5 Percutaneous Pedicle Screw Systems

Simon P. Lalehzarian, Benjamin Khechen, Brittany E. Haws, Jordan A. Guntin, Kaitlyn L. Cardinal, and Kern Singh

5 Percutaneous Pedicle Screw Systems

5.1 Introduction

Pedicle screws evolved from the original facet screw technique to improve spinal internal fixation while maintaining vertebral range of motion. ¹ With the use of plates or rods, pedicle screws can provide load sharing between vertebrae, improving overall fixation and preventing vertebral collapse. ² ^, ³ Multiple methods have been developed to improve pedicle screw fixation strength, such as improving insertional torque, cross-linking adjacent screws, and triangulating the insertion of the screw. ² In minimally invasive spine surgery, percutaneous pedicle screw insertion techniques are utilized. The pedicle screw is oriented approximately 30° from vertical to allow for accurate pedicle insertion in addition to improving screw pull-out strength (▶ Fig. 5.1). ⁴ Surgical indications for percutaneous pedicle screws are described in ▶ Table 5.1.

5.1.1 Pedicle Screw Components

A pedicle screw is composed of a head, or tulip, neck, and shaft. The head is where the rods are placed to interconnect consecutive screws. Screw heads can be fixed (monoaxial) or can allow motion at the shaft–head interface (polyaxial). While polyaxial screws allow for easier rod insertion, they are also associated with failure at lower loads and greater rates of screw-head slippage than monoaxial screws. ⁵ ^, ⁶ Monoplanar screws have also been introduced, which allow for motion in the axial plane while maintaining rigidity in the sagittal plane. ⁷ ^, ⁸ In percutaneous pedicle screw placement, tulip extensions provide a corridor through which the screw head can be accessed from outside of the patient. ⁹ These extensions can either be removable or fixed with a break-off tab for removal. Tulip extensions are traditionally rigid, but can be malleable or lie flat against the skin for increased visualization. Additionally, some systems have capabilities for reduction or deformity correction. However, literature comparing the efficacy between tulip extension designs is limited and thus, choice of system is largely based on surgeon preference.

**Fig. 5.1** Illustration of the target trajectory for pedicle screw placement.

**Table 5.1** Surgical indications for percutaneous pedicle screws
Indications
Posterior approach spinal fusion Thoracolumbar procedures Degenerative disk disease Spondylolisthesis Spinal fracture/dislocation Spinal stenosis Spinal deformity

The screw shaft can be conical or cylindrical in shape. Conical screws have previously been associated with greater insertional torque and bending performance. ¹⁰ ^, ¹¹ However, concerns exist regarding the pullout strength of conical screws when backed out 360°. ¹² The shaft also contains the threads of the screw. The height and size of the thread crests determine the inner and outer diameter of the screw shaft (▶ Fig. 5.2). The outer diameter of the screw shaft has been identified as a major determinant of pullout strength, while the inner diameter is reported to determine the fatigue strength. ¹² ^, ¹³ Different thread options are available, including double lead and dual thread screws which facilitate faster insertion and increase insertional torque. ¹² ^, ¹⁴ The distance between the crest of each thread is called the pitch, and this measurement is also thought to be crucial in determining pullout strength of the screw. When screw pullout occurs, the bone in between the crests of each thread is often fractured. As such, the amount of bone in between the threads and the quality of bone can directly impact screw strength.

Percutaneous pedicle screw placement is performed by using Kirschner wires to determine accurate trajectory under fluoroscopic guidance. Cannulated screws are then introduced over the guidewire. The hollow core of cannulated screws has raised concerns over increased risk for screw breakage as compared to solid screws. Studies have suggested that cannulated screws exhibit decreased mechanical strength, stiffness, and axial failure loads when compared to solid screws of similar diameters. ¹⁵ ^, ¹⁶ As such, it is recommended that cannulated cores not exceed 2.0 mm in diameter. ¹⁷ However, previous reports have determined that cannulated conical screws are able to maintain the bending performance of solid screws, suggesting that conical designs are preferred for cannulated screw placement. ¹⁸ ^, ¹⁹ Newer designs have been proposed that use an additional pin to fill the cannulated core in an attempt to recreate the structural stability of the solid screw. However, this design has not been determined to improve the bending performance of cannulated screws. ¹⁸

