Posterior Interosseous Nerve (pin) Injury

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Posterior Interosseous Nerve (pin) Injury

Authors, Editors, Case Information Author: Milan Oxspring, MD Affiliation: Mayo Clinic, Department of Physical Medicine and Rehabilitation Co Author(s): James B. Meiling, DO Senior Editor: Laura Mattson, DO Clinical Vignette: A 22-year-old right-handed male presented with persistent left hand weakness and dorsal proximal forearm pain 8 months after an open reduction internal fixation (ORIF) surgery for a radius and ulna fracture from a motorcycle collision. Exam was notable for significant weakness with finger extension and thumb abduction without sensory loss. Type of Probe Used: 4-18 MHz linear array ultrasound transducer. Unlabeled Long Axis View of the PIN within the Radial Tunnel Unlabeled long axis view of the left posterior interosseous nerve (PIN) within the radial tunnel. Labeled Long Axis View of the PIN within the Radial Tunnel Labeled long axis view of the left posterior interosseous nerve (PIN) within the radial tunnel. There is multifocal enlargement of the PIN (dashed line) which is hypoechoic with a complete loss of normal intraneural fascicular architecture. The architecture of the nerve appears as more of a “dumbbell” shape within the tunnel. These findings may be representative of two focal neuromas-in-continuity. Deep to the nerve, the proximal radius plate can be seen from the prior ORIF. The superficial and deep heads of the supinator muscle are both markedly hyperechoic which is likely suggestive of muscle denervation. Unlabeled Short Axis View of the PIN within the Radial Tunnel Unlabeled short axis view of the left posterior interosseous nerve (PIN) within the radial tunnel. Labeled Short Axis View of the PIN within the Radial Tunnel Labeled short axis view of the left posterior interosseous nerve (PIN) within the radial tunnel. In this view, the PIN (dashed oval) becomes focally hypoechoic and enlarged (approximate area 25.8 mm^2). It is seen situated between hyperechoic superficial and deep heads of the supinator muscle, likely secondary to muscle denervation. As seen in the above view, the PIN sits superficial to the underlying radius plate from the prior ORIF for the proximal radius fracture. #5 - Video View Video Short Axis Cine of the left posterior interosseous nerve from proximal to distal. As the nerve is scanned distally, multifocal enlargement of a hypoechoic PIN is visualized. Given the near-normal caliber nerve in between the two areas of enlargement, this may indicate the presence of two focal neuromas-in-continuity. Deep to the PIN is the proximal radius plate from the prior ORIF. Interpretation Image Interpretation: Normal radial nerve at the antecubital fossa, physiologic branching into superficial and deep radial branches, and then multifocal, markedly hypoechoic focal enlargement of the Posterior Interosseous Nerve (PIN) within the radial tunnel with an area of approximately 25.8mm^2. The two enlarged foci are immediately superficial to the proximal radius ORIF plate, with a short intervening segment of near normal caliber nerve giving a "dumbbell" appearance. Findings correlate with posterior interosseous neuropathy/neuroma in continuity in the setting of prior trauma and hardware proximity. This is consistent with prior EMG showing denervation in PIN innervated muscles and preserved sensation. Teaching Pearl: PIN syndrome is suggested by a sonographic triad of nerve enlargement at or near the arcade of Frohse, hypoechoic echotexture or loss of fascicular architecture, and denervation changes in PIN-innervated muscles. The scan should trace the radial nerve through its bifurcation and follow the deep branch through the radial tunnel. Because PIN syndrome is motor-predominant, ultrasound findings should match the clinical pattern of finger and thumb extension weakness and electrodiagnostic results when available. Side-to-side comparison is useful because normal PIN size varies and absolute thresholds are less established than for carpal tunnel syndrome. References: 1. Kim Y, Ha DH, Lee SM. Ultrasonographic findings of posterior interosseous nerve syndrome. Ultrasonography. 2017;36(4):363-369. doi:10.14366/usg.17007 2. Dietz AR, Bucelli RC, Pestronk A, Zaidman CM. Nerve ultrasound identifies abnormalities in the posterior interosseous nerve in patients with proximal radial neuropathies. Muscle Nerve. 2016;53(3):379-383. doi:10.1002/mus.24778 3. Wininger YD, Buckalew NA, Kaufmann RA, Munin MC. Ultrasound combined with electrodiagnosis improves lesion localization and outcome in posterior interosseous neuropathy. Muscle Nerve. 2015;52(6):1117-1121. doi:10.1002/mus.24782
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Authors, Editors, Case Information

Unlabeled Long Axis View of the PIN within the Radial Tunnel

Labeled Long Axis View of the PIN within the Radial Tunnel

Unlabeled Short Axis View of the PIN within the Radial Tunnel

Labeled Short Axis View of the PIN within the Radial Tunnel

#5 - Video

Interpretation

UPCOMING EVENTS

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