Abstract
Five fresh-frozen transthoracic cadaver specimens (ten upper extremities) were dissected in order to measure excursion and strain of the median nerve at the wrist and the elbow in association with different positions of the shoulder, elbow, wrist, and fingers. The relationships between motions of the joints and excursion and strain at the wrist and the elbow as well as between excursion and strain were also determined.When the wrist was moved from 60 degrees of extension to 65 degrees of flexion, the median nerve at the wrist underwent a mean total excursion of 19.6 millimeters (9.2 ± 1.38 millimeters [mean and standard deviation] in extension and 10.4 ± 1.50 millimeters in flexion). The change in strain was not measurable secondary to nerve-wrinkling. Motion of the fingers from hyperextension to full flexion caused a mean total excursion of 9.7 millimeters (6.3 ± 1.31 millimeters in hyperextension and 3.4 ± 1.04 millimeters in flexion) and a change in strain of 19.0 per cent. This change in strain became more pronounced when the wrist and the fingers moved in synergy. Such strain, if constant, has been shown to decrease microvascular perfusion and nerve conduction in rabbits.With motion of the shoulder, the mean total excursion of the median nerve at the elbow was 9.1 millimeters (4.4 ± 1.64 millimeters in abduction and 4.7 ± 1.81 millimeters in adduction) and the change in strain was 13.3 per cent. With motion of the elbow, the mean total excursion was 12.3 millimeters (12.3 ± 4.20 millimeters in flexion and zero millimeters in the resting position) and the change in strain was not measurable because of the wrinkle effect in flexion. With motion of the wrist, the mean total excursion was 5.6 millimeters (4.3 ± 1.95 millimeters in extension and 1.25 ± 0.81 millimeters in flexion) and the change in strain was 14.8 per cent. With motion of the fingers, the mean total excursion was 3.4 millimeters (2.6 ± 1.13 millimeters in hyperextension and 0.80 ± 0.92 millimeter in flexion) and the change in strain was 10.3 per cent.Motion of the wrist and the fingers induced profound excursion of the median nerve at the wrist, whereas motion of the shoulder and the elbow induced marked excursion of the median nerve at the elbow. These excursions were much more pronounced when a number of joints were moved in a composite manner. Any factor that might limit normal excursion, such as scarring, could induce strains that could adversely affect the function of the median nerve.The finding that the position and movement of the limb had a profound effect on excursion and strain of the median nerve at the wrist and the elbow may aid in the understanding of the pathophysiology of median-nerve neuropathy.
Movement of nerves in association with movement of the upper extremity has been described previously8,10,14. Additionally, the deleterious effects of strain on blood flow to nerves1,6,11, neurological function as noted by conduction velocity13, and the histological characteristics of nerves12 have been reported. To our knowledge, however, no one has performed a unified quantitative assessment of motion and concomitant strain of the median nerve as a function of both motion and the position of the shoulder, wrist, elbow, and fingers. Without an understanding of the normal relationship between excursion and strain of the median nerve, it is difficult to ascertain what occurs in the pathological condition when, for example, the median nerve is heavily entrapped in scar tissue, is repaired, or is otherwise tethered. As far as we know, it has not been demonstrated whether, in addition to direct compression, limitation of excursion of the median nerve leading to increased strain is a potential mechanism for the production of peripheral neuropathy.
The purpose of the current study was to establish the relationship between excursion and strain of the median nerve at the wrist and the elbow with movement of the shoulder, elbow, wrist, and fingers.
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. Funds were received in total or partial support of the research or clinical study presented in this article. The funding sources were a Division of Sponsored Research grant from the University of Florida and the University of Florida Tissue Bank.
†University of Florida, P.O. Box 100246, Gainesville, Florida 32610. E-mail address for Dr. Wright: thomas-wright@ufl.edu.
‡Division of Orthopaedics, Oregon Health Science University, 3181 S.W. Sam Jackson Park Road, OP13B, Portland, Oregon 97201-3098.
Five fresh-frozen transthoracic cadaver specimens were obtained and stored at -18 degrees Celsius until use. Transthoracic specimens were used in order to preserve the normal nerve-root attachment at the spinal cord. A total of ten upper extremities (five pairs) were evaluated. Immediately before testing, the specimen was thawed at room temperature and was mounted to a flat platform with a hinge at the level of the shoulder to facilitate planar abduction and adduction of the glenohumeral joint. With care being taken to center the glenohumeral joint at the center of rotation of the fixture, the humerus was fixed rigidly to the carrier's appendage with a Steinmann pin placed proximally at the center of rotation of the shoulder and a second pin placed in the mid-portion of the humerus to finalize stabilization of the segment. The thoracic spine was fixed to the main body of the carrier. In this manner, abduction and adduction of the glenohumeral joint could be produced in the plane of the body by moving the articulated carrier's appendage.
