A 3D Parameter Can Guide Concave Rod Contour for the Correction of Hypokyphosis in Adolescent Idiopathic Scoliosis

Abstract

Study Design. Retrospective. Objective. To evaluate the effect of preoperative rod shape on 3D spinal deformity correction. Summary of Background Data. Differential rod contouring is a surgeon-dependent process based on an estimate of 3D deformity correction sought and the flexibility of the spine. An objective measure to this otherwise subjective scoliosis correction technique is lacking. Methods. A series of adolescent idiopathic scoliosis (AIS) patients with right, thoracic major curves, preoperative rod contour tracings, and EOS imaging was evaluated. All patients underwent posterior spinal fusion with 5.5 mm steel rods contoured prior to insertion. 3D reconstructions were generated pre- and postoperatively using sterEOS software (EOS Imaging) and imported into MATLAB (Mathworks) for analysis. A new measurement of the maximum perpendicular distance from the preinsertion concave rod contour to the preoperative 3D sagittal spinal reconstruction was defined as rod to 3D spine distance (RSD). Linear regressions were used to identify relationships between pre and postoperative parameters, including RSD and 3D thoracic kyphosis. Results. Ninety-nine patients were included. Average preoperative concave rod angle decreased from (48 ± 10°) preoperatively to 26 ± 6° postoperatively (P < 0.001) for an average flattening of ∼20°. Average convex rod angle increased from 30 ± 6° to 34 ± 5° (P < 0.001). Average preoperative thoracic curve magnitude and apical vertebral rotation were 57 ± 8° and 16 ± 5° and decreased to 11 ± 6° and 5 ± 6° (P < 0.001). Average 2D and 3D thoracic kyphosis increased from 19 ± 14° and 2 ± 12° to 27 ± 6° and 22 ± 5° (P < 0.001). Preoperative RSD strongly correlated with 3D thoracic kyphosis change (P < 0.001, r = 0.796, R 2 = 0.633). Conclusion. On average, the concave rod flattened ∼20° after connection to the spine. Similarly, kyphosis increased ∼20° following rod insertion. A strong correlation was identified between RSD and 3D thoracic kyphosis restoration. This novel 3D parameter can guide the degree of concave rod bend necessary to achieve a desired increase in thoracic kyphosis in AIS. Level of Evidence: 2

Systematic review and meta-analysis for the impact of rod materials and sizes in the surgical treatment of adolescent idiopathic scoliosis

“…of patients Gender Patient Characteristics Study groups (no. of patients) Type of surgery Mean age at surgery (years) Follow-up Mean (SD), months) Male ( n ) Female ( n ) Machino et al (2020) [ 25 ] Cohort study 67 8 59 AIS Posterior rods and pedicle screws PLIF 14.4 mean NR Kluck et al (2020) [ 26 ] Cohort study 99 NR NR AIS Posterior rods and pedicle screws NR 14 ± 2 years NR Shen et al (2020) [ 27 ] Cohort study 19 0 19 AIS Posterior rods and pedicle screws PLIF 15.6 ± 2.1 NR Miyazaki et al (2020) [ 28 ] Cohort study 27 NR NR AIS Hypokyphotic normal–hyperkyphotic PSF with double-rod rotation NR NR Feeley et al (2019) [ 29 ] Cohort study 31 8 23 AIS …”

“…The pooled MD in change in kyphosis angle with 6 mm rods was 13.69° (95% CI: 8.54°–18.84°). Similarly, three eligible studies utilizing 5.5 mm rods reported corresponding change in kyphosis angle were also analyzed [ 26 , 52 , 67 ]. Our analysis revealed a pooled MD of 10.05° (95% CI 8.53°–11.57°) in kyphosis angle.…”

Correction of Thoracic Hypokyphosis in Adolescent Scoliosis Using Patient-Specific Rod Templating

“…The average hospital stay for our patients was 5.38 days, which was comparable to the average stay in other studies. In our cohort, we used dual differential rod contouring (DRC) in all cases, with the concave rail being over-contoured by 20 degrees [ 11 , 14 , 43 , 44 ]. This technique represents an additional correction strategy involving over-contouring of the concave rail to pull the concave side of the curve posteriorly, and under-contouring of the convex rod to push the convex side of the curve anteriorly and achieve a simultaneous correction in three planes.…”

“…Previous in vivo and biomechanical studies have demonstrated the efficacy of DRC in achieving 3D deformity correction and have demonstrated the association of increased over-correction of the concave rod with increased amounts of sagittal plane correction and the ability to achieve correction in both the instrumented thoracic spine and in the un-instrumented lumbar spine [ 14 , 18 , 43 ]. Kluck et al commented that [ 44 ] differential rod contouring in DRC is, by nature, subjective. Concave and convex rod contours are determined intraoperatively by the treating surgeon based on estimates of the desired sagittal profile and severity of the axial plane rotation.…”

The Sagittal Plane in Spinal Fusion for Adolescent Idiopathic Scoliosis

“…This effectively pulls the concave side of the curve to an overcontoured rod while pushing down on the convex side of the curve with an undercontoured rod. 33-37,40…”