Vincent Luboz

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 patients 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 11 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).

Biomechanical modeling to prevent ischial pressure ulcers

With 300,000 paraplegic persons only in France, ischial pressure ulcers represent a major public health issue. They result from the buttocks׳ soft tissues compression by the bony prominences. Unfortunately, the current clinical techniques, with - in the best case - embedded pressure sensor mats, are insufficient to prevent them because most are due to high internal strains which can occur even with low pressures at the skin surface. Therefore, improving prevention requires using a biomechanical model to estimate internal strains from skin surface pressures. However, the buttocks׳ soft tissues׳ stiffness is still unknown. This paper provides a stiffness sensitivity analysis using a finite element model. Different layers with distinct Neo Hookean materials simulate the skin, fat and muscles. With Young moduli in the range [100-500 kPa], [25-35 kPa], and [80-140 kPa] for the skin, fat, and muscles, respectively, maximum internal strains reach realistic 50 to 60% values. The fat and muscle stiffnesses have an important influence on the strain variations, while skin stiffness is less influent. Simulating different sitting postures and changing the muscle thickness also result in a variation in the internal strains.

Linear Elastic Properties of the Facial Soft Tissues Using an Aspiration Device: Towards Patient Specific Characterization

Biomechanical modeling of the facial soft tissue behavior is needed in aesthetic or maxillo-facial surgeries where the simulation of the bone displacements cannot accurately predict the visible outcome on the patient’s face. Because these tissues have different nature and elastic properties across the face, depending on their thickness, and their content in fat or muscle, individualizing their mechanical parameters could increase the simulation accuracy. Using a specifically designed aspiration device, the facial soft tissues deformation is measured at four different locations (cheek, cheekbone, forehead, and lower lip) on 16 young subjects. The stiffness is estimated from the deformations generated by a set of negative pressures using an inverse analysis based on a Neo Hookean model. The initial Young’s modulus of the cheek, cheekbone, forehead, and lower lip are respectively estimated to be 31.0xa0kPaxa0±xa04.6, 34.9xa0kPaxa0±xa06.6, 17.3xa0kPaxa0±xa04.1, and 33.7xa0kPaxa0±xa07.3. Significant intra-subject differences in tissue stiffness are highlighted by these estimations. They also show important inter-subject variability for some locations even when mean stiffness values show no statistical difference. This study stresses the importance of using a measurement device capable of evaluating the patient specific tissue stiffness during an intervention.

Foot ulcer prevention using biomechanical modelling

Foot ulcers are a common complication of diabetes and are the consequence of trauma to the feet and a reduced ability to perceive pain in persons with diabetes. Ulcers appear internally when pressures applied on the foot create high-internal strains below bony structures. It is therefore important to monitor tissue strains in persons with diabetes. We propose to use a biomechanical model of the foot coupled with a pressure sensor to estimate the strains within the foot and to determine whether they can cause ulcer formation. Our biomechanical foot model is composed of a finite element mesh representing the soft tissues, separated into four Neo-Hookean materials with different elasticity: plantar skin, non-plantar skin, fat and muscles. Rigid body models of the bones are integrated within the mesh to rigidify the foot. Thirty-three joints connect those bones around cylindrical or spherical pivots. Cables are included to represent the main ligaments in order to stabilise the foot. This model simulates a realistic behaviour when the sole is subjected to pressures measured with a sensor during bipedal standing. Surface strains around 5% are measured below the heel and metatarsal heads, while internal strains are close to 70%. This strain estimation, when coupled to a pressure sensor, could consequently be used in a patient alert system to prevent ulcer formation.

Influence of the calcaneus shape on the risk of posterior heel ulcer using 3D patient-specific biomechanical modeling.