The forearm was fixed in 30 degrees of supination with a pin across the radius and the ulna. The median nerve was exposed with minimum disruption of the surrounding soft tissues at the flexion crease of the elbow and that of the wrist, just proximal to the carpal tunnel.
A digital vernier caliper was modified to include a laser marker. This system was fixed to the skeleton with a Steinmann pin and an articulated positioning fixture allowing three degrees of freedom. The system was attached to the radius and the humerus for measurement of excursion of the median nerve at the wrist and the elbow, respectively. Measurements at both the elbow and the wrist were performed at the same setting but not simultaneously. The axis of the caliper was carefully positioned to track along the local line of the nerve (Fig. 1). A reference microsuture marker was placed in the median nerve, and the caliper with the attached laser was positioned under loupe magnification so that the laser illuminated the marker. Relative motion of the nerve was measured under loupe magnification by traversing the caliper until the marker was reilluminated by the laser.
The arbitrary initial skeletal resting position was with the neck in neutral; the face oriented straight forward; the shoulder in 90 degrees of abduction; the elbow in 10 degrees of flexion; the forearm in 30 degrees of supination; the wrist in neutral; and the metacarpophalangeal, proximal interphalangeal, and distal interphalangeal joints all at 0 degrees. The resting angle of the elbow was held by support in 10 degrees of flexion. During testing, pronation-supination of the forearm was maintained with use of crossed Kirschner wires, and the position of the wrist and the metacarpophalangeal, proximal interphalangeal, and distal interphalangeal joints was maintained with use of exchangeable plastic splints.
Excursion and strain of the median nerve were measured first at the elbow and then at the wrist for each isolated motion of the joint. All such movements began at the resting position and were measured with a goniometer. The position of the nerve was measured three times in all specimens. Excursion and strain were measured during the following sequence of movements.
1. The shoulder was moved from the resting position to 110 degrees of abduction and was returned to the resting position, with care being taken to maintain all other joints in their respective resting positions.
2. The shoulder was adducted from the resting position to 30 degrees of abduction and was returned to the resting position.
3. The elbow was moved from the resting position to 90 degrees of flexion and was returned to the resting position.
4. The wrist was moved from the resting position to 65 degrees of flexion and was then moved from the resting position to 60 degrees of extension.
5. The wrist, starting at the resting position, was radially deviated 15 degrees and moved back to the resting position; it was then ulnarly deviated 30 degrees.
6. The forearm was pronated to 60 degrees, was returned to the resting position, and was then supinated to 70 degrees.
7. The fingers were extended from the resting position to 35 degrees of hyperextension of the metacarpophalangeal joints (with the proximal and distal interphalangeal joints in 0 degrees); they then were returned to the 0-degree resting position of the metacarpophalangeal joints and were flexed fully (90 degrees of flexion of the metacarpophalangeal and proximal interphalangeal joints and 70 degrees of flexion of the distal interphalangeal joints).
Additional data on excursion and strain were obtained at the wrist and the elbow with use of the combinations of joint motions that previously resulted in the greatest amount of distal excursion as one extreme and the greatest amount of proximal excursion as the other extreme.
Strain of the median nerve, at both the wrist and the elbow, was measured with use of a Microstrain DVRT displacement gauge (Microstrain, Burlington, Vermont). This gauge was calibrated to record per cent elongation (strain) and was fixed to the median nerve, with the joints in the resting position, with use of two sharp pegs (Fig. 2). The instantaneous strain was recorded on a strip-chart recorder (model 1200; Linear Instruments, Irvine, California) and was verified with a digital voltmeter (model 73 multimeter; Fluke Manufacturing, Everett, Washington).
Each value used in the statistical calculations represents a mean of the three measurements. Pearson correlations were used to determine the relationship between the magnitudes of the excursion and strain of the median nerve. A Student t test was used to demonstrate differences between excursion at the wrist and that at the elbow and between strain at the wrist and that at the elbow. A chi-square test was used to determine the relationship between the direction of excursion (proximal or distal) at the wrist and that at the elbow. Chi-square tests were also used to evaluate the relationship between the direction of excursion and the direction of strain (increasing tension or decreasing tension) at the wrist and the elbow. SAS statistical software (SAS/Windows, version 6.07; SAS Institute, Cary, North Carolina) was used for all analyses, with the level of significance set at p = 0.05.
