Abstract
Background
In patients with advanced cubital tunnel syndrome (CuTS), conventional surgical outcomes are often suboptimal due to the long distance required for nerve regeneration. The supercharged end-to-side anterior interosseous nerve to ulnar motor nerve transfer (referred to as SETS transfer) has been proposed to enhance reinnervation and accelerate recovery. This study aimed to evaluate the electrodiagnostic and clinical outcomes of SETS transfer in patients with advanced CuTS and to determine the time course of functional recovery.
Methods
This prospective study included 11 patients with severe, non-traumatic CuTS (modified McGowan grade IIB or III) who underwent ulnar nerve decompression, subcutaneous anterior transposition, and SETS transfer. Electrodiagnostic studies were performed preoperatively and at 3, 6, and 12 months postoperatively, assessing compound muscle action potentials (CMAPs) of the abductor digiti minimi (ADM) and first dorsal interosseous muscles, sensory nerve action potentials (SNAPs) of the ulnar and dorsal ulnar cutaneous (DUC) nerves, and detection of nascent motor unit action potentials (MUAPs). Grip and key-pinch strength and Disabilities of the Arm, Shoulder, and Hand (DASH) scores were assessed preoperatively and at 12 months.
Results
The CMAP amplitude of the ADM significantly improved at the 12-month follow-up compared to the preoperative assessment (p = 0.004). Postoperative nascent MUAPs were observed in 8 patients, all of which appeared within 6 months. In contrast, among patients with no preoperative SNAP who had a postoperative response, most recorded their first SNAP at 12 months after surgery (5 of 6 for ulnar SNAP and 5 of 7 for DUC SNAP). Key-pinch strength and DASH scores improved significantly (p = 0.014 and p = 0.003, respectively), while grip strength showed no significant change (p = 0.142).
Conclusions
SETS transfer may promote early reinnervation and improve intrinsic muscle function in patients with severe CuTS. Serial electrodiagnostic studies suggest that SETS transfer may contribute to motor recovery within the first 6 months postoperatively.
Keywords: Cubital tunnel syndrome, Anterior interosseous nerve, Supercharged end-to-side nerve transfer, Electrodiagnosis
Traditionally, cubital tunnel syndrome (CuTS) is treated with surgery around the elbow, in situ decompression, epicondylectomy, or anterior transposition of the ulnar nerve.1) However, in advanced cases, the motor outcomes tend to be poor owing to the long distance required for nerve regeneration.2) To reduce the distance and time required for nerve reinnervation, a nerve transfer closer to the target motor end plates has been attempted, which is known as the supercharged end-to-side (SETS) anterior interosseous nerve (AIN) to ulnar motor nerve (UMN) transfer (referred to as SETS transfer).3) It is thought that normally functioning AIN branches augment ulnar motor endplate function and promote intrinsic reinnervation.4,5,6)
Despite growing interest, research on the outcomes of SETS transfer remains limited and conflicting. Despite the promising results reported in the majority of studies regarding this additional surgical technique,7,8,9,10,11,12,13,14,15,16) its efficacy remains unclear.17,18,19,20) Previous studies are limited in that patients with proximal ulnar neuropathy of varying severity and injury mechanisms,7,12,13,14,15,17) or those who underwent different types of surgery in conjunction with SETS transfer were included.7,9,12,14,15,17) Additionally, most studies had a retrospective design,7,9,10,11,14,17) and few used electrodiagnostic studies in addition to clinical outcomes,8,10,12,13,14,15,16) most of which were performed at a single time point after surgery.
Therefore, this prospective study aimed to evaluate the serial recovery of hand intrinsic muscles using electrodiagnostic studies in patients with advanced CuTS who underwent SETS transfer in conjunction with ulnar nerve decompression and subcutaneous anterior transposition. We hypothesized that adding SETS transfer to conventional surgery would result in early reinnervation of intrinsic muscles, thereby contributing to functional recovery.
