Ulnar nerve compressive injury due to cubital tunnel syndrome is very common. Because of
the long distance to the target muscles in the hand, functional outcome in severe cases
even with decompression surgery is often poor. Therefore, alternative treatment options
are much needed. Recently, anterior interosseous nerve reverse end to side (RETS)
transfer to the ulnar nerve above the wrist has gained popularity. However, whether a
substantial portion of motor axons in the donor nerve are indeed capable of breaching the
connective tissues in the ulnar nerve to reach the target muscles in the hand remains
untested. To answer this crucial question, in this study the investigators plan to
recruit 60 cubital tunnel syndrome patients with marked motor axonal loss who will
undergo the RETS procedure. Motor unit number estimation will be done on the ulnar and
anterior interosseous nerves at baseline and repeated at 3 and 6 months post operatively.
Hand motor function and disability scores will also be tested at the same time points.
Given the importance of this critical question and the potential utilities of distal
nerve transfers, this should be a worthwhile effort.

AIN Transfer for Cubital Tunnel Syndrome

Nerve Injury

Surgery

Introduction Peripheral nerve trauma especially through compressive injury is very
common. Of these, ulnar neuropathy at the elbow is the second most prevalent. In severe
cases, functional outcomes even with surgery are often poor. Principle reasons being that
the ulnar nerve innervates a large number of intrinsic hand muscles. At an average of 350
mm from the site of injury to the target muscles, the distance is very long, Although
peripheral nerve fibres have the ability to regenerate, it is very slow at only 1 mm/day.
That would translate to a year for the regenerating fibres to reach the hand. With the
favorable condition in the distal stump and target muscles to regrowth rapidly declining
over time, few axons will successfully reach target. Therefore, alternative methods of
surgical treatments are being sought to circumvent this major hurdle.

Rationale and purpose One potentially promising alternative is through distal nerve
transfer by using a terminal branch of the anterior interosseous nerve (AIN) to the
pronator quadratus muscle and coapt it to the ulnar nerve close to the wrist. That would
greatly shorten the distance to target by 200 mm or more. By approximating the end of the
AIN donor stump to the side of the ulnar nerve through an eipneurial window (termed
reverse end to side or RETS) transfer, it allows preservation of the integrity of the
ulnar nerve fibres thus providing a conduit for the regenerating native axons to reach
the hand. While this concept is theoretically appealing and has gained increasing
popularity in clinical practice, a crucial question of central importance remains
answered: Can the AIN donor axons indeed breach the ulnar nerve trunk and reinnervate the
hand muscles? Aside from testing the central premise of RETS, it would be important to
know the exact magnitude of reinnervation that can be achieved from the donor nerve
through this procedure.

With the above rationale in mind, the objective of this proposal is to test the
hypothesis that reinnervation of ulnar intrinsic hand muscles by the AIN can be achieved
through the RETS procedure. This will be done through quantifying the amount of
innervation by the AIN donor nerve to the ulnar intrinsic hand muscles using motor unit
number estimation, a non-invasive electrophysiological technique. The investigators will
also evaluate whether there is an associated improvement in hand function following the
procedure.

Experimental protocol Subjects For this study, the investigators aim to recruit a
prospective series of 60 patients with severe cubital tunnel syndrome causing marked
motor axonal loss.

Primary outcome measures Motor unit number estimations of native axons in the ulnar nerve
innervation the intrinsic hand muscles compared to those from the AIN.

Secondary outcomes

Motor function:

i) Pinch strength using a pinch gauge ii) Grip strength using a Jamar dynamometer iii)
Disabilities of Arm, Hand and Shoulder (DASH) Questionnaire

Procedures and follow up All outcomes will be measured at baseline and repeated at 3 and
6 months post op. Motor unit number estimation will be done using the multiple point
stimulation technique. To briefly summarize, recording surface electrodes will be placed
over the belly of the hypothenar muscles. To estimate the number of native ulnar axons
innverating the hypothenar hand muscles, supramaximal stimulation of the ulnar nerve in
the mid forearm above the site of AIN coaptation will be done. A sample of single motor
unit action potentials will be obtained by discretely stimulating individual motor axons
using finely graded stimuli. A motor unit number estimation will be calculated from the
ratio of the compound muscle action potential amplitude and the average single motor unit
action potentials. To estimate the motor unit innervation from the AIN, the same
procedure will be done by stimulating the median nerve while recording from the
hypothenar muscles.

Surgery Eligible patients will undergo surgical decompression of the ulnar nerve at the
elbow. Additionally, a reverse end-to-side transfer to the ulnar nerve will be done using
a terminal branch of the AIN to the pronator quadratus muscle. It will be coapted to the
side of the motor fascicle of the ulnar nerve just above the wrist through a 5 mm
epineurial window.

Statistical analysis To address the central question of AIN innervation, descriptive
statistics (mean and sd) will be used to quantify the extent of nerve growth through the
epineurial window created by the RETS procedure. Since there is no AIN innervation of the
hypothenar muscles at baseline, growth of motor axons through the epineurial window
should be easy to discern through motor unit number estimation. Changes in motor hand
strength and disability following surgery will be compared using repaired measure one way
ANOVA with time as the independent factor. With this being a pilot exploratory study,
further analytic statistical analysis would not be justified as the sample size will be
too small to allow for any meaningful interpretation.

Expected outcomes and significance This will be further aided by the very substantial
distance difference between the entry point of the AIN to the ulnar nerve trunk at just
above the wrist and the native ulnar motor axons that regenerate from the elbow. At less
than 100 mm from the target muscles, many AIN motor axons should reach the hypothenar
muscles by 3 months and all by 6 months. In contrast, the regenerating native ulnar motor
axons would likely not reach the hand muscles for a year or more, as shown in the
investigators' previous publications.

Given the importance of this critical question and the potential utilities of distal
nerve transfers, this should be a worthwhile effort.

nerve transfer

Efficacy of End-to-Side Nerve Transfer for Patients With Severe Cubital Tunnel Syndrome

Amplitudes of the maximum compound muscle action potentials evoked by median nerve
stimulation

Dynamometer for isometric force quantification of pinch strength between the 1st and 2nd
digits

AIN Transfer for Cubital Tunnel Syndrome

About this trial

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