Predictive structural models resulting in a trabecular bone topology closely resembling real bone would be a step toward 3D printing of sympathetic prosthetics. This study modifies an established trabecular bone structural adaptation approach, with the objective of achieving an improved adapted topology, specifically connectivity, compared to CT imaging studies; whilst retaining continuum level mechanical properties consistent with those reported in experimental studies. Strain driven structural adaptation models successfully identify trabecular trajectories, although tend to overpredict connectivity and skew trabecular radii distribution towards the smallest radius included in the adaptation. Radius adaptation of each trabecula is driven by a mechanostat approach with a target strain (1250 µɛ) below which radius is decreased (resorption), and above which radius is increased (apposition). Simulations include a lazy zone, in which neither resorption nor apposition takes place (1000 to 1500 µɛ); and a dead zone (<250 µɛ) in which complete resorption of trabeculae with the smallest included radius takes place. This study assesses the impact of increasing the dead zone threshold from <250 µɛ to <1000 µɛ, the lower limit of the lazy zone. In-silico structural models with an initial connectivity (number of trabeculae connecting at each joint) of 14 were generated using a nearest neighbour approach applied to a random cloud of points. Trabeculae were modelled using circular beams whose radii were adapted in response to normal strains caused by the axial force and bending moments due to a vertical pressure of 1 MPa applied to the top of the lattice, with the bottom of the lattice fixed in the vertical direction. Lattices in which nodes are either able (rigid jointed) or unable (pin jointed) to transmit bending moments were considered. Five virtual samples of each lattice type were used, and each simulation repeated twice: with a dead zone of either <250 µɛ or <1000 µɛ.Abstract
1.0 Objectives
2.0 Methods
Hip resurfacing remains a potentially valuable surgical procedure for appropriately-selected patients with optimised implant choices. However, concern regarding high early failure rates continues to undermine confidence in use. A large contributor to failure is adverse local tissue reactions around metal-on-metal (MoM) bearing surfaces. Such phenomena have been well-explored around MoM total hip arthroplasties, but comparable data in equivalent hip resurfacing procedures is lacking. In order to define genetic predisposition, we performed a case-control study investigating the role of human leucocyte antigen (HLA) genotype in the development of pseudotumours around MoM hip resurfacings. A matched case-control study was performed using the prospectively-collected database at the host institution. In all, 16 MoM hip resurfacing 'cases' were identified as having symptomatic periprosthetic pseudotumours on preoperative metal artefact reduction sequence (MARS) MRI, and were subsequently histologically confirmed as high-grade aseptic lymphocyte-dominated vasculitis-associated lesions (ALVALs) at revision surgery. ‘Controls’ were matched by implant type in the absence of evidence of pseudotumour. Blood samples from all cases and controls were collected prospectively for high resolution genetic a nalysis targeting 11 separate HLA loci. Statistical significance was set at 0.10 a priori to determine the association between HLA genotype and pseudotumour formation, given the small sample size.Aims
Methods
Hip resurfacing may be a useful surgical procedure when patient selection is correct and only implants with superior performance are used. In order to establish a body of evidence in relation to hip resurfacing, pseudotumour formation and its genetic predisposition, we performed a case-control study investigating the role of HLA genotype in the development of pseudotumour around MoM hip resurfacings. All metal-on-metal (MoM) hip resurfacings performed in the history of the institution were assessed. A total of 392 hip resurfacings were performed by 12 surgeons between February 1st 2005 and October 31st 2007. In all cases, pseudotumour was confirmed in the preoperative setting on Metal Artefact Reduction Sequencing (MARS) MRI. Controls were matched by implant (ASR or BHR) and absence of pseudotumour was confirmed on MRI. Blood samples from all cases and controls underwent genetic analysis using Next Generation Sequencing (NGS) assessing for the following alleles of 11 HLA loci (A, B, C, DRB1, DRB3/4/5, DQA1, DQB1, DPB1, DPA1). Statistical significance was determined using a Fisher's exact test or Chi-Squared test given the small sample size to quantify the clinical association between HLA genotype and the need for revision surgery due to pseudotumour. Both groups were matched for implant type (55% ASR, 45% BHR in both the case and control groups). According to the ALVAL histological classification described by Kurmis et al., the majority of cases (63%, n=10) were found to have group 2 histological findings. Four cases (25%) had group 3 histological findings and 2 (12%) patients had group 4 findings. Of the 11 HLA loci analysed, 2 were significantly associated with a higher risk of pseudotumour formation (DQB1*05:03:01 and DRB1*14:54:01) and 4 were noted to be protective against pseudotumour formation (DQA1*03:01:01, DRB1*04:04:01, C*01:02:01, B*27:05:02). These findings further develop the knowledge base around specific HLA genotypes and their role in the development of pseudotumour formation in MoM hip resurfacing. Specifically, the two alleles at higher risk of pseudotumour formation (DQB1*05:03:01 and DRB1*14:54:01) in MoM hip resurfacing should be noted, particularly as patient-specific genotype-dependent surgical treatments continue to develop in the future.
