Lumbrical Muscle Incursion into the Carpal Tunnel During Finger Flexion

Abstract

Carpal tunnel syndrome is one of the many so-called cumulative trauma disorders thought by some to be related to the performance of repetitive tasks in the work-place. The cause of this disorder is unknown. We have observed lumbrical muscle incursion into the carpal tunnel during finger flexion. This study was conducted to determine the amount of this incursion in normal wrists. Five cadaver upper limbs were analyzed radiographically with radiopaque markers on the flexor retinaculum and the lumbrical muscle origins in four finger positions: full extension, 50% flexion, 75% flexion, and 100% finger flexion. The lumbrical muscle origins were an average of 7.8 mm distal to the carpal tunnel in full finger extension. They moved an average of 14 mm into the carpal tunnel with 50% finger flexion, 25.5 mm with 75% flexion, and 30 mm with 100% flexion. Abnormal lumbrical muscles have been cited as a possible cause of carpal tunnel syndrome. These findings suggest that lumbrical muscle incursion during finger flexion is a normal occurrence and is a possible cause of work-related carpal tunnel syndrome.

Coexistence of multiple anomalies in the carpal tunnel

“…A study by Cobb et al (1994) indicates that lumbricals incursion into the carpal tunnel during flexion is a normal occurrence. Another study by Cobb et al (1992) found a great deal of variation in the mass of lumbricals.…”

“…A variety of congenital anatomical anomalies have been identified within the carpal tunnel by many investigators. There are also reports of lumbrical, muscles normally intrinsic to the hand, that hypertrophy to the point that a lumbrical will extend into the carpal tunnel even when the hand is in resting position (Cobb et al, 1994). Many of the studies examining variations and anomalies within the carpal tunnel focus on a single anatomical structure, however, such as the anatomical course of the median nerve and its branches, or an anomalous muscle belly.…”

Occupational Ergonomics: Principles and Applications

“…Vibration and awkward postures are additional stressors that increase the risk of carpal tunnel syndrome. Normal lumbrical muscle incursion into the tunnel may also contribute to work-related CTS (114,115). Because the size of the tunnel may decrease, rheumatoid tenosynovitis, edema, pregnancy, and hypothyroidism have been identified as conditions that may contribute to the development of CTS.…”

Biomechanical models for the pathogenesis of specific distal upper extremity disorders

“…The fourth proposed model assumes that the proximal ends of the lumbrical muscles may enter the distal end of the carpal tunnel as the flexor tendons upon which they originate move within the carpal tunnel in response to finger flexion [Cobb et al, 1994[Cobb et al, , 1995Yii and Elliot, 1994]. The increased contents within the carpal tunnel would increase intracarpal pressure and create mechanical contact pressure on the median nerve.…”

“…The tendons within the carpal tunnel are covered by one or tendon sheaths that may join to form a bursa. For some individuals, the proximal ends of the lumbrical muscles enter the distal end of the carpal tunnel, especially with digital flexion [Tanzer, 1959;Cobb et al, 1994;Yii and Elliot, 1994].…”