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N Akkas

Publications and source records attributed to N Akkas.

9 recordsLinked to original sources
PubMed1995

Do porous calcium hydroxyapatite ceramics cause porosis in bone? A bone densitometry and biomechanical study on cortical bones of rabbits.

Porous calcium hydroxyapatite (CHA) ceramics are biocompatible and present osteoconductive properties. These ceramics are widely used in orthopaedic surgery; however, it is not yet known whether they have some adverse effects on bone and bone marrow healing. Our previous radiological study revealed possible local porosis at the adjacent sites of the CHA ceramic. Histological findings of the same study revealed bone marrow swelling and depletion at the implantation site. Osteoclasts removed particles of the implant that may be the cause of local porosis. In the present study, possible local osteoporosis was evaluated by bone densitometry analyses, and compression and three-point bending tests. CHA particles were implanted into the left limbs and a sham operation was utilized on the right limbs of 75 white rabbits. The animals were followed up for 23 weeks for bone mineral density and for 6 months for biomechanical analyses. The CHA implanted area and its distal or proximal adjacent areas were evaluated with a Hologic QDR-2000 bone densitometer. Three-point bending and compression tests were performed with an M-30 K material testing device. The results revealed a time-dependent bone density increase at the CHA implantation site and no significant porosis at adjacent areas of the implant. The stiffness of CHA-implanted bones in three-point bending is larger than that of the control group. CHA-implanted rabbit bones presented a different fracture pattern from the control group. The stiffness of the control and CHA-implanted bones generally increased with time indicating no adverse effects of porous CHA ceramics in bone and bone marrow healing. The clinical relevance of this work is that porous CHA ceramics do not cause local porosis at adjacent areas when implanted into osseous sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1986

Mechanical behavior of fetal dura mater under large deformation biaxial tension.

The mechanical behavior of fetal dura mater was investigated by means of a biaxial tension test designed to simulate the constraints imposed on the membrane by the cranial bones. The experimental results are compared with the theoretical results obtained by using two published strain energy functions: one defined by Mooney and Rivlin (MR) and the other by Skalak, Tozeren, Zarda and Chien (STZC). The latter constitutive relations fit the experimental results consistently well. The STZC stiffness values from this series of tests are compared with those from membrane inflation tests performed previously and reported elsewhere by the authors.

Biomechanical Phenomena↗
PubMed1983

Two-dimensional dynamic modelling of human knee joint.

A mathematical dynamic model of the two-dimensional representation of the knee joint is presented. The profiles of the joint surfaces are determined from X-ray films and they are represented by polynomials. The joint ligaments are modelled as nonlinear elastic springs of realistic stiffness properties. Nonlinear equations of motion coupled with nonlinear constraint conditions are solved numerically. Time derivatives are approximated by Newmark difference formulae and the resulting nonlinear algebraic equations are solved employing the Newton-Raphson iteration scheme. Several dynamic loads are applied to the center of mass of the tibia and the ensuing motion is investigated. Numerical results on ligament forces, contact point locations between femur and tibia, and the orientation of tibia relative to femur are presented. The results are shown to be consistent with the anatomy of the knee joint.

Ankle Joint↗
PubMed1983

Etiology and biomechanics of hernial sac formation.

This paper, to the authors' best knowledge, presents the first attempt on the understanding of the biomechanics of hernial sac formation. First, a brief survey of the selected etiological factors and their related theories on hernia is given. Next, the results of some preliminary tensile tests conducted on normal and sac peritoneum are discussed. The third part of the paper is concerned with a theoretical model which incorporates both material and geometric nonlinearities by considering deformation of circular membrane under internal fluid pressure. The influence of the material properties of the peritoneal sac, its thickness and its initial radius of curvature, as well as the internal fluid pressure on the growth of the sac are illustrated. The existence of a critical value for a non-dimensional parameter is shown and it is proposed that the herniation process can be viewed as a biomechanically unstable phenomenon in the light of the present model.

Biomechanical Phenomena↗
PubMed1983

Mechanical behavior of fetal dura mater under large axisymmetric inflation.

The nonlinear mechanical behavior of fetal dura mater was tested experimentally and compared to two published nonlinear material strain energy functions, the Mooney-Rivlin and the Skalak, Tozeren, Zarda, and Chien (STZC). The STZC constitutive relations best fit the behavior of the dura mater and were used to describe quantitatively its stiffness. Runge-Kutta numerical procedures were used to fit the theoretical data to the experimental results. The material's stiffness was positively correlated with fetal weight (r = 0.67, p less than 0.05). These results are discussed and directions for future research indicated.

Biomechanical Phenomena↗
PubMed1978

Application of a fluid-filled spherical sandwich shell as a biodynamic head injury model for primates.

In this paper, two areas are investigated: first, the effect of pulse duration on skull fracture and brain injury and, second, under what conditions a scaling can be considered between subhuman primates and human. The skull is modeled as a spherical sandwich shell representing the general structure of the skull bones as being the compact inner and outer tables separated by a porous diploë layer. The brain and surrounding cerebrospinal fluid are modeled as an acoustic fluid with properties similar to those of water. The fluid-shell system is subjected to a local axisymmetric and arbitrary time-dependent external pulse and the transient response of the system is determined by numerical methods. The effect of pulse duration is investigated by choosing the pulse as the Heaviside function. Pulses with short duration are found to be less damaging as far as skull fracture and brain injury are concerned. Human head impact tolerance curves are obtained. For the second part of the investigation, a more realistic pulse containing both exponential and sine components is chosen to study the responses of the models of the human head and four species of primates. The numerical results are obtained for the same magnitude of the pulse as well as for equal magnitudes of initial acceleration after impact.

Animals↗