Intraobserver reliability was high for measurements of both excursion and strain, with Pearson correlation coefficients between 0.95 and 0.99 (on the basis of three trials). Interobserver variability was low for measurements of both excursion and strain, with no significant differences noted between measurements made by two observers (p > 0.05).
Excursion
A positive measurement for excursion refers to the nerve moving in a distal direction, and a negative measurement indicates proximal motion.
Wrist
With motion of the wrist, the mean total excursion of the median nerve at the wrist was 19.6 millimeters (-10.4 ± 1.50 millimeters with the wrist flexed to 65 degrees and 9.2 ± 1.38 millimeters with the wrist extended to 60 degrees). Motion of the fingers resulted in a mean total excursion of 9.7 millimeters (6.3 ± 1.31 millimeters with the fingers hyperextended to 35 degrees and -3.4 ± 1.04 millimeters with the fingers fully flexed). Smaller amounts of excursion were noted with other movements of the limb (Table I).
The composite position of the upper limb that elicited the greatest amount of distal excursion of the median nerve at the wrist, 23.8 ± 7.49 millimeters, was the shoulder adducted to 30 degrees of abduction, the elbow flexed to 90 degrees, the wrist extended to 60 degrees, the fingers hyperextended to 35 degrees, and the forearm pronated to 60 degrees. The composite position of the upper limb that created the greatest amount of proximal excursion of the median nerve at the wrist, -11.6 ± 3.37 millimeters, was the shoulder abducted to 110 degrees, the elbow in the resting position, the wrist flexed to 65 degrees and ulnarly deviated to 30 degrees, and the fingers fully flexed. Thus, the total composite excursion of the median nerve at the wrist averaged 35.4 millimeters.
Elbow
With motion of the shoulder, the mean total excursion of the median nerve at the elbow was 9.1 millimeters (-4.4 ± 1.64 millimeters with the shoulder abducted to 110 degrees and 4.7 ± 1.81 millimeters with the shoulder adducted to 30 degrees). With motion of the elbow, the mean total excursion was 12.3 millimeters (-12.3 ± 4.20 millimeters with the elbow flexed to 90 degrees and zero millimeters with the elbow in the resting position). With motion of the wrist, the mean total excursion was 5.6 millimeters (4.3 ± 1.95 millimeters with the wrist extended to 60 degrees and -1.25 ± 0.81 millimeters with the wrist flexed to 65 degrees). With motion of the fingers, the mean total excursion was 3.4 millimeters (2.6 ± 1.13 millimeters with the fingers hyperextended to 35 degrees and -0.80 ± 0.92 millimeter with the fingers fully flexed).
The composite position that created the greatest amount of distal excursion of the median nerve at the elbow, 14.7 ± 7.78 millimeters, was the shoulder in 30 degrees of abduction, the elbow in the resting position, the wrist in 60 degrees of extension, the fingers in 35 degrees of hyperextension, and the forearm in 60 degrees of pronation. The greatest amount of proximal excursion, -15.4 ± 8.80 millimeters, occurred when the shoulder was abducted to 110 degrees, the elbow was flexed to 90 degrees, the wrist was flexed to 65 degrees, and the fingers were fully flexed. Thus, the total composite excursion for the median nerve at the elbow averaged 30.1 millimeters.
Strain
Negative values represent decreases in strain and positive values, increases, compared with the amount of strain in the resting position. Strain values derived for a wrinkled nerve (one buckled by compression) are not meaningful and therefore are not reported. The wrinkle effect occurred consistently in the median nerve at the wrist during flexion of the wrist and in the median nerve at the elbow during flexion of the elbow.
Wrist
Major changes in strain for the median nerve at the wrist were 3.7 ± 0.06 per cent with the shoulder abducted to 110 degrees, -3.8 ± 0.04 per cent with the shoulder adducted to 30 degrees, -3.7 ± 0.05 per cent with the elbow flexed to 90 degrees, 9.6 ± 0.08 per cent with the wrist extended to 60 degrees, 11.0 ± 0.12 per cent with the fingers hyperextended to 35 degrees, and -8.0 ± 0.10 per cent with the fingers fully flexed. The mean total changes in strain over the entire range of motion of each joint were 7.5 per cent with motion of the shoulder, 3.7 per cent with motion of the elbow, more than 9.6 per cent with motion of the wrist, and 19.0 per cent with motion of the fingers. Other movements of the extremity caused smaller changes in strain or resulted in buckling of the nerve and were not measurable (Table I).
