2022 Volume 21 Issue 2 Pages 99-111
Carpal instability dissociative induces irreversible wrist osteoarthritis. The orthosis is essential for the treatment, but the rational wrist position has not been investigated. It needs to examine the effect of wrist muscles contraction on carpal bone under immobilization to determine the proper position of the wrist.
This study investigates the effect of contraction of carpal muscles on the position of carpal bones under the immobilization of the carpal joint. Seven healthy men participated in this study. We measured the carpal height (CH) during independent electrical stimulations of the abductor pollicis longus (APL), extensor carpi radialis longus (ECRL), extensor carpi ulnaris (ECU), flexor carpi radialis (FCR), and flexor carpi ulnaris (FCU). Further, we measured the CH during a pinch task simultaneously with the electromyogram (EMG) of each muscle activity. The three carpal joint positions during the measurements were neutral (N), radial deviation (RD), and ulnar deviation (UD).
Compared with the resting state, the CH during an electrical stimulation was not significantly shortened in RD compared with N and UD. On the contrary, it was shortened when the ECU contracted. During the pinch task, the amount of shortening of the CH increased with the increasing pinch force, but N and UD were significantly shorter than RD.
The study revealed that even under immobilization of the carpal joint, the axial pressure of the carpal joint associated with the muscular activity of the ECU causes movement of the carpal bones. These findings suggest that orthotic devices against carpal instability could be used in the radial deviation position. Besides, the ECU activity needs to be inhibited. In addition, it is necessary to refrain from using the hand during immobilization. Future work, more participants should be enrolled to investigate the kinetics of the carpal bones in a variety of motor tasks in different wrist positions.
Carpal instability dissociative (CID) is a disease that causes instability in the carpal region due to damage to the intercarpal ligaments. In CID of the proximal carpal row, there are two types of dissociation: scapholunate dissociation caused by damage to the scapholunate interosseous ligament, and lunotriquetral dissociation caused by damage to the lunotriquetral interosseous ligament damage [ 1], each of which shows a characteristic radiological deformation. scapholunate dissociation presents with dorsal intercarpal segment instability (DISI), while lunotriquetral dissociation presents with volar intercarpal segment instability (VISI) [ 2, 3].
Since the proximal carpal row is free from muscle-tendon attachment, the contractile tension of the carpal muscles attached to the metacarpal bones moves the distal row and causes movement of the proximal carpal row via the intercarpal ligaments. Long-axis pressure from the distal carpal row also produces movement of the proximal carpal row [ 4]. The proximal carpal row is tightly connected by the scapholunate interosseous ligament and the lunotriquetral interosseous ligament, and therefore moves as a single unit and has high mobility [ 5]. Therefore, when the scapholunate interosseous ligament or lunotriquetral interosseous ligament is damaged, the stability of the proximal carpal row is impaired.
Numerous treatments for CID have been reported [ 6– 8]. In the case of a minor acute injury, an orthotic device could be worn [ 9]. In contrast, for complete injury, in addition to ligament suturing, the space between the carpal bones is temporarily fixed with a K-wire, and an orthotic device is attached [ 10]. In older cases, a ligament graft does not occur; hence, ligament transplantation or reconstructive surgery is required. In addition, if arthropathic changes occur, partial immobilization of the carpal joint and proximal row carpectomy could be considered. When these procedures are performed, the function of the carpal joint is permanently impaired; thus, early detection and treatment are important for the prevention of arthropathy.
Biomechanical studies using cadavers have been conducted to develop treatments for CID [ 11– 13]. These studies investigated the effect of the axial pressure, generated by the contraction of the carpal muscles, on the proximal carpal row and found that rotational motion occurs in the proximal carpal row even when the third metacarpal is fixed. This suggests that even if an orthotic device is used to immobilize the wrist, the proximal carpal row cannot be kept at rest under the contraction of the carpal muscles. Based on these findings, we observed the movement of the triquetral bone when the carpal joint of a healthy person was externally immobilized and performed isometric ulnar deviation of the carpal joint [ 14]. The results showed that the triquetral bone underwent a translational movement of 1 to 2 mm as the muscle contraction force increased. Our next objective was to identify highly immobilizing orthotic devices that are effective for treating CID and movements that must be avoided. However, there is no evidence that the contraction of each wrist muscle contributes to CID under casting, because no study has yet examined the kinematic characteristics of the wrist joint when axial load produced by the contraction of each wrist muscle is applied actually in vivo.
