Adult Spine Deformity (ASD) is a degenerative condition of the adult spine leading to altered spine curvatures and mechanical balance. Computational approaches, like Finite Element (FE) Models have been proposed to explore the etiology or the treatment of ASD, through biomechanical simulations. However, while the personalization of the models is a cornerstone, personalized FE models are cumbersome to generate. To cover this need, we share a virtual cohort of 16807 thoracolumbar spine FE models with different spine morphologies, presented in an online user-interface platform (SpineView). To generate these models, EOS images are used, and 3D surface spine models are reconstructed. Then, a Statistical Shape Model (SSM), is built, to further adapt a FE structured mesh template for both the bone and the soft tissues of the spine, through mesh morphing. Eventually, the SSM deformation fields allow the personalization of the mean structured FE model, leading to generate FE meshes of thoracolumbar spines with different morphologies. Models can be selectively viewed and downloaded through SpineView, according to personalized user requests of specific morphologies characterized by the geometrical parameters: Pelvic Incidence; Pelvic Tilt; Sacral Slope; Lumbar Lordosis; Global Tilt; Cobb Angle; and GAP score. Data quality is assessed using visual aids, correlation analyses, heatmaps, network graphs, Anova and t-tests, and kernel density plots to compare spinopelvic parameter distributions and identify similarities and differences. Mesh quality and ranges of motion have been assessed to evaluate the quality of the FE models. This functional repository is unique to generate virtual patient cohorts in ASD. Acknowledgements: European Commission (MSCA-TN-ETN-2020-Disc4All-955735, ERC-2021-CoG-O-Health-101044828)

Aims

The most important complication of treatment of developmental dysplasia of the hip (DDH) is avascular necrosis (AVN) of the femoral head, which can result in proximal femoral growth disturbances leading to pain, dysfunction, and eventually to early onset osteoarthritis. In this study, we aimed to identify morphological variants in hip joint development that are predictive of a poor outcome.

Pre-operative 3D glenoid planning improves component placement in terms of version, inclination, offset and orientation. Version and inclination measurements require the position of the inferior angle. As a consequence, current planning tools require a 3D model of the full scapula to accurately determine the glenoid parameters. Statistical shape models (SSMs) can be used to reconstruct the missing anatomy of bones. Therefore, the objective of this study is to develop and validate an SSM for the reconstruction of the inferior scapula, hereby reducing the irradiation exposure for patients. The training dataset for the statistical shape consisted of 110 CT images from patients without observable scapulae pathologies as judged by an experienced shoulder surgeon. 3D scapulae models were constructed from the segmented images. An open-source non-rigid B-spline-based registration algorithm was used to obtain point-to-point correspondences between the models. A statistical shape model was then constructed from the dataset using principal component analysis. Leave-one-out cross-validation was performed to evaluate the accuracy of the predicted glenoid parameters from virtual partial scans. Five types of virtual partial scans were created on each of the training set models, where an increasing amount of scapular body was removed to mimic a partial CT scan. The statistical shape model was reconstructed using the leave-one-out method, so the corresponding training set model is no longer incorporated in the shape model. Reconstruction was performed using a Monte Carlo Markov chain algorithm, random walk proposals included both shape and pose parameters, the closest fitting proposal was selected for the virtual reconstruction. Automatic 3D measurements were performed on both the training and reconstructed 3D models, including glenoid version, inclination, glenoid centre point position and glenoid offset. In terms of inclination and version we found a mean absolute difference between the complete model and the different virtual partial scan models of 0.5° (SD 0.4°). The maximum difference between models was 3° for inclination and 2° for version. For offset and centre point position the mean absolute difference was 0 mm with an absolute maximum of 1 mm. The magnitude of the mean and maximum differences for all anatomic measurements between the partial scan and complete models is smaller than the current surgical accuracy. Considering these findings, we believe a SSM based reconstruction technique can be used to accurately reconstruct the glenoid parameters from partial CT scans

