The Journal of Bone and Joint Surgery (American). 2008;90:375-383.
doi:10.2106/JBJS.G.00127
© 2008 The Journal of Bone and Joint Surgery, Inc.
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Instability-Associated Changes in Contact Stress and Contact Stress Rates Near a Step-Off Incongruity

Todd O. McKinley, MD1, Yuki Tochigi, MD, PhD2, M. James Rudert, PhD2 and Thomas D. Brown, PhD2

1 Department of Orthopaedics and Rehabilitation, University of Iowa, 200 Hawkins Drive, Iowa City, IA 52242. E-mail address: todd-mckinley{at}uiowa.edu
2 Department of Orthopaedics and Rehabilitation, Biomechanics Laboratory, University of Iowa, 2181 Westlawn, Iowa City, IA 52242
Investigation performed at the Department of Orthopaedics and Rehabilitation, Biomechanics Laboratory, University of Iowa, Iowa City, Iowa

Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in excess of $10,000 from the Centers for Disease Control and Prevention (Grant R49CCR 721745) and the National Institutes of Health (P50 AR48939). Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.

Background: Intra-articular fractures can result in articular surface incongruity and joint instability, both of which can lead to posttraumatic osteoarthritis. The purpose of this study was to quantify changes in contact stresses and contact stress rates in incongruous human cadaveric ankles that were either stable or unstable. It was hypothesized that joint instability, superimposed on articular incongruity, would cause significant increases in contact stresses and contact stress rates.

Methods: Intact human cadaveric ankles were subjected to quasi-physiologic stance-phase motion and loading, and instantaneous contact stresses were captured at 132 Hz. The anterior one-third of the distal part of the tibia was displaced proximally by 2.0 mm, and testing was repeated. Anterior/posterior forces were modulated during loading to cause incongruous ankles to either remain stable or become unstable during loading. Transient contact stresses and contact stress rates were measured for seven ankles under intact, stable-incongruous, and unstable-incongruous conditions. Peak and 95th percentile values of contact stress and contact stress rates for all three conditions were compared to determine the pathomechanical effects of incongruity and instability.

Results: The addition of instability caused 95th percentile and peak contact stresses to increase approximately between 20% and 25% in the unstable-incongruous specimens compared with the stable-incongruous specimens. In contrast, the addition of instability increased the magnitude of peak positive and peak negative contact stress rates by 115% and 170% in the unstable-incongruous specimens compared with the stable-incongruous specimens. Similarly, the 95th percentile contact stress rates increased 112% in the unstable-incongruous specimens compared with the stable-incongruous specimens.

Conclusions: In human cadaveric ankles, instability superimposed on an existing articular surface incongruity causes disproportionate increases in contact stress rates compared with the increases in contact stresses.

Clinical Relevance: Cartilage injury, chondrocyte injury, and chondrocyte biosynthetic function have been shown to be particularly sensitive to the stress rate. Instability-associated increases in contact stress rates may be important pathomechanical factors that lead to posttraumatic osteoarthritis.


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