MRI of the Carpal Tunnel

Abstract

2003 Carpal tunnel syndrome (CTS) has been associated with repetitive and forceful use of the hand. While CTS is cited as the most common peripheral neuropathy, its pathomechanics are not well understood. Pressures in the carpal tunnel are known to increase with wrist deviation as is impingement of the median nerve. Detailed imaging is needed to determine the anatomical changes that could account for these pressure increases. PURPOSE: To quantify the dimensions of the carpal tunnel as a function of wrist posture and finger loading. METHODS: Using magnetic resonance imaging (MRI), 8 healthy wrists (4 male, 4 female) were examined in 7 conditions: 3 wrist postures (neutral, 30o flexion and 30o extension), each with and without exerting a 10 N pinch force, and a fist (neutral wrist). Cross-sectional area(CSA) for the carpal tunnel and its contents were calculated at 3 mm increments along the length of the tunnel and used to calculate respective tunnel volumes. Additionally, ratios between the contents and carpal tunnel itself (CTC/CT) were determined for CSA and volume. RESULTS: Gender and use of a pinch grip had no effect on area or volume measures per se, but a reduction in the CTC/CT volume ratio was found with the pinch grip (p = 0.014). A significant interaction was found between posture and level of the wrist. At the distal end of the tunnel, CSA was larger in extension (196.4 ± 34.8 mm2) than neutral (183.8 ± 31.7 mm2) and flexion (176.4 ± 37.0 mm2); the opposite was true in the proximal tunnel. Also, the smallest contents CSA was found in distal tunnel with the wrist extended (69.7 ± 9.5 mm2), with the largest in neutral (77.9 ± 14.6 mm2). Tunnel volume with extended (5564 ± 727 mm3) and neutral wrist postures (5439 ± 715 mm3) were larger than when fiexed (4928 ± 597 mm3). DISCUSSION: This is the first study to analyze carpal tunnel area and volume under a variety of conditions in vivo. Contrary to what would be expected based on carpal tunnel pressure studies, we found that both tunnel area and volume were largest, and the area of the contents was smallest, with the wrist extended. This indicates our current understanding of the relationship between pressure and volume in the carpal tunnel may be too simplistic and needs refining. Our finding that the CTC/CT volume ratio was reduced by tendon loading may relate mechanical compression of the median nerve. However, space occupied by muscle tissue was not included in the current analysis and, as a proposed mechanism in elevating carpal tunnel pressure, needs further evaluation.