Influence of screw type on initial coronal and sagittal radiological correction with hybrid constructs in adolescent idiopathic scoliosis. Correction priorities

Background: Pedicle screw constructs for spinal instrumentation in patients with adolescent idiopathic scoliosis (AIS) are effective in providing coronal plane correction but can result in loss of kyphosis, which in turn can lead to loss of lordosis. Hybrid constructs have been found superior over pedicle screw constructs in terms of thoracic kyphosis restoration. In this study, our objective was to compare outcomes with monoaxial versus polyaxial screws in an AIS population treated with hybrid constructs.

Hypothesis: Monoaxial screws provide better correction in the coronal plane but result in loss of thoracic kyphosis, whereas thoracic kyphosis is preserved when polyaxial screws are used.

Material and methods: We retrospectively analysed data from 60 patients (mean age, 15years) with Lenke 1, 2, or 3 AIS treated using a hybrid construct with self-retaining bilaminar hook claws cranially, pedicle screws between the last instrumented vertebra and T11 caudally, and sublaminar universal clamps between the two extremities of the construct. Monoaxial screws were used in the first 30 patients (MS group) and polyaxial screws in the next 30 patients (PS group). Student's t test was performed to compare the two groups in terms of thoracic Cobb angle correction and T4-T12 kyphosis 3 months after surgery.

Results: No significant preoperative differences were found between the two groups. At last follow-up, the residual Cobb angle was significantly greater in the PS group than in the MS group (20.3° versus 15°) with a percentage of correction of 72.1% in the MS group versus 64.8% in the PS group. In the sagittal plane, the thoracic kyphosis was significantly greater in the PS group than in the MS group (26.6° versus 23°).

Discussion: This preliminary study shows that, even within a population managed using hybrid constructs, which are associated with less iatrogenic hypokyphosis, differences exist according to the technique used. The importance of sagittal spinal balance has been abundantly documented in the literature, and sagittal malalignment, particularly due to iatrogenic factors, is associated with poorer clinical outcomes in adults with spinal deformities. Therefore, there is a critical need to determine whether the treatment priority is optimal correction in the coronal plane or in the sagittal plane. We believe that the main focus should be sagittal plane correction, even at the expense of a slight decrease in coronal plane correction. Long-term studies are needed to confirm our preliminary findings.