Accepted Manuscripts

Viswanath Managuli and Sitikantha Roy
J Biomech Eng   doi: 10.1115/1.4037289
A new asymptotically correct contact model has been developed for conical tip based AFM nanoindentation. This new model provides both elastic and non-specific adhesion properties of cells and soft gels by taking sample thickness at the point of indentation and its depth of indentation into consideration. The bottom substrate effect is the most common source of error in the study of "AFM force maps" of the cellular sample. The present model incorporates an asymptotically correct correction term as a function of depth of indentation to eliminate the substrate effect in the analysis. Later, the model is extended to analyze the unloading portion of the indentation curve to extract the stiffness and adhesive properties simultaneously. A comparative study of the estimated material properties using the other established contact models show that the provided corrections effectively curb the errors coming from infinite thickness assumption. Non-specific adhesive nature of cell is represented in terms of adhesion parameter (?a) based on the "work of adhesion", this is an alternative to the peak value of tip-sample attractive (negative) force commonly used as representative adhesion measurement. The simple analytical expression of the model can help in estimating more realistic and accurate biomechanical properties of cells from atomic force microscopy based indentation technique.
TOPICS: Adhesives, Biological cells, Adhesion, Atomic force microscopy, Errors, Nanoindentation, Stiffness, Biomechanics, Materials properties, Curbs (Roads)
Arnab Chanda, Isuzu Meyer, Holly E. Richter, Mark. E Lockhart, Fabia R. D. Moraes and Vinu U. Unnikrishnan
J Biomech Eng   doi: 10.1115/1.4037222
Pelvic organ prolapse (POP), downward descent of the pelvic organs resulting in a protrusion of the vagina, is a highly prevalent condition, responsible for 300,000 surgeries in the United States annually. Rectocele, a posterior vaginal wall prolapse of the rectum is the second most common type of POP after cystocele. A rectocele usually manifests itself along with other types of prolapse with multi-compartment pelvic floor defects. To date, the specific mechanics of rectocele formation are poorly understood, which does not allow its early stage detection and progression prediction over time. Recently, with the advancement of imaging and computational modeling techniques, a plethora of finite element (FE) models have been developed to study vaginal prolapse from different perspectives and allow a better understanding of dynamic interactions of pelvic organs and their supporting structures. So far, most studies have focused on anterior vaginal prolapse (or cystocele) and limited data exist on the role of pelvic muscles and ligaments on the development and progression of rectocele. In the current work, a full scale 3-dimensional (3D) computational model of the female pelvic anatomy, comprising the vaginal canal, uterus, rectum and the fibromuscular connective tissue between the rectum and the posterior vaginal wall, has been developed to study the effect of varying degrees (or sizes) of rectocele prolapse on the vaginal canal for the first time. Vaginal wall displacements and stresses generated due to the varying rectocele size were estimated objectively. Additionally, clinical relevance and implications of the results were discussed.
TOPICS: Canals, Computer simulation, Stress, Biological tissues, Finite element analysis, Surgery, Muscle, Imaging, Package on package, Anatomy
Kaushik Mukherjee and Sanjay Gupta
J Biomech Eng   doi: 10.1115/1.4037223
Bone ingrowth and remodelling are two different evolutionary processes which might occur simultaneously. Both these processes are influenced by local mechanical stimulus. However, a combined study on bone ingrowth and remodelling has rarely been performed. This study is aimed at understanding the relationship between bone ingrowth and adaptation and their combined influence on fixation of the acetabular component. Based on 3-D macroscale FE model of implanted pelvis and microscale FE model of implant-bone interface, a multiscale framework has been developed. The numerical prediction of peri-acetabular bone adaptation was based on a strain-energy density based formulation. Bone ingrowth in the microscale models was simulated using the mechanoregulatory algorithm. An increase in bone strains near the acetabular rim was observed in the implanted pelvis model, whereas, the central part of the acetabulum was observed to be stress shielded. Consequently, progressive bone apposition near the acetabular rim and resorption near the central region were observed. Bone remodelling caused a gradual increase in the implant-bone relative displacements. Evolutionary bone ingrowth was observed around the entire acetabular component. Poor bone ingrowth of 3-5% was predicted around the centro-inferio and inferio-posterio-superio-peripheral regions owing to higher implant-bone relative displacements. Whereas, the antrerio-inferior and centro- superior regions exhibited improved bone ingrowth of 35-55% due to moderate implant-bone relative displacement. For an uncemented acetabular CoCrMo component, bone ingrowth had hardly any effect on bone remodelling; however, bone remodelling had considerable influence on bone ingrowth.
TOPICS: Bone, Finite element analysis, Microscale devices, Finite element model, Density, Stress, Algorithms, Displacement
Technical Brief  
Stephen Mattucci, Jie Liu, Paul Fijal, Wolfram Tetzlaff and Thomas R Oxland
J Biomech Eng   doi: 10.1115/1.4037224
Dislocation is the most common, and severe, spinal cord injury (SCI) mechanism in humans, yet there are few preclinical models. While dislocation in the rat model has been shown to produce unique outcomes, like other closed column models it exhibits higher outcome variability. Refinement of the dislocation model will enhance the testing of neuroprotective strategies, further biomechanical understanding, and guide therapeutic decisions. The overall objective of this study is to improve biomechanical repeatability of a dislocation SCI model in the rat, through the following specific aims: i) design new injury clamps that pivot and self-align to the vertebrae; ii) measure intervertebral kinematics during injury using the existing and redesigned clamps; and iii) compare relative motion at the vertebrae-clamp interface to determine which clamps provide the most rigid connection. Novel clamps that pivot and self-align were developed based on quantitative rat vertebral anatomy. A dislocation injury was produced in 34 rats at C4/C5 using both the existing and redesigned clamps, and a high-speed x-ray device recorded kinematic motion. Relative motion between the caudal clamp and C5 was significantly greater in the existing clamps compared to the redesigned clamps in dorsoventral translation and rotation. This study demonstrates that relative motions can be of magnitudes that likely affect injury outcomes. We recommend such biomechanical analyses be applied to other SCI models when repeatability is an issue. For this dislocation model, the results show the importance of using clamps that pivot and self-align to the vertebrae.
TOPICS: Biomechanics, Dislocations, Wounds, Spinal cord, Performance, Clamps (Tools), Kinematics, Rotation, X-rays, Design, Anatomy, Testing
Technical Brief  
Nachiket M. kharalkar, Steven C. Bauserman and Jonathan W. Valvano
J Biomech Eng   doi: 10.1115/1.4026559
Effect of formalin fixation on thermal conductivity of the biological tissues is presented. A self-heated thermistor probe was used to measure the tissue thermal conductivity. The thermal conductivity of muscle and fatty tissue samples was measured before the formalin fixation and then 27 hours after formalin fixation. The results indicate that the formalin fixation does not cause a significant change in the tissue thermal conductivity of muscle and fatty tissues. In the clinical setting, tissues removed surgically are often fixed in formalin for subsequent pathological analysis. These results suggest that, in terms of thermal properties, it is equally appropriate to perform in vitro studies in either fresh tissue or formalin-fixed tissue.
TOPICS: Thermal conductivity, Biological tissues, Muscle, Probes, Surgery, Thermal properties

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