SCIENTIFIC ARTICLE
Evidence of Wrist Proprioceptive Reflexes Elicited After
Stimulation of the Scapholunate Interosseous Ligament
Elisabet Hagert, MD, PhD, Jonas K. E. Persson, MD, PhD, Michael Werner, MD, Björn-Ove Ljung, MD, PhD
Purpose Recent publications on the sensory innervation of wrist ligaments have challenged our
understanding of ligaments as mere passive restraints in wrist stability. Mechanoreceptors in
ligaments have a role in signaling joint perturbations, in which the afferent information is
believed to influence periarticular muscles. The scapholunate interosseous ligament is one of the
most richly innervated ligaments in the wrist. The purpose of our study was to investigate the possible existence of a wrist proprioceptive reflex, by which afferent information
elicited in the scapholunate interosseous ligament was hypothesized to influence the muscles
moving the wrist joint.
Methods Nine volunteers (4 women and 5 men; mean age, 26 years; range, 21–28 years)
participated in this study. Using ultrasound guidance, a fine-wire electrode was inserted into
the dorsal scapholunate interosseous ligament and stimulated with four 1-ms pulses at 200
Hz. Electromyographic activities in extensor carpi radialis brevis, extensor carpi ulnaris,
flexor carpi radialis, and flexor carpi ulnaris muscles were recorded using surface electrodes
with the wrist actively positioned in isometric extension, flexion, and radial and ulnar
deviation. The average EMGs from 30 consecutive stimulations were rectified and analyzed
using the Student’s t-test to compare the prestimulus (t1) and poststimulus (t2) EMG
activities.
Results Statistically significant changes in poststimulus EMG activity (t1– t2) were observed
at various time intervals. Within 20 ms, an excitation was seen in the flexor carpi radialis and
flexor carpi ulnaris in extension, radial and ulnar deviation, and in extensor carpi radialis
brevis in flexion. Co-contractions between agonist and antagonist muscles were observed,
with peaks around 150 ms after stimulus.
Conclusions We present evidence of wrist ligamento-muscular reactions. The early-onset reactions
may serve in a joint-protective manner, and later co-contractions indicate a supraspinal control of
wrist neuromuscular stability. These findings contribute new information to the physiologic
functions of the wrist joint, which may further our understanding of dynamic wrist stability and
serve as a foundation for future studies on proprioceptive dysfunctions after wrist ligament
injuries. (J Hand Surg 2009;34A:642–651. Copyright © 2009 by the American Society for
Surgery of the Hand. All rights reserved.)
Key words Ligament, ligamento-muscular, neuromuscular, proprioception, wrist.
From the Karolinska Institute, Department of Clinical Science and Education, Section of Hand Surgery,
Södersjukhuset, Stockholm, Sweden; Karolinska Institute, Department of Clinical Neuroscience, Section
of Neurophysiology, Stockholm, Sweden; Department of Radiology, Södersjukhuset, Stockholm, Sweden; and University of Gothenburg, Sahlgrenska Academy, Institute of Clinical Sciences, Department of
Hand Surgery, Göteborg, Sweden.
Received for publication September 11, 2008; accepted in revised form December 2, 2008.
The authors wish to express their sincere gratitude to Hans Pettersson, PhD, institute statistician at
Karolinska Institute, Department of Clinical Science and Education, Södersjukhuset, for his invaluable
advice in the statistical analysis of this material. We also wish to thank radiology nurse Tina Norström
for her superb practical assistance in the ultrasound laboratory.
642 䉬 © ASSH 䉬 Published by Elsevier, Inc. All rights reserved.
Supported by Karolinska Institute and by the Department of Hand Surgery, Södersjukhuset, Stockholm, Sweden.
No benefits in any form have been received or will be received related directly or indirectly to the
subject of this article.
Corresponding author: Elisabet Hagert, MD, PhD, Karolinska Institute, Department of Hand Surgery, Södersjukhuset, 118 83 Stockholm, Sweden; e-mail: elisabet.hagert@ki.se.
