After institutional review board approval, we performed a retrospective review of consecutive patients who underwent long-segment (≥ 6 levels) spinal fusion and pelvic fixation using IS or S2AI technique at a single institution between 2013 and 2017. The indications for surgery included frequent recurrent low-back and/or leg pain, neurological deficits, severe disability and/or progressive deformity that failed to better with conservative treatment for more than 6 months. We included ASD patients with age ≥ 60 years and at least one of the following: C7 sagittal vertical axis (SVA) > 50mm, pelvic tilt (PT) > 25°, pelvic incidence (PI) – lumbar lordosis (LL) > 10°. Patients with a history of neuromuscular diseases, malignancy, infection, and postoperative follow-up less than 24 months were excluded. Furthermore, patients were also excluded from this study if they had ankylosing spondylitis, Parkinson’s disease, or incomplete radiographic and clinical records. Eligible patients were divided into 2 groups according to the surgical method: IS group (iliac screw fixation) and S2AI group (S2-alar-iliac screw fixation).
The clinical data for this current cohort were obtained from the electronic medical records and operative database of our institution. Standard demographic data (e.g., age, gender, primary diagnosis, medical comorbidities, and American Society of Anesthesiologists [ASA] classification) were extracted. Surgical parameters included the operated levels, estimated blood loss (EBL), surgical duration, use of an osteotomy, and length of stay (LOS). The major complications requiring revision included proximal junctional kyphosis (PJK), pseudarthrosis, deep infection, SIJ pain, or other implant-related complications (pelvic screw loosening, fracture, and wound dehiscence etc.) were compared. Clinical results were evaluated using the Visual Analog Scale (VAS) of back and leg pain and Oswestry Disability Index (ODI) at preoperative and 3 months postoperative, and the last follow-up.
The IS procedure was performed base on the technique described previously.[1, 6, 17–19] With patients in the prone position, suprafascial dissection was performed via a midline incision. Routine exposure of the spinous process, lamina, facet joints, and transverse process at the index levels. The bilateral posterior superior iliac spine (PSIS) were then identified. Adequate subperiosteal dissection was performed and splitted fascia longitudinally over the PSIS midline, then stripped to the sides for fascial integrity. The ideal entry point of IS was about 10 mm between the posterior edge of the iliac crest and posterior sacrum. An osteotome was used to remove a small tri- cortical wedge around this point. A straight probe was gently and cautiously inserted into the cancellous channel of ilium, avoiding penetration of inner and outer cortex. Next, the trajectory was tapped with a ball-tip probe to verify the integrity of cortical bone. Then the screw was deeply inserted toward PSIS until its head was flush with the cortex of the ilium to lower the risk of prominence. Finally, the screw was attached to the rod underneath S1 pedicle screw using a lateral connector (Fig. 1).
The procedure of S2AI fixation was performed using a free hand technique described previously.[7, 20–24] A routine posterior exposure was performed similar as IS. S2AI technique has a sacral ala entry point locating distal to the S1 foramen with the angulation of trajectory approximately 30–45° laterally and 25–40° distally, aiming toward the greater trochanter of the femur. Using fluoroscopy to ensure position cephalad to the greater sciatic notch and tap drilling until the sacroiliac joint (SIJ), then reverse drilling until the iliac cortex was reached. Finally, the screw was inserted until its head placed in-line with S1 pedicle screw head (Fig. 2).
We commonly used the pelvic screw with a diameter of 8.5mm and a length of 7.0mm. All cases were performed as 2-stage anteroposterior fusion with an interval of 1 week by the two senior authors (KT and YC) as a team with > 20 years of experience in ASD.
All radiological analysis was performed on lateral radiographs of whole spine (36-in.) obtained preoperatively, postoperatively (first erect) and at the last follow-up according to an established positioning protocol. Spinopelvic parameters included in this analysis were SVA, PT, PI, LL (L1–S1), TK (thoracic kyphosis: T5–12), ULL (upper lumbar lordosis: L1-4), and LLL (lower lumbar lordosis: L4-S1). Kyphosis was indicated by a (+) value whereas lordosis was indicated by a (-) value. Change of parameter was calculated by subtracting the preoperative value from the postoperative value. All the parameters were measured by two surgeons who did not participate in the operation and the mean value were adopted
Additionally, we defined proximal junctional kyphosis (PJK) was considered present when proximal junctional angle was > 10° and at least 10° greater than the preoperative measurement, which was a severe complication after deformity surgery with radiographic evidence of acute PJK deformity and mechanical failure at the upper instrumented vertebra (UIV). Fusion was confirmed by the presence of bridging bone connecting the adjacent vertebral bodies either through the implants or around the implants or < 3mm of translation on flexion or extension radiographs.[28–30] Screw loosening was defined as a radiolucent area ≥ 2mm surrounding the screw on radiographic images, also known as the “double halo sign”.[31, 32]
Results were expressed as mean ± SD. We used the Student’s t-test for continuous variables, and the Chi-square test used for categorical variables. All statistical analyses were performed using SPSS 21.0 software (SPSS Inc., Chicago, IL, USA) with p values of < 0.05 considered to be statistically significant.