**Fig. 5.2** Illustration of the components of a pedicle screw.

5.1.2 Pedicle Screw Constructs

Multiple constructs have been developed to improve pedicle screw strength and efficacy. These devices are often classified as rigid, semirigid, or dynamic stabilization systems. Variations in the materials utilized to create interconnecting rods and screws are what determine the type of stabilization system. Lumbar fusion with rigid posterior instrumentation is thought to increase fusion rates in cases of degenerative spinal disorders. ²⁰ However, these rigid systems have also previously been associated with some undesirable outcomes, including loss of lumbar lordosis, stress-shielding, adjacent segment degeneration, and fatigue fractures. ²¹ ^, ²² Semirigid and dynamic systems were developed as a method of enhancing load sharing as opposed to stress shielding. Additionally, dynamic stabilization is thought to reduce susceptibility to adjacent segment degeneration. ²³ ^, ²⁴ ^, ²⁵ Despite the theoretical benefits of less rigid systems in the setting of lumbar spinal fixation, superiority over rigid systems has been difficult to elucidate. ²⁰ ^, ²⁶

5.1.3 Complications

The complications associated with pedicle screws and the accuracy of pedicle screw placement have been reviewed thoroughly throughout the literature. ²⁷ Inaccurate screw placement is a common concern, particularly with percutaneous techniques, as visualization is limited. Medially misplaced screws can result in impingement or displacement of traversing nerve roots or dural tears. Additionally, screws angled cephalad with superior facet joint violation have been suggested to increase the risk of early failure or adjacent segment disease due to increased intrapedicular bending movements. ²⁸ ^, ²⁹ While superior facet violation has been suggested to occur in approximately 18% of pedicle screw placements, similar rates have been demonstrated for both open and percutaneous techniques. ³⁰

5.2 Percutaneous Pedicle Screw Systems

**Table 5.2** Alphatec Spine Illico® MIS Posterior Fixation System
Design
Set screws specifications Standard T25 drive mechanism Designed to limit cross-threading		Reduction capabilities Zodiac® cannulated high-top screws may be used to facilitate reduction

Modular aspects and variations
Screw diameters 5.5, 6.5, 7.5 mm	Screw lengths 35–55 mm (5-mm increments)	Rod type Precontoured Straight	Rod lengths 30–100 mm (5-mm increments) 200 mm
Procedures
MIS TLIF, MIS posterior decompression
Radiographs unavailable

**Table 5.3** DePuy Synthes VIPER^® MIS Spine System
Design
Set screws specifications Standard T25 drive mechanism		Reduction capabilities X-Tab reduction screw: up to 7-mm reduction

Modular aspects and variations
Screw diameter 4.35, 5.0, 6.0, 7.0, 7.5, 8.0, 9.0 mm	Screw lengths 30–55 mm (5-mm increments)	Rod type Straight	Rod lengths 35, 40–120 (10-mm increments), 150, 200, 300, 400, 600 mm
		Kyphosed	35, 40–120 (10-mm increments), 150, 200, 300 mm
		Lordosed	30–90 (5-mm increments), 100, 110, 120, 150, 200 mm
Procedures
MIS TLIF, MIS posterior decompression

**Table 5.4** Globus Medical CREO MIS™ Posterior Stabilization System
Design
Set screw specifications Threaded locking cap with 8.0 mm of torque for final tightening		Reduction capability Reduction options include 10 and 30 mm

Modular aspects and variations
Screw diameters 4.5–8.5 (1-mm increments), 5.0 mm	Screw lengths 20–120 mm	Rod diameters 5.5 and 6 mm	Rod lengths 30–150 mm (5-mm increments) 160–300 mm (10-mm increments)
Procedures
MIS TLIF, MIS posterior decompression
Radiographs unavailable

**Table 5.5** K2 M EVEREST® Minimally Invasive Spinal System
Design
Set screws specifications Modified square thread design Designed to facilitate easy introduction and limit cross-threading		Reduction capabilities Multiple reduction constructs up to 20 mm

Modular aspects and variations
Screw diameters 5.5–8.5 mm	Screw lengths 35–55 mm (5-mm increments)	Rode type Contoured	Rod lengths 5.5 and 6 mm
Procedures
MIS TLIF, MIS posterior decompression

**Table 5.6** Medtronic CD Horizon® Longitude® II Multilevel Percutaneous Fixation System
Design
Set screws specifications Tulip designed to reduce anatomical impact		Reduction capabilities Staged reduction technique allows for accurate screw placement

