Safe treatment of trigger finger with longitudinal and transverse landmarks: an anatomic study of the border fingers for percutaneous release

Transverse landmarks have recently been determined to predict the proximal and distal edges of the A1 pulley for trigger finger release. Percutaneous A1 pulley release has been discouraged for the border digits because of the risk of injury to the neurovascular structures of the index and small fingers. The purpose of the study was to identify longitudinal surface landmarks to prevent injury to the neurovascular bundles during percutaneous A1 pulley release of the ulnar and radial border digits. Longitudinal surface landmarks were identified and marked on 29 cadaver hands. Proximal and distal landmarks for the longitudinal vector through which the A1 pulley of the small finger was released include the midline of the proximal digital crease and the scaphoid tubercle. Proximal and distal landmarks for the longitudinal line through which the index finger A1 pulley was released include the midline of proximal digital crease and radial edge of the pisiform. Longitudinal incisions were performed between these landmarks, straight through the skin and deep enough to score the A1 pulley. The distance of the medial edge of the neurovascular structures from the longitudinal incision in the A1 pulley was measured for each small finger and index finger. Using these longitudinal landmarks for the index and small fingers, none of the neurovascular structures was injured while performing these longitudinal incisions through the skin, scoring the A1 pulley. In fact, the average distance for the neurovascular structures from the longitudinal vector of the small finger was 5.4 +/- 1.4 mm radially and 6.7 +/- 1.9 mm ulnarly. The average distance for the neurovascular structures from the longitudinal line of the index finger was 8.5 +/- 1.8 mm radially and 6.2 +/- 1.7 mm ulnarly. Based on the findings of this anatomical study, these longitudinal landmarks can be used to avoid injury to neurovascular structures in the management of trigger finger involving the border digits with steroid-injection, open, or percutaneous A1 pulley release.     

New hand bones of Hadropithecus stenognathus: implications for the paleobiology of the Archaeolemuridae

A partial, associated skeleton of Hadropithecus stenognathus (AHA-I) was discovered in 2003 at Andrahomana Cave in southeastern Madagascar. Among the postcranial elements found were the first hand bones (right scaphoid, right hamate, left first metacarpal, and right and left fifth metacarpals) attributed to this rare subfossil lemur. These hand bones were compared to those of extant strepsirrhines and catarrhines in order to infer the positional adaptations of Hadropithecus, and they were compared to those of Archaeolemur in order to assess variation in hand morphology among archaeolemurids. The scaphoid tubercle does not project palmarly as in suspensory and climbing taxa, and the hamate has no hook at all (just a small tubercle), which also points to a poorly developed carpal tunnel. There is a distinctive, radioulnarly directed "spiral" facet for articulation with the triquetrum that is most similar in orientation to that of more terrestrial primates (i.e., Lemur catta, Papio, and Gorilla). The first metacarpal is very reduced and represents only 48% of the length of metacarpal V, as in Archaeolemur, which suggests that pollical grasping of arboreal supports was not important. Compared to Archaeolemur, the shaft of metacarpal V is gracile, and the head has no dorsal ridge and lacks characteristics functionally associated with digitigrade, extended metacarpophalangeal joint postures. Proximally, the articular facet for the hamate is oriented more dorsally. Thus, the carpometacarpal joint V appears to have a distinctive hyperextended set, which has no analog among living or extinct primates. The carpals of Hadropithecus are diagnostic of a pronograde, arboreal and terrestrial (although not digitigrade) locomotor repertoire that typifies Lemur catta and some Old World monkeys. No clinging, suspensory, or climbing specializations that characterize indriids or lorises can be found in the hand of this subfossil lemur. The hand of Hadropithecus likely had similar ranges of movement at the radiocarpal and midcarpal joints as of those of pronograde primates, such as lemurids, for which the hand is held in a more extended, pronated, and neutral (i.e., showing less ulnar deviation) position during locomotion in comparison to that of vertical clingers or slow climbers. Although highly autapomorphic, the hand of Hadropithecus resembles that of its sister taxon, Archaeolemur, in having a very reduced pollex and an articular facet on the scaphoid for a sizeable prepollex. These unusual hand features reinforce the monophyly of the Archaeolemuridae.     

