Advantages of Pedicle-Screw Fixation

The nature of the pedicle screw–bone junction is much more secure than the wire or hook-bone junction. A single screw provides stability in five planes of motion.¹ With transverse connection to the other side, stability is achieved in all six planes of motion.¹ The rigidity of pedicle fixation allows for the incorporation of fewer normal motion segments to achieve stabilization of an abnormal level. Because the pedicle represents the strongest point of attachment to the spine, significant forces can be applied to the spine without failure of the bone-metal junction.

Pedicle-screw fixation systems are superior in restoring and maintaining spinal alignment. Because they traverse all three columns of the spine, they provide a longer lever arm through which the longitudinal member can transmit greater corrective forces than are achieved with spine fixation systems that attach to the posterior elements alone. Just as pedicle fixation systems resist loads in multiple planes, they are able to apply multidirectional corrective forces.

Another advantage of pedicle fixation systems is that they do not require intact dorsal elements. Thus, they can be used after laminectomy or traumatic disruption of the laminae, spinous processes, or facets. Pedicle fixation systems also avoid placement of instrumentation in the spinal canal, unlike sublaminar wires and hooks. Additionally, postoperative bracing requirements may be less than with earlier, biomechanically inferior fixation devices. Finally, fusion rates are thought to be improved with the pedicle-screw systems compared with earlier devices,^14–16 as well as with noninstrumented fusions.^14–17

Disadvantages of Pedicle-Screw Fixation

A steep learning curve is required for the safe implantation of pedicle screws. Caudal or medial penetration of the pedicle cortex can result in durotomy or neural injury. Implantation of pedicle screws requires extensive tissue dissection to expose the entry points and to provide the required lateral to medial orientation for optimal screw trajectory. Lengthy operations with significant blood loss and an increased risk of infection are not uncommon. Significant osteoporosis is a relative contraindication to the use of pedicle screws because a solid screw purchase is difficult to achieve. Postoperative imaging techniques (especially MRI) are, in part, obscured by the implant. Rigid fixation can accelerate adjacent motion segment degeneration. These are costly procedures.

Pedicle Anatomy

The pedicle is a very strong, cylindrical, anatomic bridge between the dorsal spinal elements and the vertebral body. It is composed of a strong shell of cortical bone and a core of cancellous bone.

Pedicle size and angulation varies throughout the spinal column. The transverse pedicle width is narrower than the sagittal pedicle width (pedicle height) except in the lower lumbar spine.¹⁸ Pedicle width is more important than pedicle height for pedicle-screw placement (Fig. 148-1A). The transverse pedicle width increases from L1 to S1.¹⁹ Most of the pedicles below T10 are greater than 7 mm in transverse diameter, and most below L1 are greater than 8 mm in diameter.²⁰ In a study measuring the pedicle diameters with CT, Bernard and Seibert²¹ found that 20% of pedicles were less than 7 mm at L2, 15.6% at L3, and 1.9% at L4. There were no pedicles less than 7 mm in diameter at L5 and S1. They concluded that all surgeons should perform preoperative CT scans when instrumenting pedicles above L4.

FIGURE 148-1 Diagram illustrating the pedicle width (A), the transverse or coronal pedicle angle (B), and the sagittal pedicle angle (C).

The transverse pedicle angle or coronal plane angulation (Fig. 148-1B) decreases as one descends caudally in the spine until the lumbar region. The angle increases as the lumbar spine is descended. The sagittal pedicle angle (Fig. 148-1C) is steep throughout the midthoracic spine and in the upper lumbar spine.¹⁸

The intrathecal nerve roots course along the medial aspect of the pedicle. At T12, the dural sac is 0.2 to 0.3 mm away from the pedicle.¹⁸ Below L1, the medial side of the pedicle is almost touching the cauda equina. The nerve root occupies the ventral and rostral one third of the foramen. As a result, violation of the medial or caudal cortex of the pedicle risks injury to the nerve root.

Biomechanics

Rigid pedicle fixation techniques, such as rigid plate or screw-rod combinations, apply force to the spine by fixed moment arm cantilever beam fixation.²² A cantilever is a projecting beam that is supported at one end only. It resists axial loads by rigidly buttressing the spine. In the absence of load sharing with the anterior column, significant stress occurs at the screw-plate or screw-rod junction. These constructs tend to fail by screw fracture at this junction.²³ However, modifications of the spinal implants (as discussed subsequently) have reduced the incidence of screw breakage.

Pedicle-screw fixation systems in which the screws are not rigidly affixed to the plate allow toggling of the screws (Fig. 148-2A). These systems constitute nonfixed moment arm cantilever beam fixation.²² They are unable to resist axial loads without the assistance of an anterior column, which is capable of axial load bearing. Because these systems allow toggling of the screws, they may fail by screw pull-out (Fig. 148-2B).²² For this reason, a fixed moment arm system more effectively resists sagittal translation.

