Biomechanical analysis of mandibular angle fractures.

BACKGROUND: Clinical evidence has suggested that minimal fixation can reduce complications of mandibular angle fractures, though no detailed biomechanical model has yet explored this unique and somewhat unexpected finding. METHODS: The current study uses finite element analysis to biomechanically evaluate different fixation schemes used to fixate mandibular angle fractures. Three fixation scenarios were considered: a single tension band at the superior mandibular border, a single bicortical angle compression plate at the inferior border and the tension band and bicortical plate used together. RESULTS: The dual plate model incurred the lowest von Mises stresses in the plates and the lowest principal strain in the callus. The tension band model observed the highest plate and screw von Mises stresses, but had fracture-site callus strain near to that of the dual plate model. The bicortical angle compression plate model observed the highest fracture-site callus strain. CONCLUSION: The results from this study support the use of the single tension band configuration as a less invasive fixation approach to fractures of the mandibular angle. This is the first known study to explore and confirm clinical observations of angle fracture fixation outcomes with a detailed biomechanical modeling methodology.

Mechanical design optimization of bioabsorbable fixation devices for bone fractures.

Bioabsorbable bone plates can eliminate the necessity for a permanent implant when used to fixate fractures of the human mandible. They are currently not in widespread use because of the low strength of the materials and the requisite large volume of the resulting bone plate. The aim of the current study was to discover a minimally invasive bioabsorbable bone plate design that can provide the same mechanical stability as a standard titanium bone plate. A finite element model of a mandible with a fracture in the body region is subjected to bite loads that are common to patients postsurgery. The model is used first to determine benchmark stress and strain values for a titanium plate. These values are then set as the limits within which the bioabsorbable bone plate must comply. The model is then modified to consider a bone plate made of the polymer poly-L/DL-lactide 70/30. An optimization routine is run to determine the smallest volume of bioabsorbable bone plate that can perform and a titanium bone plate when fixating fractures of this considered type. Two design parameters are varied for the bone plate design during the optimization analysis. The analysis determined that a strut style poly-L-lactide-co-DL-lactide plate of 690 mm2 can provide as much mechanical stability as a similar titanium design structure of 172 mm2. The model has determined a bioabsorbable bone plate design that is as strong as a titanium plate when fixating fractures of the load-bearing mandible. This is an intriguing outcome, considering that the polymer material has only 6% of the stiffness of titanium.

Brain CT to Assess Intracranial Pressure in Patients with Traumatic Brain Injury.

INTRODUCTION: Morphologic features of computed tomography (CT) scans of the brain can be used to estimate intracranial pressure (ICP) via an image-processing algorithm. Clinically, such estimations can be used to prognosticate outcomes and avoid placement of invasive intracranial monitors in certain patients with severe traumatic brain injury. Features on a CT scan that may correlate with measurements of low ICP are sought. METHODS: A measure is proposed that is a function of the distribution of cerebrospinal fluid (CSF) in and around the brain. In our method, we present an algorithm that semiautomatically segments brain parenchyma from CSF, and apply standard image processing calculations. The ratio of CSF volume to the size of the intracranial vault (ICV) or volume inside the skull, csf(v) /icv(v) is calculated and then plotted against the actual recorded ICP, yielding a relationship between the image features and ICP. RESULTS: We analyzed a total of 45 scans from 20 patients with severe traumatic brain injury (TBI). We showed that a ratio csf(v)/icv(v) > .034 correlates with an ICP < 20 mmHg (P = .0046). For csf(v)/icv(v) ≤ .034, a distinction between low and high ICP cannot be effectively estimated by this univariate measure. CONCLUSION: This method permits a noninvasive means of identifying patients who are low risk for having elevated ICP; by following Brain Trauma Foundation guidelines strictly such a patient may be subjected to an unnecessary, invasive procedure. This work is a promising pilot study that will need to be analyzed for a larger population.

Biomechanics of the rhombic transposition flap.

