John B. Brunski, Ph.D.Professor, Biomedical Engineering |
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Research AreaIn Vivo Studies of Interfacial OverloadBased on a number of studies in animal models and humans with symptoms of overload -- such as screw loosening, implant fracture, crestal bone loss, etc. -- the hypothesis described below has evolved to explain what is happening. |
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| Very large local strains -- in excess
of the yield point of bone -- may be occurring at the micro-level, which
causes microdamage, abbreviated MDx. In turn, this MDx triggers bone
remodeling in an effort to repair the local damage. However, the
potential problem with this self-repair effort is that the remodeling
proceeds by the steps of A-R-F, Activation of osteoclasts,
Resorption
of a packet of bone, and then Formation (refilling) of the resorbed
region. While R is going on (and
R is normally much
faster than F), holes (resorption tunnels excavated by osteoclasts)
develop in the bone. This can be seen in the FE simulation above.
If loading continues while these holes exist, the local strains can worsen,
especially near the holes, thereby causing more microdamage, more A-R-F,
and so on. If this process continues unabated, it could eventually
lead to interface failure (and failure of the entire implant case) if the
damage accumulation rate exceeds the repair rate. To avoid such a
process, the goal is to design the implant loading, implant geometry, bone
contact, etc., so that interfacial strains never exceed the damage limits
during use of the implant. However, until we nail down the exact
mechanism underlying the above hypothesized events, we will not be in a
position to do this in a reliable way.
References for the photos above: top left: Prabhu A and Brunski JB (1997) "An overload failure
of a dental prosthesis: a 3D finite element nonlinear contact analysis.
In: 1997 Bioengineering Conference BED Vol 36 B. Simon (ed).
NY: ASME, pp. 141-142.
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