Brachialis Muscle and Tendon Injury

Clinical History: A 35-year-old male presents with pain and swelling in the distal arm and antecubital fossa after an ATV accident. Axial T2-weighted fat-suppressed(1A) and sagittal proton density-weighted fat-suppressed (1B) images are provided. What are your findings? What is your diagnosis?

 

 

Findings

 

 

Diagnosis

High-grade tear of the brachialis muscle with intramuscular hematoma and diffuse muscle and surrounding soft tissue edema.

 

 

Introduction

Brachialis muscle injuries may be associated with biceps brachii injuries or may occur in isolation. Isolated brachialis muscle injuries are rare, and brachialis tendon injuries are even more rare when compared to biceps brachii injuries, which make up the majority of elbow flexor injuries.^1,2  In fact, the first reported case of an isolated brachialis muscle tear was in 2001, and only a few cases have been reported since.³ Because of the rarity of the reported injury, a timely, accurate diagnosis may be missed when it does occur. MRI is very useful for the diagnosis of the type, location, and severity of brachialis muscle injury, as well as differentiating it from other pathologic entities.

 

 

Anatomy

There is a relative paucity of literature describing the anatomy of the brachialis muscle. Within the available literature, there are discrepancies regarding the anatomic makeup of the muscle and its distal insertion, including controversy regarding the number of muscle heads and the type of distal insertion. Recent anatomic studies in the past few decades have helped clarify some of the controversies.^4,5,6,7 It has been established that the brachialis muscle contains two heads: a large superficial head and a smaller deep head. The superficial head originates from the anterolateral aspect of the mid humerus and lateral intermuscular septum, and the deep head originates from the distal third of the anterior aspect of the humerus and medial intermuscular septum.^4,5 The muscle belly of the superficial head is anterior and ulnar to the muscle belly of the deep head.⁴ The superficial head forms a thick circular tendon distally, which inserts onto the distal aspect of the ulnar tuberosity. The deep head muscle fibers converge into a fan-shaped musculotendinous/musculoaponeurotic structure, which attaches more broadly to the coronoid process and proximal aspect of the ulnar tuberosity. The superficial and deep heads attach to the ulnar tuberosity overall as a single contiguous blended structure.^4,5,6 Finally, Leonello et al. described some muscle fibers attaching to the anterior elbow, while Resnick et al. saw no discrete muscle attachments.

A few accessory slips of the brachialis muscle have also been described, and variant median nerve contributions have also been reported.^6,8 The brachialis muscle is innervated by the musculocutaneous nerve, with some inferolateral deep head fibers being innervated by a branch of the radial nerve.⁵ The brachialis vascular supply is from tributaries of the brachial artery and the radial recurrent artery, and its main drainage is the brachial vein.⁸

 

 

Function

The biceps brachii, brachialis, and brachioradialis all contribute to elbow flexion. Because the brachialis lies anterior to the elbow joint and inserts onto the ulna, it only contributes to flexion of the elbow, with no role in supination or pronation.^5,8 It generates the most contractile force between the three muscles.^2,9 The biceps brachii also contributes to supination of the radioulnar joint.⁶ The brachialis has the poorest mechanical advantage of the elbow flexors due to a small lever arm when the elbow is in full extension as the line of the ulna, humerus, and brachialis fibers are parallel during extension. It has its strongest position when the elbow is in 90-degree flexion as the brachialis fibers are perpendicular to the ulna at this position.⁵ Leonello et al. postulate that the noticeable morphological differences between the superficial and deep heads suggest they have distinct functions. As the deep fibers insert more anterior to the axis of rotation of the ulnohumeral joint due to partly inserting onto the coronoid process, the deep head is thought to play a more important role in the initiation of flexion from full extension, and the superficial head is thought to generate more power once the elbow is flexed due to an increase in mechanical advantage at this position. ^2,5 Leonello et al. also postulate that since the inferolateral deep head fibers of the brachialis and the anconeus are both innervated by the radial nerve and travel in a similar direction parallel to the lateral ulnar collateral ligament, they likely provide dynamic restraint to rotatory instability. Kulig also showed that the brachialis is responsible for maintaining stability of the elbow throughout concentric and eccentric contraction and is preferentially recruited over the biceps brachii when undergoing eccentric contraction.¹⁰

 

 

Normal MRI Appearance

Axial images are very helpful in identifying the brachialis musculoaponeurotic and tendon insertions. Sagittal images are helpful in delineating the fan-shaped muscle fibers inserted onto the ulna, and sagittal as well as coronal images help identify the tendinous portion.⁴ The superficial head insertion demonstrates a typical tendinous hypointense linear signal, and the deep head insertion demonstrates a striated muscle pattern (Figure 4).