Free flap from the flexor aspect of the wrist for resurfacing defects of the hand and fingers

The distal portion of the flexor aspect of the forearm has been used as the donor site of full-thickness skin grafts, venous skin grafts, and Chinese forearm flaps. This article describes the use of a free flap harvested from the flexor aspect of the wrist and based on the superficial palmar branch of the radial artery to repair skin defects of the hand and fingers. The advantages of this flap are as follows: (1) the operative field is the same; (2) the radial artery is preserved; (3) it is thin, pliable, and hairless and thus can supply a gliding surface for tendons beneath it; (4) when it involves a palmaris longus tendon and/or the palmar cutaneous branch of the median nerve, it can be used as a vascularized tendon or nerve graft; and (5) in view of the flow-through type of the pedicle of the flap, the digital artery can be reconstructed simultaneously. However, it should be noted that a hypesthesia in the proximal central carpal area remains when the palmar cutaneous branch of the median nerve is harvested as a vascularized nerve graft. The scar of the donor site should be left in the distal wrist crease. If it is not lying in the distal wrist crease, it may suggest that the patient has tried to commit suicide.     

Innervated radial thenar flap combined with radial forearm flap transfer for thumb reconstruction

A radial thenar flap combined with radial forearm flap was used for the reconstruction of the ipsilateral thumb in four patients. Vascular supply of the combined flap was based on the radial artery and extending the vascular pedicle to the superficial palmar branch of the radial artery. The flap was sensated by the palmar branch of the superficial radial nerve. The size of the flap averaged 15 x 5 cm and the innervated region of the thenar eminence was an area approximately 5 x 3 cm located over the proximal parts of the abductor pollicis brevis and opponens pollicis muscles. The flap was transferred as a free flap in three patients and as an advancement flap in one patient. The flaps survived completely without complications. Satisfactory restoration of sensation was achieved in the flap area, as shown by 6 mm of average moving two-point discrimination. This combined flap may be a feasible reconstructive option for large palmar defects of the fingers such as degloving injuries.     

New wrist bones of the Malagasy giant subfossil lemurs

Recently discovered wrist bones of the Malagasy subfossil lemurs Babakotia radofilai, Palaeopropithecus ingens, Mesopropithecus dolichobrachion, and Megaladapis madagascariensis shed new light on the postcranial morphologies and positional behaviors that characterized these extinct primates. Wrist bones of P. ingens resemble those of certain modern hominoids in having a relatively enlarged ulnar head and dorsally extended articular surface on the hamate, features related to a large range of rotation at the inferior radioulnar and midcarpal joints. The scaphoid of P. ingens is also similar to that of the extant tree sloth Choloepus in having an elongate, palmarly directed tubercle forming a deep radial margin of the carpal tunnel for the passage of large digital flexors. In contrast, wrist remains of Megaladapis edwardsi and M. madagascariensis exhibit traits observed in the hands of extant pronograde, arboreal primates; these include a dorsopalmarly expanded pisiform and well-developed "spiral" facet on the hamate. Moreover, Megaladapis spp. and Mesopropithecus dolichobrachion possess bony tubercles (e.g., scaphoid tubercle and hamate hamulus) forming the carpal tunnel that are relatively similar in length to those of modern pronograde lemurs. Babakotia and Mesopropithecus differ from Megaladapis in exhibiting features of the midcarpal joint related to frequent supination and radioulnar deviation of the hand characteristic of animals that use vertical and quadrumanous climbing in their foraging behaviors. Comparative analysis of subfossil lemur wrist morphology complements and expands upon prior inferences based on other regions of the postcranial skeleton, and suggests a considerable degree of locomotor and postural heterogeneity among these recently extinct primates.     

