Accuracy and repeatability of joint centre location in computer-assisted knee surgery

Abstract

To properly align knee prostheses, we must accurately define the mechanical axis which joins the hip and ankle centres. Current computerassisted techniques rely on markers pinned not only to the distal femur and proximal tibia, but also to the pelvis and calcaneus, thereby increasing pain and the risk of infection. To eliminate these pins remote from the knee, we are designing non-invasive ”trackers” that are strapped to the patient’s pelvis and foot and are insensitive to skin motion. We mounted our prototype trackers on a fresh cadaver alongside conventional bone pin markers in the pelvis and calcaneus, and located the hip and ankle centres 30 times. We then dissected out and digitized the femoral head. Results from the hip tracker were excellent, the mean centre being within 0.2 mm (ML) and 1.3 mm (AP) of the physical centre of the femoral head (all results at 95% confidence limit). Compared to the digitized centre, the hip tracker introduces mechanical axis error of less than 0.07° (frontal plane) and 0.25 ° (sagittal plane) 95% of the time, while the pelvic bone pin introduces 0.03° (frontal) and 0.14° (sagittal) error. Results were not as good at the ankle: although there was no significant difference in mean ankle centre location between the foot tracker and the calcaneus bone pin (P = 0.09 in ML, P = 0.08 in AP), both means differed from the anatomical ankle centre (digitized midpoint between malleoli) by 5-7 mm, a mechanical axis difference of ~0.5°. Compared to the digitized mechanical axis, overall axis definition will be within 1.2° (frontal) and 0.9° (sagittal) using bone pins and 1.3° (frontal) and 2.0° (sagittal) using non-invasive trackers 95% of the time. For both methods, almost all error arises from bias and lack of precision at the ankle. We conclude that bone pins at the hip are unnecessary for subdegree accuracy, but that better methods of locating the ankle centre should be investigated.