Distally extended muscle belly of the flexor digitorum within the carpal tunnel: is it a risk factor for carpal tunnel syndrome?

Abstract

Background The distally extended muscle belly of the flexor digitorum (MB-FD) within the carpal tunnel has been suggested as a plausible cause for carpal tunnel syndrome (CTS) based on cadaveric studies. Purpose To evaluate whether MB-FD within the carpal tunnel is a risk factor for CTS by comparing asymptomatic volunteers and patients with CTS. Material and Methods Presence, length, and cross-sectional area of MB-FD within the carpal tunnel were evaluated in 63 wrists of 32 asymptomatic volunteers and 52 wrists of 33 patients with CTS using ultrasonography. Length of MB-FD within the carpal tunnel was measured as the distance between the distal end of MB-FD and the palmar margin of the distal radius longitudinally. On a transverse scan, the cross-sectional area of MB-FD at the proximal limit of the carpal tunnel was measured. Results MB-FD in the carpal tunnel was found in 39 (62%) asymptomatic volunteers and 35 (67%) patients with CTS ( P = 0.549). There was a female preponderancy of MB-FD within the carpal tunnel in asymptomatic volunteers ( P = 0.044), but not in patients. Length and cross-sectional area of MB-FD within the carpal tunnel were significantly greater in male-CTS patients than in male volunteers ( P = 0.022 and 0.012, respectively). These qualities were risk factors for CTS in men, though not in women. Conclusion The presence of distally extended MB-FD within the carpal tunnel was not a risk factor for CTS. In men, a longer and thicker MB-FD within the carpal tunnel increased the risk for CTS.

Identification of Aberrant Muscle Bellies in the Carpal Tunnel Using Sonography

“…Specifically, case reports suggesting a link between aberrant flexor or lumbrical muscle bellies and carpal tunnel syndrome have been reported in the literature for more than 50 years. [12][13][14][15][16][17][18][19][20][21] The continuous presence of muscle belly in the carpal tunnel even while the hand is at rest may compromise the available space within the tunnel. This loss of space may compress the median nerve, which can lead to the development of carpal tunnel syndrome.…”

“…This loss of space may compress the median nerve, which can lead to the development of carpal tunnel syndrome. 21,22 Although individuals with aberrant muscle bellies may be at increased risk for developing carpal tunnel syndrome, prospective or large-scale studies have not been conducted to establish any causative link.…”

“…28 Although evidence indicating a relationship between aberrant muscle bellies and pathology exists, the exact prevalence of these anomalous muscles within the carpal tunnel is unknown. 11,21 Proper identification of aberrant muscle bellies in the carpal tunnel is an important first step to understanding their prevalence in the population. When evaluating the presence of aberrant muscle, it is important to note the positioning of the digits, as hyperextension of the metacarpophalangeal joints will likely cause the distal ends of the long flexor muscles to travel into the tunnel.…”

Ultrasound Findings in Less Frequent Causes of Carpal Tunnel Syndrome

“…Compression of the MN by an AM located inside the CT 11 is usually related to anomalous proximal extensions of the lumbricals, 12 anatomic variations of the palmaris longus, [13][14][15] and an anomalous distal extension of the flexor digitorum superficialis. 16 Ultrasound shows an AM as a solid hypoechoic structure with a typical muscle echo texture and absence of blood flow on color Doppler imaging. A dynamic examination with finger flexion and extension is important to show the AM moving inside and outside the CT depending on the finger position ( Figure 1).…”

“…Accessory muscles (AM) are common anatomic variations that are often asymptomatic. Compression of the MN by an AM located inside the CT 11 is usually related to anomalous proximal extensions of the lumbricals, 12 anatomic variations of the palmaris longus, 13‐15 and an anomalous distal extension of the flexor digitorum superficialis 16 . Ultrasound shows an AM as a solid hypoechoic structure with a typical muscle echo texture and absence of blood flow on color Doppler imaging.…”