The anterior interval and arthrofibrosis
The anterior interval is one of those areas of the knee that can become locked up with adhesions after an episode of inflammation, resulting in a stiff knee.
Early in the process, the adhesions may be broken up with patellar and patellar tendon mobilisation exercises, but when advanced it may only be possible to free the adhesions surgically.
In this case the surgeon may undertake an 'anterior interval' release, to free the adhesions.
"....Patients with anterior interval scarring often present with lack of terminal knee extension, anterior knee pain, decreased patellar mobility, and quadriceps atrophy...."
Other relevant links -
Adhesions, Arthrofibrosis, Lysis of adhesions, Anterior interval release