Introduction

Nerve injury is associated with significant impairment of the upper limb function, thus presenting a challenge for surgical reconstruction. The technique, type of nerve reconstruction, and short period between injury and surgery are essential to achieve a satisfactory clinical effect. Ultrasound evaluation of the nerves is important in preoperative planning and qualification for surgery.
High nerve damage or advanced compression at the level of the arm or elbow is associated with a long period of regeneration for effectors-muscles located within the hand. It could lead to irreversible muscle atrophy and permanent hand dysfunction [1-4].
To protect motor plates within the muscles until the regeneration of the main nerve trunk, the RETS reconstruction (commonly called supercharge or babysitting) may be considered. The technique is similar to the classical ETS reconstruction, but the idea differs – in this situation, we transfer the distal part of a healthy nerve (commonly anterior interosseous nerve – AIN or motor branch to abductor digiti minimi – ADMI) distally to injury or compression site of a salvageable nerve (usually ulnar or median) to support regeneration or keep the viability of degenerating muscles [5,6]. The name “babysitting” describes this concept’s supportive and muscle-protective features, while “supercharge” underlines the effect of delivering motor axons into poorly regenerating nerves [7].
The idea of this technique compared to original ETS and nerve transfer is presented in Figure 1.
In clinical practice, these transfers are usually employed in ulnar or median nerve injuries in addition to treating the injury (with end-to-end coaptation, grafting, or decompression). Ideally, RETS is performed at the time of primary surgery but also can be postponed or included in cases with suspicion of inferior results.
This concept has been investigated in experimental models and its clinical applications have been published [5,8,9]. Nevertheless, it is still unclear to what extent the improvement in recovery depends on RETS and the regeneration of injured nerve, so this technique’s advantages must be further evaluated [10,11].

Aim

The paper aims to present the authors’ results after treatment of a high injury of the ulnar and median nerves with additional RETS performed in a single operation, based on the results of three clinical cases and discuss its features with recent literature.

Case series

We analysed 3 cases with ulnar or median nerve injury, where RETS was used as supercharge/babysitting technique besides treatment of the injured area.
The first patient was a 6-year-old boy with severe damage to the ulnar nerve at the elbow level after surgical treatment of the distal humerus fracture using Kirschner wires. Symptoms of ulnar nerve dysfunction include atrophy and weakness of the internal muscles of the hand, with a strength of 3 according to the Lovett scale, causing claw deformity of the IV and V fingers. Also, weakness was observed in the flexor flexor digitorum profundus (FDP) V and flexor carpi ulnaris (FCU) with Lovett 3. The little finger exhibited a disturbed delicate sensation with a two-point discrimination of 10mm.. Ultrasound showed signs of nerve thickening with disturbed nerve structure but preserved continuity.
After 6 months of the injury, due to the poor signs of nerve regeneration, neurolysis of the nerve was performed at the level of injury (elbow). The ulnar nerve was in continuity with a medium-sized neuroma and intraneural changes at a level that could be pinned with a K-wire
(Fig. 2). RETS of the anterior interosseous nerve to the ulnar motor bundle at the level of the distal part of the forearm was performed simultaneously.
The second patient was a 39-year-old female with severe damage to the ulnar nerve at the elbow level after surgical treatment of the distal humerus fracture by ORIF. After surgery due to neuralgia in the distribution of the ulnar nerve, neurolysis of the nerve at the elbow level was performed the next day, improving symptoms. Clinically progressive ulnar nerve dysfunction was observed with no significant signs of recovery over time. Atrophy and weakness of the internal muscles of the hand, with a strength of 0 according to the Lovett scale, causing claw deformity of the IV and V fingers. Also, weakness in the flexor FDP V and FCU with Lovett 3. The little finger showed a disturbed, delicate sensation with undetectable two-point discrimination on the little finger.
Ultrasound shows signs of nerve neuroma in continuity at the level of medial epicondyles. Due to the poor signs of nerve regeneration, the RETS reconstruction was performed by transferring the anterior interosseous nerve in the area of the ulnar motor bundle at the level of the distal part of the forearm and neurolysis of the ulnar nerve at the Guyon’s canal. The decision was also based on intraoperative neuromonitoring of ulnar and median nerves, which helped prove insufficient conduction of the ulnar nerve (Fig. 3).
The third patient was a 9-year-old boy with severe damage to the median nerve at the forearm level after a fracture of both forearm bone shafts; he was treated with closed reduction and intramedullary fixation using Kirschner wires. Symptoms of median nerve dysfunction include sensory disturbances on the thumb, index, and long finger with 9mm two-point discrimination and weakness of the abductor pollicis brevis with limited palmar abduction of the thumb with a strength of 3 according to the Lovett scale. Ultrasound examination showed damage and discontinuity to the median nerve at the level of the radius fracture. After 9 months after the injury, no significant nerve regeneration, and a decision about operative treatment was taken. During the surgery, the dorsoradial part of the median nerve was found to be cut by radius fragments, which healed. Due to partial continuity of the remaining part and both stumps “captured” by the bone, neurolysis of the preserved part of the nerve and resection of the neuroma with direct suture of the damaged part was possible and done. Simultaneously, due to the time from injury to the surgery, RETS transfer of the anterior interosseous nerve to the motor bundle of the median nerve at the level of the distal part of the forearm was performed (Fig. 4).
Surgical technique

