblique (a) and coronal views (b) as it tracks around the posterolateral corner of the knee. Note the further trifurcation branching occurs as it passes the fibular head into the recurrent articular and deep and superficial branches.
References: Radiology, Leicester royal infirmary, University Hospitals Leicester - Leicester/UK


Fig. 2: Axial T1W imaging of the knee at different levels. CPN is highlighted as the yellow arrow. At the level of the distal femur (a) the CPN and tibial nerve (thick red arrow) branch off the sciatic nerve. The CPN is very closely related to the medial border of the biceps femoris muscle. At the level of the femoral condyles (b) the CPN is noted inbetween the head of the lateral gastrocnemius muscle and biceps femoris muscle. At the level of the fibular head (c) the CPN tracks posteriorly ready to twist anteriorly around the fibular head.
References: Radiology, Leicester royal infirmary, University Hospitals Leicester - Leicester/UK
Fig. 3: Axial T1 weighted MRI image of the right lower leg highlighting the 4 compartments (anterior (red), lateral (green), deep posterior (blue) and superficial posterior (orange) in the leg and their relationship to the deep and superficial CPN. The deep peroneal nerve branch is noted between the extensor hallucis longus muscle and tibialis anterior muscles. The superficial peroneal branch tracks in the anterio-lateral compartment interface and anterior to the peroneus longus muscle.
References: Radiology, Leicester royal infirmary, University Hospitals Leicester - Leicester/UK


CPN Symptoms:

Compression of the CPN usually will result with patients being referred with pain in the fibular neck region, with or without neurological compromise. Weakness on eversion and dorsiflexion (and in severe cases foot drop) are well known. If there is coexistent foot inversion weakness then an L5 distribution radiculopathy or sciatic nerve injury should also be considered. Specific weakness/atrophy of the biceps femoris muscle can possible also be related to CPN injury as this is innervated by the nerve above the knee.



Pathology of the peroneal nerve:

There are several proposed mechanisms to explain CPN compression and susceptibility to injury some of which are anatomically related. The CPN is more susceptible to compression compared to other nerves of the lower limb due to its lack of perineural supporting tissue. There is said to be increased risk of neuropathy and compression in the section of the CPN that runs through the peroneal tunnel, mainly due to the lack of less fat within this region.


Normal variants of the CPN that also predispose compressive injury such as a high bifurcation/trifurcation occurring above the knee joint line Fig. 4 .

Fig. 4: Diagrammatic representation of the right knee posterolateral corner in the sagittal plane demonstrating the normal (a) and variant anatomy (b, c) of the level of bifurcation of the CPN. 81% of trifurcation of the CPN occurs at or below the level of the fibular neck (a). Less common are high bifurcation (b,c) above the fibular neck or knee joint. When bifurcation of the peroneal nerve occurs above the knee joint there is higher risk of injury.
References: Radiology, Leicester royal infirmary, University Hospitals Leicester - Leicester/UK


Trauma of the CPN usually occurs around its section around the fibular neck where compression can occur due to its superficial position. Impingement/entrapment of the nerve within the fibular tunnel (which is prone to pathological ossification) is also a key pathology to identify. The fibular tunnel is formed by the arch of the peroneus longus/soleus tendon with the bony floor of the fibula. This is difficult to appreciate on imaging and should be considered when MRI findings of CPN neuropathy are noted without an obvious cause. A common location of a nerve sheath ganglion is at the entrance port for the recurrent articular branch after the CPN trifurcates.



Imaging findings and unusual CPN compressive lesions:



In addition to a focal lesion causing compression of the CPN, the muscles of the knee provide addition information relating to denervation. This is particularly well depicted on MRI and has at least three patterns that can have considerable overlap.

1)       Oedema within a muscle (usually after 48hrs) and corresponds to the acute phase of denervation and are well depicted as high signal intensity on fluid sensitive sequences with normal muscle signal intensity on T1-weighted images.

2)       This is usually followed by atrophy, relating to muscular tissue loss occurring after 7 days and is seen on all imaging sequences. At this point the pathophysiology is reversible within the muscle.

