Sanders, A. P., and R. M. Brannon. (2012). “ Scaled surrogate Hertzian bearing pairs for contact and wear testing.” Transactions of the Orthopaedic Research Society 2012 Annual Meeting, San Francisco, CA, Feb. 4-7, Poster 2070. 2012 ORS poster 01 small
New implant bearing materials require extensive laboratory testing before clinical use, but the currently practiced contact and wear test methods impose limitations. Screening wear tests of prototype materials are typically done using simple bearing shapes (such as a ball-on-flat pair) and low loads. These tests are relatively simple and inexpensive, but they lack representative bearing shapes and contact stresses. Simulator wear tests on full-scale components overcome this shortcoming by implementing higher loads and complex, physiologic motion patterns. However, these tests are lengthy and expensive; so, they are reserved for final design testing. Surrogate test specimens that would mimic the contact mechanics of full-scale bearing pairs could improve the relevance of early screening tests. This research examines the hypothesis that a reduced-scale surrogate Hertzian contact pair can elicit a smaller scale, equal stress version of the contact response of a larger original contact pair. A chosen original contact pair mimics a knee implant femoral-tibial condylar interface, and a full-scale surrogate pair is found using recently published formulas. New formulas were derived to find a smaller version of the surrogate pair. The contact pairs were tested in quasi-static normal loading, and their contact patches were measured to evaluate the hypothesis.
Kakarla, D., A. P. Sanders, S. Siskey, K. Ong, N. Ames, J. O. Ochoa, and R. M. Brannon. (2012). “Modeling, Testing, and Analysis of Impulse Response of Femoral Head Reduction in Ceramic Hip Prostheses.” Transactions of the Orthopaedic Research Society 2012 Annual Meeting, San Francisco, CA, Feb. 4-7, Poster 2076.
Hip simulator wear tests including micro-separation conditions have revealed that abnormal loading events can outweigh normal loading conditions in causing wear of hard-on-hard bearings. Yet, there is a paucity of data to describe the mechanics of abnormal events such as edge loading by femoral neck impingement or femoral head subluxation. Though the magnitude of head subluxation has been measured in-vivo for a variety of human activities, there are apparently no corresponding reports of the concurrent head-liner contact forces; accurate measurements of the same may be rendered difficult by the transient, impulsive nature of edge loading. This report provides initial laboratory results of an in-vitro and in-silico study of impulsive femoral head reduction whose ultimate aim is to quantify dynamic edge-loading contact forces and stresses. The study implements an engineering model of proximal-lateral head subluxation and edge loading as could occur in a lax hip during the swing phase of gait. Rapid reduction is caused by applying a sudden cranio-caudal motion to the acetabular liner. In the laboratory, the femur’s response to this input is measured with strain gages and a laser vibrometer.
Spectral analysis of ceramic hip squeaking
Sanders, A., I. Tibbitts, and R. Brannon. (2012). “Concomitant evolution of wear and squeaking in dual-severity, lubricated wear testing of ceramic-on-ceramic hip prostheses.” Journal of Orthopaedic Research: DOI 10.1002/jor.22080.
Ceramic-on-ceramic (CoC) hip bearings were tested in short-term wear tests with a systematically varied contact force. Continuous vibration and intermittent surface roughness measurements were obtained to elucidate potential causes of in vivo hip joint squeaking. The three-phase test comprised alternating cycles of edge loading (EL) and concentric articulation (CA), always using ample serum lubricant. A 50,000-cycle wear trial in which the contact force during CA was distant from the head’s wear patch yielded no squeaking and practically no liner roughening. In 10-cycle trials of an edge-worn head coupled with a pristine liner, the contact force was varied in magnitude and point of application; immediate, recurrent squeaking occurred only when the contact force exceeded a critical threshold value and was centered upon the head’s wear patch. In a 27,000-cycle wear trial with the contact force applied near the margin of the head’s wear patch, recurrent squeaking emerged progressively as the liner’s inner surface was roughened via its articulation with the worn portion of the head. The results reveal key conditions that yield recurrent squeaking in vitro in various scenarios without resorting to implausible dry conditions. A fundamental theory explains that hip squeaking is induced by myriad stress waves emanating from asperity collisions; yet, the root cause is edge loading.