Statistical shape modeling (SSM) and statistical density modeling (SDM) are tools capable of describing the main modes of deviation in the shape and density distribution of the shoulder using a set of uncorrelated variables called principal components (PCs). We hypothesize that the first PC of the SDM, which scales overall density up/down, will be inversely correlated with age and will, on average, be greater for males than females. We also hypothesize that there is a correlation between some PCs of shape and density. SSM and SDM were developed for scapulae and humeri by segmenting surface meshes from computed tomographic images of 75 cadaveric shoulders. Bones were co-registered and defined by the same surface mesh. Volumetric tetrahedral meshes were defined for one of the specimens serving as base meshes for SDM. Base meshes were morphed to each individual bone's surface and superimposed upon the corresponding CT data to determine image intensity in Hounsfield units at each node. Principal component analysis was performed on the exterior shape and internal density distribution of bones. T-tests were performed to find any differences in PC scores between males and females, and Pearson correlation coefficients were calculated for age and PC scores. Finally, correlation coefficients between each of the PCs of the shape and density models were calculated. For the humerus, the first three PCs of the SDM were significantly correlated with age (ρ = 0.40, −0.46, and 0.36, all p ≤ 0.007). For the scapula, the first and ninth PCs showed such correlation (ρ = −0.31, and −0.32, all p ≤ 0.02). Statistically significant differences due to sex were found for the second to sixth SDM PCs of the humerus, with differences in average PC scores of 1, 1, −0.7, −0.8, and −0.6 standard deviations, respectively, for males relative to females. For the scapula, the second, fifth and seventh SDM PCs were significantly different between males and females, with average PC scores differing by 1.1, 0.7, and −0.6 standard deviations. Finally, for both bones, the first PC of SSM showed a weak but significant correlation with the second PC of the SDM (ρ = 0.47, p < 0.001 for the humerus, and ρ = 0.39, p < 0.001 for the scapula). The results of this study suggest that age has a significant influence on the first PC of the SDM, associated with scaling the density in the cortical boundary. Moreover, the negative correlation of age with the second PC of the humerus in SDM which mostly influences the thickness of the cortical boundary implies cortical thinning with age. The second PC of both bones differed significantly between males and females, implying that cortical thickness differs between sexes. Also, there was a significant correlation between the size of the bones and the thickness of the cortical boundary. These findings can help guide the designs of population-based prosthesis components

INTRODUCTION. Statistical shape models (SSM) have become a common tool to create reference models for design input and verification of total joint implants. In a recent discussion paper around Artificial Intelligence and Machine Learning, the FDA emphasizes the importance of independent test data [1]. A leave-one-out test is a standard way to evaluate the generalization ability of an SSM [2]; however, this test does not fulfill the independence requirement of the FDA. In this study, we constructed an SSM of the knee (femur and tibia). Next to the standard leave-one-out validation, we used an independent test set of patients from a different geographical region than the patients used to build the SSM. We assessed the ability of the SSM to predict the shapes of knees in this independent test set. METHODS. A dataset of 82 computed tomography (CT) scans of Caucasian patients (42 male, 40 female) from 11 different geographic locations in France, Germany, Austria, Italy and Australia were used as training set to make an SSM of the femur and tibia. A leave-one-out test was performed to assess the ability of the SSM to predict shapes within the training set. A test dataset of 4 CT scans of Caucasian patients from Russia were used for the validation. The SSM was fitted onto each of the femur and tibia shapes and the root mean square error (RMSE) was measured. RESULTS. The leave-one-out tests showed that the femur and tibia SSMs were able to predict patients in the input population with an RMSE of 0.59 ± 0.1 mm (average ± standard deviation) for the femur and 0.70 ± 0.1 mm for the tibia. The validation test showed that the femur and tibia SSMs were able to predict the shapes of the Russian patients with an RMSE 0.62 ± 0.1 mm for the femur and 0.71 ± 0.1 mm for the tibia. DISCUSSION. There were no significant differences in the ability of the SSM to predict femur and tibia shapes of patients in a new geographic region compared to the ability of the SSM to predict shapes within the training set. CONCLUSIONS. Based on this study, 11 different geographic locations in France, Germany, Austria, Italy and Australia provide a complete sample of the Caucasian population. Using an independent set of CT scans is a valuable tool to further validate the generalization ability of an SSM. For any figures or tables, please contact authors directly