The area covering both nerves (donor and recipient) is evaluated, and the incision is marked, enabling comfortable access to both areas. Typically, it is an area separate from the nerve injury site. First, the recipient nerve is exposed and evaluated. Neurostimulation can be used to target specific areas of the nerve trunk to confirm the absence of conduction or muscle response. Based on preoperative and intraoperative findings, the decision is made about RETS or nerve transfer. Additionally, at this stage of the surgery, neuromonitoring can also be planned and used to evaluate the above features with more adequate methods and higher sensitivity, including the opinion of an experienced neurophysiologist as a valuable support of the decision.
After deciding on RETS, the recipient nerve is further prepared – based on nerve anatomy and previous studies on the location of specific fascicles, the area is opened, and an appropriate part of the nerve is selected for donor nerve transfer.
Then, the donor nerve is located, prepared, and followed as distally as possible (taking into consideration terminal branching and calibre). This shortens the distance of supercharge regeneration, the same as in the nerve transfers principle. The donor nerve is cut distally, then transferred and sutured to the recipient’s nerve’s side (to the part previously selected), usually creating an epineural window, depending on the surgeon’s preference and calibre of the nerve (nerve trunk or specific fascicle). The sutures are done under microscopic magnification with Nylon 8-0, 9-0 or 10-0 in a tension-free fashion, which is evaluated by passively moving adjacent joints and observing the gliding of the suture site. This evaluation is crucial for optimal nerve regeneration and the possibility of early exercises.
The following steps of this technique are shown in Fig. 5.

Results

In patients with ulnar nerve injury, a significant improvement in nerve function was obtained after surgery. Half a year after the operation, full motor regeneration was achieved, claw deformity disappeared, and muscle atrophy was reduced with the improvement of muscle strength of the internal muscles, FDP V, and FCU. Partial sensory recovery was noted, with persistent but less severe numbness in the little finger. In patient 2 – complete motor and sensory recovery was achieved in the last follow-up after 18 months.
Partial recovery was achieved in a patient with a median nerve injury. One year after surgery, a significant improvement in motor regeneration was found with improved palmar thumb abduction, better strength and thumb motion despite the persistent muscle belly atrophy. Improvement of sensation with preserved delicate feeling on the thumb index and long finger with persistent but less severe numbness on the tips.
The clinical pre and intraoperative features and the outcomes, are presented in Table 1.

