In severe cases of total knee arthroplasty which cannot be treated with off-the-shelf implants anymore custom-made knee implants may serve as one of the few remaining options to restore joint function or to prevent limb amputation. Custom-made implants are specifically designed and manufactured for one individual patient in a single-unit production, in which the surgeon is responsible for the implant design characteristics in consultation with the corresponding engineer. The mechanical performance of these custom-made implants is challenging to evaluate due to the unique design characteristics and the limited time until which the implant is needed. Nevertheless, the custom-made implant must comply with clinical and regulatory requirements. The design of custom-made implants is often based on a underlying reference implant with available biomechanical test results and well-known clinical performance. To support surgeons and engineers in their decision whether a specific implant design is suitable, a method is proposed to evaluate its mechanical performance. The method uses
Background. Children suffering from primary bone cancer necessitating resection of growth plates, may suffer progressive leg length discrepancy, which can be attenuated with extendable prostheses. A serious complication is catastrophic implant failure. Over time, bone will remodel, altering the stress pattern in the implant. By using
Introduction and aims. Biomechanical testing has been a cornerstone of the development of surgical implants for fracture stabilisation. To date most fracture surgery implant design and testing has been dominated by the use of standard bench top biomechanical testing. Although such methods have been used to successfully reproduce certain clinical observations, there are very clear limitations. More recently however, computerised engineering technology using
Abstract. Background. Proximal fibular osteotomy (PFO) was defined to provide a treatment option for knee pain caused by gonarthrosis(1). Minor surgical procedure, low complication rate and dramatic pain relief were the main reasons for popularization of this procedure(2, 3). However, changes at the knee and ankle joint after PFO were not clarified objectively in the literature. Questions/purposes. We asked: 1) Does PFO change the maximum and average pressures at the medial and lateral chondral surface of the tibia plateau? 2) Are chondral surface stresses redistributed at the knee and ankle joint after PFO? 3)Does PFO change the distribution of total load on the knee joint? 4) Can PFO lead to change in alignment of lower limb?. Methods. This study was conducted at Maltepe University Faculty of Medicine Hospital, Orthopedics and Traumatology Department and Yildiz Technical University Mechanical Engineering Department in Istanbul, Turkey, between September 2019 and February 2020.
Background. Comminuted fractures involving the tibia are associated with a high level of complications including delayed healing and non-union, in conjunction with dramatically increased healthcare costs. Certain clinicians utilise a Pixel Value Ratio (PRV) of 1 to indicate such fracture healing. The subjectivity of this method has led to mixed outcomes including regenerate fracture. The poor prognosis of complex load bearing fractures is accentuated by the fact that no quantitative gold standard currently exists to which clinicians can reference regarding the definition of a healed fracture. The aim of the current study is to use patient specific
To date, the fixation of proximal humeral fractures with angular stable locking plates is still insufficient with mechanical failure rates of 18% to 35%. The PHILOS plate (DePuy Synthes, Switzerland) is one of the most used implants. However, this plate has not been demonstrated to be optimal; the closely symmetric plate design and the largely heterogeneous bone mineral density (BMD) distribution of the humeral head suggest that the primary implant stability may be improved by optimizing the screw orientations.
Summary Statement. An MRI-derived subject-specific finite element model of a knee joint was loaded with subject-specific kinetic data to investigate stress and strain distribution in knee cartilage during the stance phase of gait in-vivo. Introduction.
Femoro-acetabular impingement involves a deformity of the hip joint and is associated with hip osteoarthritis. Although 15% of the asymptomatic population exhibits a deformity, it is not clear who will develop symptoms. Current diagnostic imaging measures have either low specificity or low sensitivity and do not consider the dynamic nature of impingement during daily activities. The goal of this study is to determine stresses in the cartilage, subchondral bone and labrum of normal and impinging hips during activities such as walking and sitting down. Quantitative CT scans were obtained of a healthy Control and a participant with a symptomatic femoral cam deformity (‘Bump’). 3D models of the hip were created from automatic segmentation of CT scans. Cartilage layers were added so the articular surface was the mid-line of the joint. Finite element meshes were generated in each region. Bone elastic modulus was assigned element-by-element, calculated from CT intensity converted to bone mineral density using a calibration phantom. Cartilage was modelled as poroelastic, E=0.467 MPa, v=0.167, and permeability 3×10-16 m4/N s. The pelvis was fixed while rotations and contact forces from Bergmann et al. (2001) were applied to the femur over one load cycle for walking and sitting in a chair. All analyses were performed in FEBio. High shear stresses were seen near the acetabular cartilage-labrum junction in the Bump model, up to 0.12 MPa for walking and were much higher than in the Control. Patient-specific modelling can be used to assess contact and tissue stresses during different activities to better understand the risk of degeneration in individuals, especially for activities that involve high hip flexion. The high stresses at the cartilage labrum interface could explain so-called bucket-handle tears of the labrum.