Introduction. A good anatomic fit of a Total Knee Arthroplasty is crucial to a good clinical outcome. The big variability of anatomies in the Asian and Caucasian populations makes it very challenging to define a design that optimally fits both populations. Statistical Shape Models (SSMs) are a valuable tool to represent the morphology of a population. The question is how to use this tool in practice to evaluate the morphologic fit of modern knee designs. The goal of our study was to define a set of bone geometries based on SSMs that well represent both the Caucasian and the Asian populations. Methods. A Statistical Shape Model (SSM) was built and validated for each population: the Caucasian Model is based on 120 CT scans from Russian, French, German and Australian patients. The Asian Model is based on 80 CT scans from Japanese and Chinese patients. We defined 7 Caucasian and 5 Asian bone models by using mode 1 of the SSM. We measured the antero-posterior (AP) and medio-lateral (ML) dimensions of the distal femur on all anatomies (input models and generated models) to check that those bone models well represent the studied population. In order to cover the whole population, 10 additional bone models were generated by using an optimization algorithm. First, a combined Asian-Caucasian SSM was generated of 92 patients, equally balanced between male and female, Caucasian and Asian. 10 AP/ML dimensions were defined to obtain a good coverage of the population. For a given AP/ML dimension, Markov chain Monte Carlo sampler was used to find the most average shape with AP/ML dimensions as close as possible to the target dimensions. The difference of the AP/ML dimensions of the generated models to the target dimensions was computed. A chi-squared distribution was used to assess how average the resulting shapes were compared to typical patient shapes. Results. The AP-ML dimensions of the 7 Caucasian bones and the 5 Asian bones well cover the range of the respective populations. For the Caucasian Femur, the AP/ML dimensions range from (53,6/64,9mm) for size 1 to (67,7/80,7mm) for size 7. For the Asian Femur, the AP/ML dimension range from (53,0/62,4mm) for size 1 to (60,5/72,4mm) for size 5. The dimensions of the 10 additionally generated bones differed in average (± 1 standard deviation) by 0,2±0,4mm in AP and 0,5±0,5mm in ML to the target dimensions. The maximal deviation was 0,9mm in AP and 1,0mm in ML. All 10 bones had a P-value of P < 10. -27. according to the chi-squared distribution. Conclusion. The proposed models of 7 Caucasian and 5 Asian bones well represent both populations. The 10 additional geometries enable to get a complete coverage of the population. Since they are very close to average, all these bone models provide more generalized reference shapes compared to individual patients. By performing a virtual implantation on those anatomies, the anatomical fit of implants to these populations can be evaluated. For any figures or tables, please contact authors directly

Purpose and Background. Both overall spine shape and the size and shape of individual vertebrae undergo rapid growth and development during early childhood. Motor development milestones such as age of walking influence spine development, with delayed ambulation linked with spinal conditions including spondylolysis. However, it is unclear whether associations between motor development and spine morphology persist into older age. Therefore, these associations were examined using data from the MRC National Survey of Health and Development, a large nationally-representative British cohort, followed up since birth in 1946. Methods and Results. Statistical shape modelling was used to characterise spinal shape (L5-T10) and identify modes of variation in shape (SM) from dual energy x-ray absorptiometry images of the spine taken at age 60–64 years (N=1327 individuals; 51.8% female). Associations between walking age in months (reported by mothers at 2 years) and SMs were examined with adjustment for sex, birthweight, socioeconomic position, height, lean mass and fat mass. Later onset of independent walking was weakly associated with greater lordosis (SM1; P=0.05) and more uniform antero-posterior vertebral size along the spine (SM6, P=0.07). Later walking age was also associated with smaller relative anterior-posterior vertebral dimensions (SM3) among women whereas the opposite was found for men (P <0.01 for sex interaction). Conclusions. Spinal morphology in early old age was associated with the age that individuals began walking independently in childhood, potentially due to altered mechanical loading. This suggests that motor development may have a persisting effect on clinically-relevant features of spine morphology throughout life. Conflict of interest: None. Funded by the UK Medical Research Council (Grant MR/L010399/1) which supported FRS, SGM and AVP

Aims. Little is known about the risk factors that predispose to a rupture of the posterior cruciate ligament (PCL). Identifying risk factors is the first step in trying to prevent a rupture of the PCL from occurring. The morphology of the knee in patients who rupture their PCL may differ from that of control patients. The purpose of this study was to identify any variations in bone morphology that are related to a PCL. Patients and Methods. We compared the anteroposterior (AP), lateral, and Rosenberg view radiographs of 94 patients with a ruptured PCL to a control group of 168 patients matched by age, sex, and body mass index (BMI), but with an intact PCL after a knee injury. Statistical shape modelling software was used to assess the shape of the knee and determine any difference in anatomical landmarks. Results. We found shape variants on the AP and Rosenberg view radiographs to be significantly different between patients who tore their PCL and those with an intact PCL after a knee injury. Overall, patients who ruptured their PCL have smaller intercondylar notches and smaller tibial eminences than control patients. Conclusion. This study shows that differences in the shape of the knee are associated with the presence of a PCL rupture after injury. A smaller and more sharply angled intercondylar notch and a more flattened tibial eminence are related to PCL rupture. This suggests that the morphology of the knee is a risk factor for sustaining a PCL rupture. Cite this article: Bone Joint J 2019;101-B:1058–1062