Several experimental studies derived relationships between density and macroscale material properties of trabecular bone, taking the form E=αρβ, where E is Young's modulus, ρ is density, and α and β are constants. Classical structural mechanics demonstrates β can vary between 1 (behaviour of the trabecular lattice is dominated by the axial stiffness of individual trabeculae) and 3 (behaviour is dominated by the bending stiffness of individual trabeculae). The ratio between rods (round trabeculae characterised by radius) and plates (flat trabeculae characterised by thickness) is also believed to govern the macroscale material properties of trabecular bone. To assess feasible ranges of α and β for trabecular bone, and their dependence on rod to plate ratio, 25 virtual samples of trabecular bone were generated as Voronoi lattices. Each 8×8×8mm sample was composed of 320 randomly generated Voronoi cells forming a foam like structure. Edges formed the rod network. Faces formed the plate network. Tissue level Young's modulus was set to 18,000MPa. Relative density was varied: 0.05, 0.1, 0.15, 0.2, 0.25. Rod to plate ratio was varied: 100:0, 75:25, 50:50, 25:75, 0:100. Macroscale Young's modulus was averaged in three orthotropic directions and used to find α and β. Around 14,000 3-noded quadratic beam elements represented rods, with average length of 0.63mm, and around 42,000 8-noded quadratic shell elements represented plates, with average area of 0.10mm2. Results for α and β were 3274 and 1.463 for 100% rods, 3646 and 1.067 for 50:50 rods and plates, and 4981 and 1.062 for 100% plates, showing the presence of plates improves the stiffness characteristics of trabecular bone. Work investigating the impact of element based geometry optimisation is ongoing. The work has important implications for the onset of conditions including osteoporosis and osteoarthritis, as well as those designing 3D printed scaffolds and implants.Abstract
Ceramic on Ceramic bearings in Total Hip Arthroplasty (THA) afford a low friction coefficient, low wear rates and extreme hardness. Significant complications include hip squeak, ceramic fracture and poor polyethylene performance in revision procedures due to imbedding of abrasive microscopic ceramic fragments. We report on the results of this bearing at a minimum of 10 years. A single-centre retrospective review of 449 THAs was performed. Primary outcome measures included aseptic revision and all-cause revision rates at a minimum of 10 years post operatively. Evaluation of functionality was performed with WOMAC and SF-36 scores which were performed pre-operatively and at intervals of 6 months, one year, 2 years, 5 years and 10 years post operatively.Introduction and Objective
Materials and Methods
Bone shape and internal architecture are accepted as optimised to resist joint contact and muscle forces the skeleton is subjected to through daily living and more demanding activities. Finite element studies to predict bone architecture, either using continuum or structural approaches have made assumptions common in structural optimisation, that lead to trabecular bone effectively being modelled as a truss-type structure, with compressive or tensile strains, present due to axial forces driving adaptation. These models are successful in predicting bone fracture, and trends in bone degradation associated with disuse or unloading osteopenia but tend to overpredict bone mineral density reduction compared to clinical observations. A new structural model of bone adaptation, including both trabeculae (element) cross-section adaptation in response to axial force and biaxial bending moments, and alteration of joint (node) positions within the trabecular network, was developed using a Voronoi space partition to define the initial network. This was compared to results from a structural bone adaptation using a truss-type network generated by connecting each node to its nearest 16 neighbours [1].Abstract
Objectives
Methods
