Objectives Primary objective: Determine if there is a clinically significant difference in percent Cobb curve correction in a low- vs. high-implant density cohort through a prospective randomized controlled trial.

Design and Outcomes Randomized clinical trial of equivalence to test the efficacy and safety of low vs. high implant density instrumentation for spine deformity surgery in AIS patients with Lenke IA curve patterns.

Interventions and Duration Intervention: low-implant density group or high-implant density group. Duration: 2 years. Sample Size and Population Target population: 10 to 17 years old with AIS who will undergo instrumented spinal fusion. Sample size needed (power = 90%) is 174 subjects with 87 in each group.

The MIMO Study Group proposes to conduct a prospective randomized controlled trial in order to determine whether a low implant density cohort has equivalent clinical and radiographic results to a high implant density cohort. This study is proposed as a trial of equivalence. The null hypothesis is that significant differences exist in percent major Cobb angle correction between high- and low-implant density cohorts for Lenke 1A curve types, the most common scoliotic curve pattern (73). Consecutive qualifying patients will be screened and enrolled by high-volume spine surgeons at 14 sites and randomized to spinal instrumentation with a low- or high-density screw pattern. Patients will return at 3-months, 1-year, and 2-years for exam, radiographs, and collection of patient-reported outcome scores. The Harms Study Group Foundation will serve as a collaborating organization and will provide database development and management, secure online data collection, centralized radiographic measurements, and data quality control. The University of Minnesota team will lead study design, protocol development, IRB/data use agreement contracts, patient randomization, and statistical analysis, including quarterly data review. Imaging will be obtained either with an EOS system or using a calibration belt during the radiographs, which allows for subsequent with the 3D modeling of the deformity. Six enrolling sites currently have this capability (EOS - Montreal, St. Louis, Iowa, Mayo, DuPont, and San Diego). An additional five sites already have calibration belts - Dallas, New York, Baltimore, Boston, U. of Rochester), and new calibration belts will be fabricated for the additional sites. The Montreal site will supervise collection of radiographic data with 3D capability and assist with troubleshooting and radiographic quality control. Although the study is primarily powered to detect change in percent correction of the major coronal curve, preliminary data will be gathered on axial and sagittal plane correction as well as complications, surgical factors, and patient-reported outcomes.

Inclusion Criteria:

• Patients aged ≥10 and ≤ 18 years
• Male or Female
• Diagnosis of idiopathic scoliosis for which surgery is recommended to prevent the curvature or to correct trunk disfigurement Lenke 1A curve pattern
• Curve cobb of 45° to 65°
• T5-T12 kyphosis measuring 0° to 40°
• Spina bifida Oculta is permitted
• Spondylolisthesis and Spondylolysis are permitted, as long as non- operative

Exclusion Criteria:

• Prior spinal surgery
• MRI abnormalities (including >4mm of Syrinx and/or Chiari malformation)
• Neuromuscular or other serious co-morbidities
• Thoracogenic or cardiogenic scoliosis
• Associated syndrome or developmental delay
• Unable or unwilling to firmly commit to returning for required follow-up