International Conference and Expo on Biomechanics and Implant Design July 27-29, 2015 Florida, USA

The mechano-regulation algorithm using interstitial fluid velocity (FV) and octahedral shear strain (OSS) is well-accepted for predicting normal fracture healing in a poroelastic medium (PM). However, the ability of OSS as a single bio-feedback variable to predict the normal healing pattern in a PM is still a place of debate. This work is an attempt to check the validity of the hypothesis that the OSS could solely regulate tissue differentiation in a PM, as well as to investigate whether or not the OSS could solely regulate tissue differentiation in an elastic medium (EM). For this purpose, a 2D model of a broken tibia, with a 3mm fracture gap, was made and a biphasic finite element analysis was done to simulate bone healing process. The differentiation of stem cells and development of tissues was implemented as a biofeedback loop by use of Python scripting in Abaqus software. In the poroelastic model, results of this study showed that OSS could predict bone formation at the callus tip and the intramedullary canal, as well as its progress toward intracortical gap successfully. However, it failed to predict bony bridge at the external callus and a narrow strip area was left with fibrous tissue, which restrained completion of healing pattern. In the elastic model, OSS correctly predicted healing process, but the completion of process occurred much faster than in normal healing. In conclusion, results of this study indicate that OSS cannot be considered as a single regulator of tissue development in the normal fracture healing process.

Potential for malalignment is the main disadvantage of IM nailing, which can occur mainly as a result of reduced bone-implant contact at the distal quarter and high shear interfragmentary movements (IFM). The goal of this study was to evaluate the performance of a new designed IM nailing system in which an extra screw could be inserted right above the fracture gap. For this purpose, a three-dimensional finite element model of tibia-nail system was constructed by considering a 3mm transverse gap at the distal quarter. Physiological boundary conditions were applied, and three different materials, i.e. stainless steel, titanium, and carbon/epoxy, were assumed for the IM nail. The von Mises stress, interfragmentary movements, and the specific production of different tissue phenotypes were obtained to make a comparison among different bone-nail constructs. Results of this study showed that inserting an extra screw above the fracture gap can cause a dramatic reduction in the shear IFM. However, since axial IFM mainly occurred as a result of bony fragments’ tilting, insertion of extra screw also restricted axial movements. Thus, in order to preserve stimulatory axial micromotion, the proposed design is recommended when an IM nail with a low young modulus, such carbon/epoxy, is used. The finite element model was validated with the results of in vitro tests on cadaver.

Prof. (Dr.) D. M. Kulkarni is a Professor of Mechanical Engineering Dept at BITS Pilani, K K Birla Goa Campus, Goa, India. He is also Dean Administration and Associate Dean, Faculty Affairs. He has his Masters from IIT Kharagpur and PhD from BITS Pilani. He has 18 years of teaching and 3 years of industrial experience. His research area is Biotribology and Fracture Mechanics. He has research grants from various industries and DST in the area of BioTribology. He has published 40+ key papers in the international journal of repute and International conferences in India and abroad. He was a certified Instructor of American Society of Mechanical Engineering (ASME) to design the courses and train the industrial professionals in India.

With an increasing demand for prosthesis hip implantation, there is an increasing need for a more sophisticated wear testing device which would simulate the geometrical parameters, kinematic motions, load profile and biological lubricating conditions across the articulating surfaces of pair of materials of hip implants. Investigations on wear mechanisms and quantifying the wear depth distribution in acetabular cups have been extensively undertaken by researchers over the last two decades using in-vitro studies. While the load profile is well defined in ISO–14242 for a gait cycle, numerous investigations are performed using hip simulators and computational techniques to propose wear rate, wear tracks and wear depth distribution in acetabular cups. However, there is a need to understand wear depth distribution and wear tracks from in-vivo studies to integrate kinematic motion and load profile across articulating surfaces of hip joint and design a wear mechanism for a wear screening device.\r\nIn this work, retrieval study is carried out on a scanning white light interferometer using 20 retrieved acetabular cups from Indian patients and compared with wear depth distribution and wear tracks using hip simulators and computational techniques. \r\nMaximum linear wear was observed to be in the supero-posterior region of the cup. New wear sliding track is extracted from the wear depth distribution using Archard’s wear law and load profile in a gait cycle (as per ISO–14242). Procedure has been devised to plot the most dominant wear sliding track and a complete protocol of wear sliding tracks has been proposed to customize wear screening device (like ball-on-disc or pin-on-disc tribometer) in order to get results similar to the clinical study. Comparative results show that wear distribution and wear track is primarily dependent on the materials of the contact pair constituting the acetabular cup and the femur head, type of motion and load profile during human gait cycle over the functional life of the artificial hip joint.\r\n