The 'loge de Guyon'. A contribution to the clinical anatomy of the human hand

In 40 hands of adults the 'loge de Guyon', a narrow bounded area within the proximal hypothenar region, has been dissected to realize an exact determination of the important characteristics of size. Beside measurements of the wall structures in the region of the pisiform bone, the hook of hamate and the entrances of the loge, variations of muscles and the position of the ulnar artery and nerve with their terminal branches have also been examined. The deep palmar branch of the ulnar artery crosses the rami of the ulnar nerve on the palmar side in 65% of cases. The branch of the artery crosses on the dorsal side in 30% and in 5% the ramus profundus of the ulnar artery runs between both terminal branches of the ulnar nerve. The clinical significance of the loge is emphasized, whereas the irregular nomenclature in the national and international literature is discussed in detail.     

Finite element analysis for transverse carpal ligament tensile strain and carpal arch area

Mechanics of carpal tunnel soft tissue, such as fat, muscle and transverse carpal ligament (TCL), around the median nerve may render the median nerve vulnerable to compression neuropathy. The purpose of this study was to understand the roles of carpal tunnel soft tissue mechanical properties and intratunnel pressure on the TCL tensile strain and carpal arch area (CAA) using finite element analysis (FEA). Manual segmentation of the thenar muscles, skin, fat, TCL, hamate bone, and trapezium bone in the transverse plane at distal carpal tunnel were obtained from B-mode ultrasound images of one cadaveric hand. Sensitivity analyses were conducted to examine the dependence of TCL tensile strain and CAA on TCL elastic modulus (0.125–10 MPa volar-dorsally; 1.375–110 MPa transversely), skin-fat and thenar muscle initial shear modulus (1.6–160 kPa for skin-fat; 0.425–42.5 kPa for muscle), and intratunnel pressure (60–480 mmHg). Predictions of TCL tensile strain under different intratunnel pressures were validated with the experimental data obtained on the same cadaveric hand. Results showed that skin, fat and muscles had little effect on the TCL tensile strain and CAA changes. However, TCL tensile strain and CAA increased with decreased elastic modulus of TCL and increased intratunnel pressure. The TCL tensile strain and CAA increased linearly with increased pressure while increased exponentially with decreased elastic modulus of TCL. Softening the TCL by decreasing the elastic modulus may be an alternative clinical approach to carpal tunnel expansion to accommodate elevated intratunnel pressure and alleviate median nerve compression neuropathy.     

Thenar and hypothenar muscles and their innervation by the ulnar and median nerves in the human hand.

The thenar and hypothenar muscles as well as their supplying nerves were analyzed with an improved dissecting method. Among the four thenar muscles, the m. abductor pollicis brevis (AbPB) has a separate muscle belly, whereas the m. opponens pollicis (OP), the superficial and deep heads of the flexor pollicis brevis (sFPB and dFPB), and the adductor pollicis (AdP) are fused with each other to make a single mass (deep thenar muscle group). These muscles are innervated by branches of the recurrent nerve and the accessory recurrent nerve from the median nerve as well as by terminal branches of the deep branch (ramus profundus) of the ulnar nerve. These three nerves frequently form a loop within the deep thenar muscle group (thenar loop), and a branch to the OP and one to deep parts of the sFPB often make a smaller loop (intrathenar loop), whereas the AbPB receives a separate nerve branch. Among the hypothenar muscle, the m. abductor digiti minimi and the m. flexor digiti minimi brevis are fused with each other, and their supplying nerves frequently form a loop in these muscles (intrahypothenar loop), whereas the m. opponens digiti minimi is separated from the others and receives a separate nerve branch. In the distribution pattern of supplying nerves to the thenar and hypothenar muscles, we find regularities in that they branch off in a regular manner from the ulnar and the median nerve, and that nerve branches to those muscles with fused bellies frequently communicate with each other to make loops.     

