I've got trigger finger and I've got it bad. My finger is starting to get stuck in that position. Just what's holding it back?
Little by little, researchers are coming to understand what's behind the problem of trigger finger. Trigger finger (and trigger thumb) are conditions affecting the movement of the tendons as they bend or flex the fingers or thumb toward the palm of the hand.
The tendons that move the fingers are held in place on the bones by a series of ligaments called pulleys. These ligaments form an arch on the surface of the bone that creates a sort of tunnel for the tendon to follow along the bone. To keep the tendons moving smoothly under the ligaments, the tendons are wrapped in a slippery coating called tenosynovium. The tenosynovium reduces the friction and allows the flexor tendons to glide through the tunnel formed by the pulleys as the hand is used to grasp objects. When the flexor tendon gets pinched or stuck under the A1 pulley, a trigger digit develops.
A series of studies have been done examining this pulley mechanism called the A1 pulley under high-powered microscope called light microscopy. So far, the scientists have found that normal pulleys have three layers: an outer, middle, and inner layer. Each layer is made up of different types of cells and a different mixture of those cells. For example, the outer layer is formed by loose connective tissue with a good blood supply. The middle layer contains dense connective tissue. And the inner layer is the gliding layer made up of fibrous chondrocytes (cartilage cells).
Light microscopy has made it possible to look at the tissues with increasing magnification. The borders between each layer are visible, thickness of the layers can be measured, and the surfaces are easy to see, too. Pulleys from trigger fingers show various amounts of damage and destruction of the fibrocartilaginous layers of the pulley. Sometimes it is so thin, it disappears. Tiny cracks called fissures are common along with scar tissue and increased blood supply to the damaged area. Despite all of those changes, there are no signs of inflammation anywhere in the area.
It appears that increased mechanical stress from repetitive use of the fingers results in mild abnormalities that can gradually get worse over time. As the condition progresses, more and more of the gliding surface is destroyed and replaced by fibrosis (scar tissue) that has its own blood supply. As the pulley gets thicker and thicker, the tendon is less able to slide and glide underneath it.
K. Drossos, MD, et al. Correlations Between Clinical Presentations of Adult Trigger Digits and Histologic Aspects of the A1 Pulley. In The Journal of Hand Surgery. October 2009. Vol. 34-A. No. 8. Pp. 1429-1435.
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