Ehsan Basafa

Intraoperative Image-based Multiview 2D/3D Registration for Image-Guided Orthopaedic Surgery: Incorporation of Fiducial-Based C-Arm Tracking and GPU-Acceleration

Intraoperative patient registration may significantly affect the outcome of image-guided surgery (IGS). Image-based registration approaches have several advantages over the currently dominant point-based direct contact methods and are used in some industry solutions in image-guided radiation therapy with fixed X-ray gantries. However, technical challenges including geometric calibration and computational cost have precluded their use with mobile C-arms for IGS. We propose a 2D/3D registration framework for intraoperative patient registration using a conventional mobile X-ray imager combining fiducial-based C-arm tracking and graphics processing unit (GPU)-acceleration. The two-stage framework 1) acquires X-ray images and estimates relative pose between the images using a custom-made in-image fiducial, and 2) estimates the patient pose using intensity-based 2D/3D registration. Experimental validations using a publicly available gold standard dataset, a plastic bone phantom and cadaveric specimens have been conducted. The mean target registration error (mTRE) was 0.34±0.04 mm (success rate: 100%, registration time: 14.2 s) for the phantom with two images 90° apart, and 0.99±0.41 mm (81%, 16.3 s) for the cadaveric specimen with images 58.5° apart. The experimental results showed the feasibility of the proposed registration framework as a practical alternative for IGS routines.

Preliminary development of a workstation for craniomaxillofacial surgical procedures: introducing a computer-assisted planning and execution system.

IntroductionFacial transplantation represents one of the most complicated scenarios in craniofacial surgery because of skeletal, aesthetic, and dental discrepancies between donor and recipient. However, standard off-the-shelf vendor computer-assisted surgery systems may not provide custom features to mitigate the increased complexity of this particular procedure. We propose to develop a computer-assisted surgery solution customized for preoperative planning, intraoperative navigation including cutting guides, and dynamic, instantaneous feedback of cephalometric measurements/angles as needed for facial transplantation and other related craniomaxillofacial procedures. MethodsWe developed the Computer-Assisted Planning and Execution (CAPE) workstation to assist with planning and execution of facial transplantation. Preoperative maxillofacial computed tomography (CT) scans were obtained on 4 size-mismatched miniature swine encompassing 2 live face-jaw-teeth transplants. The system was tested in a laboratory setting using plastic models of mismatched swine, after which the system was used in 2 live swine transplants. Postoperative CT imaging was obtained and compared with the preoperative plan and intraoperative measures from the CAPE workstation for both transplants. ResultsPlastic model tests familiarized the team with the CAPE workstation and identified several defects in the workflow. Live swine surgeries demonstrated utility of the CAPE system in the operating room, showing submillimeter registration error of 0.6 ± 0.24 mm and promising qualitative comparisons between intraoperative data and postoperative CT imaging. ConclusionsThe initial development of the CAPE workstation demonstrated that integration of computer planning and intraoperative navigation for facial transplantation are possible with submillimeter accuracy. This approach can potentially improve preoperative planning, allowing ideal donor-recipient matching despite significant size mismatch, and accurate surgical execution for numerous types of craniofacial and orthognathic surgical procedures.

Patient-specific finite element modeling for femoral bone augmentation

The aim of this study was to provide a fast and accurate finite element (FE) modeling scheme for predicting bone stiffness and strength suitable for use within the framework of a computer-assisted osteoporotic femoral bone augmentation surgery system. The key parts of the system, i.e. preoperative planning and intraoperative assessment of the augmentation, demand the finite element model to be solved and analyzed rapidly. Available CT scans and mechanical testing results from nine pairs of osteoporotic femur bones, with one specimen from each pair augmented by polymethylmethacrylate (PMMA) bone cement, were used to create FE models and compare the results with experiments. Correlation values of R(2)=0.72-0.95 were observed between the experiments and FEA results which, combined with the fast model convergence (~3 min for ~250,000 degrees of freedom), makes the presented modeling approach a promising candidate for the intended application of preoperative planning and intraoperative assessment of bone augmentation surgery.

