Stefan M. Gabriel

In vivo knee kinematics derived using an inverse perspective technique.

Sixty-four subjects having implanted and nonimplanted knees were studied using fluoroscopic videos. Each subject, flexing in the sagittal plane, performed successive deep knee bends under fluoroscopic surveillance. Femorotibial contact in the sagittal plane was then determined using image matching and discrete digitization. At full extension, the mean contact point of the normal and posterior stabilized implanted femurs was anterior to the tibial midpoint in the sagittal plane. The average position was 6.49 mm (+3 - +13 mm) for the normal knees and 0.30 mm (0 - +4 mm) for the posterior stabilized knees. The implanted posterior cruciate retaining and anterior cruciate ligament deficient knees differed from the other knee types. Their average initial contact was posterior. The average contact at full extension for the posterior cruciate retaining and anterior cruciate ligament deficient knees was -5.13 mm (-2 - -8 mm) and -5.45 mm (-2 - -14 mm), respectively. The femur of the normal knee contacts the tibia anterior to the midpoint in the sagittal plane in full extension and translates posteriorly during flexion. The femur of the posterior stabilized knee contacts the tibia anteriorly, slightly less than the normal knee, and rolls back posteriorly during flexion similar to normal knees. The femurs of the posterior cruciate retaining and anterior cruciate ligament deficient knees contact the tibia posterior in extension, but translate anteriorly during midflexion in a substantial number of cases, which is kinematically opposite of the normal knees. The abnormal anterior femoral translation observed in the posterior cruciate retaining knees may be a factor in the premature polyethylene wear seen in retrieval studies.

Three-dimensional determination of femoral-tibial contact positions under in vivo conditions using fluoroscopy.

OBJECTIVE: A method has been developed to accurately measure three-dimensional (3-D) femoral-tibial contact positions of artificial knee implants in vivo from X-ray fluoroscopy images using interactive 3-D computer vision algorithms. DESIGN: A computerized graphical (CAD) model of an implant component is displayed as an overlay on the original X-ray image. An image matching algorithm matches the silhouette of the implant component against a library of images, in order to estimate the position and orientation (pose) of the component. The operator further adjusts the pose of the graphical model to improve the accuracy of the match. BACKGROUND: Previous methods for in vivo measurement of joint kinematics make only indirect measurements of joint kinematics, require invasive procedures such as markers or pins, or make simplifying assumptions about imaging geometry which can reduce the accuracy of the resulting measurements. METHODS: Fluoroscopic videos are taken of implanted knees in subjects performing weight-bearing motion. Images from the videos are digitized and stored on a computer workstation. Using computerized model matching, the relative pose of the two knee implant components can be determined in each image. The resulting information can be used to determine where the two components are contacting, the area of the contact region, liftoff angle, and other kinematic data. RESULTS: Accuracy tests done on simulated imagery and in vitro real imagery show that the pose estimation method is accurate to less than 0.5 mm of error (RMS) for translations parallel to the image plane. Orientation error is less than or equal to 0.35 degrees about any axis. Errors are larger for translations perpendicular to the image plane (up to 2.25 mm). In a clinical study, the method was used to measure in vivo contact points, and characterize the kinematic patterns of two different knee implant designs. CONCLUSIONS: The ability to accurately measure knee kinematics in vivo is critical for the understanding of the behavior of knee implant designs and the ultimate development of new, longer lasting implants. RELEVANCE: This work shows that it is possible to accurately measure the three-dimensional position and orientation (pose) of artificial knee implants in vivo from X-ray fluoroscopy images using interactive 3-D computer graphics. The method can be applied to any joint when accurate CAD models are available. The resulting data can be used to characterize the kinematics of current knee implant designs.

Polyethylene wear on the distal tibial insert surface in total knee arthroplasty

Abstract We studied polyethylene wear damage on articular (superior) and distal (inferior) surfaces of 27 tibial inserts after an average of 50 months in vivo. The damage scoring method overestimated the subtle damage on the inferior surface, but the average superior surface score was still four times the inferior surface score. Inferior surface damage correlated significantly only to implantation time and was highest for inserts having metal backings with holes. Wear damage occurs on the inferior surface of tibial inserts. In the absence of holes in the metal backing and large insert/tray relative motions, however, this wear appears insignificant.