Bone health deterioration is a major public health issue. General guidelines for the limitation of bone loss prescribe a healthy lifestyle and a minimum level of physical activity. However, there is no specific recommendation regarding targeted activities that can effectively maintain lumbar spine bone health. To provide a better understanding of such influencing activities, a new predictive modelling framework was developed to study bone remodelling under various loading conditions. The approach is based on a full-body subject-specific musculoskeletal model [1] combined with structural finite element models of the lumbar vertebrae. Using activities recorded with the subject, musculoskeletal simulations provide physiological loading conditions to the finite element models which simulate bone remodelling using a strain-driven optimisation algorithm [2]. With a combination of daily living activities representative of a healthy lifestyle including locomotion activities (walking, stair ascent and descent, sitting down and standing up) and spine-focused activities involving twisting and reaching, this modelling framework generates a healthy bone architecture in the lumbar vertebrae. The influence of spine-focused tasks was studied by adapting healthy vertebrae to an altered loading scenario where only locomotion activities were performed.Abstract
OBJECTIVES
METHODS
Osteoporosis of the pelvis and femur is diagnosed in a high proportion of lower-limb amputees which carries an increased fracture risk and subsequently serious implications on mobility, physical dependency and morbidity. Through the development of biofidelic musculoskeletal and finite element (FE) models, we aim to determine the effect of lower-limb amputation on long-term bone remodelling in the hip and to understand the potential underpinning mechanisms for bone degradation in the younger amputee population. Our models are patient specific and anatomically accurate. Geometries are derived from MRI-scans of one bilateral, above-knee, amputee and one body-matched control subject. Musculoskeletal modelling enables comparison of muscle and joint reaction-forces throughout gait. This provides the loading scenario implemented in FE. FE modelling demonstrates the effect of loading on the amputated limb via a prosthetic socket by comparing bone mechanical stimulation in amputee and control cases.Abstract
Objectives
Methods
An understanding of anatomic variability can help guide the surgeon on intervention strategies. Well-functioning thumb metacarpophalangeal joints (MCPJ) are essential for carrying out typical daily activities. However, current options for arthroplasty are limited. This is further hindered by the lack of a precise understanding of the geometric variation present in the population. In this paper, we offer new insight into the major modes of geometric variation in the thumb MCP using Statistical Shape Modelling. Ten participants free from hand or wrist disease or injury were recruited for CT imaging (Ethics Ref:14/LO/1059)1. Participants were sex matched with mean age 31yrs (range 27–37yrs). Metacarpal (MC1) and proximal phalanx (PP1) bone surfaces were identified in the CT volumes using a greyscale threshold, and meshed. The ten MC1 and ten PP1 segmented bones were aligned by estimating their principal axes using Principal Component Analysis (PCA), and registration was performed to enable statistical comparison of the position of each mesh vertex. PCA was then used again, to reduce the dimensionality of the data by identifying the main ‘modes’ of independent size and shape variation (principal components, PCs) present in the population. Once the PCs were identified, the variation described by each PC was explored by inspecting the shape change at two standard deviations either side of the mean bone shape.Introduction
Methods
Long term acetabular component fixation is dependent on bone ingrowth, which is affected by initial stability and the contact area between the bone and acetabular component. Mismatch between the component and cavity size has been shown to be one reason for component loosening. Furthermore, the potential of acetabular fracture during insertion of oversized components is larger than line-to-line components. An ideal cavity preparation would be a true hemispherical cavity that can provide maximum contact area between the shell and bone while also achieving adequate press fit for implant initial stability. The goal of this study was to characterize the cavity morphology produced by a commercially available reamer and compare it to a new reamer design. 