3)       Irreversibility is usually determined by the final pattern seen on MRI which is of fatty replacement suggesting chronic denervation. The whole muscle can be affected or partial involvement relating to preserved collateral motor innervation.  On MRI, fatty atrophy is best visualised on T1 weighted imaging as high signal fatty replacement within the atrophic muscle and usually occurs after several weeks to months of denervation injury. Denervation hypertrophy can also occur in chronic cases and also provides a useful indicator to ensure that there is no compressive lesion causing this.


Examples of CPN denervation:

A) CPN mass lesion causing denervation Fig. 5


Fig. 5: 61 Year old female patient presenting with a enlarging lump in the right lower leg and popliteal fossa. Initial ultrasound imaging (a) demostreated a mixed-predominantely hypoechoic lesion with no evidence of increased doppler neovascularity. Subsequent MRI was performed with various sequences (b-f) which confirmed a heterogenous signal mass in the posteriolateral corner of the right knee between the lateral head of gastrocnemius and the biceps femoris musculotendinous junction (WHITE ARROW HEADS). On the fluid sensitive sequences (b, d, e) the mass is of heterogenous high signal with intermediate SI on the T1 W imaging (c). The CPN is noted to arise from the distal aspect of the mass suggestive involvement, and there is abnormal oedema and fatty denervation in the extensor and peroneal compartment muscles (WHITE ARROW). Findings overall were in keeping with a neurogenic mass (confirmed MPNST) arising from the CPN. Appropriate MDT referral and approach is recommended. This case highlights the pattern of denervation on MRI and the need to consider neurogenic tumours of the CPN when anterior compartment lower leg denervation changes are seen.
References: Radiology, Leicester royal infirmary, University Hospitals Leicester - Leicester/UK



B) Fibular head lesion and intraneural ganglion causing CPN neuropathy and compression Fig. 6


Fig. 6: 51 year old male patient presenting with knee pain and foot drop. Initial plain radiographs of the left lower leg (a) demonstrate a lytic lesion in the fibular head extending into the neck(WHITE ARROW). Subsequent various MRI sequences of the left knee (b-f) and lower leg confirmed the fluid intense region in the fibula head (WHITE ARROW) which is well defined and subarticular in location most likely in keeping with a degenerative geode. There is no cortical breach and no extraosseous soft tissue component. Not that there is no surrounding bone marrow oedema. There is an intra neural ganglion cysts (WHITE ARROW HEAD) arising from the central tendon of the peroneus longus muscle and is noted to surrounding the common peroneal nerve and likely the cause of neuropathy (DASHED WHITE ARROW). Denervation changes noted in the anterior compartment muscles of the lower leg (RED ARROW) and would account for the patients foot drop.
References: Radiology, Leicester royal infirmary, University Hospitals Leicester - Leicester/UK


Having discussed the relevant anatomy and relevant pathophysiology, we highlight  4 cases of unusual neurovascular compression that the radiologist may encounter.



Case 1 : CPN compression by wallerian degeneration of Nerve sheath ganglion Fig. 7


Fig. 7: MRI T2 FS axial (a) and sagittal (b) sequences of the left knee of a patient demonstrating the classical denervation pattern oedema in the anterior extensor compartment (WHITE ARROW). A longitudinal multi loculated fluid signal intensity nerve sheath ganglion is noted adjacent to the fibular neck along the course of the CPN (RED ARROW). Note the nerve sheath oedema and disruption of the nerve in keeping with wallerian degeneration (ARROW HEADS).
References: Radiology, Leicester royal infirmary, University Hospitals Leicester - Leicester/UK


In the knee the CPN is the most common location of a nerve sheath ganglion and the presence of the recurrent articular branch can explain the high incidence. The schematic diagram in Fig. 8 demonstrates the common locations of the nerve sheath ganglia forming in the CPN and its branches.  The intraneural ganglion follows the path of least resistance and extends proximally in the affected nerves.  On MRI imaging the nerve sheath ganglion appears as a tubular structure within the nerve with high T2SI and low T1 SI. There is no post contrast enhancement and no focal capsule is noted. We highlight a common example of CPN entrapment by a large multilobed ganglion cyst Fig. 9