METHODS
This prospective single-arm study was approved by the Institutional Review Board of Korea University Anam Hospital (IRB No. 2023AN0478). Written informed consent was obtained from all the participants.
Participants
Participants were selected from patients who visited the Department of Orthopedic Surgery at our institution between July 2019 and June 2023 according to the following inclusion criteria: (1) non-traumatic, compressive ulnar neuropathy at the elbow (CuTS); (2) clinically severe CuTS, defined as modified McGowan grade IIB or III,21) with Froment and Wartenberg signs observed in all patients; and preoperative electrodiagnostic evidence of (3) CuTS with intrinsic muscle denervation (fibrillations); and (4) a normal-functioning AIN branch to the pronator quadratus. Patients with (1) a history of CuTS surgery; (2) traumatic ulnar nerve injury, fracture- or tumor-related ulnar neuropathy, or iatrogenic ulnar nerve transection; (3) C8–T1 radiculopathy, medial cord/lower trunk plexopathy, or any electrodiagnostic findings suggestive of double-crush syndrome; (4) a space-occupying lesion or an anomalous anconeus epitrochlearis muscle identified on preoperative ultrasonography; or (5) a follow-up period of less than 12 months were excluded. All patients underwent the same surgical procedure: ulnar nerve decompression, subcutaneous anterior transposition, and SETS transfer.
Electrodiagnostic Studies
Electrodiagnostic studies were performed using standardized methods outlined by the American Association of Electrodiagnostic Medicine,22) with a Nicolet EDX apparatus (Natus Medical Inc.). Skin temperature was maintained between 32 ℃ and 34 ℃ throughout the procedure.
For motor and sensory nerve conduction studies (NCS), supramaximal stimuli with a square-wave pulse of 0.1 ms duration were delivered. For motor NCS, the standard belly–tendon technique was used with filter settings of 2–10,000 Hz. Median compound muscle action potentials (CMAPs) were recorded from the abductor pollicis brevis following stimulation 7 cm proximal to the active electrode (E1) and at the elbow. Ulnar CMAPs were recorded from the abductor digiti minimi (ADM) and first dorsal interosseous (FDI) muscles, with stimulation 8 cm proximal to E1, 3 cm distal to the medial epicondyle, and 7 cm proximal to the medial epicondyle, while the elbow was flexed at 90°. Distal latency, conduction velocity, and baseline-to-peak amplitude were measured.
Sensory NCS were performed antidromically. Sensory nerve action potentials (SNAPs) from the median, ulnar, and dorsal ulnar cutaneous (DUC) nerves were recorded using a bar electrode with a 3-cm interelectrode distance and a filter range of 20–3,000 Hz. Recordings were obtained over the middle finger (median), little finger (ulnar), and the dorsum between the fourth and fifth metacarpals (DUC nerve), with stimulation at 14 cm (median and ulnar) or 10 cm (DUC) proximal to the E1. Onset and peak latencies and baseline-to-peak amplitudes were evaluated.
Needle electromyography (EMG) was performed to evaluate (1) muscles innervated by the ulnar nerve, including the flexor carpi ulnaris, flexor digitorum profundus, ADM, and FDI muscles and (2) the pronator quadratus muscle to verify an intact AIN branch. In addition, non-ulnar C8–T1/lower-trunk–innervated muscles and the C8–T1 paraspinals were examined to exclude lower-trunk brachial plexopathy or C8–T1 radiculopathy. The EMG filter settings were 20–10,000 Hz.
Outcome Measures
Electrodiagnostic studies were performed preoperatively and at 3 postoperative time points (3, 6, and 12 months after surgery). At each time point, the following data were collected: (1) the amplitude of ulnar nerve CMAPs recorded from the ADM and FDI muscles and (2) the amplitude of SNAPs of the ulnar and DUC nerves. In addition, during the needle EMG performed at the 3 postoperative time points, the time at which nascent motor unit action potentials (MUAPs): small, multiphasic, and unstable potentials that arise from axonal regrowth - first appeared in either the ADM or FDI muscle was recorded.