36mm and 52mm reamers (n=6) were selected from conventional reamers (Stryker, Mahwah, NJ), which have successful clinical history exceeding 20 years, and Smooth Cut Reamers (Tecomet, Warsaw, IN and Stryker, Mahwah, NJ), which are a new design. Hemispherical cavities were created in 30 pcf polyurethane foam blocks (Pacific Research Laboratories, WA) using a custom software for the Mako System (Stryker, Mahwah, NJ), with new reamers of both designs. A reamer 2mm smaller in diameter than the final size was used to create a pilot cavity to replicate a clinically relevant reaming scenario. The resulting cavities were scanned using a Triple Scan high resolution 3D Scanner (ATOS, Purchase, NY) to generate 3D models of each cavity. The models were then post processed, and the following dimensions were collected:
Gaussian best fit spherical diameter of the entire cavity (Dimension A) Gaussian best fit diameter at the rim of the cavity (measured at a distance of 0.25mm from the top surface of the foam block) (Dimension B) One-sided two sample T-tests were conducted to determine statistical significance.Introduction
Materials & Methods
Low back pain (LBP) is the top leading global cause of years lived with disability. In order to examine LBP, researchers have typically viewed the spine in isolation. Clinically, it is imperative that the lower limbs are also considered. The aim of this study was to design a holistic and reliable multi-segmental kinematic model of the spine and lower limbs. The spine was modelled according to easily identifiable anatomical landmarks, including upper thoracic (T1-T6), lower thoracic (T7-T12) and lumbar (L1-L5) segments. Pelvis, thigh, shank and foot segments were included. A 10-camera 3D motion capture system was used to track retro-reflective markers, which were used to define each segment of 10 healthy participants as they walked 3 times at a comfortable speed over a 6km walkway. The relative peak angles between each segment were calculated using the Joint Coordinate System convention and Intraclass Correlation Coefficients (ICCs) were used to determine intra-rater and inter-rater reliability (between an experienced clinician and biomechanical scientist).Introduction
Method
It is well known that individuals with a history of low back pain (hLBP) exhibit altered movement patterns that are caused by changes in neuromuscular control. Postural disturbance provides an effective method for creating these differentiable movement patterns. This study has explored the response of the lower limb and spine to a translational perturbation similar to that experienced on public transport in healthy volunteers and those with hLBP. Healthy volunteers (n=16) and subjects with hLBP (n=10) were subjected to 31 identical postural disturbances at varying time intervals while standing atop a moving platform. Skeletal kinematics and muscle activation were recorded using a 10-camera Vicon system (Oxford, UK) and Myon electromyography (EMG) at the trunk (lumbar, lower thoracic, and upper thoracic segments), pelvis, thigh, calf, and foot. Joint angles were calculated using Body Builder (Vicon) and a unilateral seven-segment custom model.Statement of Purpose
Methods
We sought to validate a method of measuring the range of motion of knees on radiographs as part of a new system of “Virtual Knee Clinics”. The range of motion of 52 knees in 45 patients were first obtained clinically with goniometers and compared to radiographs of these patients' knees in full active flexion and extension. Four methods of plotting the range of motion on the radiographs were compared. The intra-class correlation coefficient (ICC) for inter-rater reliability using the goniometer was very high; ICC=0.90 in extension and 0.85 in flexion. The best ICC for radiographic measurement in extension was 0.86 indicating substantial agreement and best ICC in flexion was 0.95 (method 4). ICC for intra-rater reliability was 0.98 for extension and 0.99 for flexion on radiographic measurements. Measuring range of motion of the knee has never previously been validated in the literature. This study has allowed us to set up a “Virtual Knee Clinic,” combining postal questionnaires and radiographic measurements as a surrogate for knee function. We aim to maintain high quality patient surveillance following knee arthroplasty, reduce our new to follow-up ratios in line with Department of Health guidelines and improve patient satisfaction through reduced travel to hospital outpatients.
