Carpal Tunnel Syndrome Screening Device

The present invention relates to a device for measuring muscular strength and more particularly to the measurement of the strength of the intrinsic hand muscles innervated by the median nerve after it passes through the carpal tunnel in order to provide an objective indication of the presence of carpal tunnel syndrome.

Generally, the median nerve supplies sensation to the thumb, index finger, middle finger and half of the ring finger. The nerve passes through the wrist beneath the flexor retinaculum or transverse carpal ligament. The transverse carpal ligament and tendons of the flexor muscles define a tunnel through which the median nerve passes. The median nerve innervates three thenar and two lumbrical muscles. Other short muscles of the hand are innervated by the ulnar nerve. The muscles innervated by the median nerve include the abductor pollicis brevis, the opponens pollicis and the superficial head of the flexor pollicis brevis.

Should the median nerve become swollen as it passes through the carpal tunnel or should the carpal ligament become abnormally thickened, pinching or compression of the median nerve results. Swelling of the nerve or surrounding tissues may result from certain diseases, such as diabetes mellitus or rheumatoid arthritis. Unusual strain on the wrist such as that caused by repetitive and strenuous use of the hand may also result in such swelling. When such occurs, the resulting symptoms are referred to as carpal tunnel syndrome. The symptoms include pain, loss of sensation and weakness in the hand.

Generally, diagnosis of carpal tunnel syndrome will not occur until long after onset of the symptoms. Presently, there is no objective way to measure weakness in the hand which might be related to carpal tunnel syndrome and which would provide an early identification of the problem. The earlier diagnosis is accomplished, the more effective treatment procedures will be. If detected early, carpal tunnel syndrome may be treated and relief obtained short of surgical procedures. Surgical procedures are not always effective. Carpal tunnel syndrome is a significant factor in the work place resulting in lost work time and compensation claims. Early identification and treatment may reduce the cost associated with th disease.

A need exists for a device and method which can detect weakness in the hand which may be related to carpal tunnel syndrome and which will provide objective parameters which indicate the need for further evaluation.

In accordance with the present invention, the aforementioned needs are fulfilled. Essentially, a device is provided which measures the combined force vector or strength of the intrinsic hand muscles innervated by the median nerve after it passes through the carpal tunnel. The device includes a base defining a palm rest or support surface and a thumb guide surface. The surfaces are configured to position the hand so that movement of the thumb is restricted to substantially a vertical plane with the thumb positioned at an approximately 45.degree. angle with respect to the index finger. A force measuring means receives the thumb at the interphalangeal joint. Restraining and positioning the thumb in such a fashion permits measurement of the strength of or force generated by the intrinsic hand muscles supplied by the median nerve and lessens or omits input from other muscles, including the flexor pollicis longus which flexes the distal phalanx of the thumb but is not innervated by the median nerve after going through the carpal tunnel.

The present invention allows the generation of criteria indicating normal strength. An individual may be tested or screened and the strength indication compared with the predetermined normal strength measurements. Should the individual test indicate weakness, further evaluation would be indicated. An abnormality involving hand strength is likely to worsen if activities are unrestricted. Highly repetitive and strenuous use of the hands may be limited to prevent further degradation. Abnormal weakness or strength may be caused by other factors besides carpal tunnel syndrome. Such include, for example, peripheral neuropathy, cerebral damage, cervical cord damage and neuromuscular disease. The screening procedure permits the physician to evaluate and diagnose cause of the abnormality. The device in accordance with the present invention permits effective screening techniques to be developed, a reduction in lost work time and workers compensation claims and an improvement in the effectiveness of treatment through early identification of the problem.

FIG. 1 is a side, elevational view of a carpal tunnel screening device in accordance with the present invention;

FIG. 2 is a front, elevational view of the device;

FIG. 3 is a cross-sectional view taken generally along line III--III of FIG. 2;

FIG. 4 is a top, plan view of the device; and

FIG. 5 is a palmar view of a partially dissected hand showing the median nerve and the principal muscles innervated thereby.

A preferred embodiment of the carpal tunnel syndrome screening device in accordance with the present invention is illustrated in FIGS. 1-4 and generally designated by the numeral 10. Device 10 includes a base 12 and a tower or support structure 14 joined to the base. Base 12 defines a right hand palm support or rest surface 16, a left hand palm support or rest surface 18, a wrist support surface 20 and a longitudinally extending, centrally positioned thumb slot 22. Opposed sides 24, 26 of base 12 extend upwardly and are joined at a truncated apex 28 positioned directly above thumb slot 22 to form tower 14. Both palm support surfaces 16, 18 are configured so that they assume an angle "a" (FIG. 2) with horizontal of approximately 35.degree.. Surfaces 16, 18, as shown in FIG. 4, join vertical surfaces 19, 21 which assume an angle "b" of approximately 45.degree. with a longitudinal centerline 34 of the device. Surfaces 16, 18 slope downwardly from a lower edge 23 of each surface 19, 21. The wrist support surface 20, as shown in FIG. 3, assumes an angle "c" of approximately 35.degree. with respect to horizontal.

