A surface-marker imaging system to measure a moving knee´s rotational axis pathway in the sagittal plane

Describes a new surface-marker imaging system designed to measure the rotational axis pathway (RAP) of a moving knee in the sagittal plane. Measurement of this parameter can provide important information about a knee´s slipping and rolling action that can aid clinical assessment. Seated subjects are video recorded as they actively extend their legs. A series of stills is then captured and analyzed to extract the coordinates of markers plated on the subjects: upper and lower legs. These coordinates are then processed to deduce an instant center of rotation (ICR) for each still. These ICRs are then plotted to derive the joint´s RAP. The system has been validated with a mechanical model and tested in a clinical study of ten patients with unilateral anterior cruciate ligament (ACL) ruptures. The study found that the system could consistently measure differences between a patients normal and injured knees. Leg extension caused the normal knees ICRs to displace anteriorly with a mean value of 17.4 mm, whereas the injured knees had a mean displacement of 7.5 mm. This loss of roll in the ACL-deficient knees is consistent with their abnormal biomechanical arrangement.