Background. Degeneration of the shoulder joint is a frequent problem. There are two main types of shoulder degeneration: Osteoarthritis and cuff tear arthropathy (CTA) which is characterized by a large rotator cuff tear and progressive articular damage. It is largely unknown why only some patients with large rotator cuff tears develop CTA. In this project, we investigated CT data from ‘healthy’ persons and patients with CTA with the help of 3D imaging technology and statistical shape models (SSM). We tried to define a native scapular anatomy that predesignate patients to develop CTA. Methods. Statistical shape modeling and reconstruction:. A collection of 110 CT images from patients without glenohumeral arthropathy or large cuff tears was segmented and meshed uniformly to construct a SSM. Point-to-point correspondence between the shapes in the dataset was obtained using non-rigid template registration. Principal component analysis was used to obtain the mean shape and shape variation of the scapula model. Bias towards the template shape was minimized by repeating the non-rigid template registration with the resulting mean shape of the first iteration. Eighty-six CT images from patients with different severities of CTA were analyzed by an experienced shoulder surgeon and classified. CT images were segmented and inspected for signs of glenoid erosion. Remaining healthy parts of the eroded scapulae were partitioned and used as input of the iterative reconstruction algorithm. During an iteration of this algorithm, 30 shape components of the shape model are optimized and the reconstructed shape is aligned with the healthy parts. The algorithm stops when convergence is reached. Measurements. Automatic 3D measurements were performed for both the healthy and reconstructed shapes, including glenoid version, inclination, offset and critical shoulder angle. These measurements were manually performed on the mean shape of the shape model by a surgeon, after which the point-to-point correspondence was used to transfer the measurements to each shape. Results. The critical shoulder angle was found to be significantly larger for the CTA scapulae compared to the references (P<0.01). When analyzing the classified scapulae significant differences were found for the version angle in the scapulae of group 4a/4b and the critical shoulder angle of group 3 when compared to the references (P<0.05). Conclusion. Patients with CTA have a larger critical shoulder angle compared with reference patients. Some significant differences are found between the scapulae from patients in different stages of CTA and healthy references, however the differences are smaller than the accuracy of the SSM reconstruction. Therefore, we are unable to conclude that there is a predisposing anatomy in terms of glenoid version, inclination or offset for CTA

Introduction. Acetabular bone defects are still challenging to quantify. Numerous classification schemes have been proposed to categorize the diverse kinds of defects. However, these classification schemes are mainly descriptive and hence it remains difficult to apply them in pre-clinical testing, implant development and pre-operative planning. By reconstructing the native situation of a defect pelvis using a Statistical Shape Model (SSM), a more quantitative analysis of the bone defects could be performed. The aim of this study is to develop such a SSM and to validate its accuracy using relevant clinical scenarios and parameters. Methods. An SSM was built on the basis of segmented 66 CT dataset of the pelvis showing no orthopedic pathology. By adjusting the SSM's so called modes of shape variation it is possible to synthetize new 3D pelvis shapes. By fitting the SSM to intact normal parts of an anatomical structure, missing or pathological regions can be extrapolated plausibly. The validity of the SSM was tested by a Leave-one-out study, whereby one pelvis at a time was removed from the 66 pelvises and was reconstructed using a SSM of the remaining 65 pelvises. The reconstruction accuracy was assessed by comparing each original pelvis with its reconstruction based on the root-mean-square (RMS) surface error and five clinical parameters (center of rotation, acetabulum diameter, inclination, anteversion, and volume). The influence of six different numbers of shape variation modes (reflecting the degrees of freedom of the SSM) and four different mask sizes (reflecting different clinical scenarios) was analyzed. Results. The Leave-one-out study showed that the reconstruction errors decreased when the number of shape variation modes included in the SSM increased from 0 to 20, but remained almost constant for higher numbers of shape variation modes. For the SSM with 20 shape variation modes, the RMS of the reconstruction error increased with increasing mask size, whereas the other parameters only increased from Mask_0 to Mask_1, but remained almost constant for Mask_1, Mask_2 and Mask_3. Median reconstruction errors for Mask_1, Mask_2, and Mask_3 were approximately 3 mm in Center of Rotation (CoR) position, 2 mm in Diameter, 3° in inclination and anteversion, as well as 5 ml in volume. Discussion. This is the first study analyzing and showing the feasibility of a quantitative analysis of acetabular bone defects using a SSM-based reconstruction method in the clinical scenario of a defect or implant in both acetabuli and incomplete CT-scans. Validation results showed acceptable reconstruction accuracy, also for clinical scenarios in which less healthy bone remains. Further studies could apply this method on a larger number of defect pelvises to obtain quantitative measures of acetabular bone defects

