Arm prostheses offer patients with malformation or amputation of the upper limb many advantages. Restoration of natural appearance to a great extent is of essential psychological importance for the patient. In addition, prostheses provide weight compensation to help to reduce different deficits of clinical parameters such as posture insufficiencies, backache, etc.1 Finally, arm prostheses also reconstruct lost functions in a largely physiological manner. Compared with the sound side, the range of motion with transradial prosthesis is limited by the fit of prosthetic socket.2 This results in functional limitations for the patient requiring compensatory movements to perform essential motion patterns in daily life.3 The incorporation of a wrist joint that allow flexion completes the myoelectric prosthetic hand system by an additional degree of freedom. In the present study, biomechanical parameters are used to objectify a myoelectric arm prosthesis system with an integrated wrist joint and adjustable flexion and extension. This allows for more physiological motion patterns, increasing the patient's quality of life.
MATERIALS AND METHODS
Investigations were conducted with three patients affected by traumatic below elbow amputation and three patients with congenital deficiencies of the forearm. All patients have been experienced users of myoelectric arm prostheses (23 ± 15 years). The patients' mean age was 38.7 ± 21.4 years, the average height was 166.0 ± 2.4 cm, and the mean weight was 70.0 ± 19.1 kg.
All prostheses were provided with Muenster socket designs. For prosthetic fittings, myoelectric systems with Transcarpal-Hands (Otto Bock, Germany) were used. Mechanical wrist joint systems MyoWrist Transcarpal (Otto Bock) were integrated into the prostheses to allow flexible adjustment of wrist flexion. The MyoWrist Transcarpal permits flexion up to 40° corresponding to the free natural functional range of the human hand described by Kapandji.4 The wrist joint can be locked at 0° as well as at flexion and extension of 20° and 40°. Compared with the patients' own prostheses, the combination of MyoWrist Transcarpal and Transcarpal-Hand does not cause excess length or additional weight. Therefore, the patients do not require time for adaptation.
The measurements were conducted in a motion analysis laboratory by means of an optoelectronic camera system recording motion kinematics (VICON 460, VICONPEAK Oxford, GB). According to the “VICON Upper Limb Model,”5 25 passive markers were placed on the upper part of the subjects' body (Figure 1). On the basis of configuration of these markers, the 3D angles of shoulder, elbow, and wrist joint were calculated, similar to Murgia et al.,6 following the Euler convention.7 In addition, the coordinates of the markers allowed identification of significant compensatory movements of the upper body part using external software.8
To obtain conclusive results, motion patterns of daily living were chosen. In each test, the starting position was standing posture with hanging arm in neutral-0-position and prosthetic hand closed (Figure 1). Tests were performed with both the sound and affected limb. The following motion patterns were performed at moderate speed without physical effort.
Hand to Contralateral Anterior Superior Iliac Spine
The subject moves the hand to the contralateral anterior superior iliac spine (Figure 1) and returns the arm to the initial position. This movement simulates actions at the contralateral side, e.g., threading a belt or tucking in a shirt.
Hand to Mouth
The subject is standing in front of a table. The table board is level with the anterior superior iliac spine. A half filled mug with water is placed in a defined position. The subject takes the mug and moves it to the mouth. The mug is placed back to the initial position after taking a sip. This movement simulates actions near the face, e.g., eating, drinking.
Hand to Sternum
The subject moves its hand to a defined point in the middle of sternum and returns the arm to the initial position. This movement simulates actions in the body midline, e.g., buttoning of shirt or zipping up a zipper.
Hand to Ipsilateral Hip Pocket9
The subject moves the hand to ipsilateral hip pocket and returns the arm to the initial position. This motion simulates actions at the rear body side, e.g., removing a wallet from a pocket, personal hygiene.
Hand at Face Level
For this test, the subject stands in front of a tripod and moves the prosthesis to a defined place at head level and returns the arm to the initial position. This motion pattern represents actions away from the body level with the head, e.g., driving a nail into the wall, taking objects from a rack or cupboard.
At first, the single motion patterns were performed twice with the sound side followed by the same motion with the prosthetic side. These movements were repeated six times at 0°, 20° and 40° wrist joint flexion. Hand rotation was adjusted to a position most favorable for the patient before starting the tests with these flexion angles. Based on the single tests conducted with all subjects, mean values of selected biomechanical parameters were calculated. In addition, the patients were asked to give a subjective assessment of quality of motion performance.
