Background

Surgical reconstruction of the anterior cruciate ligament is a common practice to treat the disability or chronic instability of the knee. Several factors associated with success or failure of the ACL reconstruction, including surgical technique and graft material and graft tension. We aimed to show how we can optimize the graft properties and achieve better post surgical outcomes during ACL reconstruction using 3-dimensional computational finite element simulation.

High tensile stress has been considered as a contributing factor to the rim fracture of polyethylene acetabular cup liner. We performed the 3 D Finite Element Analysis (FEA) to compare the stress patterns at the polyethylene liner rim as a function of polyethylene thicknesses and whether or not rim was supported by the titanium acetabular shell extension. Two 3.1 mm thick generic 52 mm titanium alloy acetabular shells with and without 2 mm high rim support extension were modelled. Six corresponding Ultra High Molecular Weight Polyethylene (UHMWPE) liners with inner bearing diameters ranging from 22 mm to 44 mm and same outer diameters, were fixed in the shells. A 2 450 N load was applied through the corresponding CoCr femoral heads to the rims of liners while the acetabular shells were fixed on the outer spherical surface. The FEA was performed in half body of the assembly. The maximum principal stresses at the rim regions of UHMWPE liners were recorded. The results showed that in all rim supported conditions, the maximum principal stress were in compressive patterns, a preferred pattern to reduce the potential polyethylene liner fracture. In rim unsupported conditions, the stresses was in tensile on the internal bearing surface when polyethylene liner thickness was bellow 5 mm, or was bellow 9 mm if the average maximum principal stress cross the rim was considered. We conclude that the metal rim support changes the stress pattern in the rim region of UHMWPE liner to compressive for all liner thicknesses. The stress pattern turns to tensile, or there will be a higher potential for rim fracture, if UHMWPE liner is unsupported and the polyethylene rim thickness is less than 9 mm. Although components used this study did not include the locking details which add higher stress concentrations, the trend of stress patterns should follow the results found in this study

Introduction: Despite a very high fusion rate (90%) achievable by present techniques, the clinical success rate for curing back pain is in the range of 50%. We hypothesise that disc degeneration gives rise to abnormal stress patterns in the bone. Although the cages integrate fully, load is taken by the cage producing abnormal stress patterns in the vertebrae. Unless a near normal stress pattern in the vertebrae is established, pain may continue. Method: A simple finite element model of a disc and its adjacent vertebral bodies was developed using ANSYSS software. The dimensions of the model were based on the human lumbar disc. The normal disc was modelled as a fluid with a bulk modulus of 1720 MPa. The degenerate disc was modelled as having the same material properties for the nucleus and the annulus. Fusion of the disc was modelled by replacing the nucleus with commonly used cages. In all the models, the material properties of the cancellous bone (E=100 MPa; v=0.3) and the cortical bone (E=12000 MPa; v=0.3) remained the same. The model was loaded axially with 1.5 kN. Results: The vertical and horizontal stress patterns around a loaded degenerate disc showed areas of increased loading in the endplate and the cancellous bone confirming the authors’ previous work using load transducers. The introduction of the cages in the model changed the stress distribution – they caused an increase in the compressive stresses in the cancellous bone, and a high concentration of tensile and compressive stresses at the point of contact with the cages. Conclusion: This study has shown that fusion cages alter the pattern of stress distribution in the adjacent vertebral bodies similar to that of a degenerate disc. It supports the concept that abnormal weight transfer is a more significant cause of back pain as compared to abnormal mobility

