The contact volume between the cortical bone and screws differed for screws of different diameters placed in the same screw trajectory. Therefore, the screw diameter would affect the fixation strength. Two parameters were introduced in combination with the characteristics of CBT screw placement, namely, the TRW and TRH. Since the height of the vertebral pedicle was greater than the width, the TRW was the main factor limiting the screw diameter. When TRW < 4.0 mm, the pedicle was very narrow, and the screw could extremely easily pierce the cortical bone in the screw trajectory; thus, the placement of this screw was not recommended. When 4.0 mm ≤ TRW < 4.5 mm, a screw with a diameter of 4.0 mm was recommended; when 4.5 mm ≤ TRW < 5.0 mm, a screw with a diameter of 4.5 mm was recommended; when 5.0 mm ≤ TRW < 5.5 mm, a screw with a diameter of 5.0 mm was recommended; when 5.5 mm ≤ TRW, a screw with a diameter of 5.5 mm was recommended (Table (Table11).

Relationship between the TRW and the diameter of the screw trajectory.

TRW = trajectory reference width.

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