Compressive fracture of osteoporotic vertebrae has been one of the most important health problems in aged societies because severely injured spin might be a reason of bedridden for elderly people. Osteoporosis has been widely assessed by averaged bone mineral density of vertebrae measured using DEXA, however, BMD sometimes does not reflect the strength of vertebrae. CT imaged based finite element method (CT-FEM) has been applied to evaluate the strength of vertebrae based on the biomechanics theory and approved by a part of the highly advanced medical treatment in Japan. In the present study, compressive strength of more than 100 vertebrae were evaluated using CT-FEM, and the correlation between BMD and the strength was thoroughly investigated. It was found that some vertebrae with high BMD could have low strength which may cause fracture easily. Thus, a controversial point of the BMD based diagnosis of osteoporosis was clearly indicated. In this invited talk, some basic theories of CT-FEM and fracture assessment and some key results from the recent study will be presented

We investigated the effect of the location and the number of distal screws in the efficiency of an intramedullary nail implementing the finite element method (FEM). The left proximal femur of a 93-year old man was scanned and two series of full 3D models were developed. The first series, consisting of five models, concerned the use of a single distal screw inserted in five different distal locations. The second series, consisting of four models, concerned the use of four different pairs of distal screws. Each model was analyzed with the (FEM) twice, first considering that the femur is fractured and then considering that the femur is healed. For nails with a single distal screw, stresses around the nail hole were reduced with proximal placement of the distal screw but the area around the nail hole where the lag screw is inserted is stressed more. Furthermore, for nails with a pair of distal screws, placing the pair of distal screws at a specific location is most beneficial for the mechanical behavior of the femur/nail assembly. The distal area of the nail generally gets less stressed when a pair of distal screws is introduced, while the presence of two distal screws far away from each other results in lower proximal femoral head displacements. The stress field at the area of fracture is not influenced significantly by the presence of a single distal screw or a pair of distal screws

Squeaking ceramics bearing surfaces have been recently recognised as a problem in total hip arthroplasty. The position of the acetabular cup has been alluded to as a potential cause of the squeaking, along with particular combinations of primary stems and acetabular cups. This study has used the finite element method to investigate the propensity of a new large diameter preassembled ceramic acetabular cup to squeaking due to malpositioning. A verified three-dimensional FE model of a cadaveric human pelvis was developed which had been CT scanned, and the geometry reconstructed; this was to be used to determine the behaviour of large diameter acetabular cup system with a thin delta ceramic liner in the acetabulum. The model was generated using ABAQUS CAE pre-processing software. The bone model incorporated both the geometry and the materials properties of the bone throughout based on the CT scan. Finite element analysis and bone material assignment was performed using ABAQUS software and a FORTRAN user subroutine. The loading applied simulated edge loading for rising from a chair, heel-strike, toe off and stumbling. All results of the analysis were used to determine if the liner separated from the shell and if the liner was toggling out of the shell. The results were also examined to see if there was a propensity for the liner to demobilise and vibrate causing a squeaking sound under the prescribed loading regime. This study indicates that there is a reduction in contact area between the ceramic liner and titanium shell if a patient happens to trip or stumble. However, since the contact between the liner and the shell is not completely lost the propensity for it to squeak is highly unlikely