Back pain may be related to abnormal segmental movement and suggested treatment is segmental fusion. Recent techniques using cages can achieve fusion rates of over 90% but the clinical results are no better. We hypothesise that the cages integrate fully to adjacent vertebrae taking all the load, producing abnormal stress patterns in the vertebrae producing pain. In this study a simple FE model of a disc and its adjacent vertebral bodies was developed using ANSYS software. The dimensions of the model were based on those of a human lumbar disc. The normal disc was modelled as having nucleus with fluid properties (bulk modulus 1720 MPa). To model the degenerate disc, the material properties of the nucleus were changed to be the same as the annulus (Young’s modulus, E=5 Mpa; Poisson’s ratio, n=0.49). To model fusion of the disc, the nucleus was replaced with a simple representation of a one of three of the commonly used cages. In all the models the material properties of the cancellous bone (E = 100 MPa; n = 0.3) and the cortical bone (E=12000MPa;n=0.3) remained the same. The model was loaded axially with 1.5 kN. The vertical and horizontal stress patterns around a loaded degenerate disc showed areas of increased loading in the endplate and cancellous bone. The inclusion of cages in the model showed high concentration of tensile and compressive stresses at the point of contact with the cages and in the cancellous bone of the vertebral bodies. The stress pattern showed more similarity to that of degenerate disc, than a normal one. Fusion cages alter the pattern of stress distribution in the adjacent vertebral bodies from that of the normal disc. The excellent fusion rates of the cages are not mirrored by improvement in clinical results. It supports the concept that abnormal load transfer may be a more significant cause of back pain than abnormal movement

Hip resurfacing arthroplasty is emerging as an increasingly popular, conservative option for the treatment of end-stage osteoarthritis in the young and active patient. Despite the encouraging clinical results of hip resurfacing, aseptic loosening and femoral neck fracture remains concerns for the success of this procedure. This study used finite element analysis (FEA) to analyse the stresses within proximal femoral bone resulting from implantation with a conservative hip prosthesis. FEA is a computational method used to analyse the performance of real-world structures through the development of simplified computational models using essential features. The aim of this study was to examine the correlation between the orientation of the femoral component of a hip resurfacing prosthesis (using the Birmingham Hip Resurfacing as a model) and outcomes during both walking and stair climbing. The outcomes of interest were stresses in the femoral neck predisposing to fracture, and bone remodelling within the proximal femur. Multiple three-dimensional finite element models of a resurfaced femur were generated, with stem-shaft angles representing anatomic (135°), valgus (145°), and varus (125°) angulations. Applied loading conditions included normal walking and stair climbing. Bone remodelling was assessed in both the medial and lateral cortices. Analyses revealed that amongst all orientations, valgus positioning produced the most physiological stress patterns within these regions, thereby encouraging bone growth. Stress concentration was observed in cortical and cancellous bone regions adjacent to the rim of the prosthesis. As one would expect, stair climbing produced consistently higher stress than walking. The highest stress values occurred in the varus-orientated femur during both walking and stair climbing, whilst anatomic angulation resulted in the lowest stress values of all implanted femurs in comparison to the intact femur. This study has shown through the use of FEA that optimising the stem-shaft angle towards a valgus orientation is recommended when implanting a hip resurfacing arthroplasty. This positioning produces physiological stress patterns within the proximal femur that are conducive to bone growth, thus reducing the risk of femoral neck fracture associated with conservative hip arthroplasty

Introduction: Homogenous cell distribution and suffi-cient initial scaffold stability remain key issues for successful tissue engineered osteochondral constructs. The purpose of this study was to investigate the application of initial compression forces during the first 24 hours of cell culture followed by different stress patterns. Methods: Bone marrow stromal cells were harvested from the iliac crest during routine trauma surgery. The cells were expanded in a 2-dimensional culture and then seeded into the biologic hybrid scaffold with a concentration of 1x10E6 cells per ml. Pressure and vacuum forces were applied in a specially developed glass kit. The constructs were exposed to two different protocols of compression combined as oteochondral matrices of CaReS (collagen I) and Tutobone (Ars Arthro, Esslingen, Germany and Tutogen Medical GmbH, Neunkirchen a. Br., Germany). Controls were resected osteochondral fragments from patients with articular fractures and uncompressed constructs. These effects were evaluated using light microscopy after standard staining to identify matrix penetration. Biomechanical tests were conducted, too using a modified biomechanical testing machine. The ‘constrained compression’, maximum load to failure, modulus, and strain energy density were determined. Results: Histology: Penetration and cell distribution was demonstrated homogenous and vital, respectively. Mechanical tests showed a significant enhancement of primary matrix stability. The following stress patterns did not enhance significantly stability over seven days. Discussion: The aim of this project was to investigate the response and cell distrubution of human bone marrow stromal cells seeded on a 3-dimensional biologic hybrid scaffold using compression and vacuum forces. The integration of mechanical stimulation in the tissue engineering process may lead to a progress in the structural and biomechanical properties of these tissues and offers new possibilities in the management of bone injuries and degenerative diseases