Biomechanical analysis is important to evaluate the effect of orthopaedic surgeries. CT-image based finite element method (CT-FEM) is one of the most important techniques in the computational biomechanics field. We have been applied CT-FEM to evaluate resorptive bone remodeling, secondary to stress shielding, after total hip arthroplasty (THA). We compared the equivalent stress and strain energy density to postoperative BMD (bone mineral density) change in the femur after THA, and a significant correlation was observed between the rate of changes in BMD after THA and equivalent stress. For periacetabular osteotomy cases, we investigated mechanical stress in the hip joint before and after surgery. Mechanical stress in the hip joint decreased significantly after osteotomy and correlated with the degree of the acetabular coverage. For arthroscopic osteochondroplasty cases, we examined mechanical strength of the proximal femur after cam resection using CT-FEM. The results suggested that both the depth and area of the resection at the distal part of femoral head-neck junction correlated strongly with fracture risk after osteochondroplasty. This talk consists of our results of clinical application studies using CT-FEM, and importance of application of CT-FEM to biomechanical studies to assess the effect of orthopaedic surgeries.
The annulus fibrosus (AF) of the intervertebral disc (IVD) has a unique, complex structure. If engineered tissues for the IVD are to be successfully developed, it is essential that the constituent level mechanics of the tissues in their natural form are fully understood (Nerurkar, J. Biomech. 2010). Published finite element (FE) models of the IVD do not represent lamellae behaviour and are validated using bulk mechanics of the intervertebral joint. This study aims to develop models of the IVD that include representation of the lamellae structure of the AF and the behaviour of this tissue within the disc. Three FE models of a vertebra-disc-vertebra section were developed considering the following scenarios of the AF: Homogenous AF. Concentric rings representing AF's lamellae structure with frictionless contact between rings. Concentric rings with ‘interface’ elements representing the interlamellar space; properties were derived through calibration of a separate model of an AF tissue sample with histological studies of the AF (Gregory, J. Biomechs. 2009). Displacements, stiffness and disc bulge were compared with the literature. The properties derived for the interface elements were stiffer than those for the AF tissue. this is in agreement with in vitro studies that have examined the mechanisms by which the lamellae fail prior to the interlamellar interaction (Veres, Spine, 2010). The macro-scale performance of the disc was sensitive to how the interlamellar interactions were modelled. Disc stiffness reduced by 7.1% between the homogenous and frictionless models. Use of the interface model improved the agreement with the in vitro performance of the disc: 5.8% error was recorded for disc stiffness and 2.1% error for disc bulge. The mechanics of the lamellae within the AF changed significantly between the frictionless and interface models. The relative displacement of adjacent lamellae was reduced by 15% between the frictionless and interface models. This study shows that the representation of the lamina structure of the AF affects the mechanics of the whole disc. Discrepancies in the modelling of interlamellar mechanics could have a significant effect on the interpretation of several important aspects of the biomechanics of the IVD.METHODS
RESULTS & CONCLUSIONS
Finite element (FE) models are frequently used in biomechanics to predict the behaviour of new implant designs. To increase the stability after severe bone loss tibial components with long stems are used in revision total knee replacements (TKR). A clinically reported complication after revision surgery is the occurrence of pain in the stem-end region. The aim of this analysis was the development of a validated FE-model of a fully cemented implant and to evaluate the effect of different tibial stem orientations. A scanned 4th generation synthetic left tibia (Sawbones) was used to develop the FE-model with a virtually implanted fully cemented tibial component. The 500 N load was applied with medial:lateral compartment distributions of 60:40 and 80:20. Different stem positons were simulated by modifying the resection surface angle posterior to the tibias shaft axis. The results were compared with an experimental study which used strain gauges on Sawbones tibias with an implanted tibial TKR component. The locations of the experimental strain gauges were modelled in the FE study.Background
Methods