Subject-specific planning of femoroplasty: An experimental verification study

The risk of osteoporotic hip fractures may be reduced by augmenting susceptible femora with acrylic polymethylmethacrylate (PMMA) bone cement. Grossly filling the proximal femur with PMMA has shown promise, but the augmented bones can suffer from thermal necrosis or cement leakage, among other side effects. We hypothesized that, using subject-specific planning and computer-assisted augmentation, we can minimize cement volume while increasing bone strength and reducing the risk of fracture. We mechanically tested eight pairs of osteoporotic femora, after augmenting one from each pair following patient-specific planning reported earlier, which optimized cement distribution and strength increase. An average of 9.5(±1.7) ml of cement was injected in the augmented set. Augmentation significantly (P<0.05) increased the yield load by 33%, maximum load by 30%, yield energy by 118%, and maximum energy by 94% relative to the non-augmented controls. Also predicted yield loads correlated well (R(2)=0.74) with the experiments and, for augmented specimens, cement profiles were predicted with an average surface error of <2 mm, further validating our simulation techniques. Results of the current study suggest that subject-specific planning of femoroplasty reduces the risk of hip fracture while minimizing the amount of cement required.

Control of the Coupled Motion of a 6 DoF Robotic Arm and a Continuum Manipulator for the Treatment of Pelvis Osteolysis

The paper addresses the coupled motion of a 6 degree of freedom robot and a snake-like dexterous manipulator (SDM) designed for the treatment of bone defects behind the implant during total hip arthroplasty revision surgery. We have formulated the problem as a weighted, multi-objective constraint, linear optimization. A remote center of motion (RCM) acts as a virtual constraint for the robot. The coupled robot kinematics does not assume piecewise-constant curvature for the SDM. We have evaluated our method by simulating the coupled system inside a potential lesion area.

Optimizing Hybrid Occlusion in Face-Jaw-Teeth Transplantation: A Preliminary Assessment of Real-Time Cephalometry as Part of the Computer-Assisted Planning and Execution Workstation for Craniomaxillofacial Surgery.

Background: The aesthetic and functional outcomes surrounding Le Fort–based, face-jaw-teeth transplantation have been suboptimal, often leading to posttransplant class II/III skeletal profiles, palatal defects, and “hybrid malocclusion.” Therefore, a novel technology—real-time cephalometry—was developed to provide the surgical team instantaneous, intraoperative knowledge of three-dimensional dentoskeletal parameters. Methods: Mock face-jaw-teeth transplantation operations were performed on plastic and cadaveric human donor/recipient pairs (n = 2). Preoperatively, cephalometric landmarks were identified on donor/recipient skeletons using segmented computed tomographic scans. The computer-assisted planning and execution workstation tracked the position of the donor face-jaw-teeth segment in real time during the placement/inset onto recipient, reporting pertinent hybrid cephalometric parameters from any movement of donor tissue. The intraoperative data measured through real-time cephalometry were compared to posttransplant measurements for accuracy assessment. In addition, posttransplant cephalometric relationships were compared to planned outcomes to determine face-jaw-teeth transplantation success. Results: Compared with postoperative data, the real-time cephalometry–calculated intraoperative measurement errors were 1.37 ± 1.11 mm and 0.45 ± 0.28 degrees for the plastic skull and 2.99 ± 2.24 mm and 2.63 ± 1.33 degrees for the human cadaver experiments. These results were comparable to the posttransplant relations to planned outcome (human cadaver experiment, 1.39 ± 1.81 mm and 2.18 ± 1.88 degrees; plastic skull experiment, 1.06 ± 0.63 mm and 0.53 ± 0.39 degrees). Conclusion: Based on this preliminary testing, real-time cephalometry may be a valuable adjunct for adjusting and measuring “hybrid occlusion” in face-jaw-teeth transplantation and other orthognathic surgical procedures.

Modeling the Biomechanics of Swine Mastication – An Inverse Dynamics Approach

A novel reconstructive alternative for patients with severe facial structural deformity is Le Fort-based, face-jaw-teeth transplantation (FJTT). To date, however, only ten surgeries have included underlying skeletal and jaw-teeth components, all yielding sub-optimal results and a need for a subsequent revision surgery, due to size mismatch and lack of precise planning. Numerous studies have proven swine to be appropriate candidates for translational studies including pre-operative planning of transplantation. An important aspect of planning FJTT is determining the optimal muscle attachment sites on the recipients jaw, which requires a clear understanding of mastication and bite mechanics in relation to the new donated upper and/or lower jaw. A segmented CT scan coupled with data taken from literature defined a biomechanical model of mandible and jaw muscles of a swine. The model was driven using tracked motion and external force data of one cycle of chewing published earlier, and predicted the muscle activation patterns as well as temporomandibular joint (TMJ) reaction forces and condylar motions. Two methods, polynomial and min/max optimization, were used for solving the muscle recruitment problem. Similar performances were observed between the two methods. On average, there was a mean absolute error (MAE) of <0.08 between the predicted and measured activation levels of all muscles, and an MAE of <7 N for TMJ reaction forces. Simulated activations qualitatively followed the same patterns as the reference data and there was very good agreement for simulated TMJ forces. The polynomial optimization produced a smoother output, suggesting that it is more suitable for studying such motions. Average MAE for condylar motion was 1.2mm, which reduced to 0.37 mm when the input incisor motion was scaled to reflect the possible size mismatch between the current and original swine models. Results support the hypothesis that the model can be used for planning of facial transplantation.

