Peter J. Keir

Effects of finger posture on carpal tunnel pressure during wrist motion

Persistent elevations in carpal tunnel pressure may aggravate carpal tunnel syndrome. This study examined the effects of finger posture on carpal tunnel pressure during wrist motion. Carpal tunnel hydrostatic pressure was measured using a saline-filled catheter inserted into the nondominant wrists of 14 healthy individuals. Range of motion tasks of wrist flexion-extension and radioulnar deviation were repeated with metacarpophalangeal (MCP) joint angles of 0 degrees, 45 degrees, and 90 degrees flexion. Pressures were significantly greater with the fingers straight (MCP = 0 degrees) than when the MCP joints were flexed to 45 degrees for all radioulnar deviation angles and from 10 degrees of wrist flexion to all angles of wrist extension tested. Pressures were also significantly higher with MCP joints at 0 degrees than at 90 degrees for wrist extension angles from 10 degrees to 40 degrees. Pressures increased to over 30 mm Hg (4.0 kPa) in some wrist extension and ulnar and radially deviated postures. Finger and wrist postures should be considered when designing splints or evaluating tasks for patients with carpal tunnel syndrome.

The effects of tendon load and posture on carpal tunnel pressure

Two pressure measurement techniques (catheter and bulb) were used to decompose the effects of tendon loads on carpal tunnel pressure (CTP). The catheter technique measures true hydrostatic pressure, whereas the bulb technique is a estimate of contact force or pressure on the median nerve. Eight cadaveric wrists were moved through a range of flexion-extension (0 degrees, 10 degrees, 20 degrees, 30 degrees, and 45 degrees of each) and radioulnar deviation (10 degrees and 20 degrees radial and 0 degrees, 10 degrees, 20 degrees, and 30 degrees ulnar) while CTPs were measured under 4 muscle loading conditions with the thumb, index, and long finger in a pinch-grip posture. The first of these was zero load. Then a 1-kg mass was applied in turn to both flexors of the index and long fingers, the palmaris longus (PL); and the flexor pollicis longus. The hydrostatic pressure was found to be affected by both wrist posture and tendon load. With no load, highest pressures were seen in wrist extension. Muscular loading elevated CPT, particularly the loading of palmaris longus with the wrist in extension and the digital flexors with the wrist flexed. Bulb pressure measurements, related to local contact forces by the digital flexors, indicated the highest loads on the median nerve with the wrist flexed. Palmaris longus loading created the highest pressures in extension and only moderate pressure in flexion, indicating that it may alter the geometry of the transverse carpal ligament. In view of the data from this study, it is necessary to incorporate measures of hydrostatic pressure and local contact forces to describe possible trauma to the median nerve in the carpal tunnel, as neither appears sufficient when used independently.

Effects of posture, movement and hand load on shoulder muscle activity.

The influence of external factors such as arm posture, hand loading and dynamic exertion on shoulder muscle activity is needed to provide insight into the relationship between internal and external loading of the shoulder joint. Surface electromyography was collected from 8 upper extremity muscles on 16 participants who performed isometric and dynamic shoulder exertions in three shoulder planes (flexion, mid-abduction and abduction) covering four shoulder elevation angles (30 degrees , 60 degrees , 90 degrees and 120 degrees). Shoulder exertions were performed under three hand load conditions: no load, holding a 0.5 kg load and 30% grip. It was found that adding a 0.5 kg load to the hand increased shoulder muscle activity by 4% maximum voluntary excitation (MVE), across all postures and velocities. Performing a simultaneous shoulder exertion and hand grip led to posture specific redistribution of shoulder muscle activity that was consistent for both isometric and dynamic exertions. When gripping, anterior and middle deltoid activity decreased by 2% MVE, while posterior deltoid, infraspinatus and trapezius activity increased by 2% MVE and biceps brachii activity increased by 6% MVE. Increased biceps brachii activity with gripping may be an initiating factor for the changes in shoulder muscle activity. The finding that hand gripping altered muscle activation, and thus the internal loading, of the shoulder may play an important role in shoulder injury development and rehabilitation.

Guidelines for wrist posture based on carpal tunnel pressure thresholds.

