J.A. Simões

The influence of different tibial stem designs in load sharing and stability at the cement–bone interface in revision TKA

Total Knee Arthroplasty (TKA) changes mechanical loading of the knee joint. Bone loss in the tibia is commonly encountered at the time of the revision TKA. Restoration of lost bone support and joint stability are the primary challenges in revision TKA. Normally, these defects are treated with non-living structures like metallic augments or bone grafts (autografts or allografts). Alone, neither of these structures can provide the initial support and stability for revision implants. In the latter, the use of intramedullary stems can provide the necessary load sharing and protect the remaining host bone and graft from excessive stress, increasing component stability. The purpose of this study was to evaluate comparatively load sharing (cortical rim, cancellous bone and stem) and stability at the cement-bone interface under the tibial tray induced by the use of cemented and press-fit tibial component stem extensions. Furthermore the study of the desirable option in cases where the bone defect is cavitary (cancellous bone defect contained by an intact cortical rim) or uncontained bone defect (bone loss involving the supporting cortical rim) was carried out. Because in vitro evaluation of these biomechanical parameters is difficult we used finite element (FE) models to overcome this. The biomechanical results suggest an identical behaviour in case of cavitary defects for both types of stems assessed. In the case of uncontained defect treated with bulk allografts the cemented stem may be a prudent clinical option.

The influence of cement mantle thickness and stem geometry on fatigue damage in two different cemented hip femoral prostheses

Experimental models can be used for pre-clinical testing of cemented and other type of hip replacements. Total hip replacement (THR) failure scenarios include, among others, cement damage accumulation and the assessment of accurate stress and strain magnitudes at the cement mantle interfaces (stem-cement and cement-bone) can be used to predict mechanical failure. The aseptic loosening scenario in cemented hip replacements is currently not fully understood, and methods of evaluating medical devices must be developed to improve clinical performance. Different results and conclusions concerning the cement micro-cracking mechanism have been reported. The aim of this study was to verify the in vitro behavior of two cemented femoral stems with respect to fatigue crack formation. Fatigue crack damage was assessed at the medial, lateral, anterior and posterior sides of the Lubinus SPII and Charnley stems. All stems were loaded and tested in stair climbing fatigue loading during one million cycles at 2 Hz. After the experiments each implanted synthetic femur was sectioned and analyzed. We observed more damage (cracks per area) for the Lubinus SPII stem, mainly on the proximal part of the cement mantle. The micro-cracking formation initiated in the stem-cement interface and grew towards the direction of cortical bone of the femur. Overall, the cement-bone interface seems to be crucial for the success of the hip replacement. The Charnley stem provoked more damage on the cement-bone interface. A failure index (maximum length of crack/maximum thickness of cement) considered was higher for the cement-stem interface of the Lubinus SPII stem. For a cement mantle thickness higher than 5 mm, cracking initiated at the cement-bone interface and depended on the opening canal process (reaming procedure and instrumentation). The analysis also showed that fatigue-induced damage on the cement mantle, increasing proximally, and depended on the axial position of the stem. The cement thickness is an important factor for the success of THR and this study evidenced that cement thickness higher than 2 mm apparently does not affect the mechanical behavior of the cement mantel and induce more crack formation on the cement-bone interface.

Finite Element and Experimental Cortex Strains of the Intact and Implanted Tibia

Finite Element (FE) models for the simulation of intact and implanted bone find their main purpose in accurately reproducing the associated mechanical behavior. FE models can be used for preclinical testing of joint replacement implants, where some biomechanical aspects are difficult, if not possible, to simulate and investigate in vitro. To predict mechanical failure or damage, the models should accurately predict stresses and strains. Commercially available synthetic femur models have been extensively used to validate finite element models, but despite the vast literature available on the characteristics of synthetic tibia, numerical and experimental validation of the intact and implant assemblies of tibia are very limited or lacking. In the current study, four FE models of synthetic tibia, intact and reconstructed, were compared against experimental bone strain data, and an overall agreement within 10% between experimental and FE strains was obtained. Finite element and experimental (strain gauge) models of intact and implanted synthetic tibia were validated based on the comparison of cortex bone strains. The study also includes the analysis carried out on standard tibial components with cemented and noncemented stems of the P.F.C Sigma Modular Knee System. The overall agreement within 10% previously established was achieved, indicating that FE models could be successfully validated. The obtained results include a statistical analysis where the root-mean-square-error values were always <10%. FE models can successfully reproduce bone strains under most relevant acting loads upon the condylar surface of the tibia. Moreover, FE models, once properly validated, can be used for preclinical testing of tibial knee replacement, including misalignment of the implants in the proximal tibia after surgery, simulation of long-term failure according to the damage accumulation failure scenario, and other related biomechanical aspects.

