BIOE 3200 - Fall 2015

Hard and Soft Tissue Biomechanics

BIOE 3200 Fall 2015

BIOE 3200 - Fall 2015

Identify tissue structures and components that contribute and/or explain viscoelastic properties for different biologic materials◦ These slides focus on bone◦ In class we will focus on ligament

Learning Objective:

σ= f(ε, ἐ); E = f(ε, ἐ) Strain rate in daily activities

increases as activity becomes more strenuous◦ Slow walking ~ 0.001/sec◦ Brisk walking ~ 0.01/sec

For typical daily activities, E changes by ~15%

◦ Slow running ~ 0.03/sec◦ Fast running ~ 0.06/sec

We can estimate the % change in bone strength for brisk walking versus slow walking

BIOE 3200 - Fall 2015

Cortical bone: strain rate sensitivity

Loaded slower

Loaded faster

BIOE 3200 - Fall 2015

Graph of bone strain over time for adult human cortical bone in tension shows…◦ Bone will continue to deform if

under constant stress for an extended period of time

◦ If loaded for long enough time, cortical bone will break at a stress well below yield and ultimate strengths.

◦ Creep without fracture: permanent deformation (viscoplastic behavior)

Creep: cortical bone continues to deform under constant stress.

BIOE 3200 - Fall 2015

As loading rate increases by 6 orders of magnitude:◦ Modulus (E) increases

by a factor of 2 ◦ Strength (σ) increases

by a factors of 3

Ultimate strength of cortical bone is more sensitive to strain rate than modulus.

BIOE 3200 - Fall 2015

Mechanical Properties of Cortical and Trabecular Bone

BIOE 3200 - Fall 2015

Mechanical properties of trabecular bone depend on apparent density

BIOE 3200 - Fall 2015

Effects of Age on Mechanical Properties of Bone

From Orthopaedic Biomechanics, Bartel, Davy and Keaveny (2006).

BIOE 3200 - Fall 2015

Quick review: Composition and Structure of Bone

Bone tissue:◦ Mineral phase (70% by

weight) Calcium phosphate

(hydroxyapatite) Calcium carbonate

◦ Organic matrix (20% by weight) with cells embedded Type I collagen (90%) Other glycoproteins

and glycosaminoglycans (10%)

◦ Water (10% by weight)

BIOE 3200 - Fall 2015

Question: What are combined effects of

viscoelastic behavior and age on bone?Consider effects of factors such as strain rate, structure, and level of osteoporosis for, say, a 90 year old compared to a 20 year old.