Length Redundancy and Twist Improve the Biomechanical Properties of Polytetrafluoroethylene Bypass Grafts.

BACKGROUND: The iliofemoropopliteal artery significantly changes path length during normal hip and knee flexion. Prosthetic bypass grafts, such as polytetrafluoroethylene (PTFE) grafts, are relatively stiff and thus can subject graft anastomoses to high tension when the path length increases. The aim of this study was to examine the influence of length redundancy and twist on the biomechanical properties of PTFE bypass grafts. METHODS: Unreinforced and ring-reinforced PTFE grafts were loaded in an axial mechanical testing machine to measure the tensile and compressive axial forces with varying levels of length redundancy and axial twist. RESULTS: Adding 5-15% length redundancy to a graft decreases the force to cause 5% extension by > 90% without substantially increasing shortening forces. Adding 4.5°/cm of axial twist imparts a corkscrew shape to the graft without increasing extension or shortening forces in the presence of length redundancy. Ring-reinforced PTFE grafts require more length redundancy to experience these reductions in forces especially in the presence of axial twist. CONCLUSIONS: A modest amount of length redundancy and twist (i.e., a cork-screw condition) confers improved biomechanical properties in a PTFE graft, especially in ring-reinforced grafts. This should be taken into consideration when fashioning an arterial bypass graft in the iliofemoropopliteal segment.

Quantitative biomechanical comparison of ankle fracture casting methods.

The incidence of ankle fractures is increasing rapidly due to the ageing demographic. In older patients with compromised distal circulation, conservative treatment of fractures may be indicated. High rates of malunion and complications due to skin fragility motivate the design of novel casting systems, but biomechanical stability requirements are poorly defined. This article presents the first quantitative study of ankle cast stability and hypothesises that a newly proposed close contact cast (CCC) system provides similar biomechanical stability to standard casts (SC). Two adult mannequin legs transected at the malleoli, one incorporating an inflatable model of tissue swelling, were stabilised with casts applied by an experienced surgeon. They were cyclically loaded in torsion, measuring applied rotation angle and resulting torque. CCC stiffness was equal to or greater than that of SC in two measures of ankle cast resistance to torsion. The effect of swelling reduction at the ankle site was significantly greater on CCC than on SC. The data support the hypothesis that CCC provides similar biomechanical stability to SC and therefore also the clinical use of CCC. They suggest that more frequent re-application of CCC is likely required to maintain stability following resolution of swelling at the injury site.