Subject-Specific Planning of Femoroplasty: A Combined Evolutionary Optimization and Particle Diffusion Model Approach

A potential effective treatment for prevention of osteoporotic hip fractures is augmentation of the mechanical properties of the femur by injecting it with agents such as (PMMA) bone cement - femoroplasty. The operation, however, is only in research stage and can benefit substantially from computer planning and optimization. We report the results of computational planning and optimization of the procedure for biomechanical evaluation. An evolutionary optimization method was used to optimally place the cement in finite element (FE) models of seven osteoporotic bone specimens. The optimization, with some inter-specimen variations, suggested that areas close to the cortex in the superior and inferior of the neck and supero-lateral aspect of the greater trochanter will benefit from augmentation. We then used a particle-based model for bone cement diffusion simulation to match the optimized pattern, taking into account the limitations of the actual surgery, including limited volume of injection to prevent thermal necrosis. Simulations showed that the yield load can be significantly increased by more than 30%, using only 9 ml of bone cement. This increase is comparable to previous literature reports where gross filling of the bone was employed instead, using more than 40 ml of cement. These findings, along with the differences in the optimized plans between specimens, emphasize the need for subject-specific models for effective planning of femoral augmentation.

A Particle Model for Prediction of Cement Infiltration of Cancellous Bone in Osteoporotic Bone Augmentation

Femoroplasty is a potential preventive treatment for osteoporotic hip fractures. It involves augmenting mechanical properties of the femur by injecting Polymethylmethacrylate (PMMA) bone cement. To reduce the risks involved and maximize the outcome, however, the procedure needs to be carefully planned and executed. An important part of the planning system is predicting infiltration of cement into the porous medium of cancellous bone. We used the method of Smoothed Particle Hydrodynamics (SPH) to model the flow of PMMA inside porous media. We modified the standard formulation of SPH to incorporate the extreme viscosities associated with bone cement. Darcy creeping flow of fluids through isotropic porous media was simulated and the results were compared with those reported in the literature. Further validation involved injecting PMMA cement inside porous foam blocks — osteoporotic cancellous bone surrogates — and simulating the injections using our proposed SPH model. Millimeter accuracy was obtained in comparing the simulated and actual cement shapes. Also, strong correlations were found between the simulated and the experimental data of spreading distance (R2 = 0.86) and normalized pressure (R2 = 0.90). Results suggest that the proposed model is suitable for use in an osteoporotic femoral augmentation planning framework.

Ancillary procedures necessary for translational research in experimental craniomaxillofacial surgery.

Introduction Swine are often regarded as having analogous facial skeletons to humans and therefore serve as an ideal animal model for translational investigation. However, there is a dearth of literature describing the pertinent ancillary procedures required for craniomaxillofacial research. With this in mind, our objective was to evaluate all necessary procedures required for perioperative management and animal safety related to experimental craniomaxillofacial surgical procedures such as orthotopic, maxillofacial transplantation. Methods Miniature swine (n = 9) were used to investigate perioperative airway management, methods for providing nutrition, and long-dwelling intravenous access. Flap perfusion using near-infrared laser angiography and facial nerve assessment with electromyoneurography were explored. Results Bivona tracheostomy was deemed appropriate versus Shiley because soft, wire-reinforced tubing reduced the incidence of tracheal necrosis. Percutaneous endoscopic gastrostomy tube, as opposed to esophagostomy, provided a reliable route for postoperative feeding. Femoral venous access with dorsal tunneling proved to be an ideal option being far from pertinent neck vessels. Laser angiography was beneficial for real-time evaluation of graft perfusion. Facial electromyoneurography techniques for tracing capture were found most optimal using percutaneous leads near the oral commissure. Experience shows that ancillary procedures are critical, and malpositioning of devices may lead to irreversible sequelae with premature animal death. Conclusions Face-jaw-teeth transplantation in swine is a complicated procedure that demands special attention to airway, feeding, and intravascular access. It is critical that each ancillary procedure be performed by a dedicated team familiar with relevant anatomy and protocol. Emphasis should be placed on secure skin-level fixation for all tube/lines to minimize risk for dislodgement. A reliable veterinarian team is invaluable and critical for long-term success.