Péter Bujtár

Finite element analysis of the human mandible at 3 different stages of life

OBJECTIVE This study analyzed detailed models of human mandibles at 3 different stages of life with simulation of supra normal chewing forces at static conditions. METHODS AND MATERIALS Finite element analysis (FEA) was used to generate models from cone-beam computerized tomograms (CBCT) of 3 patients aged 12, 20, and 67 years, using numerically calculated material parameters. Estimated chewing forces were then applied to the simulations. RESULTS The results reflect higher elasticity in younger models in all regions of the mandible. The experimental models show that physiologic load stress and strain distributional changes of the mandible vary according to age. CONCLUSION The CBCT-based model generation used in this study provided high-quality model definition of the 3 individual patients of different ages. FEA has great potential to predict bone responses to paradigms of mechanical activity. Future applications of FEA will include surgical planning, surgical hardware testing, and the design of scaffolds and tissue-engineered constructs.

Porous Hydroxyapatite and Aluminium-Oxide Ceramic Orbital Implant Evaluation Using CBCT Scanning: A Method for In Vivo Porous Structure Evaluation and Monitoring

Objective. This study aimed to define CBCT as a technique for postimplantation in vivo examination of porous hydroxyapatite and aluminium-oxide orbital implant shape, volume and density changes. Methods and Materials. CBCT was used to evaluate 30 enucleated patients treated with spherical polyglactin 910 wrapped hydroxyapatite and aluminum-oxide orbital implants. The mean duration of patient followup was 3.2 years or 1338 days with a range of 0.2 to 7.2 years or 79 to 2636 days in a population with an average age of 40.8 years. Results. The resolution of currently clinically used CBCT equipment allowed detailed structural observation of the orbital hydroxyapatite implants with some modifications. Volume and shape estimations were possible while density evaluation was more complicated compared to medical source computed tomography. The mean densities of the orbital implants were followed and a consistent gradual decrease identified from the beginning of implantation which was better defined after the applied correction procedure. Conclusion. CBCT with lower dosages of radiation exposure can be used to follow changes in implanted high-density porous structures. The density evaluation is possible with calibration modifications. Changes in orbital implant densities identified in this study may correspond to healing and maturation of soft tissues surrounding and penetrating the implants.

A finite element analysis of bone plates available for prophylactic internal fixation of the radial osteocutaneous donor site using the sheep tibia model.

INTRODUCTION The strengthening effect of prophylactic internal fixation (PIF) with a bone plate at the radial osteocutaneous flap donor site has previously been demonstrated using the sheep tibia model of the human radius. This study investigated whether a finite element (FE) model could accurately represent this biomechanical model and whether stress or strain based failure criteria are most appropriate. METHODS An FE model of an osteotomised sheep tibia bone was strengthened using 4 types of plates with unilocking or bicortical screw fixation. Torsion and 4-point bending simulations were performed. The maximum von Mises stresses and strain failure criteria were studied. RESULTS The strengthening effects when applying stress failure criteria [factor 1.76-4.57 bending and 1.33-1.80 torsion] were comparable to the sheep biomechanical model [factor 1.73-2.43 bending and 1.54-2.63 torsion]. The strongest construct was the straight 3.5mm stainless steel unilocking plate. Applying strain criteria the strongest construct was the straight 3.5mm stainless DCP plate with bicortical screw fixation. CONCLUSIONS The FE model was validated by comparison with the sheep tibia model. The complex biomechanics at the bone-screw interface require further investigation. This FE modelling technique may be applied to a model of the human radius and other sites.

Refinements in osteotomy design to improve structural integrity: a finite element analysis study

Osteotomy cuts are typically made using a saw, and the meeting point acts as a focus for the concentration of stress and failure. We have studied the impact of different designs of osteotomy cut. Cadaver sheep tibias were scanned by computed tomography (CT) and transformed into a computer-aided design (CAD) model. A standard marginal resection defect was created and then modified, and a finite element analysis made. The relative stress concentrations at the intersection of osteotomy cuts were recorded using principal stresses S1, S3, and von Mises stress, von Mises under both 4-point bending and torsion testing. The osteotomy designs studied were: right-angled and bevelled osteotomy end cuts, overcutting, and a stop drill hole. Peak stress values for 4-point bending and torsion were 24-30% greater at the right-angled osteotomy than the bevelled end cut. Overcutting dramatically increased peak stress values caused by bending and torsion by 48% and 71%, respectively. Substantially lower concentrations of stress were noted with a stop hole using both a 90° (bending 38% and torsion 56%), and a tangential (bending 58% and torsion 60%) cut. A bevelled osteotomy has substantially lower concentrations of stress than a right-angled osteotomy. It is important to avoid creating an overcut as this causes an appreciable increase in the concentration of stress, while a stop drill hole substantially reduces the stress. The creation of a stop hole and the use of judicious bevelling techniques are modifications in the design of an osteotomy that are readily applicable to surgical practice.

