An anatomical term, interosseous finds its etymology in the Latin words inter, meaning between, and os, meaning bone. Specifically, interosseous refers to any structure or area connecting or lying between bones. Many of these structures are located in the spine between vertebrae, in the leg immediately above the ankle, and directly above the wrist in the arm. These structures can include membranes, ligaments, nerves, or arteries.
Found between the tibia and fibula bones in the lower leg and between the radius and ulna bones in the forearm, the interosseous membrane is a fibrous sheet of tissue that is a primary component of the tibiofibular syndesmosis joint in the lower leg and the radioulnar syndesmosis joint in the forearm. In both locations, the membrane runs the length of the bones it connects, becoming the interosseous ligament in the lower leg just above the ankle. Its main function is to connect the adjacent bones and transfer forces among those bones, as well as act as an attachment site for muscles. In addition, the membrane divides the muscles of these locations into separate compartments.
Accompanying the membrane in the forearm are interosseous nerves. The anterior interosseous nerve (AIN) branches off the median nerve and connects to several muscles deep in the anterior, or inside, forearm. Specifically, the AIN innervates the flexor pollicis longus muscle, the pronator quadrates muscle, and the lateral half of the flexor digitorum profundus. This nerve can suffer compression injury much like carpal tunnel syndrome, known as Anterior Interosseous Syndrome. Symptoms include muscle weakness in the thumb and forefinger, and sometimes in the middle finger; however, as the AIN is only a motor nerve and not a sensory nerve, this syndrome rarely produces the numbness associated with carpal tunnel.
The posterior interosseous nerve (PIN), on the other hand, branches off the radial nerve and innervates most of the muscles of the dorsal, or back, side of the forearm, including the extensor digitorum and extensor pollicis muscles. A PIN can become entrapped by the forearm supinator muscle — an injury sustained by tennis players and other athletes who regularly and forcefully engage in forearm supination. This neuropathy can present as wrist pain and decreased motor function, such as dropped fingers. Like the AIN, the PIN is only a motor nerve — it has no sensory function so no numbness is reported.