The wear phenomenon of ultra-high molecular weight polyethylene (UHMWPE) in knee and hip prostheses is one of the major restriction factors on the longevity of theses implants. Despite quite a number of studies on the wear of UHMWPE, the wear mechanism is not clear yet. In order to minimize the wear of UHMWPE and to improve the longevity of artificial joints, it is necessary to clarify the factors influencing the wear mechanism of UHMWPE. Especially for the artificial knee joint with anatomical design, the contact stresses in the UHMWPE tibial insert are generally higher than the yield stress of the material during normal gait. In addition, the predominant types of wear on reported simulator-tested and retrieved UHMWPE tibial inserts are delamination and pitting. These facts suggest that the fatigue fracture that causes micro-cracks both on and below the surface of the UHMWPE tibial insert and the generation of wear particles as fatigue type are closely related to the repeated plastic deformation. On the metallic femoral components of the retrieved knee prostheses with anatomical design, a number of microscopic scratches caused by various factors were observed. It is thought that microscopic surface asperities caused by this surface damage contribute to increasing and/or accelerating wear of the UHMWPE tibial insert. The primary objective of this study was to investigate the factors influencing the wear mechanism of UHMWPE tibial insert in knee prosthesis. In this study, macroscopic and microscopic elasto-plastic contact analyses of the UHMWPE tibial insert based on macroscopic and microscopic geometrical measurements from retrieved knee prosthesis were performed using finite element method (FEM) in order to investigate the mechanical state, plastic deformation behavior in the UHMWPE tibial insert and microscopic wear of the polyethylene caused by microscopic surface asperity. For this purpose, the determinative method of the contact position between the femoral component and the UHMWPE tibial insert for the retrieved knee prosthesis was developed. The three-dimensional FEM model of the retrieved knee prosthesis with worn contact surfaces was produced. Three-dimensional microscopic surface profile measurements of damaged surface of a retrieved metallic femoral component by using a laser microscope and reproduction of the femoral component surface by using 3D CAD software were performed in order to produce the 3D FEM models of the microscopic asperity based on actual measurement data. The analytical findings of this study suggest that maximum plastic strain below the surface is closely related to subsurface crack initiation and delamination of the retrieved UHMWPE tibial insert. The worn surface whose macroscopic geometrical congruity had been improved due to wear after joint replacement showed lower contact stress at the macroscopic level. The aspect ratio, shape ratio and indentation depth of the microscopic asperity have a significant effect on increasing and/or accelerating wear on the UHMWPE. Higher aspect ratios, shape ratios and indentation depths cause higher contact stresses and plastic strains in the UHMWPE. These are therefore significant factors influencing the wear mechanism of UHMWPE

Aims: The purpose of this study was to clarify how the mechanical characteristics of the lengthened bone changes with time by means of the analyses using the CT based finite element method. Methods: CT images were obtained from the bilateral tibiae of five patients who had undergone unilateral tibia. The average time interval from completion of lengthening to CT scanning was 30 months. There were two patients had CT examinations twice. The analyses were made using the Mechanical Finder®(Mitsubishi Space Software, Osaka, Japan). 3-D finite element models were made from axial CT images of the whole tibiae. The models were 3mm tetrahedron elements for a cancellous bone and 3 nodal-points shell elements with a thickness of 0.3 mm for a cortical bone. The uni-axial compressive load was applied on the tibial plateau, while the distal part of the tibia was fully restrained. The elastic moduli at the middle of the lengthened bones and the maximum principal strains were calculated using the elastic analysis. Results: The elastic moduli of the lengthened bones were significantly smaller than those of the contra-lateral bones, while the maximum principal strains of the lengthened bones tended to be larger. The ratios of the elastic modulus disparity between the lengthened bone and the paired contra-lateral bone to the elastic modulus of the contra-lateral bone decreased significantly with time. Likewise, the ratios of the maximum principal strains calculated as above decreased identically. Conclusions: The results indicated that the stiffness of the lengthened bone got closer to that of the contra-lateral with time, which means the lengthened bones were in the process of modeling during these follow up time intervals

Finite element (FE) modelling has been widely used to create and assess musculoskeletal models. However to achieve a high degree of resolution in describing the structure, significant computational power and time are required. The objective of this study was to introduce a complimentary approach to FE modelling using structural beam theory. This requires far less computational power and models can be analyzed in a fraction of a second, offering quick, intuitive results for engineers and surgeons.

Beam theory was first introduced as a method for analyzing the stresses in long bones in 1917. It was used as the de facto method for several decades. The introduction of FE modelling offered great advances; beam theory calculations were considered laborious and less accurate. However with the advances in computational power so too comes the ability to create modern automated beam theory models.

