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Repetitive Strain Injury (RSI) &
Work-Related Upper Limb Disorder (WRULD)
RSI and the MouseBean® Hand Rest by David Tibbs MS, FRCS.
Consulting Surgeon. Oxford.
Repetitive Strain Injury (RSI). A variety of conditions
nowadays are included under the non-specific terms of RSI or WRULD, that is
to say, a painful, and possibly disabling condition in or near
the wrist, and associated with an occupation involving prolonged
or repeated manipulation of the hand or wrist. However, each of
these various states is distinct and has its own particular
occupational associations. In the case of the computer mouse, RSI
is quite often attributed to its use and the Carpal Tunnel
Syndrome is the usual variety found here.
There are many other possible causes for Carpal Tunnel
Syndrome but here we are only concerned with it in relation
to the prolonged use of a computer mouse and its prevention or
relief by a device specifically designed for this purpose. Let us
first consider the ways in which the median nerve becomes
involved to cause this syndrome.
What is Carpal Tunnel Syndrome?
The hand is a miracle of compact engineering. It can grip or
manipulate with power and delicacy and is provided with a nerve
system giving a remarkable array of sensors governing its use and
protection. Its scope of activity is greatly enhanced by the wide
range of wrist movement, giving virtually a universal joint.
The real power in the hand comes from the forearm muscles and
is transmitted by strong tendons to the digits. To permit free
movement at the wrist, these tendons are channelled through a
narrow tunnel, the carpal tunnel, centrally situated at
the base of the hand, and allowing a sliding movement of the
tendons in every position of the wrist. The tunnel is shared by a
large nerve, the median nerve, which is some 5mm across
and responsible for receiving sensation from the thumb and the
first three fingers and also controlling some of the small
muscles in the hand (especially the thumb) which give the fine
adjustment movements. The nerve is nourished by an artery and
vein running along its length. Compression of this nerve and its
blood supply is the basic cause of the syndrome.
[Two other nerves, the ulnar and the radial, are responsible
for supplying the remainder of the hand but these nerves enter it
by different routes and are not prone to the same stresses.]
The carpal tunnel has unyielding walls of bone and is
roofed over by a very tough ligament. It acts as a pulley,
centralising the direction of pull by the tendons in any position
of the wrist. The tendons are lubricated by synovial sheaths
which can become inflamed by excessive use. The nerve and tendons
are uneasy companions in the tunnel. The tunnel is a tight fit
and the median nerve can be compressed when the strong tendons
alongside it are tensed. The tendency to compression of the nerve
is accentuated by use of the wrist in a bent position during
which the limits of tolerance may be reached. In the working
hand, the most favourable position is for the wrist to be
relatively straight, and the least favourable is with the wrist
bent strongly backwards.
Another factor giving the median nerve a hard time is
its central alignment in front of the tendons, giving it an
exposed position on the front surface of the wrist (it is the
first structure to be divided if the wrist is slashed in a
suicide attempt). In this way, the nerve is vulnerable to
external pressure just before it enters the carpal tunnel. This
occurs when the wrist and base of the hand (and weight of the
arm) are rested heavily on a flat surface, especially if the
nerve is made prominent by a backward bending of the wrist. This
happens all too easily when using a mouse and the arm is tiring
from prolonged use.
The Manifestations of Carpal Tunnel Syndrome
The symptoms arise from distress in the median nerve.
In the early stages there is tingling in the thumb and first
three fingers. This is soon replaced by numbness in the same
area. On ceasing to use the mouse, the symptoms will subside in a
few minutes but are a clear warning of compression causing
distress in the median nerve. Unless the method of using the
mouse is altered, the symptoms will occur with increasing
frequency and severity, often accompanied by weakness of the
small muscles at the base of the thumb which can affect its
control of gripping and fine movements. Eventually, use of the
mouse has to be abandoned and recovery usually occurs slowly,
although some loss of sensation and muscle control may persist.
At this stage, surgical operation to open up the tunnel (by
dividing the strong ligament spanning over it) may be the only
way of restoring full use. The condition should not be allowed to
progress this far.
There are considerable variations in the dimensions and
proportions of the wrist structures, and for this reason, some
people are much more likely to sustain median nerve
compression. Nevertheless, all users would be wise to find
ways to ease the stresses on this nerve by any simple device that
spares it from the unfavourable factors described above. The
MouseBean® Hand Rest offers a good solution.
The MouseBean® Hand Rest
The inventor, John Crocker, set out to provide a device with
the following features:
- It would support the hand and weight of the arm on a sliding
device fully able to move with the mouse.
- The surface on which the base of the hand rested was to be
well cushioned but with a space between the cushions to spare the
line of the vulnerable median nerve from any external
- The hand would be supported with the wrist in a near-straight
- Finger movement for fine adjustment of the mouse position, or
when pressing its buttons, should be easily possible without
raising the hand, and, when required, wide movement of the mouse
is easily permitted by sliding the device over the mouse
The final version was developed from a series of devices and
was christened the MouseBean® Hand Rest. It has been tried by
a number of full-time mouse users, including some who were known
to be prone to carpal tunnel syndrome.