The purpose of this study was to investigate whether patients who had had excision of the Ligamentum Teres as part of a surgical hip dislocation for femoro-acetabular impingement exhibited symptoms of acute Ligamentum Teres rupture post-operatively. Recent reports in the literature suggest that injury to the Ligamentum Teres can cause instability, severe pain and inability to walk. We present the results of a postal questionnaire to 217 patients who had undergone open surgical hip dislocation for femoro-acetabular impingement where the LT was excised. This included seven patients who had undergone bilateral surgery. The questionnaire was designed to enquire about specific symptoms attributed to LT injuries in the literature; gross instability, incomplete reduction, inability to bear weight and mechanical symptoms. 161 patients responded (75%), with a total of 168 (75%) questionnaires regarding 224 hips completed. There were 104 females and 64 males. Median age was 34 and median follow-up was 52 months. All patients were found to have cam deformities, 72% (n=121) had associated labral tears. All patients were able to fully weight bear after surgery. 77% experienced no groin pain and 61% experienced no pain on exercise. 35% of patients experienced popping and locking in their operated hip and 24% had subjective feeling of their hip giving way. Oxford Hip scores and Nonarthritic Hip scores improved by 12 and 28 points respectively (n=47). Our results show that the symptoms of pain and instability described with LT pathology can be present but are by no means universal. This leads us to conclude that their symptoms may be attributed to labral pathology which is frequently noted to coexist.
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.
Recently finite element studies have incorporated bone remodelling algorithms in an attempt to simulate bone's mechano-adaptation to loading conditions. In order to simplify these analyses, bone is usually considered to be isotropic, which does not explain the directionality of its internal structures; neither the orthotropic properties measured at the continuum level. Furthermore, simplified loading is usually applied to the bone models, which can result in an unrealistic remodelling stimulus. However, free boundary condition modelling of the femoral and pelvic constructs has been shown to produce more physiological stress and strain distributions. This paper describes the application of a 3D remodelling algorithm (with bone modelled as a strain-adaptive continuum with local orthotropic material properties) to a free boundary model of the femoral construct, where the hip and knee joints, as well as muscles and ligaments crossing the joints were included explicitly. Two load cases were analysed: single leg stance and standing up. Material properties and directionality distributions were produced for the whole femur, showing good agreement with observed structures from clinical studies. This indicates that the loading conditions modelled correspond to those experienced in vivo. In addition, the impact of the different load cases in bone structure modelling could be compared. Observations of the material properties distribution and orientation for standing up indicate that it promotes changes in bone stiffness in the anterior regions of the femoral neck and cortical shaft and the posterior side of the condyles. Development of this approach to modelling and bone structure prediction can lead to a better understanding of bone's mechanical behaviour and to the development and public release of orthotropic heterogeneous models for different constructs. These can be applied in many areas of interest in orthopaedic biomechanics, such as the study of bone-implant interfaces, improvement of the currently used surgical tools and techniques and the influence of certain activities in affecting local bone strength and mineralisation.
Finite element models of the musculoskeletal system have the possibility of describing the in vivo situation to a greater extent than a single in vitro experimental study ever could. However these models and the assumptions made must be validated before they can be considered truly useful. The object of this study was to validate, using digital image correlation (DIC) and strain gauging, a novel free boundary condition finite element model of the femur. The femur was treated as a complete musculoskeletal construct without specific fixed restraint acting on the bone. Spring elements with defined force-displacement relationships were used to characterize all muscles and ligaments crossing the hip and knee joints. This model was subjected to a loading condition representing single leg stance. From the developed model muscle, ligament and joint reaction forces were extracted as well as displacement and strain plots. The muscles with the most influence were selected to be represented in the simplified experimental setup. To validate the finite element model a balanced in vitro experimental set up was designed. The femur was loaded proximally through a construct representative of the pelvis and balanced distally on a construct representing the tibio-femoral joint. Muscles were represented using a cabling system with glued attachments. Strains were recorded using DIC and strain gauging. DIC is an image analysis technique that enables non-contact measurement of strains across surfaces. The resulting strain distributions were compared to the finite element model. The finite element model produced hip and knee joint reaction forces comparable to in vivo data from instrumented implants. The experimental models produced strain data from both DIC and strain gauging; these were in good agreement with the finite element models. The DIC process was also shown to be a viable method for measuring strain on the surface of the specimen. In conclusion a novel approach to finite element modeling of the femur was validated, allowing greater confidence for the model to be further developed and used in clinical settings.
