In the order from fingertip to finger base there is an upper, a middle and a lower phalanx. The lower phalanx (proximal phalanx) is connected to the second metacarpal bone by tendons, which gives the index finger both stability and freedom of movement. The bones are surrounded by tendons, fatty tissue and skin that are essential for movement. The skin of the index finger is completely covered on the side of the palm and on the side of the back of the hand with the exception of the fingertip section by the median nerve (median arm nerves) with sensitive fibres which are important for sensory perception. The fingertip on the side of the back of the hand is supplied sensitively by the radial nerve (spoke nerve).
Tendons and ligaments
Numerous tendons end at the index finger, which are very important for its motor function, but also for its stability. Most of the tendons originate from muscles that originate in the elbow or forearm, run over it and finally attach to the bone of the index finger. If these muscles contract, this leads to certain finger movements such as bending, stretching, spreading and pulling up.
The tendons that are responsible for stretching the finger are attached to the knuckles of the fingers on the side of the back of the hand. There are two muscles responsible for this movement, the index finger extensor (Musculus extensor indicis) and the general finger extensor (Musculus extensor digitorum communis). The tendons of the muscles responsible for flexion are attached to the knuckles of the fingers on the side of the palm.
Here too, there are two muscles that are mainly responsible for movement. One is the superficial (Musculus flexor digitorum superficialis), the other the deep finger flexor (Musculus flexor digitorum profundus). The flexor tendons are reinforced by a ring-shaped ligament (Ligamentum anulare). The ring ligament is part of the tendon sheath in which the tendons are embedded and which ensures their gliding ability. The Ligamentum anulare prevents the tendons from protruding from the bone like an arcuate tendon during bending, as the motor function and the function of the index finger would otherwise be immensely restricted.
A numb finger can have many causes. Mostly it is the result of disturbances in the blood circulation or in the supply of nerves, especially if the corresponding nerve is pinched. This can be accompanied by tingling sensations, a cold index finger and stabbing pain.
These are summarised as paraesthesias or paresthesias. Each of these manifestations can be experienced as very stressful for those affected, which is only one of the reasons why a medical clarification of the symptoms, especially if they occur frequently, is advisable. The numbness is certainly known to all of us as falling asleep fingers.
If the feeling is temporary and disappears when the index finger is moved, there is usually no serious illness behind it. A very common disease that leads to pain and sensation in the area of the index finger is the carpal tunnel syndrome. This involves a narrowing of the median nerve.
Typically, the problem, especially the pain, occurs more frequently during the night. Motor disturbances can accompany advanced carpal tunnel syndrome. This manifests itself among other things in a reduction in the strength of the hand.
The closure of the fist is only possible to a limited extent. Therapeutically, cortisone can be injected in the vicinity of the nerve. However, many affected persons are only relieved of their suffering by a minor operation.
During this procedure, a ligament is split in the area of the crook of the hand under which the nerve runs and which is partly responsible for its constriction. In addition to a reduced nerve transmission due to constriction and pressure on the nerve, a disturbed blood flow can also cause numbness in the area of the index finger. One should be particularly careful if the complaints occur only on one side of the body and are accompanied by other sensory disturbances on the same side.
If, in addition to the numb fingers, there are also sensations of numbness in the area of the same side of the face, arm or leg, as well as a reduction in strength, which manifests itself as a hanging corner of the mouth, weak arm or weak leg, these can be indications of a stroke. Quick action is essential for survival. In case of doubt, the emergency doctor should be informed at the slightest suspicion of a stroke.
Not only a disturbance of the nerves in the area of the hand can lead to numbness in the fingers and especially in the area of the index finger. If there is a constriction and compression of the nerves at the exit in the area of the cervical spine, for example due to a slipped disc in the cervical spine, this can also lead to the same problem. One should take seriously sensations that appear suddenly and do not disappear again or those that persist over a longer period of time and possibly get worse. Accompanying pain or paralysis symptoms should also be clarified by a doctor. The therapy depends on the triggering cause, insofar as this can be determined.