Aims

Metaphyseal 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.

Objectives

Elevated proximal tibial bone strain may cause unexplained pain, an important cause of unicompartmental knee arthroplasty (UKA) revision. This study investigates the effect of tibial component alignment in metal-backed (MB) and all-polyethylene (AP) fixed-bearing medial UKAs on bone strain, using an experimentally validated finite element model (FEM).

Aims

Loosening is a well-known complication in the fixation of fractures using devices such as locking plates or unilateral fixators. It is believed that high strains in the bone at the bone-screw interface can initiate loosening, which can result in infection, and further loosening. Here, we present a new theory of loosening of implants. The time-dependent response of bone subjected to loads results in interfacial deformations in the bone which accumulate with cyclical loading and thus accentuates loosening.

Objectives

Secondary fracture healing is strongly influenced by the stiffness of the bone-fixator system. Biomechanical tests are extensively used to investigate stiffness and strength of fixation devices. The stiffness values reported in the literature for locked plating, however, vary by three orders of magnitude. The aim of this study was to examine the influence that the method of restraint and load application has on the stiffness produced, the strain distribution within the bone, and the stresses in the implant for locking plate constructs.

Objectives

Up to 40% of unicompartmental knee arthroplasty (UKA) revisions are performed for unexplained pain which may be caused by elevated proximal tibial bone strain. This study investigates the effect of tibial component metal backing and polyethylene thickness on bone strain in a cemented fixed-bearing medial UKA using a finite element model (FEM) validated experimentally by digital image correlation (DIC) and acoustic emission (AE).

25–40% of unicompartmental knee replacement (UKR) revisions are performed for unexplained pain possibly secondary to elevated proximal tibial bone strain. This study investigates the effect of tibial component metal backing and polyethylene thickness on cancellous bone strain in a finite element model (FEM) of a cemented fixed bearing medial UKR, validated using previously published acoustic emission data (AE).

FEMs of composite tibiae implanted with an all-polyethylene tibial component (AP) and a metal backed one (MB) were created. Polyethylene of thickness 6–10mm in 2mm increments was loaded to a medial load of 2500N. The volume of cancellous bone exposed to <−3000 (pathological overloading) and <−7000 (failure limit) minimum principal (compressive) microstrain (µ∊) and >3000 and >7000 maximum principal (tensile) microstrain was measured.

Linear regression analysis showed good correlation between measured AE hits and volume of cancellous bone elements with compressive strain <−3000µ∊: correlation coefficients (R= 0.947, R2 = 0.847), standard error of the estimate (12.6 AE hits) and percentage error (12.5%) (p<0.001). AP implants displayed greater cancellous bone strains than MB implants for all strain variables at all loads. Patterns of strain differed between implants: MB concentrations at the lateral edge; AP concentrations at the keel, peg and at the region of load application. AP implants had 2.2 (10mm) to 3.2 (6mm) times the volume of cancellous bone compressively strained <−7000µ∊ than the MB implants. Altering MB polyethylene insert thickness had no effect. We advocate using caution with all-polyethylene UKR implants especially in large or active patients where loads are higher.

Joint registries report that 25–40% of UKR revisions are performed for pain. Proximal tibial strain and microdamage are possible causes of this “unexplained” pain. The aim of this study was to examine the effect of UKR implant design and material on proximal tibial cortical strain and cancellous microdamage.

Composite Sawbone tibias were implanted with cemented UKR components: 5 fixed bearing all-polyethylene (FB-AP), 5 fixed bearing metal backed (FB-MB), and 5 mobile bearing metal backed implants (MB-MB). Five intact tibias were used as controls. Tibias were loaded in 500N increments to 2500N. Cortical surface strain was measured using digital image correlation (DIC). Cancellous microdamage was measured using acoustic emission (AE), a technique which detects elastic waves produced by the rapid release of energy during microdamage events.