A study was conducted using the commercially available general structural analysis software Oasys GSA. A synthetic biomechanical femur was CT scanned and the solid model constructed. This model was sectioned into approximately seventy sections in the regions of the shaft and condyles, thirty in the neck and thirty in the head. Line plots of the shape of each of the sections, for both cortical and trabecular parts, were then imported into Oasys GSA. The centroid, area, second moments of area and torsion constant were calculated for each section. The sections were plotted at the position of the cortical centroid and parallel axis theorem was used to plot the trabecular section in the same position. A force representing the hip joint reaction force was applied to a node corresponding to the centre of the femoral head. Muscular forces were applied to stiff radial elements according to those active at the point of peak joint contact force during gait.

Oasys GSA produced instant results showing moment and deflection characteristics of the femur. This data was then used to predict strain plots, which were directly compared to FE results. Initial results compare favourably.

This study has demonstrated an updated fast, efficient and intuitive alternative to finite element modelling.

Latarjet procedure (transfer of coracoid process to the anterior glenoid rim) has been widely used for severe anterior shoulder instability. The purpose of the present study was to investigate the intraarticular stress distribution after this procedure to clarify the pathomechanism of its postoperative complications. CT-DICOM data of the contralateral healthy shoulder in 10 patients with unilateral anterior shoulder instability (9 males and 1 female, age: 17–49) was used for the present study. Three-dimensional finite element models of the glenohumeral joint was developed using software, Mechanical Finder (RCCM, Japan). In each shoulder, a 25% bony defect was created in the anterior glenoid cavity, where coracoid process was transferred using two half-threaded screws. The arm position was determined as 0-degree and 90-degree abduction. While medial margin of the scapula was completely constrained, a standard compressive load (50 N) toward the centre of the glenoid was applied to the lateral wall of the greater tuberosity. A tensile load (20N) was also applied to the tip of coracoid process along the direction of conjoint tendon. Then, elastic analysis was performed, and the distribution pattern of Drucker-Prager equivalent stress was investigated in each model. The proximal half of the coracoid represented significantly lower equivalent stress than the distal half (p < 0.05). In particular, the lowest mean equivalent stress was seen in its proximal-medial-superficial part. On the other hand, a high stress concentration newly appeared in the antero-inferior aspect of the humeral head exactly on the site of coracoid bone graft. We assumed that the reduction of mean equivalent stress in the proximal half of the coracoid was caused by the stress shielding, which may constitute one of the pathogenetic factors of its osteolysis. A high stress concentration in the humeral head may eventually lead shoulder joint to osteoarthritis.

The design philosophy of polished tapered total hip replacements (THR), such as the Exeter, intends for them to migrate distally within the cement mantle. As well as migration, dynamically induced micromotion (DIMM) occurs as a result of functional activity between the implant and the cement. The aim of the current study was to develop and validate a finite element (FE) model of the Exeter/cement/bone system which can be used to predict DIMM and investigate the stresses induced in the cement mantle during functional activity.

In the context of the current study, DIMM is defined as the displacement of the implant component relative to the bone when moving from double leg stance to single leg stance on the operated limb. Using Roentgen Stereo-photogrammetric Analysis (RSA), DIMM was measured in 21 patients implanted with Exeter stems 3 months post-operatively. A previous study, using a reduced FE model of the Exeter stem and the surrounding cement mantle focused on the solution of the contact problem at the stem-cement interface. It was demonstrated that sliding contact combined with Coulomb friction and an appropriate parameter setting could be used to predict DIMM of a polished tapered stem. For the purposes of the current study, the previous simple model was incorporated into the FE model of the Muscle Standardised Femur and validated against the RSA measurements for DIMM. For the current extended model, loading included muscle forces representing all active muscles acting on the femur. The effect of initial cement stresses and interdigitation was also considered.

The Exeter stem demonstrated significant DIMM (p< 0.017). The FE model, accounting for sliding contact at the cement–implant interface was able to predict similar distal migration of the head and the tip. The results of both the calculations and the measurements showed that the femoral head moves medially, distally and posteriorly relative to the bone. In the cement mantle, maximum principal stresses were oriented circumferentially, minimum principal stresses were oriented radially. When the taper got engaged, submicroscopic movements which did not recover following unloading still took place and accumulated.

