Evaluation of Tension at Median and Ulnar Nerve Repairs at the Wrist in a Cadaveric Model

Abstract

Background: Median and ulnar nerve lacerations at the wrist are often combined with zone 5 tendon injury. The inability to provide early range of motion leads to increased adhesions. Current therapy protocols recommend the wrist be held in 30° of flexion post operatively to protect the nerve repair. However, if tension and elongation across the nerve repair stay under a critical level in less wrist flexion, postoperative splinting in more extension could allow for better tendon excursion and less adhesions. Methods: Six cadaveric specimens were used. After appropriate dissection, the median and ulnar nerves were transected and repaired with a single 10-0 nylon suture. The wrist was ranged from 30° flexion to 45° extension to see if the repair would fail. Next, an epineural repair was accomplished with 9-0 nylon suture. The percent elongation along the nerve repair was measured at set increments from 30° flexion to 45° extension. Results: In all 6 specimens, median and ulnar nerve repairs with a single 10-0 nylon suture did not fail with wrist range of motion from 30° flexion to 45° extension. Mean percent elongation stayed under critical levels in up to 30° of extension. Conclusions: Both median and ulnar nerve repairs stayed under critical levels of tension and elongation in up to 30° of wrist extension. We believe it is possible to be more aggressive with wrist positioning in wrist level median and ulnar nerve repairs.

Introduction

Injuries to the median and/or ulnar nerve at the wrist are common in trauma setting, particularly with vehicle accidents and penetrating trauma.¹ Often, these nerve injuries have a concomitant zone 5 flexor tendon injury, proximal to the carpal tunnel.² Management of combined nerve and tendon injuries at the wrist can be particularly challenging, given the high potential for scar burden during an immobilization period. Many current therapy protocols recommend that the wrist be held in approximately 30° of flexion for 3 weeks after a median and/or ulnar nerve repair to prevent excessive elongation and protect the repair (Figure 1).³ Lundborg and Rydevik found that less than 5% elongation on a peripheral nerve repair allows for retention of adequate blood flow for healing. However, elongation between 5% and 10% leads to continuous impairment of intraneural microvascular flow, potentially diminishing the ability of nerve fibers to heal.⁴ Clark et al⁵ found that nerve elongation greater than 8% can result in ischemia that is detrimental to nerve regeneration. Furthermore, elongation greater than 16% can result in mechanical failure of the repair. Sunderland and Brenner studied the effects of tension on nerve regeneration and found that the critical threshold at which peripheral nerve primary repair is at risk for non-healing falls between 0.39 N and 0.56 N. They concluded that a 6- to 9-mm gap between nerve ends held together by a single 10-0 suture falls just below this critical threshold.⁶

Figure 1.

Wrist held in approximately 30° of flexion after median and/or ulnar nerve repair.

Previous studies have shown poor functional outcomes in the setting of combined peripheral nerve and flexor tendon repairs, as patients are left with diminished mobility and grip strength.⁷ The inability to provide early range of motion and excursion leads to increased peritendinous adhesions and decreased long-term range of motion.⁸ This study aims to determine if wrist positioning can be modified after simultaneous peripheral nerve and flexor tendon repairs to allow for better excursions while keeping the associated nerve repair under critical levels of elongation and tension. It would also suggest that early active range of motion of the digits could be initiated, as in the early active flexor tendon protocol, where active place and hold exercises are initiated with the wrist in slight extension as early as 5 days postoperatively.⁹ In addition, early passive range of motion of the wrist from flexion to slight extension could produce profound median and ulnar nerve excursion at the wrist, which would prevent scarring and poor functioning of the median and/or ulnar nerve.

We hypothesize that nerve tension and elongation will remain within acceptable parameters in 15° to 30° of wrist extension. Acceptable parameters are defined as: (1) the ability of the nerve repair to remain intact with a single 10-0 nylon suture; and (2) elongation less than 5% across the nerve after nerve repair with a single 10-0 nylon suture and concomitant epineural repair with multiple 9-0 nylon suture.

