Anterior Vertebral Body Screw Position Placed Thoracoscopically: A Function of Anatomy and Surgeon Experience in a Porcine Model

Study Design. Manual digital measurements, in a porcine model of thoracoscopic anterior instrumentation, divided into two studies: 1) comparing the actual screw position within each vertebra with the perceived ideal screw position and correlation with the learning curve of the surgeon; and 2) a quantitative analysis of the vertebral bodies from T3 to T12 to determine the ideal screw insertion point and trajectory. Objective. To investigate whether the position of thoracoscopically placed screws is related to the portion of the thoracic spine instrumented and to the experience of the surgeon and to define the ideal starting position and direction of anterior thoracic screws in a porcine model. Summary of Background Data. Anterior screw insertion using a thoracoscopic approach is generally considered to be technically demanding and potentially dangerous. To our knowledge, there is no study analyzing the proper position of anterior vertebral screws using thoracoscopic technique and no study analyzing the ideal starting position of anterior vertebral body screws. Methods. In Study 1, 26 pigs were assigned to two groups (early experience, n = 16; late experience, n = 10) and underwent thoracoscopic anterior instrumentation and fusion from T5 to T10. The screw position was determined in the sagittal plane and in the frontal plane, respectively. In Study 2, 10 thoracic pig spines (T3-T12) were obtained. Each vertebra was digitally measured in the midtransverse plane. Using a point directly adjacent to the rib head as the ssumed ideal screw starting point, the percent of vertebral body "obscured" by the rib head and the maximum safe anterior and posterior screw insertion angles were determined. Results. In Study 1, screw positions of Group 1 were significantly more anterior from T5 to T7 compared with the ideal screw position (P < 0.05). Most screws were in the ideal position in Group 2. In both groups, the screw position within T5 was too inferior (P < 0.05). The majority of posterior cortical disruptions occurred in the distal vertebrae. Most anterior disruptions occurred in the proximal vertebrae. In Study 2, the percent of vertebral body behind the rib head significantly decreased from T3 (61.5%) to T12 (25.7%) (P < 0.0001). The maximum anterior insertion angle significantly increased from T3 (11°) to T12 (24°) (P < 0.0001) while the maximum posterior insertion angle significantly decreased from T3 (28°) to T12 (11°) (P < 0.0001). Conclusions. The position of thoracoscopically placed vertebral 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 too inferior while posterior placement of screws occurs in the distal thoracic spine. When using the rib head as the starting point for anterior screw insertion, a slight anterior angle is required in the distal thoracic spine while a slight posterior insertion angle is required in the proximal thoracic spine. A learning curve does exist, however; the proximal screw positioning continues to be a challenge.