Clinical anatomy of the m. flexor carpi radialis tendon sheath

At the transitional zone from the forearm to the hand the insertion tendon of the m.flexor carpi radialis (FCR) glides on a fibrous and fatty cushion, which is connected dorsally with the joint capsule of the radiocarpal articulation. The tendon distally crosses the palmar side of the scaphoid tubercle and enters the dorsally curved rim of the trapezoid tubercle. At the level of the wrist joint the narrow tendon sheath begins, which extends to the insertion at the metacarpus. Immediately after entering the gliding tunnel the tendon branches off radially as a rule with an accessory fibre strand 8 mm in width to the scaphoid, trapezium and the joint capsule between these two bones. The insertion tendon regularly is attached to the palmar and radial surfaces of the second and third metacarpal bones. The wall of the osteofibrous gliding tunnel can be prominent following trauma, inflammation or arthrosis deformans in the trapezio-scaphoideal joint and may irritate the tendon (tendovaginosis stenosans). Against resistance forces pain will occur in the wrist joint during palmar flexion. The typical point of tenderness is situated at the entering of the tendon in the thenar region. Operative decompression will be effective by opening the radial wall of the tendon sheath from the carpal tunnel.     

Correction of first web space deficiency in congenital deformities of the hand with the pseudokite flap

The authors describe a new flap to enlarge or create a first web in patients with congenital deficiencies of the hand. This lozenge-shaped flap is harvested from the dorsoradial aspect of the index finger based on a narrow proximal skin bridge protecting its axial vascularization. A dorsal skin graft of the donor site is avoided by closure using a rhomboid flap. Procedures in 16 patients were performed and reviewed; the only complication was insertion of a small split-thickness graft in five patients to avoid tension at the metacarpophalangeal joint level. The flap lengthened the web fold an average of 3.2 cm without "pseudolengthening" the thumb.     

The arteria radialis superficialis. An unusual variation of the arteria radialis of man and its phylogenetic significance

During the clinical investigation of 570 soldiers of the German army, we were not able to feel the pulse in 5 cases at the typical place of the radial pulse in the distal part of the forearm. In these 5 cases we were able to find a subcutaneous artery which coursed superficial to the anatomical snuffbox and crossed superficial to the tendon of the extensor pollicis longus muscle. This superficial radial artery enters the deep aspect of the palm between the first and the second metacarpale bones. In three cases we found a bilateral occurrence of this artery, the other variations were observed unilaterally, two on the right side and one on the left side of the forearm. In the family of two patients other members were found who had the same variation of the radial artery. In one of the cases an arteriography of the vessels was made to find the exact anatomical course of the observed variation. We compared our results with the literature on this variation of the radial artery and found agreement on the following course for this vessel. The radial artery divides in the distal fourth of the forearm (5-7 cm proximal to the wrist joint) into two branches. The dorsal branch courses subcutaneously over the tendon of the brachioradialis muscle and runs over the tendon of the extensor pollicis longus muscle to enter the deep aspect of the palm in the first metacarpal space. This dorsal branch courses parallel to the superficial branch of the radial nerve. The palmar branch can be regarded as the 'normal' radial artery, which continues along the medial border of the brachioradialis muscle and courses deep under the tendons of the dorsal muscles of the thumb. The rare appearance (frequency approximately 1%) of a superficial radial artery in man has probably some phylogenetic importance. This is proven by studies on the comparative anatomy of mammals. This variation of the radial artery seems to be homologous to the superficial radial artery which is described in many lower mammals. In human embryos a superficial radial artery is found as well which courses parallel to the superficial branch of the radial nerve and ends on the dorsal side of the hand. Taking all the anatomical and embryological facts into consideration we propose to name this variation of the radial artery the 'arteria radialis superficialis'.     

Resection of the hook of the hamate. Its place in the treatment of median and ulnar nerve entrapment in the hand.

