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Timely Fracture-Healing Requires Optimization of Axial Fixation Stability

¹ Center for Musculoskeletal Surgery, Charité-Universitätsmedizin Berlin, Free and Humboldt-University of Berlin, Augustenburger Platz 1, D – 13 353 Berlin, Germany. E-mail address for G.N. Duda: georg.duda{at}charite.de
² Trauma PD Center Europe, Synthes, Güterstrasse 5, CH-2544 Bettlach, Switzerland

Investigation performed at the Center for Musculoskeletal Surgery, Charité-Universitätsmedizin Berlin, Berlin, Germany

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 AO Foundation and the German Research Foundation (KFO102/2). 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.

Methods: A standardized midshaft osteotomy of the right tibia was performed on Merino-mix sheep and was stabilized with either one of four monolateral external fixators or one of two tibial nails inserted without reaming. The in vitro stiffness of fixation was determined in six loading conditions (axial compression, torsion, as well as bending and shear in the anteroposterior and mediolateral planes) on ovine tibial specimens. Stiffness was calculated by relating displacements of the fracture fragments, determined by means of attached optical markers, and the loads applied by a materials testing machine. Torsional testing until failure of the explanted tibiae was performed with use of a standard materials testing machine after nine weeks of healing to determine the failure moment and the torsional stiffness of the healed tibia.

Results: External fixation in sheep generally resulted in higher fixation stiffness than did conventional unreamed tibial nailing. The use of angle-stable locking screws in tibial nailing resulted in fixation stiffness comparable with that of external fixation. The highest torsional moment to failure was observed for the external fixator with moderate axial stiffness and high shear stiffness. The fixator with the highest axial stability did not result in the highest failure moment. Low axial stability in combination with low shear stability resulted in the lowest failure moment.

Conclusions: In this study, a clear relationship between the stability of fixation and the mechanical strength of the healing tibia was seen. Moderate levels of axial stability were associated with the highest callus strength and stiffness.

Clinical Relevance: Optimizing axial stability and limiting shear instability appear to be important for creating conditions for timely fracture-healing.

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