Therefore, in this study, we analyzed the movement of the proximal carpal row when inducing the contraction of the wrist muscles electrically and the contraction voluntarily with pinch task in different wrist positions to identify the carpal joint positions that are suitable for immobilization against CID. The muscle contraction by electrical stimulation was used to investigate the effects of individual muscle actions. The pinch task was employed because it is an activity that is frequently performed in daily life, even under casting.
1. Participants
Seven healthy adults with no history of neurological or orthopedic disease participated in the study. All were male. The median age of the participants was 32.1 years (SD ± 6.3). All were right-handed. The study was approved by the Ethics Committee of Niigata University of Health and Welfare (Approval No.: 18723-210913). The participants received a written explanation of the purpose and methods of the study, and each gave written consent before participating.
2. Methods
The study consisted of two experiments. In the first experiment, independent contraction of muscles was induced by an electrical stimulation, and in the second experiment, the participants were each assigned a task of performing a pinching movement. In each experiment, the carpal height (CH) was measured and the effect of muscle activity on the proximal carpal row was analyzed.
1) Insertion of electrodes
Wire electrodes were inserted into the target muscles for electrical stimulation and electromyography (EMG) measurements. The target muscles were abductor pollicis longus (APL), extensor carpi radialis longus (ECRL), extensor carpi ulnaris (ECU), flexor carpi radialis (FCR), and flexor carpi ulnaris (FCU). The wire electrodes were bipolar wire electrodes (TN204-123, Unique Medical, Tokyo, Japan) made of a 0.05-mm diameter tungsten wire (uninsulated part: 2 mm, the distance between electrodes: 5 mm) coated with rigid urethane. The tip of the electrode was bent on a hook of approximately 15 mm to follow the muscle contraction. A 25-gauge needle was used as a guide for electrode insertion, and only the guide needle was removed after the electrode implantation.
2) Measurement of CH during electrical stimulation
(1) Electrical stimulation
For the electrical stimulation, an electrical stimulator (SEN-3301; Nihon Kohden, Tokyo, Japan) and an isolator (SS-202J; Nihon Kohden, Tokyo, Japan) were used. The stimulation pulse width was 200 µs, the stimulation frequency was 20 Hz, and the stimulation time was 3 s. The stimulus intensity was set to the supramaximal stimulus, which was calculated from the intensity that induced the maximum M wave measured in advance. The positions of the jointed appendages during the electrical stimulation were as follows. The forearm was set to an intermediate position, and a total of three carpal joint positions were set: neutral (N), 15-degree radial deviation (RD), and 15-degree ulnar deviation (UD) ( Figure 1). Electric stimulation was applied separately to each muscle in each carpal joint position, and independent contraction was induced.
(2) Measurement of CH
Ultrasound images of the carpal region were taken in brightness mode (B-mode) before and during the electrical stimulation using an ultrasound imaging system (LOGIQ e, GE Healthcare, Tokyo, Japan) with a 14-MHz linear probe. Imaging was performed by grounding the probe on the back of the hand and taking a long-axis image of the carpal region. During imaging, the probe was fixed with an arm to maintain the imaging area. The collected ultrasound images were analyzed using open-source digital measurement software (Image J, NIH) to measure the CH, defined as the distance from the distal dorsal edge of the capitate to the distal dorsal edge of the radius ( Figure 2). Each examination was repeated 3 times, and the average value was obtained. Intra-observer reliability of each measurement was 0.93. The obtained CH was normalized based on the pre-stimulation value (%CH).