Poor results can be anticipated with conventional surgical decompression of nerves entrapped within the hand in (1) those with underlying systemic disease causing primary neuropathy, (2) those with combined median and ulnar nerve palsies, and (3) those who have been previously operated upon for nerve entrapment within the hand. Eighteen patients belonging in these categories were surgically treated by resection of the hook of the hamate and (in some) by intraneural neurolysis. Using this technique, we have decreased our failure rate from 20 percent to less than one percent. We believe that failures can be eliminated if the patients destined to have poor results from the usual treatment are identified preoperatively and a more aggressive surgical decompression is used on this "at risk" group.     

Refinements in pollicization: a 30-year experience

The thumb is a specialized organ with unique functions that cannot be replicated by any other digit. The most powerful technique for construction or reconstruction of a lost or missing thumb is index finger pollicization. In this article, the authors outline five technical refinements in this procedure that have evolved over the past 30 years in 313 cases. These refinements improve appearance and function, and include (1) modification of the incisions to produce a well-contoured web space, (2) metacarpal head positioning for optimal recreation of the carpal arch, (3) extrinsic tendon repositioning for improved pronation of the new thumb, (4) intrinsic tendon repositioning for maximal strength, and (5) thenar augmentation with an adipofascial flap in select cases for improved appearance.     

Optimizing the correction of severe postburn hand deformities by using aggressive contracture releases and fasciocutaneous free-tissue transfers

Severe postburn hand deformities were classified into three major patterns: hyperextension deformity of the metacarpophalangeal joint of the fingers with dorsal contracture of the hand, adduction contracture of the thumb with hyperextension deformity of the interphalangeal joint, and flexion contracture of the palm. Over the past 6 years, 18 cases of severe postburn hand deformities were corrected with extensor tenotomy, joint capsulotomy, and release of volar plate and collateral ligament. The soft-tissue defects were reconstructed with various fasciocutaneous free flaps, including the arterialized venous flap (n = 4), dorsalis pedis flap (n = 3), posterior interosseous flap (n = 3), first web space free flap (n = 3), and radial forearm flap (n = 1). Early active physical therapy was applied. All flaps survived. Functional return of pinch and grip strength was possible in 16 cases. In 11 cases of reconstruction of the dorsum of the hand, the total active range of motion in all joints of the fingers averaged 140 degrees. The mean grip strength was 16.5 kg and key pinch was 3.5 kg. In palm reconstruction, the wider contact area facilitated the grasping of larger objects. In thumb reconstruction, key-pinch increased to 5.5 kg and the angle of the first web space increased to 45 degrees. Jebsen's hand function test was not possible before surgery; postoperatively, it showed more functional recovery in gross motion and in the dominant hand. Aggressive contracture release of the bone,joints, tendons, and soft tissue is required for optimal results in the correction of severe postburn hand deformities. Various fasciocutaneous free flaps used to reconstruct the defect provide early motion, appropriate thinness, and excellent cosmesis of the hand.     

Quantification of fingertip force reduction in the forefinger following simulated paralysis of extensor and intrinsic muscles

Objective estimates of fingertip force reduction following peripheral nerve injuries would assist clinicians in setting realistic expectations for rehabilitating strength of grasp. We quantified the reduction in fingertip force that can be biomechanically attributed to paralysis of the groups of muscles associated with low radial and ulnar palsies. We mounted 11 fresh cadaveric hands (5 right, 6 left) on a frame, placed their forefingers in a functional posture (neutral abduction, 45° of flexion at the metacarpophalangeal and proximal interphalangeal joints, and 10° at the distal interphalangeal joint) and pinned the distal phalanx to a six-axis dynamometer. We pulled on individual tendons with tensions up to 25% of maximal isometric force of their associated muscle and measured fingertip force and torque output. Based on these measurements, we predicted the optimal combination of tendon tensions that maximized palmar force (analogous to tip pinch force, directed perpendicularly from the midpoint of the distal phalanx, in the plane of finger flexion–extension) for three cases: non-paretic (all muscles of forefinger available), low radial palsy (extrinsic extensor muscles unavailable) and low ulnar palsy (intrinsic muscles unavailable). We then applied these combinations of tension to the cadaveric tendons and measured fingertip output. Measured palmar forces were within 2% and 5° of the predicted magnitude and direction, respectively, suggesting tendon tensions superimpose linearly in spite of the complexity of the extensor mechanism. Maximal palmar forces for ulnar and radial palsies were 43 and 85% of non-paretic magnitude, respectively (p<0.05). Thus, the reduction in tip pinch strength seen clinically in low radial palsy may be partly due to loss of the biomechanical contribution of forefinger extrinsic extensor muscles to palmar force. Fingertip forces in low ulnar palsy were 9° further from the desired palmar direction than the non-paretic or low radial palsy cases (p<0.05).     