0363-5023/09/34A04-0007$36.00/0
doi:10.1016/j.jhsa.2008.12.001
EVIDENCE OF WRIST PROPRIOCEPTIVE REFLEXES
T
of the wrist joint, an equilibrium of both static and dynamic functions
must be maintained. Whereas static stability is
determined by the articular congruency and ligamentous restraints of the wrist, dynamic stability is determined by the neuromuscular and proprioceptive influences on the wrist joint.1 The term proprioception was
first described by Sir Charles Scott Sherrington in 1906
as sensations arising in the deep areas of the body,
contributing to conscious sensations, postural equilibrium, and joint stability.2 Since then, the field of proprioceptive research has come to entail any and all of
these sensory modalities. To promote the research field
concerning joint stability, a more precise terminology,
sensorimotor function,3 has been advocated. This term
implies the total integration of sensory, motor, and
central functions in processes pertaining to joint homeostasis.
Our understanding of the role of ligaments in joint
stability has evolved during the past century from that
of passively stabilizing to sensory dynamic structures.4,5 Whereas thorough research into the innervation
of ligaments and their contributions to joint equilibrium
has been performed on the knee, ankle, spine, shoulder,
and elbow joints,4,6 –9 the sensorimotor functions of the
wrist are still largely unknown.
Recent investigations have mapped the mechanoreceptor populations in wrist ligaments, and distinct variations with regard to receptor density and ligament structure were found, signifying differential roles in functional
joint stability.10,11 One of the richest innervated ligaments
is the scapholunate interosseous ligament, which is recognized as a key structure for maintaining carpal stability.12
The dorsal region of the scapholunate interosseous ligament, in particular, has properties enabling the resistance of extreme loads,13 as well as an abundance
of sensory nerve endings.10,14
Because the scapholunate interosseous ligament is
consistently regarded as a biomechanically important
ligament, as well as recognized as a richly innervated
ligament, we hypothesized that it has a role in the
proprioceptive control of the wrist. The purpose of our
study was to investigate the possible existence of wrist
ligamento-muscular reflexes, in which sensory information from the scapholunate interosseous ligament is
assumed to influence the forearm muscles associated
with wrist motion.
O ACHIEVE STABILITY
MATERIALS AND METHODS
Subjects
The design of this study was approved by the regional
ethical review board. Nine healthy volunteers, 4 women
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and 5 men (mean age, 26 years; range, 21–28 years)
were included, and all subjects gave informed consent
to participate in the study. The participants were all
healthy, with no reported history of wrist trauma and no
clinical signs of wrist dysfunction upon examination by
a specialist hand surgeon. The dominant hand was
examined in all subjects.
Ultrasound techniques
An Acuson Sequoia 512 (Siemens Medical Solutions Inc.,
Malvern, PA) with a transverse 8-MHz transducer was
used. A fine, stainless steel wire electrode (model
1512A-M; Life-Tech Inc., Stafford, TX) with a distal,
uninsulated hook was preinserted in a 0.5-mm puncture
needle. Using sterile conditions, we identified the dorsal
scapholunate interosseous ligament in a transverse-dorsal
view between the dorsal surfaces of the scaphoid and
lunate, as previously described,15,16 and the puncture needle subsequently advanced into the ligament. As the needle was retracted, the hook of the electrode remained fixed
in the dorsal scapholunate interosseous ligament. Ultrasound imaging confirming correct needle position was
obtained from all subjects (Fig. 1).
Ultrasound was additionally used to identify the extensor carpi radialis brevis (ECRB), extensor carpi ulnaris (ECU), flexor carpi radialis (FCR), and flexor
carpi ulnaris (FCU) muscles. The EMG surface electrodes were correctly positioned according to Perotto
and Delagi17 at the center of each muscle belly.
EMG recordings
EMG signals were recorded by Ag-AgCl surface electrodes (NE-113A; Nihon Kohden Co., Tokyo, Japan;
diameter, 10 mm; interelectrode distance, 20 mm). The
reference electrode was placed on the dorsum of the
hand, just distally to the insertion point of the stimulating electrode located in the ligament. The ground surface electrode (Rec. No. 019-4009100; Nicolet Biomedical, Madison, WI) was placed between the
stimulating and the surface electrodes, which were then
connected to a 4-channel EMG (KeyPoint, v5.09;
Medtronic, Copenhagen, Denmark). Visual monitoring
of EMG signals on the oscilloscope while the subject
voluntarily contracted each respective forearm muscle
was used to check the accuracy of electrode placement.