Clinical assessment included measurements of grip and key-pinch strength using handheld and pinch dynamometers (Jamar Dynamometers). Functional evaluation of the upper extremity was performed using the Disabilities of the Arm, Shoulder, and Hand (DASH) questionnaire. Clinical and functional assessments were obtained preoperatively and at 12 months postoperatively.
The primary endpoint was the improvement in the amplitude of the ulnar nerve CMAP recorded from the ADM muscle at 12 months postoperatively. Secondary endpoints included (1) the improvement in the amplitude of the ulnar nerve CMAP recorded from the FDI muscle at 12 months postoperatively, (2) the initial emergence of nascent MUAPs detected in the ADM or FDI muscle, (3) the time interval between the initial emergence of nascent MUAPs and the improvement in SNAPs, and (4) improvements in clinical outcomes, including grip strength, key-pinch strength, and DASH scores.
Statistical analyses were performed using the IBM SPSS version 24 (IBM Corp.). This study was designed as a prospective exploratory cohort study. Due to the rarity of eligible severe cases and the intensive burden of serial electrodiagnostic follow-up, a formal a priori sample size or power calculation was not feasible. Given the small sample size (n = 11), normality of continuous variables was assessed using the Shapiro-Wilk test. Outcomes that were not normally distributed were analyzed using nonparametric methods. Specifically, the Wilcoxon signed-rank test was used to compare paired measurements between preoperative and 12-month postoperative time points for CMAP amplitudes, grip and key-pinch strength, and DASH scores, and effect sizes (r) as well as median changes with 95% confidence intervals were reported. For repeated-measures CMAP data, the Friedman test was performed to assess overall time effects, and for variables showing a significant overall effect, post hoc pairwise comparisons were conducted using the Wilcoxon signed-rank test with Bonferroni correction. For the SNAP amplitudes of the ulnar and DUC nerves, the number of patients with recordable preoperative responses was small, resulting in an insufficient number of paired observations for statistical comparison using the Wilcoxon signed-rank test. Accordingly, SNAP amplitudes were summarized using only descriptive statistics (median [interquartile range]) at each time point, and the numbers of patients with recordable and unrecordable SNAP responses were also reported.
Surgical Technique
With the patient’s arm abducted and externally rotated, a posteromedial incision was made along the course of the ulnar nerve at the elbow. The incision began approximately 5 cm proximal to the medial epicondyle and extended distally for about 3 cm. The ulnar nerve was identified and decompressed by incising the overlying Osborne’s ligament. It was then transposed anteriorly and positioned over the medial epicondyle. To ensure stability, the nerve was placed on the fascia of the flexor-pronator muscle group and secured beneath a thick adipose layer using a fascial sling.
For the AIN transfer, a second incision was made on the volar aspect of the forearm, starting from the ulnar side of the wrist crease and extending proximally for 5 to 7 cm. The flexor tendons were retracted radially to expose the pronator quadratus muscle. Dissection proceeded over the muscle to approximately its midpoint, where the motor branches of the AIN were identified. The AIN was carefully isolated along its course from proximal to distal, and the distal end was transected at the point where it branches into the muscle.
To prepare for nerve transfer, interfascicular neurolysis was performed on the ulnar nerve to distinguish its motor and sensory components. Motor fascicle selection and end-to-side AIN coaptation were performed according to the standardized SETS technique at the distal forearm level proximal to the pronator quadratus, and all patients were screened preoperatively for Martin-Gruber anastomosis by electrodiagnostic testing. The motor fascicle was consistently located between the main sensory fascicle and the dorsal cutaneous branch. Under an operating microscope, a perineural window was created in the epineurium of the motor fascicle. An end-to-side neurorrhaphy was then performed between the distal end of the AIN and the motor fascicle of the ulnar nerve using 9-0 Nylon suture (Fig. 1). All incisions were subsequently irrigated and closed in layers.