DIC showed significant differences in anteromedial cortical strain between implants at 1500N and 2500N in the proximal 10mm only (p<0.001) with strain shielding in metal backed implants. AE showed significant differences in cancellous microdamage (AE hits), between implants at all loads (p=0.001). FB-AP implants displayed significantly more hits at all loads than both controls and metal backed implants (p<0.001). FB-AP implants also differed significantly by displaying AE hits on unloading (p=0.01), reflecting a lack of implant stiffness. Compared to controls, the FB-AP implant displayed 15x the total AE hits, the FB-MB 6x and the MB-MB 2.7x. All-polyethylene medial UKR implants are associated with greater cancellous bone microdamage than metal backed implants even at low loads.

Objectives

Because of the contradictory body of evidence related to the potential benefits of helical blades in trochanteric fracture fixation, we studied the effect of bone compaction resulting from the insertion of a proximal femoral nail anti-rotation (PFNA).

Introduction

Osteoarthritis continues to be a major cause of pain and disability. The pathological processes leading to the end-stage of joint degeneration remain poorly understood. Advances in radiological imaging have the potential to improve understanding of the structural and functional changes observed in OA. The aim of this study was to describe the microarchitecture of the femoral head in osteoarthritis.

Objectives

Orthopaedic surgeons use stems in revision knee surgery to obtain stability when metaphyseal bone is missing. No consensus exists regarding stem size or method of fixation. This in vitro study investigated the influence of stem length and method of fixation on the pattern and level of relative motion at the bone–implant interface at a range of functional flexion angles.

Finite element modelling is being extensively used to evaluate the biomechanical behaviour of fractured bone treated with fixation devices. Appropriate modelling of the bone-implant interface is key to quality biomechanical prediction.

The present study considers this interface modelling in the context of locking plates. A majority of previous studies assume the interface to be represented by a tied constraint or a fully bonded interface. Many other studies incorporate a frictional interface but ignore screw threads. This study compares the various interface modelling strategies. An interface with screw threads explicitly included is also considered.

The study finds that interface modelling has significant impact on both the global and local behaviour. Globally, the load-deflection behaviour shows considerable difference depending on the interface model. Locally, the stress-strain environment within the bone close to the screws is significantly altered.

The results show that the widely used tie constraint can overestimate stiffness of a construct which must be correctly predicted to avoid non-union or periprosthetic re-fracture, especially in osteoporotic bone. In addition, the predictions of screw loosening, bone damage and stress shielding are very different when screw threads are included in the model.

A significant source of failure for external fixation devices is loosening of the fixation implant. As bone competence drops with ageing or disease such as osteoporosis, the risk of loosening is likely to increase. However it is not clear how fixator configuration should be adapted to minimise loosening in weaker bone. The aim of this study was to assess the effect of bone competence on the yielding of bone tissue surrounding fixation implants, and thereby inform the selection of fixator configuration to minimise loosening. External fixation of the tibial midshaft using half-pins and Illizarov wires was modelled using finite-element analysis. Half-pin configurations of two and three stainless steel and titanium pins pins were assessed. Illizarov wire configurations of two and four wires were studied, over a range of wire tensions. Bone competence was varied by changing the cortical thickness and elastic properties of the bone fragments to approximate: a) young, high-density bone, b) middle-aged, mid-porosity bone and c) old-aged, severely porous bone. Bone elastic properties were taken from a recent study of cortical bone conducted by the authors. The interaction between implants and bone was modelled with contact analysis, enabling realistic separation. Implant loosening was included using a bone-specific, strain-based yield criterion. Regions where bone tissue yielded were identified as likely sites of loosening. In all cases loading was applied to simulate a one-legged stance.

Several previous studies have examined the mechanical environment in the femur using computational modelling. In particular the proximal femur has been extensively studied using finite element (FE) analyses. This study considers the issues associated with modelling with special interest in the distal femur. FE models require appropriate input on the geometry of the system being considered, material properties of different components, loading regimes and boundary conditions (i.e. the manner in which the system is supported). This study focuses on the last two of the above. A number of models with variable levels of complexity; and different boundary and loading conditions were considered. The simplest loading and boundary conditions considered comprised load applications at the tibio-femoral joint with the proximal femur artificially restrained. More complex models had the femur fully supported on muscles and ligaments. In each case the stress-strain environment in the femur was examined. The results show that the sophistication of the model needs to be based on the answers being sought from the analysis. Some good predictions on the mechanical environment can be made with relatively crude models. For example the stress-strain behaviour in the vicinity of the knee joint was found to be reasonably well predicted by the model that was artificially restrained in the mid-femoral region. Further while different models can be used for comparing different scenarios (e.g. forces during the gait cycle) true quantitative measures are strongly dependent on experimental loading data. The study also shows that it is important to generate and evaluate models of increasing complexity in order to maintain transparency with respect to the influence of different parameters associated with loading and boundary conditions.