Discussion

RETS has been studied on animal models by several authors. Kale et al. evaluated the regenerative effects of the RETS reconstruction in rats by comparing the results to the end-to-end (ETE) suture [5]. 13 rats were evaluated in each group. They performed a resection of the tibial nerve fragment in the control group, and in one study group, an ETE suture of a peroneal nerve to the distal tibial nerve stump was performed. In the other study group, a RETS reconstruction was performed using a peroneal nerve transfer to the side of the distal tibial nerve stump. Based on the histomorphometric analysis, nerve fibres were regenerated in the ETE and RETS groups to a comparable extent, with no statistical differences. Analysis of normalized muscle mass from ETE and RETS animals confirmed statistically significant preservation of muscle mass compared to the control group, without statistical differences between study groups after 10 weeks. The regeneration of nerve fibres in the RETS group was assessed using confocal imaging. Over time, axonal regeneration was observed along the length of the recipient nerve, oriented perpendicular to the repair site. By day 35, robust axonal regeneration was observed, with axons projecting distally.
Farber et al. evaluated in an experimental study on rats regenerative effects in 3 study groups after resection of the tibial nerve: incomplete recovery model (IRM) using free graft reconstruction, IRM with RETS using peroneal nerve transfer, and RETS alone [9]. They found that in histomorphometry analysis, the myelinated fibre counts in the SETS-IRM (supercharged end-to-side-incomplete recovery model) group were significantly higher than those in the IRM and SETS groups. Cross-sections of the nerve in electron microscopy qualitatively revealed a substantial amount of large myelinated fibres in the SETS-IRM group compared to the IRM and SETS groups alone. After sciatic nerve stimulation, the gastrocnemius muscle-specific force was significantly higher in the SETS-IRM group than the IRM group, but with no significant difference compared to the SETS group exclusively.
Besides experimental findings, this concept already has several published clinical applications. Contrary to our study, most authors focused on augmenting regeneration in advanced high ulnar neuropathy. They usually combined in-situ ulnar nerve decompression or anterior transposition with distal RETS with AIN to the ulnar motor branch. The results are encouraging – Doherty [12] analyzed 30 patients after transposition and supercharge transfer, with 73% improving from MRC grade 1 to 3,47% to grade 4.
Chen et al. evaluated 24 patients with high ulnar nerve injury (Sunderland grade IV or V) and found that RETS of AIN transfer to the ulnar nerve significantly improves the outcome compared to repair alone [13].
In a retrospective analysis, Baltzer et al. found better results in patients with RETS augmentation to ulnar nerve repair [14]. He compared RETS vs No-RETS in a cohort of high ulnar transection or compressive injuries and found better outcomes in intrinsic function in RETS group in transected injury patients. Similar findings were reported by Koriem et al. – better results in the RETS group [15]. Moreover, they also observed faster recovery with this technique.
On the contrary, in the Western Canadian multicenter study based on 62 patients with ulnar nerve compression or laceration, the authors haven’t found many advantages in performing RETS in these cases. The patients were followed-up for 3 years, and only key pinch was significantly improved in the RETS group. They concluded that the justification for this technique should be reevaluated [10].
Patients who were operated on significantly improved motor function. This can result from 3 mechanisms: 1. natural regeneration that occurred despite initial injury, 2. supportive and muscle-protective mechanism of RETS, and 3. reinnervation from the transferred nerve that is taking over the function of the motor fibers of the original (recipient) nerve. Despite experimental studies, the exact mechanism remains unconfirmed and could be a mix [16,17].
Partial sensory recovery was noted, with persistent but less severe numbness in the little finger. Although authors in several experimental studies on rodents also showed improved sensory activity, the last finding in our study is not a result of the RETS technique but the natural regeneration of the repaired/neurolysed nerve. It could serve as evidence supporting the safety of the reverse end-to-side technique, despite potential damage to the recipient nerve by an epineural window or placement of stitches [5,7].
Also, the timing of this procedure is not strictly defined. Chen et al. looked at early (before 8 weeks) and delayed transfer in 24 high ulnar injury patients and found no statistical differences in the outcome between these two groups. In clinical practice, it usually depends on the type of nerve lesion and the surgeon’s attitude. Imaging confirms continuity in closed nerve injuries accompanying fractures, and monitors nerve regeneration. In the unsuccessful course of 6 months, the decision is made about exploration, most often neurolysis and RETS if indicated. The second scenario is nerve laceration, e.g., high ulnar nerve injury, which is unlikely to regenerate and impacts intrinsic function – in these cases, RETS can be performed during end-to-end or graft reconstruction. The decision is not clear because AIN to a deep ulnar motor branch in an end-to-end manner is well established [18,19].
In our case series, we haven’t observed any donor morbidity. The distal part of AIN was used, responsible for the innervation of the pronator quadratus muscle, profound pain and proprioceptive sensation from the part of the wrist. To date, no objective studies have tested the possible loss of pronation strength. In future studies, it may be worthwhile to explore this matter and evaluate its impact (if any) on daily activities. Proprioception testing is challenging, as most research concentrates on EARJP (active reproduction error of joint position), specifically in shoulder problems like instability, while studies on the wrist are still limited [20,21].
This donor also differs from the second most common option – ulnar nerve branch to ADMI muscle, where some weakness of abduction can be observed, not influencing activities of daily living.
We also want to draw attention to the third case we described, in which an unusual combination of RETS was performed. We did not find the described AIN transfer to the median thenar motor branch of the ulna in an end-to-side fashion that reflects the anatomy and specificity of the injury in this case. The patient had partial median transection in mid-forearm fracture, distally to the division of AIN from the median nerve. In this case, AIN could be used to augment the regeneration of the motor part innervating the thenar muscles. Usually, in this kind of deficiency, transfer of the ulnar ADMI branch to the thenar branch of the median nerve is usually concerned [22]. Regarding the literature, this particular RETS application is the first case described.

Conclusions

All presented patients had significant signs of improvement, with full to partial motor recovery of the nerves supported by supercharge/babysitting RETS technique. It is valuable, especially in borderline cases when the decision is between neurolysis/repair alone, or nerve transfers are hard to take and not clearly defined.
Nonetheless, based on in vitro and clinical studies, the impact of RETS and nerve regeneration on recovery improvement is unclear. Therefore, it is essential to assess the advantages of this technique and establish guidelines for its clinical use.

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