Fig. 1. Intraoperative views of the supercharged end-to-side anterior interosseous nerve (AIN) to ulnar motor nerve transfer. (A) Interfascicular neurolysis was performed to identify the motor fascicle (blue arrow) between the main sensory fascicle (yellow arrow) and the dorsal cutaneous branch. The AIN was isolated and tagged (red arrow). (B) Under the microscope, a perineural window was created in the motor branch of the ulnar nerve, and end-to-side neurorrhaphy was performed between the AIN and motor fascicle using 9-0 Nylon suture.
Postoperative rehabilitation was performed according to the standardized protocol for SETS nerve transfer. The operated upper extremity was initially immobilized in a long-arm splint with the elbow flexed at 70°–90° and the forearm in neutral rotation for 2 weeks, after which a short-arm splint was applied for 2 more weeks. Gentle passive and active assisted range of motion exercises of the elbow, wrist, and forearm (including pronation–supination) were started at 4 weeks postoperatively, followed by progressive active range of motion. Nerve-gliding exercises were initiated at 4 weeks, and intrinsic hand muscle strengthening was begun at approximately 8–10 weeks postoperatively as reinnervation was detected clinically or electrophysiologically. All patients followed the same standardized hand therapy protocol under the supervision of certified hand therapists.
RESULTS
The patient demographics and clinical data are presented in Table 1. All patients reported subjective improvements in hand numbness and weakness after surgery. At 12 months postoperatively, Medical Research Council (MRC) muscle strength increased by 1 grade compared with preoperative values in 5 patients for the ADM and 7 patients for the FDI. Importantly, no donor-site morbidity was observed in this cohort. At the 3-month postoperative follow-up, no patients reported subjective forearm pronation weakness, and no objective pronation weakness was detected on physical examination. In addition, no newly developed neuropathic pain involving the forearm or hand, infections, or reoperations were identified during the follow-up period.
Table 1. Demographic and Clinical Data (n = 11).
| Characteristics | Preoperative | Postoperative 12 mo | ||
|---|---|---|---|---|
| Sex (male : female) | 9 : 2 | |||
| Mean age (range, yr) | 64 (54–76) | |||
| Underlying disease | ||||
| Diabetes mellitus | 6 | |||
| Chronic kidney disease | 0 | |||
| Cancer | 1 | |||
| Dominant hand (right : left) | 11 : 0 | |||
| Side affected (right : left) | 9 : 2 | |||
| MRC muscle strength grade | ||||
| ADM | ||||
| 2 | 1 | 0 | ||
| 3 | 4 | 2 | ||
| 4 | 6 | 8 | ||
| 5 | 0 | 1 | ||
| No. of patients with MRC grade increase | - | 5 | ||
| FDI | ||||
| 2 | 2 | 0 | ||
| 3 | 4 | 2 | ||
| 4 | 5 | 8 | ||
| 5 | 0 | 1 | ||
| No. of patients with MRC grade increase | - | 7 | ||
| Modified McGowan grade | ||||
| IIB | 6 | - | ||
| III | 5 | - | ||
| Symptom duration (mo) | 18.6 (2–42) | |||
| Concomitant peripheral polyneuropathy | 1 | |||
Values are presented as number or mean (range).
MRC: medical research council, ADM: abductor digiti minimi, FDI: first dorsal interosseous.