Biomechanical Analysis Comparing Natural and Alloplastic Temporomandibular Joint Replacement Using a Finite Element Model

PURPOSE Prosthetic materials and bone present quite different mechanical properties. Consequently, mandible reconstruction with metallic materials (or a mandible condyle implant) modifies the physiologic behavior of the mandible (stress, strain patterns, and condyle displacements). The changing of bone strain distribution results in an adaptation of the temporomandibular joint, including articular contacts. MATERIALS AND METHODS Using a validated finite element model, the natural mandible strains and condyle displacements were evaluated. Modifications of strains and displacements were then assessed for 2 different temporomandibular joint implants. Because materials and geometry play important key roles, mechanical properties of cortical bone were taken into account in models used in finite element analysis. RESULTS The finite element model allowed verification of the worst loading configuration of the mandibular condyle. Replacing the natural condyle by 1 of the 2 tested implants, the results also show the importance of the implant geometry concerning biomechanical mandibular behavior. The implant geometry and stiffness influenced mainly strain distribution. CONCLUSION The different forces applied to the mandible by the elevator muscles, teeth, and joint loads indicate that the finite element model is a relevant tool to optimize implant geometry or, in a subsequent study, to choose a more suitable distribution of the screws. Bone screws (number and position) have a significant influence on mandibular behavior and on implant stress pattern. Stress concentration and implant fracture must be avoided.

Experimental modal analysis of a synthetic composite femur

In this paper we describe the experimental characterization of the modal parameters of a synthetic composite femur model widely used in biomechanical research studies. The objective of the experimental procedure was to identify the natural frequencies and mode shapes of an unconstrained (free-free) femur. The experimental data were compared with the same obtained in an analog study performed with a fresh cadaveric femur bone. Other objective of the study was to investigate modal analysis as a technique to validate a finite element model of a composite femur with isotropic material properties.

Simulation of Physiological Loading in Total Hip Replacements

The determination of biomechanical force systems of implanted femurs to obtain adequate strain measurements has been neglected in many published studies. Due to geometric alterations induced by surgery and those inherent to the design of the prosthesis, the loading system changes because the lever arms are modified. This paper discusses the determination of adequate loading of the implanted femur based on the intact femur-loading configuration. Four reconstructions with Lubinus SPII, Charnley Roundback, Muller Straight and Stanmore prostheses were used in the study. Pseudophysiologic and nonphysiologic implanted system forces were generated and assessed with finite element analysis. Using an equilibrium system of forces composed by the Fx (medially direction) component of the hip contact force and the bending moments Mx (median plane) and My (coronal plane) allowed adequate, pseudo-physiological loading of the implanted femur. We suggest that at least the bending moment at the coronal plane must be restored in the implanted femur-loading configuration.

Tribological characterisation of carbon nanotubes/ultrahigh molecular weight polyethylene composites: the effect of sliding distance

The tribological characterisation of metal-on-polymer (MOP) or ceramic-on-polymer (COP) couple is required to prevent osteolysis and loosening of the prosthesis which leads to subsequent failure of the implants. An attempt was made to enhance the tribological properties of ultrahigh molecular weight polyethylene (UHMWPE) by adding the carbon nanotubes (CNTs). The chemically treated CNTs were homogeneously mixed with UHMWPE using a ball milling process and the mixed raw materials were used to prepare a compression moulded sheet. Tribological characterisation of the test sample as a function of sliding distance was carried out in a tribometer using a ball on plate configuration. Different types of wear trend and friction coefficient were observed in polymer and nanocomposites. It was also observed that wear volume and wear coefficient decreases significantly with an addition of CNTs in the polymer and they follow a linear relation with sliding distance.