Sanders, A. P., P. J. Dudhiya, and R. M. Brannon. (2012). “Thin Hard Crest on the Edge of Ceramic Acetabular Liners Accelerates Wear in Edge Loading.” Journal of Arthroplasty 27(1): 150-152.
Ceramic acetabular liners may exhibit a small, sharp crest—an artifact of discontinuous machining steps—at the junction between the concave spherical surface and the interior edge. On 3 ceramic liners, this crest was found to form a 9° to 11° deviation from tangency. Edge loading wear tests were conducted directly on this crest and on a smoother region of the edge. The crest elicited 2 to 15 times greater volumetric wear on the femoral head. The propensity of the crest to rapidly (<2000 wear cycles) cause elevated wear under low contact force (200 N) suggests that the crest artifact of prevailing machining protocols might be a root cause of stripe wear and squeaking in ceramic acetabular bearings.
These are power-point slides that I used during my lectures in the Computational Constitutive Modelling on October 25 and 27, 2011. The power-point slides can be found here.
The following tutorial provides instructions for both the host (CSM group) and guest to set up videoconferencing.
METHOD 1 (for impromptu small meetings without graphics sharing)
Remote guest can make the request to Dr. Brannon, whose Skype name is rebecca.brannon
METHOD 2 (for extended multi-participant meetings with graphics sharing)
The Interactive Video Conferencing (IVC) equipment at the University of Utah allows us to connect to other people and places throughout the state and the world.
Host (CSM personnel) instructions:
The following steps are necessary for an IVC meeting:
- To schedule an IVC meeting, the CSM personnel should contact the IVC through one of the following options:
1. call 435-879-4762
2. e-mail firstname.lastname@example.org
3. fill the forms here.
- The IVC staff find an available room on campus and arrange a test call with the guest.
- If the test connection is successful, the IVC staff schedule a connection for the actual meeting.
- The CSM personnel should be trained on how to use the equipment. For this purpose, the IVC staff provide a short training session for the CSM personnel.
The guest should have the required equipment, and provide its IP number to the CSM personnel. The guest and the CSM personnel should be in contact to schedule a test call and troubleshoot any issue.
Some ceramic-on-ceramic hip implants have been shown to squeak in vivo. While many researchers have investigated the squeaking phenomenon, the root cause is still debated. The most widely accepted hypotheses postulate that squeaking occurs as a result of edge-loading, stripe-wear, vibrations that are amplified by the femoral stem, dryness, or a combination of the foregoing. In our custom test apparatus to asses wear related squeaking, we found that even when both implants are severely worn, squeaking only occurs under dry conditions as shown in the attached video.
Ceramic-on-Ceramic Hip Implants Squeak Only When Dry
After you have uploaded a picture, there may be a chance that you will want to crop or re-size it to make it look better; you may also want to change what portion of the image shows up in the thumbnail. The following steps will help you with these goals:
1)In the media library, click on the image you would like to edit. On the next screen click the “Edit Image” button underneath the picture.
2)Select whether the changes you are about to make should apply to the full image or the thumbnail.
3)Drag a box, on the image, over the part that you would like to keep (or show up in the thumbnail).
4)Click the crop icon above the image. This will then show you what the cropped image looks like.
5)Once satisfied with the cropping, click the save button below the image. This will take you out of the edit page.
6)Finally, click the update image button and you are done.
Red numbers correspond with the steps above
However, if you are editing an image that you have already put in a post you will need to take one additional step. Go into the edit page for the post and remove the current image, then insert the image you just edited. It should carry over the previous caption/settings. As always, check the post to make sure it looks good and you are done!