The outcomes of the electrodiagnostic study and clinical assessments are presented in Table 2 and Fig. 2. The CMAP amplitude of the ADM significantly improved at the 12-month follow-up compared with the preoperative assessment (p = 0.004, effect size r = 0.86; median change, 1.5 [95% CI, 1.0–2.5]). The CMAP amplitude of the FDI also showed a tendency to improve but did not reach statistical significance (p = 0.056, effect size r = 0.61; median change, 1.0 [95% CI, 0.0–2.0]). The CMAP amplitude of the ADM muscle showed progressive improvement over the follow-up period (Fig. 2A, Table 3). The ADM CMAP demonstrated a significant overall time effect (Friedman p < 0.05). At 12 months postoperatively, CMAP amplitudes were significantly higher compared with preoperative values (p = 0.004, r = 0.86; median change, 1.5 [95% CI, 1.0–2.5]) and with 3- and 6-month values (p = 0.002–0.004, r = 0.87–0.91; median change, 1.5 [95% CI, 1.0–2.0]). However, no significant change was observed between preoperative and 3- or 6-month measurements (p = 0.739–0.336, r = 0.10–0.29), and the differences between the 3- and 6-month postoperative time points were also not statistically significant (p = 0.414, r = 0.25).
Table 2. Outcomes of Electrodiagnostic Study and Clinical Assessments.
| Preoperative | Postoperative 3 mo | Postoperative 6 mo | Postoperative 12 mo | p-value (preoperative vs. postoperative 12 mo) | Effect size (r) | Median change (95% CI) | |||
|---|---|---|---|---|---|---|---|---|---|
| Electrodiagnostic study | |||||||||
| CMAP amplitude of ADM | |||||||||
| Mean ± SD (mV) | 3.7 ± 1.9 | 3.9 ± 2.6 | 4.1 ± 2.8 | 5.5 ± 2.8 | |||||
| Median (IQR) (mV) | 4.0 (2.0 to 5.0) | 3.0 (2.0 to 5.0) | 4.0 (2.0 to 5.0) | 4.0 (3.0 to 6.0) | 0.004* | 0.859 | 1.5 (1.0 to 2.5) | ||
| CMAP amplitude of FDI | |||||||||
| Mean ± SD (mV) | 3.2 ± 2.8 | 3.5 ± 4.3 | 3.7 ± 4.3 | 4.3 ± 3.5 | |||||
| Median (IQR) (mV) | 2.0 (1.0 to 5.0) | 1.0 (1.0 to 6.0) | 1.0 (1.0 to 7.0) | 2.0 (2.0 to 7.0) | 0.056 | 0.605 | 1.0 (0.0 to 2.0) | ||
| SNAP amplitude of ulnar nerve | |||||||||
| Median (IQR) (µV) | 0.0 (0.0 to 6.0) | 0.0 (0.0 to 5.0) | 2.0 (0.0 to 4.0) | 4.0 (3.0 to 6.0) | |||||
| Recordable (n) | 4 | 4 | 5 | 10 | |||||
| Non-recordable (n) | 7 | 7 | 6 | 1 | |||||
| SNAP amplitude of DUC nerve | |||||||||
| Median (IQR) (µV) | 0.0 (0.0 to 10.0) | 0.0 (0.0 to 7.0) | 0.0 (0.0 to 3.0) | 4.0 (3.0 to 6.0) | |||||
| Recordable (n) | 3 | 4 | 5 | 10 | |||||
| Non-recordable (n) | 8 | 7 | 6 | 1 | |||||
| Postoperative change (n) | |||||||||
| First detection of nascent MUAPs | 5 | 3 | 0 | ||||||
| Newly detectable Ulnar nerve SNAP | 0 | 1 | 5 | ||||||
| Newly detectable DUC nerve SNAP | 1 | 1 | 5 | ||||||
| Clinical outcome | |||||||||
| Grip strength (lb) | |||||||||
| Mean ± SD | 47.2 ± 17.6 | - | - | 52.9 ± 16.0 | |||||
| Median (IQR) | 46.0 (38.0 to 55.0) | - | - | 50.0 (40.0 to 64.0) | 0.142 | 0.443 | 6.5 (–2.0 to 14.0) | ||
| Key-pinch strength (lb) | |||||||||
| Mean ± SD | 7.0 ± 4.1 | - | - | 10.5 ± 3.8 | |||||
| Median (IQR) | 6.0 (4.0 to 10.0) | - | - | 10.0 (8.0 to 13.0) | 0.014* | 0.742 | 3.5 (1.0 to 6.0) | ||
| Functional outcome | |||||||||
| DASH score | |||||||||
| Mean ± SD | 44.9 ± 17.1 | - | - | 10.9 ± 12.1 | |||||
| Median (IQR) | 41.0 (39.0 to 50.0) | - | - | 4.0 (0.0 to 23.0) | 0.003* | 0.885 | –33.3 (–44.5 to –22.5) | ||
CMAP: compound muscle action potential, ADM: abductor digiti minimi, SD: standard deviation, IQR: interquartile range, FDI: first dorsal interosseous, SNAP: sensory nerve action potential, DUC: dorsal ulnar cutaneous, MUAP: motor unit action potentials, DASH: Disabilities of the Arm, Shoulder, and Hand.