Aims

The aim of this study was to optimize screw hole placement in an acetabulum cup implant to improve secondary initial fixation by identifying the region of thickest acetabulum bone. The “scratch fit” of modern acetabular cup implants with highly porous coatings is often adequate for initial fixation in primary total hip arthroplasty. Initial fixation must limit micromotion to acceptable levels to facilitate osseointegration and long term cup stability. Secondary initial fixation can be required in cases with poor bone quality or bone loss and is commonly achieved with bone screws and a cup implant with multiple screw holes. To provide maximum secondary initial fixation, the cup screw holes should be positioned to allow access to the limited region of thick pelvic bone.

An increased prevalence of osteoarthritis (OA) in post-menopausal women has led to the suggestion that hormonal factors may play a role in the pathogenesis. This study aims to examine if undergoing a hysterectomy, both with retention and removal of ovaries, predisposes women to OA and secondly if the development is influenced by hormone replacement therapy (HRT). Statistical shape modelling (SSM) is a method of image analysis allowing for detection of subtle shape variation described by landmark points. Through the generation of linearly independent modes of variation, each image can be described in terms of numerical scores. 149 radiographs from female participants of the Osteoarthritis Initiative (OAI) were examined to compare hip morphology in those who had undergone hysterectomies compared to controls. No differences were observed in BMI, age, height or weight between groups. ANOVA and Games-Howell post-hoc analysis showed that modes 3 and 5 were statistically significant. Lower mode 3 scores were associated with hysterectomy (p=0.019), with narrowing of the femoral neck and increased acetabular coverage. Lower mode 5 scores were associated with hysterectomy and oophorectomy (p=0.049), displaying reduced coverage of the femoral head, superolateral migration of the femoral head and larger greater trochanter. No associations were observed between HRT use and OA. The subtle morphologic features of hip OA present in only hysterectomised women suggests undergoing a hysterectomy may be a predisposing factor and a clinical consideration. The use of HRT was not observed to influence the development of OA and thus cannot be suggested as a protective measure

Objectives

It has been hypothesized that patellofemoral pain, a common knee condition in adolescents and young adults, may be a precursor of degenerative joint changes and may ultimately lead to patellofemoral osteoarthritis. Since both conditions share several mechanical disease characteristics, such as altered contact area between the femur and patella and increased joint stress, we investigated whether these conditions share similar and different shape characteristics of the patella compared with normal controls.

Aims

Restoring the pre-morbid anatomy of the proximal humerus is a goal of anatomical shoulder arthroplasty, but reliance is placed on the surgeon’s experience and on anatomical estimations. The purpose of this study was to present a novel method, ‘Statistical Shape Modelling’, which accurately predicts the pre-morbid proximal humeral anatomy and calculates the 3D geometric parameters needed to restore normal anatomy in patients with severe degenerative osteoarthritis or a fracture of the proximal humerus.

Inter-subject variability is inherently present in patient anatomy and is apparent in differences in shape, size and relative alignment of the bony structures. Understanding the variability in patient anatomy is useful for distinguishing between pathologies and to assist in surgical planning. With the aim of supporting the development of stratified orthopaedic interventions, this work introduces an Articulated Statistical Shape Model (ASSM) of the lower limb. The model captures inter-subject variability and allows reconstructing ‘virtual’ knee joints of the lower limb shape while considering pose. A training dataset consisting of 173 lower limbs from CT scans of 110 subjects (77 male, 33 female) was used to construct the ASSM of the lower limb. Each bone of the lower limb was segmented using ScanIP (Simpleware Ltd., UK), reconstructed into 3D surface meshes, and a SSM of each bone was created. A series of sizing and positioning procedures were carried out to ensure all the lower limbs were in full extension, had the same femoral length and that the femora were aligned with a coincident centre. All articulated lower limbs were represented as: (femur scale factor) × (full extension articulated lower limb + relative transformation of tibia, fibula and patella to femur). Articulated lower limbs were in full extension were used to construct a statistical shape model, representing the variance of lower limb morphology. Relative transformations of the tibia, fibula and patella versus the femur were used to form a statistical pose model. Principal component analysis (PCA) was used to extract the modes of changes in the model. The first 30 modes of the shape model covered 90% of the variance in shape and the first 10 modes of the pose model covered 90% of the pose variance. The first mode captures changes of the femoral CCD angle and the varus/valgus alignment of the knee. The second mode represents the changes in the ratio of femur to tibia length. The third mode reflects change of femoral shaft diameter and patella size. The first mode characterising pose captures the medial/lateral translation between femur and tibia. The second mode represents variation in knee flexion. The third mode reflects variation in tibio-femoral joint space. An articulated statistical modelling approach was developed to characterize inter-subject variability in lower limb morphology for a set of training specimens. This model can generate large sets of lower limbs to systematically study the effect of anatomical variability on joint replacement performance. Moreover, if a series of images of the lower limb during a dynamic activity are used as training data, this method can be applied to analyse variance of lower limb motion across a population