Materials and Methods

Six fresh frozen above-elbow cadaveric specimens were used for this study. Each specimen was carefully mounted palmar side up on a table top. Using a single 10-cm longitudinal incision across the volar forearm and wrist, dissections were made down to the median and ulnar nerves, respectively. Both the carpal tunnel and Guyon’s canal were released to allow for adequate nerve exposure. Minimal variability was noted in terms of the sizes of the median and ulnar nerves among the specimens, with the exception of a bifid median nerve found in one specimen. Marking sutures (8-0 nylon) were placed 1 cm proximal and distal to the site of the future nerve transection at the wrist. The nerves were subsequently transected 1 cm proximal to the wrist flexion crease and repaired with a single 10-0 nylon suture. A simple epineural suture was positioned centrally in the nerve, as demonstrated in Figure 2. The wrist was gently ranged manually from 30° flexion to 45° extension to monitor suture failure; a goniometer was used to measure range of motion. Gapping of the nerve ends was measured with the wrist in 30° flexion, 15° flexion, neutral, 15° extension, 30° extension, and 45° extension. Structural supports were used to position the wrist, and the position was confirmed by measurement with a goniometer.

Figure 2.

Marking suture placed 1 cm proximal and distal to site of future nerve transection and nerve repair with a single 10-0 nylon suture.

Next, an epineural repair was accomplished with 9-0 nylon suture (Figure 3). Between 8 and 10 interrupted simple sutures were placed in the epineurium, depending on the size of the nerve. With the wrist in a neutral position, the distance between the two 8-0 nylon marking sutures proximal and distal to the nerve repair was measured using a micrometer. This measurement was used as a baseline to calculate percent elongation. The distance between the marking sutures was again measured with the wrist in 30° flexion, 15° flexion, 15° extension, 30° extension, and 45° extension. At each wrist position, the percent elongation was reported as the difference in the distance between marking sutures relative to the baseline measurement made with the wrist in a neutral position. The percent elongation for the median and ulnar nerves was reported as a mean of the 6 samples.

Figure 3.

Repair of nerve with single 10-0 suture and epineural repair with 9-0 suture.

Results

Table 1 shows the average percent elongation for the median and ulnar nerves across the nerve repair in varying degrees of wrist extension and wrist flexion; a range of percent elongation is also provided. Negative values indicate shortening of the nerve. Figure 4 shows a graphical representation of average percent elongation for the median and ulnar nerves from neutral to 45° of wrist extension; range of percent elongation at each wrist position is demonstrated by vertical bars at the respective data points.

Table 1.

Median and Ulnar Nerve Mean Percent Elongation at Various Wrist Positions.

Nerve	45° extension	30° extension	15° extension	Neutral	15° flexion	30° flexion
Median	6.2 (4.6 to 12.1)	4.1 (1.5 to 9.4)	2.6 (0.5 to 5.0)	0	−3.0 (−0.3 to −9.3)	−6.1 (−0.8 to −9.3)
Ulnar	7.3 (3.9 to 11.9)	4.3 (0.6 to 9.8)	2.8 (0.3 to 5.6)	0	−2.5 (−0.3 to −5.8)	−5.4 (−0.6 to −8.9)

Note. Percent elongation reported as a mean of 6 samples; negative values indicate shortening of nerve.

Figure 4.

Average percent elongation for the median and ulnar nerves across the repair from neutral to 45° of wrist extension.

With 9-0 nylon suture maintaining the epineural repair, the mean percent elongation in 30° of wrist extension was 4.1% in the median nerve and 4.3% in the ulnar nerve, both of which fall under the critical level defined by this study (5%). The mean percent elongation in 45° of wrist extension was 6.2% in the median nerve and 7.4% in the ulnar nerve, both of which exceed the critical level defined by this study. On average, shortening of the median and ulnar nerve was seen in both 15° and 30° of wrist flexion.

In all specimens, none of the 10-0 suture ruptured or pulled out when the wrist was gently ranged from 30° of flexion to 45° of extension. From 30° of flexion to neutral wrist position, no gapping was noted between the repaired nerve ends. Within the first 30° of extension, 6 of the 12 repaired nerves demonstrated a gap of 2 to 5 mm between the transected ends. At the terminal point of wrist extension for this study, 45°, all specimens demonstrated a gap of 5 to 10 mm between the repaired nerve ends. This gap was more prominent in ulnar nerve repairs compared with median nerve repairs and at the peripheries of the nerve repair (Figure 5).

Figure 5.

With the wrist in 45° of extension, a gap between the repaired nerve ends was seen (white arrow indicates median nerve, yellow arrow indicates ulnar nerve).