Purpose. The purpose of this study is to report the results of the first 1000 cases hip arthroplasty using the Bencox. ®. hip stem, the first hip prosthesis developed and manufactured in Korea. Material & Method. This study reviewed 1000 cases retrospectively who underwent arthroplasty using Bencox. ®. hip system. The Bencox. ®. hip stem is the first hip prosthesis developed and manufactured in Korea. This stem have a double-tapered, wedge shape figure with a rectangular-shaped cross-section and specially designed neck shape, which is design to achieve normal stress pattern of the proximal femur and to increase initial stability and to increase range of motion. Surface is treated with MAO (Micro Arc Oxidation) coating. From the first arthroplasty with this system in September 2006, sequentially 1000 arthroplasties were performed by single surgeon until the July 2014. This material included 439 men and 561 female. Average age of patients was 65 year old. Follow up period was average 72.1 month (minimum 34 months to maximum 120 months). 1000 cases consisted of 569 hips in patients with femoral neck or intertrochanteric fracture or subtrochanteric fracture (fracture group), 155 hips in osteoarthritis, 192 hips in patients with osteonecrosis of the femoral head (arthritis group), 84 hips in revision surgery (revision group). Revision cases consisted of 58 hips with aseptic loosening and 26 hips with loosening due to infection sequelae. They were underwent hip arthroplasty using a Bencox. ®. hip stem in combination with Bencox. ®. bipolar cup and Bencox. ®. acetabular cup. Patients in the fracture group usually underwent bipolar hip arthroplasty, and those in the arthritis group and revision group underwent total hip arthroplasty. They were reviewed by medical records, clinically and radiologically. Results. During the follow-up period, there were no cases of revision of the femoral stem. Radiographically, there were no cases of radiolucent line except very proximal part of the stem endosteal bone ongrowth was found in most cases. Postoperative complications such as stem loosening, infection, dislocation, and ceramic breakage were not noted. But periprosthetic fracture was encountered in 7 hips due to slip down. They were treated by open reduction and internal fixation with plate and cables. There were no cases of failure of these treatments. Conclusion. Clinical and radiographic evaluations of hip arthroplasty using the Bencox. ®. hip system showed excellent outcomes with average 72.1 month follow-up in 1000 case

The screw fastening torque applied during bone fracture fixation has a decisive influence on subsequent bone healing. Insufficient screw tightness can result in device/construct instability; conversely, excessive torques risk damaging the bone causing premature fixation failure. This effect is even more prominent in osteoporotic bone, a condition associated annually with almost 9 million fractures worldwide. During fracture fixation, screw tightening torque is applied using subjective feel. This approach may not be optimal for patient”s recovery, increasing risk of fixation failure, particularly in osteoporotic bone, and potentially require revision surgical interventions. Besides bone density, various factors influence the performance of screw fixation. These factors include bone geometry, cortical thickness and time-dependant relaxation behaviour of the bone. If the influence of screw fastening torque on the bone and relationships between these factors was better understood, the surgical technique could be optimised to reduce the risk of complications. Within this study, we developed an axisymmetric finite element (FE) model of bone screw tightening incorporating viscoelastic behaviour of the cortical bone such as creep and stress relaxation. The model anticipated time-dependent behaviour of the bone for different bone thickness and density after a typical bone fixation screw had been inserted. The idealised model has been developed based on CT scans of bones with varying densities and inserted screws. The model was validated through a series of experiments involving bovine tibiae (4–5 months) to evaluate the evolution of surface strains with time (Ncorr v1.2). Stress distribution was assessed in photoelastic experiments using acrylic analogues. Relaxation tests have been performed in aqueous environment for up to 48 hours to ensure the relaxation would be complete. The creep behaviour (maximum principal strain) was compared against computational predictions. Our early simulations predicted relaxation strains on the surface of the bone to be 1.1% within 24 hours comparing favourably to 1.3% measured experimentally. Stress distribution patterns were in agreement with photoelastic results. Using experimentally derived viscoelastic properties, the model has the potential to predict creep and stress relaxation patterns after screw insertion with different fastening torques for bones with varying density and geometry. We aim to develop this into a planning tool providing guidance to surgeons for optimal tightening when using screw fixation, particularly in reduced quality bone

