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Compression
Shortens an object.
Load pushing molecules more tightly together
Bones are designed for compression. As you stand, your bones (Femur, tibia, etc.) are
supporting your body weight. This body weight is a result of gravity acting on your
mass. This causes compressive stress (load) on your bones.
Can lead to bruising of soft tissues or crushing fractures of bones
Tension
Pulling apart of molecules, resulting in elongation of material.
More likely to lead to bone fracture.
https://www.youtube.com/watch?v=FT8Xg98uw1o
Types of Fractures
Closed – skin not punctured. Open or Compound – skin
punctured and bone exposed.
Transverse – perpendicular to long axis of bone.
Comminuted – 3 or more fractures in same bone.
Undisplaced – broken pieces aligned.
Displaced – broken pieces not aligned.
Greenstick – fracture on 1 side of bone causing other side to bend.
Healing Nature of Bones
Bones have the ability to heal
Key Players: Osteoclasts – break down bone Osteoblasts – build new bone
http://www.exogen.com/physicians/how-it-works/
Inflammation
1) Fracture Hematoma forms – helps keep the bones aligned & cuts off oxygen from the fractured bone (killing it)
Source: http://science.howstuffworks.com/life/human-biology/heal-broken-bones.htm
Soft Callus
Several days later, the fracture hematoma turns into a soft callus Fibroblasts develop collagen & chondroblasts
begin developing fibrocartilage Transforms from soft callus to fibrocartilage
callus – bridges gap between bones (3 wks)
Source: http://science.howstuffworks.com/life/human-biology/heal-broken-bones.htm
Bone Callus (Hard Callus)
Formation of the bone callus Osteoblasts begin building new bone 3-4 month process Provides enough stability for the bone to
finish healing
Source: http://science.howstuffworks.com/life/human-biology/heal-broken-bones.htm
Remodeling
Not done yet… Osteoclasts and Osteoblasts spend months
remodeling the bone by replacing bone callus with compact bone
Overtime, remodeling reduces the bulge
Weight-lifting
Weight-lifting causes small fractures to occur in our bones, but DON’T panic!
These fractures signal osteoclasts to come remove the damaged bone and osteoblasts to build new bone.
Overtime, you end up with stronger, healthier bones!
Also, remember, the more muscles your have, the more bone you need at your joints to hold those muscles, further strengthening your bones.
Casting
Some fractures only require casting to aid healing.
Casts provide stability and prevent the bones from misaligning during healing.
Repairing Bones: Overview
Some serious breaks need the aid of engineers because: Need to restore function and position Likely not to heal correctly High risk of infection Very long healing time
Biomedical engineers use internal and external fixation approaches
While beneficial, added challenges and possible complications exist
Internal vs. External Fixation
Internal fixation: Temporary or
permanent fixtures directly attached to the bone under the skin, for alignment and support pins rods or nails plates screws wires grafting
External fixation:
Temporary repair supports outside of the skin that stabilize and align bone while the body heals screws in bone to hold
in place metal braces or casts can be externally
adjusted
Internal Fixation
To determine the best repair technique, the break type and location are considered
Image source: US National Library of Health, National Institutes of Health, Medline Plus, http://www.nlm.nih.gov/medlineplus/ency/presentations/100077_3.htm
External Fixation
Installing temporary repair supports outside of the skin to stabilize and align bone while the body heals.
Examples: screws in bone, metal braces, casts, slings.
Image source: US National Library of Medicine, National Institutes of Health, MedlinePlus, http://www.nlm.nih.gov/medlineplus/ency/imagepages/18021.htm
Example: Rods, Plates and Screws
Rods are used for alignment and support of long and large bones
Plates hold together loose pieces of bone and support smaller bones
Screws hold plates and rods in place
Image source: US National Library of Medicine, National Institutes of Health, Medline Plus, http://www.nlm.nih.gov/medlineplus/ency/imagepages/18023.htm
More Plates and Screws
X-ray example of shattered dog femur that was repaired with a plate and seven screws
Image source: David M. Nunamker, University of Pennsylvania School of Veterinary Medicine. http://cal.vet.upenn.edu/projects/saortho/chapter_39/39mast.htm Used with permission.
Medical Implant Materials
Bone is an amazing material: strong and flexible
Most human-made materials that are strong are also brittle
To be accepted by the body and not cause other problems, the materials for rods, pins, screws and plates must also be biocompatible.
Engineers design materials especially for medical implants that are made of: Surgical stainless steels
(blends of nickel, chrome and molybdenum) Titanium alloys Polymers
Results
After a few months, patients should be back on their feet, ready to participate in everyday activities
Image source: US National Library of Medicine, National Institutes of Health, MedlinePlus, http://www.nlm.nih.gov/medlineplus/ency/presentations/100077_4.htm
Your Design Challenge
You will engineer a device to support the broken bone throughout healing.
Your device will be tested in the same way the bone was broken. Can you make it stronger?
Things to consider: Strong? Minimally invasive? Compatible? Inexpensive? Ease of implementation?