The carpal tunnel is an opening in the wrist bones that allows the nerves to the hands to pass from the forearm through the wrist. There's a band of fibrous tissue called the retinaculum across the carpal tunnel at the base of the wrist. Carpal tunnel release takes the pressure off the median nerve by cutting through the retinaculum.

An incomplete decompression can occur in one of three ways. First, the retinaculum may not be cut through completely. The cut may not go all the way from the bottom at the base of the wrist to the top near the palm.

Second the cut may go from bottom to top but doesn't cut through the full thickness of the retinaculum. This leaves a thinner but fully intact retinaculum in place.

Third, the retinaculum can be cut completely but scar tissue forms as the body tries to repair the damage. Fibrous scarring fills in the spot, once again putting pressure on the nerve.