A computational and experimental mechanical study of nanocomposites for 3D printed scaffolds with a new geometry.

We present a combined study of the mechanical properties of 3D printed scaffolds made by nanocomposite materials based on polycaprolactone (PCL). The geometry and dimensions of the three different systems is the same. Τhe porosity is 50% for all systems. Distributions of von-Mises strains and stresses, and total deformations were obtained through Finite Element Analysis (FEA) for a maximum amount of force applied, in a compressive numerical experiment. Also compressive experiments were performed for both raw and 3D nanoconposite scaffolds.

SAFETRANS: a system for 3D visibility estimation and cloud cover detection.

The estimation of visibility is of significant importance in aviation safety and forms part of the measurements routinely collected in real time to provide safety guidelines and decisions. Our work concerns the creation and implementation of a lidar-based visibility estimation system as part of the SAFETRANS research program. We created a reproducible system to (1) support standard airport equipment, (2) serve simultaneously as a visibility meter and a ceilometer reporting on cloud cover, (3) offer increased accuracy and improved capabilities compared to standardized equipment currently in use while (4) requiring minimal user training to function. This work presents the visibility estimation and cloud cover algorithms and subsequently reports on results of field tests in a number of Greek airports under various atmospheric conditions.

Flux measurements in the surface Marine Atmospheric Boundary Layer over the Aegean Sea, Greece.

Micro-meteorological measurements within the surface Marine Atmospheric Boundary Layer took place at the shoreline of two islands at northern and south-eastern Aegean Sea of Greece. The primary goal of these experimental campaigns was to study the momentum, heat and humidity fluxes over this part of the north-eastern Mediterranean Sea, characterized by limited spatial and temporal scales which could affect these exchanges at the air-sea interface. The great majority of the obtained records from both sites gave higher values up to factor of two, compared with the estimations from the most widely used parametric formulas that came mostly from measurements over open seas and oceans. Friction velocity values from both campaigns varied within the same range and presented strong correlation with the wind speed at 10 m height while the calculated drag coefficient values at the same height for both sites were found to be constant in relation with the wind speed. Using eddy correlation analysis, the heat flux values were calculated (virtual heat fluxes varied from -60 to 40 W/m(2)) and it was found that they are affected by the limited spatial and temporal scales of the responding air-sea interaction mechanism. Similarly, the humidity fluxes appeared to be strongly influenced by the observed intense spatial heterogeneity of the sea surface temperature.

Wavelet analysis of head acceleration response under dirac excitation for early oedema detection.

The present work deals with the application of an innovative in-house developed wavelet-based methodology for the analysis of the acceleration responses of a human head complex model as a simulated diffused oedema progresses. The human head complex has been modeled as a structure consisting of three confocal prolate spheroids, whereas the three defined regions by the system of spheroids, from the outside to the inside, represent the scull, the region of cerebrospinal fluid, and the brain tissue. A Dirac-like pulse has been used to excite the human head complex model and the acceleration response of the system has been calculated and analyzed via the wavelet-based methodology. For the purpose of the present analysis, a wave propagation commercial finite element code, LS-DYNA 3D, has been used. The progressive diffused oedema was modeled via consecutive increases in brain volume accompanied by a decrease in brain density. It was shown that even a small increase in brain volume (at the level of 0.5%) can be identified by the effect it has on the vibration characteristics of the human head complex. More precisely, it was found that for some of the wavelet decomposition levels, the energy content changes monotonically as the brain volume increases, thus providing a useful index of monitoring an oncoming brain oedema before any brain damage appears due to uncontrolled intracranial hypertension. For the purpose of the present work and for the levels of brain volume increase considered in the present analysis, no pressure increase was assumed into the cranial vault and, associatively, no brain compliance variation.

A new method for the early diagnosis of brain edema/brain swelling. An experimental study in rabbits.