*p < 0.05.
Fig. 2. Longitudinal changes in outcomes of electrodiagnostic studies and clinical assessments. (A) Compound muscle action potential (CMAP) amplitude of the abductor digiti minimi. (B) CMAP amplitude of first dorsal interosseous muscle. (C) Grip strength. (D) Key-pinch strength. (E) Disabilities of the Arm, Shoulder, and Hand (DASH) score. CMAP amplitudes (A, B) were measured preoperatively and at 3, 6, and 12 months postoperatively, whereas grip strength, key-pinch strength, and DASH score (C-E) were measured preoperatively and at 12 months postoperatively. Gray lines represent individual patient trajectories, and the blue line represents the trajectory of the group mean.
Table 3. Longitudinal Changes in Compound Muscle Action Potential Amplitudes at 0, 3, 6, and 12 Months: Friedman Test, with Post-Hoc Wilcoxon Signed-Rank Tests and Bonferroni Correction Performed When Friedman p < 0.05.
| Outcome | Median (IQR) (mV) | Friedman p | Post-hoc | |||
|---|---|---|---|---|---|---|
| Preop | Postop 3 mo | Postop 6 mo | Postop 12 mo | |||
| CMAP amplitude of ADM | 4.0 (2.0–5.0) | 3.0 (2.0–5.0) | 4.0 (2.0–5.0) | 4.0 (3.0–6.0) | < 0.001* | Preop vs. Postop 3 mo: p = 0.739, r = 0.100 (median change, 0.0; 95% CI, –0.5 to 1.0) |
| Preop vs. Postop 6 mo: p = 0.336, r = 0.290 (median change, 0.0; 95% CI, –0.5 to 1.0) | ||||||
| Preop vs. Postop 12 mo: p = 0.004†, r = 0.859 (median change, 1.5; 95% CI, 1.0 to 2.5) | ||||||
| Postop 3 mo vs. Postop 6 mo: p = 0.414, r = 0.246 (median change, 0.0; 95% CI, 0.0 to 0.5) | ||||||
| Postop 3 mo vs. Postop 12 mo: p = 0.002†, r = 0.912 (median change, 1.5; 95% CI, 1.0 to 2.0) | ||||||
| Postop 6 mo vs. Postop 12 mo: p = 0.004†, r = 0.868 (median change, 1.5; 95% CI, 1.0 to 2.0) | ||||||
| CMAP amplitude of FDI | 2.0 (1.0–5.0) | 1.0 (1.0–6.0) | 1.0 (1.0–7.0) | 2.0 (2.0–7.0) | 0.066 | |
Preop: preoperative, Postop: postoperative, IQR: interquartile range, CMAP: compound muscle action potential, ADM: abductor digitiminimi, FDI: first dorsal interosseous.
*Friedman p < 0.05. †Post-hoc p < 0.0083 (Bonferroni-corrected).