Anatomical consideration of reverse-flow island flap transfers from the midpalm for finger reconstruction.

Primary soft-tissue coverage for large palmar defects of the fingers is a difficult problem for cases in which homodigital or heterodigital flaps cannot be used. The aim of this study was to explore the vascular and neural anatomy of the midpalmar area to assess the possibility of reverse island flaps from this area. In 24 cadaver hands perfused with a silicone compound, the arterial pattern of the superficial palmar arch and common palmar digital artery was examined. The cutaneous perforating arteries and nerve branches supplying the midpalmar area were dissected, and the number, location, and arterial diameter of these branches were measured. In six other specimens, the common palmar digital artery was injected to determine the skin territory supplied by the artery. The superficial palmar arch contained the three common palmar digital arteries and its terminal branch coursed along the radial margin of the index metacarpus. This terminal branch had three to six cutaneous perforators (diameter range, 0.1 to 0.5 mm) and supplied the radial aspect of the midpalmar area located over the ulnar half of the adductor pollicis muscles. The midpalmar area was divided into two regions-the proximal and distal-according to the vascular distributions. The proximal region contained dense aponeurosis and thin subcutaneous tissue, and the cutaneous perforators were rather sparse (between three and nine) and had a small diameter (0.1 to 0.3 mm). The distal region, which had loose aponeurosis and abundant subcutaneous tissue, had a rich vascular supply from the common and proper digital artery. Perforating arteries of this region coursed frequently in an oblique fashion and the number of perforators (between eight and 15) and their arterial diameters (diameter range, 0.1 to 0.5 mm) were higher than those of the proximal region. The area of skin perfused by the common palmar digital artery was 5 x 3 cm at the distal midpalmar region. There were three to five cutaneous nerve branches from the palmar digital nerve supplying the midpalmar area. From this study, two different reverse flaps were proposed. First, a 5 x 2 cm flap from the distal midpalmar region was elevated on the basis of the common and proper palmar digital artery. Measurement of the rotation arc revealed that the pivot point of this flap was located at the proximal interphalangeal joint level and could cover the finger pulp of the digits. The second flap candidate was that from the radial aspect of the midpalm, which was supplied by the terminal branch of the superficial palmar arch. In studies with cadaver hands, connection of this artery with the deep arterial system enabled this flap to reach the thumb pulp. These flaps may be a useful reconstruction option for significant palmar soft-tissue loss of the fingers.     

Struthers' ligament and associated median nerve variations in a cadaveric specimen.

A Ligament of Struthers has been identified and studied in a cadaveric arm. Its relationships to a rudimentary supracondylar process, the pronator teres muscle, the median nerve, and the medial epicondyle of the humerus are described. Compared to an extensively studied series of normals, the ligament in this case was associated with abnormally proximal branching of the median nerve, a finding which is surgically significant and not addressed in the literature to date.     