The aim of the present work is to develop a non-destructive, non-invasive technique for the early diagnosis of an oncoming brain edema based on the variation of vibration characteristics of the head system (i.e. eigenfrequency spectrum and modal damping). Besides the theoretical model that supports the basic principle, the proposed technique has been verified experimentally in animal tests. The advantage of such an approach is that the relative information is available well in advance an increase of intracranial pressure is detected. The uncontrolled intracranial hypertension is associated with increased mortality or vegetative state in head trauma. Traumatic lesions located on temporal lobe render particularly impeding the transtendorial herniation. From the medical point of view, intracranial pressure (ICP) monitoring represents an effective way for early consideration of neurological decompensation in various neurosurgical conditions particularly in the head-injured setting. However, the use of ICP monitoring is not an effective way of brain edema detection, since ICP increase very often causes irreversible problems to the patient's brain. Therefore, the determination of an earlier, less invasive and more sensitive indicator of the oncoming intracranial hypertension and of the impeding neurological deterioration is of profound importance. The present work aims at experimental verification of both eigenfrequency shifting and modal damping increase of the spectral response of the head system of rabbits, wherever a mass increase in the content of cranial shell appears. The conducted analysis concludes that the eigenfrequency spectrum and its modal damping characteristics are sufficiently sensitive parameters in order to characterize mass increase in the cranial shell. Therefore the combination of both the above parameters could be used with confidence for the early diagnosis of brain edema.

Impact energy absorption by specimens from the upper end of the human femur.

A cadaveric biomechanical study was performed to investigate the fracture energy absorbed by strips of bone from the proximal femur in relation to age and gender, under impact loading conditions. Four groups (young male, young female, old male, old female) of four cadaveric proximal femurs were used in each case. Four bone strips were taken from the neck and four from the subtrochanteric area and these were tested under dynamic-impact conditions using the Charpy impact test. The fracture energy was calculated as the energy needed to achieve fracture per unit area, and expressed in J/m2. Bone specimens from young males are significantly tougher under impact conditions to those of females (p = 0.001), whereas between the old male and female groups, fracture energy does not significantly differ (p = 0.165). There was also significant difference (p < 0.0005) between the young and the old groups in both genders. The fracture energy absorption of the subtrochanteric area compared to that of the femoral neck for the same group of age and gender is in general slightly higher for all groups. In conclusion, gender in the young age group played a significant role in bone resistance in breaking whereas in the older age group it played a less important role.

Viscoelastic properties of cartilage-subchondral bone complex in osteoarthritis.

OBJECTIVE: This study examines viscoelastic properties of the 'unaffected' tibial condyle in unicompartmental osteoarthritic knees, in order to determine whether to preserve it or not in knee replacement operations. DESIGN: The viscoelastic properties of longitudinal strips of cartilage and subchondral bone from osteoarthritic and healthy knees were studied. METHODS: Nine medial compartment osteoarthritic and nine cadaveric knees, all age- and gender-matched, were studied. Samples from polyethylene and methacrylate cement were also obtained and all then tested in the DMA (Dynamic Mechanical Analysis) testing apparatus. The dynamic modulus of elasticity (E(dyn)) and the loss factor (tan delta) were measured. RESULTS: The medial compartment in osteoarthritic knees had lost its viscoelastic properties, having a significantly higher E(dyn) compared to the lateral one, and lower values of loss factor. In healthy knees there was no significant difference (p=0.18) in viscoelastic properties between both compartments and with the unaffected side of the osteoarthritic knee. Polyethylene and cement appear to have a high modulus of elasticity compared to both the healthy and the osteoarthritic knees. CONCLUSIONS: The 'unaffected' (lateral) tibial condyles in medial compartment osteoarthritic knees seem to preserve their viscoelastic properties; it is worth considering a hemiarthroplasty. The significant modulus of elasticity difference between the condyles-polyethylene-cement results in different deformation in the interface between them and induces micromotion and loosening.

Experimental investigation of the tension band in fractures of the patella.

