TRAUMA SURGERY Underneath the cerclage: an ex vivo study on the cerclage-bone interface mechanics Mark Lenz • Stephan Marcel Perren • Boyko Gueorguiev • Robert Geoff Richards • Fabian Krause • Alberto Fernandez dell’Oca • Dankward Ho ¨ntzsch • Markus Windolf Received: 4 April 2012 / Published online: 28 June 2012 Ó Springer-Verlag 2012 Abstract Introduction Cerclages regain interest due to a rising number of periprosthetic fractures. The contact distribution at the circumferential cerclage-bone interface is still unknown. Local interface pressure depends on the amount of contact area. Cortical damage at the interface would provoke cerclage loosening. Therefore, the contact area, the bone pressure along the interface and the cortical resistance underneath loaded cerclages were determined in an ex vivo model. Materials and methods Human diaphyseal femoral bone was used with differing cross-sectional geometry. Bone contact points of fixed 1.5 mm wire and 1.7 mm cable cerclages were identified from axial radiographs. Pressure distribution at the cerclage-bone interface was recorded with a pressure-measuring film using a distraction setup with two cortical half shells. Bone shells with installed cerclages were separated with up to 400 N force and were subsequently analyzed histologically to detect cortical damage. Results Both cerclage types exhibited a point contact fixation with non-loaded spanned zones in-between. Cables cover larger contact areas. Both cerclages exhibited an inhomogeneous interface pressure distribution depending on the bone surface geometry. Histology revealed intact cortical bone without cortical affection after loading of both cerclage types. Conclusion Point contact fixation of the cerclages installs non-loaded, spanned zones where the periosteum is not compressed, rendering a strangulation of the blood supply unlikely. Cortical bone withstands static concentric pres- sure produced by the cerclage. Cortical groove formation is attributed to instability under functional load and not to weakness of the cortex itself. Keywords Cerclage Á Point contact fixation Á Instability Á Fracture fixation This work was performed at the AO Research Institute Davos, Switzerland. M. Lenz (&) Á S. M. Perren Á B. Gueorguiev Á R. G. Richards Á M. Windolf AO Research Institute Davos, Clavadelerstrasse 8, 7270 Davos-Platz, Switzerland e-mail: mark.lenz@aofoundation.org S. M. Perren e-mail: stephan.perren@aofoundation.org B. Gueorguiev e-mail: boyko.gueorguiev@aofoundation.org R. G. Richards e-mail: geoff.richards@aofoundation.org M. Windolf e-mail: markus.windolf@aofoundation.org F. Krause Department of Orthopaedic Surgery, Inselspital, University of Berne, Freiburgstrasse, 3010 Bern, Switzerland e-mail: fabian.krause@insel.ch A. Fernandez dell’Oca Department of Traumatology, British Hospital, Av. Italia, 2420 Montevideo, Uruguay e-mail: AaFernan@netgate.com.uy D. Ho ¨ntzsch Department of Medical Technology Development, BG Trauma Hospital Tu ¨bingen, Schnarrenbergstrasse 95, 72076 Tu ¨bingen, Germany e-mail: DHoentzsch@bgu-tuebingen.de 123 Arch Orthop Trauma Surg (2012) 132:1467–1472 DOI 10.1007/s00402-012-1572-x