Experimental protocol
The subject was seated in an examination chair with the
arm placed in an armrest. The position of the arm was
20° shoulder abduction, 80° elbow flexion, and pronated forearm. During the experiment, the wrist was actively positioned in an average of 40° wrist extension,
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FIGURE 1: A A transverse-dorsal ultrasound image of the dorsal surfaces of the scaphoid (SCA) and lunate (LUN). The inserted
needle point is visualized (arrows) in the scapholunate interval, positioned in the dorsal region of the scapholunate interosseous
ligament. B Same image colorized to facilitate the visualization of the scaphoid (blue), lunate (green), and dorsal scapholunate
interosseous ligament (purple). The needle point is graphically illustrated in the ligament (blue arrow).
20° flexion, 20° radial deviation, and 30° ulnar deviation, respectively. The subject performed isometric contraction, using submaximal force, in each wrist position,
while the motion was resisted manually by one of the
examiners.
The electrical stimulus consisted of four 1-ms pulses
delivered at 200 Hz by the fine-wire electrode placed in
the ligament. The individual threshold for consciously
sensing the stimulus was determined and the test stimulus set at 2 to 3 times this sensory threshold (mean, 2.0
mA; range, 1.2–5.0 mA) yet consistently below the
nociceptive threshold. The scapholunate interosseous ligament was subsequently stimulated with 30 consecutive
stimulations at 2-second intervals. The raw EMG signals
were amplified, band-pass filtered, and digitally sampled from 100 ms before to 700 ms after the stimulus.
As an internal control, stimulation of the dorsal
scapholunate interosseous ligament was performed
while the forearm muscles were relaxed, which resulted
in no reflex reactivity in the ECRB, ECU, FCR, and
FCU muscles. Additionally, to rule out stimulation of
nerve endings in the skin, 2 mL lidocaine hydrochloride
was infiltrated intradermally around the stimulating
electrode in 2 subjects and the experiment was subsequently repeated, which resulted in no change in reflex
reactivity.
MA). After rectification, each subject and all 4 muscles
were analyzed independently in each separate wrist
position. The root mean square was subsequently used
for statistical analysis using the Student’s t-test (2tailed, for independent variables). Poststimulus changes
in EMG activity (recorded as changes in EMG amplitude) that were statistically significant (p ⬍ .05) in a
majority of subjects (5 or more) were noted as peaks of
EMG activity. These peaks were subsequently plotted
in poststimulus time histograms. The statistical models
were chosen in collaboration with the institute statistician.
RESULTS
General observations
Patterns of muscle reactivity resembling varying recruitment orders of the forearm muscles were observed
at various time intervals after electrical stimulation of
the scapholunate interosseous ligament. The median
values of all statistically significant changes (p ⬍ .05) in
amplitude (t1 – t2) are plotted in poststimulus time
histograms (Figs. 2– 6), which depict the sequence of
events occurring in each wrist position. The time-related changes in amplitude may be divided into the
following reaction patterns:
1. Immediate reaction. The primary muscle response,
Statistical analysis
The average EMG data from the 30 consecutive trials
were exported to and mounted in a software program
(Microsoft Excel 2004, v.11.3.7; Microsoft Corp., Redmond, WA). The EMG data were divided into time
intervals, for which the prestimulus 100-ms interval
was regarded as baseline (t1), and the poststimulus
EMG (t2) was divided into 20-ms intervals, up to 500
ms after stimulation.
The EMG spreadsheets were imported into software
(OriginPro v.7.5714; Origin Lab Corp., Northampton,
which is observed within the first 20-ms interval
after stimulation of the ligament. This reaction
was of a monophasic type and presented in the
antagonist muscles, as related to each wrist position.
2. Reciprocal activation. The immediate reaction was
frequently followed by a corresponding activation in
the agonist muscle(s) for each wrist position, from
around 20 to 60 ms after stimulation, with or without
concurrent antagonist activation.
