| Wrist is the most important joint of human hand. Any abnormalities or pathological changes of the wrist will affect the normal functions of the hand. First described by Sir James Paget in 1865, carpal tunnel syndrome (CTS) has become one of the most commonly reported nerve compression syndrome of the wrist. It is defined as an entrapment neuropathy of the median nerve, which is constrained and predisposed to compression when it passes through the space of carpal tunnel created by the carpal bones and the transverse carpal ligament (TCL), often resulting in a loss of sensory and motor nerve function. It is characterized clinically by pain, numbness, and tingling in the hands. The surgical technique for carpal tunnel syndrome is carpal tunnel release (CTR), in which the transverse carpal ligament is cut to relieve pressure on the median nerve. However, many complications of carpal tunnel release include incomplete release, neuropraxia or injury to the median or ulnar nerve, and inadvertent entrance into Guyon's canal, injury to digital nerves, the ulnar artery etc. have been generally reported. It is estimated that the economic burden of carpal tunnel syndrome to US industry exceeds two billion dollars per year. The high social and economic costs of carpal tunnel syndrome lead to a surge in the studies on it especially during the past ten years.The biomechanical researches related to carpal tunnel release and carpal tunnel itself are mainly experimental studies to date. Clinical evidence shows that sectioning of the transverse carpal ligament causes widening of the transverse carpal arch and the decrease in carpal stiffness. Other studies find that increased excursion that is consumed by motion of the wrist after division of the TCL results in fewer excursions for flexion of the other joints and may contribute to the weakness of grip strength that has been noted after carpal tunnel release. Studies of the excursion of digital flexor tendons and median nerve find that inter-compressive load conditions exist between the tendons and the median nerve, and also between them and the transverse carpal ligament. Due to the considerable complications after carpal tunnel release, lengthening of the transverse carpal ligament by sustained tensile loads or using transposition flap technique has also been studied experimentally as an exploration of an alternative treatment method. However, the quantitative effects of dividing the transverse carpal ligament on the mechanical behavior of the carpal tunnel remain unclear.Finite Element (FE) method has been used extensively in the engineering sciences for nearly forty years as a technique to solve differential equations over arbitrary domains, and now it has been widely utilized to analyze musculoskeletal system such as the knee, hip, shoulder, and spine. However, due to the number of bodies involved and the complexity of the soft tissue interaction, biomechanical model of the whole wrist that takes into consideration of the three-dimensional geometrical structure and the mechanical properties of the transverse carpal ligament is rarely reported, as well as the three-dimensional finite element simulation of the wrist that aims to study the biomechanical effects of open carpal surgery. By establishing a three-dimensional finite element model of the whole wrist, the present study has studied the in-vivo morphological changes of the transverse carpal ligament and the carpal tunnel under tensile loads, and analyzed the effects of dividing the transverse carpal ligament on the mechanical behavior of the wrist. The three parts of this paper are briefly described as follows:(1) Based on the CT scan images of the right wrist of a 24-year-old healthy male volunteer, a three dimensional surface model of the whole wrist is reconstructed using commercial medical remodeling software. Then a geometrical solid model is developed using reverse engineering software. Finite element pre-process software is used to convert the meshes of bones and cartilages geometry into a three-dimensional solid finite element model. The transverse carpal ligament is also modeled and meshed three-dimensionally by referring to anatomic research data. Other wrist ligaments are modeled using non-linear, tension-only spring elements by connecting the corresponding attachment nodes on the carpal bones. Different element properties and mechanical properties are assigned to elements that represent different tissues of the wrist. Combining the bone, the cartilage and the ligament structures together completed assembly of the overall model. By compare to the experimental results that are in good agreement with the calculational results of the current nonlinear model, this three-dimensional finite element model is validated.(2) By using the three-dimensional finite element model of the carpal tunnel extracted from the whole wrist model, the transforming abilities of the transverse carpal ligament and the typical sectional areas of the carpal tunnel under palmarly directed stretch force on the TCL are studied, together with the stress and strain distribution, all of them are found to increase non-linearly as the stretch force increases. But the transverse carpal ligament itself is found to be rather unstretchable. The effects of different elastic modulus and Poisson's ratio of the transverse carpal ligament on the morphological changes of carpal tunnel and on TCL itself are discussed too. With the elastic modulus and the Poisson's ratio increases respectively, the arch height of the transverse carpal ligament and the sectional areas of the carpal tunnel decrease in different ways.(3) The contact areas, contact pressures and force transmission between the radiocarpal joint are calculated using the three-dimensional finite element model of the wrist. The effects of dividing the transverse carpal ligament on the displacements of the carpal bones and the contact stress distributions in the midcarpal joints have also been studied using finite element method. By dividing the transverse carpal ligament, the scaphoid moved radially and dorsally with a radially deviation and dorsal-palmar flexion. The whole structure of the carpus deviated radially at the same time with the capitate and the lunate being the approximate center axial of this deviation. The effects of dividing the transverse carpal ligament on the contact stresses of the midcarpal joints are quite in consistent with the displacement results. The radially movement and the deviation and the flexion of the scaphoid decreased the contact stress in the STT joint, and the dorsal movement of the scaphoid move the peak contact stress in the scaphocapitate joint dorsally. The contact stress between the triquetrum and the hamate increased reasonably as the axial displacement of the triquetrum decreased more than that of the hamate.In sum up, this paper makes an initial attempt to set up a three-dimensional finite element model of the wrist including the tunnel structure of the carpal tunnel, distal radius and ulna and the proximal ends of metacarpals, with the transverse carpal ligament being modeled as three-dimensional structure for the first time. And this model is validated by comparing to experimental study results. Transforming abilities of the transverse carpal ligament and the carpal tunnel under mechanical loads are studied, with the effects of different mechanical properties of TCL on them being analyzed using finite element method. The effects of dividing the transverse carpal ligament on the mechanical behavior of the wrist have also been studied using this three-dimensional finite element wrist model. The methodology developed in this paper provides useful reference for individual specified modeling of wrist and hand, and also for in-vivo biomechanical researches in musculoskeletal system in the future. The analysis results of this paper also provides insights to further study of in-vivo mechanical behavior of the wrist under all kinds of mechanical loads, and they can also serve as guides to clinical diagnosis and treatment of carpal tunnel syndrome as well. |
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