Stemless shoulder implants have recently gained increasing popularity. Advantages include an anatomic reconstruction of the humerus with preservation of bone stock for upcoming revisions. Several implant designs have been introduced over the last years. However, only few studies evaluated the impact of the varying designs on the load transfer and bone remodeling. The aim of this study was to compare the differences between two stemless shoulder implant designs using the micro finite element (µFE) method. Two cadaveric human humeri (low and high bone mineral density) were scanned with a resolution of 82µm by high resolution peripheral quantitative computer tomography (HR-pQCT). Images were processed to allow virtual implantation of two types of reverse-engineered stemless humeral implants (Implant 1: Eclipse, Arthrex, with fenestrated cage screw and Implant 2: Simpliciti, Tornier, with three fins). The resulting images were converted to µFE models consisting of up to 78 million hexahedral elements with isotropic elastic properties based on the literature. These models were subjected to two loading conditions (medial and along the central implant axis) and solved for internal stresses with a parallel solver (parFE, ETH Zurich) on a Linux Cluster. The bone tissue stresses were analysed according to four subregions (dividing plane: sagittal and frontal) at two depths starting from the bone-implant surface and the distal region ending distally from the tip of Implant 1 (proximal, distal)Introduction
Materials and Methods
Osteochondral (OC) grafting is one available method currently used to repair full thickness cartilage lesions with good results clinically when grafting occurs in patients with specific positive prognostic factors. However, there is poor understanding of the effect of individual patient and surgical factors. With limited tissue availability, development of Finite Element (FE) models taking into account these variations is essential. The aim of this study was to evaluate the effect of altering the material properties of OC grafts and their host environment through computer simulation. A generic FE model (ABAQUS CAE 2017) of a push-out test was developed as a press-fit bone cylinder (graft) sliding inside a bone ring (host tissue). Press-fit fixation was simulated using an interference fit. Overlap between host and graft (0.01mm–0.05mm) and coefficient of friction (0.3–0.7) were varied sequentially. Bone Young's moduli (YM) were varied individually between graft and host within the range of otherwise derived tissue moduli (46MPa, 82MPa, 123MPa). Increasing both overlap and frictional coefficient increased peak dislodging force independently (overlap: 490% & frictional coefficient: 176% across range tested). Increasing bone modulus also increased dislodging force, with host bone modulus (107%, 128%, and 140% increase across range, when Graft YM = 123MPa, 82 MPa, and 46MPa, respectively) having a greater influence than graft modulus (28%, 19% and 10% increase across range, when Host YM = 123 MPa, 82MPa and 46MPa, respectively). As anticipated increasing overlap and friction caused an increase in force necessary to dislodge the graft. Importantly, differentially changing the graft and host material properties changed the dislodging force indicating that difference between graft and host may be an important factor in the success or failure clinically of osteochondral grafting.
To analyse bone stresses in humerus-megaprosthesis construct in response to axial loading under varying implant lengths in proximal humeral replacement following tumour excision. CT scans of 10 cadaveric humeri were processed in 3D Slicer to obtain three-dimensional (3D) models of the cortical and cancellous bone. Megaprostheses of varying body lengths (L) were modelled in FreeCAD to obtain the 3D geometry. Four FE models: group A consisting of intact bone; groups B (L=40mm), C (L=100mm) and D (L=120mm) comprising of humerus-megaprosthesis constructs were created. Isotropic linear elastic behaviour was assigned for all materials. A tensile load of 200N was applied to the elbow joint surface with the glenohumeral joint fixed with fully bonded contact interfaces. Static analysis was performed in Abaqus. The bone was divided at every 5% bone length beginning distally. Statistical analysis was performed on maximum von Mises stresses in cortical and cancellous bone across each slice using one-way ANOVA (0-45% bone length) and paired t-tests (45-70% bone length). To quantify extent of stress shielding, average percentage change in stress from intact bone was also computed. Maximum stress was seen to occur distally and anteriorly above the coronoid fossa. Results indicated statistically significant differences between intact state and shorter megaprostheses relative to longer megaprostheses and proximally between intact and implanted bones. Varying levels of stress shielding were recorded across multiple slices for all megaprosthesis lengths. The degree of stress shielding increased with implant lengthening being 2-4 times in C and D compared to B. Axial loading of the humerus can occur with direct loading on outstretched upper limbs or indirectly through the elbow. Resultant stress shielding effect predicted in longer megaprosthesis models may become clinically relevant in repetitive axial loading during activities of daily living. It is recommended to use shorter megaprosthesis to prevent failure.
