Patellar tracking patterns measurement by analytical X-ray photogrammetry

Abstract

Variants of normal patellar tracking patterns have been implicated in the pathomechanics of patellar chondromalacia. Since these patterns are dependent upon the dynamic interaction of the quadriceps mechanism and knee geometry, a method for measuring three-dimensional patellar displacement during knee motion is needed before patellar pathomechanics can be fully understood. Once this information is available, a more rational basis for adjusting abnormal tracking patterns can be formulated. A method for accurately measuring patellar displacements using X-ray photogrammetry has been developed. Stainless steel 1 mm balls are implanted percutaneously into the patella and distal femur to provide a radiopaque landmark system which represents the two structures. In order to provide optimal measurement geometry, an analytical photogrammetry system is used. This means that the perspective centers of the X-ray anodes must be calculated before the three-dimensional space coordinates of each landmark can be determined. The displacements between the patella and femoral condyles are derived by computing the relative motion between their respective ball systems. Anterior-posterior, medial-lateral and tilting displacements of the patella were determined for the valgus osteotomy patients shown in this research. The positional accuracy of the patella can be determined by comparing distances between two fixed points (steel balls) in one position with the same distances as computed independently from the other positions. The mean square error is a measure of the patellar position accuracy. Since the implanted markers remain in place permanently, the repeatability of this reference system for future measurements is assured. This method is applicable to a variety of situations requiring the determination of small, three-dimensional displacements between two rigid structures.