Fig. 8: Diagrammatic representation of the common sites of nerve sheath ganglion (depicted by three blue shaded zones (1,2 & 3)). The blue shaded zones from left to right refers to the proximal common peroneal nerve (1), the horizontal portion of the recurrent articular CPN branch (2) and the ascending portion of the recurrent articular CPN branch(3).
References: Radiology, Leicester royal infirmary, University Hospitals Leicester - Leicester/UK
Fig. 9: 58 Year old male referred for meniscal injury. Routine MRI sequences of the knee were obtained (a-e) demonstrating a large multi-loculated ganglion cyst (WHITE ARROW HEADS), coursing around the fibular neck, and directed retrograde along the course of the CPN. The source of the cyst of the cyst is likely related to the anterior aspect of the proximal tibio-fibular joint. Fatty atrophy of the peroneus longus (c), peroneus brevis and the anterior compartment is well noted on the T1W imaging (c), (with only preservation of the extensor digitorum longus) noted. Increased signal on the fluid sensitive sequences of the muscles are due to oedematous changes (RED ARROW) again in keeping with denervation.
References: Radiology, Leicester royal infirmary, University Hospitals Leicester - Leicester/UK


CASE 2 - CPN compression secondary to popliteal aneurysm Fig. 10


Fig. 10: CPN compression secondary to a popliteal aneurysm at the origin of the anterior tibial artery branch. MRI axial T2FS (a), T1 (b) and coronal STIR (c) of the right knee demonstrate marked oedema and atrophy of the peroneal muscles (WHITE ARROWS) with fatty infiltration. Note the high fatty signal within the muscles affected on the T1W imaging (b). A small popliteal artery aneurysm (RED ARROW) is noted in the course of the popliteal artery at the level of the bifurcation and origin of the anterior tibial artery just medial to the fibular neck. This is causing extrinsic compression of the adjacent deep peroneal nerve branch of the CPN. A vascular opinion is suggested as recommendation in this case.
References: Radiology, Leicester royal infirmary, University Hospitals Leicester - Leicester/UK


CASE 3 - CPN denervation secondary to parameniscal cyst extrinsic compression  Fig. 11

Fig. 11: MRI series of a patient presenting with CPN denervation secondary to a parameniscal cyst. Sagittal PDW MRI (a) demonstrates an undisplaced horizontal oblique tear (WHITE ARROW) of the posterior horn reaching the inferior articular surface of the lateral meniscus. On the coronal T1 (b) and STIR (c) weighted MRI sequences, a poorly marginated parameniscal cyst is seen at the lateral aspect of the joint (WHITE ARROW HEAD) and eroding the lateral tibial condyle (DASHED WHITE ARROW) with intraosseus extension of the cyst (as seen on the sagittal STIR (d)). Axial STIR images at the level of the parameniscal cyst (e) and fibular head (f) demonstrates oedema involving the proximal digitorum longus muscle (RED ARROW) but no significant muscle bulk loss in keeping with early features of denervation. Intramuscular cystic dilatation is also noted. Overall findings are suggestive of intraneural extension of the parameniscal cyst involving a branch of the CPN to EDL.
References: Radiology, Leicester royal infirmary, University Hospitals Leicester - Leicester/UK



CASE 4 - Parameniscal cysts causing popliteal artery entrapment Fig. 12

Fig. 12: Patient presenting with pain and claudication in the left leg. MRI was performed demonstrating popliteal artery entrapment secondary to extrinsic compression from parameniscal cysts. MRI axial STIR images at different levels of the knee. High signal fluid parameniscal cyst (RED ARROWHEAD) noted in the medial compartment (a) and extending into the popliteal fossa posterior compartment (b) of the knee arising from a medial meniscal tear. The posterior compartment paramensical cyst is noted to compress the popliteal artery. Subsequent transverse Doppler US (e-f) demonstrates compression of the popliteal artery at the level (e) and below the level (f) of the parameniscal cyst. The patient subsequently underwent arthroscopy, partial mensicetomy and excision of the parameniscal cyst. The patient reported complete resolution of symptoms.
References: (Image taken from Botchu R, Shah A, Jakanani G, Esler CN, Rennie WJ. Parameniscal Cyst - A Rare Cause of Popliteal Artery Compression: Treatment with Ultrasound-guided Decompression. West Indian Med J. 2015;64(2):172–173. doi:10.7727/wimj.2013.218)
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