Biomechanical comparison of the screw-bone interface: optimization of 1 and 2 screw constructs by varying screw diameter.

Study Design: In vitro biomechanical investigation of 1 and 2-screw anterior scoliosis constructs with varying screw diameters.

Objective: To determine a possible optimal configuration of screw number and diameter at varying levels within the thoracic spine for anterior vertebral body fixation.

Summary Of Background Data: Single-rod systems are typical in anterior thoracic and thoracolumbar correction of adolescent idiopathic scoliosis; although dual rod systems may offer more flexural stability. Loss of fixation remains problematic, particularly in the proximal thoracic vertebrae, and it remains unclear how screw diameter or the number of screws within the vertebrae affect fixation.

Methods: Individual vertebral levels from 10 cadaveric thoracic spines were randomly assigned to either 1 or 2 screws of 5, 6, or 7-mm diameter. Bone-screw interface failures were created in coronal plane cantilever plow, and failure loads were compared across vertebral levels for each instrumentation method.

Results: Two-screw constructs had significantly higher failure loads than single-screw constructs, while increasing screw diameter also produced significant changes in fixation strength. Two-screws had improved performance in the mid and lower thoracic spine, while a single screw was more stable in the upper thoracic spine.

Conclusions: Failure modes for 1-screw constructs almost entirely (89%) showed gradual plowing through the bone, whereas acute fracture through the vertebral body or pedicles were common forms of failure (85%) for 2-screw constructs.