Ultrasound appearance of regenerative peripheral nerve interface with clinical correlation

Objective To describe the ultrasound (US) appearance of regenerative peripheral nerve interfaces (RPNIs) in humans, and correlate clinically and with histologic findings from rat RPNI. Materials and methods Patients (≥ 18 years) who had undergone RPNI surgery within our institution between the dates of 3/2018 and 9/2019 were reviewed. A total of 21 patients (15 male, 6 female, age 21–82 years) with technically adequate US studies of RPNIs were reviewed. Clinical notes were reviewed for the presence of persistent pain after RPNI surgery. Histologic specimens of RPNIs in a rat model from prior studies were compared with the US findings noted in this study. Results There was a variable appearance to the RPNIs including focal changes involving the distal nerve, nerve-muscle graft junction, and area of the distal sutures. The muscle grafts varied in thickness with accompanying variable echogenic changes. No interval change was noted on follow-up US studies. Diffuse hypoechoic swelling with loss of the fascicular structure of the nerve within the RPNI and focal hypoechoic changes at the nerve-muscle graft junction were associated with clinical outcomes. US findings corresponded to histologic findings in the rat RPNI. Conclusion Ultrasound imaging can demonstrate various morphologic changes involving the nerve, muscle, and interface between these two biological components of RPNIs. These changes correspond to expected degenerative and regenerative processes following nerve resection and muscle reinnervation and should not be misconstrued as pathologic in all cases. N5 and N1 morphologic type changes of the RPNI were found to be associated with symptoms.