Objective: To develop work guidelines for wrist posture based on carpal tunnel pressure. Background: Wrist posture is considered a risk factor for distal upper extremity musculoskeletal disorders, and sustained wrist deviation from neutral at work may be associated with carpal tunnel syndrome. However, the physiologic basis for wrist posture guidelines at work is limited. Methods: The relationship of wrist posture to carpal tunnel pressure was examined in 37 healthy participants. The participants slowly moved their wrists in extension-flexion and radioulnar deviation while wrist posture and carpal tunnel pressure were recorded. The wrist postures associated with pressures of 25 and 30 mmHg were identified for each motion and used to determine the 25th percentile wrist angles (the angles that protect 75% of the study population from reaching a pressure of 25 or 30 mmHg). Results: Using 30 mmHg, the 25th percentile angles were 32.7° (95% confidence interval [CI] = 27.2°—38.1°) for wrist extension, 48.6° (37.7°—59.4°) for flexion, 21.8° (14.7°—29.0°) for radial deviation, and 14.5° (9.6°—19.4°) for ulnar deviation. For 25 mmHg, the 25th percentile angles were 26.6° and 37.7° for extension and flexion, with radial and ulnar deviation being 17.8° and 12.1°, respectively. Conclusion: Further research can incorporate the independent contributions of pinch force and finger posture into this model. Application: The method presented can provide wrist posture guidelines for the design of tools and hand-intensive tasks.

Effect of wrist posture on carpal tunnel pressure while typing.

Long weekly hours of keyboard use may lead to or aggravate carpal tunnel syndrome. The effects of typing on fluid pressure in the carpal tunnel, a possible mediator of carpal tunnel syndrome, are unknown. Twenty healthy subjects participated in a laboratory study to investigate the effects of typing at different wrist postures on carpal tunnel pressure of the right hand. Changes in wrist flexion/extension angle (p = 0.01) and radial/ulnar deviation angle (p = 0.03) independently altered carpal tunnel pressure; wrist deviations in extension or radial deviation were associated with an increase in pressure. The activity of typing independently elevated carpal tunnel pressure (p = 0.001) relative to the static hand held in the same posture. This information can guide the design and use of keyboards and workstations in order to minimize carpal tunnel pressure while typing. The findings may also be useful to clinicians and ergonomists in the management of patients with carpal tunnel syndrome who use a keyboard.

Continuous assessment of work activities and posture in long-term care nurses

The high prevalence of low back injuries in nursing has prompted the use of mechanical lift assists while overall assessment of activities and postures remains limited. The purpose of this study was to chronicle trunk posture and work tasks of long-term healthcare professionals. An inclinometer monitored trunk posture for 27 workers, 20 of whom were also observed continuously throughout their shift. Patient lifts and transfers accounted for less than 4% of the shift while patient care, unloaded standing and walking and miscellaneous tasks accounted for 85%. Manual lifts and transfers occurred twice as often as mechanically assisted lifts but took only half the time. The workers had a median trunk flexion angle of 9.2°, spent 25% of their time flexed beyond 30° and had peak flexion angles greater than 75° in many tasks. Analysis of posture throughout the entire working shift indicates that, in addition to lifts and transfers, emphasis needs to be placed on patient care and miscellaneous activities when assessing injury risk for nurses. Statement of Relevance: Patient handling has been the focus in the effort to reduce back pain and injury in nursing. In addition to the use of mechanical lifts, there is a need to examine other aspects of nursing, including patient care and other ancillary tasks, which comprise the majority of the work shift and, while often unloaded, exhibit extreme postures that may also lead to injury.

The effect of typing posture on wrist extensor muscle loading.

High static loading of the forearm extensor musculature has been observed during keying tasks. To reduce the level of loading, one must first understand the contributing factors. A simulation of the human finger was used to determine muscle force contributions during a static index finger key press at several wrist postures. The planar model included active and passive muscle forces of the intrinsic and extrinsic finger muscles. The model was expanded to include the passive forces from the other fingers as well as the weight of the hand to determine the exertion required of the wrist extensor muscles to maintain the given wrist and finger postures. Model results indicated that greater than 25% of maximal exertion is required of the wrist extensors when the wrist is extended to 30°. The increased moment contribution from passive forces of the extrinsic finger flexor muscles was responsible for the majority of the increased wrist extensor contribution as the wrist was extended. These findings are in relative agreement with previous electromyographic studies and may indicate a mechanism for forearm extensor pain in office workers. Potential applications of this research include ergonomic modeling of the upper limb to determine internal loads that may lead to work-related disorders.

