IN-VITRO BIOMECHANICAL COMPARISON OF MULTISTRAND CABLES WITH CONVENTIONAL CERVICAL STABILIZATION

Study Design. The biomechanical stability of six different methods of cervical spine stabilization, three using multistrand cables, were eva luated in a bovine model. Objectives. To quantify and compare in the i n vitro biomechanical properties of multistrand cables used for poster ior cervical wiring to standard cervical fixation techniques. Summary of Background Data. Fixation of the posterior cervical spine with mono filament stainless steel wire is a proven technique for stabilization of the cervical spine. Recently, multistrand braided cables have been used as a substitute for monofilament stainless steel wires. These cab les, made of stainless steel, titanium, or polyethylene, are reported to be stronger, more flexible, and fatigue resistant than are monofila ment wired based on mechanical testing. However, no in vitro biomechan ical studies have been performed testing a standard posterior cervical wiring technique using multistrand cables. Methods. Thirty-six fresh frozen cervical calf spines consistent in size and age were mounted an d fixed rigidly to isolate the C4-C5 motion segment. Six different rec onstruction techniques were evaluated for Rogers' posterior cervical w iring technique using: 1) 20-gauge stainless steel monofilament wire, 2) stainless steel cable, 3) titanium cable, 4) polyethylene cables, 5 ) anterior locking plate construct with interbody graft, and 6) poster ior plate construct. Six cervical spines were included in each group ( n = 6), with each specimen statically evaluated under three stability conditions: 1) intact, 2) reconstructed, and 3) postfatigue. The insta bility model created before the reconstruction consisted of a distract ive flexion Stage 3 injury at C4-C4. Nondestructive static biomechanic al testing, performed on an material testing machine (MTS 858 Bionix t est system, Minneapolis, MN), included axial compression, axial rotati on, flexion-extension, and lateral bending. After reconstruction and s tatic analysis, the specimens were fatigued for 1500 cycles and then s tatically retested. Data analysis included normalization of the recons tructed and postfatigue data to the intact condition. The calculated s tatic parameters included operative functional unit stiffness and rang e of motion. Results. Posterior cervical reconstruction with stainless steel monofilament wire proved inadequate under fatigue testing. Two of the six specimens failed with fatigue, and this construct permitted the greatest degree of flexion-extension motion after fatigue in comp arison with all other constructs (P < 0.05). There were no significant differences in flexural stiffness or range of motion between stainles s steel, titanium, or polyethylene cable constructs before or after fa tigue testing. The posterior cervical plate constructs were the stiffe st constructs under flexion, extension, and lateral bending modes, bef ore and after fatigue testing (P > 0.05)> Conclusions. Multistrand cab les were superior to monofilament wire with fatigue testing using an i n vitro calf cervical spine model. There were no failures or detectabl e differences in elongation after fatigue testing between the stainles s steel, titanium, an polyethylene cables, as shown by the flexion-ext ension range of motion. The posterior cervical plate construct offered the greatest stability compared with all other constructs.