The objective of this study was to assess the reliability and appropriateness of statistical shape modelling for capturing variation in thoracic vertebral anatomy for future use in assessing scoliotic vertebral morphology. Magnetic resonance (MR) images of the thoracic vertebrae were acquired from 20 healthy adults (12 female, 8 male) using a 1.5 T MR scanner (Intera, Philips). A T1 weighted spin-echo sequence (repetition time = 294 ms, echo time = 8 ms, number of signal averages = 3) was used. A set of slices (number = 27, thickness = 1.9 mm, gap = 1.63 mm, pixel size = 0.5 mm) were acquired for each vertebrae, parallel to the mid-transverse plane of the vertebral body. Repeated imaging, including participant repositioning, was performed for T4, T8 and T12 to assess reliability. Landmark points were placed on the images to define anatomical features consisting of the vertebral body and foramen, pedicles, transverse and spinous processes, inferior and superior facets. A statistical shape model was created using software tools developed in MATLAB (R2013a, The MathWorks Inc.). The model was used to determine the mean vertebral shape and ‘modes of variation’ describing patterns in vertebral shape. Analysis of variance was used to test for differences between vertebral levels and subjects and reliability was assessed by determining the within-subject standard deviation from the repeated measurements. The first three modes of variation, shown below (green = mean, red and blue = ±2 standard deviations about the mean), accounted for 70% of the variation in thoracic vertebral shape (Mode 1 = 44%, Mode 2 = 19%, Mode 3 = 4%). Visual inspection indicated that these modes described variation in anatomical features such as the aspect ratio of the vertebral bodies, width and orientation of the pedicles, and position and orientation of the processes and facet points. Variation in shape along the thoracic spine, characterised by these modes of variation, was consistent with that reported in the literature. Significant differences (p< 0.05) between vertebral levels and between some subjects were found. The reliability of the method was good with low relative error (Mode 1 = 5%, Mode 2 = 8%, Mode 3 = 19%). Statistical shape modelling provides a reliable method for characterizing many anatomical features of the thoracic vertebrae in a compact number of variables. This is useful for robustly assessing morphological differences between scoliotic and non-scoliotic vertebrae and in assessing entry points and trajectories for pedicle screws

Summary. This work proposes a novel, automatic method to obtain an anatomical reconstruction for 3D segmented bones with large acetabular defects. The method works through the fitting of a Statistical Shape Model to the non-defect parts of the bone. Introduction. Patient-specific implants can be used to treat patients with large acetabular bone defects (IIa-c, IIIb, Paprosky 1994). These implants require a full 3D preoperative planning that includes segmentation of volumetric images (CT or MRI), extraction of the 3D shape, reconstruction of the bone defect into its anatomic (non-defect) state, design of an implant with a perfect fit and optimal placement of the screws. The anatomic reconstruction of the bone defect will play a key role in diagnosing the amount of bone loss and in the design of the implant. Previous reconstruction methods rely on a healthy contralateral (Gelaude 2007); however this is not always available (e.g. partial scan or implant present). Statistical shape models (SSM) of healthy bones can help to increase the accuracy and usability of the reconstruction and will decrease the manual labor and user dependency. Skadlubowicz (2009) illustrated the use of an SSM to reconstruct pelvic bones with tumor defects; however tumors generally affect a smaller region of the bone so that the reconstruction will be easier than in large acetabular bone defects. Also, the tumor reconstruction method uses 80 manually indicated landmarks, while the proposed method only uses 14. Patients & Methods. CT-scans from subjects with a healthy hemi-pelvis (15 male, 33 female, mean age: 69±20) were used to generate an SSM. The CT-scans were segmented using Mimics (Materialise NV, Belgium) to create a triangulated mesh. Preprocessing of the meshes ensured that the triangulation was smooth and uniform to help solve the corresponding point problem. An algorithm based on Redert (1999) was used to morph the template hemi-pelvis onto each dataset entity, creating a dataset with corresponding points. From this dataset the SSM was calculated using principal component analysis, so that the principal components serve as parameters for the mathematical model of the hemi-pelvis. To fit the SSM to a new defect hemi-pelvis, a matching algorithm was used. The algorithm varies the Principal Components independently optimizing the distance of the non-defect parts of the defect hemi-pelvis to the SSM sample. To validate the reconstruction method, 6 healthy bone meshes were used to generate a synthetic defect in the acetabular region. The original mesh was used as ‘golden standard’ to measure the reconstruction error. To illustrate the clinical use of the reconstruction method, one hemi-pelvis with a substantial defect was reconstructed. Results. The correspondence error for the morphing algorithm was 4.68±0.78 mm. The leave-one-out error for the SSM was 1.30±0.96 mm. The reconstruction error for the non-defect part was 1.44±1.13mm and for the reconstructed part 2.15±1.53mm. Discussion/Conclusion. The proposed method performs comparable to the contralateral method and the tumor reconstruction method, without the need of a healthy contralateral geometry. Consequently, the validation and the clinical illustration show that the proposed method is promising for automatic reconstruction of large acetabular defects

