It is a saddle type of synovial joint but functions as a plane joint.

The proximal tibiofibular joint is a small plane joint.

Plane joints permit sliding or gliding movements in the plane of articular surfaces.

(The rest of the sternocostal joints are synovial plane joints.)

Plane joints are numerous and are nearly always small, such as the acromioclavicular joint between the acromion of the scapula and the clavicle.

A plane joint (arthrodial joint, gliding joint, plane articulation) is a synovial joint which, under physiological conditions, allows only gliding movement.