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 finite element analysis we can model different bone remodeling conditions to ascertain the effect that this will have on stress distribution and magnitude. A finite element analysis was performed. Simplified computer generated models were designed of a cemented femoral Stanmore growing massive endoprosthesis. Three scenarios were designed, modelled on post-operative radiographs. Scenario 1 had a gap between the end of the femur and the implant collar, scenario 2 had no gap, but with no bone attachment into the collar, and scenario 3 had growth of the bone over the length of the collar with attachment. Physiological loading conditions were applied. The resultant stress in the implant for each scenario was measured, and compared to the strength of the material. Peak stresses were recorded at the stem-collar junction. The maximum stress recorded in the implant in scenario 1 was 3104.2Mpa, compared to 1054.4Mpa in scenario 2, and 321.2Mpa in scenario 3. Conclusions. Both accurate reduction and bone growth with attachment to the stem of a massive endoprosthesis will greatly reduce the resultant stress in the implant under loading conditions. The load is redistributed throughout the length of the bone. This may help to prevent catastrophic failure in the implant under loading conditions. Further investigations of patient findings are needed to ensure the model findings are verified. Level of Evidence. IIb (Theoretical)

Anterior cruciate ligament (acl) reconstruction is one of the most commonly performed procedures in orthopedics for acl injury. While literature suggest short-term good-to-excellent functional results, a significant number of long-term studies report unexplained early oa development, regardless type of reconstruction. The present study reports the feasibility analysis and development of a clinical protocol, integrating different methodologies, able to determine which acl reconstruction technique could have the best chance to prevent oa. It gives also clinicians an effective tool to minimize the incidence of early oa. A prospective clinical trial was defined to evaluate clinical outcome, biochemical changes in cartilage, biomechanical parameters and possible development of oa. The most common reconstruction techniques were selected for this study, including hamstring single-bundle, single-bundle with extraarticular tenodesis and anatomical double-bundle. Power analysis was performed in terms of changes at cartilage level measurable by mri with t2 mapping. A sample size of 42 patients with isolated traumatic acl injury were therefore identified, considering a possible 10% to follow-up. Subjects presenting skeletal immaturity, degenerative tear of acl, other potential risk factors of oa and previous knee surgery were excluded. Included patients were randomized and underwent one of the 3 specified reconstruction techniques. The patients were evaluated pre-operatively, intra-operatively and post-operatively at 4 and 18 months of follow-up. Clinical evaluation were performed at each time using subjective scores (koos) and generic health status (sf-12). The activity level were documented (marx) as well as objective function (ikdc). Preliminary results allow to verify kinematic patterns during active tasks, including level walking, stair descending and squatting using dynamic roentgen sterephotogrammetric analysis (rsa) methodology before and after the injured ligament reconstruction. Intra-operative kinematics was also available by using a dedicated navigation system, thus to verify knee laxity at the time of surgery. Additionally, non-invasive assessment was possible both before the reconstruction and during the whole follow-up period by using inertial sensors. Integrating 3d models with kinematic data, estimation of contact areas of stress patterns on cartilage was also possible. The presented integrate protocol allowed to acquired different types of information concerning clinical assessment, biochemical changes in cartilage and biomechanical parameters to identify which acl reconstruction could present the most chondroprotective behavior. Preliminary data showed all the potential of the proposed workflow. The study is on-going and final results will be shortly provided