When composite positioning was performed, as described previously, the amount of strain in the position that caused less tension was not measurable because of nerve-wrinkling; however, the amount of strain in the position that caused increased tension (adduction of the shoulder, flexion of the elbow, extension of the wrist, hyperextension of the fingers, and pronation of the forearm) was 18 per cent.
Elbow
Strain values of greater than 4 per cent were noted with many positions, including the shoulder abducted to 110 degrees (9.1 ± 0.09 per cent), the shoulder adducted to 30 degrees (-4.2 ± 0.08 per cent), the wrist extended to 60 degrees (7.4 ± 0.10 per cent), the wrist flexed to 65 degrees (-7.4 ± 0.11 per cent), the fingers hyperextended to 35 degrees (6.2 ± 0.08 per cent), and the fingers fully flexed (-4.1 ± 0.08 per cent). The mean total changes in strain over the entire range of motion of each joint were 13.3 per cent with motion of the shoulder, 14.8 per cent with motion of the wrist, and 10.3 per cent with motion of the fingers (Table I).
Relationships Involving Excursion and Strain at the Wrist and the Elbow
Analysis of the relationship between the magnitudes of excursion and strain yielded two significant relationships, both involving the wrist. First, the composite position consisting of 30 degrees of abduction of the shoulder, 90 degrees of flexion of the elbow, 60 degrees of extension of the wrist, 35 degrees of hyperextension of the fingers, and 60 degrees of pronation of the forearm significantly increased tension in the nerve as well as distal excursion (p < 0.05). The second significant finding was that extension of the wrist significantly increased both tension in and distal excursion of the median nerve (p < 0.05).
When the direction of excursion of the median nerve at the elbow and the wrist was evaluated, it was noted that opposite movements of a joint (flexion and extension) caused significant, opposite excursions at both sites (p < 0.05). For example, extension of the wrist caused distal movement of the nerve at the wrist and the elbow, and flexion of the wrist caused proximal movement at the wrist and the elbow (Table II). Movements proximal (involving the shoulder) or distal (involving the wrist and the fingers) to the sites of measurement resulted in a predictable direction of excursion. Flexion of the elbow and pronation and supination of the forearm, which occur between the sites of measurement, did not behave in a predictable manner; the movement that transpired between the two sites resulted in minimum or opposite directed excursions at the elbow and the wrist.
When the relationship between the direction of the strain (increased or decreased tension) and the direction of the excursion were evaluated at the wrist (Table II), there were a number of significant findings involving movements of the distal joints (flexion and extension of the fingers and the wrist) as well as of the proximal joints (abduction of the shoulder). Similar but less pronounced activity occurred at the elbow (Table II).
There were numerous significant relationships (p < 0.05) between the directions of strain at the wrist and the elbow (Table II). It appears that strain is propagated in the same direction throughout the length of the nerve when the movement occurs proximal or distal to the two measured sites.
A number of movements created a similar change in the magnitude and the direction of strain at both the wrist and the elbow. These movements included flexion of the fingers, abduction of the shoulder to 110 degrees, and adduction of the shoulder to 30 degrees of abduction.
To evaluate the consequences of the use of paired specimens, we used a t test to determine right-left differences. With regard to excursion, only three of the many combinations of movements studied showed differences. With regard to strain, however, almost all of the movements were associated with right-left differences. Thus, the power of our observation is probably overstated with regard to excursion but appropriate with regard to strain.
It has long been believed that carpal tunnel syndrome is predominantly a compressive neuropathy involving the median nerve2,7. However, synovitis or other compressive structures in the carpal tunnel are seldom seen in patients who have idiopathic carpal tunnel syndrome. Little attention has been focused on the possible contribution of biomechanical effects, such as motion or gliding of the nerve or stretching associated with routine movement of the joints of the upper extremity, to the pathogenesis of peripheral neuropathies.
