Abstract
Introduction
Numerous fluoroscopic studies have been conducted to investigate kinematic variabilities of total knee arthroplasty (TKA). In those studies, subjects having a posterior stabilized (PS) TKA experience greater weightbearing knee flexion and posterior femoral rollback of the lateral condyle. In those same studies, subjects did experience a high incidence of variable medial condyle motion and reverse axial rotation, especially occurring when the cam engaged the post. More recently, a PS TKA was designed to accommodate both gender and ethnicity. Therefore, the objective of this study was to assess in vivo kinematics for subjects having this TKA type to determine if subjects having this PS TKA experienced more optimal knee kinematics.
Methods
Twenty-five subjects in this study were asked to perform a deep knee bend to maximum knee flexion and a step-up maneuver while under fluoroscopic surveillance. All subjects were patients of one experienced surgeon and received the same PS TKA. Using a 3D-2D registration technique, the CAD models, supplied by the sponsoring company, were superimposed over x-ray images at specified increments throughout the fluoroscopic footage. The kinematics were then analyzed to evaluate lateral anterior/posterior (LAP) and medial anterior/posterior (MAP) condyle translation as well as axial rotation of the femur with respect to the tibia.
Results
During the DKB activity, the average flexion for the PS TKA subjects was 1108°. On average subjects experienced a lateral condyle motion in the posterior direction of 7.3mm, with the maximum amount of posterior rollback being 12.8 mm. These same subjects experienced an average medial condyle motion in the posterior direction of 4.8 mm with the maximum amount of posterior motion being 7.8 mm. Therefore, with the lateral condyle rolling more posterior than the medial condyle, these subjects experienced an average amount of 7.1° of axial rotation, with a maximum of 12.0°. Only one subject in this study experienced a reverse axial rotation from full extension to maximum knee flexion. During the step-up maneuver, subjects consistently experienced a roll forward motion of both their condyles.
Discussion
Subjects in this study experienced a high incidence and magnitude of lateral condyle posterior femoral rollback, leading a normal-like axial rotation pattern, although less in magnitude compared to the normal knee. There was variability occurring with the medial condyle as some experience experienced an anterior slide while others rolled in the posterior direction. As seen in previous studies, during mid flexion both condyles experienced a more variable motion pattern. Twenty-five subjects having a posterior cruciate retaining TKA are being added to this study to determine if retention of the PCL in a similarly designed TKA leads to more normal-like kinematic patterns.
