Heiko Ramm

Fast Generation of Virtual X-ray Images for Reconstruction of 3D Anatomy

We propose a novel GPU-based approach to render virtual X-ray projections of deformable tetrahedral meshes. These meshes represent the shape and the internal density distribution of a particular anatomical structure and are derived from statistical shape and intensity models (SSIMs). We apply our method to improve the geometric reconstruction of 3D anatomy (e.g. pelvic bone) from 2D X-ray images. For that purpose, shape and density of a tetrahedral mesh are varied and virtual X-ray projections are generated within an optimization process until the similarity between the computed virtual X-ray and the respective anatomy depicted in a given clinical X-ray is maximized. The OpenGL implementation presented in this work deforms and projects tetrahedral meshes of high resolution (200.000+ tetrahedra) at interactive rates. It generates virtual X-rays that accurately depict the density distribution of an anatomy of interest. Compared to existing methods that accumulate X-ray attenuation in deformable meshes, our novel approach significantly boosts the deformation/projection performance. The proposed projection algorithm scales better with respect to mesh resolution and complexity of the density distribution, and the combined deformation and projection on the GPU scales better with respect to the number of deformation parameters. The gain in performance allows for a larger number of cycles in the optimization process. Consequently, it reduces the risk of being stuck in a local optimum. We believe that our approach will improve treatments in orthopedics, where 3D anatomical information is essential.

Radiological method for measuring patellofemoral tracking and tibiofemoral kinematics before and after total knee replacement

Objectives Numerous complications following total knee replacement (TKR) relate to the patellofemoral (PF) joint, including pain and patellar maltracking, yet the options for in vivo imaging of the PF joint are limited, especially after TKR. We propose a novel sequential biplane radiological method that permits accurate tracking of the PF and tibiofemoral (TF) joints throughout the range of movement under weightbearing, and test it in knees pre- and post-arthroplasty. Methods A total of three knees with end-stage osteoarthritis and three knees that had undergone TKR at more than one year’s follow-up were investigated. In each knee, sequential biplane radiological images were acquired from the sagittal direction (i.e. horizontal X-ray source and 10° below horizontal) for a sequence of eight flexion angles. Three-dimensional implant or bone models were matched to the biplane images to compute the six degrees of freedom of PF tracking and TF kinematics, and other clinical measures. Results The mean and standard deviation for the six degrees of freedom of PF tracking and TF kinematics were computed. TF and PF kinematics were highly accurate (< 0.9 mm, < 0.6°) and repeatable. Conclusions The developed method permitted measuring of in vivo PF tracking and TF kinematics before and after TKR throughout the range of movement. This method could be a useful tool for investigating differences between cohorts of patients (e.g., with and without pain) impacting clinical decision-making regarding surgical technique, revision surgery or implant design.

A computed tomographic data-based vibrant bonebridge visualization tool

Abstract Aim Information about the temporal bone size and variations of anatomical structures are crucial for a safe positioning of the Vibrant Bonebridge B-FMT. A radiological based preoperative planning of the surgical procedure decreases the surgical time and minimizes the risk of complications. Materials and methods We developed a software tool, which allows a catch up of foreign DICOM data based CT temporal bone scans. The individual CT scan is transmitted into a 3D reconstructed pattern of the temporal bone. In this 3D reconstruction the individually favored position of the B- FMT should be found. Results The software allows a determination of a safe B-FMT position by identifying the individual relation of middle fossa, jugular bulb and external auditory canal. Skull thickness and screw length are contained parameters for the surgical planning. Conclusion An easy to handle software tool allows a radiologically data based safe and fast surgical positioning of the B-FMT.

