Abstract
Introduction
Cementless tibial fixation has been used for over 30 years. 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 micrmotions 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. Finite element analyses has been used to assess primary stability, however, it is becoming increasing difficult to differentiate performance. The aim of this study was too examine the micromotion for a variety of different activities for three commercially available tibial tray designs.
Methods
A finite element model of the implanted proximal tibia was generated form CT scans of a 72 year old male and material properties were assigned based on the Hounsfield units. Three tray designs were evaluated: LCS, Duofix and Sigma (DePuy Inc, Warsaw USA). The implants were assumed to be debonded, with a coefficient of friction of 0.4 applied to the bone-implant interface except for the porous coated region of the Duofix design, which was assumed to be 0.6. The distal portion the tibia was rigidly constrained. Five activities were simulated based on data from Orthoload.com (patient K1L) including walking, stair ascent, stair descent, sitting down and a deep knee bend. The three force and three moment time histories were discritised to give between 44 and 48 individual load steps. Custom written scripts were used to generate composite peak micromotion plots, which report the peak micromotion that occurs at each point of the contact surface during the gait cycle. The primary stability was then assessed by reporting the maximum micromotion, the average peak micromotion and the percentage of the contact area experiencing micromoitons less than 50 microns.
Results and discussion
Similar trends were observed for all three designs across the range of activities. Stair ascent and descent generated the highest micromotions, closely followed by level gait. Across these three activities the mean peak (maximum) micromotions ranged from 64-78 (186-239) microns for PFC Sigma, 61-72 (199-251) microns for Duofix and 92-106 (229-264) microns for LCS. The peak micromotions did not necessarily occur at the peak loads. For instance, for level walking the peak micromotions occurred when there were low axial forces, but moderate varus-valgus moments. This highlights the need to examine the whole gait cycle in order to properly determine the initial stability tibiae tray designs. By exploring a range of activities and interrogating the entire contact surface, it is easier to differentiate between the relative performance of different implant designs.