Study Aim

Femoral components used in total knee arthroplasty (TKA) are primarily designed on the basis of kinematics and ease of fixation. This study considers the stress-strain environment in the distal femur due to different implant internal geometry variations (based on current industry standards) using finite element (FE) analyses. Both two and three dimensional models are considered for a range of physiological loading scenarios – from full extension to deep flexion. Issues associated with micro-motion at the bone-implant interface are also considered.

The aim of the study is to investigate the biomechanical effects on the pelvis of the anterolateral and posterolateral approaches at the time of hip arthroplasty. In particular the study investigates the change in stress distribution, and the change in muscle recruitment pattern following surgery.

The study uses an advanced finite element model of the pelvis, in which the role of muscles and ligaments in determining the stress distribution in the pelvis is included. The model is altered for the posterolateral approach by excision of the external rotators. Different levels of gluteal damage for the anterolateral approach are modelled by excising in turn the anterior third, half, and two-thirds of the gluteus medius and minimus. Although attempt is generally made to repair gluteal damage at the time of surgery, it is clear the muscle volume will be compromised immediately after surgery.

In support of previous clinical studies indicating an increased risk of limp, and pelvic tilt following the anterolateral approach, significant differences were found in the muscle recruitment pattern following the anterolateral, compared to the posterolateral approach. During single leg stance and walking force transfer to the iliacus and pectineus was observed. Required levels of muscle force, to maintain coronal balance, following the anterolateral approach were found to be close to maximum sustainable levels. In addition significant alteration to the pelvic stress distribution was found following the anterolateral approach. The effects of increasing gluteal damage for the anterolateral approach were progressive, and became more pronounced when more than fifty percent of the gluteus medius and minimus were damaged. Increases in stresses around the acetabulum were observed for the posterolateral, compared to the anterolateral approach.

Thus, based on a biomechanical evaluation, the anterolateral approach presents increased risk of limp, and pelvic tilt, in comparison to the posterolateral approach.

Previous experimental studies of the pelvis have been carried out on cadaveric samples stripped of soft tissue. Investigations of the stress concentrations present in the pelvis due to the application of force through the hip joint have been conducted with the superior iliac crests cast in resin or cement. Thus stress concentrations are observed towards the superior iliac crests, and to some extent the pubic symphysis (these being the areas in which force transfer can occur). Due to the rigid fixing of the pelvis in these experiments, the pelvic bone has become viewed as a ‘sandwich beam’ acting between the sacro-iliac and the pubic joints. Numerical models employing similar fixed conditions have shown good agreement with the experimental studies.

However it is clear that these experiments, and the accompanying computational models are not representative of the in-vivo situation, in which the muscles and ligaments of the pelvis and hip joint provide resistance to movement, and in the case of muscles place additional forces on the pelvis, not addressed in the experimental studies. This study presents a finite element model of the pelvis in which novel techniques have been used to include the pelvic ligaments, and hip joint muscles using realistic attachment areas on the cortex, providing a more realistic comparison to the in-vivo environment. Joint interactions at the pubic symphysis and sacro-iliac joints are also simulated. A fixed boundary condition model is also presented for comparison.

The resulting stress concentrations in the pelvis for single leg stance observed in the in-vivo boundary condition model are dramatically different to those presented in studies in which the pelvis is rigidly fixed in place. The abductor muscles are seen to play a significant role in reducing stress concentrations towards the sacro-iliac joints and superior to the acetabulum, in comparison to fixed boundary condition analyses. Stress reductions away from the acetabulum are also observed in the underlying trabecular bone for the in-vivo boundary condition model. Similar stresses are observed within the acetabular region for the fixed, and in-vivo boundary condition models.

