Hybrid surgery (HS) allows surgeons to tailor fusion and arthroplasty in the treatment of multiple-level cervical disc degeneration. However, the decision making of selecting either ACDF or ADR for each level in three-level HS remains controversial and has not been fully investigated. This study was aimed to optimize three-level cervical hybrid constructs by systematically investigating their biomechanical properties and their effect on adjacent levels. A finite element model of cervical spine (C2-C7) was developed, and eight C3-C6 surgical models including six HS were constructed. The range of motion (ROM) in flexion, extension, lateral bending, and axial rotation under 2.0 Nm moments with 30 N follower load were simulated. The von Mises stress, strain energy at the adjacent intervertebral disc (IVD) and force at the adjacent facet were calculated. The ROM of the hybrid constructs and adjacent levels was close to that of the intact spine. HS with arthroplasty performed at C5-6 had better performance in terms of ROM reduction at the inferior adjacent level (C6-7). Moreover, C-D-D and 3ADR had best performance in reducing the von Mises stress and strain energy at C6-7. All HS reduced the facet burden at both C2-3 and C6-7 levels. However, the major drawback of HS revealed here is that the effect of C6-7 protection is at the cost of increased C2-3 IVD burden. In conclusion, we recommend C-D-D and 3ADR for patient with C3-C6 disc degeneration without predisposing C2-3 condition. C-C-D could be a good alternative with a lower medical cost. This analysis guides the decision making in three-level cervical HS before future cadaver studies or human clinical trials.

The management of thoracolumbar (TL) burst fractures remained challenging. Due to the complex nature of the fractured vertebrae and the lack of clinical and biomechanical evidence, currently, there was still no guideline to select the optimal posterior fixation strategy for TL burst fracture. We utilized a T10-L3 TL finite element model to simulate L1 burst fracture and four surgical constructs with one or two-level suprajacent and infrajacent instrumentation (U1L1, U1L2, U2L1, and U2L2). The present study was aimed to compare the biomechanical properties and find an optimal fixation strategy for TL burst fracture in order to minimize motion in the fractured level without exerting significant burden in the construct. Our result showed that two-level infrajacent fixation (U1L2 and U2L2) resulted in greater motion reduction ranging from 66.0 to 87.3 precents compared to 32.0 to 47.3 percents in one-level infrajacent fixation. Flexion produced the largest pathological motion in the fractured level but the differences between the constructs were small, all within 0.26 degrees. Comparisons in implant stress showed that U2L1 and U2L2 had an average 25.3 and 24.8 percent less von Mises stress in the pedicle screws compared to U1L1 and U1L2, respectively. The construct of U2L1 had better preservation of physiological motion while providing sufficient ROM reduction at the fractured level. We suggested that U2L1 was a good alternative to the standard long-segment fixation with better preservation of motion and without an increased risk of implant failure.