We investigated the strain pattern developed in the anterior and posterior part of the fixed patella during knee motion. Eight fresh cadaver knees were used but two were excluded because of non reliable measurements due to misplacement of gauges. Two strain gauges were bonded in the midline of the anterior and two in the posterior surface of the patella. Threaded steel rods were cemented into the intramedullary femoral and tibial canals. The knee was placed on a special device. The quadriceps tendon was gripped and a 4.5 kg weight was attached to the tibial rod 16.5 cm distal to the joint line. Ten flexion/extension cycles were performed before testing. Initially the intact patella was tested. A transverse osteotomy was performed before being stabilized by the AO recommended tension band technique. The knee was retested again as above. Finally an additional circular wire was passed around the patella and the knee was tested again under the same loading configuration. The intact patella showed weak tensile strain on the anterior and compressive strain on the posterior surface through the range of knee motion. Tension band fixation produced weak tensile strains in the first few degrees of flexion and then weak compressive strains in the posterior surface. The presence of the additional circular wire significantly increased the compressive strain. The classical tension band is highly effective for the fixation of the fractured patella but is improved by an additional circular wire.

Patellar fractures associated with medial-third bone-patellar tendon-bone autograft ACL reconstruction.

Patella fractures following anterior cruciate ligament (ACL) reconstruction are a recognized but rarely reported complication. To our knowledge, 24 reports of patella fractures after ACL reconstruction using the central-third patella-tendon autograft have been reported in the literature. Patellar fractures associated with the use of the medial-third bone-patellar tendon-bone autograft have not been reported. This article describes four cases of patellar fractures in 478 ACL reconstructions between 1992 and 1999, using the medial third of the patellar tendon graft. All of them were transverse fractures of the patella but only one was displaced. All patients suffered local injury to the donor knee between 2 and 4 months postoperatively. No significant differences in the final outcome were noticed between the cases complicated with patellar fracture and those with uncomplicated ACL reconstructions.

Strain development in carpal scaphoid for various wrist positions: a cadaveric study using strain gauges.

In order to investigate the ideal position in which the wrist should be immobilized during scaphoid fracture, treatment the strains which are developed in the carpal scaphoid for various wrist positions has been recorded in cadaveric wrists, using strain gauges. The data obtained shows that during radial deviation with neutral or slight palmar flexion of the wrist the waist of the scaphoid tends to compress because of the development of strong compressive strains, while the strain development parallel to the fracture site that tends to shift the scaphoid waist is minimum. This position seems to be the best for stable scaphoid fracture immobilization.

Brain eigenfrequency shifting as a sensitive index of cerebral compliance in an experimental model of epidural hematoma in the rabbit: preliminary study.

OBJECTIVE: To verify brain eigenfrequency shifting after the occurrence of a lesion producing mass effect into the cranial vault. DESIGN: Experimental animal study. SETTING: Laboratory of experimental surgery affiliated with a university critical care department. SUBJECTS: Six adult male New Zealand white rabbits. INTERVENTIONS: A Camino ICP monitor was placed in the parenchyma, and a 5-Fr balloon-tipped catheter and accelerometer were placed into the epidural space. MEASUREMENTS: Before and after the introduction of successive 0.1-mL increments of autologous blood into the balloon, intracranial pressure (ICP) was recorded along with the accelerometer signal obtained during free vibration of the skull triggered by a calibrated hammer. Fast Fourier transformation of the digitized signal provided the eigenfrequency spectrum. The eigenfrequency showing the sharpest decrease after the initial 0.1-mL volume addition was considered as the best frequency, and its variation in response to subsequent 0.1-mL increments represents the brain eigenfrequency shifting. MAIN RESULTS: Brain eigenfrequency shifting to lower values occurs for small blood volume increments (up to 0.2 mL). When volume addition becomes >0.3 mL, brain eigenfrequency shifting to higher values is exhibited. The decrease in best frequency after the initial introduction of 0.1 mL is statistically significant (p = .003), in a range of volume in which no significant intracranial pressure difference appears. The respective variation of ICP is explained using a quadratic curve. For volumes of 0 to 0.1 mL, the change in ICP is not statistically significant (p = .08). CONCLUSIONS: Changes of the brain's physical characteristics by mass addition in the cranial vault can be expressed by brain eigenfrequency shifting. The method seems advantageous because it reliably detects mass additions at low levels where no ICP change occurs. Additionally, it provides serial measurements, and it is less invasive than the currently used methods for intracranial compliance.

