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.

An apparatus for applying pure nonconstraining moments to spine segments in vitro.

STUDY DESIGN: This article reports on the design and use of a new apparatus for creating and monitoring pure, relatively nonconstraining moments to induce flexion/extension, lateral bending, and axial rotation in cadaveric spine segments of two or more vertebrae. OBJECTIVE: The apparatus was designed to take advantage of the precision and control available in a servo-hydraulic testing frame to efficiently create and monitor testing moments. SUMMARY OF BACKGROUND DATA: Other laboratories have reported methods of flexibility testing that also use cables and pulleys. However, instead of loading the cables and pulleys using a mechanical testing frame, previous systems have used pneumatic actuators or dead weights. METHODS: Force from a uniaxial mechanical testing frame is converted to torque applied to the specimen through a system of cables and pulleys. The cable orientation is monitored to ensure that pure moments are created. Applied moments are recorded using one or two load cells. RESULTS: Sketches of the apparatus are presented and its operation is described. CONCLUSION: Because the materials required to build this apparatus are inexpensive and the equipment needed for its operation is common in mechanical testing labs, this design may be useful for researchers interested in beginning in vitro spine flexibility testing with minimal expenditure.

Biomechanical effects of transoral odontoidectomy.

The acute biomechanical effects of transoral odontoidectomy were studied by using qualitative and quantitative methods to assess atlantoaxial motion. In vitro biomechanical testing was performed on the upper cervical spines of eight baboon and five human cadaveric specimens. Using an unconstrained testing apparatus, we performed a flexibility method of testing. Physiological range loading was applied to atlantoaxial specimens, and three-dimensional motion was analyzed with stereophotogrammetry. Force-deformation relationships were delineated in intact specimens and again after surgical removal of the anterior C1 arch, odontoid process, and transverse atlantal ligament. We studied the total range of rotational and linear motions, the behavior of the neutral zone and elastic zone, the flexibility coefficients, and the instantaneous axes of rotation during flexion, extension, bilateral lateral bending, and bilateral axial rotation. Odontoidectomy produced several distinct alterations in motion and in force-deformation responses at C1-C2 that were almost identical in the baboon and human specimens. After odontoidectomy, the atlas developed significantly increased translational movements, which were most prominent in the anteroposterior direction. The total angular range of motion increased significantly during flexion, extension, and lateral bending but not during axial rotation. When the total range of motion was altered, the neutral zone was affected selectively and the elastic zone was spared. Surgery produced mobile, widely spread, unconstrained instantaneous axes of rotation that were in a constrained, fixed position in intact specimens. Clinically, transoral odontoidectomy may predispose patients to spinal instability. Even if acute spinal instability is not apparent, the patients may be susceptible to the delayed effects of the surgery because of the altered anatomy and biomechanical responses.

Morphology and kinematics of the baboon upper cervical spine. A model of the atlantoaxial complex.

STUDY DESIGN: Quantitative and qualitative analyses were performed to compare the anatomy and biomechanics of baboon and human upper cervical spines. OBJECTIVES: This study examined the baboon as a potential model for in vivo and in vitro atlantoaxial research. SUMMARY OF BACKGROUND DATA: A variety of animal models have been used for spine research; however, no species have been used for C1-C2 research. Most species have remarkably different C1-C2 morphology compared with that of humans. METHODS: Twenty baboon and seven human normal adult cadaveric upper cervical spines were studied morphologically. C1-C2 motion segments were analyzed biomechanically using a flexibility method of testing with physiologic range, nondestructive loading. Motion and load-deformation relationships were studied during flexion, extension, bilateral lateral bending, and bilateral axial rotation. RESULTS: The bones and ligaments of the baboon and human upper cervical vertebrae have similarly proportioned structures, identical individual components, and similar geometric configurations. The average size of the baboon vertebrae was 50% to 60% of the human specimens. There were several minor anatomical differences. Baboons had more horizontal C2-C3 facet joints and more vertical C1-C2 articular surfaces; the vertebral arteries were encased in a continuous bony canal in C1. Biomechanical testing demonstrated that baboons and humans had similarly proportioned neutral zones and elastic zones. Compared with humans, baboons had a 2 degrees to 9 degrees wider range of motion in all directions. CONCLUSIONS: The baboon and human upper cervical anatomy and biomechanics are similar. The baboon may be useful to study atlantoaxial biomechanics and pathology.

The effects of pedicle screw fit. An in vitro study.

STUDY DESIGN: The effects of pedicle screw size (major diameter and length) on fixation stiffness in osteoporotic and nonosteoporotic vertebrae were evaluated in vitro. METHODS: Lumbar vertebrae were obtained from two fresh frozen human spines. Bone mineral densities were determined using dual energy radiograph absorptiometry, followed by nondestructive mechanical testing of the specimens instrumented with pedicle screws. A loading technique was used that more closely mimics loading of pedicle screws in vivo. RESULTS: Testing revealed that for good quality bone, screw size had a significant effect on fixation stiffness, but the effect of penetration depth depended on pedicle fill, and vice versa. In nonosteoporotic bone, the use of a longer screw increased fixation stiffness if the screw filled up the pedicle by 70% or more. The use of wider screws increased the fixation stiffness if the penetration depth was 80% or more. CONCLUSION: Screw size had little or no effect on fixation stiffness in osteoporotic bone.