Comparative mechanical properties of spinal cable and wire fixation systems.

STUDY DESIGN

Surgical spinal cable and wire fixation systems were tested mechanically using standardized methodologies.

OBJECTIVES

To compare 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 different 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 mechanical properties. No reports have compared the relative properties of different cable systems.

METHODS

Nine spinal cable and wire fixation systems were mechanically tested to compare their static tensile strength, stiffness, fatigue strength, creep, conformance, and abrasion properties. Titanium and stainless steel Codman cable, Danek cable, and AcroMed cable, polyethylene Smith & Nephew cable, and 20- and 22-gauge stainless steel monofilament Ethicon wire were tested using identical methodologies. The cable or wire was connected into loops with methods that simulated in vivo clinical applications.

RESULTS

Under static tensile testing, titanium cables had 70% to 90% of the ultimate tensile strength of the comparable steel cables; the different cables were 100% to 600% stronger than monofilament wire; the ultimate strength of the polyethylene cable was similar to that of the strongest available steel cable. Fatigue testing delineated important differences among the different materials. For a given manufacturer, titanium cables were always more susceptible to fatigue than stainless steel cables of comparable diameter. Polyethylene cable withstood cyclical loading without breaking better than all of the metal cables and wires. The mechanisms of failure differed substantially among materials and types of tests. Polyethylene cables exhibited significant stretching or "creep" at loads 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 cables failed by breaking. Monofilament wire and cables conformed least to a solid surface; polyethylene cable conformed the most and flattened out against solid surfaces. Abrasion properties depended on the surface characteristics of the implants. Polyethylene cable was abraded by (and eventually failed by wearing against) the simulated bone, a result that did not occur with any metal cables or wires. The steel and titanium cables and the monofilament wires all had an ability to abrade through simulated bone.

CONCLUSIONS

Titanium, steel, and polyethylene cable systems all behave substantially differently mechanically compared with monofilament wire. The relative advantages and disadvantages of each particular products should be considered when selecting an implant for a specific clinical use.