Lorenz Fieten

Anatomically constrained deformation for design of cranial implant: methodology and validation

We present a new approach for cranial implant design which uses anatomical constrained deformation based on reference models. The methodological framework contains three steps: patient-specific generation of the reference model containing the anatomical constraints about the skull shape; determination of the spatial correspondence between the patient skull and the reference model by 3D matching; adaptive deformation of the fragment on the reference model corresponding to the defect area on the patient skull for implant design. The proposed method was validated by simulating the reconstruction of artificially generated defects on healthy skulls. The validation results show that this approach can generate implant geometry very fast and with satisfactory quality. This approach also outperforms the surface interpolation method in reconstructing cranial defects.

Surface-based determination of the pelvic coordinate system

In total hip replacement (THR) one technical factor influencing the risk of dislocation is cup orientation. Computer-assisted surgery systems allow for cup navigation in anatomy-based reference frames. The pelvic coordinate system most used for cup navigation in THR is based on the mid-sagittal plane (MSP) and the anterior pelvic plane (APP). From a geometrical point of view, the MSP can be considered as a mirror plane, whereas the APP can be considered as a tangent plane comprising the anterior superior iliac spines (ASIS) and the pubic tubercles. In most systems relying on the pelvic coordinate system, the most anterior points of the ASIS and the pubic tubercles are selected manually. As manual selection of landmark points is a tedious, time-consuming and error-prone task, a surface-based approach for combined MSP and APP computation is presented in this paper: Homologous points defining the MSP and the landmark points defining the APP are selected automatically from surface patches. It is investigated how MSP computation can benefit from APP computation and vice versa, and clinical perspectives of combined MSP and APP computation are discussed. Experimental results on computed tomography data show that the surface-based approach can improve accuracy.

Automatic extraction of the mid-sagittal plane using an ICP variant

Precise knowledge of the mid-sagittal plane is important for the assessment and correction of several deformities. Furthermore, the mid-sagittal plane can be used for the definition of standardized coordinate systems such as pelvis or skull coordinate systems. A popular approach for mid-sagittal plane computation is based on the selection of anatomical landmarks located either directly on the plane or symmetrically to it. However, the manual selection of landmarks is a tedious, time-consuming and error-prone task, which requires great care. In order to overcome this drawback, previously it was suggested to use the iterative closest point (ICP) algorithm: After an initial mirroring of the data points on a default mirror plane, the mirrored data points should be registered iteratively to the model points using rigid transforms. Finally, a reflection transform approximating the cumulative transform could be extracted. In this work, we present an ICP variant for the iterative optimization of the reflection parameters. It is based on a closed-form solution to the least-squares problem of matching data points to model points using a reflection. In experiments on CT pelvis and skull datasets our method showed a better ability to match homologous areas.

Application and evaluation of biomechanical models and scores for the planning of total hip arthroplasty

Intimate knowledge of the biomechanics of a given individual hip joint provides a potential advantage during the planning of total hip arthroplasty, and would thus have a positive influence over the outcome of such an intervention. In current clinical practise, the surgical planning is based solely on the status of the individual hip and its radiographic appearance. However, additional information could be gathered from the radiography to be used as input data for biomechanical models aimed at calculating the resultant force FR within the hip joint. An investigation of the biomechanical models by Pauwels, Debrunner and Iglič was performed, where the magnitude of FR calculated by the models showed a favourable comparison to the in-vivo data from instrumented prostheses by Bergmann. The Blumentritt model returned abnormally high results. The computational results showed large variations for FR orientation, which tends to depend more on the model used than on patient-specific parameters. Furthermore, a discrepancy was found between the data gathered from instrumented prostheses and the Standing Human Model within the ‘AnyBody Modeling System™’ software by AnyBody Tech. Additionally, the variations in inter-rater and intra-rater errors made while localizing radiographic landmarks were analysed with respect to their influence on Babisch-Layher-Blumentritt (BLB)-scoring using the Blumentritt hip model.

An ICP variant with anisotropic weighting to accommodate measurement errors in A-Mode ultrasound-based registration