Extensor tendon injuries: acute management and secondary reconstruction

LEARNING OBJECTIVES: After reviewing the article, the participant should be able to: (1) Describe the anatomy of the extensor tendons at the level of the forearm, wrist, hand, and fingers. (2) Recognize variations in the anatomy. (3) Master the hand examination and define the relevant findings in acute injuries of the extensor tendon(s). (4) Delineate the techniques for extensor repair in both acute and secondary (delayed) management. SUMMARY: Extension of the fingers is an intricate process that reflects the combined action of two independent systems. The interossei and lumbricals constitute the intrinsic musculature of the hand. These muscles innervated by the median and ulnar nerves extend the proximal interphalangeal and distal interphalangeal joints and flex the metacarpophalangeal joints. The extrinsic extensors are a group of muscles innervated by the radial nerve, originating proximal to the forearm. The extrinsic digital extensor muscles include the extensor digitorum communis, extensor indicis proprius, and extensor digiti quinti. The digital extensors function primarily to extend the metacarpophalangeal joints, but also extend the proximal interphalangeal and distal interphalangeal joints. Normal extensor physiology reflects a delicate balance between these two unique extensor systems. In the injured hand, a functioning intrinsic system may potentially compensate for an extrinsic deficit. An understanding of the relevant anatomy and an appreciation for the complex interplay involved in extensor physiology is necessary to recognize and manage these injuries.     

Comparison of the Effects of Flexion and Extension of the Thumb and Fingers on the Position and Cross-Sectional Area of the Median Nerve

Objective

To assess the separate effects of thumb and finger extension/flexion on median nerve position and cross-sectional area.

Methods

Ultrasonography was used to assess median nerve transverse position and cross-sectional area within the carpal tunnel at rest and its movement during volitional flexion of the individual digits of the hand. Both wrists of 165 normal subjects (11 men, 4 women, mean age, 28.6, range, 22 to 38) were studied.

Results

Thumb flexion resulted in transverse movement of the median nerve in radial direction (1.2±0.6 mm), whereas flexion of the fingers produced transverse movement in ulnar direction, which was most pronounced during flexion of the index and middle fingers (3.2±0.9 and 3.1±1.0 mm, respectively). Lesser but still statistically significant movements were noted with flexion of the ring finger (2.0±0.8 mm) and little finger (1.2±0.5 mm). Flexion of the thumb or individual fingers did not change median nerve cross-sectional area (8.5±1.1 mm²).

Conclusions

Volitional flexion of the thumb and individual fingers, particularly the index and middle fingers, produced significant transverse movement of the median nerve within the carpal tunnel but did not alter the cross-sectional area of the nerve. The importance of these findings on the understanding of the pathogenesis of the carpal tunnel syndrome and its treatment remains to be investigated.     

The sensitivity of endpoint forces produced by the extrinsic muscles of the thumb to posture

This study utilizes a biomechanical model of the thumb to estimate the force produced at the thumb-tip by each of the four extrinsic muscles. We used the principle of virtual work to relate joint torques produced by a given muscle force to the resulting endpoint force and compared the results to two separate cadaveric studies. When we calculated thumb-tip forces using the muscle forces and thumb postures described in the experimental studies, we observed large errors. When relatively small deviations from experimentally reported thumb joint angles were allowed, errors in force direction decreased substantially. For example, when thumb posture was constrained to fall within ±15° of reported joint angles, simulated force directions fell within experimental variability in the proximal–palmar plane for all four muscles. Increasing the solution space from ±1° to an unbounded space produced a sigmoidal decrease in error in force direction. Changes in thumb posture remained consistent with a lateral pinch posture, and were generally consistent with each muscle’s function. Altering thumb posture alters both the components of the Jacobian and muscle moment arms in a nonlinear fashion, yielding a nonlinear change in thumb-tip force relative to muscle force. These results explain experimental data that suggest endpoint force is a nonlinear function of muscle force for the thumb, support the continued use of methods that implement linear transformations between muscle force and thumb-tip force for a specific posture, and suggest the feasibility of accurate prediction of lateral pinch force in situations where joint angles can be measured accurately.