Entrapment Neuropathies

The ulnar nerve is well seen on axial MR images as it passes through the cubital tunnel.61,62 Anatomic variations of the cubital tunnel retinaculum may contribute to ulnar neuropathy.63 These variations in the cubital tunnel retinaculum and the appearance of the ulnar nerve itself can be identified with MRI. The retinaculum may be thickened in 22% of the population, resulting in dynamic compression of the ulnar nerve during elbow flexion. In 11% of the population, the cubital tunnel retinaculum may be replaced by an anomalous muscle, the anconeus epitrochlearis, resulting in static compression of the ul-nar nerve (Fig. 4.14).63 The cubital tunnel retinaculum may be absent in 10% of the population, allowing anterior dislocation of the ulnar nerve over the medial epi-condyle during flexion with subsequent friction neuritis

FIGURE 4.13. Rupture of the biceps tendon and bicipital aponeurosis (lacertus fibrosus). (A) T2-weighted sagittal image reveals the retracted biceps tendon (arrow) within the antecubital fossa. (B) There is increased signal and poor definition of the lacertus fibrosus (small black arrows) on this T2-weighted axial image. There is increased signal at the expected attachment site of the distal biceps tendon to the radial tuberosity (open arrows). Hypertrophy of the radial tuberosity is seen contributing to narrowing of the space between the ulna and radius where the torn distal biceps tendon was previously impinged. (C, capitel-lum, R, radius)

FIGURE 4.13. Rupture of the biceps tendon and bicipital aponeurosis (lacertus fibrosus). (A) T2-weighted sagittal image reveals the retracted biceps tendon (arrow) within the antecubital fossa. (B) There is increased signal and poor definition of the lacertus fibrosus (small black arrows) on this T2-weighted axial image. There is increased signal at the expected attachment site of the distal biceps tendon to the radial tuberosity (open arrows). Hypertrophy of the radial tuberosity is seen contributing to narrowing of the space between the ulna and radius where the torn distal biceps tendon was previously impinged. (C, capitel-lum, R, radius)

(Fig. 4.15).64 This subluxation of the ulnar nerve, as well as subluxation of the medial head of the triceps in cases of snapping elbow syndrome, may be depicted with MR images of the elbow in flexion.65

MRI signs of ulnar neuritis and entrapment include displacement and flattening of the nerve adjacent to a mass, swelling and enlargement of the nerve proximal or distal to a mass, infiltration of the perineural fat, and increased signal intensity within the nerve on T2-weighted images.62,66 Peripheral nerves are normally intermediate in signal intensity on T2-weighted images. The ulnar nerve must be followed carefully to avoid mistaking it for enlargement of the adjacent veins. The posterior ul-nar recurrent artery and the deep veins that accompany it are normally small structures that course with the ulnar nerve through the cubital tunnel. Enlargement of a deep vein may appear as a bright tubular structure on T2-weighted, gradient-echo, or STIR sequences and may mimic an edematous ulnar nerve.66

Entrapment of the median nerve and radial nerve also may be evaluated with MRI. Median nerve entrapment may be due to a variety of uncommon anatomic variations about the elbow including the presence of a supra-

condyloid process with a ligament of Struthers, anomalous muscles, an accessory bicipital aponeurosis, and hypertrophy of the ulnar head of the pronator teres.67 These anatomic variants and abnormal mass lesions, such as an enlarged radial bicipital bursa, may entrap the median nerve and may be identified with MRI. Radial nerve entrapment may occur due to thickening of the arcade of Frohse along the proximal edge of the supinator muscle. Ganglion cysts may arise from the anterior margin of the elbow joint and compress the radial nerve.68

MRI may be complementary to electromyography and nerve conduction studies in cases of nerve entrapment about the elbow.61 In subacute denervation, the affected muscles have prolongation of T1 and T2 relaxation times secondary to muscle fiber shrinkage and associated increases in extracellular water.69 Entrapment of a nerve about the elbow may therefore cause increased signal within the muscles innervated by that nerve on T2-weighted or STIR images. These changes may be followed to resolution or progressive atrophy and fatty infiltra-tion.70,71 Moreover, the site and cause of entrapment may be discovered with MRI by following the nerve implicated from the distribution of abnormal muscles on MRI.72

FIGURE 4.14. Anconeus epitrochlearis muscle replacing the cubital tunnel retinaculum. A T2-weighted axial image reveals the ulnar nerve (white arrow) deep to an anomalous anconeus epitrochlearis muscle (black arrow) and superficial to the posterior bundle of the medial collateral ligament (curved arrow).

FIGURE 4.14. Anconeus epitrochlearis muscle replacing the cubital tunnel retinaculum. A T2-weighted axial image reveals the ulnar nerve (white arrow) deep to an anomalous anconeus epitrochlearis muscle (black arrow) and superficial to the posterior bundle of the medial collateral ligament (curved arrow).

FIGURE 4.15. Ulnar neuritis in a patient with questionable ulnar nerve subluxation on physical examination. A T1-weighted axial image reveals prominent enlargement and medial subluxation of the ulnar nerve (arrow). The overlying cubital tunnel retinaculum is developmental^ absent, allowing anterior dislocation of the ulnar nerve during elbow flexion with subsequent friction neuritis. The nerve was increased in signal intensity on the T2-weighted images (not shown).

FIGURE 4.15. Ulnar neuritis in a patient with questionable ulnar nerve subluxation on physical examination. A T1-weighted axial image reveals prominent enlargement and medial subluxation of the ulnar nerve (arrow). The overlying cubital tunnel retinaculum is developmental^ absent, allowing anterior dislocation of the ulnar nerve during elbow flexion with subsequent friction neuritis. The nerve was increased in signal intensity on the T2-weighted images (not shown).

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