Radiographic anatomy of the tibial plateau

Radiographically distinguishable screws were centered 5 mm below the articular surface in the anterior and posterior halves of the tibial plateau in 8 fresh-frozen cadaveric knees. X-rays were taken with the knees in 0 degrees, 10 degrees, 20 degrees, 40 degrees, and 60 degrees of flexion. The screws were positioned relative to each other and to the articular surface. The posterior slope of the tibial plateau was measured from a lateral x-ray. Full extension of the knee caused the anterior screw to appear inaccurately placed--superior to the posterior screw in all 8 knees and intra-articular in 5 of these knees. In 6 cases, the screws appeared collinear and accurately subchondral on the x-ray taken with the knee in 10 degrees of flexion. In the other 2 cases, the screws were super-imposed on each other on the x-ray taken with the knee in 20 degrees of flexion. In all 8 cases, the knee's being in more than 20 degrees of flexion caused the anterior screw to inaccurately appear inferior to the posterior screw on x-ray. Anteroposterior x-rays assessing the position of screws or wires in the tibial plateau should be taken with the knee in 10 degrees of flexion, rarely in 20 degrees, to achieve accurate assessment. Radiographic evaluation of tibial plateau hardware with the knee in full extension or in more than 20 degrees of flexion will produce inaccurate results.