Partial movements of the shoulder and elbow joint are appropriate for describing and comparing the results to the sound limb. The focus is on anteversion and abduction movements of the shoulder joint, upper arm rotation, and flexion movement of the elbow joint. In Table 1, the mean differences between sound side and prosthetic arm with and without wrist flexion for the motion amplitudes of the analyzed movements are indicated. In contrast to the sound side, significant compensatory movements can be observed on the prosthetic side. Furthermore it becomes obvious that the motion pattern may be considerably optimized by the flexion possibilities offered by a wrist joint. In 18 of the 20 investigated cases (four partial movements for five motion tasks), the difference between prosthesis with and without wrist flexion is drastically reduced. It is striking that particularly the abnormal wide anteversion and humeral rotation angles may be reduced by prosthetic hand flexion. As an example, the temporal structure of anteversion movement of a patient for the first motion task and all investigated flexion positions of the wrist joint is shown in Figure 2. Without wrist flexion, anteversion increased by approximately 35° when compared with the motion pattern of the sound limb. Motion performance is not changed significantly with a wrist flexion of 20°. However, a flexion angle of 40° reduces compensatory anteversion by approximately 18° coming noticeably close to the physiological motion pattern. In Figure 3, an example for simple compensatory movements of the trunk is calculated by the use of markers in addition to the joint angles. The mean motion amplitudes for the angle in frontal plane between the marker at sternoclavicular joint and at acromioclavicular joint are shown. By using the example of motion task 2, graphically illustrates that shoulder elevation on the prosthetic side is abnormally strong. This compensatory movement may be reduced by approximately 7° when using a wrist joint with 40° flexion.
In the two right columns of Table 1, the results of subjective evaluation are presented. A number of patients perceived an influence on the motion pattern by the adjusted wrist flexion. The generally preferred flexion angles are shown and the tendency is similar to the results of the motion analysis.
Figure 4 demonstrates that 15 of 30 conducted motion patterns (six patients with five motion tasks) could be most comfortably performed with a wrist flexion of 40°. Wrist flexion of 20° was most convenient for nine motion patterns. Patients perceived that 80% of motions were positively attributed to wrist joint flexion. Only in two cases a patient felt more comfortable without wrist flexion. On four motion patterns, the patients did not comment regarding differences.
The results of the study suggest that the integration of wrist flexion in the prosthetic hand of a myoelectric system allows the patient to perform motion patterns of daily life more naturally. It also suggests that certain movements would not be at all possible without wrist flexion. A survey of prostheses wearers using a flexion wrist joint conducted by Sears et al.10 shows that 76% of the interviewed persons benefit from a wider activity range.
Analyzing motion, some studies3,11–13 focus on investigating a fixed position of the wrist joint. In this study, it could be shown that different positioning of the wrist joint in one motion direction may result in reduced compensation movements. A generally optimal position of the prosthetic hand minimizing compensation movements, as searched for by Landry,12 could not be identified as well.
It was observed in this study that in many cases, the parallel optimization of rotation was required to take full advantage of wrist flexion. Without the combination of flexion and rotation some motion patterns could not be performed. The degree of pronation and supination of the prosthetic hand was highly dependent on the patient. This could be attributed to the different angles of the individual forearm sockets.
The tests show that changed wrist flexion not only influences residual arm joints but other body regions as well. Similar results are also presented in the study by Zinck et al.13 It is explained that the trunk also compensates for the lost joints caused by splints combined with a joint integrated socket. In addition, compensatory movements in torso bending of prosthesis users occurring while opening a door is shown in the study by Carey et al.3 Therefore it can be assumed that even slight optimizations of the arm prosthesis minimize compensatory movements of the arm, but also lead to a more physiological motion pattern of the body in general.
Even slight differences of the structure of angles strengthen the subjective positive impression of the patients. Sears et al.10 confirm the subjective positive perception when using a flexion wrist joint. In his survey, 71% of the interviewed prosthetic users reported that they move the prosthesis with wrist flexion in a more natural manner.
The relevance of the present study results for fitting practice may be increased by future investigations. Some tests could be specified and combined with certain activities. The destinations to be reached in the motion tests were located in a relatively large spatial area and this could be restricted to gain further information.
Sacchetti14 supposed in his investigation that the missing flexion wrist joint and a prosthetic socket with condyle suspension caused the deviations measured in the motion pattern. The range of motion that is limited by a prosthetic socket can be increased by a prosthetic fitting with an arm liner in the forearm area. This allows the enhanced use of remaining pronation and supination. Furthermore, an extension of the elbow is not restricted by a socket that encompasses the joint. Knowledge about motion patterns of patients with such fittings would further enhance the present results.
In the present pilot study, motion patterns typically performed in the patients' daily life were selected. The results of motion analysis show that compensatory movements may be reduced by wrist flexion in most of the cases. This is noted considerably by kinematic characteristics of the shoulder joint on the prosthetic side. Even if only slight differences of few degrees were measured, the patients perceived an optimization of the motion pattern. Reduced compensatory movements support more physiological loading of the unaffected joints of the locomotor system. The more natural subjective impression is an important psychological aspect for the prosthetic user.
The authors thank Anett Elsner, Sandra Ramdial, and Sean Toren for the preparation of this manuscript.
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