This work was motivated by the need to capture the spectrum of anatomical shape variability rather than relying on analyses of single bones. A novel tool was developed that combines image-based modelling with statistical shape analysis to automatically generate new femur geometries and measure anatomical parameters to capture the variability across the population. To demonstrate the feasibility of the approach, the study used data from 62 Caucasian subjects (31 female and 31 male) aged between 43 and 106 years, with CT voxel size ranging 0.488 × 0.488 × 1.5 mm to 0.7422 × 0.7422 × 0.97 mm.

The scans were divided into female and male subgroups and high-quality subject-specific tetrahedral finite element (FE) meshes resulting from segmented femurs formed the so-called training samples. A source mesh of a segmented femur (25580 nodes, 51156 triangles) from the Visible Human dataset [Spitzer, 1996] was used for elastic surface registration of each considered target male and female subjects, followed by applying a mesh morphing strategy.

To represent the variations in bone morphology across the population, gender-based Statistical Shape Models (SSM) were developed, using Principal Component Analysis. These were then sampled using the principal components required to capture 95% of the variance in each training dataset to generate 1000 new anatomical shapes [Bryan, 2010; Blanc, 2012] and to automatically measure key anatomical parameters known to critically influence the biomechanics after hip replacement (Figure 1).

Analysis of the female and male training datasets revealed the following data for the five considered anatomical parameters: anteversion angle (12.6 ± 6.4° vs. 6.2 ± 7.5°), CCD angle (124.8 ± 4.7° vs. 126.3 ± 4.6°), femoral neck length (48.7 ± 3.8 mm vs. 52 ± 5 mm), femoral head radius (21.5 ± 1.3 mm vs. 24.9 ± 1.5 mm) and femur length (431.0 ± 17.6 mm vs. 474.5 ± 26.3 mm). However, using the SSM generated pool of 1000 femurs, the following data were computed for females against males: anteversion angle (10.5 ± 14.3° vs. 7.6 ± 7.2°), CCD angle (123.9 ± 5.8° vs. 126.7 ± 4°), femoral neck length (46.7 ± 7.7 mm vs. 51.5 ± 4.4 mm), femoral head radius (21.4 ± 1.2 mm vs. 24.9 ± 1.4 mm) and femur length (430.2 ± 16.1 mm vs. 473.9 ± 25.9 mm).

The highest variability was found in the anteversion of the females where the standard deviation in the SSM-based sample was increased to 14.3° from 6.4° in the original training dataset (Figures 2 & 3). The mean values for both females (10.5°) and males (7.6 °) were found close to the values of 10° and 7° reported in [Mishra, 2009] in 31 females and 112 males with a [2°, 25°] and [2°, 35°] range, respectively.

Femoral neck length of the female (male) subjects was 47.3 ± 6.2 mm (51.8 ± 4.1 mm) compared to 48.7 ± 3.8 mm (52 ± 5 mm) in the training dataset and 63.65 ± 5.15 mm in [Blanc, 2012] with n = 142, 54% female, 46% male and a [50.32–75.50 mm] range. For the measured CCD angle in both female (123.9 ± 5.8°) and male (126.7 ± 4°) subjects, a good correlation was found with reported values of 128.4 ± 4.75° [Atilla, 2007], 124.7 ± 7.4° [Noble, 1988] and 129.82 + 5.37° [Blanc, 2012].