Modular aspects and variations
Screw diameters 4.5 mm 5.5 mm 6.5 and 7.5 mm 8.5, 9.5, 10.5 mm	Screw lengths 30, 35, 40 mm 30–50 mm (5-mm increments) 35–55 mm (5-mm increments) 90, 100, 110 mm	Rod type Prebent Straight	Rod lengths 30–80 mm (5-mm increments) 70–260 mm (10-mm increments)
Procedures
MIS TLIF, MIS posterior decompression

**Table 5.7** Medtronic CD Horizon® Solera® Spinal System
Design
Set screw specifications Reverse angle thread locking mechanism		Reduction capability Multiple reduction devices available for different diameter rods

Modular aspects and variations
Screw diameter 4–6 mm (0.5-mm increments), 6.5–9.5 mm (1.0-mm increments)	Screw lengths 20–60 mm (5-mm increments)	Rod type Pre-bent Straight	Rod lengths 30–120 mm (5-mm increments) 500, 600 mm
Procedures
MIS TLIF, MIS posterior decompression
Radiographs unavailable

**Table 5.8** Medtronic CD Horizon® Solera® Voyager Spinal System
Design
Set screws specifications Utilizes tulip design to reduce anatomical impact		Reduction capabilities Break-off section of tab extenders allows for 13.8 mm of reduction

Modular aspects and variations
Screw diameters 4.5 mm 5.5 mm 6.5 and 7.5 mm	Screw lengths 35, 40, 45 mm 35–50 mm (5-mm increments) 35–55 mm (5-mm increments)	Rod type Percutaneous Capped	Rod lengths 30–90 mm (5-mm increments) 30–80 mm (5-mm increments)
Procedures
MIS TLIF, MIS posterior decompression
Radiographs unavailable

**Table 5.9** NuVasive Reline® Posterior Fixation System
Design
Set screw specifications Helical flange and metric thread locking mechanisms		Reduction capability Reduction capabilities ranging from 9 to 50 mm

Modular aspects and variations
Screw diameters 5.5, 6.5, 7.5, 8.5 mm	Screw lengths 35–55 mm (5-mm increments)	Rod type Lordotic Straight	Rod lengths 25–100 (5-mm increments), 110, 120, 140, 160, 300 mm
Procedures
MIS TLIF, MIS posterior decompression
Radiographs unavailable

**Table 5.10** NuVasive SpheRx® DBR III Spinal System
Design
Set screw specifications Designed to limit cross-threading and allow “instrument-free” compression		Reduction capability Reduction can be achieved using DBR III counter-torque/reduction sleeve, reduction T-handle, and reducer extension

Modular aspects and variations
Screw diameter 5.5, 6.5, 7.5 mm	Screw lengths 30–55 mm (5-mm increments)	Rod type Pre-bent Pre-bent dual ball Straight dual ball	Rod lengths 20–110 (5-mm increments), 120, 130, 140, 150 mm 25–70 mm (2.5-mm increments) 17.5, 20, 22.5, 72.5, 75, 77.5, 80 mm
Procedures
MIS TLIF, MIS posterior decompression

**Table 5.11** RTI Surgical Streamline® MIS Spinal Fixation System
Design
Set screws specifications Standard T25 drive mechanism Designed to limit cross-threading		Reduction capabilities Simple rod inserter: up to 15-mm reduction Rod reducers: up to 30-mm reduction

Modular aspects and variations
Screw diameter 4.5–8.5 mm (1-mm increments)	Screw lengths 30–55 mm (5-mm increments)	Rod type Pre-bent Straight Long	Rod lengths 35–80 mm (5-mm increments) 90–150 mm (10-mm increments) 160–250 mm (10-mm increments)
Procedures
MIS TLIF, MIS posterior decompression

**Table 5.12** Zimmer Biomet PathFinder NXT™ Pedicle Screw Fixation System
Design
Set screws specifications Standard T25 drive mechanism Designed to limit cross-threading		Reduction capability Reduction forceps: up to 10-mm reduction Power knob reducer: up to 30-mm reduction

Modular aspects and variations
Screw diameters 4.5–7.5 mm (1-mm increments)	Screw lengths 30–60 mm (5-mm increments)	Rod type Pre-bent Straight	Rod lengths 30–100 mm (5-mm increments) 100–240 mm (20-mm increments)
Procedures
MIS TLIF, MIS posterior decompression