3. Co-contraction. An excitatory reaction occurring
in 2 or more of the recorded forearm muscles,
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FIGURE 2: EMG amplitude (y axis) in all forearm muscles
along a time line (x axis), depicting the simultaneous muscle
activity in the ECRB, ECU, FCR, and FCU muscles after
stimulation of the ligament with the wrist in isometric
extension. Positive values indicate an increase in muscle
amplitude, an excitation, whereas negative values denote a decrease
in amplitude, an inhibition, compared with the prestimulus median
value (baseline, t1). Note the peaks of co-contraction that occur
around 50, 150, and 250 ms.
indicating a simultaneous contraction of both agonist and antagonist muscles. This intermediate
reaction generally lasted about 100 ms, with peak
activity around 150 ms after stimulation. The largest changes in muscle amplitude were noted during this reaction.
4. Receding reactions. From time intervals around
250 –300 to 500 ms after stimulation, the general
muscle pattern was one of receding activity. If
excitation occurred, the amplitudes were generally
smaller than those previously noted. Additionally,
inhibition of EMG activity was frequently seen
during this interval.
Specific patterns in each wrist position
Extension: The muscle pattern seen simultaneously
in the 4 forearm muscles is illustrated in Figure 2,
and the precise change in each muscle is depicted in
poststimulus time histograms in Figure 3.
The immediate reaction after stimulation with the
wrist in extension was observed in the FCR and FCU
muscles. This primary reaction was instantly followed by a reciprocal activation of the ECRB and
ECU muscles from 20 to 60 ms after stimulus, and,
additionally, of the FCR muscle from 40 to 60 ms
after stimulation.
A longer period of co-contraction occurs in all 4
muscles at time intervals from 60 – 80 to approximately 160 –180 ms. This period of co-contraction
contains the largest changes in amplitude seen in the
645
ECRB muscle in extension (median, 42.29 V). A
third phase of co-contraction is seen in the ECRB,
ECU, and FCU muscles, peaking around 250 ms,
during which time the excitatory amplitudes are
smaller than the preceding recordings.
The final phase is one of receding activity, with
inhibitory reactions in the FCR, ECU, and ECRB
muscles. Figure 7 depicts the actual rectified EMG in
extension from 1 subject and illustrates the co-contraction patterns described above.
Flexion: The immediate poststimulus reaction in
flexion was observed in the ECRB muscle (Fig. 4).
After this initial reaction, only intermittent reactions
were seen in the ECRB and ECU muscles. A subsequent reciprocal activation in the FCR and FCU
muscles was noted from 20 to 60 ms. This reaction
pattern was again repeated from 120 to 200 ms and
contained the largest changes in amplitude observed
in the FCU muscle (median, 9.76 V).
Only a brief period of co-contraction is observed
in the ECU and FCR muscles, occurring at 240 to
260 ms. The receding activity, approximately 320 ms
after stimulus, is characterized by inhibitory reactions in
the FCR and FCU muscles, followed by sporadic excitatory responses in the FCR muscle.
Radial deviation: The immediate reaction is observed
in the FCR and FCU muscles during the initial 20 ms
(Fig. 5). In this position, the largest changes in
immediate amplitude in the FCR and FCU muscles
were observed (median, 8.12 and 5.82 V, respectively).
This is directly followed by a co-contraction in the
ECRB, ECU, and FCR muscles from 20 to 60 ms. A
phase of co-contraction is once again seen approximately 100 to 160 ms after stimulus, with excitation
of all wrist muscles. As in the extended wrist position,
this phase contains the largest amplitude changes in the
ECRB muscle (median, 7.47 V). In the final phase
of receding activity, co-contraction with smaller amplitudes is seen in the ECRB and FCR muscles
around 250 ms, followed by inhibitory reactions in
the ECRB and ECU muscles and excitatory reactions
in the FCR muscle.
