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Undergraduate Research Project Management System

Spinal Rod Fatigue Testing and Analysis

Status Complete
Seeking Researchers No
Start Date 01/01/2009
End Date 06/30/2009
Funding Source Dr. Alex Hills Engineering Research Award and Alaska Heart Institute Biomedical Research Fellowship Supplemental Grant
Funding Amount
Community Partner
Related Course
Last Updated 09/25/2010 01:49AM
Keywords Spinal rod, biomechanical properties, titanium, stainless steel, fatigue, fracture, failure

People

Faculty
  Anthony Paris

Student Researchers
  Alex Bergeron

Abstract

The objectives of the proposed study are to measure and compare the fatigue behavior of the three available alloys used for surgically implanted spinal rods and to publish the results for possible use by physicians, patients, engineers, and scientists. Orthopedic surgeon Andres Munk, M.D., is a collaborating researcher on the proposed study.

Medical patients with spinal injuries or conditions such as scoliosis, degenerative disc disease, spinal trauma or hernia(s) may require surgery to implant spinal rods to give the vertebra and spine needed structural support and to ensure proper bone growth. With movement, these rods are subjected to cyclical loading and fatigue, the primary cause of spinal rod failure. A person with a low intensity level of daily activity can easily walk one million steps and subject a spinal rod to one million loading cycles in six months. Over time, the cyclic loading can lead to fatigue crack initiation and growth and even fracture of the rod. For physicians and patients considering spinal rod implants, there are three different metallic alloy rods available: titanium, stainless steel and vitallium. However, little has been published on the behavior of the alloys or their relative merits. By using fatigue crack growth rate testing, this study will provide a basis for the comparison of the fatigue behavior of the available metallic rods.

The fatigue process will be initiated by introducing a circumferential pre-crack to the rod. Rotating bending fatigue will imitate the loading that a spinal rod experiences while implanted. One rotation will be considered to be one cycle and will simulate one walking step of a spinal rod patient. The load and number of cycles will be applied, and the amount of crack growth will be measured. To accurately measure crack growth, a technique called heat tinting will be used—when the alloy is heated to a moderately high temperature, the external surfaces of the alloy oxidize and change color. This will mark the initial crack length. After cycling, the specimen will be broken, and both the initial and final fatigue crack lengths will be measured. The amount of crack growth will be determined from the difference between the initial and final crack lengths. Testing will consist of varying the magnitude of the load and number of cycles.

The surgical spinal rods are expensive. To reduce costs, the rotating bending fatigue test protocol will be refined using common round bar stock specimens—the actual surgical spinal rod specimens will not be used until the test method is sufficiently refined to ensure the successful testing of each surgical spinal rod sample.

The fatigue crack growth rate will be correlated with the load, number of cycles, and crack length for each material. It is anticipated that the fatigue behavior will depend upon the metallic alloy. The results of the study will be useful to surgeons and patients considering spinal rod implantation and to engineers and scientists working in biomedicine.

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