Following hip arthroplasty carried out using the Slooff-Ling impaction grafting technique micro-motion of the acetabular cup is frequently seen within the bone graft bed. In some cases this can lead to gross migration and rotation of the acetabular cup, resulting in failure of the arthroplasty. The movement of the cup is thought to be due to the irrecoverable deformation of bone graft under shear and compressive forces. Previous experimental studies have addressed ways in which the behaviour of the bone graft material may be improved, for example through washing and the use of improved particle size distribution. However there has been a limited amount of research carried out into assessing the behaviour of the acetabular construct in-vivo.

This study presents a 3D finite element model of the acetabular construct and hemi-pelvis following impaction grafting of a cavitory defect. A sophisticated elasto-plastic material model was developed based on research carried out by the group to describe the bone graft bed. The material model includes the non-linear stiffness response, as well as the shear and consolidation yield response of the graft. Loading associated with walking, sitting down, and standing up is applied to the model. Distinct patterns of migration and rotation are observed for the different activities. When compared in a pseudo-quantitative manner with clinical observations results were found to be similar. Walking is found to account for superior migration, and rotation in abduction of the acetabular cup, while sitting down and standing up are found to account for posterior migration, and lateral rotation. The developed 3D model can be used in the assessment of cup designs and fixation devices to reduce the rate of aseptic failure in the acetabular region.

Micro level finite element models of bone have been extensively used in the literature to examine its mechanical behaviour and response to loads. Techniques used previously to create these models involved CT attenuations or images (e.g. micro-CT, MRI) of real bone samples. The computational models created using these methods could only represent the samples used in their construction and any possible variations due to factors such as anatomical site, sex, age or degree of osteopo-rosity cannot be included without additional sample collection and processing. This study considers the creation of virtual finite element models of trabecular bone, i.e. models that look like and mechanically behave like real trabecular bone, but are generated computationally.

The trabecular bone is anisotropic both in terms of its micro-architecture and its mechanical properties. Considerable research shows that the key determinants of the mechanical properties of bone are related to its micro-architecture. Previous studies have correlated the apparent level mechanical properties with bone mineral density (BMD), which has also been the principal means of diagnosis of osteoporosis. However, BMD alone is not sufficient to describe bone micro-architecture or its mechanical behaviour. This study uses a novel approach that employs BMD in conjunction with micro-architectural indices such as trabecular thickness, trabecular spacing and degree of anisotropy, to generate virtual micro-architectural finite element models. The approach permits generation of several models, with suitable porous structure, for the same or different levels of osteoporosity. A series of compression and shear tests are conducted, numerically, to evaluate the apparent level orthotropic elastic properties. These tests show that models generated using identical micro-architectural parameters have similar apparent level properties, thus validating this initial bone modelling algorithm. Numerical tests also clearly illustrate that poor trabecular connectivity leads to inferior mechanical behaviour even in cases where the BMD values are relatively high. The generated virtual models have a range of applications such as understanding the fracture behaviour of osteoporotic bone and examining the interaction between bone and implants.

Morsellised cortico-cancellous bone (MCB) is used extensively in impaction grafting procedures, such as the filling of cavitory defects on the femoral and acetabular sides during hip arthroplasty. Several experimental studies have attempted to describe the mechanical behaviour of MCB in compression and shear, and it has been found that it’s properties can be improved by washing and rigorous impaction at the time of surgery. However their focus has not been on the development of constitutive models that can be used in computational simulation.

The results of serial confined compaction tests are presented and used to develop constitutive models describing the non-linear elasto-plastic behaviour of MCB, as well as its time dependent visco-elastic behaviour. It is found that the elastic modulus, E of MCB increases linearly with applied pressure, p, with E achieving a value of around 30 MPa at a pressure of around 1 MPa. The plastic behaviour of MCB can be described using a Drucker Prager Cap yield criterion, capable of describing yielding of the graft in shear and compression. The time dependent visco-elastic behaviour of MCB can be accurately modelled using a spring and dashpot model that can be numerically expressed using a fourth order Prony series. The role of impaction in reducing subsequent plastic deformation was also investigated. The developed relationships allow the constitutive modelling of MCB in finite element simulations, for example of the acetabular construct following impaction grafting. The relationships also act as a gold standard against which to compare synthetic graft and graft extender materials.