Introduction. Although total hip arthroplasty (THA) has been one of the most successful, reliable and common prosthetic techniques since the introduction of cemented low-friction arthroplasty by Charnley in the early 1960s, aseptic loosening due to stem-cement and cement-bone interface failures as well as cement fractures have been known to occur. To overcome this loosening, the stem should be mechanically retentive and stable for long term repetitive loading. Migration studies have shown that all stems migrate within their cement mantle, sometimes leading to the stem being debonded from the cement [1]. If we adopt the hypothesis that the stems debond from the cement mantle, the stem surface should be polished. For the polished stem, the concept of a double taper design, which is tapered in the anteroposterior (AP) and mediolateral (ML) planes, and a triple-tapered design, which has trapezoidal cross-section with the double tapered, have been popularized. Both concepts performed equally well clinically [2]. In this study, we aimed to analyze stress patterns for both models in detail using the finite element (FE) method. Methods. An ideal cemented stem with bone was made using three dimensional FE analyses (ANSYS 13). The cortical bone was 105 mm long and 7 mm thick and the PMMA cement mantle was 5 mm in thickness surrounding the stem. Young's modulus was set at 200 GPa for the bone and 2.2 GPa for the cement. Poisson's ratio was 0.3 for both materials. The bone-cement interface was completely bonded and cement-stem interface was not bonded in cases where a polished stem surface was used. The two types of stems were compared. One being the double tapered (Fig 1 left) and the other the triple tapered (Fig 1 right). The coefficient of friction (μ) at the stem-cement interface was set at 0 for both models. The distal ends of the stems were not capsulated by the PMMA and therefore the stems were free to subside. All materials were assumed to be linearly isotropic and homogeneous. The distal ends of the bone were completely constrained against any movements and rotations. An axial load of 1200 N and a transverse load of 600 N were applied at the same time simulating the bending condition [3]. Results. Although the stress distribution differences between the designs were minor, the positions where higher stresses and absolute values in the cement were observed varied. For double tapered model, the highest maximum principal stress was 1.98 MPa observed around the corner of the stem at the proximal region. For the triple tapered model, the highest maximum principal stress was 1.67 MPa observed at more medial side than the double tapered model

Objective. To investigate the biomechanical basis and report preliminary clinical efficacy of eccentric rotational acetabular osteotomy (ERAO) when treating developmental dysplasia of the hip (DDH). Methods. Biomechanical model of the hip joint was established on cadaveric hips. After performed ERAO on the biomechanical model, we explored the impact of this surgery on biomechanics of the hip joint. Meanwhile, we reported postoperative follow-up cases who underwent ERAO in our hospital between November 2007 to July 2012. A total of 14 patients (15 hips) were reported, including 4 males and 10 females, mean age was 30 years old. Harris hip score was defined as clinical evaluation standard and radiographic assessment was based on the measurement and further comparison of pre- and post-operative AHI (Acetabular-head index), CE angle (Center-edge angle) and Sharp angle. Results. The established biomechanical model was accord with the physiological state of normal hip joint. Postoperative stress was not statistically significant compared with the preoperative stress. Meanwhile, by the end of follow-up, 13 patients (14 hips) were followed for an average time of 26 months, thus, the follow-up rate was 92.9%. Harris hip score improved from preoperative (67.1 ± 8.7) points to (88.1 ± 7.3) points; postoperative AHI increased an average of 39.6%, CE angle increased an average of 33.2 ° and sharp angle reduced an average of 9.6 °. Conclusions. Both biomechanical study and preliminary clinical observation show that ERAO has the ability to correct the deformity of acetabulum. It enlarges the acetabular coverage of the femoral head and thus corrects the abnormal stress pattern. No bone graft is needed during the operation and postoperative rehabilitation is short, therefore, ERAO may have good curative effect when treating the DDH