3) Measurement of muscle activity and CH during pinch task
(1) Pinch task
The pinch task was defined as a lateral pinch with the right hand ( Figure 3). The participants were instructed to maintain 30-% maximum voluntary contraction (MVC), 60-% MVC, and 90-% MVC of the maximum-effort pinching force for 3 s. A pinch gauge (SPR6720, Sakai Medical, Tokyo, Japan) was used to measure the pinch force. The positions of the jointed appendages during the pinching movements were as follows. The forearm was set to an intermediate position, and a total of three carpal joint positions were set: neutral (N), 15-degree radial deviation (RD), and 15-degree ulnar deviation (UD). The carpal joint positions were randomized before the measurements. Participants took a one-minute break between measurements to avoid muscle fatigue.
(2) Derivation and analysis of EMG
The EMG signals of each muscle obtained from the wire electrodes were amplified 100-fold by an EMG amplifier (DL-1440; 4 Assist, Tokyo, Japan), A/D-converted at a sampling rate of 2 kHz through an analog output box (DL-270, 4 Assist, Tokyo, Japan), and transferred to a personal computer. A data acquisition and analysis system (Power Lab 8/35; AD Instrument, Australia) was used to capture the data. The recorded EMG signals were processed by a 10–1000 Hz bandpass filter. The EMG time interval that was analyzed was 200 ms starting from the point when the pinch force was stabilized. After full-wave rectification, the muscle integral value (IEMG) was calculated. The IEMG obtained at each measured limb position was normalized based on the maximum IEMG value of each participant (normalized IEMG; NIEMG). Finally, the mean NIEMG and standard deviation among the participants were calculated for each measured limb position.
(3) Measurement of CH
Ultrasound images of the carpal region were taken at rest and during the pinch task. The CH was measured using the same imaging equipment and image analysis system used during the electrical stimulation, and then, the %CH was calculated.
4) Data analysis and statistics
(1) CH during electrical stimulation
The participants' %CH was compared using repeated-measures two-way ANOVA with the stimulated muscle and carpal joint position as factors. The Bonferroni test was employed as a post hoc test for the carpal joint position and stimulated muscle factors. The significance level of the post hoc test was set at p=0.05. For statistical processing, SPSS analysis software (IBM SPSS statistics Ver.18; SPSS Japan Inc, IBM, Japan) was used.
(2) Muscle activity and CH during pinch task
The NIEMG and %CH of each muscle were compared by repeated-measures two-way ANOVA with the carpal joint position and the percentage of maximum voluntary contraction (%MVC) as factors. The Bonferroni test was employed as a post hoc test for the carpal joint position and %MVC factors. The signifi cance level of the post hoc test was set at p=0.05. For statistical processing, SPSS analysis software as same as previous before was used.
1) %CH during electrical stimulation
The %CH during the electrical stimulation was significantly influenced by both the carpal joint position and the stimulated muscle (Position: F(1,2)=8.377, p<0.001, and η 2=0.093; Muscle: F(1,4)=5.508, p=0.001, and η 2=0.140). In addition, a significant difference was observed in their interaction (F(1,8)=4.031, p<0.001, and η 2=0.209). In multiple comparisons, in the case of the carpal joint positions, significant differences were found between RD and UD (RD>UD, p<0.001) among carpal joint positions, and in the case of stimulated muscles, between the APL and FCU, ECU and FCR, ECU and FCU, FCR and FCU, and ECRL and ECU ( Figure 4, Table 1).
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2) Muscle activity and %CH during pinch task
For all the stimulated muscles, the NIEMG during the pinch task was not significantly affected by the carpal joint position ( Figure 5). In terms of %MVC, there was a significant effect on muscles except APL and ECU ( Figure 5, Table 2).
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The %CH during the pinching movement was significantly influenced by both the carpal joint position and the %MVC (Position; F(1,2)=3.259, p<0.001, and η 2=0.373; %MVC; F(1,2)=3.555, p<0.001, and η 2=0.220). In multiple comparisons, significant differences were found between carpal joint positions RD and UD (RD > UD, p = 0.035) and between 30% and 90% MVC (30% MVC > 90% MVC, p = 0.014) ( Figure 6, Table 3).
