Influence of the fixation stability on the healing time--a numerical study of a patient-specific fracture healing process.
BACKGROUND
The healing outcome of long bone fractures is strongly influenced by the interfragmentary movement of the bone fragments. This depends on the fixation stability, the optimum value of which is still not known. The aim of this study was to simulate a patient-specific human healing process using a numerical algorithm and to retrospectively analyse the influence of the fixation stability on the healing time.


METHODS
The healing simulation was processed as an initial value problem. This was iteratively solved based on two mechanical (invariants of the strain tensor, calculated through a finite element analysis) and five biological state variables (local tissue composition and blood perfusion) using a previously published fuzzy logic algorithm. For validation purposes, the calculated interfragmentary movement was compared to in vivo measurements of this patient. By changing clinically adjustable parameters of the fixation device, the influence of the fixation stability on the healing time was analysed.


RESULTS
The time course showed good agreement of the interfragmentary movement compared with the in vivo measurements. The predicted healing time was strongly influenced by the fixation stability, i.e. by changing the parameters of the fixation device, it was possible to significantly reduce the healing time.


CONCLUSIONS
The time to heal could be greatly reduced by modification of the fixator design, i.e. increasing the fixation stiffness. When using external fixation devices, this could be achieved by decreasing the free bending length of the pins, using a stiff fixation body and a stiff connection between the pins and the body.
Wehner
Tim
T
Institute of Orthopaedic Research and Biomechanics, Ulm University, Helmholtzstrasse 14, Ulm, Germany. timwehner@web.de
Claes
Lutz
L
Niemeyer
Frank
F
Nolte
Daniel
D
Simon
Ulrich
U
D000465:Algorithms; D001696:Biomechanical Phenomena; D004867:Equipment Design; D016267:External Fixators; D005592:Fracture Fixation / Q000295:instrumentation; D017102:Fracture Healing; D050723:Fractures, Bone / Q000601:surgery; D017143:Fuzzy Logic; D006801:Humans; D016012:Poisson Distribution; D012189:Retrospective Studies; D013978:Tibial Fractures / Q000601:surgery; D013997:Time Factors