OBJECTIVE: To develop a computational model of cutaneous wound closures comparing variations of the rhombic transposition flap. STUDY DESIGN: A nonlinear hyperelastic finite element model of human skin was developed and used to assess flap biomechanics in simulated rhombic flap wound closures as flap geometric parameters were varied. SETTING: In silico. METHODS: The simulation incorporated variables of transposition angle, flap width, and tissue undermining. A 2-dimensional second-order Yeoh hyperelastic model was fit to published experimental skin data. Resultant stress and strain fields as well as local surface changes were evaluated. RESULTS: For the rhombus defect, closure stress and strain were minimized for the transposition flap with a distal flap angle of 30° by recruiting skin from opposing sides of the defect. Alteration of defect dimensions showed that peak stress and principal strain were minimized with a square defect. Likelihood of a standing cutaneous deformity was driven by the magnitude of angle closure at the flap base. Manipulation of the transposition angle reoriented the primary skin strain vector. Asymmetric undermining decoupled wound closure tension from strain, with direct effects on boundary deformation. CONCLUSIONS: The model demonstrates that flap width determines the degree of secondary tissue movement and impact on surrounding tissues. Transposition angle determines the orientation of maximal strain. Local flap design requires consideration of multiple factors apart from idealized biomechanics, including adjacent "immobile" structures, scar location, local skin thickness, and orientation of relaxed skin tension lines. Finite element models can be used to analyze local flap closures to optimize outcomes.

Purtscher retinopathy: an alternative etiology supported by computer fluid dynamic simulations.

PURPOSE: To explore an alternative etiology for Purtscher retinopathy by literature review and fluid dynamic computational simulations of wall shear stress (WSS) profiles. METHODS: Computer simulations were developed, incorporating posterior pole retinal microvascular flow parameters, to demonstrate WSS profiles at 90° and 45° angle artery/arteriolar branching. RESULTS: Computer simulations reveal WSS profiles dependent on artery/arteriolar branching angles. At high flow rates an area of changed WSS and flow swirling and reversal was noted at the proximal fillet of the 90° arteriolar branching. These changes did not appear at the 45° arteriolar branching until the flow rate was increased an additional 30%. CONCLUSIONS: Computer simulation data, as well as review of the history and clinical findings of Purtscher and Purtscher-like retinopathy, present evidence that an additional etiology for Purtscher retinopathy may be a rheological event at a retinal posterior pole foci of vascular endothelial dysregulation, followed by downstream endothelin-induced vasculopathy.

Engineering analysis of an unreported complication of septoplasty.

OBJECTIVES: To describe a cause of recurrent nasal obstructive symptoms after septoplasties including the creation of a sizable submucous window and to suggest treatments for this complication. METHODS: Case report of a woman presenting with side-changing nasal dyspnea approximately 1 year after undergoing septoplasty and engineering analysis of nasal cavity airflow. We created a computer model of the airway, analyzed varying sizes of surgical defects, and optimized the geometry of the submucous window. RESULTS: An optimum area of resection to maximize the area of cartilage and/or bone resected and to minimize deflection of the septal area of iatrogenic litheness is a rectangular shape approximately 44 mm long by 12 mm high in our model. CONCLUSIONS: A large submucous window can result in obstruction of nasal airflow after septoplasty owing to displacement of this compliant area with respiration under the forces described in the Bernoulli theorem. Treatment may include turbinate reduction and/or septal reconstruction.

Comparison of plate-screw systems used in mandibular fracture reduction: finite element analysis.

A finite element model of the human dentate mandible has been developed to provide a comparison of fixation systems used currently for fracture reduction. Volume domains for cortical bone, cancellous bone, and teeth were created and meshed in ANSYS 8.0 based on IGES curves created from computerized tomography data. A unilateral molar clench was loaded on the model with a fracture gap simulated along the symphysis. Results based on Von Mises stress in cortical and cancellous bone surrounding the screws, and on fracture surface spatial fixation, show some relative differences between different screw-plate systems, yet all were judged to be appropriate in their reduction potential.