The results of the present study showed that it is possible to measure DIMM in the Exeter stem and combine this with FE modelling of the contact mechanism. Future studies will include various activities, such as walking or stair climbing. Based on accumulated submicroscopic movements, short-, mid- or long-term migration patterns will be predicted.

Introduction: There is a clear need for the development of more sensitive risk assessment tools for clinical predictors of fractures. Bone densitometries are limited in the ability to account for complex geometry, architecture, and heterogeneity of bone. Quantitative computed tomography (QCT)-based finite element (FE) Methods: (QCT/FEM) are able to perform structural analyses taking these factors into consideration to accurately predict bone strength. However, no basic data have been available regarding predicted strength (PS) of the proximal femur by QCT/FEM with reference to age in a normal population. The purpose of this study was thus to create a database on PS in a normal population as a preliminary trial. With these data, parameters that affect PS were also analyzed.

Methods: Participants in this study comprised individuals who participated in a health checkup program with computed tomography (CT) at our hospital in 2008. Participants included 487 men and 237 women (age range, 40–87 years). Exclusion criteria were provided. Scan data of the proximal femur were isolated and taken from overall data from CT of each participant with simultaneous scans of a calibration phantom containing hydroxyapatite rods. A FE model was constructed from the isolated data using Mechanical Finder software. For each of the FE models, loading and boundary conditions as well as the definition of PS were exactly the same as described by Bessho et al. (Bone 2009). For each participant, height, weight, and abdominal circumference (AC) were measured. The analyses included linear regression analysis relating age and PS, one-way analysis of variance to compare average PS among the groups of participants who were divided into 5-year age brackets, and multiple regression analysis to determine how PS was affected by age, height, weight, and AC. Differences were considered significant for values of p< 0.05.

Result: The following results were obtained. First, average PS was lower in women than in men for all age ranges. Second, PS in men under stance configuration, and those in women under stance and fall configurations significantly decreased with age. Third, weight positively affected PS in both men and women.

Discussion: This was the first study to investigate changes in PS with age in a normal population. Whether PS by QCT/FEM correlates more closely with fracture risk for osteoporotic patients in comparison to other bone densitometries remains unclear, but the our results did not contradict any existing concept of risk factors for fragility fracture. More baseline data for PS in normal populations need to be accumulated by increasing the number of participants in studies like this.

Aims

A functional anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL) has been assumed to be required for patients undergoing unicompartmental knee arthroplasty (UKA). However, this assumption has not been thoroughly tested. Therefore, this study aimed to assess the biomechanical effects exerted by cruciate ligament-deficient knees with medial UKAs regarding different posterior tibial slopes.

Aims

Unicompartmental knee arthroplasty (UKA) has become a popular method of treating knee localized osteoarthritis (OA). Additionally, the posterior cruciate ligament (PCL) is essential to maintaining the physiological kinematics and functions of the knee joint. Considering these factors, the purpose of this study was to investigate the biomechanical effects on PCL-deficient knees in medial UKA.

Aims

Commonly performed unicompartmental knee arthroplasty (UKA) is not designed for the lateral compartment. Additionally, the anatomical medial and lateral tibial plateaus have asymmetrical geometries, with a slightly dished medial plateau and a convex lateral plateau. Therefore, this study aims to investigate the native knee kinematics with respect to the tibial insert design corresponding to the lateral femoral component.

Aims. There is ambiguity surrounding the degree of scaphoid union required to safely allow mobilization following scaphoid waist fracture. Premature mobilization could lead to refracture, but late mobilization may cause stiffness and delay return to normal function. This study aims to explore the risk of refracture at different stages of scaphoid waist fracture union in three common fracture patterns, using a novel finite element method. Methods. The most common anatomical variant of the scaphoid was modelled from a CT scan of a healthy hand and wrist using 3D Slicer freeware. This model was uploaded into COMSOL Multiphysics software to enable the application of physiological enhancements. Three common waist fracture patterns were produced following the Russe classification. Each fracture had differing stages of healing, ranging from 10% to 90% partial union, with increments of 10% union assessed. A physiological force of 100 N acting on the distal pole was applied, with the risk of refracture assessed using the Von Mises stress. Results. Overall, 90% to 30% fracture unions demonstrated a small, gradual increase in the Von Mises stress of all fracture patterns (16.0 MPa to 240.5 MPa). All fracture patterns showed a greater increase in Von Mises stress from 30% to 10% partial union (680.8 MPa to 6,288.6 MPa). Conclusion. Previous studies have suggested 25%, 50%, and 75% partial union as sufficient for resuming hand and wrist mobilization. This study shows that 30% union is sufficient to return to normal hand and wrist function in all three fracture patterns. Both 50% and 75% union are unnecessary and increase the risk of post-fracture stiffness. This study has also demonstrated the feasibility of finite element analysis (FEA) in scaphoid waist fracture research. FEA is a sustainable method which does not require the use of finite scaphoid cadavers, hence increasing accessibility into future scaphoid waist fracture-related research. Cite this article: Bone Jt Open 2023;4(8):612–620