In conclusion, the present study demonstrates that the proposed methodology based on gender-specific statistical shape modelling can be a valuable tool for automatically generating a large specific population of femurs to support implant design and planning of femoral reconstructive surgery.

With the increasing use of 3D medical imaging, it is possible to analyze 3D patient anatomy to extract features, trends and population specific shape information. This is applied to the development of ‘standard implants’ targeted to specific population groups. INTRODUCTION. Human beings are diverse in their physical makeup while implants are often designed based on some key measurements taken from the literature or a limited sampling of patient data. The different implant sizes are often scaled versions of the ‘average’ implant, although in reality, the shape of anatomy changes as a function of the size of patient. The implant designs are often developed based on a certain demographic and ethnicity and then, simply applied to others, which can result in poor design fitment [1]. Today, with the increasing use of 3D medical imaging (e.g. CT or MRI), it is possible to analyze 3D patient anatomy to extract features, trends and population specific shape information. This can be applied to the development of new ‘standard implants’ targeted to a specific population group [2]. PATIENTS & METHODS. Our population analysis was performed by creating a Statistical Shape Model (SSM) [3] of the dataset. In this study, 40 full Chinese cadaver femurs and 100 full Caucasian cadaver femurs were segmented from CT scans using Mimics®. Two different SSMs, specific to each population, were built using in-house software tools. These SSMs were validated using leave-one-out experiments, and then analyzed and compared in order to enhance the two population shape differences. RESULTS. An SSM is typically represented by an average model and a few independent modes of variation that capture most of the inherent variations in the data. Based on these main modes of variations, the shape features, e.g. length, thickness, curvature neck angle and femoral version, presenting largest variations were determined, and correlations between these features were calculated. Figure 1 represents the Caucasian and Chinese average models, and shows that while the length of these two models was significantly different, the AP and ML dimensions were similar, indicating a difference of morphology (other than a scaling) between the two populations. Figure 2 represents the first mode of variation that illustrates the variation of Chinese femur shape with size. As an example, the neck angle increases of 26° with an increase of 139 mm in femur length, indicative of the effect of changes in loading conditions on geometry as a function of size. CONCLUSION. The advantage of using more advanced statistical analyses is that the 3D data are probed in an unbiased fashion, allowing the most important parameters of variation to be determined. These analyses are thus particularly effective to compare different populations, to evaluate how well existing implant designs fit specific populations, and to highlight the design parameters that need to be adapted for good fitment of specific populations

INTRODUCTION. Understanding bone morphology is essential for successful computer assisted orthopaedic surgery, where definition of normal anatomical variations and abnormal morphological patterns can assist in surgical planning and evaluation of outcomes. The proximal femur was the anatomical target of the study described here. Orthopaedic surgeons have studied femoral geometry using 2D and 3D radiographs for precise fit of bone-implant with biological fixation. METHOD. The use of a Statistical Shape Model (SSM) is a promising venue for understanding bone morphologies and for deriving generic description of normal anatomy. A SSM uses measures of statistics on geometrical descriptions over a population. Current SSM construction methods, based on Principal Component Analysis (PCA), assume that shape morphologies can be modeled by pure point translations. Complicated morphologies, such as the femoral head-neck junction that has non-rigid components, can be poorly explained by PCA. In this work, we showed that PCA was impotent for processing complex deformations of the proximal femur and propose in its place our Principal Tangent Component (PTC) analysis. The new method used the Lie algebra of affine transformation matrices to perform simple computations, in tangent spaces, that corresponded to complex deformations on the data manifold. RESULTS. Both PCA and PTC were applied to the proximal femur dataset, from which selected femurs were reconstructed using the accumulation of components. PCA was deemed to have failed to reconstruct the surfaces because it required 65 components to achieve high coverage of the dataset. An important observation was that the head-neck junction was the most difficult section in the femur, requiring more components than other anatomical regions to reconstruct. This finding is consistent with the surgical observation that deformations occur in this junction for abnormal hip morphologies. PTC was successful in recovering 100% of the medical data using the only the first 5 components. We note that the encoding of deformation in PTC accounting for the performance increase. PTC outperformed PCA on the dataset in descriptive compactness. CONCLUSION. A standard SSM construction method was not adequate for analysing proximal femur surfaces because it could not easily model the complexity of non-rigid deformations at the head-neck junction. Principal tangent components, a novel method for using exponential maps on manifolds, accurately reconstructed the anatomical surfaces with very few components. Future work may include extending these concepts to describe joint diseases based on the shape of surfaces derived from volumetric data, such as CT or MRI. In conclusion, we have shown that differential geometry may be provide new insights to computational anatomy applications