The 'anterior interval' is the region of the knee between the fat pad and the front of the tibia bone (anterior tibial plateau) and the patellar tendon.
The importance of the anterior interval
The anterior interval is one of those areas of the knee that can become locked up with adhesions, resulting in a stiff knee. In this case the surgeon may undertake an 'anterior interval' release, to free the adhesions.
Surgical Technique for Release of Anterior Interval Scarring of the Knee After Anterior Cruciate Ligament Reconstruction Rose M, McNeilan R, Genuario J and Schlegel T. Arthrosc Tech. 2018 Sep; 7(9): e887–e891.