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)

A predictive mechano-biological model of the bone-implant healing

The quality of the fixation orthopaedic implant to its surrounding bone determines its clinical longevity. Up to 20% of hip replacement operations are currently revisions for aseptic loosening. While this fixation quality is determined primarily by the bone and tissue anchoring the implant, conditions influencing bone growth in the early post-operative period include the surgical technique and coupled mechanical and biochemical factors. The aim of the study was to propose an original mechano-biological formulation of the healing process of periprosthetic tissue. The multiphasic porous model involved the solid osseous matrix, the extracellular fluid phase, the osteoblastic cellular phase responsible from the bone formation and the growth factor phase promoting the cellular activity. To derive the non-linear convective-diffuse governing equations, mass balance was associated to cell active haptotactic and chemotactic migration, growth factor diffusion, cell proliferation (logistic law) and bone formation (reactive medium). The in-vivo application concerned a canine axisymmetric implant which was stable and mechanically unloaded. Predictive numerical results were compared to ex-vivo data from a histologic study. The generic healing pattern involving two main oscillations of the radial bone formation was well predicted. In the future, the model could assist in evaluating the role of growth factor concentrations and their temporal delivering as far as the role of pertinent sources such as bioactive coating or additional biomaterials

Disc volume properties from MRI in adolescent idiopathic scoliosis: correlation to surgical outcome

In young scoliotic patients, the post-operative consequence of spine fusion upon the free lower lumbar spine is one of the major concerns of the surgical treatment. The remodeling of free-motion segment and the role of discs below thoraco-lumbar fusions remains unknown. However, disc hydration and mass exchange flow between disc and vertebral body should play a significant role in the mechano-biology of the vertebral segment. Magnetic resonance imaging is relevant to study intervertebral discs in young scoliotic patients since related to hydration and non-radiant

A solution of torsional problem by energy method in case of anisotropic cross-section

We proposed an original method to investigate the problem of torsion of anisotropic cross-section. We implemented an energy method to calculate the stress function represented by infinite series of trigonometric functions adapted to rectangular cross-section. After validation, we implemented a parametric sensitivity study to investigate the influence of the cross-section aspect ratio and the anisotropy level on the stress function, the strain energy density and the torsion stiffness. The process showed a fast convergence with a very good accuracy. The model showed a potential interest for the experimental identification of anisotropic material properties

Effective mechanical properties of diaphyseal cortical bone in the canine femur

The effective elastic modulus, yield strength, yield strain, ultimate strength, ultimate strain, strain energy density at yield and strain energy density at ultimate failure of femoral diaphyseal cortical bone were investigated on canine femurs. Four femurs representative of the canine population were selected from four statistically-determined clusters based on increasing size and weight comprising the Toy poodle (5 kg), Poodle (12 kg), German shorthaired pointer (25 kg) and Doberman (50 kg). The zones of interest were the lateral, medial, cranial, and caudal quadrants of the mid-diaphysis. Effective mechanical properties were measured using quasi-static three-point bending tests on strips. The averages +/- SD were 15.6 +/- 2.6 GPa for effective elastic modulus, 1743 +/- 32.1 MPa for yield strength, 0.012 +/- 0.003 for yield strain, 251.0 +/- 49.1 MPa for ultimate strength, 0.021 +/- 0.005 for ultimate strain, 10.7 +/- 4.0 J m(-3) x 10(5) for strain energy density at Yield and 33.0 +/- 14.1 J M-3 X 10(5) for strain energy density at ultimate failure. Significant differences were found between dogs and the effective elastic modulus increased with breed weight and size (13.9 GPa for the Toy poodle to 17.2 GPa for the Doberman). The ultimate strength sigma(u) and strain energy density at ultimate failure U-u were significantly lower in the Toy poodle than in the Poodle and German shorthaired pointer indicating that the cortical bone material in the Toy poodle differed from that of the other dogs. Examination of the zones of interest revealed that the cranial quadrant showed the greatest stiffness, whereas strength was highest at the medial site. The caudal cortex was less stiff and strong than the cranial cortex

Measurement of implant deployment and related forces in kyphoplasty by percutaneous approach

BACKGROUND: The treatment of osteoporotic vertebral compression fractures using a transpedicular approach and cement injection has grown significantly over the last two decades. METHODS: The aim was to study the deployment of an implant dedicated to the vertebral augmentation by percutaneous approach (kyphoplasty). Its kinematics and the related forces have been investigated. A theoretical model of deployment has been proposed and the ancillary was instrumented with strain gauges and Hall effect sensors to measure kinematics and force in the deployment actuator (tensile rod). The methodology was first evaluated ex-vivo in a test-bench with boundary conditions monitored by a tensile machine. Then, a cadaver study was carried out in three lumbar and thoracic vertebral segments of normal and osteoporotic spines. FINDINGS: The relationships between ancillary internal forces, deployment, and cranio-caudal pushing force have been obtained. The test-bench experiment showed quasi-proportional relationship between force distribution and kinematics during the deployment. Ex-vivo cranio-caudal pushing forces were measured. Cadaver studies showed cranio-caudal pushing forces comprised between 100N and 200N. These forces were dependent upon the implant location in the vertebral body and bone stock. INTERPRETATION: The methodology was related to the analysis of load distribution and kinematics of a deployable implant for vertebral augmentation. The ancillary instrumentation contributed to the objective quantification of the surgical technique. The cadaver study in normal and osteoporotic spines exhibited the role of bone properties and implant location in implant deployment. This pilot study showed a methodology to improve the kyphoplasty surgery and patient comfort in clinical routine