COMPARATIVE MECHANICAL-PROPERTIES OF SPINAL CABLE AND WIRE FIXATION SYSTEMS

Study Design. Surgical spinal cable and wire fixation systems were tes ted mechanically using standardized methodologies. Objectives. To comp are the relative mechanical properties and biomechanical performances of the different commercially available spinal wire and cable fixation devices, and to provide information that will help in selecting diffe rent cables for different clinical applications. Summary of Background Data. Spinal cables have become extensively used for spinal fixation; however, there are few published accounts delineating their mechanica l properties. No reports have compared the relative properties of diff erent cable systems. Methods. Nine spinal cable and wire fixation syst ems were mechanically tested to compare their static tensile strength, stiffness, fatigue strength, creep, conformance, and abrasion propert ies. Titanium and stainless steel Codman cable, Danek cable, and AcroM ed cable, polyethylene Smith & Nephew cable, and 20- and 22-gauge stai nless steel monofilament Ethicon wire were tested using identical meth odologies. The cable or wire was connected into loops with methods tha t simulated in vivo clinical applications. Results. Under static tensi le testing, titanium cables had 70% to 90% of the ultimate tensile str ength of the comparable steel cables; the different cables were 100% t o 600% stronger than monofilament wire; the ultimate strength of the p olyethylene cable was similar to that of the strongest available steel cable. Fatigue testing delineated important differences among the dif ferent materials. For a given manufacturer, titanium cables were alway s more susceptible to fatigue than stainless steel cables of comparabl e diameter. Polyethylene cable with stood cyclical loading without bre aking better than all of the metal cables and wires. The mechanisms of failure differed substantially among materials and types of tests. Po lyethylene cables exhibited significant stretching or ''creep'' at loa ds that were much lower than the static failure loads. In contrast, no wire cable demonstrated creep. Monofilament wires demonstrated little creep. Polyethylene cables failed by elongating and loosening; wire c ables failed by breaking. Monofilament wire and cables conformed least to a solid surface; polyethylene cable conformed the most and flatten ed out against solid surfaces. Abrasion properties depended on the sur face characteristics of the Implants. Polyethylene cable was abraded b y (and eventually failed by wearing against) the simulated bone, a res ult that did not occur with any metal cables or wires. The steel and t itanium cables and the monofilament wires all had an ability to abrade through simulated bone. Conclusions. Titanium, steel, and polyethylen e cable systems all behave substantially differently mechanically comp ared with monofilament wire. The relative advantages and disadvantages of each particular product should be considered when selecting an imp lant for a specific clinical use.