Kinematic Differences Between Gender Specific and Traditional Knee Implants

In the ongoing debate about gender-specific (GS) vs. traditional knee implants, there is limited information about patella-specific outcomes. GS femoral component features should provide better patellar tracking, but techniques have not existed previously to test this accurately. Using novel computed tomography and radiography imaging protocols, 15 GS knees were compared to 10 traditional knees, for the 6 degrees of freedom of the patellofemoral and tibiofemoral joints throughout the range of motion, plus other geometric measures and quality of life (QOL). Significant differences were found for patellar medial/lateral shift, where the patella was shifted more laterally for the GS femoral component. Neither group demonstrated patellar maltracking. There were no other significant differences in this well-functioning group.

Pelvic tilt compensates for increased acetabular anteversion

PurposePelvic tilt determines functional orientation of the acetabulum. In this study, we investigated the interaction of pelvic tilt and functional acetabular anteversion (AA) in supine position.MethodsPelvic tilt and AA of 138 individuals were measured by computed tomography (CT). AA was calculated in relation to the anterior pelvic plane (APP) and relative to the table plane. We analysed these parameters for gender-specific and age-related differences.ResultsThe mean pelvic tilt was -0.1 ± 5.5°. Pelvic sagittal rotation displayed no gender nor age related differences. Females showed higher angles of AA compared with males (20.0° vs 17.2°, p < 0.001; AA relative to the APP). Anterior tilting of the pelvis positively correlated with AA and individuals with high AA had a higher anterior pelvic tilt compared with those with low AA (p < 0.0001; AA relative to the APP).ConclusionsAA has to be calculated regarding pelvic sagittal rotation for correct acetabular orientation. Pelvic tilt is dependent on acetabular orientation and compensates for increased AA.

[Development of a computed tomography data-based Vibrant Bonebridge viewer].

BACKGROUND Because of the anatomy of the mastoid and the size of the actuator, positioning of the Vibrant Bonebridge B-FMT can be difficult without prior evaluation of the individual computed tomography (CT) scan of the temporal bone. Development of a user-friendly CT data viewer to enable positioning of the B-FMT in the temporal bone model, whilst identifying individual, potential anatomic conflicts and offering possible solutions could provide a useful tool for preoperative positioning. OBJECTIVES Aim of the study was to define the requirements of a Vibrant Bonebridge viewer and construct a prototype. MATERIALS AND METHODS Based on a ZIBAmira software version and inclusion of a B-FMT model upon creation of a model of the temporal bone-which allows the intuitive estimation of individual, anatomic conflicts-a Vibrant Bonebridge viewer was constructed. RESULTS The segmentation time of the individual digital imaging and communications in medicine (DICOM) data set is about 5 min. Positioning within the individual three-dimensional temporal bone model allows quantitative and qualitative estimation of conflicts (sigmoid sinus, middle cranial fossa) and determination of a preferred position for the B-FMT. Lifting of the B-FMT can be simulated with the help of a virtual washer. CONCLUSION The Vibrant Bonebridge viewer reliably allows simulation of B-FMT positioning. The clinical value of the viewer still has to be evaluated.

Entwicklung eines CT-Daten-basierten Vibrant-Bonebridge-Viewers

BACKGROUND Because of the anatomy of the mastoid and the size of the actuator, positioning of the Vibrant Bonebridge B-FMT can be difficult without prior evaluation of the individual computed tomography (CT) scan of the temporal bone. Development of a user-friendly CT data viewer to enable positioning of the B-FMT in the temporal bone model, whilst identifying individual, potential anatomic conflicts and offering possible solutions could provide a useful tool for preoperative positioning. OBJECTIVES Aim of the study was to define the requirements of a Vibrant Bonebridge viewer and construct a prototype. MATERIALS AND METHODS Based on a ZIBAmira software version and inclusion of a B-FMT model upon creation of a model of the temporal bone-which allows the intuitive estimation of individual, anatomic conflicts-a Vibrant Bonebridge viewer was constructed. RESULTS The segmentation time of the individual digital imaging and communications in medicine (DICOM) data set is about 5 min. Positioning within the individual three-dimensional temporal bone model allows quantitative and qualitative estimation of conflicts (sigmoid sinus, middle cranial fossa) and determination of a preferred position for the B-FMT. Lifting of the B-FMT can be simulated with the help of a virtual washer. CONCLUSION The Vibrant Bonebridge viewer reliably allows simulation of B-FMT positioning. The clinical value of the viewer still has to be evaluated.

