• Dennis Winge Hallager, Martin Gehrchen, Benny Dahl, Jonathan A Harris, Manasa Gudipally, Sean Jenkins, Ai-Min Wu, and Brandon S Bucklen.
• *Department of Orthopaedic Surgery, Copenhagen University Hospital, Copenhagen, Denmark†Musculoskeletal Education and Research Center, A Division of Globus Medical, Inc., Audubon, PA‡Department of Spinal Surgery, 2nd Affiliated Hospital of Wenzhou Medical University, Wenzhou, China.
• Spine. 2016 Apr 1; 41 (7): E388-95.

Study DesignIn vitro cadaveric biomechanical study.ObjectiveTo assess effects of 4-rod reconstruction, rod material, and anterior column support on motion and surface rod strain in a pedicle subtraction osteotomy model.Summary Of Background DataPedicle subtraction osteotomy (PSO) can correct significant sagittal deformity of the lumbar spine; however, revision rates are high. To reduce rod strain and the incidence of rod fracture, clinical use of multi-rod construction, cobalt chrome (CoCr) alloy rods, and interbody spacers adjacent to PSO has been proposed. Investigating both motion and rod strain is necessary to determine the biomechanical efficacy of these techniques.MethodsFive specimens (T12-S1) underwent PSO at L3 with pedicle screw stabilization at L1-S1. Pedicle subtraction was adjusted to achieve a final lordosis of 70°. Flexion-extension (FE), lateral bending, and axial rotation were applied. Linear strain gauges measured surface rod strain during FE on primary and accessory rods at PSO level. Testing evaluated (1) accessory rods (4-Rod) added at PSO level versus primary rods (2-Rod); (2) Ti versus CoCr rods; and (3) lateral interbody spacers (S) inserted adjacent to PSO. One-way and three-way analysis of variance was performed (P ≤ 0.05).ResultsAll constructs significantly reduced FE and lateral bending motion relative to intact (P < 0.001). The main effect of accessory rods in reducing FE motion was significant (P = 0.021). Accessory and CoCr rods reduced relative surface strain on the primary rod, irrespective of construct (P < 0.001). CoCr 4-Rod + S provided the greatest reduction in strain (76% decrease; P = 0.003).ConclusionAccessory and CoCr rods provided greatest reduction in motion and rod strain at PSO level. Interbody devices minimally affected motion-induced strain and might act primarily to maintain disc height. Clinicians must assess whether surface strain and motion reduction minimize the incidence of rod fracture.Level Of EvidenceN/A.

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