Study sample
A retrospective review of patients over 40 years old with rigid AdIS who underwent surgery with Hi-PoAD technique was undertaken.
Follow-up evaluations were performed post-operatively, at 1 and 2 years and up to final follow-up. Informed consents for participation in the study and for publication of clinical images were obtained from each patient.
Data collection
The patients enrolled were all affected by AdIS of the thoracic and/or lumbar spine, with less than 25% of reduction in the main curve. Patients with lack of flexibility due to anterior vertebral bony fusion (congenital or acquired) evaluated by routinely performed pre-operative MRI were excluded from the study.
Operative time, blood loss, length of stay, intra- and post-operative complications were recorded. Coronal Cobb’s angle of each curve, coronal flexibility (difference in Cobb’s angle of the main coronal curve between the pre-operative full-length standing and lateral supine side-bending X-rays, expressed as a percentage), thoracic kyphosis (TK) and lumbar lordosis (LL) angles were measured on pre- and post-operative full-length standing and lateral side-bending radiographs. C7 plumb line (C7PL)/central sacral vertical line (CSVL) and sagittal vertical axis (SVA) were used to assess coronal and sagittal imbalance. The rotation angle sagittal (RAsag) was calculated in the 22 patients whose pre- and post-operative CT-scans were available. SRS-22 questionnaire was administered pre-operatively and at last follow-up.
Surgical planning
A surgical planning was performed on pre-operative radiographs in order to assess the fusion area, the correct levels for Ponte osteotomies and the rods contour.
The fusion area was pre-operatively defined according to the criteria by Lenke et al. [18]. In the presence of disc degeneration at the level distal to the planned lower instrumented vertebra (LIV) and when compensatory curves showed low flexibility, the fusion area was extended distally.
The first Ponte osteotomy was planned at the apex of the main curve and at the two periapical levels (above and below the apex) in all cases. Further extension proximally and distally was performed in all the curves with a Cobb angular value over 70°. When osteotomies were planned distal to T12, they were performed according to Smith-Petersen.
The rods were asymmetrically moulded, to obtain a synergic action on sagittal profile restoration, with a lifting effect at the physiological apex of thoracic kyphosis and a lowering effect in the lumbar spine to restore lumbar lordosis; at the scoliosis apex, the concavity rod was higher (over-bended) than the convexity rod in order to obtain a derotational effect (Figs. 1, 2, 3 and 4).
Surgical technique
All surgeries were performed by the first author. Patient was placed prone on the Allen table with multimodal evoked potentials neuromonitoring. Subperiosteal exposure of the posterior elements was performed. The inferior articular facets were excised at every level of the desired fusion area, to allow spinal release and increase spinal flexibility with the exception of the two most cephalad instrumented vertebrae to reduce the risk of proximal junction syndrome. A 2.7-mm drill hole was performed at every level included in the fusion area adopting a funnel technique in the thoracic tract. In presence of type C and D pedicles [19], an in–out-in trajectory was chosen. The drilled tract was tapped for 1.5 cm with tap smaller than the screw diameter (e.g. 3.5 mm for a 4.5 mm screw).
With the aim of optimizing force transmission from the tulip to the screw, uniplanar pedicle screws (K2M® MESA® Stryker) were adopted in 24 cases, while polyaxial lockable screws (Must®, Medacta®) were used in 16 cases. At T12, given the more medial entry point at this level, a polyaxial screw was always chosen to facilitate rod engaging in the tulip. Ponte osteotomies were then performed according to the pre-operative planning with resection of the spinous process, removal of the residual superior facets, wide inferior and superior laminectomies with the removal of the entire ligamentum flavum extending from pedicle to pedicle to promote release in concavity and compression in convexity.
Two different 5.5-mm cobalt-chrome rods were then asymmetrically moulded (with the convexity rod less bended than the concavity rod), placed through the crickets (K2M® MESA® Stryker) or through the locking towers (Must® Medacta®) and simultaneously rotated with the curves consensually oriented on the sagittal plane. Extensors were applied over the crickets or the locking towers at periapical levels both in concavity and in convexity and direct vertebral rotation was performed. Maintaining a derotative force applied over the extensors, the translation manoeuvre was performed progressively, tightening the crickets or the locking towers. This allowed to sum the corrective power of DVR to that offered by translational manoeuvre over differently moulded rods. The translation manoeuvre over the rods was performed dissipating the corrective forces over all the instrumented levels, especially at the apex of the concavity, where the distance between the tulips and the rod is maximal, as well as the mechanical stresses at the screw–bone interface. When the distance between the rod and the tulips was considered too high by the Surgeon, with the risk of screw pull-out, rod curvature was slightly reduced using in situ rod benders. Before engaging the rods in the tulips, selective compression and distraction were performed at apical and periapical levels to close gaps at the Ponte osteotomy sites. Final engagement of the rods was then performed. In this phase, if a minimal amount of pull-out was observed at the apical and periapical levels in concavity, it was considered clinically irrelevant. In situ rod benders were used to complete coronal correction if needed. An extensive decortication of the posterior elements of the instrumented vertebrae was performed. Subfascial drain was placed, and standard suture was performed.
Early mobilization out of bed started on post-operative day 1. For the first 6 weeks after surgery, a thoracic lumbar sacral orthosis (TLSO) to restrict spinal movements and facilitate initial bone graft fusion was prescribed.
Patient’s characteristics
Forty patients (8 males and 32 females) were included, with a follow-up of 2.9 ± 0.4 (range 2–3.5). The average age was 51.4 ± 6 (range 41–63). Patient’s characteristics are summarized in Table 1.
Table 1 Patient’s characteristics Statistical analysis
Parametric test was used to compare samples in case of continuous variables, normal distribution and appropriate numerousness. The Shapiro–Wilk test was used to verify normal distribution. The Levene test was used to evaluate homogeneity of the variances. As parametric test, we used two-tailed student t test to compare the average of the variables for homoscedastic paired groups. As nonparametric test, we used the two-tailed Wilcoxon signed-rank test for paired group. Continuity correction was applied in case of discrete distribution. P-values < 0.05 were considered to be significant. SPSS 17.0 statistical analysis software (SPSS Inc., Chicago, Illinois, USA) was used to perform statistical analysis.