American Society for Peripheral Nerve

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Do Distal AIN to Ulnar Nerve Transfers Affect Forearm Prosupination Strength?
Emily C Liu, MD, FRCSC1; Douglas C Ross, MD MEd FRCSC2; Christopher Doherty, MD, FRCSC1; TA Miller, MD1
1Western University, London, ON, Canada, 2The Peripheral Nerve Clinic, London, ON, Canada

Introduction: Distal AIN-to-ulnar motor end-to-side (ETS) nerve transfers are being completed with increasing frequency as indications have grown to include proximal ulnar nerve injuries as well as severe (McGowan 3) compression neuropathies. Although the effect of denervating pronator quadratus (PQ) on forearm prosupination is thought to be minimal, there is no literature to support this assumption.

Purpose: The purpose of this study is two-fold: 1. Establish normative data for forearm pronation and supination strength as measured by peak torque. 2. Measure any loss of forearm pronation and supination strength after denervation of the PQ muscle after AIN-to-ulnar ETS transfer.

Methods: Isokinetic testing was performed using a Biodex® kinematic dyanamometer on 20 healthy volunteers to determine peak torque values of supination and pronation in the dominant and non-dominant (ND) extremities. Four measurements were taken with the elbow flexed to 90° as well as in full extension with the average utilized for subsequent calculations. Ratios of ND to dominant extremities were determined. Twenty patients who had undergone unilateral AIN-to-ulnar ETS nerve transfer were similarly tested and the ratio of ND to dominant peak torque values were determined and compared to normative values using ANOVA.

Results: Peak pronation torque values for males in the normative group were 11.3 N-m with the elbow at 90° and 9.4 N-m with the elbow fully extended (p<0.001). Similar values for women were 4.6 and 4.4 respectively (p=0.32). Peak supination torque values for men were 9.3 and 7.0 (p<0.001) for men and 3.9 and 3.5 for women (p=0.14). Torque values were highly correlated in all subjects between dominant and ND extremities (Pearson correlation coefficient 0.9). The ratio of ND to dominant pronation peak torque was 1.05 with the elbow flexed and 0.90 with the elbow extended. In the post-operative group, the ratio of pronation peak torque between operated and non-operated sides showed a trend (p=0.08 with elbow flexed, 0.06 with elbow extended) towards an increase (consistent with a decrease in pronation strength). Unexpectedly, the peak torque ratios for supination were more affected (p=0.06 with elbow flexed; p=0.02 with elbow extended).

Conclusions: There appears to be a modest decrease in pronation strength after AIN-to-ulnar ETS transfer which demonstrates a trend towards-but does not achieve-statistical significance in this cohort. There is a statistically significant decrease in supination strength after AIN transfer which may reflect the role of PQ as a DRUJ stabilizer during forearm rotation.

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