The Effects of Microdamage and Aging on Loadbearing Bone

Posted on | August 4, 2011 | No Comments

Osteoporotic Triggers

Glen L. Niebur

  • Glen L. Niebur
    Roth-Gibson Associate Professor
    Aerospace and Mechanical Engineering

Some of nature’s strongest structures can also be the most fragile. Consider the egg; it cannot be broken from its domed ends but cracks easily on its side. Bone, particularly loadbearing bone, is similar in that it can become more directionally dependent and susceptible to microcracks in its structure as it ages. This microdamage affects its mechanical abilities and has been linked to osteoporosis. Notre Dame faculty are studying the causes of microdamage, its relationship to osteoporosis, and its effect on the mechanical properties of bone, the results of which will be applied to the diagnosis and treatment of osteoporosis, prosthesis loosening, the development of artificial bone substitutes, and the design of drugs to combat bone disease.

No one can explain why a 54-year old athlete participating in Scottish Highland Games can survive being hit in the thigh by an errant hammer with nothing more than a hefty bruise, while a 58-year old disc jockey breaks a hip slipping in an icy parking lot. It’s not something that can be attributed solely to good genes or even an elevated level of physical activity. The truth is that one in every 30 men and one in every nine women will fracture a hip. And about half of the women who do so will be unable to walk unaided afterward. Because of this, it is imperative to develop a keener understanding of the biomechanics of bone, specifically trabecular bone.

A living tissue, bone is always adjusting to its environment, including healing the microcracks caused by everyday activities. This is part of what it is designed to do. Trabecular bone is an extremely porous form of bone that bears the brunt of the load in the hip and spine, which are the two most common and debilitating sites of osteoporotic fractures. As humans age, trabecular bone weakens. Changes occur in the arrangement of the plates and rods of its structure, and tiny cracks accumulate in the remaining material. Unfortunately, treating either the structural issues or the cracks in trabecular bone can aggravate the other condition.

Microdamage image

Microdamage in bone can be labeled with fluorescent agents visualized under microscopy, and compared to computational models created from micro-CT images of the bone to identify microscopic structures where fractures are most likely to initiate.

Neibur research image

Three-dimensional visualization was used to better characterize osteoporotic bone. The blue regions indicate tissue yield under compressive strain and the red under tensile strain, while the arrows indicate the loading direction. Loading orientation and strain both affect bone mechanics, particularly in osteoporotic bone which has thinner trabeculae, providing insights into bone fragility and the mechanisms that affect bone performance.

In light of the country’s aging population (every day for the next 19 years approximately 10,000 Baby Boomers will turn 65) and reflective of the 300,000 hip fractures occurring in the U.S. annually (approximately 90 percent of which are found in people over 60), the clinical and economic implications related to the biomechanical behavior of trabecular bone is staggering. “We know that trabecular architecture plays a role in bone mechanics,” says Glen L. Niebur, the Roth-Gibson Associate Professor of Aerospace and Mechanical Engineering. “We also know that it changes as the body ages and can deteriorate through osteoporosis so that some individuals fracture their bone during routine daily activities.” What Niebur, Associate Professor Ryan Roeder, Professor Tim Ovaert, and their team (A. Simon Turner, Colorado State University; X. Edward Guo, Columbia University; and Notre Dame graduate students Jacqueline Garrison, Ziheng Wu, and Xiutao Shi) are investigating is how the changes in the pore space and the bone material affect the likelihood of damage formation and risk of fracture. Because while it’s true that micro-damage can be repaired through natural bone remodeling, if the damage accumulation is beyond the bone’s ability to heal itself, it can change the mechanical properties of the bone.

Throughout the project, which is funded by the National Institutes of Health, the team has been studying the architectural features of the bone using an Individual Trabeculae Segmentation technique and comparing the findings to the microdamage assessed via computer models and experimental measurements. Both in vitro and in vivo microcrack densities were related to the Structure Model Index, a measure of the degree to which the bone is composed of plate-like or rod-like elements. Their results suggest that the more rod-like regions in trabecular bone, the more susceptible it is to microdamage formation, fatigue, and cracks that cannot be repaired through normal biological processes.

In a parallel project, Niebur and Roeder are exploring new methods to image damage formation in bone via X-ray contrast agents and high-resolution CT imaging. Together, these projects will allow Notre Dame’s Orthopaedics research group to better understand the fracture susceptibility of bone in order to improve diagnosis and treatment of osteoporosis.

Suggested Reading

Shi, Xiutao; Liu, X. Sherry; Wang, Xiang; Guo, X. Edward; and Niebur, Glen L., “Type and Orientation of Yielded Trabeculae during Overloading of Trabecular Bone along Orthogonal Directions,” Journal of Biomechanics, 2010, 43, 13, 2460-66.

Shi, Xiutao; Liu, X. Sherry; Wang, Xiang; Guo, X. Edward; and Niebur, Glen L., “Effects of Trabecular Type and Orientation on Microdamage Susceptibility in Trabecular Bone,” Bone, 2010, 46, 5, 1260-66.

Garrison, Jacqueline G.; Slaboch, Constance L.; and Niebur, Glen L., “Density and Architecture Have Greater Effects on the Toughness of Trabecular Bone than Damage,” Bone, 2009, 44, 5, 924-49.

Shi, Xiutao; Wang, Xiang; and Niebur, Glen L., “Effects of Loading Orientation on the Morphology of the Predicted Yielded Regions in Trabecular Bone,” Annals of Biomedical Engineering, 2009, 37, 2, 354-362.

Wang, Xiang; Masse, Daniel B.; Leng, Huijie; Hess, Kevin P.; Ross, Ryan D.; Roeder, Ryan K.; and Niebur, Glen L., “Detection of Trabecular Bone Microdamage by Micro-computed Tomography,” Journal of Biomechanics, 2007, 40, 15, 3397-3403.

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