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Draper, UT, United States

Sanders A.P.,University of Utah | Sanders A.P.,Ortho Development Corporation | Brannon R.M.,University of Utah
Journal of Biomechanics | Year: 2011

The components of prosthetic hip bearings may experience in-vivo subluxation and edge loading on the acetabular socket as a result of joint laxity, causing abnormally high, damaging contact stresses. In this research, edge-loaded contact of prosthetic hips is examined analytically and experimentally in the most commonly used categories of material pairs. In edge-loaded ceramic-on-ceramic hips, the Hertzian contact theory yields accurate (conservatively, <10% error) predictions of the contact dimensions. Moreover, the Hertzian theory successfully captures slope and curvature trends in the dependence of contact patch geometry on the applied load. In an edge-loaded ceramic-on-metal pair, a similar degree of accuracy is observed in the contact patch length; however, the contact width is less accurately predicted due to the onset of subsurface plasticity, which is predicted for loads >400. N. The Hertzian contact theory is shown to be ill-suited to edge-loaded ceramic-on-polyethylene pairs due to polyethylene's nonlinear material behavior. This work elucidates the methods and the accuracy of applying classical contact theory to edge-loaded hip bearings. The results help to define the applicability of the Hertzian theory to the design of new components and materials to better resist severe edge loading contact stresses. © 2011 Elsevier Ltd.

Chyr A.,University of Utah | Sanders A.P.,University of Utah | Sanders A.P.,Ortho Development Corporation | Raeymaekers B.,University of Utah
Wear | Year: 2013

The statistical survivorship of total knee replacement (TKR) devices declines dramatically after fifteen years of use because the articulating surfaces wear, and biologically active wear particles may induce osteolysis and subsequent loosening of the implant. This lack of durability leads to revision surgery or surgery postponement. Thus, friction and wear evaluation is of primary concern when designing the next generation of TKR bearings. Presently, knee simulators are used to aid in the development of TKR devices, mimicking six degrees of freedom of the knee joint under simulated gait. However, these simulator experiments are lengthy and, thus, do not allow for efficient testing of large numbers of bearing material pairs and designs. Conversely, inexpensive pin-on-disk (POD) tests allow for efficient testing, but the results may have limited clinical relevance. This paper presents the design of a hybrid friction and accelerated wear testing apparatus that combines elements of both a POD apparatus and a knee simulator. The apparatus simulates dynamic axial loading and flexion/extension rotation, and it enables fast and inexpensive screening of TKR bearing materials with boundary and loading conditions that are more clinically relevant than those of a POD apparatus. © 2013 Elsevier B.V.

Sanders A.P.,Ortho Development Corporation | Brannon R.M.,University of Utah
Journal of Tribology | Year: 2011

Laboratory testing of contact phenomena can be prohibitively expensive if the interacting bodies are geometrically complicated. This work demonstrates means to mitigate such problems by exploiting the established observation that two geometrically dissimilar contact pairs may exhibit the same contact mechanics. Specific formulas are derived that allow a complicated Hertzian contact pair to be replaced with an inexpensively manufactured and more easily fixtured surrogate pair, consisting of a plane and a spheroid, which has the same (to second-order accuracy) contact area and pressure distribution as the original complicated geometry. This observation is elucidated by using direct tensor notation to review a key assertion in Hertzian theory; namely, geometrically complicated contacting surfaces can be described to second-order accuracy as contacting ellipsoids. The surrogate spheroid geometry is found via spectral decomposition of the original pair's combined Hessian tensor. Some numerical examples using free-form surfaces illustrate the theory, and a laboratory test validates the theory under a common scenario of normally compressed convex surfaces. This theory for a Hertzian contact substitution may be useful in simplifying the contact, wear, or impact testing of complicated components or of their constituent materials. © 2011 American Society of Mechanical Engineers.

Ortho Development Corporation | Date: 2014-08-07

Orthopedic implants, prosthesis and instrumentation, namely, artificial orthopedic joint implants, joint prostheses and surgical instruments all for total joint replacement.

Ortho Development Corporation | Date: 2014-06-10

Orthopedic implants and prosthesis, namely knee prosthesis.

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