Rigid or sliding plate. A mechanical evaluation of osteotomy fixation in sheep.

Fracture fixation using rigid plates leads to direct bone union, but it also may lead to complications because of stress protection osteopenia. This study aims to compare the mechanical characteristics restored during the callus formation after an osteotomy is fixed with two types of internal plate fixation. Twenty-four adult female sheep were divided randomly into three groups of eight each, which were euthanized at 2, 4, and 6 months after operation. Half of them had their osteotomized radius fixed with a seven hole dynamic compression plate, whereas in the remainder a sliding plate was used. The sliding plate consists of two halves connected together in such a way as to permit axial sliding of the one within the other, thus allowing cyclic axial load transfer at the fracture site. Bone strips obtained from the healthy (control) and the surgically treated side were subjected to four-point bending tests. The effective modulus of elasticity, ultimate bending strength, and energy absorption to fracture (toughness) were calculated. All parameters were restored more quickly in the sliding plate group, but there was no statistically significant difference observed at 6 months when all the osteotomies were united completely. Thus, the sliding plate, by allowing axial loading at the fracture site, led to a faster callus maturation and hence bony union, which, hopefully, will permit earlier full weightbearing and functional recovery of the injured limb.

Pattern of screw loosening in fractures fixed with conventional and functional plates.

Plate fixation depends mainly on the holding power of the screws. In the present study the pattern of screw loosening was investigated. Thirty-two adult female sheep divided into four groups were used. A mid-diaphyseal transverse osteotomy was made on the right radius of each animal, and then plated on the anterior (tension) surface. Half of each group were plated using a standard narrow 7-hole AO Dynamic Compression Plate (DCP), whereas in the remaining animals a 6-hole newly designed sliding plate (SP) was applied. The required torque of tightening intraoperatively, as well as for releasing the screws after killing the animals was recorded with a tension-calibrated screwdriver. The pattern of loosening was similar for all the screws and the three phases could be recognized. An initial loosening was observed 1 month after the operation. A slow recovery of the torque was measured from the second month onwards, becoming highest by the fourth month after the operation, whereas a slow decrease in torque was observed from the fourth to the sixth month. In the SP group, the overall loosening was much lower than the DCP group (P < 0.05), whereas there was no difference in the loosening between the proximally placed screws and the distally placed ones (P > 0.05) for both plates.

Comparative study of callus performance achieved by rigid and sliding plate osteosynthesis based upon dynamic mechanical analysis.

Operative management of long bone fractures is considered to be the treatment of choice all over the world. This could be achieved either by intramedullary nailing or plating. The former allows delivery of stresses into the fracture site, but it requires a well-equipped operating theatre and an experienced surgeon who is, unfortunately, continuously exposed to radiation. On the other hand, plates could easily be applied in every orthopaedic clinic, but the existing ones are rigid and protect the fracture site from stresses to which it is ordinarily exposed. Recently, various experimental attempts have been made to provide plates which allow partial loading on the fracture site and enhance callus formation (secondary fracture healing). Bearing that in mind, a two-part sliding plate (SP) has been developed at the Orthopaedic Clinic of Patras University, which allows intermittent loading into the fracture site. Both the conventional AO rigid plate and SP were applied to osteotomized sheep radii. The dynamic properties of the callus were estimated with its two dynamic mechanical characteristics (dynamic modulus of elasticity and the relative loss factor), whereas its static properties (strength) were evaluated by measuring the ultimate bending strength. A superiority of callus produced by the sliding plate was observed concerning all the parameters under consideration.