Magnetic resonance imaging as a research tool for biomechanical studies of the wrist.

The field of biomechanics has welcomed magnetic resonance imaging (MRI) as a research tool to provide quantified anatomy of various body parts in vivo. The ability to view, reconstruct, and analyze images of an intact system under varying conditions has improved our knowledge of functional anatomy. This article forms a review of MRI use in biomechanics research, with examples from several areas and an emphasis on the distal upper extremity. Biomechanical parameters such as muscle fascicle directions of pull, moment arms in three dimensions, muscle cross-sectional areas, and detailed muscle geometry data are prevalent because of advances in imaging technology. This has resulted in improved anatomic realism in biomechanical models. Wrist biomechanics research has benefited greatly using MRI. The unique anatomy of the carpal tunnel, and the concerns regarding carpal tunnel syndrome, have prompted numerous studies examining the contents of the carpal tunnel, its shape, and its volume. These studies are presented, as is an analysis of the finger flexor tendons as they pass through the carpal tunnel. These imaging-based studies all examine the aspects of the potential mechanisms for median nerve compression at the wrist. MRI is a tremendously valuable tool in biomechanics research, especially in the search for the mechanisms of carpal tunnel syndrome and wrist function, providing both visual representation and quantitative evaluation of anatomic phenomena.

Canadian musculoskeletal fitness norms.

The purpose of this report is to provide representative norms for measurements of musculoskeletal fitness (partial curl-ups, vertical jump, and leg power) for which Canadian norms are not currently available. Partial curl-ups, vertical jump, trunk flexion (sit and reach), grip strength, muscular endurance (push-ups), body mass index, and subcutaneous adiposity (sum of five skinfolds) were assessed, and leg power was calculated in 571 self-reportedly healthy participants (312 females and 259 males) aged 15-69 yr. The representativeness of the sample was confirmed by statistically comparing the fitness characteristics of the participants in the present study to those in the Canada Fitness Survey of 1981 and the Campbell's Survey of 1988. Normative data for partial curl-ups, vertical jump, and leg power were generated for males and females in six age groups (15-19, 20-29, 30-39, 40-49, 50-59, 60-69) for use in fitness appraisal protocols for the Canadian population.

Flexor muscle incursion into the carpal tunnel: a mechanism for increased carpal tunnel pressure?

PURPOSE: To explore the hypothesis that the extrinsic finger flexor muscles have the potential to move into the proximal end of the carpal tunnel with wrist extension. METHODS: The most distal muscle fibres from the deep and superficial finger flexors were measured relative to the pisiform bone in 18 cadaveric specimens. Muscle excursions during wrist extension were calculated using regression equations previously reported in the literature. RESULTS: The mean distances from the pisiform were 9.3 and 4.9 mm for the deep and superficial flexors, respectively. Ten flexor muscle bellies were at the level of or distal to the pisiform bone in the anatomical position, while 17 of 36 were within 5 mm. DISCUSSION: The excursions expected with wrist extension indicate that many muscles have the potential to enter the carpal tunnel, especially those within 5 mm of the pisiform bone. Comparing the expected excursions to recent pressure data, corroborating support for the pressure increase is found. CONCLUSION: Although not directly measured, the results of this study indicate incursion of the flexor muscles into the carpal tunnel space, particularly with wrist extension, is a plausible mechanism for increased carpal tunnel pressure. RELEVANCE: Proposing a mechanism by which carpal tunnel pressure is elevated during wrist and finger extension is a stepping stone to determining the etiology of the disease itself. Finding that the flexor muscle bellies appear to enter the carpal tunnel with wrist extension indicates that use of the flexor muscles should be avoided when the wrist and fingers are extended.

Effects of computer mouse design and task on carpal tunnel pressure.

Computer mouse use has become an integral part of office work in the past decade. Intensive mouse use has been associated with increased risk of upper extremity musculoskeletal disorders, including carpal tunnel syndrome. Sustained, elevated fluid pressure in the carpal tunnel may play a role in the pathophysiology of carpal tunnel syndrome. Carpal tunnel pressure was measured in 14 healthy individuals while they performed tasks using three different computer mice. Participants performed a multidirectional dragging ('drag and drop') task starting with the hand resting (static posture) on the mouse. With one mouse, an additional pointing ('point-and-click') task was performed. All mice were associated with similar wrist extension postures (p = 0.41) and carpal tunnel pressures (p = 0.48). Pressures were significantly greater during dragging and pointing tasks than when resting the hand (static posture) on the mouse (p = 0.003). The mean pressures during the dragging tasks were 28.8-33.1 mmHg, approximately 12 mmHg greater than the static postures. Pressures during the dragging task were higher than the pointing task (33.1 versus 28.0 mmHg), although the difference was borderline non-significant (p = 0.06). In many participants the carpal tunnel pressures measured during mouse use were greater than pressures known to alter nerve function and structure, indicating that jobs with long periods of intensive mouse use may be at an increased risk of median mononeuropathy. A recommendation is made to minimize wrist extension, minimize prolonged dragging tasks and frequently perform other tasks with the mousing hand.

Changes in geometry of the finger flexor tendons in the carpal tunnel with wrist posture and tendon load: an MRI study on normal wrists.