Stress shielding of the proximal femur occurs in stemmed implants. Resurfacing implant does not invade the intramedullary region. We studied the stress patterns in conventional and nonstemmed designs. Methods. FE model geometry was based on standard femur from the international Society of Biomechanics Mesh Repository. Loading simulated for one- legged stance with body weight of 826 N. 2 regions were defined, R1 (40 mm from tip of head) and R2 41 mm–150 mm) of the intramedullary part of the stemmed model's interface with bone. 2 different loading conditions bending and torsion were compared for stress and strain. The FE model was solved with ANSYS version 6.1 on a single processor NT station. Results. With conventional implants, stem shields cortical bone from being loaded. In nonstemmed implants, Von Misses stress contours show a similar distribution as intact bone, transferring loads to the cortical shell but with higher stresses and a maximum displacement of 17.39% higher than that of intact bone. 15–23 mm proximal to R2 and around 110 mm, region of the stem tip, there were higher stress and strain concentrations. Conclusion. Based on simulations, nonstemmed implants provide more physiological loading compared to conventional implants though stress shielding increases in region of the stem in resurfacing implant

Over the last two decades, design modifications in cementless total hip arthoplasty have led to longer lasting implants and an increased success rate. However, there remains limitations to the cementless femoral stem implant. Traditional cementless femoral components require large amounts of bone to be broached prior to stem insertion (. 1. ). This leads to a decrease in host bone stock, which can become problematic in a young patient who may eventually require a revision operation during his or her lifetime. Osteopenia, only second to distal stress shielding can lead to aseptic loosening of the implant and stem subsidence, which also accelerates the need for a revision operation (. 2. –. 4. ). Recent literature suggests that thigh pain due to distal canal fixation, micro-motion, uneven stress patterns or cortex impingement by the femoral stem is directly correlated to increased stem sizes and often very disabling to a patient (. 5. –. 8. ). In this study, we sought to determine whether reducing stem length in the femoral implant would produce more physiologic loading characteristics in the proximal femur and thus eliminate any remaining stress shielding that is present in the current design. We analyzed the surface strains in 13 femurs implanted with. no implants,. stemless,. ultra short and. short stem proximal fill implants in a test rig designed to assimilate muscle forces across the hip joints, including the ilio-tibial band and the hip abductors. Analysis of the resulting surface strains was performed using the photoelastic method. For each femur, intact and with the different stem length components in place, the fringe patterns were compared at the same applied loads. The highest fringe orders observed for all tests were located on the lateral proximal femur and medial proximal femur. The fringes decreased as they approached the neutral axis of bending (posterior and anterior). Distal fringe patterns were more prominent as the stem length increased. The results demonstrate that the stemless design most closely replicated normal strain patterns seen in a native femur during simulated gait. The presence of a stemless, ultra short and short stem reduced proximal strain and increased distal strain linearly, thereby increasing the potential for stress shielding. The stemless design most closely replicated normal strain patterns observed in a native femur and for this reason has the potential to address the shortcomings of the traditional cementless femoral implant

Introduction: Implantation of a THA changes the strain distribution pattern in the proximal femur, with a massive loss of stress in the proximal part of the femur and an increase in stress at the distal part of the femoral component. Aim: The purpose of this study was to quantify the changes in BMD during long-term follow-up, i.e. five years, after insertion of the collarless, two-side conical, cemented Exeter stem. Material & Methods: Eighteen patients (all women), aged 55 to 80 years, undergoing THA were included in the study after informed consent. BMD was measured in 7 regions of interest according to Gruen et al., using Dual Energy X-ray Absorptiometry postoperatively, after 18 and 60 months of follow-up. At the same time, the contra lateral hip and spine were scanned. Results were tested using Wilcoxon matched-pairs signed-rank test. P values below 0.05 were considered significant. Results: During the first 18 months, a significant decrease in BMD was present in Gruen zones 2, 3, 6, and 7. No significant changes were seen in BMD of the zones 4, and 5 in the contra lateral hip, nor at the spine. In zone 1 there was a small but significant rise in BMD. From 18 to 60 months of follow up we observed a significant rise in BMD in all Gruen zones but zone 4 and 7. Despite this the total periprostetic BMD decreased during the study periode. There was no significant decrease in BMD in the contra lateral hip. In the spine, we observed a significant rise in BMD. Conclusion: During short-term follow-up (i.e. 18 months) after THA, BMD decreased in Gruen zones 2, 3, 6 and 7. The bone loss is similar to findings in other implants and seems to be related to the changes in stress pattern within the proximal femur. During long-term follow-up (i.e. 5 years) BMD increased again in these zones, however, BMD remained lower than baseline

