Study Design. Biomechanical in vitro laboratory study.Objective. To compare the biomechanical performance of 3 fixation concepts used for anterior instrumented scoliosis correction and fusion (AISF).Summary of Background Data. AISF is an ideal estimate for selective fusion in adolescent idiopathic scoliosis (AIS). Correction is mediated using rods and screws anchored in the vertebral bodies. Application of large correction forces can promote early weakening of the implant-vertebra interfaces with potential postoperative loss of correction, implant dislodgment and non-union. Therefore, improvement of screw-rod anchorage characteristics with AISF is valuable.Methods. 111 thoracolumbar vertebrae harvested from seven human spines completed a testing protocol. Age of specimens was 62.9±8.2 years. Vertebrae were potted in PMMA and instrumented using 3 different devices with identical screw length and unicortical fixation: single constrained screw fixation (SC-fixation), non-constrained dual screw fixation (DNS-fixation) and constrained dual screw-fixation (DC-fixation) resembling a novel implant type. Mechanical testing of each implant-vertebra unit (IVU) using cyclic loading and pullout tests were performed after stress tests were applied mimicking surgical maneuvers during AISF. Test order was as follows: 1) Preload-Test-1 simulating screw-rod locking and cantilever forces; 2) Preload-Test-2 simulating compression/distraction maneuver; 3) Cyclic loading tests with IVU subjected to stepwise increased cyclic loading (max.:200N) protocol with 1000 cycles at 2Hz, tests were aborted if displacement >2mm occurred before reaching 1000 cycles; 4) Coaxial pullout tests at a pullout rate of 5mm/min. With each test the mode of failure, i.e. shear versus fracture, was noted as well as the ultimate load to failure (N), number of IVUs surpassing 1000 cycles, number of cycles and related loads applied.Results. 33% of vertebrae surpassed 1000 cycles, 38% in the SC-group, 19% in the DNS-group and 43% in the DC-group. The difference between the DC-group and DNS-group yielded significance (p = .04). For vertebrae not surpassing 1000 cycles, the number of cycles at implant displacement >2mm in the SC-group was 648.7±280.2 cycles, in the DNS-group 478.8±219.0 cycles, and in the DC-group 699.5±150.6 cycles. Differences between the SC-group and DNS-group were significant (p = .008), as between the DC-group and DNS-group (p = .0009). Load to failure in the SC-group was 444.3±302N, in the DNS-group 527.7±273N and in the DC-group 664.4±371.5N. The DC-group outperformed the other constructs. The difference between the SC-group and DNS-group failed significance (p = 0.25), while there was a significant difference between the SC-group and DC-group (p = .003). The DC-group showed a strong trend towards increased load to failure compared to the DNS-group but without significance (p = .067). Surpassing 1000 cycles had a significant impact on the maximum load to failure in the SC-group (p = 0.0001), the DNS-group (p = .01), but not in the DC-group (p = .2), which had the highest number of vertebrae surpassing 1000 cycles.Conclusion. Constrained dual-screw fixation characteristics in modern AISF implants can improve resistance to cyclic loading and pullout forces. DC-constructs bear the potential to reduce the mechanical shortcomings of AISF.
Useful keywords (using NLM MeSH Indexing)
Bone Screws/adverse effects
Range of Motion, Articular/physiology
Spinal Fusion/adverse effects
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