AbstractMost posterior heel ulcers are the consequence of inactivity nand prolonged time lying down on the back. They appear when pressures applied on the heel create high internal strains and the soft tissues are compressed by the calcaneus. It is therefore important to monitor those strains to prevent heel pressure ulcers. Using a biomechanical lower leg model, we propose to estimate the influence of the patient-specific calcaneus shape on the strains within the foot and to determine if the risk of pressure ulceration is related to the variability of this shape. The biomechanical model is discretized using a 3D Finite Element mesh representing the soft tissues, separated into four domains implementing Neo Hookean materials with different elasticities: skin, fat, Achilles’ tendon, and muscles. Bones are modelled as rigid bodies attached to the tissues. Simulations show that the shape of the calcaneus has an influence on the formation of pressure ulcers with a mean variation of the maximum strain over 6.0 percentage points over 18 distinct morphologies. Furthermore, the models confirm the influence of the cushion on which the leg is resting: a softer cushion leading to lower strains, it has less chances of creating a pressure ulcer. The methodology used for patient-specific strain estimation could be used for the prevention of heel ulcer when coupled with a pressure sensor.

Clinical workflow for personalized foot pressure ulcer prevention.

Foot pressure ulcers are a common complication of diabetes because of patients lack of sensitivity due to neuropathy. Deep pressure ulcers appear internally when pressures applied on the foot create high internal strains nearby bony structures. Monitoring tissue strains in persons with diabetes is therefore important for an efficient prevention. We propose to use personalized biomechanical foot models to assess strains within the foot and to determine the risk of ulcer formation. Our workflow generates a foot model adapted to a patients morphology by deforming an atlas model to conform it to the contours of segmented medical images of the patients foot. Our biomechanical model is composed of rigid bodies for the bones, joined by ligaments and muscles, and a finite element mesh representing the soft tissues. Using our registration algorithm to conform three datasets, three new patient models were created. After applying a pressure load below these foot models, the Von Mises equivalent strains and cluster volumes (i.e. volumes of contiguous elements with strains above a given threshold) were measured within eight functionally meaningful foot regions. The results show the variability of both location and strain values among the three considered patients. This study also confirms that the anatomy of the foot has an influence on the risk of pressure ulcer.

Simulation of the Exophthalmia Reduction Using a Finite Element Model of the Orbital Soft Tissues

This paper proposes a computer-assisted system for the surgical treatment of exophthalmia. This treatment is classically characterized by a decompression of the orbit, by the mean of an orbital walls osteotomy. The planning of this osteotomy consists in defining the size and the location of the decompression hole. A biomechanical model of the orbital soft tissues and its interactions with the walls are provided here, in order to help surgeons in the definition of the osteotomy planning. The model is defined by a generic Finite Element poro-elastic mesh of the orbit. This generic model is automatically adapted to the morphologies of four patients, extracted from TDM exams. Four different FE models are then generated and used to simulate osteotomies in the maxillary or ethmoid sinuses regions. Heterogeneous results are observed, with different backwards movements of the ocular globe according to the size and/or the location of the hole.

Conception and evaluation of a 3D musculoskeletal finite element foot model.

This paper introduces a new patient-specific musculoskeletal and Finite Element (FE) model of the foot aimed to be used in the context of deep pressure ulcer prevention, orthopedic and motion analysis. This model is evaluated in both static and dynamic frameworks.

Foot biomechanical modelling to study orthoses influence.

Several pathologies of the foot can be solved simply by adding an orthosis under the patients foot. Defining the geometry and the size of such orthosis is key in optimizing its influence on the foot. Unfortunately, most of the orthoses produced today are not specifically design for a patient. They allow improvements to some degrees but could be more efficient if they were patient specific. We propose to use a patient-specific finite element foot model to study the influence of such orthoses and to help designing them in a better way, in accordance with the patients anatomy and pathology.

Using a 3D biomechanical model to improve a light aspiration device for in vivo soft tissue characterisation

Estimation of living tissue constitutive law is needed for patient-specific simulations during surgery. However, this task remains very challenging. The device used for characterisation must undergo sterilisation and must give real-time results to characterise tissues that can only be accessed in the operating room (e.g. brain tissues). This paper presents the light aspiration device for in vivo soft tissue characterisation (LASTIC) device that was designed to fulfil these expectations. The paper compares the axisymmetric 2D model with a 3D model of LASTICs aspiration procedure.