Femoral nails are thought to be load sharing devices. However, the specific load sharing characteristics and associated stress concentrations have not yet been reported in the literature. The purpose of this study was to use a validated, three dimensional finite element model of a nailed femur subjected to gait loads in order to determine the resulting stresses in the femur and the nail. The results showed that load was shared between the nail and the bone throughout the gait cycle. In addition, high stress concentrations were noted in the bone around the screw holes, and dynamization was of minimal benefit. To determine the stresses in the bone and nail in a femur with a locked, retrograde, intramedullary nail. The retrograde femoral nail is a load sharing device. High stress concentrations occur in the bone around locking screw holes. When only one locking screw is used proximally and distally, stresses in the implant are excessive and may lead to failure. Dynamization was of minimal benefit. This is the first study to use a validated three dimensional finite element model to provide a detailed biomechanical analysis of stress patterns in a retrograde nailed femur under gait loads. The results can help resolve issues of stress shielding, implant removal, number of locking screws and dynamization. In the fully locked condition, loads in the femur were significantly higher than those in the nail for most of the gait cycle. Removal of locking screws to obtain dynamization only increased axial load in the femur by 17 %. However, stresses in the locking screws increased by as much as 250% when fewer than 4 screws were used. Maximum stresses in the bone were found around screw holes. A three dimensional finite element model of the femur and nail was developed. The model was validated by comparing results to a physical saw bone model instrumented with strain gages and subjected to a simple a compressive load. Once good correlation with simple loading patterns was demonstrated, gait loading patterns obtained from literature were incorporated and simulations were run for various conditions

Notching of the anterior femoral cortex during total knee arthroplasty is thought to be a possible risk factor for subsequent periprosthetic femoral fracture. Understanding the stress pattern caused by notching may help the orthopedic surgeon reduce the risk of fracture. A validated, three dimensional, finite element model of the femur using gait loads has been used to analyze the stress concentrations caused by anterior femoral cortex notching. Three factors that increase these stresses were identified. The notch depth, radius of curvature, and its proximity to the end of the femoral prosthesis influence the state of stress in the surrounding bone. The purpose of this study was to characterize the stress concentration caused by anterior femoral notching during total knee replacement (TKR) in order to determine when a patient is at risk for a periprosthetic fracture of the femur. We concluded that notches greater than 3 mm with sharp corners located directly at the proximal end of the femoral implant produced the highest stress concentrations and may lead to a significant risk of periprosthetic femur fracture. One complication that can occur during TKR is notching of the anterior femoral cortex which results in a stress concentration. It is important to characterize this stress riser in order to determine when a stemmed femoral component should be used to minimize the risk of fracture. Three factors that affected the stress concentration were identified. First, increasing the notch depth lead to significant increased stress concentrations. When the depth was greater than 3 mm, local stresses increased markedly. Second, the radius of curvature was found to be inversely related to stress concentration. As the radius decreased, the local stress increased. Third, the proximity of the notch to the prostheses affected the stress concentration. Notches that were 1 mm proximal to the implant resulted in much larger stresses than those that were 10 mm away. A validated, three dimensional finite element model of a femur subjected to a gait loading pattern was used to characterize the stress concentration caused by anterior femoral notching. The results compared well to previous work reported in the literature

