Biomechanical properties of double-headed pedicle screws and parallel connectors in thoracolumbar extension revision fixation: a finite element analysis

BACKGROUND: Revision surgery of the spine frequently requires removal of the original instrumentation, which can cause substantial iatrogenic tissue injury. To address this issue, we developed a revision construct utilizing double-headed pedicle screws and parallel connectors, which can obviate the need for removal of the original internal fixation. This study aimed to investigate the biomechanical properties of these constructs. METHODS: An intact finite element model of T12-L5 was established. Then, four surgical models were developed. Constructs A and C simulated the traditional long and short-segment revision surgeries, respectively, which removed the original surgical connecting rods and replaced longer ones. Constructs B and D used novel double-headed pedicle screws and parallel connectors to achieve long and short-segment extended revision fixation. Finally, the range of motion (ROM), segmental displacement, and the maximum rod stress among different constructs were analyzed. RESULTS: The ROM was 0.62°-1.83° in Construct A, 0.50°-1.57° in Construct B, 0.01°-0.08° in Construct C, and 0.02°-0.12° in Construct D. The maximum displacement of Constructs A, B, C, and D was 1.50–3.91 mm, 1.21–3.38 mm, 2.08–7.44 mm, and 2.10–7.49 mm, respectively. The maximum stress on rods in Constructs A, B, C, and D was 184.70-437.70 MPa, 170.72-383.49 MPa, 70.73-174.87 MPa, and 65.90- 216.74 MPa, respectively. CONCLUSIONS: The stability and rod stress of extension revision constructs based on double-headed pedicle screws and parallel connectors was found to be comparable to that of conventional constructs. The device may make extended fixation in revision surgery possible without the need to remove the original surgical instruments.