The carpal tunnel is a bony groove on the inside of the carpus through which a total of 9 tendons and the median nerve pass. To the outside, the bony groove is protected by a tight band of connective tissue called the retinaculum flexorum, forming a tunnel-like passage called the carpal tunnel. Common problems result from narrowing of the tunnel, which leads to compression of the median nerve and causes carpal tunnel syndrome.
What is carpal tunnel?
The carpal tunnel is formed by a special deformation of the carpal bones on the inside of the carpal joint and is bounded on the outside by a tight band of tissue called the retinaculum flexorum. The bony groove and the tissue ligament, also called the transverse wrist ligament, together form a tunnel-like passage called the carpal tunnel. It accommodates the nine tendons of the finger flexors and the median nerve, the middle arm nerve. The main significance of the carpal tunnel is that the tendons of the finger flexor muscles, even when the wrist is bent inward, are forcibly guided through the predetermined course of the tunnel and thus run close to the body. This greatly reduces the risk of injury to the tendons when the hand is bent inward and promotes the necessary precise fine motor skills of the fingers. Directly below the retinaculum flexorum runs the median nerve, which contains afferent motor and efferent sensory fibers. When swelling of tissue structures occurs in the carpal tunnel area due to injury or inflammatory reactions, the median nerve enters a compression situation that is the trigger for the well-known carpal tunnel syndrome.
Anatomy and structure
The bony groove of the carpal tunnel is dictated by appropriate deformation of several carpal bones. The size and shape of the groove are largely determined by genetic predisposition. Internally, as well as on both sides, the structure is directly adjacent to the periosteum of the carpal bones. Outwardly, the groove is covered by the retinaculum flexorum, creating a tunnel-like structure. The tissue ligament forms a common tendon sheath for the eight tendons of the deep and superficial finger flexors and a separate tendon sheath for the tendon of the long thumb flexor. In the tendon sheaths, synovial fluid, also known as gliding fluid or synovial fluid, ensures that the tendons can move with as little friction as possible. In addition, the synovial fluid supplies the tendons and the tendon sheaths with nutrients. Above the tendons, just below the retinaculum flexorum, on the thumb side runs the median nerve, which usually still within the carpal tunnel gives off a small motor branch to part of the thumb muscles.
Function and Tasks
The most important functions of the carpal tunnel are to protect and constrain the eight tendons of the finger flexors and the thumb-side wrist flexor, and to physically protect the tendons. Without the carpal tunnel, they would have no support when the hand is flexed inward, and the conversion of contraction of the individual finger flexors into corresponding flexion of the fingers could not function when the hand is flexed inward. The fact that the median nerve also passes through the carpal tunnel is solely for the mechanical protection of the nerve, especially during inward and outward flexions of the hand. However, the course of the median nerve through the carpal tunnel directly below the retinaculum flexorum sometimes also makes itself felt negatively when the underlying structures “spread out” a little and thus put the nerve “under pressure”, i.e. by displacing it they leave no more room for the nerve. This can result in a typical nerve compression, which in this case is called carpal tunnel syndrome. The retinaculum flexorum, which delimits the carpal tunnel to the outside, is part of the hand fasciae and thus performs tasks in conjunction with them to stabilize the carpal joints and the entire wrist.
Diseases
The most common complaints and problems observed in connection with the carpal tunnel are usually effects of carpal tunnel syndrome. The syndrome usually results from inflammatory reactions to structures within the carpal tunnel. For example, tendon sheaths can become inflamed and easily swell due to overuse or incorrect strain.This is enough to compress the median nerve and trigger typical symptoms. Because the median nerve carries not only motor but also sensory fibers, initial symptoms may consist of sensory disturbances such as ant tingling in the palm or decreased sensitivity. Large portions of the palm receive sensory input from the median nerve. Other symptoms include motor problems and deficits in the fingers and pain. For example, the index and middle fingers can no longer be closed when trying to make a fist, a symptom known as “swear hand.” In cases of prolonged carpal tunnel syndrome, an externally visible deterioration of the muscles of the ball of the thumb (muscle atrophy) is also typical. The risk of developing carpal tunnel syndrome also depends on the genetically determined anatomical conditions within the carpal tunnel. This means that the risks of developing carpal tunnel syndrome are unevenly distributed. Very often, recurring incorrect postures such as resting the wrist on the edge of a table when operating a computer mouse cause irritation of the median arm nerve and thus the first symptoms of carpal tunnel syndrome. Wrist fractures or fractures of the radius near the wrist are more difficult and complex. They can lead to a narrowing of the carpal tunnel even after years and cause carpal tunnel syndrome. Any space-occupying changes in the wrist area such as osteoarthritis, hormonal changes, certain medications and more can be culprits.