Determination of sagittal instability of the knee joint.

For measuring sagittal instability in the knee joint a radiologic method has been developed which requires no complicated equipment, is easy to apply, and is relatively insensitive to the sources of error commonly associated with determination of sagittal instability, such as difference of flexion angle and changes in the projection. Measurements are made between reference points located in the femur and the tibia. The reproducibility of the method is high enough to make it suitable for testing instability before and after reconstructive surgery. By the use of constructed reference points sagittal instability can be distinguished from rotational instability.