Abstract. Objectives. Spinal disorders such as back pain incur a substantial societal and economic burden. Unfortunately, there is lack of understanding and treatment of these disorders are further impeded by the inability to assess spinal forces in vivo. The aim of this project is to address this challenge by developing and testing a novel image-driven approach that will assess the forces in an individual's spine in vivo by incorporating information acquired from multimodal imaging (magnetic resonance imaging (MRI) and biplane X-rays) in a subject-specific model. Methods. Magnetic resonance and biplane X-ray imaging are used to capture information about the anatomy, tissues, and motion of an individual's spine as they perform a range of everyday activities. This information is then utilised in a subject-specific computational model based on the finite element method to predict the forces in their spine. The project is also utilising novel machine learning algorithms and in vitro, six-axis mechanical testing on human, porcine and bovine samples to develop and test the modelling methods rigorously. Results & Discussion. MRI sequences have been identified that provide high-quality image data and information on different tissue types which will be used to predict subject-specific disc properties. In-vivo protocols to capture motion analysis, EMG muscle activity, and video X-rays of the spine have been designed with planned data collection of 15 healthy volunteers. Preliminary modelling work has evaluated potential machine learning approaches and quantified the sensitivity of the models developed to material properties. Conclusion. The development and testing of these image-driven subject-specific spine models will provide a new tool for determining forces in the spine. It will also provide new tools for measuring and modelling spine movement and quantifying the properties of the spinal tissues. Acknowledgments. Funding from the EPSRC: EP/V036602/1 (Meakin, Holsgrove & Javadi) and EP/V032275/1 (Holt & Williams). Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

To unravel the relation between mechanical loading and biological response, cell-seeded hydrogel constructs can be used in bioreactors under multi-axial loading conditions that combines compressive with torsional loading. Typically, considerable biological variation is observed. This study explores the potential confounding role of mechanical factors in multi-directional loading experiments. Indeed, depending on the material properties of the constructs and characteristics of the mechanical loading, the mechanical environment within the constructs may vary. Consequently, the local biological response may vary from chondrogenesis in some parts to proteoglycan loss in others. This study uses the finite element method to investigate the effects of material properties of cell-seeded constructs and multiaxial loading characteristics on local mechanical environment (stresses and strains) and relate these to chondrogenesis (based on maximum compressive principal strain (MCPS) - Zahedmanesh et al., 2014) and proteoglycan loss (based on fluid velocity (FV) - Orozco et al., 2018). The construct was modelled as a homogenized poro-hyperelastic (using a Neohookean model and Darcys law) cylinder of 8mm diameter and equal height using Abaqus. The bottom surface was fully constrained and dynamic unconfined compression and torsion loading were applied to the top surface. Free fluid flow was allowed through the lateral surface. We studied the sensitivity of the maximum values of the target parameters at 9 key locations to the material parameters and loading characteristics. Six input parameters were varied in preselected ranges: elastic modulus (E=[20,80]kPa), Poissons ratio (nu=[0.1,0.4]), permeability (k=[1,4]e-12m4/Ns), compressive strain (Comp=[5,20]%), rotation (Rot=[5,20]°) and loading frequency (Freq=[1,4]Hz). A full-factorial design of experiment method was used and a first-order polynomial surface including the interactions fitted the responses. MCPS varies between 7.34% and 33.52% and is independent of the material properties (E, nu and k) and Freq but has a high dependency on Comp and a limited dependency on Rot. The maximum value occurs centrally in the construct, except for high values of Rot and low Comp where it occurs at the edges. FV vary between 0.0013mm/sec and 0.1807mm/sec and dominantly depends on E, k and Comp, while its dependency on Rot and Freq is limited. The maximum value usually occurs at the edges, although at high Freq it may move towards the center of the superficial and deep zones. This study can be used as a guideline for the optimized selection of mechanical parameters of hydrogel for cell-seeded constructs and loading conditions in multi-axial bioreactor studies. In future work, we will study the effect in intact and injured cartilage explants