Feature-Preserving, multi-material mesh generation using hierarchical oracles

This paper presents a method for meshing multi-material domains with additional features curves. This requirement arises for instance in situations where smooth objects (e.g. anatomical structures) are combined with technical objects (e.g. implants, surgical screws). Our approach avoids the tedious process of generating a single consistent input surface by means of an implicit representation, called oracle. Input features are preserved in the output mesh and termination of the algorithm is proved for certain conditions. We show that our method provides good element quality while at the same time keeping the number of elements in the output mesh low.

Changes in Knee Shape and Geometry Resulting from Total Knee Arthroplasty

Changes in knee shape and geometry resulting from total knee arthroplasty can affect patients in numerous important ways: pain, function, stability, range of motion, and kinematics. Quantitative data concerning these changes have not been previously available, to our knowledge, yet are essential to understand individual experiences of total knee arthroplasty and thereby improve outcomes for all patients. The limiting factor has been the challenge of accurately measuring these changes. Our study objective was to develop a conceptual framework and analysis method to investigate changes in knee shape and geometry, and prospectively apply it to a sample total knee arthroplasty population. Using clinically available computed tomography and radiography imaging systems, the three-dimensional knee shape and geometry of nine patients (eight varus and one valgus) were compared before and after total knee arthroplasty. All patients had largely good outcomes after their total knee arthroplasty. Knee shape changed both visually and numerically. On average, the distal condyles were slightly higher medially and lower laterally (range: +4.5 mm to −4.4 mm), the posterior condyles extended farther out medially but not laterally (range: +1.8 to −6.4 mm), patellofemoral distance increased throughout flexion by 1.8–3.5 mm, and patellar thickness alone increased by 2.9 mm (range: 0.7–5.2 mm). External femoral rotation differed preop and postop. Joint line distance, taking cartilage into account, changed by +0.7 to −1.5 mm on average throughout flexion. Important differences in shape and geometry were seen between pre-total knee arthroplasty and post-total knee arthroplasty knees. While this is qualitatively known, this is the first study to report it quantitatively, an important precursor to identifying the reasons for the poor outcome of some patients. Using the developed protocol and visualization techniques to compare patients with good versus poor clinical outcomes could lead to changes in implant design, implant selection, component positioning, and surgical technique. Recommendations based on this sample population are provided. Intraoperative and postoperative feedback could ultimately improve patient satisfaction.

Influence of pelvic tilt on functional acetabular orientation

BACKGROUND Pelvic tilt influences acetabular orientation (AO). Anatomical AO can be measured in relation to the anterior pelvic plane (APP), functional AO can be calculated relative to tables plane. OBJECTIVE To assess to what extent functional AO is determined by pelvic tilt and if APP and table plane give equal information for correct AO. METHODS AO was evaluated by computed tomography (CT) scans of 138 patients. Pelvic tilt, anatomical and functional AO were measured, differences between the two reference planes were calculated. RESULTS Anatomical and functional acetabular anteversion (AA) were found to be different in 21% of individuals with an enhanced extent of pelvic tilt. Functional AA was increased compared to anatomical AA at high posterior pelvic tilt (p < 0.001). Enlarged anterior tilting of the pelvis reduced APP-related AA (p < 0.002). Anatomical AA positively correlated with pelvic tilt, particularly in females (p < 0.01, correlation coefficient = 0.698, R2 = 0.523). CONCLUSIONS APP and table plane do not provide equal information about AO at enhanced pelvic tilt. Functional orientation of the acetabulum is dependent on pelvic tilt, which itself is influenced by anatomical AA and should therefore be analyzed for precise AO.