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1. CH under electrical stimulation of each muscle
When long-axis pressure is applied to the carpal joint, the carpal row rotates in three dimensions [ 15] and moves in the proximal direction [ 16, 17]. However, until now, the details of how the carpal row is affected by the axial pressure caused by the contraction of the muscles that produce movement of the carpal joint are yet to be understood. To address this question, recent studies using cadavers have been conducted to observe the kinetics of the carpal bones by using traction techniques on the carpal muscle tendons. However, these methods give limited insight into the precise movements of the carpal row, and the results have not directly led to the development of orthotic device therapies for CID.
Therefore, in this study, we measured the CH with all the metacarpals externally immobilized, as if the patient were wearing an orthotic device on the wrist. The results show that in the neutral position and ulnar deviation position, the contraction of all the muscles except the FCR shortened the CH, and in particular, the contraction of the ECU in the ulnar deviation position most shortened the CH. In the radial deviation position, there was little change in the CH. Salva-Coll et al. [ 11] performed an experiment in which the carpal joint was immobilized by inserting a pin intramedullary into the third metacarpal while traction was applied to the carpal muscle tendon. The results showed that traction of the APL, ECRL, and FCU caused supination of the proximal and distal carpal rows, while that of the ECU caused pronation. They also showed that traction of the APL, ECRL, and FCU reduced the gap between the scaphoid lunate bones when the scapholunate interosseous ligament was severed. Furthermore, León-Lopez et al. [ 18] showed that traction of the ECU reduced the gap between the lunate triquetral bones when the lunotriquetral interosseous ligament was severed. Based on the above reports, the contraction of each of the carpal muscles applied long-axis pressure to the carpal joint, thereby inducing supination/pronation, flexion/extension, and radial deviation/ulnar deviation of the carpal row. This suggests that the changes in the CH shown in this study are the result of these three-dimensional rotational movements. In addition, we can infer that the contraction of the ECU in the ulnar deviation position could produce the shortest CH not only by the pronation of the distal carpal row but also by the dorsal flexion of the triquetral bone associated with ulnar deviation of the proximal carpal row. Furthermore, the fact that the ECU's moment arm from the radial and ulnar deviation axes of the carpal joint increases in the ulnar deviation position [ 12, 19] may explain why the ulnar deviation position more effectively induced the carpal row movement, resulting in a significant shortening of the CH. Other factors that may be responsible for increasing the movement of the carpal row include the fact that the hamate triquetral joint located on the ulnar side is a biaxial elliptical joint [ 20], the fact that the 4th and 5th carpometacarpal joints have extensive mobility, and the laxity of the soft tissue in the ulnar carpal region. In contrast, the FCR did not affect the CH in any of the positions because it traverses the palmar side of the scaphoid bone, which means that the muscle tension of the FCR limits the movement of the proximal carpal row by breaking the palmar rotational movement of the scaphoid bone. This may explain why the FCR is known to be a dynamic stabilizer of the carpal joint [ 13, 20].
Concerning the fact that the contraction of all the muscles including the FCR did not affect the CH in the radial deviation position, the most likely reason is that in the radial deviation position, the CH was already shortened owing to palmar flexion in the scaphoid bone associated with the radial deviation of the proximal carpal row, and as a result, the CH does not change significantly even when the carpal muscles are contracted in this state. The scaphotrapezio-trapezoidal and scaphocapitate joints, which are spiral joints connecting the distal and proximal carpal rows, are found on the radial side of the center part of the carpal joint, and the scaphotrapezium and scaphocapitate ligaments firmly connect the distal and proximal carpal rows [ 22, 23]. The movement of the distal carpal row caused by the contraction of the carpal muscles moves the proximal carpal row through these ligaments. However, in the radial deviation position, the mobility of the distal carpal row is reduced, which likely explains why the movement does not propagate to the proximal carpal row.
2. Muscle activity and CH during the pinch task
In this study, we showed that the CH was shortened in all the carpal joint positions during the pinch task and that it increased with the increasing pinch force. The fact that the activity of all the muscles except the APL increased with the increase in pinch force suggests that the shortening of the CH during the pinching was increased by muscle contractions other than the APL. However, by observing the activity of the individual muscles, the FCR and FCU are less active in all the carpal joint positions, while the ECRL and ECU are significantly active. In other words, the shortening of the CH during pinching was due to the ECRL and ECU, and the impact of the ECU, which becomes more active in the neutral and ulnar deviation positions, appears to be particularly significant. This can also be inferred from the fact that the amount of the CH shortening was greater in the ulnar deviation position.