Preoperatively, the SNAPs of the ulnar and DUC nerves were detected in only 4 and 3 patients, respectively. At 12 months postoperatively, SNAPs of both nerves were detected in 10 patients. Newly detected ulnar nerve SNAP responses were not observed at 3 months, were identified in 1 patient at 6 months, and in an additional 5 patients by 12 months (cumulative proportions at 3, 6, and 12 months: 0%, 9.1%, and 54.5%). Newly detected DUC nerve SNAP responses were observed in 1 patient at both 3 and 6 months postoperatively, and in an additional 5 patients at 12 months (cumulative proportions at 3, 6, and 12 months: 9.1%, 18.2%, and 63.6%). In contrast, postoperative nascent MUAPs were detected in a total of 8 patients, all of which appeared within 6 months. The nascent MUAPs were first observed in 5 patients at 3 months postoperatively and in the remaining 3 patients at 6 months postoperatively (cumulative proportions at 3, 6, and 12 months: 45.5%, 72.7%, and 72.7%).
Key-pinch strength showed significant improvement postoperatively (p = 0.014, effect size r = 0.74; median change, 3.5 [95% CI, 1.0–6.0]); however, the changes in grip strength were not significant (p = 0.142, effect size r = 0.44; median change, 6.5 [95% CI, 2.5–14.0]). The DASH score decreased significantly postoperatively (p = 0.003, effect size r = 0.89; median change, –33.3 [95% CI, –44.5 to –22.5]).
DISCUSSION
This study investigated the effect of SETS transfer on intrinsic functional recovery in advanced stage CuTS. We recruited 11 patients with severe CuTS and performed serial electrodiagnostic studies during a 1-year follow-up period to explore the contribution of SETS transfer to clinical and functional improvement. Unlike many previous studies,7,9,11,12,14,15,17) this investigation employed a prospective design and sought to establish a homogeneous cohort with respect to the etiology and severity of CuTS. In addition, all patients underwent an identical surgical protocol—ulnar nerve decompression, subcutaneous anterior transposition, and SETS transfer—thereby minimizing surgical variability. Collectively, these efforts allowed us to explore the potential contribution of SETS transfer in patients with advanced non-traumatic compressive ulnar neuropathy at the elbow. Furthermore, whereas many prior studies have either reported electrodiagnostic findings at a single time point or focused primarily on clinical recovery,8,10,12,13,14,15,16) the present study incorporated longitudinal electrodiagnostic evaluations. This design enabled a time-resolved assessment of (1) progressive changes in CMAP amplitude, (2) the timing of nascent MUAP appearance, and (3) patterns of SNAP recovery, and revealed a temporal dissociation between motor and sensory recovery, providing further evidence that SETS transfer may contribute to motor recovery through early reinnervation of intrinsic muscles.
All patients reported subjective improvements in hand weakness postoperatively. Objectively, improvement in pinch strength exceeding the minimal clinically important difference (MCID) was observed in 9 of the 11 patients (82%), which was statistically significant. The median increase in key-pinch strength of 3.5 lb exceeds the commonly reported MCID for pinch strength (approximately 2–3 lb),23) indicating that this improvement is not only statistically significant but also clinically meaningful in daily functional activities. Additionally, all patients demonstrated significant functional improvement in the DASH score, with 100% of patients exceeding the MCID.24) This finding is important as outcomes of conventional surgery in severe CuTS are typically poor, regardless of the surgical technique.2) However, these results alone do not fully reflect the extent of improvement attributable to the additional effect of SETS transfer.
