Issue: February 2021
Source: Cheesborough JE, et al. Hand (N Y). 2014;doi:10.1007/s11552-014-9602-5. Kuiken TA, et al. Tech Orthop. 2017;doi:10.1097/BTO.0000000000000194.
Disclosures: Gaston and Loeffler report the Reconstructive Center for Lost Limbs visiting surgeon program receives educational and financial support from Hanger Clinic. Niedermeier reports no relevant financial disclosures.
February 17, 2021
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Targeted muscle reinnervation, regenerative nerve techniques aid transradial amputation

Issue: February 2021
Source: Cheesborough JE, et al. Hand (N Y). 2014;doi:10.1007/s11552-014-9602-5. Kuiken TA, et al. Tech Orthop. 2017;doi:10.1097/BTO.0000000000000194.
Disclosures: Gaston and Loeffler report the Reconstructive Center for Lost Limbs visiting surgeon program receives educational and financial support from Hanger Clinic. Niedermeier reports no relevant financial disclosures.
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Aside from the hand, the most common level of amputation in the upper extremity is the transradial amputation; and these injuries typically result from major trauma.

Patients with transradial amputations have higher prosthetic utilization rates than patients who undergo upper extremity amputation at other levels. Unfortunately, patients with transradial amputations also have the highest rates of phantom limb pain and neuropathic pain compared with patients who underwent upper extremity amputation at other levels. Targeted muscle reinnervation (TMR) is a surgical technique whereby amputated nerves that no longer have targets are transferred to motor nerves that innervate remaining muscles. Studies have shown this technique can be used to decrease neuroma formation and phantom/neuropathic pain. Surgical options for TMR are determined by the level of amputation and presence of apposite muscles. For example, patients who underwent transhumeral amputation can impart prosthesis control via TMR using median, distal radial and ulnar nerves for “hand close,” “hand open” and possibly “wrist control,” respectively. This is done in addition to using the native, non-TMR innervation of the biceps and triceps for elbow flexion and extension signals, respectively.

Emily N. Morgan, MD, and colleagues have one of the first described surgical techniques for TMR in the setting of transradial amputation. They transferred the median nerve to a motor branch to the flexor digitorum superficialis and the ulnar nerve to a motor branch of the flexor carpi ulnaris (FCU). This technique often requires two separate approaches: one for the transradial amputation and then a more proximal, volar incision to perform TMR. We have found the reinnervated FCU to be a common source of ongoing pain given the direct pressure applied to this area by the prosthesis. Given current prosthetic capabilities, presently the only additional functional benefit from forearm-level TMR is thumb opposition from the low median nerve transfer. Therefore, TMR for transradial amputation currently is primarily for neuroma control and potentially for future prosthetic control as myoelectric technology continues to develop.

Alternate technique

More recently, the technique of creating a regenerative peripheral nerve interface (RPNI) has been described, offering a simple and effective alternative technique for reducing neuroma and phantom pain. This technique involves implantation of peripheral nerves into small, devascularized muscle grafts that serve as targets for the nerve ingrowth.

We describe a surgical technique for transradial amputation-level TMR and RPNI in patients with preoperative limbs that possess a viable pronator quadratus (PQ). This technique adds the benefit of using a novel, single-incision approach while performing TMR with a pedicled PQ that can be used for myoelectric thumb opposition and RPNI to prevent neuroma formation and neuropathic pain. It also spares the proximal forearm of additional surgery, which improves prosthetic comfort in this region. The reduced dissection also allows soft tissue volume to reach equilibrium faster, which results in earlier prosthetic wear in our experience.

Surgical technique

The patient is placed supine on the operating table with an upper arm tourniquet. Proximal regional anesthesia is not contraindicated and can be performed preoperatively without interfering with intraoperative stimulation of recipient motor nerves.

A fish-mouth incision is preferred for raising long volar and dorsal flaps (Figure 1); however, this can be modified as needed to accommodate the soft tissue status of the operative limb. The goal is to obtain a stable residual limb with substantial soft tissue coverage to allow for prosthetic fitting and avoid wound complications. After sharply incising through the skin, the volar and dorsal fascia is incised, and the tendons of the volar and dorsal compartments are transected approximately 7 cm to 8 cm proximal to the radiocarpal joint (Figure 2). The radial and ulnar arteries are identified, bluntly dissected and suture ligated at a level proximal to that of the incision. The median, ulnar and superficial radial nerves are dissected, but are transected distally to maximize subsequent length for transfer (Figure 3). The medial and lateral antebrachial cutaneous nerves have typically innervated the skin already at this level and do not require management.