Ulnar deviation: As in the other wrist positions, an
immediate reaction is seen, occurring in the FCR and
FCU muscles (Fig. 6). In general, however, the reaction pattern observed in ulnar deviation differs
greatly from that of the other positions. No periods of
co-contraction were observed, and only sporadic excitatory responses in the FCR and ECRB muscles
occurred. In fact, the predominant muscular response
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FIGURE 3: Poststimulus time histograms of the significant (p ⬍ .05) changes in amplitude (t1 – t2) observed after stimulation (stim) of
the scapholunate interosseous ligament with the wrist in a position of isometric extension. Each column represents the statistically
significant (p ⬍ .05) median value for a specific time interval. From top to bottom: ECRB, ECU, FCR, and FCU muscles. Note the
early reaction in FCR and FCU muscles. The largest excitation is seen in the primary motor, ECRB muscle, around 150 ms.
is one of inhibition of the primary motor in ulnar
deviation—the ECU muscle. A total of 5 inhibitory
phases are seen between 80 and 420 ms after stimulus.
DISCUSSION
In this study, we have used a protocol of electrical
stimulation of the scapholunate interosseous ligament
while monitoring the EMG activities in 4 forearm muscles to investigate possible wrist ligamento-muscular
connections. Our experimental procedure was based on
previously published reports on ligamento-muscular reflexes in the shoulder and knee joints.8,18,19 We have
been able to confirm our hypothesis and, for the first
time, present evidence of reactions between a wrist
ligament (scapholunate interosseous ligament) and 4
muscles responsible for wrist motion (ECRB, ECU,
FCR, and FCU). Furthermore, these ligamentomuscular reflexes exhibit linear reaction patterns, suggesting time-dependent recruitment phases of the forearm muscles. In the subsequent sections, we will
discuss the possible role of these recruitment phases in
wrist neuromuscular stability.
Immediate muscle reactions
The findings of mechanoreceptor populations in ligaments have elicited hypotheses regarding the sensory
role of ligaments in providing reflex stability in the
event of joint perturbations. These concepts of jointprotective reflexes were first studied by Palmer in
195820 and have subsequently been confirmed in the
knee, ankle, shoulder, and elbow joints.4,5,9,21 Controversy exists regarding protective reflexes because the
efficacy of a defensive reflex is entirely dependent on
the immediacy of the ligamento-muscular reaction.
Hence, to be joint protective, an adequate reaction
would need to be equivalent in response time to a
monosynaptic stretch reflex (ie, patellar reflex), when
the antagonist muscles act as first line of defense to
break a potentially damaging joint motion.
Recordings from the receptive fields of mechanoreceptors in human glabrous skin have shown that these
nerve endings belong to the fastest group of nerve
fibers, group I, or the A␣-fiber.22 In the joint, however,
mechanoreceptors are believed to belong to group II, or
A-afferents, with conduction velocities of approximately 70 m/s.23 Additional investigations of these
nerve conduction velocities in the human forearm have
confirmed compound nerve velocity rates indicative of
fast-reacting and proprioceptive afferents.24,25
Furthermore, experiments on the excitation of human flexor motoneurons have shown that stimulation of
the median and/or ulnar nerves at the level of the wrist
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647
FIGURE 4: Poststimulus time histograms of the significant (p ⬍ .05) changes in amplitude (t1 – t2) observed after stimulation
(stim) of the scapholunate interosseous ligament with the wrist in a position of isometric flexion. From top to bottom: ECRB,
ECU, FCR, and FCU muscles. The early-onset reaction is observed in the ECRB muscle. The predominant activity is observed in
the wrist flexors, the FCR and FCU muscles, with the largest excitation in the primary motor, FCU muscle, around 150 ms. Only a
brief period of co-contraction is seen around 250 ms.
joint elicits monosynaptic excitations of the FCR and
FCU muscles, occurring within 20 ms after stimulation.26,27 These reaction times are similar to the immediate reactions observed in our study, indicating that
these, too, may be of monosynaptic origin.