The relationship between tension of the nerve and blood flow to the nerve was well delineated by Clark et al. In a study of the sciatic nerves of rats, they found that an 8 per cent strain was associated with a 50 per cent diminution of blood flow and that a 15 per cent elongation reduced the blood flow to the nerve by approximately 80 per cent. Ogata and Naito, using a hydrogen washout technique in the sciatic nerves of rabbits, demonstrated that a 15 per cent strain resulted in complete arrest of blood flow. These observations were further supported by the work of Lundborg and Rydevik6. Wall et al. also showed that nerve conduction was adversely affected by increasing nerve strain; a 6 per cent strain for more than one hour resulted in a 70 per cent decrease in nerve conduction velocity, and a 12 per cent strain blocked nerve conduction completely. When these strains were removed, the nerve conduction returned after a period of recovery.
The results of the present study indicate that the median nerve at the wrist and the elbow frequently undergoes strains of approximately 10 per cent or greater, approaching some of the deleterious levels just discussed. The position of 30 degrees of abduction of the shoulder with flexion of the elbow, pronation of the forearm, and extension of the wrist and the fingers is the common position of a keyboard operator and results in a mean of fifteen millimeters of distal excursion of the median nerve with a tensile strain of 18 per cent at the wrist. This suggests the possibility that strain may contribute to decreased nerve conduction and blood flow, particularly in the repetitive-motion disorders. Although these positions are usually held for only brief periods of time, cumulative damage may result in a nerve injury. Thus, external compression may work in synergy with traction, with both potentiating injury to the median nerve.
Nerve traction may help to explain why some patients with carpal tunnel syndrome who have had an appropriate carpal tunnel release often have a recurrence of the symptoms when they return to the same type of work that initiated the problem. These patients may be more prone to have a recurrence postoperatively if normal excursion of the median nerve is prevented by postoperative scarring around the nerve. This neurodesis effect may cause ongoing injury to the nerve with normal movement of the wrist and the fingers by tethering, which could increase strain distal to the tether. This is a concern when any nerve undergoes a neurolysis or is repaired. A particular problem with a repaired nerve is that early mobilization is not an option. The tethering effects of peripheral nerves of the upper extremity have been demonstrated clinically with the upper-limb tension test described by Kenneally et al. and the median-nerve stress test reported by Marin et al. and by LaBan et al.4,5.
Our results indicate that, in a normal individual, the median nerve glides proximally and distally with movement of the upper extremity. With flexion and extension of the wrist alone (from 60 degrees of extension to 65 degrees of flexion), the median nerve at the wrist moved a mean total of 19.6 millimeters. This finding is consistent with that of Wilgis and Murphy, who reported a mean of 15.5 millimeters of excursion of the median nerve in association with flexion and extension of the wrist. Similarly, in the current study, flexion and extension of the fingers created motion of the median nerve of 9.7 millimeters just proximal to the carpal tunnel.
When the effects of several changes in the positions of the joints were combined, there was a mean total of 35.4 millimeters of excursion of the median nerve at the wrist. It is interesting to note that the mean total composite excursion at the wrist was nearly the same as that at the elbow. McLellan and Swash recorded excursion of the nerve with use of electromyographic needle electrodes in live subjects; however, other than the fact that the direction of nerve movement in their study was similar to that in the current study, the data are not comparable because the combinations of movements of the upper extremity were different.
Movement of the upper extremity proximal to the elbow and distal to the wrist affects excursion and strain of the median nerve at the elbow and the wrist in a predictable manner; for example, abduction of the shoulder to 110 degrees pulls the nerve proximally, increasing tension, whereas extension of the wrist pulls the nerve distally, also increasing tension.
It can readily be seen that, with full motion of all joints of the upper extremity, the median nerve requires more than thirty millimeters of unimpeded motion at both the elbow and the wrist. If this normal excursion is impeded, a neurodesis effect and increased strain occur across the portion of tethered nerve. This increase in strain may cause direct mechanical damage to the nerve, or it may cause injury as a result of ischemia, which probably occurs when a nerve undergoes strain of more than 10 per cent.
The current study had several shortcomings. Measurement of strains required dissection about the nerve in order to apply the strain-gauge. Although the study was performed with use of fresh-frozen cadaver specimens and attempts were made to keep the tissues moist, some properties of the nerve and the surrounding soft tissues may differ from the in vivo situation. Nerve-wrinkling also posed a problem with regard to measurement of strain in that accurate assessment of changes in length under this circumstance was not possible. Another limitation was that data were not obtained in a continuous fashion for all attainable positions but only at specific positions of the joint. Finally, five paired cadaver specimens were used rather than ten different specimens.
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