Sidewalls 24, 26 define opposed, spaced, parallel thumb guide surfaces 38, 40. The spacing between surfaces 38, 40 is selected to accommodate the thumb. Surfaces 16, 18, 19, 20, 21, 38 and 40 are configured so that the patient's thumb 48 is fixed and constrained to move in a substantially vertical plane coincident with longitudinal centerline 34. The slot is dimensioned to limit side to side movement of the thumb.

As shown in FIGS. 3 and 4, the device accommodates both the left and right hands 44, 46 of the individual to be tested. When the hand is positioned on the device with the wrist on surface 20 and the palm on one of the surfaces 16, 18, the hand is angled downwardly and outwardly with respect to the thumb slot. Thumb 48 is positioned between the respective guide surfaces 38, 40 and parallel to a vertical plane extending through centerline 34. The sidewall portions defining surfaces 19, 21 and the guide surfaces 38, 40 define wedges which properly position the thumb at a 45.degree. angle with respect to the adjacent index finger, as shown in FIG. 4. The hand is positioned and fixed so that the direction of thumb movement is limited. Movement is permitted substantially only in a direction caused by the intrinsic hand muscles innervated by the median nerve after it passes through the carpal tunnel.

In order to measure the strength of the muscles or the force vector generated by these muscles in such movement of the thumb, a force measuring means is provided. In the presently preferred form, the force measuring means includes a generally semicircular or half moon thumb cradle 50. Cradle 50 is suspended by a force transmission member, rod, line or chain 52. An upper end 54 of member 52 is connected to a force transducer or dynamometer 56. Dynamometer 56 measures the force generated by the downward movement of the thumb, as illustrated in FIG. 3. Transducer 56 is preferably a commercially available electric, miniature force transducer. It is connected to an amplifier or other electronic instruments (not shown) for the measurement of forces. Such measurements are performed without significant downward movement or motion of the thumb with respect to the transducer. One type of acceptable miniature force transducer is manufactured for measurement of tensile forces. The force applied to cradle 50 is transmitted to the transducer 56 by member 52. In one available transducer, the force is applied to a measuring body whose deformation is converted by four full-bridge strain gauges into a force proportional electrical signal. This electrical signal or output from the transducer may be amplified by readily available components to provide a readout. Other force transducers may be used. Any device which is capable of measuring the forces involved and providing a readout proportional thereto may be used.

FIG. 5 illustrates a partially dissected human hand. As shown, the transverse carpal ligament 60 extends over the carpal bones of the wrist. A median nerve 62 extends beneath ligament 60. Nerve 62 innervates the abductor pollicis brevis muscle 64 (shown cut in FIG. 5), the opponens pollicis muscle 66 and the superficial head of the flexor pollicis brevis muscle 68. In order to isolate the force measured by the device, the thumb and hand are positioned on device 10 so that cradle 50 is at the interphalangeal joint. Sidewalls 24, 26 and their respective surfaces 19, 21 define a wedge which is inserted into the space between the thumb and the index finger. The hand is positioned and fixed. The force measured upon downward movement of the thumb is that generated by such intrinsic hand muscles supplied by the median nerve.

By testing a statistically significant number of individuals from appropriate control groups, measurements of the strength of these muscles may be made to determine a "normal" strength value. Once a value or range of values which indicate "normal" strength is determined, the device may be used to screen or test individuals and compare their results against such normal values.

In a typical screening procedure, multiple readings would be taken to determine an average strength value for the individual. If the strength is that indicated for the normal individual, no objective weakness would be indicated. This would provide the physician with an objective indication that there is no carpal tunnel syndrome associated weakness with the individual. The objective indication of normal strength would not, however, exclude the presence of mild disease.

If the user tests at an abnormally low strength level, further evaluation would be advisable. Such low levels would indicate that highly repetitive, strenuous use of the hands be eliminated. Various tests could then be conducted on the individual to identify the precise nature of the disease and eliminate other causes besides those associated with carpal tunnel syndrome. The present invention provides, for the first time, an objective indication or screening capability. An early identification of a potential problem may be made and the effectiveness of treatment increased. Restriction of the individual's activities may prevent worsening of the problem and avoid the need for surgical treatment. In the work environment, the individual could be moved to a different job in which strain on the wrist would be minimized. This would limit the costs involved with lost work time and the like before a severe problem occurs.

In view of the foregoing description, those of ordinary skill in the art may envision various modifications which would not depart from the inventive concepts disclosed herein. For example, shims may be provided to reduce the spacing between thumb guide surfaces 38, 40 to adapt the device to a smaller than average hand. The principal consideration is to limit movement of the thumb to a defined plane along a defined vector. Some limited nonvertical movement should not affect the validity of the output since the distances involved are extremely small. Further, it is believed that other force measuring means besides the dynamometer, cradle and flexible transmission means disclosed could be employed. This approach is, however, presently preferred. It is, therefore, expressly intended that the above description should be considered as only that of the preferred embodiment. The true spirit and scope of the present invention may be determined by reference to the appended claims.