title: The Role of Fluid Dynamics in Distributing Ankle Stresses in Anatomic and Injured States.
abstract: BACKGROUND
  In 1976, Ramsey and Hamilton published a landmark cadaveric study demonstrating a dramatic 42% decrease in tibiotalar contact area with only 1 mm of lateral talar shift. An increase in maximum principal stress of at least 72% is predicted based on these findings though the delayed development of arthritis in minimally misaligned ankles does not appear to be commensurate with the results found in dry cadaveric models. We hypothesized that synovial fluid could be a previously unrecognized factor that contributes significantly to stress distribution in the tibiotalar joint in anatomic and injured states.
  
  
  METHODS
  As it is not possible to directly measure contact stresses with and without fluid in a cadaveric model, finite element analysis (FEA) was employed for this study. FEA is a modeling technique used to calculate stresses in complex geometric structures by dividing them into small, simple components called elements. Four test configurations were investigated using a finite element model (FEM): baseline ankle alignment, 1 mm laterally translated talus and fibula, and the previous 2 bone orientations with fluid added. The FEM selected for this study was the Global Human Body Models Consortium-owned GHBMC model, M50 version 4.2, a model of an average-sized male (distributed by Elemance, LLC, Winston-Salem, NC). The ankle was loaded at the proximal tibia with a distributed load equal to the GHBMC body weight, and the maximum principal stress was computed.
  
  
  RESULTS
  All numerical simulations were stable and completed with no errors. In the baseline anatomic configuration, the addition of fluid between the tibia, fibula, and talus reduced the maximum principal stress computed in the distal tibia at maximum load from 31.3 N/mm2to 11.5 N/mm2. Following 1 mm lateral translation of the talus and fibula, there was a modest 30% increase in the maximum stress in fluid cases. Qualitatively, translation created less high stress locations on the tibial plafond when fluid was incorporated into the model.
  
  
  CONCLUSIONS
  The findings in this study demonstrate a meaningful role for synovial fluid in distributing stresses within the ankle that has not been considered in historical dry cadaveric studies. The increase in maximum stress predicted by simulation of an ankle with fluid was less than half that projected by cadaveric data, indicating a protective effect of fluid in the injured state. The trends demonstrated by these simulations suggest that bony alignment and fluid in the ankle joint change loading patterns on the tibia and should be accounted for in future experiments.
  
  
  CONCLUSIONS
  Synovial fluid may play a protective role in ankle injuries, thus delaying the onset of arthritis. Reactive joint effusions may also function to additionally redistribute stresses with higher volumes of viscous fluid.
authors.affiliation: Rush University Medical Center, Chicago, IL, USA hamidinnovation@gmail.com.
authors.forename: Kamran S
authors.initials: KS
authors.lastname: Hamid
authors.affiliation: Wake Forest School of Medicine, Winston-Salem, NC, USA.
authors.forename: Aaron T
authors.initials: AT
authors.lastname: Scott
authors.affiliation: Hospital for Special Surgery, New York, NY, USA.
authors.forename: Benedict U
authors.initials: BU
authors.lastname: Nwachukwu
authors.affiliation: Wake Forest School of Medicine, Winston-Salem, NC, USA.
authors.forename: Kerry A
authors.initials: KA
authors.lastname: Danelson
mesh_terms: D000842:Ankle / Q000502:physiology* / Q000503:physiopathology; D016512:Ankle Injuries / Q000503:physiopathology*; D001696:Biomechanical Phenomena; D005360:Fibula / Q000502:physiology; D020342:Finite Element Analysis; D006801:Humans; D057446:Hydrodynamics; D008954:Models, Biological; D013314:Stress, Mechanical; D013582:Synovial Fluid / Q000502:physiology*; D013628:Talus / Q000502:physiology; D013977:Tibia / Q000502:physiology; D016474:Weight-Bearing
keywords: ankle; cadaver; cadaveric; contact area; distribution; finite element analysis; finite element model; finite element modeling; force; joint; load; redistribution; stress; tibiotalar