Anterior Vertebral Body Screw Position Placed Thoracoscopically: A Function of Anatomy and Surgeon Experience

PURPOSE: Thoracoscopic anterior instrumentation and fusion for spinal deformity is technically demanding and may result in less than ideal screw position. The purpose of this study was to compare the screw position within each vertebra with the perceived ideal position, while comparing early versus later experiences in a porcine model of thoracoscopic anterior spinal fusion.

METHOD: 26 pigs underwent thoracoscopically-assisted fusion and instrumentation by the same surgical team. The animals were assigned to two groups; Group 1, early experience (n=16) and Group 2, later experience (n=10). Discectomies were performed from T5 to T10 followed by anterior instrumentation. Spines were harvested at 4 months postoperatively. Screw holes found to be within 3 mm of the vertebrae cortex, spinal canal, or endplate were considered to have disrupted the safe margins of the vertebral body. In the sagittal plane, the ideal screw position was considered to be equidistant from the anterior and posterior vertebral margins and in frontal plane, at a location equidistant from the superior and inferior endplates.

RESULTS: Saggital Plane Analysis: group 1 (early experience) screw positions were significantly more anterior from T5 to T7 compared with the ideal screw position (p<0.05) while the screws from T8 to T10 were ideal. In group 2 (later experience) all screws were in the ideal position. In group 1, 5% of the screws disrupted the posterior cortex, compared with 1.7% of the screws in group 2 (p=0.28). The majority of posterior cortical disruptions occurred in the distal vertebrae. Anterior cortical disruptions occurred in 6% of the screws in group 1, compared with 0% in group 2 (p=0.05). Most anterior disruptions occurred in the proximal vertebrae. Frontal Plane Analysis: In both groups, the screw position within T5 was consistently too inferior when compared with the ideal screw position (p<0.05). One screw (1%) in group 1 and no screw in group 2 disrupted the inferior endplate (p=0.44).

CONCLUSIONS: The position of thoracoscopically-placed screws is dependent on the level of the spine instrumented and surgeon experience. Screws placed in the proximal thoracic spine tend to be too anterior and inferior. Posterior placement of screws tends to occur in the distal thoracic spine. A learning curve does exist with respect to the ideal versus actual vertebral screw position and requires training and practice in cadaver and/or animal labs.