from Part II - Mechanics
Published online by Cambridge University Press: 05 June 2012
Inquiry
How would you safely design a femoral component of a total hip replacement to prevent fatigue fracture of the stem without causing stress shielding to the adjacent bone?
The inquiry posed above represents a realistic design challenge that one might face in the field of orthopedics. Stress shielding is defined as bone loss that is owed to insufficient stress transfer from the implant to the neighboring bone. Stress shielding can be minimized through a combination of material selection and device design that provides improved compliance match to the tissue. Device design is a multifactorial process and it is equally important to ensure that the femoral stem is able to safely provide the requisite fatigue properties for successful performance of the implant. For a total hip replacement to function for two decades, the femoral component should offer a minimum of 20 million fatigue cycles to provide the estimated one million walking cycles per year for an average person. This fatigue life is further complicated by the anatomy, weight, health, and physical activity of the specific patient. The performance in vivo also depends on factors such as surgical placement and tissue healing as well as immunological response to the material. In order to design a fatigue-resistant implant, one would want to know the magnitude and nature of the expected stresses in the implant. For example, the implant may experience a combination of tensile, compressive, and bending stresses that are cyclic in nature. At a minimum, the peak stress ranges in the femoral stem should be below the endurance limit for the material and mean stresses should be incorporated into the life calculations. Further, the design should mitigate the use of sharp corners or other stress concentrations that can lead to fatigue crack propagation and fracture; if these elements exist within the design, then the fatigue life should be estimated conservatively using fracture mechanics methodologies.
Overview
Fatigue damage and associated failures are of critical concern to the medical device community as sustained loading of structural implants is inevitable. There are numerous cases of fatigue failures in medical devices owing to poor material selection, unsuitable sterilization methods, deficiencies in manufacturing processes, careless designs with inherent stress concentrations or crack initiating sites, and insufficient stress analysis given the complex multiaxial stresses of the body. The case example at the end of this chapter highlights a fatigue failure owing to the coupling of a poor component design and a material with inferior fatigue resistance.
To save this book to your Kindle, first ensure [email protected] is added to your Approved Personal Document E-mail List under your Personal Document Settings on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part of your Kindle email address below. Find out more about saving to your Kindle.
Note you can select to save to either the @free.kindle.com or @kindle.com variations. ‘@free.kindle.com’ emails are free but can only be saved to your device when it is connected to wi-fi. ‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
Find out more about the Kindle Personal Document Service.
To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Dropbox.
To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Google Drive.