 

 

Clinical Presentation and Pathophysiology

Acute brachialis injuries are postulated to occur with eccentric contraction during pronation and resistance of hyperextension.^1,11,12 They have been described in hockey, rugby, football, and lifting of heavy objects.^{3,13,11,14,15,16,17,2,1,12} Most, if not all, reported cases in the literature describe muscle tears as opposed to insertional tears, although in our practice, we have seen insertional tears. On physical exam, there is pain and swelling anteriorly and proximal to the elbow without discernible bruising or neurologic deficit. There is also pain with flexion and extension, including inability to fully extend the elbow. There is no finger motion restriction, and there is usually normal range of motion in pronation and supination. A palpable mass may be felt. In general, it is difficult to palpate the brachialis due to its deep location. Of note, a biceps brachii tendon gap is not present on physical exam, which can be a helpful clue.^1,11,14 More chronically, repetitive pull-ups, especially in climbers, which involve the use of arms predominantly in the pronated semi-flexed position, can result in strain and tendinopathy of the brachialis, called “climber’s elbow.” ¹⁸

 

 

MRI Findings

MRI is an excellent tool for the characterization of brachialis injuries, including location and severity, and for simultaneously evaluating the remainder of the elbow for additional or alternative pathology. Acute muscle tears are easily seen on fluid-sensitive images, demonstrating discontinuity of the muscle fibers with architectural distortion, intervening fluid and/or hemorrhagic signal collection, muscle edema, and surrounding soft tissue edema, with or without evidence of insertional signal abnormality. Lower-grade muscle strains will demonstrate feathery edema signal in the muscle belly on fluid-sensitive sequences without muscle fiber disruption (Figure 5).

Insertional tendinopathy will usually demonstrate intermediate signal on all sequences, with possible thickening of the tendon (Figure 6). Insertional partial tears will usually demonstrate higher signal and focal discontinuity of the insertional fibers with likely fluid within the tear gap (Figure 7).  

 

Differential Considerations

Distal biceps tendon rupture: Distal biceps tendon injuries are much more common than brachialis injuries. Distal biceps tendon injuries most commonly occur in the dominant arm of males over the age of 40 or in younger athletes sustaining a heavy load on the distal biceps during flexion.^19,20 Distal biceps tendon ruptures account for 3-10% of biceps injuries. Distal biceps tendon ruptures are readily demonstrated by MRI (Figure 8).

Bicipitoradial bursitis: Bicipitoradial bursitis presents as a mass at the antecubital fossa, sometimes with pain or limited range of motion.²¹ Patients can also present with neurologic symptoms due to irritation or mass effect on the adjacent radial nerve or sometimes the median nerve.²¹ Bicipitoradial bursitis most commonly occurs due to mechanical friction and is often associated with distal biceps pathology. It can also occur due to inflammatory arthropathies such as rheumatoid arthritis, tumoral calcinosis, synovial osteochondromatosis, and infectious etiologies.²¹ MRI can clearly demonstrate bicipitoradial bursitis (Figure 9). Other muscle injuries: Brachioradialis and pronator teres injuries are also readily diagnosed with MRI (Figures 10 and 11).    

Treatment

There is no standard treatment protocol. Both surgical and non-surgical treatments have been described, but most brachialis injuries have been treated conservatively in the literature, with report of one high-level athlete undergoing surgical repair to facilitate early return to sport.¹⁴ Conservative therapy in acute injuries involves cast immobilization with flexion to bring the torn brachialis muscle fibers in close approximation for 1-3 weeks, followed by progressive range of motion exercises to regain function and range of motion.^1,14,15

 

 

Conclusion

The diagnosis of brachialis muscle or insertional tear may be difficult clinically due to its rare nature and deep location in the anterior elbow, precluding a thorough physical exam. MRI allows evaluation of the brachialis muscle and its insertion, can assist in differentiation of muscle belly or insertional tears, and exclude pathology in the adjacent structures.

 

 

References