János Simonovics

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.

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.

How bilateral iliolumbar fusion increases the stability of horizontal osteosynthesis in unstable pelvic ring injuries

ObjectiveIn type C pelvic ring injuries, the operative stabilization of the posterior ring is absolutely indicated. There exist four different types of operative methods: iliosacral screw fixation, transsacral plate synthesis, ventral plate fixation (primarily for sacroiliac luxations), and local plate synthesis performed on the dorsal cortex of the sacrum. In our current article, we analyzed the stability of fixation methods used together with bilateral iliolumbar techniques.MethodsWe analyzed a finite element pelvic model attached to lumbar 4–5 vertebrae. By imitating a standing position on two feet, we measured the differences in tension and displacement in T1 and T2 thoracic vertebrae fractures with and without iliolumbar fusion in cases of iliosacral screw fixation, transsacral plate synthesis and KFI-H (small fragment-H) plate synthesis.ResultsThe osteosynthesises reinforced via Galveston technique were rather stable; the amount of displacement measured in the fracture gap was significantly less than in the cases without iliolumbar fusion. The tension in the implants were below the allowed values, therefore they were capable of withstanding the imposed loads without permanent deformation.ConclusionsIn unilateral pelvis injuries, if a non-weight bearing status cannot be achieved on the injured side, unilateral iliolumbar fusion reinforcement is justified, since the contralateral lower limb must also be non-weight bearing due to the pelvis injury itself. In the case of the most unstable sacrum fracture—“jumper’s fracture”, bilateral iliolumbar fusion is necessary, in which case the patient will be able to bear weight during the early postoperative period.