How do stress risers affect bone healing and fractures?
The image to the right is one of those moments where my wife just shakes her head. We were driving through northern New Mexico when I pulled over to the side of the road and got out of the car - in the middle of nowhere. And I took a picture of the guardrail on the side of the road. Getting back into the car my wife said to me, "Honey, what are we doing?" My response was to say, "It's all about stress risers, honey."
What is the mechanical definition of a stress riser?
Take a close look at this image and what you'll see is that the highway department intentionally drilled holes (stress risers) in the base of each of the first five or six wooden posts holding up the guardrail. The idea behind the holes is to decrease the impact that a car and driver would sustain upon hitting the guardrail. The posts are intentionally drilled to snap at the hole. The holes purposely make the post snap, resulting in a slower, more gradual impact. In this case, stress risers can have a very positive effect and may actually save lives.
Historically though, stress risers have been studied in engineering due to their negative impact on structures. Two of the saddest examples of negative stress risers (or stress concentrations) was the design of the Liberty Ships of WWIIand of the De Havallin CometAirplanes of the late 1950's and early 1960's.
Bone - how it responds to stress
Bone is remarkably designed to provide internal support and act as a reservoir for vitamins, minerals and bone marrow. Bone is only one of the two tissues in the body (also the liver) that has the capacity to generate more bone in response to stress. Bone also is remarkable in it's ability to configure itself in a way that provides the best design for support and yet still allow motion in all three body planes.
Stress risers in bone
Stress risers in bone develop in areas of weakness that concentrate stress. Stress risers in bone may include the following;
- Previous site of fracture
- Site of bone tumor or bone infection
- Current or previous implant site
These examples of stress risers in bone are the sites that will first undergo deformity, leading to fracture. Just like the posts on the highway, stress risers in bone will be a focal area for fracture. That's why your doctor will often remove your hardware after internal fixation is used to treat a fracture. Stress risers are probably the only downside of the science of fixation as described by the AO Foundationin Davos, Switzerland.
Thanks honey for stopping on the side of the road in New Mexico. I don't think I disturbed her knitting one bit.