Preservation of both anterior and posterior cruciate ligaments in total knee arthroplasty (TKA) can lead to near-normal post-operative joint mechanics and improved knee function. We hypothesised that a patient-specific bicruciate-retaining prosthesis preserves near-normal kinematics better than standard off-the-shelf posterior cruciate-retaining and bicruciate-retaining prostheses in TKA. We developed the validated models to evaluate the post-operative kinematics in patient-specific bicruciate-retaining, standard off-the-shelf bicruciate-retaining and posterior cruciate-retaining TKA under gait and deep knee bend loading conditions using numerical simulation.Objectives
Methods
Prophylactic treatment is advised for metastatic bone disease patients with a high risk of fracture. Clinicians face the task of identifying these patients with high fracture risk and determining the optimal surgical treatment method. Subject-specific finite element (FE) models can aid in this decision process by predicting the mechanical effect of surgical treatment. In this study, we specifically evaluated the potential of FE models to simulate femoroplasty, as uncertainty remains whether this prophylactic procedure provides sufficient mechanical strengthening to the weight-bearing femur. In eight pairs of human cadaveric femurs artificial metastatic lesions were created. In each pair, an identical defect was milled in the left and right femur. Four pairs received a spherical lesion in the neck and the other four an ellipsoidal lesion in the intertrochanteric region, each at the medial, superior/lateral, anterior and posterior side, respectively. One femur of each pair was augmented with polymethylmethacrylate (5–10 ml), while the contralateral femur was left untreated. CT scans were made at three different time points: from the unaffected intact femurs, the defect femurs with lesion and the augmented femurs. Bone strength was measured by mechanical testing until failure in eight defect and eight augmented femurs. Nonlinear CT-based FE models were developed and validated against the experimentally measured bone strength. Subsequently, the validated FE model was applied to the available CT scans for the three different cases: intact (16 scans), defect (16) and augmented (8). The FE predicted strength was compared for the three different cases. The FE models predicted the experimental bone strength with a strong correspondence, both for the defect (R2 = 0.97, RMSE= 0.75 kN) and the augmented femurs (R2 = 0.90, RMSE = 0.98 kN). Although all lesions had a “moderate” to “high” risk for fracture according to the Mirels’ scoring system (score 7 or 8), three defect femurs did not fracture through the lesion (intertrochanteric anterior, lateral and posterior), indicating that these lesions did not act as a critical weak spot. In accordance with the experimental findings, the FE models indicated almost no reduction in strength between the intact and defect state for these femurs (0.02 ± 0.1%). For the remaining “critical” lesions, bone strength was reduced with 15.7% (± 14.9%) on average. The largest reduction was observed for lesions on the medial side (up to 43.1%). For the femurs with critical lesions, augmentation increased bone strength with 29.5% (± 29.7%) as compared to the defect cases, reaching strength values that were 2.5% (± 3.7%) higher than the intact bone strength. Our findings demonstrate that FE models can accurately predict the experimental bone strength before and after augmentation, thereby enabling to quantify the mechanical benefit of femoroplasty. This way FE models could aid in identifying suitable patients for whom femoroplasty provides sufficient increase in strength. For all lesions evaluated in this study, femoroplasty effectively restored the initial bone strength. Yet, additional studies on larger datasets with a wide variation of lesion types are required to confirm these results.
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. This 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.Aims
Methods
In severe cases of total knee & hip arthroplasty, where off-the-shelf implants are not suitable (i.e., in cases with extended bone defects or periprosthetic fractures), 3D-printed custom-made knee & hip revision implants out of titanium or cobalt-chromium alloy represent one of the few remaining clinical treatment options. Design verification and validation of such custom-made implants is very challenging. Therefore, a methodology was developed to support surgeons and engineers in their decision on whether a developed design is suitable for the specific case. A novel method for the pre-clinical testing of 3D-printed custom-made knee implants has been established, which relies on the biomechanical test and
Implant manufacturers develop new products to improve existing fracture fixation methods or to approach new fracture challenges. New implants are commonly tested and approved with respect to their corresponding predecessor products, because the knowledge about the internal forces and moments acting on implants in the human body is unclear. The aim of this study was to evaluate and validate implant internal forces and moments of a complex physiological loading case and translate this to a standard medical device approval test. A finite elements model for a transverse femur shaft fracture (AO/OTA type 32-B2) treated with a locked plate system (AxSOS 3 Ti Waisted Compression Plate Broad, Stryker, Kalamazoo, USA) was developed and experimentally validated. The fractured construct was physiologically loaded by resulting forces on the hip joint from previously measured in-vivo loading experiments (Bergmann et. al). The forces were reduced to a level where the material response in the construct remained linear elastic. Resulting forces, moments and stresses in the implant of the fractured model were analysed and compared to the manufacturers’ approval data. The FE-model accurately predicted the behaviour of the whole construct and the micro motion of the working length of the osteosynthesis. The resulting moment reaction in the working length was 24 Nm at a load of 400 N on the hip. The maximum principle strains on the locking plate were predicted well and did not exceed 1 %. In this study we presented a protocol by the example of locked plated femur shaft fracture to calculate and validate implant internal loading using
Orthopaedic impaction-instruments are used to drive implants into the bone of the patient. Pre-clinical experimental testing protocols and computer models of those are used to assess robustness and functional efficiency of such instruments. This generally involves impaction of the instrument mounted on a substrate that should represent the mechanics of the patient. In this study, the effects of the substrate on stressing of the impaction-instruments were investigated using dynamic