Development of Six Degree of Freedom Mechanical Testing for Assessing The Primary Stability of Cementless Tibial Trays

Introduction: Cementless tibial fixation has been used for over 30 years in total knee arthroplasty. There are several potential advantages including preservation of bone stock and ease of revision. More importantly, for young active patients there is the potential for increased longevity of fixation. However, the clinical results have been variable, with reports of extensive radiolucent lines, rapid early migration and aseptic loosening. Problems appear to stem from a failure to become sufficiently osseointegrated, which in turn suggests a lack of primary stability. In order to achieve boney ingrowth, interface micromotions should be less than 50 microns, whereas fibrous tissue formation is known to occur if micromotions are in excess of 150 microns. The degree of micromotion at the bone-implant interface are dependent on the kinematics and kinetics of the replaced joint. The tibial tray experiences complex six degree of freedom (6DOF) loading as a consequence of activities of daily living (ADL) comprising a combination of axial, anterior-posterior and medial-lateral loads as well as flexion-extension, varus-valgus and internal-external moments. Until recently our knowledge of the magnitude and temporal variation of these forces and moments has been limited, but recent data from telemetric implants [1, 2] have now given detailed information that will prove invaluable for pre-clinical testing. In vitro studies investigating primary stability have used simplified loading conditions incorporating a static axial load in combination with internal-external moments [3, 4], AP shear forces [4, 5] and ML forces [5]. Typically, these studies have used a force equivalent to the peak load which occurs during the stance phase of gait. To date only computational studies have been able to simulate the 6DOF loading conditions for a variety of activities [6]. This study aims to develop an in vitro testing protocol capable of applying six degree of freedom loads for ADL's in order to assess the primary stability of cementless tibial trays. Methods: A cadaveric tibia, with no signs of disease or gross defects, was retrieved and stored at-20 degrees Celsius. Prior to testing, the tibia was defrosted and cleaned of all soft tissue. The tibia was implanted with an idealized stainless steel, cementless tibial tray, which had a single conical keel (based on the geometry of the LCS, DePuy Inc). There were no coatings applied to the tray and cone, which only had a course grit blasted finish. The tibia was resected approx. 100mm below the distal tip of the prostheses …