OBJECTIVES: (1) To develop a methodology to determine the trajectories of the digital flexor tendons using MRI. (2) To examine changes in tendon trajectories due to wrist posture, with and without pinch force. (3) To calculate the radius of curvature of the flexor tendons and note implications for contact forces on the median nerve. (4) To assess the use of Landsmeer's models at the wrist. DESIGN: Finger flexor tendon centroids were digitized from magnetic resonance images of the carpal tunnel and the tendon paths were determined analytically. Radii of curvature were calculated from the tendon paths. BACKGROUND: Landsmeer's models of joint-tendon interaction (Landsmeer, 1961) have been used to determine moment arms and radius of curvature of the tendon paths about articulations. An explanation for a biomechanical cause of work-related carpal tunnel syndrome originated from these models. METHODS: Three healthy male participants had their right wrist scanned while splinted in four wrist postures (flexed to 20 degrees, 45 degrees, neutral, extended to 20 degrees ) with and without maintaining a 10 N pinch grip. 20-24 cross-sectional images were used for each condition. RESULTS: Volar movement of the tendons was seen with wrist flexion and the opposite was true with extension. Tendon intersection angles were calculated between the tendon as it entered the carpal tunnel and as it exited the tunnel and were 50-65% of the wrist angle (R(2)=0.81-0.96). The radius of curvature was smallest (mean=82-127 mm) with an active pinch grip with the wrist splinted at 45 degrees of flexion (mean actual wrist angle 37 degrees ). CONCLUSIONS: The radius of flexor tendon curvature is not constant as previously assumed and is larger than previous estimates. The addition of tendon force with the wrist flexed acts to reduce the radius of curvature which further increases the contact stress on the median nerve and other wrist structures. The use of MRI to determine the tendon paths has provided new insight into the relationships between the finger flexor tendons and other structures at the wrist. RELEVANCE: These findings provide data for biomechanical models of the carpal tunnel and predict the possible pathophysiology of work-related carpal tunnel syndrome.

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.

Fingertip loading and carpal tunnel pressure: differences between a pinching and a pressing task.

Carpal tunnel syndrome may be caused by repeated or sustained elevated carpal tunnel pressure. This study examined the relationship between carpal tunnel pressure, posture, and fingertip load. In 20 healthy individuals, carpal tunnel pressure was measured with a catheter inserted into the carpal tunnel of the dominant hand and connected to a pressure transducer. With the wrist in a pressure-neutral position, the subjects pressed on a force transducer with the index finger to levels of 0, 5, 10, and 15 N. They then pinched the transducer at the same levels of force. For both fingertip-loading postures, the carpal tunnel pressure increased with increasing fingertip load. Carpal tunnel pressures were significantly greater (p < 0.015) for the pinching task (14.2, 29.9, 41.9, and 49.7 mm Hg [1.89, 3.99, 5.59, and 6.63 kPa] for 0, 5, 10, and 15 N force levels, respectively) than for simple finger pressing (7.8, 14.1, 20.0, and 33.8 mm Hg [1.04, 1.88, 2.67, and 4.51 kPa]). This study indicates that although the external load on the finger remained constant between the two tasks, the internal loading, as measured by carpal tunnel pressure, experienced a near 2-fold increase by using a pinch grip. These findings should be given consideration in designing work tasks and tools because relatively low fingertip forces, especially in a pinch grip, elevate carpal tunnel pressures to levels that, if prolonged, may lead to the development or exacerbation of 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 static fingertip loading on carpal tunnel pressure.

The purpose of this study was to explore the relationship between carpal tunnel pressure and fingertip force during a simple pressing task. Carpal tunnel pressure was measured in 15 healthy volunteers by means of a saline-filled catheter inserted percutaneously into the carpal tunnel of the nondominant hand. The subjects pressed on a load cell with the tip of the index finger and with 0, 6, 9, and 12 N of force. The task was repeated in 10 wrist postures: neutral; 10 and 20 degrees of ulnar deviation; 10 degrees of radial deviation; and 15, 30, and 45 degrees of both flexion and extension. Fingertip loading significantly increased carpal tunnel pressure for all wrist angles (p = 0.0001). Post hoc analyses identified significant increase (p < 0.05) in carpal tunnel pressure between unloaded (0 N) and all loaded conditions, as well as between the 6 and 12 N load conditions. This study demonstrates that the process whereby fingertip loading elevates carpal tunnel pressure is independent of wrist posture and that relatively small fingertip loads have a large effect on carpal tunnel pressure. It also reveals the response characteristics of carpal tunnel pressure to fingertip loading, which is one step in understanding the relationship between sustained grip and pinch activities and the aggravation or development of median neuropathy at the wrist.

Passive properties of the forearm musculature with reference to hand and finger postures.

OBJECTIVE: To quantify individual forearm muscle passive forces and evaluate their impact on hand function. DESIGN: The passive force-length properties of the 24 extrinsic hand and wrist muscles were determined in five fresh frozen cadaver arms. BACKGROUND: Muscle force production is a summation of the active and passive force components. The passive properties of the extrinsic finger musculature and wrist musculature appear to strongly affect both hand posture and hand movement. METHODS: The passive force-length properties of extrinsic hand and wrist muscles were determined by applying a slow, continuous extension to each muscle and recording the resulting tension. Each force-length curve was fit using exponential regression and were related to specific joint rotations and seven hand postures by calculating the muscle excursions for those postures. RESULTS: The exponential passive force-length relationship explained over 97% of the experimental variance. The largest passive forces were elicited in the digital extensors in grips involving large flexion angles such as tip pinch, key pinch, and a briefcase grip. CONCLUSIONS: The passive properties of the extrinsic finger musculature and wrist musculature affect both hand posture and movement especially in postures with flexed wrist and fingers.