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The tibial slope is an important datum in knee surgery; the levelling by a uni- or bicompartmental prosthesis preserving both cruciate ligaments requires to maintain this slope. The postoperative rehabilitation of the A.C.L. plasties, especially those performed with a patellar tendon and the surgical cure of a genu recurvatum, must integrate this notion. Even if the anatomical slopes are usable in prosthetic surgery, they do not seem precise enough in the field of A.C.L. and tibia recurvatum surgery. In these indications, the authors propose to determine the functional tibial slope defined as being the complement of the angle formed by the tangent to the tibial medial plateau and the lateral mechanical axis of the leg. Measured to an average of 7 degrees on 238 knees, the average median functional slope cannot be deduced from anatomical slopes; it must be measured on a lateral teleradiographic view of the lower limb, the knee being in full extension and X-rayed in true posterior and inferior lateral view.

The classical X-Ray exploration of the patellar instability is done through incidences performed with knees in a 30 degrees flexion and a 0 degrees rotation. To this basic investigation the authors propose to add original, more physiological incidences performed with knees in external rotation. Because of their extreme sensitiveness, these incidences with a 30 degrees external rotation allow a more accurate diagnosis of the patellar instability (and thereby a better breaking up of the still too nebulous chapter of patellar chrondropathies) and a more critical checking of the surgical centering of the patella. Technically easy to perform they should enter into the usual practice of X-Ray exploration of any mechanical knee.

The posterior inclination of the tibial plateaus relative to the longitudinal axis of the bone, also called tibial slope, is important to know for the pathology of the cruciate ligaments and to lay some knee prostheses. We have chosen a reproducible method to measure it, on the basis of a large radiograph of the lower limb. A radio-anatomical analysis of the morphology of the tibia has first been carried out to properly choose axes that are easy to find and useful in practice. The accuracy of the measurement is to within one degree. In adults, the slope ranges from 0 to 18 degrees, according to the subjects, with variations form one knee to the other.