Computer simulation of nerve transfer strategies for restoring shoulder function after adult C5 and C6 root avulsion injuries

Abstract

Purpose: Functional ability after nerve transfer for upper brachial plexus injuries relies on both the function and magnitude of force recovery of targeted muscles. Following nerve transfers targeting either the axillary nerve, suprascapular nerve, or both, it is unclear whether functional ability is restored in the face of limited muscle force recovery.

Methods: We used a computer model to simulate flexing the elbow while maintaining a functional shoulder posture for 3 nerve transfer scenarios. We assessed the minimum restored force capacity necessary to perform the task, the associated compensations by neighboring muscles, and the effect of altered muscle coordination on movement effort.

Results: The minimum force restored by the axillary, suprascapular, and combined nerve transfers that was required for the model to simulate the desired movement was 25%, 40%, and 15% of the unimpaired muscle force capacity, respectively. When the deltoid was paralyzed, the infraspinatus and subscapularis muscles generated higher shoulder abduction moments to compensate for deltoid weakness. For all scenarios, movement effort increased as restored force capacity decreased.

Conclusions: Combined axillary and suprascapular nerve transfer required the least restored force capacity to perform the desired elbow flexion task, whereas single suprascapular nerve transfer required the most restored force capacity to perform the same task. Although compensation mechanisms allowed all scenarios to perform the desired movement despite weakened shoulder muscles, compensation increased movement effort. Dynamic simulations allowed independent evaluation of the effect of restored force capacity on functional outcome in a way that is not possible experimentally.

Clinical relevance: Simultaneous nerve transfer to suprascapular and axillary nerves yields the best simulated biomechanical outcome for lower magnitudes of muscle force recovery in this computer model. Axillary nerve transfer performs nearly as well as the combined transfer, whereas suprascapular nerve transfer is more sensitive to the magnitude of reinnervation and is therefore avoided.

Figure 1. The upper extremity model represented five degrees of freedom at the shoulder and elbow: shoulder abduction, shoulder rotation, shoulder flexion/extension, elbow flexion, and forearm pronation/supination (not shown)

The model is shown with the limb in the initial (left and upper right) and final (lower right) postures of the movement.

Figure 2. Shoulder abduction moment contributions by deltoid, infraspinatus, subscapularis, and supraspinatus at the point in the motion when the net shoulder joint moments were maximal

The moments are shown as a percent of the net shoulder abduction moment and can exceed 100% due to the simultaneous action of antagonist muscles. In the unimpaired, axillary, and combined scenarios, the deltoid was the primary contributor to net shoulder abduction moment. When the deltoid remained paralyzed in the suprascapular scenario, the infraspinatus, subscapularis, and supraspinatus muscles were the primary contributors to net shoulder abduction moment.

Figure 3. Shoulder rotation moment contributions by deltoid, infraspinatus, subscapularis, and supraspinatus at the point in the motion when the net shoulder joint moments were maximal

Shoulder rotation moments are shown as a percent of net shoulder rotation moment, respectively. Positive values indicate an internal rotation moment, while negative values indicate an external rotation moment.

Figure 4. Relationship between restored force capacity and movement effort

The movement effort was expressed as the ratio of the effort required by the nerve transfer scenario to perform the desired movement to the effort required by the unimpaired scenario to perform the desired movement. The suprascapular scenario exhibited the highest movement effort, while the combined scenario exhibited the lowest movement effort. Movement effort decreased as restored force capacity increased.