Forearm posture and grip effects during push and pull tasks

Direction of loading and performance of multiple tasks have been shown to elevate muscle activity in the upper extremity. The purpose of this study was to evaluate the effects of gripping on muscle activity and applied force during pushing and pulling tasks with three forearm postures. Twelve volunteers performed five hand-based tasks in supinated, neutral and pronated forearm postures with the elbow at 90° and upper arm vertical. All tasks were performed with the right (dominant) hand and included hand grip alone, push and pull with and without hand grip. Surface EMG from eight upper extremity muscles, hand grip force, tri-axial push and pull forces and wrist angles were recorded during the 10 s trials. The addition of a pull force to hand grip elevated activity in all forearm muscles (all p < 0.017). During all push with grip tasks, forearm extensor muscle activity tended to increase when compared with grip only while flexor activity tended to decrease. Forearm extensor muscle activity was higher with the forearm pronated compared with neutral and supinated postures during most isolated grip tasks and push or pull with grip tasks (all p < 0.017). When the grip dynamometer was rotated so that the push and pull forces could act to assist in creating grip force, forearm muscle activity generally decreased. These results provide strategies for reducing forearm muscle loading in the workplace. Statement of Relevance: Tools and tasks designed to take advantage of coupling grip with push or pull actions may be beneficial in reducing stress and injury in the muscles of the forearm. These factors should be considered in assessing the workplace in terms of acute and cumulative loading.

Tendon and nerve excursion in the carpal tunnel in healthy and CTD wrists.

BACKGROUND During hand and finger motions, friction between flexor digitorum superficialis tendon and the median nerve is thought to play a role in the development of cumulative trauma disorders. This study investigated three methods to determine excursions of the flexor digitorum superficialis tendon and median nerve using several motions. METHODS Twenty-five participants (mean age 37.2 years SD 13.4) were classified as healthy (n=16), self-reported distal upper extremity cumulative trauma disorders (6), or wheelchair users (3). Static carpal tunnel measurements were taken and displacements of the index flexor digitorum superficialis tendon and median nerve were determined via the velocity time integral and post hoc integration of the Doppler ultrasound waveform using a 12-5 MHz linear array transducer, as well as using predictive equations. FINDINGS Median nerves in symptomatic wrists were larger than healthy wrists by 4.2 mm(2) (left) and 4.1 mm(2) (right) proximally to less than 1.4 mm(2) distally. In healthy wrists, left-right tendon excursion differences ranged from 0.7 mm to 4.3 mm depending on the motion while left to right differences in symptomatic wrists ranged over 22.2 mm. Ultrasound measures of tendon excursion overestimated those determined using predictive equations and were poorly correlated. The ratio of median nerve excursion to tendon excursion was lower in finger only motions compared to wrist motions with or without finger motion. INTERPRETATION Spectral Doppler ultrasound imaging provided insights into tendon excursion that was not apparent with mathematical modeling. The difference in excursion between finger motions and wrist motions could be beneficial in therapeutic techniques.

Continuous assessment of low back loads in long-term care nurses

Considerable effort has been spent evaluating aspects of low back injury risk in nursing yet comprehensive evaluation of all work tasks has been limited. The purpose of this study was to evaluate peak and cumulative lumbar spine loads experienced by personal support workers. A total of 20 female long-term care workers were observed and had trunk posture monitored via an inclinometer throughout their shift. When adjusted for an 8-h workday, workers experienced cumulative loads of 21.3 ± 4.6 MNs, 1.8 ± 0.6 MNs and 2.9 ± 1.4 MNs for compression, lateral and anterior shear, respectively. Patient care, unloaded standing, walking and miscellaneous tasks accounted for almost 80% of cumulative compression, while lifts and transfers accounted for less than 10%. Mechanical lift assists reduced peak loads and contributed minimally to cumulative loading. These findings suggest that both peak and cumulative spine loads should be considered when evaluating injury risk in the nursing profession. Statement of Relevance: This study has shown that tasks other than patient transfers and lifts are important in the assessment of low back injury risk in nurses. The method developed is a relatively straightforward approach that can be used to estimate peak and cumulative spine load to provide insight to risk of injury in many occupational settings.