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.

Introduction
A spondylitis developed after a surgery has been usually treated with drainage and curettage through anterior approach and autoiliac strut bone graft. However, anterior support with titanium cage combined with posterior pedicle screw fixation has been attempted. Implanting a foreign material is usually prohibited at an active pyogenic infection site. We tried to prove the usefulness of chip bone graft with metal cages in surgical treatment of spondylitis developed in posterior lumbar intebody fusion cases.
Materials and Methods
This is a retrospective study. The patients who received posterior lumbar interbody fusion (PLIF) between Jan. 2007 and Dec 2017 and had a spondylitis around the cage were reviewed. There were 1,831 PLIFs during the study period. There were 32 cases of surgical site infection and 20 of them were spondylitis around the cage. Ten out of the 20 cases had a revision surgery. All implants removal, drainage and curettage were done and interbody bone graft and pedicle screw re-fixation was done simultaneously. Five cases used autoiliac strut bone (Group I) and the other 5 cases used titanium cage and autoiliac chip bone (Group II) as interbody graft materials. The demographic, diagnostic and microbiological characteristics were investigated and the results of treatment were compared between the two groups.
Results
The diagnosis of infection was made at 282.0±106.1 (180~410) days in group I and 209±118.4 (75~335) days in group II after the PLIF. All cases had neither general fever nor local manifestations like heating and redness etc. All patients had back pain, however, only 2 cases of group II had neurological symptoms. C-reactive protein (CRP) level was elevated at 2 weeks from the PLIF in all cases (p<0.001). All cases had implant loosening at the time of their diagnosis. There was no failure of infection control. All cases showed normalization of CRP and radiological interbody fusion. The final Oswestry disability index (ODI) showed no difference between the two groups. ODI improved from 54.6±11.5 to 42.2±6.8 in group I (p=0.095) and from 63.6±6.9 to 44.8±11.7 in group II (p=0.025).
Conclusion
For the surgical treatment of spondylitis that were developed in PLIF, a comprehensive one stage operation that comprised all implants removal, drainage and curettage followed by simultaneous intebody bone graft with metal cages and pedicle screw re-fixation was useful to control the infection.

Vertebral compression fracture is one of the most common osteoporotic fractures along with hip fractures. In some patients, pain was improved through conservative treatment including pharmaceuticals, bracing and physiotherapy. But it has been reported that conservative therapy is insufficient in many patients with vertebral compression fracture. Accordingly, interest in vertebroplasty has increased and many studies have been reported that vertebroplasty have therapeutic effects in acute vertebral compression fracture. On the other hand, some studies have been reported that the effect of vertebroplasty was not significant. So whether vertebroplasty is useful or not is still controversial. Therefore, in this review article, we will review open-label randomized controlled trials and blinded randomized controlled trials that provide high-level evidence and provide an opinion on which patients it is appropriate to perform vertebroplasty.

Surgical method for cervical myelopathy caused by ossification of posterior longitudinal ligament depends on cervical sagittal alignment and shape of ossification mass. For K-line (-) patients, indirect decompression method such as laminoplasty may not be effective since posterior shift of spinal cord would not occur. Therefore, other surgical method is frequently needed in this patient group. In this article, we would review surgical methods that could be applied for patients with K-line (-) cervical myelopathy caused by ossification of posterior longitudinal ligament.

Spinal subdural hematoma (SDH) is a rare complication after spinal surgery. Only a few cases are reported on spinal SDH following open lumbar spinal decompression or fusion surgery. Moreover, there has been no case report on spinal SDH following percutaneous transforaminal endoscopic lumbar discectomy. We report a case of spinal SDH following endoscopic discectomy, review the literature of this complication and discuss the etiology to it and methods to prevent it. A 63-year-old woman presented with severe radiating pain. Pain was not improved with conservative management. Lumbar magnetic resonance imaging (MRI) was checked and revealed right L3-4 ruptured disc with severe L4 root compression. Percutaneous transforaminal endoscopic decompression was performed and the pain subsided promptly after the endoscopic procedure. On 7th post-operative day, pain on Rt. buttock, anterior thigh was deteriorated severely, more than in pre-operatively. Deteriorated pain was not controlled by oral medications and epidural block. Repeat MRI showed no definite recurrence of disc herniation at decompressed level but spinal SDH, severely compressing cauda equina was seen on T12-sacral area. Spinal SDH is a rare complication following spine surgery, including percutaneous endoscopic surgery. A spine surgeon should be aware of the possibility of spinal subdural hematoma, having severe sequel.