3. Clinical application of study results
In this study, we showed that the contraction of the ECU in the ulnar deviation position shortens the CH in both the electrical stimulation of the muscle and pinching movement experiments. This contraction represents a condition that can easily induce the movement of the carpal row, which may be a factor that adversely affects CID. Garcia-Elias et al. [ 24] reported that the dartsthrow motion (dorsal flexion and radial deviation/palmar flexion and ulnar deviation) was the motion that most aggravated scapholunate dissociation, and recommend the reversed darts-throw motion (palmar flexion and radial deviation/ dorsal flexion and ulnar deviation). There is also research investigating whether lunotriquetral dissociation is worsened by ulnar deviation of the carpal joint [ 25]. These reports support the finding of the present study that ulnar deviation is the worst position for this disease. Although there has been little discussion of ECU so far. However, the results of the present study suggest that the radial deviation position, which is the position that does not cause contraction of the ECU, should be the recommended position. Furthermore, it is effective to encourage the contraction of the FCR, which is known as a dynamic stabilizer, in the radial deviation position.
For exercise therapy for lunotriquetral dissociation, León-Lopez MM et al. [ 18] recommended ECU training, based on the idea that the contraction of the ECU stabilizes the triquetral bone by rotating the distal carpal row. However, the results of the present study demonstrate that the contraction of the ECU moves the carpal row under immobilization of the carpal joint. We, therefore, recommend that ECU training should not be performed with isometric contractions when the patient is wearing an orthotic device on the wrist. Furthermore, since low-intensity pinching may induce instability of the proximal carpal row, we recommend that patients should limit the use of their fingers as much as possible for a period while wearing an orthotic device on the wrist.
4. Study limitations
This study has several limitations. First, in this study, we used an ultrasound imaging system for electrical stimulation of the body and observation during voluntary movements. Therefore, the amount of rotation of the proximal carpal row could not be measured, and an observable CH was used. This means that the obtained changes in the CH and the amount and direction of rotation of the carpal row are only estimates. To obtain detailed kinetics, it would be necessary to employ a different measurement method.
Next, the present study involves data obtained by muscle contraction via electrical stimulation using wire electrodes. Because the maximal contraction using electrical stimulation is different from the maximal voluntary contraction, selective muscle contraction by voluntary movement may produce different results.
Due to the limited sample size, our research group positioned these results as a preliminary study. Therefore, in the future, we plan to increase the sample size and conduct more reliable statistical analysis. In future work, more participants should be enrolled to investigate the kinetics of the carpal bones in a variety of motor tasks in different wrist positions.
In this study, we investigated the changes in the CH during electrical stimulation and pinching to elucidate the kinetics of the carpal bones under immobilization by the contraction of the carpal muscles. The results demonstrated that the contraction of the ECU shortens the CH. Moreover, during pinching, the CH was shortened regardless of the carpal joint position, although the results were most pronounced in the ulnar deviation position. We propose that an orthotic device against CID should be designed in such a way as to immobilize the carpal joint in the radial deviation position to minimize the impact of the ECU contraction. In addition, we hypothesized that exercise therapy in the form of training of the FCR and non-isometric training of the ECU may enhance the support of the carpal row. In future work, more participants should be enrolled to investigate the kinetics of the carpal bones in a variety of motor tasks in different wrist positions.
We thank Dr. Y. Ehara Y and Dr. M. Kubo for helpful comments on the manuscript. We would like to thank Editage ( www.editage.com) for English language editing. This work was supported by Grant-in-Aid for Research Activity Start-up and Challenging Research from the Niigata University of Health and Welfare, 2021.
The study was registered in the University Hospital Medical Information Network (UMIN) Clinical Trials Registry, trial no. UMIN000046605 ( http://www.umin.ac.jp/ctr/index.htm).
The study was approved by the Ethics Committee of Niigata University of Health and Welfare (Approval No: 18723-210913).
There are no conflicts of interest to declare.
