Background
S2-alar-iliac (S2AI) screws are one of the options for spinopelvic fixation to improve stability across the lumbosacral junction. The S2AI screws cross the cortical surfaces of the sacroiliac joint, which can increase the biomechanical strength of the instrumentation. Objective: To investigate the durability and failure types of S2AI screw by finite element model (FEM) analysis.
Methods
Through the FEM, complex material and geometrical properties of the biological system can be evaluated, and various physical variables, such as stress, and fracture, can be analyzed. We examined the biomechanical stress distribution at the set screw and screw head by using a FEM. Von Mises (V.M.) stress (MPa) is derived from 3-dimensional status of stress. The finite element software Abaqus^® version 6.5 (ABAQUS Inc., Johnston, RI, USA) was used to create a FEM.
Results
We quantified the peak V.M. stress applied to the set screw and screw head when rod to S2AI screw trajectory angle was 30º angled and perpendicular. In FEM analysis, at an angle of 30 degrees rather than perpendicular, the stress increased further around the area where the screw head and rod contacted and the displacement distribution of set screw also increased.
Conclusion
S2AI screw fixation has several drawbacks such as screw fracture and dislodgement of the set screw. This FEM analysis can support the result.

Purpose
The purpose of this review is the current understanding of proximal junctional kyphosis (PJK) and proximal junctional failure (PJF) following adult spinal deformity (ASD) surgery.
Materials and Methods
We carried out a systematic search of PubMed for literatures published up to September 2016 with “proximal junctional kyphosis” and “proximal junctional failure” as search terms. A total of 57 literatures were searched. Finally, the 33 articles were included in this review.
Result
PJK and PJF are recognized complications after long instrumented posterior fusion in ASD surgery. PJK is multifactorial in origin and likely results from surgical, radiographic, and patient related risk factors. PJF is a progressive form of the PJK spectrum including bony fracture of uppermost instrumented vertebra (UIV) or UIV+1, subluxation between UIV and UIV+1, failure of fixation, neurological deficit, which may require revision surgery for proximal extension of fusion. Variable risk factors for PJK and PJF have been investigated, and they can be categorized into surgical, radiographic, and patient-related factors. There are several strategies to minimize PJK and PJF. Soft tissue protections, adequate selection of the UIV, prophylactic rib fixation, hybrid instrumentation such as hooks, vertebral cement augmentation at UIV and UIV+1, and age-appropriate spinopelvic alignment goals are worth consideration.
Conclusion
The ability to perform aggressive global realignment of spinal deformities has also led to the discovery of new complications such as PJK and PJF. Continuous research on PJK and PJF should be proceeded in order to comprehend the pathophysiology of these complications.