Influences of implant condyle geometry on bone and screw strains in a temporomandibular implant.

A 3D finite element model of an in vitro implanted mandible was analysed. The load point was placed on the condyle in three positions (inside the mouth, centred and outside) to simulate different contact points between the mandible condyle and the temporal bone. The strain fields in the condyle were assessed and detailed around the surgical screws. The temporomandibular implant studied here was modelled on a commercial device that uses four screws to fix it in vivo in a very similar position. The boundary conditions of the numerical model simulated a load on the incisors with a 15 mm mouth aperture. The same contact loads were applied to the two condyles. Numerical results were successfully obtained for the three different contact points: the inside contact produced lower strains on the condyle. The first screw created a critical strain distribution in the bone, just under the screw. The study shows that centred and inside contact induces lower strain distributions. This suggests that spherical condyle geometry should be applied in order to reduce the strains in fixation. As the top screw was observed to play the most critical role, the third screw is in fact unnecessary, since the lower strain distribution suggests that it will be loosened.

Biomechanical evaluation of different reconstructive techniques of proximal tibia in revision total knee arthroplasty: An in-vitro and finite element analysis

BACKGROUND Bone loss and subsequent defects are often encountered in revision total knee arthroplasty. In particular, when the cortical rim of proximal tibia is breached, the surgical decision on the reconstructive options to be taken is challenging due to the variety of defects and the lack of data from clinical or experimental studies that can support it. The purpose of this study is to assess how different reconstructive techniques, when applied to an identical defect and bone condition, can be associated to dissimilar longevity of the revision procedure, and the role of a stem in this longevity. METHODS Proximal cortex strains and implant stability were measured in ten reconstructive techniques replicated with synthetic tibiae. The cancellous bone strains under each construct were assessed with finite element models which were validated against experimental strains. FINDINGS The measured strains and stability showed that the proximal cortex is not immune to the different reconstructive techniques when applied to an identical defect. The largest cancellous strain differences between modular and non-modular techniques indicate a distinct risk between reconstructive techniques, associated to the supporting capacity of cancellous bone at long term. INTERPRETATION The main finding of the present study is the observation that modular augments increases, on a long term basis, the potential risk of bone resorption relative to the non-modular techniques. In addition, the use of a press-fit stem in the scope of non-modular techniques can lead to improved stability and load transfer, which can contribute positively to the life expectancy of these techniques.

A new press-fit stem concept to reduce the risk of end-of-stem pain at revision TKA: A pre-clinical study

PURPOSE Revision total knee arthroplasty presents numerous technical challenges, with lower patient outcomes compared with those obtained in primary surgery. Extended stems have been used in revision total knee arthroplasty to improve component alignment and fixation. Hybrid fixation with cemented tibial tray and press-fit stem has shown good results. One of the disadvantages of this technique is pain related to the presence of a cementless diaphyseal engaging stem, often designated as end-of-stem pain. Patients with this pain have reported a decrease in overall satisfaction, as well as demonstrate a lower clinical outcome score. Clinical findings suggest that stem material and design are important factors in the development of end-of-stem pain. Therefore, a question can be raised: can a novel press-fit stem concept minimize bone strain changes at the stem tip? The hypothesis here considered lies upon the fact, that if periosteal cortex strain changes are minimized at the stem tip comparatively to the intact situation, the risk of end-of-stem pain might be minimized. SCOPE This pre-clinical study was accomplished using synthetic tibiae to experimentally predict the periosteal cortex strains at the proximal and stem tip regions, with a commercial press-fit stem and a new stem concept. CONCLUSIONS The results demonstrated that the new stem concept has the ability to minimize strain changes induced by the stem tip at the distal periosteal cortex and consequently, at the periosteal layer of bone tissue, which is highly pain sensitive, probably contributing to the reduction of the risk of end-of-stem pain.