22 research outputs found
Numerical and experimental studies on the mechanical behaviour of the distal femur following total knee arthroplasty
The history of total knee arthroplasty stretches back over 70 years. Many studies
have shown that TKA is, in general, a successful operation for the relief of joint pain,
with patient satisfaction rates of 90-95% and implant survival rates at 10-15 years of
greater than 90%. However, a number of studies have also shown the potential for
failures or complications arising post-implantation leading to revision surgery.
This thesis presents finite element (FE) models of the distal femur following primary
and revision total knee arthroplasty. Pre-arthroplasty models are also developed for
comparison. Particular attention is given to how femoral component design and
method of fixation impacts the mechanical environment of the distal femur and
stability of the prosthesis. FE analyses with fully bonded interfaces indicate that
femoral components are subject to areas of low stress (stress shielding) immediately
under the anterior flange and chamfer regardless of internal implant features.
However, internal implant features were found to play a role in the pattern and
magnitude of stress concentrations. Both stresses and motions were observed to
increase with increasing flexion angle, indicating the importance of testing at
multiple angles.
The initial models of the distal femur were extended to incorporate the effects of
ageing and endosteal thinning of the femoral cortex, through novel application of
pre-existing FE modelling techniques, specifically the ability to assign variable
material properties corresponding to the nodal temperatures output from a heat
transfer analysis. The findings from this study indicate that older patients with
osteoporosis may be at increased risk of periprosthetic fracture compared to younger
healthy patients. The use of a revision femoral component with a cemented stem as a
means to mitigate this fracture risk was also investigated.
FE analyses using frictional interfaces were employed to determine the influence of
femoral component design on micromotion at the interface. These models showed
that all primary implants were subject to similar magnitudes of relative motion at the
interface, however, the distinct internal implant features led to very different regional
variations. Furthermore, certain internal implant features (i.e. femoral box) were
found to be highly sensitive to errors in surgical bone cuts. This aspect of the thesis
also concluded that the addition of a stem served to significantly reduce motions at
the interface in comparison to primary stemless implants. Long stemmed prostheses
were found to result in the smallest levels of interface motion.
This study also detailed the design and creation of an in vitro test setup for the
purposes of determining the influence of stem length and fixation on the stability of
revision prostheses. Experimental results using this test rig showed that a cemented
short stem provides as much initial stability as the uncemented long stem, and is
easier to fit surgically. Corresponding FE models incorporating a virtual
representation of the test rig and in vitro loading conditions revealed that the relative
motion at the multi-planar bone-prosthesis interface cannot be adequately described
using a single reference point. However, in vitro setups may be used to predict a
general measure of implant stability and to provide a source of calibration for FE.
The distal femur models were further modified to investigate how the presence of
condylar defects as classified by AORI defect classification system (Engh 2006) and
weak osseous support due to osteoporosis may adversely affect the survival of the
prosthesis. These investigations revealed that fixation of the femoral component, the
presence of a large condylar defect and the level of osseous support all had an impact
on stress in the implant, it is concluded that a non-modular approach should be
adopted in older patient groups with severe osteoporosis to mitigate the risk of
component junction failure and distal femoral fracture
An Efficient Method to Capture the Impact of Total Knee Replacement on a Variety of Simulated Patient Types: A Finite Element Study
Computational Modelling of Motion at the Bone-Implant Interface after Total Knee Arthroplasty: The Role of Implant Design and Surgical Fit
In-lab three-dimensional printing:an inexpensive tool for experimentation and visualization for the field of organogenesis
The development of the microscope in 1590 by Zacharias Janssenby and Hans Lippershey gave the world a new way of visualizing details of morphogenesis and development. More recent improvements in this technology including confocal microscopy, scanning electron microscopy (SEM) and optical projection tomography (OPT) have enhanced the quality of the resultant image. These technologies also allow a representation to be made of a developing tissue’s three-dimensional (3-D) form. With all these techniques however, the image is delivered on a flat two-dimensional (2-D) screen. 3-D printing represents an exciting potential to reproduce the image not simply on a flat screen, but in a physical, palpable three-dimensional structure. Here we explore the scope that this holds for exploring and interacting with the structure of a developing organ in an entirely novel way. As well as being useful for visualization, 3-D printers are capable of rapidly and cost-effectively producing custom-made structures for use within the laboratory. We here describe the advantages of producing hardware for a tissue culture system using an inexpensive in-lab printer
Patient-Specific Modelling of Abdominal Aortic Aneurysms: The Influence of Wall Thickness on Predicted Clinical Outcomes
Metaphyseal cones in revision total knee arthroplasty: The role of stems
AimsMetaphyseal tritanium cones can be used to manage the tibial bone loss commonly encountered at revision total knee arthroplasty (rTKA). Tibial stems provide additional fixation and are generally used in combination with cones. The aim of this study was to examine the role of the stems in the overall stability of tibial implants when metaphyseal cones are used for rTKA.MethodsThis computational study investigates whether stems are required to augment metaphyseal cones at rTKA. Three cemented stem scenarios (no stem, 50 mm stem, and 100 mm stem) were investigated with 10 mm-deep uncontained posterior and medial tibial defects using four loading scenarios designed to mimic activities of daily living.ResultsSmall micromotions (mean < 12 µm) were found to occur at the bone-implant interface for all loading cases with or without a stem. Stem inclusion was associated with lower micromotion, however these reductions were too small to have any clinical significance. Peak interface micromotion, even when the cone is used without a stem, was too small to effect osseointegration. The maximum difference occurred with stair descent loading. Stress concentrations in the bone occurred around the inferior aspect of each implant, with the largest occurring at the end of the long stem; these may lead to end-of-stem pain. Stem use is also found to result in stress shielding in the bone along the stem.ConclusionWhen a metaphyseal cone is used at rTKA to manage uncontained posterior or medial defects of up to 10 mm depth, stem use may not be necessary
Biomechanical assessment predicts aneurysm-related events in patients with abdominal aortic aneurysm
Objective To test whether aneurysm biomechanical ratio (ABR; a dimensionless ratio of wall stress and wall strength) can predict aneurysm related events. Methods In a prospective multicentre clinical study of 295 patients with an abdominal aortic aneurysm (AAA; diameter ≥ 40 mm), three dimensional reconstruction and computational biomechanical analyses were used to compute ABR at baseline. Participants were followed for at least two years and the primary end point was the composite of aneurysm rupture or repair. Results The majority were male (87%), current or former smokers (86%), most (72%) had hypertension (mean ± standard deviation [SD] systolic blood pressure 140 ± 22 mmHg), and mean ± SD baseline diameter was 49.0 ± 6.9 mm. Mean ± SD ABR was 0.49 ± 0.27. Participants were followed up for a mean ± SD of 848 ± 379 days and rupture (n = 13) or repair (n = 102) occurred in 115 (39%) cases. The number of repairs increased across tertiles of ABR: low (n = 24), medium (n = 34), and high ABR (n = 44) (p = .010). Rupture or repair occurred more frequently in those with higher ABR (log rank p = .009) and ABR was independently predictive of this outcome after adjusting for diameter and other clinical risk factors, including sex and smoking (hazard ratio 1.41; 95% confidence interval 1.09–1.83 [p = .010]). Conclusion It has been shown that biomechanical ABR is a strong independent predictor of AAA rupture or repair in a model incorporating known risk factors, including diameter. Determining ABR at baseline could help guide the management of patients with AAA