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December 2009 Case of the Month
Compiled by: Robert Cargile, M.D.
History: 18-year-old female with pain radiating from her elbow into her 5th digit for past three years. Presumptive clinical diagnosis of cubital tunnel syndrome versus medial epicondylitis.
Exam: MRI of the right right elbow was performed. T1 and proton density fat-suppressed sequences were performed in multiple planes without gadolinium. Arrows denote lesion.
Findings: Orthogonal short axis T1 (Fig. 1), coronal T1 (Fig. 2), and sagittal PD fat-suppressed (Fig. 3) images demonstrate an accessory muscle in lieu of the normal cubital tunnel retinaculum.
Images:
Diagnosis: Anconeusepitrochlearis
Discussion: The anconeus epitrochlearis muscle takes the same course as the cubital tunnel retinaculum, running from the medial cortex of the olecranon to the inferior surface of the medial epicondyle. Care must be taken to distinguish the anconeus epitrochlearis from the ulnar head of the flexor carpi ulnaris muscle, which occurs more distally and merges with the humeral head of that muscle (1).
The cubital tunnel retinaculum (Osborne’s fascia) is confluent with the deep investing fascia of the flexor carpi ulnaris and is postulated to be the phylogenetic remnant of the anconeus epitrochlearis (2). The latter has a reported prevalence of 11 percent (3). In fact, Dellon’s study of 104 elbows in 64 cadavers demonstrated presence of an Osborne's fascia in 77 percent, of some degree of ulnar subluxation in 25 percent, the ulnar nerve beneath the medial head of the triceps in 24 percent, the medial head of the triceps within the floor of the cubital tunnel in 28 percent, the presence of a rudimentary supracondylar process in 1.5 percent, and a high origin of the pronator teres in 17 percent of the cadavers. There was a significant association between the presence of an anconeus epitrochlearis muscle and the ulnar nerve being completely covered by the medial head of the triceps muscle.
The more recent MRI based work of Husarik et al. (4) involving 120 elbows of 60 asymptomatic volunteers revealed an anconeus epitrochlearis prevalence of 23 percent. Perhaps more interesting is the prevalence of increased signal intensity in 60 percent of ulnar nerves on fluid-sensitive sequences. While not demonstrated in the radial or median nerves, this substantiates the predisposition of the ulnar nerve to subclinical injury in the cubital tunnel.
Images from a different patient are provided to better illustrate the relationship of the cubital tunnel and ulnar collateral ligament. This axial T1 arthrogram with microscopy coil of the proximal (Fig. 4) and mid (Fig. 5) cubital tunnel clearly demonstrates the posterior bundle of the ulnar collateral ligament functions as the floor of the cubital tunnel, sandwiching the ulnar nerve between itself and Osborne’s fascia.By contrast, the more commonly discussed anterior band of the ulnar collateral ligament runs parallel to the course of the cubital tunnel and inserts on the sublime tubercle of the ulna (Fig. 6; Long axis T1 with microscopy coil), disavowing itself of any role in ulnar nerve compression.
Treatment: Masear et al. recommend simple excision of the anomalous anconeus epitrochlearis muscle when no other cause of ulnar nerve neuropathy at the elbow is identified. Anterior transposition of the nerve is unnecessary (5).
References:
1. Jeon IH, Fairbairn KJ, Neumann L, Wallace WA. MR imaging of edematous anconeus epitrochlearis: another cause of medial elbow pain? Skeletal Radiol 2005;34:103 107.
2. O’Driscoll SW, Horii E, Carmichael SW, Morrey BF. The Cubital Tunnel and Ulnar Neuropathy. J Bone Joint Surg Br. 1991 Jul;73(4):613-7.
3. Dellon AL. Musculotendinous variations about the medial humeral epicondyle. J Hand Surg [Br] 1986;11:175–181.
4. Husarik DB, Saupe N, Pfirrmann CW, Jost B, Hodler J, Zanetti M. Elbow nerves: MR findings in 60 asymptomatic subjects--normal anatomy, variants, and pitfalls. Radiology. 2009 Jul;252(1):148-56. Epub 2009 May 18.
5. Masear VR, Hill JJ Jr, Cohen SM. Ulnar compression neuropathy secondary to the anconeus epitrochlearis muscle. J Hand Surg [Am] 1988;13: 720–724.
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