Therefore, the time frame of recovery must also be considered when interpreting the results of the SETS transfer. Axonal degeneration, observed in severe compressive neuropathies, was confirmed in our study participants by clinical muscle atrophy and denervation potentials of hand intrinsic muscles noted on preoperative electrodiagnostic tests. Given that the expected rate of nerve regeneration is 1 inch per month, reinnervation from the SETS transfer coaptation site is expected to occur in approximately 6–7 months (range, 3–9 months).7,9) Davidge et al.7) reviewed the records of 55 patients with ulnar neuropathy who underwent SETS transfer. They reported a significant improvement in FDI muscle strength, with over 70% of the patients achieving an improvement of MRC grade 3 or higher. The researchers suggested that SETS transfer contributed to intrinsic recovery, as the most notable improvement occurred within the 6-month follow-up period, which is the expected time frame of recovery for nerve transfer, rather than axon regrowth from the elbow level. Jarvie et al.8) and Dengler et al.9) also reported improvements in clinical and electrodiagnostic outcomes within a similar recovery period. A prospective, randomized comparison study conducted by Xie et al.16) reported that the recovery effect of the additional SETS transfer group was better than that of the conventional surgery alone group at 6 months after surgery, with a significant increase in pinch strength and CMAP amplitude of the FDI and ADM.
In our study, nascent MUAPs were observed in 8 of the 11 patients, and all were detected at 3 or 6 months postoperatively. Nascent MUAPs are small, multiphasic, unstable potentials that arise from axonal regrowth during the recovery process after nerve injury.25) Considering the timing of the nascent MUAPs observed, the SETS transfer possibly contributed to some degree of recovery. Doherty et al.10) also reported similar results in that a significant proportion of patients demonstrated nascent MUAPs in the intrinsic ulnar-innervated hand within 7 months after surgery, which was considerably earlier than the expected time for axonal regrowth in the cubital tunnel. A disproportionate recovery between motor and sensory functions in the electrodiagnostic study is also considered as an indication of the recovery capabilities of SETS transfer. No significant change was noted in SNAP amplitude, as anticipated owing to surgery at the elbow, during the period when nascent MUAPs were detected.
In the current study, the CMAP amplitude demonstrated progressive improvement over the follow-up period and showed a statistically significant change, suggesting that continuous nerve recovery occurred during this timeframe. Considering that all 11 patients enrolled in this study had incomplete injuries, recovery likely involved not only axonal regrowth via SETS transfer but also distal axonal sprouting of residual ulnar nerve axons.25) MUAPs arising from distal sprouting may also appear polyphasic and unstable early in the process due to immature terminal branches and poorly myelinated neuromuscular junctions. However, unlike the small-amplitude nascent MUAPs, these MUAPs have a normal initial size, and their amplitude and duration increase as the motor unit territory expands. This electrophysiological feature allows differentiation between recovery mediated by distal axonal sprouting and that mediated by SETS transfer. Therefore, while distal sprouting likely contributed to the observed recovery, the observation of nascent MUAPs may indicate that SETS transfer additionally contributed to motor recovery within the first 6 months postoperatively. In severe CuTS, the appearance of nascent MUAPs within 6 months could be considered a potential exploratory prognostic marker, which requires further investigation in prospective studies.
The limitations of this study include the small sample size and the lack of serial clinical assessments beyond electrodiagnostic tests, as evaluations were not performed at 3 and 6 months, which limited the ability to analyze the relationship between electrophysiologic changes and functional outcomes. Clinically intuitive intrinsic function metrics, such as clawing severity and Froment/Wartenberg sign changes, were also not systematically included, further restricting interpretation. The small sample size and non-normal data limited the analysis of factors potentially influencing outcomes, such as age, comorbidities, and symptom duration. Moreover, the 12-month follow-up may be insufficient to fully assess sensory recovery and intrinsic muscle reinnervation, and the absence of a control group prevents precise attribution of improvements to SETS transfer. Finally, the relatively older age and higher prevalence of diabetes in this cohort may limit generalizability. Future studies with larger cohorts, longer follow-up, and control groups are needed to clarify these effects.
Footnotes
CONFLICT OF INTEREST: No potential conflict of interest relevant to this article was reported.
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