Utilizing the Posterior Deltoid Muscle as a Receptor and Intraoperative Electrophysiological Quality Control in Targeted Muscle Reinnervation for High-Level Upper Extremity Amputees
Targeted muscle reinnervation (TMR) has emerged as a transformative surgical approach to enhance prosthetic control for high-level upper extremity amputees. By redirecting severed peripheral nerves to reinnervate residual muscle segments, TMR enables intuitive myoelectric signal generation for multifunctional prosthetic devices. Despite its potential, challenges persist in ensuring successful nerve transfers, particularly for smaller nerves like the ulnar nerve, and in identifying viable receptor muscles for optimal signal acquisition. This study introduces a novel approach by incorporating the posterior deltoid muscle as an additional receptor and implementing intraoperative electrophysiological assessments to improve surgical precision and outcomes.
Anatomical and Technical Basis for Posterior Deltoid Utilization
The deltoid muscle, innervated by distinct branches of the axillary nerve, is divided into anterior, middle, and posterior segments. While traditional TMR protocols prioritize muscles like the biceps, triceps, or pectoralis major as receptors, the posterior deltoid offers unique advantages. Its anatomical separation from other deltoid segments allows selective reinnervation without compromising shoulder function. Additionally, its posterior location provides a distinct electromyography (EMG) signal source, reducing cross-talk and enhancing prosthetic control specificity.
Intraoperative Electrophysiological Quality Control
A critical innovation in this study is the use of somatosensory evoked potential (SEP) and compound muscle action potential (CMAP) testing to evaluate nerve stump viability and receptor muscle suitability. SEP testing, conducted with scalp electrodes, identified functional proximal nerve stumps by detecting cortical responses to electrical stimulation (5.0 mA, 5 Hz, 100 sweeps). This approach ensured nerve transfers were performed at sites with preserved axonal integrity. CMAP testing (3.0 mA, three averaged trials) further validated the neuromuscular integrity of receptor muscles, enabling precise identification of viable axillary nerve branches for posterior deltoid reinnervation.
Clinical Application in Transhumeral Amputees
Case 1
A 34-year-old male with a right transhumeral amputation underwent TMR via anterior and posterior axillary incisions. Intraoperative SEP confirmed functional median, ulnar, and radial nerve stumps. CMAP testing revealed robust posterior deltoid innervation via the posterior axillary nerve branch. Nerve transfers were performed as follows:
- Median nerve → medial biceps head (motor branch).
- Distal radial nerve → lateral triceps head.
- Ulnar nerve → posterior axillary nerve branch (posterior deltoid).
Case 2
A 45-year-old male with a right transhumeral amputation presented limited receptor options due to atrophy in traditional target muscles. SEP and CMAP evaluations identified intact posterior deltoid innervation. Nerve transfers included:
- Median nerve → clavicular pectoralis major.
- Radial nerve → sternal pectoralis major.
- Ulnar nerve → posterior axillary nerve branch (posterior deltoid).
Postoperative Rehabilitation and Outcomes
Patients engaged in daily phantom limb movement exercises to stimulate reinnervation. At three months, electrophysiological assessments confirmed successful reinnervation:
- SEP: Cortical responses from all transferred nerves.
- CMAP: Evoked potentials in posterior deltoid (1.2–1.8 mV amplitude).
- sEMG: Distinct signal patterns from the posterior deltoid enabled proportional prosthetic control.
Prosthetic integration utilized five bipolar surface EMG electrodes, strategically placed over reinnervated muscles. The posterior deltoid provided a unique control signal for hand opening/closing and finger abduction. Functional improvement was quantified via the Action Research Arm Test (ARAT), with scores increasing from preoperative baselines of 12/57 (Case 1) and 10/57 (Case 2) to 25/57 and 22/57, respectively, at 12 months. Patients demonstrated proficient object manipulation across shapes and sizes, as documented in supplementary video recordings.
Advantages of Posterior Deltoid Integration
- Functional Preservation: Selective reinnervation of the posterior deltoid avoids sacrificing anterior/middle deltoid function, preserving shoulder mobility.
- Signal Specificity: The posterior deltoid’s isolated location reduces EMG interference, enhancing prosthetic control accuracy.
- Expanded Receptor Options: This approach addresses cases where traditional receptors (biceps/triceps) are unsuitable due to atrophy or injury.
Role of Intraoperative Electrophysiology
Traditional nerve assessment relies on visual inspection of fascicle health, which is subjective and prone to error. SEP and CMAP testing provided objective, real-time feedback, ensuring:
- Optimal nerve stump selection for transfer.
- Confirmation of receptor muscle innervation capacity.
- Identification of distinct axillary nerve branches for posterior deltoid targeting.
Clinical Implications and Future Directions
This study highlights the posterior deltoid’s viability as a TMR receptor and underscores the necessity of intraoperative electrophysiological monitoring. Future research should explore:
- Long-term outcomes of posterior deltoid reinnervation in larger cohorts.
- Integration of advanced signal processing algorithms to leverage posterior deltoid EMG patterns.
- Application of this approach in shoulder disarticulation and bilateral amputees.
Conclusion
Incorporating the posterior deltoid into TMR protocols expands prosthetic control options for high-level amputees. Combined with intraoperative SEP/CMAP testing, this method enhances surgical precision, reduces reinnervation failure risks, and improves functional outcomes. As prosthetic technology evolves, optimizing receptor muscle selection and nerve transfer techniques will remain pivotal in restoring upper extremity function.
doi.org/10.1097/CM9.0000000000001261
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