Abstract
Introduction
The number of total knee joint replacements has increased dramatically, from 28,000 in 2004 to over 73,000 in 2008 in the UK. This increase in procedures means that there is a need to assess the performance of an implant design in the general population. For younger, more active patients, cementless tibial fixation is an attractive alternative means of fixation and has been used for over 30 years. However, the clinical results with cementless fixation have been variable, with reports of extensive radiolucent lines, rapid early migration and aseptic loosening [1]. This study investigates the inter-patient variability of bone strain at the implant-bone interface of 130 implanted tibias over a full gait cycle.
Methods
A large scale FE study of a full gait cycle was performed on 130 tibias implanted with a cementless tibia tray (PFC Sigma, DePuy Inc, USA). A population of tibias was generated from a statistical shape and intensity (SSI) model [2].
The tibia tray was automatically positioned and implanted using ZIBAmira (Zuse Institute Berlin, Germany). Cutting and implanting were performed using Boolean operations on the meshed surfaces of the tibia and implant. After generation of a volume mesh from the resulting surface, the bone modulus was mapped onto the new mesh.
The FE models were processed in Abaqus (SIMULIA, RI, USA). Associated force data (axial, anterior-posterior and medial-lateral forces and flexion-extension, varus-valgus and internal-external moments) was sampled from a statistical model of the gait cycle derived from musculoskeletal modelling of 20 elderly healthy subjects. Patient weight was estimated using the length of the tibia and a BMI sampled from NHANES data.
Loads were applied to four groups of nodes on the tibia tray (anterior, posterior, medial and, lateral) for 51 steps in the gait cycle. The bone and implant were assumed to be bonded, simulating the osseointegrated condition.
Results
The equivalent strain was computed for each element in the model. The peak strain in each element over all the gait cycle was found. The mean strain, for all implanted tibiae, at the bone-implant interface was found to be 477 microstrain, with a 95th percentile of 1370 microstrain. The maximum and minimum mean interface strains of each individual tibia were 1243 microstrain and 221 microstrain respectively. A one-way ANOVA test was carried out to see if there was any significant difference of mean strain levels between implant sizes. No significant difference was shown between the implant sizes and mean strain (p = 0.37).
Discussion
There is a large variability of the mean strain within the population, a range of 1000 mircostrain. The implant size does not appear to influence the mean strain of the population. With a large scale study, it is possible to investigate the effect of other factors which might influence the strain field at the contact interface, such as modulus, bone shape, or loading.
Acknowledgements
This project is funded by EPSRC and DePuy.