Notching of the anterior femoral cortex during total knee arthroplasty is thought to be a possible risk factor for subsequent periprosthetic femoral fracture. Understanding the stress pattern caused by notching may help the orthopedic surgeon reduce the risk of fracture. A validated, three dimensional, finite element model of the femur using gait loads has been used to analyze the stress concentrations caused by anterior femoral cortex notching. Three factors that increase these stresses were identified. The notch depth, radius of curvature, and its proximity to the end of the femoral prosthesis influence the state of stress in the surrounding bone. The purpose of this study was to characterize the stress concentration caused by anterior femoral notching during total knee replacement (TKR) in order to determine when a patient is at risk for a periprosthetic fracture of the femur. We concluded that notches greater than 3 mm with sharp corners located directly at the proximal end of the femoral implant produced the highest stress concentrations and may lead to a significant risk of periprosthetic femur fracture. One complication that can occur during TKR is notching of the anterior femoral cortex which results in a stress concentration. It is important to characterize this stress riser in order to determine when a stemmed femoral component should be used to minimize the risk of fracture. Three factors that affected the stress concentration were identified. First, increasing the notch depth lead to significant increased stress concentrations. When the depth was greater than 3 mm, local stresses increased markedly. Second, the radius of curvature was found to be inversely related to stress concentration. As the radius decreased, the local stress increased. Third, the proximity of the notch to the prostheses affected the stress concentration. Notches that were 1 mm proximal to the implant resulted in much larger stresses than those that were 10 mm away. A validated, three dimensional finite element model of a femur subjected to a gait loading pattern was used to characterize the stress concentration caused by anterior femoral notching. The results compared well to previous work reported in the literature

Introduction. The use of larger femoral heads in Total Hip Arthroplasty has increased in order to reduce the risk of dislocation and to improve the range of motion of the joint. In 2003, within the UK, the “standard” head size of 28mm was used in 73% of all hip procedures, whereas by 2012, this figure dropped to 36%. Concerns regarding the impact of this increment in head size on the cement and bone stresses have arisen; however, this has yet to be clearly determined. Methods. To understand the relationship between femoral head size and cement mantle and bone stress in cemented hip arthroplasty, 3D-Finite-Element models of a hemipelvis with cemented cup[tb6] (50mm outer-diameter) were developed. Loading conditions of single-leg-stance (average and overweight) were simulated for three head sizes (28, 32 and 36mm). The models were validated with an in-vitro experiment using the average loading condition. Results. Stresses were evaluated at the periacetabular bone and cement mantle. In the pelvic bone the peak von Mises stress value presented no change in magnitude due to change in head size for the average patient; for the overweight patient, there was a small increment. In the cement mantle, there was a noticeable difference in the pattern distribution and magnitude of the stresses for the two loading conditions[tb7]. For the average patient, average stresses in the cement were 1.7MPa, 1.8MPa and 1.9MPa for 28, 32 and 36mm heads, respectively; whereas for the overweight patient the stresses were 3.4MPa, 3.6MPa and 3.8MPa. Conclusions. Pelvic bone remained largely unaffected by the changes in femoral head size. The major effect of femoral head size occurs in the stress level and stress distribution pattern in the cement mantle. The current predicted cement stresses are below the cement endurance limit, this indicates that the cement fatigue life is not affected by the increasing head size

Introduction and Aims: Hip arthroplasty alters stress patterns in the proximal femur, thereby influencing femoral bone remodelling. The purpose of our study was to determine long-term skeletal response to wellfixed total hip arthroplasty. Method: Seventy-two hips in 66 patients (mean age 57, range 25–72; 29 male, 37 female) were evaluated with standardised measurement protocol after arthroplasty with cemented Charnley (32 hips) or uncemented 5/8 coated AML stem (40 hips). Inclusion criteria: stable implants and complete radiographic record with minimum follow-up 15–20 years. 3159 measurements were made with power calipers and normalised for magnification. Results: There was time dependent loss of proximal cortical thickness around both stems (AML greater than Charnley; proximal medial greater than proximal lateral cortex, (p< 0.05, all parameters). At 15–20 years, median proximal medial cortical thickness decreased by 12% for Charnley and 70% for AML stems. Median proximal lateral cortical thickness decreased by 9% for Charnley and 21% for AML stems. Median cortical thickness changes around the mid and distal prosthesis for both stems was mild, with a non-statistically significant trend (p> 0.05) towards more cortical loss (2–9%) around Charnley than AML stems (0–8%). The median intramedullary width increased by 1–10%, depending on level (no difference by prosthesis type, p> 0.05). Changes continued progressively over the entire observation period. Conclusion: This paper provides the first detailed long-term information on the effect of well-functioning hip arthroplasty on femoral morphology in a large patient group. Morphologic changes are most pronounced in the proximal medial femur and vary by implant type. Also, the medullary canal widens around a replaced hip as the patient ages