|ECR 2015 / C-0654|
|Anatomical peculiarities and common pathologies of distal biceps brachii tendon|
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Findings and procedure details
The biceps brachii muscle consists of two heads, the short head and the long head, which originate from the coracoid process of the scapula and the supraglenoid tubercle, respectively. The two heads fuse with each other inferior to the bicipital groove.
Anatomical peculiarities of distal biceps brachii tendon (Fig. 1):
Some studies noted complete separation of the two muscle bellies along their entire course or some degree of interdigitation of the muscle bellies proximal to the distal tendon but with two separate tendons till the point of insertion on the radial tuberosity3-7.
The tendon of the short head attaches distally and anteriorly whereas the tendon of the long head attaches proximally3-4.
Distal biceps tendon is not invested by a synovial sheath but covered by a paratenon8.
A bursa, called the bicipitoradial bursa, surrounds the distal biceps tendon and decreases friction between the tendon and the radial tuberosity during pronation and supination9.
A thin fibrous structure known as the bicipital aponeurosis or lacertus fibrosus arises from the biceps tendon at the myotendinous juction and extends medially to blend with the fascia of the forearm3,6,7. The lacertus fibrosus when intact, may prevent a ruptured biceps tendon from retracting proximally. Transaxial MR images are useful in evaluation of lacertus fibrosus (Fig. 2).
The distal biceps brachii tendon anatomy and pathologies can be best evaluated by ultrasound and MRI.
With ultrasound, three approaches are possible for visualisation of the tendon: anterior approach with arm extended and forearm supinated and the medial and lateral approaches with elbow in 90 degree flexion and supinated forearm. The oblique course of the tendon causes anisotropy and difficulty in visualisation of distal-most part due to anisotropy with anterior approach. The medial and lateral approaches minimise the anisotropy effect and improve visualisation of the tendon1,10,11.
With MR imaging, transaxial images from the musculotendinous junction to the insertion of the biceps tendon on the radial tuberosity are most useful. Sagittal MR images are useful in some cases8.
Tear: The tendon usually tears at its insertion on the radial tuberosity and less comonly at the myotendinous junction1. The tear can be partial or complete and may involve one or both tendon heads12-14. Acute complete tear is most commonly caused by a strong contraction of the biceps against resistance as in heavy weight lifters. Early diagnosis and surgical repair is important to prevent chronic weakness.
Findings of an acute complete tendon tear include tendon discontinuity with proximal retraction of the tendon stump and hemorrhage or hematoma in the tendon gap (Fig. 3). On ultraound, posterior acoustic shadowing may be seen at the retracted tendon stump. Absence or minimal retraction of the tendon may occur if the lacertus fibrosus remains intact. The status of lacertus fibrosus can be assessed on transaxial MR images.
Partial tear is usually non traumatic. The pathogenesis is not clear8. Imaging features of partial tendon tear include thinning or thickening of the tendon, focal areas of altered echogenicity or increased signal intensity (Fig. 4). On ultrasound, loss of fibrillar pattern of the tendon is seen. An associated distension of the bicipitoradial bursa may be seen.
Tendinosis: tendinosis, defined as intrasubstance degeneration15 may occur in the distal biceps tendon in a relatively hypovascular zone, approximately 10mm from its insertion on the radial tuberosity. This zone is subject to mechanical impingement due to repititive pronation movement and subsequent degeneration which is a predisposing factor for tendon tear16. On ultrasound, tendinosis is seen as thickened hypoechoic tendon. On MRI, the tendon is thickened and shows increased signal intensity. Partial tendon tear may co-exist with tendinosis. Both conditions also have common imaging features hence differentiation between tendinosis and partial tears can often be challenging (Fig. 5).
Calcifying tendinitis: acute calcific tendinitis in distal biceps tendon occurs very rarely17. Fig. 6 is an illustration of acute calcifying tendinitis seen on ultrasound as thickened hypoechoic distal biceps tendon with focal calcification and hyperemia in a 22 year old female patient who presented with acute pain in the cubital fossa without any history of previous trauma or surgery.
Bicipitoradial bursitis: Normally, the bicipitoradial bursa is flattened and not visualised with ultrasound or MRI. Distension of the bursa may occur due to bursitis caused by repetitive trauma or overuse, inflammatory conditions such as rheumatoid arthritis and infection; or in association with partial tendon tear and tendinosis2,18. It is seen as a fluid filled structure aound the distal biceps tendon on ultrasound and MRI (Fig. 7). It could compress the radial nerve and less commonly the median nerve causing neural symptoms.
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