In our study, a monophasic immediate reflex was
present in all wrist positions within the first 20 ms after
stimulation of the ligament. In extension and in radial
and ulnar deviation, this first response was seen in the
FCR and FCU muscles. In these 3 wrist positions, the
dorsal scapholunate interosseous ligament is in a state
of elongation,28 leaving the ligament vulnerable to disruption. The simultaneous contraction of FCR and FCU
muscles as paired antagonists would counteract a potentially noxious extension or deviation motion. Similarly, with the wrist in a flexed position, the immediate
response was seen in the primary antagonist, the ECRB
muscle. The short-onset latency and the specific recruitment of muscle groups to disrupt potentially damaging
wrist positions may indicate that the immediate reactions recorded in our study serve in a joint-protective
manner. The precise afferent pathways of these early
reflexes, however, are not delineated in our study.
Based on previous publications, we speculate that these
reactions are of a monosynaptic spinal origin6 or, pos-
sibly, modulations of ongoing muscle activity through
local gamma-fusimotor systems.4
Co-contraction
After the immediate reflex, a pattern of coactivation of
agonist and antagonist muscles, co-contraction, was
observed. These reactions generally commenced
around 50 ms after stimulation, with peak amplitudes of
EMG activity around 150 ms, the largest amplitudes
being observed in the dominant muscle for each wrist
position. The later-onset latency of these reactions suggest that they differ from the immediate responses described above.
Although we currently lack information on the precise path of proprioceptive information from the wrist
joint, studies of cortical somatosensory evoked potentials from the median and radial nerves29 and of the
effects of transcranial magnetic stimulation on reflexes
in the forearm muscles30 indicate afferent and efferent
time intervals to correspond with these periods of coactivation. This intermediate reaction pattern is most
likely the result of a polysynaptic spinal reflex arc
influenced by supraspinal pathways.31
The long-onset latency of the co-contraction reaction
suggests that it has importance in the long-term control
of joint neuromuscular stability as opposed to the sim-
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FIGURE 5: Poststimulus time histograms of the significant (p ⬍ .05) changes in amplitude (t1 – t2) observed after stimulation (stim) of
the scapholunate interosseous ligament with the wrist in a position of isometric radial deviation. From top to bottom: ECRB, ECU, FCR,
and FCU muscles. Note the early reactions in the FCR and FCU muscles. The largest excitations are seen in the FCR muscle within the
first 20 ms, and in ECRB muscle around 150 ms. Peaks of co-contraction are observed around 50, 150, and 250 ms.
ple joint-protective reflex described above. Co-contraction has previously been seen as an effective method of
stability in the knee joint, where perturbation of the anterior cruciate ligament has elicited reflex actions in both
hamstring and quadriceps musculature.32,33 Global contraction of agonist and antagonist muscles around a joint
will result in general joint stiffness, effectively reducing the
risk of joint damage.34
Studies of the biomechanical adaptations of the wrist
have shown a substantial increase in co-contraction
after unstable loading of the wrist,35 as well as a dynamic ability to adapt these co-contractions to variations in destabilizing actions.36 Muscles that are preactivated have a general stiffness at the onset of joint
perturbations, and thus will respond more quickly and
with greater amplitudes.3 This corresponds with our
observations that the largest excitatory changes in amplitude are seen in the agonist muscles (ECRB in extension and radial deviation, FCU in flexion) during the
intermediate period of co-contraction.
The functional implication of co-contraction is not
merely one of joint stability after external loads. More
importantly, the delicate balance of co-contraction is
believed to be important in maintaining smooth joint
motions. This ability to sustain an adequate joint equilibrium has been shown to be impaired in anterior
cruciate ligament– deficient knees,37 where an inade-
quate neuromuscular recruitment renders changes in
knee kinematics potentially harmful to the joint.38
Whether similar disturbances occur in wrist proprioceptive functions after ligament injuries is currently unknown.
Inhibition
Although co-contraction is an effective strategy to increase joint stiffness and maintain neuromuscular joint
stability, it is correlated with a large expenditure of
energy.36 Additionally, a prolonged loading of ligaments has been associated with an increase of reflex
thresholds and a reciprocal decrease in EMG activity.5
These combined observations may in part explain the
receding EMG activity observed during the final stages
of our experiments. This period of general fatigue is
paradoxically a phase in which the efforts to maintain
joint stability may result in a joint susceptible to damage and unable to adequately respond to external forces.