The number of knee replacement surgeries have increased rapidly over the past few years. However, these implants can have limited life due to the issue of wear. An accurate lubrication model is an important component in understanding and designing joints to deliver lower joint wear and the risks associated with such wear. One of the main challenges in tribological modelling of the knee implant is capturing the effects of the complex geometry on the joint performance. Most current models assume a single point of contact, with zero pressure and deformation assumed elsewhere. Unlike the hip implant, which can be described as a circular or elliptical contact, the knee implant involves a geometry that cannot be easily approximated into a regular shape. For this reason, the elastohydrodynamic lubrication equations become computationally expensive and challenging to solve. Finite element methods are required to capture the complex geometry and calculate deformations and how they vary spatially over the joint surface. Furthermore, the irregularity and asymmetry of the geometry provides no guarantee that well-defined contact points exist. A mixed lubrication model for a human knee implant is presented, incorporating the irregularity of the knee geometry. Tribological conditions in the mixed lubrication regime are calculated using a statistically representative description of surface roughness. This approach involves using the flow factors approach of Patir and Cheng (1978), and the Greenwood and Tripp (1970) approach for asperity contact. From this, the evolution of both the gross geometry and the change in surface roughness due to wear is determined

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

Introduction. The human wrist is a highly complex joint, offering extensive motion across various planes. This study investigates scapholunate ligament (SLL) injuries’ impact on wrist stability and arthritis risks using cadaveric experiments and the finite element (FE) method. It aims to validate experimental findings with FE analysis results. Method. The study utilized eight wrist specimens on a custom rig to investigate Scapho-Lunate dissociation. Contact pressure and flexion were measured using sensors. A CT-based 3D geometry reconstruction approach was used to create the geometries needed for the FE analysis. The study used the Friedman test with pairwise comparisons to assess if differences between testing conditions were statistically significant. Result. The study found significant variations in scaphoid and lunate bone movement based on ligament condition. Full tears increased scapholunate distance in the distal-proximal direction and decreased in the medial-lateral direction. Lunate angles shifted from flexion to extension with fully torn ligaments. Conversely, the scaphoid shifted significantly from extension to flexion with full tears. A proximal movement was observed in the distal-proximal direction in all groups, with significant differences in the partial tear group. Lateral deviation of the scaphoid and lunate occurred with ligament damage, being more pronounced in the partial tear group. All groups exhibited statistically significant movement in the volar direction, with the full tear group showing the least movement. Also, radiocarpal joint and finger contact pressure and contact area were studied. Whereas the differences in contact area were not significant, scapholunate ligament tears resulted in significantly decreased finger contact pressures. FEA confirmed these findings, showing notable peak radiocarpal contact pressure differences between intact and fully torn ligaments. Conclusion. Our study found that SLL damage alters wrist stability, potentially leading to early arthritis. The FEA model confirmed these findings, indicating the potential for the clinical use of computer models from CT scans for treatment planning

