Orbital and Maxillofacial Computer Aided Surgery: Patient-Specific Finite Element Models To Predict Surgical Outcomes

This paper addresses an important issue raised for the clinical relevance of Computer-Assisted Surgical applications, namely the methodology used to automatically build patient-specific Finite Element (FE) models of anatomical structures. From this perspective, a method is proposed, based on a technique called the Mesh-Matching method, followed by a process that corrects mesh irregularities. The Mesh-Matching algorithm generates patient-specific volume meshes from an existing generic model. The mesh regularization process is based on the Jacobian matrix transform related to the FE reference element and the current element. This method for generating patient-specific FE models is first applied to Computer-Assisted maxillofacial surgery, and more precisely to the FE elastic modelling of patient facial soft tissues. For each patient, the planned bone osteotomies (mandible, maxilla, chin) are used as boundary conditions to deform the FE face model, in order to predict the aesthetic outcome of the surgery. Seven FE patient-specific models were successfully generated by our method. For one patient, the prediction of the FE model is qualitatively compared with the patient's post-operative appearance, measured from a Computer Tomography scan. Then, our methodology is applied to Computer-Assisted orbital surgery. It is, therefore, evaluated for the generation of eleven patient-specific FE poroelastic models of the orbital soft tissues. These models are used to predict the consequences of the surgical decompression of the orbit. More precisely, an average law is extrapolated from the simulations carried out for each patient model. This law links the size of the osteotomy (i.e. the surgical gesture) and the backward displacement of the eyeball (the consequence of the surgical gesture)

Personalized modeling for real-time pressure ulcer prevention in sitting posture

, Ischial pressure ulcer is an important risk for every paraplegic person and a major public health issue. Pressure ulcers appear following excessive compression of buttock's soft tissues by bony structures, and particularly in ischial and sacral bones. Current prevention techniques are mainly based on daily skin inspection to spot red patches or injuries. Nevertheless, most pressure ulcers occur internally and are difficult to detect early. Estimating internal strains within soft tissues could help to evaluate the risk of pressure ulcer. A subject-specific biomechanical model could be used to assess internal strains from measured skin surface pressures. However, a realistic 3D non-linear Finite Element buttock model, with different layers of tissue materials for skin, fat and muscles, requires somewhere between minutes and hours to compute, therefore forbidding its use in a real-time daily prevention context. In this article, we propose to optimize these computations by using a reduced order modeling technique (ROM) based on proper orthogonal decompositions of the pressure and strain fields coupled with a machine learning method. ROM allows strains to be evaluated inside the model interactively (i.e. in less than a second) for any pressure field measured below the buttocks. In our case, with only 19 modes of variation of pressure patterns, an error divergence of one percent is observed compared to the full scale simulation for evaluating the strain field. This reduced model could therefore be the first step towards interactive pressure ulcer prevention in a daily setup. Highlights-Buttocks biomechanical modelling,-Reduced order model,-Daily pressure ulcer prevention

CT Scan Merging to Enhance Navigation in Interventional Radiology Simulation

International audienceWe present a method to merge two distinct CT scans acquired from dif- ferent patients such that the second scan can supplement the first when it is missing necessary supporting anatomy. The aim is to provide vascular intervention simula- tions with full body anatomy. Often, patient CT scans are confined to a localised region so that the patient is not exposed to more radiation than necessary and to increase scanner throughput. Unfortunately, this localised scanning region may be limiting for some applications where surrounding anatomy may be required and where approximate supporting anatomy is acceptable. The resulting merged scan can enhance body navigation simulations with X-ray rendering by providing a com- plete anatomical reference which may be useful in training and rehearsal. An ex- ample of the use of our CT scan merging technique in the field of interventional radiology is described

3D musculo-skeletal finite element analysis of the foot kinematics under muscle activation with and without ankle arthrodesis

International audienceThe choice between arthrodesis and arthroplasty in the context of advanced ankle arthrosis remains a highly disputed topic in the field of foot and ankle surgery. Arthrodesis, however, represents the most popular option. Biomechanical modeling has been widely used to investigate static loading of cadaveric feet as well as consequences of arthrodesis on bony structures. Although foot kinematics has been studied using motion analysis, this approach lacks accuracy in capturing internal joints motion due to limitations inherent to external “marker sets” and the fact that it imposed the foot to be considered as a rigid solid. The consequences of arthrodesis on kinematics of the unloaded foot are not well understood although it is of crucial importance during the swing phase and at heel contact. Investigating ankle mobility during muscle contraction with and without arthrosis could explain how the motion is produced by extrinsic muscles activations affected by an arthrodesis. This study aims at defining if a biomechanical model with Finite Elements could help arthrodesis understanding

Comparison of LASTIC (Light Aspiration device for in vivo Soft TIssue Characterization) with classic Tensile Tests.

International audienceLASTIC is a device estimating in vivo soft tissue elasticity. It uses negative pressure to deform the tissue surface and captures several deformation stages to trace the behavioral curve. Using Finite Element inverse analysis and a Neo Hookean constitutive law, the tissue's Young modulus is evaluated. This paper compares LASTIC capabilities with standard tensile tests on four samples with elastic properties ranging from 10 kPa to 100 kPa. Although LASTIC overestimates Young modulus by an average of 24 %, it allows a first estimation of the elastic modulus of different materials

Dynamic biomechanical modelling for foot ulcer prevention.

International audienceThis paper introduces a 3D Dynamic Finite Element biomechanical model of the human foot used for diabetic foot pressure ulcer prevention. The model estimates the internal strains and send an alert to the user in case of high strains values