Screw

Screw definition:

A mechanism that converts rotational motion to linear motion, and a torque (rotational force) to a linear force.

Screw features

Pitch

distance between threads

Lead

distance advanced with one revolution

Screw working distance (length)

defined as the length of bone traversed by the screw

Outer diameter

Pullout strength is proportionate to outer (major) diameter^2

Maximised by

large outer diameter difference
fine pitch

Pedicle screw pullout most affected by quality of bone (degree of osteoporosis)

Root (inner) diameter

bending strength is proportionate to inner (minor) diameter^3

Pedicle screws are placed with the guidance of pilot holes.

Tapping

Aim:

tapping was used to cut threads into the bone along the pilot hole, matching the screw’s threads and easing screw insertion.

Pros

Reduce insertion torque
Make advancement of the screw more predictable

Cons

Tapping reduces the insertion torque and pullout resistance of pedicle screws, especially in spongy/trabecular bone (like vertebrae), meaning the screw is easier to insert but is anchored less strongly

Pedicle screws can be inserted into their pilot holes

Without tapping
with undertapping (0.5 or 1 mm)

1mm undertapping has comparable pullout strength to that of screws inserted without tapping
Pros

helps to redefine the pilot hole for easier screw insertion

Cons

Increase operative time
have potential trauma

with same size tapping

Not recommended: significantly reduce screw purchase.

Pedicle Screw effectiveness is determined by:

Core geometry
Thread design
Material properties
Insertion technique

The correct trajectory of pilot holes can be verified by

meticulous attention to technical detail,
tactile pedicle sounding,
radiographic imaging,
intraoperative monitoring techniques

Pedicle Screws types:

Monoaxial Screws:

Have a fixed head, providing the highest rigidity and stability.
Ideal for precise deformity correction like direct vertebral rotation (DVR).
Require exact rod contouring, which can increase stress at the bone-screw interface and raise the risk of fixation failure.
Studies show superior axial plane correction compared to polyaxial screws.
Facilitate superior scoliosis correction and improved rib cage symmetry during direct vertebral body derotation (DVBD) due to their rigid head-body connection.

Uniaxial Screws:

Allow motion in a single plane (typically sagittal) while maintaining rigidity in other directions.
Offer a balance between rigidity and ease of rod insertion, suitable for axial deformity correction.
Provide improved apical vertebral rotation compared to polyaxial screws, making them preferable when axial control is crucial.
Their pivoting head, moving only in the sagittal plane, enables easier rod loading while retaining derotational benefits similar to monoaxial screws.

Polyaxial Screws:

Feature a spherical joint allowing multi-directional movement, simplifying rod placement and accommodating coronal plane malalignment.
Are user-friendly but provide less vertebral derotational forces compared to monoaxial or uniplanar screws.
Some studies suggest they can achieve coronal, sagittal, and rotational correction outcomes comparable to monoaxial pedicle screws in moderate AIS cases.

Screw anchor Density:

Defined as the number of spinal anchors per fusion level.
Higher anchor density pros: better fixation and minimises rod deformation, especially in large or rigid curves.
High anchor density cons: longer operative time, higher costs, and increased potential for complications.
Current literature shows conflicting views on whether higher screw density consistently leads to better clinical and radiographic outcomes in AIS patients.

A systematic review and meta-analysis of Lenke I AIS patients found low-density pedicle screws could achieve similar clinical and radiographic outcomes to high-density constructs, potentially reducing costs.
Another review of various Lenke curve types reported better radiographic and patient-reported outcomes with high-density constructs (>1.54 anchors per level).

Screw Impact on Derotation Techniques:

Monoaxial pedicle screws facilitate superior scoliosis correction and improve rib cage symmetry during DVR due to their rigid head-body connection, which eliminates rotational freedom at the screw-rod interface.
Uniaxial pedicle screws also retain derotational benefits while offering easier rod loading due to their pivoting head that moves in only the sagittal plane.
Emerging systems like the Multi-Degree-of-Freedom (MDOF) system offer increased flexibility with six degrees of freedom, enhancing the surgeon's ability to achieve the desired spinal configuration.
The selection of pedicle screw types should be tailored to individual patient anatomy and deformity characteristics.

Revision screw

Varghese 2019

the pullout strength increased by 121% when the original index screws were replaced with screws that were two millimeters greater in diameter