American Association for Hand Surgery

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AIN Palsy in Both Bone Forearm Fracture: A Case Based and Cadaveric Evaluation
Nicholas Munaretto, MD1, Chelsea C Boe, MD1, Nicholas Pulos, M.D.1 and Marco Rizzo, MD2, (1)Mayo Clinic, Rochester, MN, (2)Orthopedics, Mayo Clinic, Rochester, MN

Introduction. Complete palsy affecting the anterior interosseous nerve (AIN) is a rare complication of both bone forearm fracture (BBFF) fixation. The goal of this study was to identify cases of this complication and identify anatomic branching patterns to ascertain any identifiable risk factors and provide insight about management and expected outcome.

Materials and Methods. A retrospective review was performed, identifying BBFFs in patients from 2003-2018 at a level I trauma center. Only patients who were treated with plate and screw osteosynthesis were included. Demographic and injury data, surgical approach and documented post-operative nerve deficits were collected.

Additionally, we performed cadaveric dissections of 10 specimens to evaluate the anatomy of the AIN. Specifically, we studied the variability in branching pattern and distance from palpable elbow landmarks to the AIN takeoff, branch to the flexor policis longus (FPL) and branch to the index flexor digitorum profundus (FDP).

Results. : 94 patients met inclusion criteria. Five patients were identified with post-operative nerve deficits affecting the AIN. Average age was 34 (16-66). Two patients had documented deficits preoperatively, and 3 were noted in the immediate post-operative period. Four patients underwent fixation through a volar approach and one patient through a dorsal approach. No complications were noted at the time of surgery. All palsies were managed with observation. There was complete recovery in all cases at an average of 50 days (6-105).

In our cadaveric dissections, the distance from the lateral epicondyle to the take-off of the AIN was never less than 4 cm (4-7). The branch to the FDP took off at the same level or before the branch to the FPL, though there was great variability in the distance of branching from the take-off of the AIN with 30% of cases branching at the same level or within 1.5 cm. Gantzer's muscle was identified in 30% our specimens, though this did not seem to have any effect on the branching pattern of the AIN.

Conclusion. AIN palsy is a rare complication of BBFF. This appears to be a transient neuropraxia with predictable recovery. The etiology remains unclear, however the variability in AIN branching noted in our dissection suggests that in certain specimens the nerve to the index FDP and FPL may branch at the same level as the AIN, potentially creating an anatomic susceptibility to injury with proximal both bone fractures, fracture hematoma or excessive traction during the surgical approach for fixation.

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