Abstract In computer-assisted surgery, navigation based on pre-operative images and intra-operative tracking requires fusion of data from different coordinate systems. Intra-operative registration is used to determine the spatial relationship between these coordinate systems. Feature-based registration methods rely on reference structures localized both in the pre-operative images and in the surgical site. Optically tracked A-Mode ultrasound (US) allows for non-invasive and cost-efficient digitization of bone surface points needed as input data for registration algorithms. It is especially attractive in combination with surface-based registration algorithms, such as the Iterative Closest Point algorithm and its variants, because they automatically localize the corresponding points, which are covered by soft tissue and, hence, not visible in the site. However, as transcutaneous palpation relies on some assumptions, e.g., regarding the average speed of sound of the scanned soft tissue, that are only partly justified in practice, errors occur that make transcutaneous palpation less reliable than direct palpation. Furthermore, optical tracking causes errors that have to be considered, especially if a so-called dynamic reference base is attached to the patient. The present work investigates how to reduce the effect of important error sources in A-Mode US-based registration. The major contributions are techniques for application-specific error modeling and new methods for surface-based registration with anisotropic weighting. This includes a Newton-like optimization scheme for point-to-point registration and a modified kd-tree-based algorithm for closest point computation. Various combinations of registration algorithms and modeling techniques are tested in a simulation study addressing periacetabular osteotomy, and it is clearly demonstrated that standard methods are not recommendable. On the contrary, the new algorithms allow for a substantial increase in registration accuracy and encourage further research in that field.

Integration of model-based weighting into an ICP variant to account for measurement errors in intra-operative A-Mode ultrasound-based registration

This paper addresses error modeling in A-Mode ultrasound- (US-) based registration and integration of model-based weighting into the Random-ICP (R-ICP) algorithm. The R-ICP is a variant of the Iterative Closest Point (ICP) algorithm, and it was suggested for surface-based registration using A-Mode US in the context of skull surgery. In that application area the R-ICP could yield high accuracy even in case of a small number of data points and a very inaccurate user-interactive pre-registration. However, it cannot cope with unequal point uncertainty, which is an important drawback in the context of hip surgery: Uncertainty about the average speed of sound is an error source, whose impact on the registration accuracy increases with the thickness of the scanned soft tissue. It can, therefore, lead to considerable localization errors if a thick soft tissue layer is scanned, and it might vary a lot from data point to data point as the soft tissue thickness is inhomogeneous. The present work investigates how to account for this error source considering also other error sources such as the establishment of point correspondences. Simulation results show that registration accuracy can be substantially improved when model-based weighting is integrated into the R-ICP.

Ultrasound-based registration of the pelvic coordinate system in the lateral position using symmetry for total hip replacement

Abstract In total hip replacement, patient placement in the lateral position is preferred by many surgeons. However, it complicates registration of the so-called pelvic coordinate system that is the standard reference for surgeons to measure cup orientation. This coordinate system comprises the anterior pelvic plane and the mid-sagittal plane, and it is conventionally registered on the basis of bony anatomical landmarks including the left and the right anterior superior iliac spine (ASIS). Ultrasound has been suggested for transcutaneous palpation of the bone surface. The difficulty in registration of the pelvic coordinate system with the patient in the lateral position arises because the contralateral ASIS cannot be reached easily by a mechanical pointer and is not accessible by means of an ultrasound probe. Up to now, methods to compensate for these missing data have not been used in clinical routine. This paper describes a new ultrasound-based method that requires neither image segmentation nor statistical shape models and uses symmetry to approximate the position of the contralateral ASIS. A detailed analysis based on computed tomography data of 60 hips following a cadaver study is presented to show the ability of our method to reliably reconstruct the pelvic coordinate system. The median angles between ground truth planes and the “reconstructed” planes were <2°. By choosing a standard cup orientation w.r.t. the “reconstructed” planes, the median abduction and version angle errors were <2°, too.

System architecture of ultrasound-based real-time bone thickness determination for synergistically operated cutting tools

Cutting bone is one of the most important tasks in many surgical interventions (e.g. craniotomy, sternotomy). It is often performed close to critical structures such as central nervous structures and vessels with an inherent high risk of serious damage. One solution for cutting the bone safely is a hand-held synergistic bone cutting tool using a soft-tissue preserving saw and a sensor-controlled cutting tool, which allows an automatic adjustment of the cutting depth with respect to the local bone thickness. In this contribution an ultrasound-based concept for real-time bone thickness measurement using coded excitation and pulse compression is introduced. A first laboratory system has been implemented and evaluated in both transmission and impulse-echo mode using high attenuating bone mimicking material (40 dB/cm at 5 MHz) within the signal path. The first results have demonstrated the superiority of the system in terms of penetration depth compared to single burst technology. Furthermore, with a runtime of 11 ms for the entire signal processing chain, the system satisfies the real-time requirements of the hand-held sawing tool.

A Symmetry-Based Approach for Non-invasive Determination of the Pelvic Coordinate System Using Ultrasound with the Patient in the Lateral Position

The pelvic coordinate system (CS) plays an important role in total hip replacement (THR). It is defined by the mid-sagittal plane (MSP) and the so-called anterior pelvic plane (APP) comprising the anterior superior iliac spines (ASISs) and the pubic tubercles (PTs).