Three-dimensional computer-aided surgical workflow to aid in reconstruction: from diagnosis to surgical treatment

The development of three-dimensional computer-aided surgical workflow has simplified the planning of complex reconstruction cases. It can also be helpful in planning distraction osteogenesis cases. This article examines the evolving role of three-dimensional computer-aided surgical workflow in maxillofacial surgery.

High-quality image acquisition by double exposure overlap in cone beam computed tomography

OBJECTIVE With a double exposure overlapping cone beam computed tomography (CBCT) scan technique, using CBCT acquisition radiation dose, the objective was to obtain apparent density similar to that of multidetector computed tomography (MDCT). STUDY DESIGN Factory quality-assurance phantom and water phantom were used for the evaluation of apparent density fidelity of iCAT scans in different modes. Each scans apparent density was analyzed for identical regions using ImageJ, version 1.42q. RESULTS The iCAT Classic extended height acquisition with 4-cm central overlap and reconstruction of 2 groups of 300 projections per rotation for the water and quality-assurance CBCT phantoms resulted in improved apparent density fidelity. This apparent density accuracy was superior to that of iCAT scan at high resolution (600 projections during 1 rotation). CONCLUSIONS Using double exposure overlapping CBCT scans allows the analysis quality to be comparable with that of MDCT.

Stop-hole osteotomy technique

r e j s o d d s v i a i he cuts that are required to create a step osteotomy of the andible or a section defect are primarily in a linear plane. he point at which 2 cuts meet is a focus for the concenration of stress and the area is prone to failure. The “stop rill hole method” has been used to block the propagation f existing crack lines during aircraft maintenance1 and in ider mechanical engineering practice, but has not been ommonly applied to avoid iatrogenic fractures in surgical ractice. A stop-hole of sufficient diameter will prolong the ime to fatigue failure under cyclical loading.2 We have developed a finite element analysis technique to imulate stress levels within bone. We tested the mandible nder normal biting forces,3 long bones with plate fixation,4 nd differing designs of osteotomy end cuts5 for bending nd torsional loading. When compared with a baseline rightngled osteotomy (maximum stress level 100%), the peak tress concentration at a bevelled osteotomy was substanially lower (70–81%). It was further reduced when the orner of a right-angled osteotomy was rounded out with stop-hole, which could be entered tangentially (40–47%) Fig. 1) or at 90◦ (44–69%) (Fig. 2). In contrast, an overut oblique osteotomy (104–158%) substantially increased he stress concentration and implied a weakening effect.5 A top-hole at the corner of an osteotomy will therefore have a

Emerging manufacturing bioengineering technologies 2: Scaffold designing experiment using titanium scaffolds

Substantial volume defects of the head and neck oftenrequire customized solutions to improve quality of life likefree flap transfers.Titanium and its alloys are versatile materialsproviding the feature of osteointegration. The conditionswhich facilitate the deposition of lamellar bone are underextensive research. Our project aimed to determine whethertitanium can function as a scaffold - unlike simple plates - toenhance bone regeneration for load bearing structures. Thereaction of stem cells to scaffolds with varying stiffness willbe presented.Additive manufacturing were used to produce a variety ofscaffolds to optimize titanium structures. Electric beam melting(EBM) manufacturing allowed us to optimize the elasticmodulus (Young) of the titanium to match with cadaveric bone from a previous project. Multidirectional mechanicaltests were performed on the various designs of titanium cellstructures (n=80). The predictability and quality of manufacturingwas assessed statistically and also with scanningelectron microscope (SEM).The results demonstrated structures matching the mechanicalproperties of bone and even anisotropy as our resultssuggest 3GPa elasticity. This allows the possibility to buildregenerating bone with predictable properties. In addition,predictable patterning - unlike etching and sandblasting - ofmicroscopic (nano) features found to be significant and nonhomogenous simple repetitive patterns provide better cellularresponse.The benefit that tissue engineering techniques offer isdecreased morbidity, relative independence from donor site,with a highly specific and customized shape. Titanium basedreconstruction constructs seems to offer an alternative futurefor bony reconstruction.