Defining the number and type of fixation anchors for optimal main curve correction in posterior surgery for adolescent idiopathic scoliosis

Background Context

The number and type of fixation anchors to use during posterior surgery for adolescent idiopathic scoliosis (AIS) is still debated, and the relationship with curve correction remains unclear.

Purpose

This study aimed to determine the number and type of fixation anchors associated with optimal curve correction following posterior surgery for AIS.

Study Design

A retrospective study of the relationship between fixation anchors and main curve correction in AIS surgery was carried out.

Patient Sample

A cohort of 137 AIS patients operated from a posterior-only approach using hooks and pedicle screws comprised the study sample.

Outcome Measures

Correction of the main scoliotic curve was the outcome measure.

Methods

Implant density (ID) was defined as the number of fixation anchors divided by the number of available anchor sites within the main curve. Pedicle screw ratio (PSR) was defined as the number of pedicle screws divided by the total number of fixation anchors within the main curve. Multiple linear regressions were performed to analyze the influence of ID and PSR on main curve correction, while taking into account age, gender, curve type, preoperative main Cobb angle, main curve reducibility, number of fused levels, and number of levels within the main curve.

Results

Main coronal curve correction was significantly related only to ID for all patients and for the subgroup of patients with a main thoracic curve. Constructs with an ID ≥70% and <90% provided a correction similar to that obtained with an ID ≥90%. However, main coronal curve correction was inferior for constructs with an ID <70%, when compared with constructs with ID ≥90%. Implant density and PSR were not related to the change in thoracic kyphosis in the multiple linear regressions.

Conclusions

Implant density is an important predictor of main coronal curve correction in posterior surgery for AIS. Increasing the number of fixation anchors within the main curve—rather than favoring screws over hooks—can lead to better correction in the coronal plane. However, after reaching an ID of ≥70% in the main curve, adding fixation anchors is not likely to result in significantly greater correction of the main curve in the coronal plane.