Discussion

In this cadaveric study, we evaluated the tension at median and ulnar nerve repairs at the wrist in varying degrees of wrist flexion and extension. Based on previous studies, we stated that a peripheral nerve repair with a single 10-0 nylon suture successfully holding the repair and less than 5% elongation after nerve and epineural repair would allow for appropriate nerve healing. We found that positioning the wrist in 15° and 30° of extension maintains the nerve repair and keeps nerve elongation below a mean of 5% in both median and ulnar nerves, as we hypothesized. Our findings are suggestive that wrist positioning after peripheral nerve and flexor tendon repair can be modified to slight extension to reduce loss of motion and improve clinical outcomes while maintaining mechanical parameters that will allow for nerve healing. Positioning of the wrist in slight extension would allow early active range of motion of the digits with less load on flexor tendons, which would facilitate increased tendon and nerve gliding, range of motion, and optimal nerve functioning.¹⁰ Early passive range of motion of the wrist could also be initiated within the immobilization orthosis to provide median and ulnar nerve excursion at the wrist without tensioning, thus leading to increased local tissue nutrition, blood flow, nerve conduction, and nerve mobility.¹¹

We did note some variability in our elongation measurements, especially in the ulnar nerve. At 15° of wrist extension, all specimens had median nerve elongation below 5%, and 2 specimens demonstrated ulnar nerve elongation above 5% (5.3%, 5.6%). At 30° of wrist extension, 1 specimen demonstrated median nerve elongation above 5% (9.4%), and 3 specimens demonstrated ulnar nerve elongation above 5% (5.6%, 6.1%, 9.8%). These results indicate that the ulnar nerve may be less tolerant of aggressive wrist positioning after nerve repair in comparison with the median nerve. However, given the small sample size of the study, it is difficult to make this conclusion with certainty.

As a cadaver study, we do not know how the dynamic in vivo environment, postoperative swelling, friction issues from adjacent tendon repairs, or other issues may change the nature of strain on actual clinical nerve repairs. In the clinical setting, traumatic zone 5 lacerations of the median and/or ulnar nerve often result in frayed nerve ends, which require trimming prior to primary repair.¹² Preparation of the nerve ends would increase the gap at the transection site, which could increase tension on the repair and/or yield greater nerve elongation. In our study, positioning the wrist in 15° of extension resulted in no gap in any of the 12 repaired nerves. Positioning the wrist in 30° of extension resulted in a gap of 2 to 5 mm in 6 repaired nerves and no gap in the remaining 6 repaired nerves. As mentioned previously, Sunderland and Brenner⁶ demonstrated that a gap of 6 to 9 mm after repair with a single 10-0 nylon suture will maintain tension below a critical threshold and allow for adequate healing. Slatter studied the repair of traumatic nerve lacerations in an animal model and noted the importance of nerve end debridement prior to surgical anastomosis. He found that the proximal and distal nerve stumps generally need to be debrided by 1 to 2 mm each before proceeding with repair.¹³ Using this information, we can extrapolate that “freshening the ends” of the nerves after a laceration and performing a subsequent primary repair will still allow for wrist positioning in 15° of extension. Positioning the wrist in 30° of extension could push tension near a critical threshold, as some repaired nerves in our study had a gap of 5 mm here. Including a debridement of 2 mm on each stump would theoretically result in a gap of 9 mm, which is the high end of the safe range described by Sunderland and Brenner. Ultimately, any decision on mobilization has to incorporate the specifics of a particular case. If extensive debridement is needed to achieve healthy nerve ends, then it is certainly possible that no early postoperative mobilization is advisable or that the nerve would need to be grafted.

Another factor of significant importance is the change in tensile strength that occurs in nerves during the healing process after repair in vivo. Temple et al studied load to gapping and load to failure in the first 8 weeks after primary nerve repair in an animal model. They found that load to gapping increased significantly from 0 to 1 week. From 1 to 2, 2 to 4, and 4 to 8 weeks, the load to gapping increased but was not significant. At 8 weeks, the nerve repair achieved 63% of the strength of a healthy control.¹⁴ The gapping in our study was mentioned in the context of the single epineurial suture, which was done as an assessment of nerve tension, not as a representation of how a complete nerve repair would be performed. In vivo, we could expect some improvement in tensile strength as nerve regeneration occurs. The repaired nerve may become more tolerant of increasing load; however, the assessment of these longitudinal changes during healing is beyond the scope of this study.

Our study provides encouraging findings that early wrist range of motion within predescribed ranges may allow some flexibility for hand surgeons and therapists in the rehabilitation of wrist-level nerve lacerations.