1. The fish-mouth incision is shown.
Source: Steven R. Niedermeier, MD

Pronator quadratus is pedicled

The PQ is carefully identified and exposed to visualize its attachments to the volar surface of the distal radius and ulna. Carefully, the PQ is sharply elevated off of the radius and ulna (Figure 4, page 4) in a distal to proximal direction being sure not to compromise its innervation and blood supply via the anterior interosseous nerve (AIN) and artery (AIA) (Figure 5, page 4). This pedicle is moistened with normal saline and protected.

The pedicled PQ and its neurovascular pedicle are protected while the volar and dorsal compartment musculature are divided at the planned level of bone resection. The residual muscle is elevated 1 cm to 2 cm proximal to the planned amputation level, and the radius and ulna are then osteotomized with an oscillating saw approximately 7 cm to 8 cm from the radiocarpal joint. This leaves the patient with a limb that is roughly two-thirds its natural length, which helps facilitate prosthetic wear by leaving sufficient room distally for the patient’s prosthetic componentry.

2. The level of tenotomy is shown. 3. Identification, dissection and transection of the median, ulnar and superficial radial nerves are shown. 4. The PQ (white asterisk) is shown being elevated off of the volar surface of the radius (red arrow). 5. The AIN (white arrow) and the AIA (black arrow) to the PQ are shown.

Nerves enclosed in muscle

Muscle grafts are next selected for RPNI from the residual volar or dorsal musculature. A graft size of 3 cm x 1 cm x 1.5 cm has been shown to maximize the area for nerve ingrowth and permit graft survival (Figure 6). The superficial radial nerve end is freshened to healthy fascicles with an 11-blade scalpel on a sterile tongue depressor or with a nerve cutting device. The nerve is then sutured to the center of the muscle graft using 8-0 monofilament epineural-to-muscle sutures (Figure 7); then, two 6-0 monofilament sutures are placed for support from the edge of the muscle graft to the adjacent epineurium. Finally, the muscle is wrapped around the nerve end using 6-0 monofilament epimysial-to-muscle sutures to enclose the nerve in muscle. This process is repeated for the ulnar nerve which is typically separate into two fascicular groups for two distinct RPNI grafts (Figure 8). These RPNIs are then buried proximally in the residual limb.

Reinforced coaptation

The median nerve end is freshened using an 11-blade scalpel. The AIN and AIA are carefully separated from one another maintaining the vascular supply to the PQ. The AIN is dissected and sharply transected proximal to its innervation of the PQ. The median nerve is of larger caliber than the AIN; so, the median nerve is sutured to the terminal AIN using 8-0 monofilament suture placed in the middle of the AIN and is then loosely brought into the center of the median nerve. The coaptation is reinforced with 6-0 monofilament epineural-to-epimysial stitches to support the target nerve similar to what was done for RPNI (Figure 9). This technique ensures the entire cross-sectional area of the transected donor nerve is coapted into recipient nerve or muscle. The pedicled TMR is then mobilized proximally within the interosseous space or over the distal end of the radius/ulna.

6. Resection of a muscle graft that is 3 cm x 1.5 cm for RPNI is shown. 7. Epineural-to-epimysial RPNI of the superficial radial nerve to a free muscle graft is shown. 8. Final RPNI of the split ulnar nerve after the muscle graft is wrapped around the nerve is shown. 9. Median nerve to AIN TMR coaptation (white arrow) is shown with a pedicled PQ (black arrow). 10. Final closure is shown.

Myodesis is performed with braided, absorbable sutures between the volar and dorsal compartments to create firm and protective coverage over the radius and ulna. The incision is meticulously closed with interrupted, subcutaneous monofilament suture and interrupted, monofilament skin closure to prevent dehiscence (Figure 10).

Conclusion

This article introduces a new, effective surgical technique utilizing TMR and RPNI in the setting of transradial amputation. This novel technique has the benefit of eliminating a second, more proximal incision that is typically made to perform TMR. Additionally, the techniques utilized in this approach have been shown to diminish postoperative neuroma formation and neuropathic pain while improving patient control of a myoelectric prosthesis. In the future, implantable myoelectric sensors in RPNI muscle grafts may be used for added prosthetic signals.