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Abstract
– Abstract – Correct alignment of pedicle screws is imperative in multilevel instrumentation. However, there has been no report addressing the technical aspects of this subject. If the head diameter of a pedicle screw is D, the head height is H, and the convergence angle of the screw being inserted is α, then the distance between the extension line of the medial borders of the inserted screw heads and the insertion point of the adjacent screw (A) is described by the following formula: A = ½Dcos α - Hsin α If an L3 pedicle screw (D=13 mm, H=15 mm) is to be inserted with a convergence angle of 14 degrees after the insertion of L4 and L5 screws, its insertion point should be 3.6 mm medial to the extension line of the centers of the L4 and L5 screws and 2.7 mm lateral to the extension line of their medial borders for all screw heads, so it can be aligned in the coronal plane. In order to achieve the best alignment, a pedicle screw is inserted between the extended line of the centers and that of the medial borders of the inserted adjacent screw heads. For the routine range of convergence angles, it is essential to move the entry point medially toward the extended line connecting the medial borders of the inserted adjacent screw heads.
REFERENCES
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Fig. 1.
If screw heads are not correctly aligned, rod insertion could be difficult (bottom, B). (A) The best alignment, (B) An inappropriate alignment.
Fig. 2.
(A) The first point is located in the extended line connecting the centers of adjacent screw heads (red), and the second point is in the extended line connecting their medial borders (blue). (B) The screw being inserted was lateralized. (C) The best coronal alignment was obtained.
Fig. 3.
Relationship between the base (adjacent) screw and the screw that will be inserted. (A) If a pedicle screw is directed in line with the center of the base screw head, it will be lateralized in coronal plane. (B) The entry point must be located in line with the medial border of the base screw head to get the best alignment of screw heads in coronal plane. These schemas demonstrate the same situations as B and C in figure 2, respectively.
Fig. 4.
The suggested numerical formula describes the distances between the entry point and the medial border (A) and the center (B) of a screw head, respectively.
Fig. 5.
Adjustment of entry point according to convergence angle. The entry point should be slightly lateralized in case that the convergence angle is smaller than that of a base (adjacent) screw to align the screw heads (A, B). If the convergence angle of a screw being inserted is larger than that of a base (adjacent) screw, its entry point must be slightly medialized (C, D).
Table 1.
The distance between the entry point and the medial border of a screw head (A), the distance between the entry point and the center of a screw head (B), and average convergence angle (C)1) according to the level of each lumbar vertebra. The diameter and height of a screw head are 13 mm and 15 mm, respectively.
| A(mm) | B(mm) | C(degrees) | |
|---|---|---|---|
| L1 | +3.5 | 2.9 | 10.9 |
| L2 | +3.2 | 3.1 | 12.0 |
| L3 | +2.7 | 3.6 | 14.4 |
| L4 | +1.5 | 4.6 | 17.7 |
| L5 | -1.9 | 7.5 | 29.8 |
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