Metal on metal press-fit acetabular cups are the worst performing acetabular cup type with severe failure consequences compared to cups made from more inert materials such as polyethylene or ceramic. The cause of failure of these cup types is widely acknowledged to be multi-factorial, therefore creating a complex scenario for analysis through clinical studies. A factorial analysis has been carried out using an experimentally validated finite element analysis to investigate the relative influence of four input factors associated with acetabular cup implantation on output parameters indicating potential failure of the implantation. These input factors were: cup material stiffness; cup inclination; cup version; cup seating; and level of press-fit. The output parameter failure indicators were: wear; tensile strains in the underlying bone; bone remodelling; and cup-bone micromotions. The factorial analysis concluded that the most significant influence was that of cup inclination on wear, and the second most significant was the influence of the level of press-fit on bone remodelling at the acetabular rim. Significant influence was also observed between version angle and wear, and cup-seating and micro-motion. The results demonstrated the clear multi-factorial nature of implant failure and highlighted the importance of correct implant positioning and fit.
Human recombinant Osteogenic Protein 1 or rhBMP-7 is licensed for use in tibial non-union where autologous bone grafting has failed. Through its osteoconductive and osteoinductive properties, its application may be more widely applied. We audited our use of rhBMP-7 and present the largest series currently reported in the literature. We reviewed 107 consecutive patients on whom rhBMP-7 was used over a 5-year period (2002–2007). Demographic and clinical details (e.g indication, site, use of adjuncts, previous surgery, smoking status, time to union, mean follow up etc) were entered into an electronic spreadsheet. RhBMP-7 was used in 112 sites on 107 patients (65 male, 42 female). Ages ranged from 16yrs to 89yrs (mean 47.6). Non-union was the main indication for surgery (82 cases). RhBMP-7 was used alone in 39 cases and with autologous bone graft (56 cases). In other cases demineralised bone matrix, USS and bone allograft were used as adjuncts. Tibia (42 cases), femur (29 cases), humerus (21 cases) were the most common sites of administration. Mean number of operations prior to use of rhBMP-7 was 1.6 (range 1–20). In all cases, union was achieved in 65% (73/112) with a mean union time 5.8 months. The ‘rhBMP-7 alone’ subgroup demonstrated union in 83% (30/36), mean union time 5.15 months. 68% (56/82) of cases treated for nonunion subsequently united with rhBMP-7. Our results suggest rhBMP-7 is useful in the management of fracture non-union and limb reconstruction surgery irrespective of site. It promotes bone healing of non-unions subjected to multiple operations previously. It may be indicated in those patients in whom autologous bone graft harvest is undesirable or not possible or as an adjunct to bone grafting. Moreover we did not detect any adverse reactions specific to the administration of rhBMP-7.
Previous experimental studies of the pelvis have been carried out on cadaveric samples stripped of soft tissue. Investigations of the stress concentrations present in the pelvis due to the application of force through the hip joint have been conducted with the superior iliac crests cast in resin or cement. Thus stress concentrations are observed towards the superior iliac crests, and to some extent the pubic symphysis (these being the areas in which force transfer can occur). Due to the rigid fixing of the pelvis in these experiments, the pelvic bone has become viewed as a ‘sandwich beam’ acting between the sacro-iliac and the pubic joints. Numerical models employing similar fixed conditions have shown good agreement with the experimental studies. However it is clear that these experiments, and the accompanying computational models are not representative of the in-vivo situation, in which the muscles and ligaments of the pelvis and hip joint provide resistance to movement, and in the case of muscles place additional forces on the pelvis, not addressed in the experimental studies. This study presents a finite element model of the pelvis in which novel techniques have been used to include the pelvic ligaments, and hip joint muscles using realistic attachment areas on the cortex, providing a more realistic comparison to the in-vivo environment. Joint interactions at the pubic symphysis and sacro-iliac joints are also simulated. A fixed boundary condition model is also presented for comparison. The resulting stress concentrations in the pelvis for single leg stance observed in the in-vivo boundary condition model are dramatically different to those presented in studies in which the pelvis is rigidly fixed in place. The abductor muscles are seen to play a significant role in reducing stress concentrations towards the sacro-iliac joints and superior to the acetabulum, in comparison to fixed boundary condition analyses. Stress reductions away from the acetabulum are also observed in the underlying trabecular bone for the in-vivo boundary condition model. Similar stresses are observed within the acetabular region for the fixed, and in-vivo boundary condition models.