Background. Periprosthetic femoral fractures following total hip arthroplasty are relatively uncommon but are associated with significant morbidity. With an increasing number of total hip arthroplasties being carried out in an aging population we need to ensure correct implants are chosen for our patients. A recent review of NJR data suggested a significantly higher revision risk for the Zimmer CPT stems due to periprosthetic fractures when compared to the Stryker Exeter stems. Objectives. Our aim was to compare the biomechanics of periprosthetic fractures around the CPT and Exeter V40 stems in a composite saw bone model to identify if a difference in fracture risk exists between the two stems. We also compared the engineering design of the two implants in order to analyse the possible effect this may have on fracture risk. Study Design & Methods. Fourteen composite femurs were divided into two groups and cemented using Palacos R cement with either the CPT or Exeter V40 stem by a single surgeon. The implanted femurs were then mounted onto an Instron machine and were axially loaded and torqued to fracture with an axial compressive force of 2000N over 10 seconds followed by a rotation of 40 degrees applied over 1 second. A power calculation from a previous composite saw bone model study suggested that a minimum of 6 implanted femurs would be required in each group. Results. The implanted femurs invariably sustained fracture patterns similar to the Vancouver B2 periprosthetic fracture which are commonly seen in clinical practice. Implanted femurs with CPT stems suffered periprosthetic fractures with less rotation when compared to those femurs with the Exeter V40 stem (20.10 versus 33.60, p<0.01). We also found that CPT implanted femurs were fracturing at significantly lower torque values when compared to the Exeter V40 implanted femurs (124Nm Versus 174Nm, p<0.01). The energy release rate (G111) for CPT stems was 21.8Nm compared to 61.2Nm for Exeter V40 stems. The higher energy release with Exeter stems led to more comminuted fractures in Exeter implanted femurs when compared to the CPT femurs, which fractured earlier, but with simpler fracture patterns. Finite element method (FEM) simulation analysis showed that fractures initiated between the prosthesis and cement at the proximal end of the femur. Two dimensional sections at the same height showed a difference in bone-cement-implant geometrics at the critical point of failure suggesting that a design cause may be the reason for the higher risk of periprosthetic fractures in CPT implanted femurs. Conclusions. Our observations may explain the higher revision risk secondary to periprosthetic fractures that has been observed with the CPT stem when compared to the Exeter V40 stem

Introduction. Wear phenomenon of ultra-high molecular weight polyethylene (UHMWPE) in hip and knee prostheses is one of the major restriction factors on the longevity of these implants. In retrieved hip prostheses with screw holes in the metal acetabular cup for fixation to the pelvis, the generation of cold flow into the screw holes is frequently observed on the backside of the UHMWPE acetabular cup liner. In most retrieved cases, the protruded areas of cold flow on the backside were located on the reverse side of the severely worn and deformed surface of the polyethylene liner. It would appear that the cold flow into screw holes contributes to increase of wear and damages of the polyethylene liner in hip prosthesis. Methods. In a previous study (Cho et al., 2016), we pointed out the generation of cold flow into the screw holes on the backside of the retrieved UHMWPE acetabular cup liner as shown in Figure 1. The primary purpose of this study was to investigate the influence of the cold flow into the screw holes on the wear of the polyethylene liner in hip prosthesis. In this study, computer simulations of the generation of cold flow were performed using the finite element method (FEM) in order to propose the design criteria about the cold flow of the hip prosthesis for improving the wear resistance of the polyethylene liner. We especially focused on the influence of polyethylene thickness and contact surface conformity on the generation of cold flow into the screw hole. Results. An example of the results of a series of the FEM simulations performed in this study is shown in Figure 2. This figure shows the distributions of the contact stress in the polyethylene liners. The graphs shown in Figure 3 are the summary of results of a series of the FEM simulations performed in this study. The graph in Figure 3(a) shows the changes in the maximum contact stress in the polyethylene liner with the thickness of polyethylene liner. The graph in Figure 3(b) shows the changes in the maximum contact stress in the polyethylene liner with the radial clearance between the femoral head and the polyethylene liner. Discussion and Conclusions. It was found that the magnitudes of cold flow and maximum contact stress in the polyethylene liner had a tendency to increase with decreasing the thickness of polyethylene liner. It was also found that the magnitude of cold flow and maximum contact stress in the polyethylene liner had a tendency to increase with increasing the radial clearance between the femoral head and the polyethylene liner. The results of this study suggest that polyethylene thickness and contact surface conformity have a significant influence on the generation of cold flow into the screw holes and wear of the polyethylene liner. For any figures or tables, please contact authors directly