Clinical relevance
The findings of specific ligamento-muscular reflexes
elicited from electrical stimulation of the scapholunate
interosseous ligament confirm our hypothesis that this
ligament has a sensory and proprioceptive function. The
consequence of our findings is the realization that a
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649
FIGURE 6: Poststimulus time histograms of the significant (p ⬍ .05) changes in amplitude (t1 – t2) observed after stimulation (stim) of
the scapholunate interosseous ligament with the wrist in a position of isometric ulnar deviation. From top to bottom: ECRB, ECU, FCR,
and FCU muscles. The earliest reactions are seen in the FCR and FCU muscles. No periods of co-contraction are observed, and the
predominant activity is one of inhibition in the ECU muscle, where a total of 5 inhibitory phases are seen from 80 to 420 ms.
scapholunate interosseous ligament injury entails a loss
of both structural and sensory function in the carpus.
Additionally, the reflexes found indicate both a simple joint-protective purpose and a refined muscle control of the wrist. By analyzing these patterns of neuromuscular stability and co-contraction further, we hope
to enhance our understanding of the long-term effects
of wrist ligament injuries and generate adequate neuromuscular rehabilitation techniques to promote proprioception reeducation.
The scapholunate interosseous ligament, however, is
only one of several richly innervated wrist ligaments.
We still lack substantial information regarding ligamento-muscular control from other parts of the carpus
and distal radioulnar joint, which must be obtained
before we truly can understand the complex mechanisms of wrist sensorimotor functions.
Assumptions and limitations
Although elicitation of ligament reflexes through electrical stimulation is an accepted methodology in proprioception research, the technique has several limitations.39 The primary concern is the assumption that the
electrical discharge will, in some manner, correspond
with a physiologic joint perturbation. This is unknown,
leaving us with the possibility that the reactions ob-
served are artificial. In our protocol, the scapholunate
interosseous ligament was stimulated at intensities 2 to
3 times the sensory, but below the nociceptive, threshold. None of our subjects reported pain during application of the electrical stimuli, indicating that no A⭸- or
C-fibers were stimulated. Because fine afferents require
high levels of stimulation to elicit a nociceptive response,40 we believe our low-intensity stimulus was
within an acceptable physiologic range to elicit responses from mechanoreceptive A-afferents.
The second concern is the position of the electrode.
Because our aim was to study reflexes elicited in the
scapholunate interosseous ligament, a displacement of
the electrode may cause an actual stimulation of receptors in the skin and/or dorsal wrist capsule. To eliminate
the possibility of cutaneous contributions, the skin
around the stimulating fine-wire electrode was anesthetized in 2 of our subjects after completion of the first
experiment. After the skin was desensitized, a second
series of stimulations of the dorsal scapholunate interosseous ligament was performed. Skin desensitization did not alter the muscular reactions, which led us to
believe that the reactions observed were indeed elicited
from the ligament itself. The dorsal capsule, however,
cannot be desensitized without risk of influencing the
afferent information from the dorsal scapholunate in-
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FIGURE 7: Rectified EMG (averaged data from 30 consecutive stimulations) from 1 subject, with the wrist in isometric extension.
From top to bottom: A ECRB, B ECU, C FCR, and D FCU muscles. The baseline (horizontal line) depicts the prestimulus root
mean square value of each muscle. The stimulation artifacts are clearly seen in FCR and FCU muscles. The onset of cocontraction (dotted vertical line) is observed in all 4 muscles around 50 ms after stimulation of the scapholunate interosseous
ligament.
terosseous ligament. Furthermore, it cannot be identified using sonographic techniques, which eliminated
the possibility of inserting a capsular stimulating electrode to evaluate possible reflex reactions. The dorsal
wrist capsule, innervated by terminal branches from the
posterior interosseous, dorsal radial and ulnar sensory
nerves,41 has not been studied with regard to presence
of proprioceptive nerve endings. Hence, we currently
lack information on its possible role in wrist proprioception.
Finally, the number of subjects included in this study
was small, thus limiting the statistical merit of our
findings. The experimental setup is considered invasive,
and institutional review board approval was confined to
a descriptive study. In comparison with previously
published experiments on joint ligamento-muscular reflexes,8,18,42 our study is one of the largest
series conducted on human subjects.
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