FIGURE 148-2 Pedicle fixation systems in which the screws are not rigidly affixed to the plate allow toggling of the pedicle screws (A). These systems may fail by screw pull-out (B).

Pedicle-screw fixation can be applied with either a flexion component or an extension component to the applied moment arm, which is usually a rigid pedicle fixator.²² These constructs are used to reduce deformities. Extension moment arm application is their most common mode of application in this regard.

Pedicle fixation techniques may fail during axial loading. This is in part due to a tendency toward the development of a parallelogram-like translational deformity. Toeing-in of the screws and the use of transverse connectors help prevent this mechanism of construct failure.²⁴

The most frequently encountered problems with pedicle-screw fixation are hardware failure or failure at the screw-bone junction. Screw pull-out, breakage, and toggle are often the result of biomechanically inappropriate applications.

Screw Characteristics

Most pedicle screws have a cancellous thread pattern. In general, screw outside diameters range from 4.5 to 7 mm. Screw lengths are measured from the tip of the screw to the base of the screw head. Pedicle-screw lengths range from 30 to 55 mm, with 5-mm increments. There are two predominant types of pedicle screws: self-tapping and nontapping screws. With nontapping screws, a separate tap is used to cut threads into the pedicle.

Screw strength is proportional to the cube of the core (minor) diameter.²⁵ The larger the minor diameter, the greater the resistance to screw bending or breakage. The outside (major) diameter is an important factor in screw pull-out resistance. Other important components of pull-out resistance are the thread depth, pitch, and shape. Thread pitch is the distance from one point on the screw thread to the corresponding point on the adjacent thread. Pull-out resistance is directly proportional to the volume of bone between the threads that is determined by the thread depth and pitch. The angle or shape of the thread can affect the interthread volume and hence the pull-out resistance.²⁵

Some systems (e.g., compact Cotrel-Dubousset [CD]) have a smooth extended neck without threads. The diameter of this portion of the neck is greater than the tip and the body. This construction increases the strength of the neck of the screw where most breaks occur. Some systems use screws with a conical inner diameter. This conical shape minimizes screw fracture by increasing the diameter of the screw where it is most likely to fail.

Polyaxial heads have made the pedicle screw more versatile. Therefore, the screws may be easily connected to the rod. However, polyaxial pedicle screws have lower strength than conventional pedicle screws, and they are vulnerable to fatigue failure.²⁶ A biomechanical study has demonstrated that the polyaxial head coupling to the screw is the first to fail and may be a protective feature of the pedicle screw, preventing pedicle-screw breakage.²⁷

Preoperative Planning and Surgery

Usually, preoperative plain radiographs and a CT scan should be examined to determine bone quality, pedicle transverse diameter, and screw trajectory. Because intraoperative radiographs are usually necessary, preoperative planning should include preparation for anteroposterior (AP) and lateral radiographs. Lateral radiographs can usually be obtained without difficulty whether either a table or frame is used. Special radiolucent spinal frames are useful, particularly for AP radiographs.

The patient is placed prone on rolls, a frame, or a table with the abdomen hanging as freely as possible to indirectly decompress the epidural venous plexus. A midline incision is used and carried down to the level of the spinous processes. An adequate incision length facilitates proper trajectory during screw hole preparation and screw placement. All soft tissue is dissected off subperiosteally to minimize blood loss. Muscle dissection should allow for a lateral exposure of the transverse processes on both sides, leaving the intertransverse membrane intact at all levels of the planned instrumentation. A crank retractor helps maintain the lateral exposure. Release of the retractor every 30 minutes reduces muscle necrosis and thereby decreases the risk of infection. The spinal level is identified either anatomically or radiographically, and a decompressive laminectomy is performed if appropriate.

After placement of the pedicle screws, the dorsolateral bone graft bed is prepared by decorticating the lateral aspect of the facet joints and the transverse processes with a combination of a high-speed drill with a cutting bur and sharp, cupped curets. The synovium can be removed from the facet joints that are included in the fusion and packed with cancellous bone. The importance of meticulous decortication to achieving bony fusion cannot be overstated.

Pedicle Screw Entry Sites and Trajectory

Every spine surgeon should have a thorough knowledge of pedicle anatomy, including the coronal and sagittal plane angulation of the pedicle (see Fig. 148-1). The sagittal pedicle angle increases in the thoracic spine from an average of 0 degrees at T1 to 10 degrees at T8 and then decreases to 0 degrees at T12.²⁸ Usually the L4 sagittal pedicle angle is 0 degrees, and subsequent rostral and caudal levels are associated with progressively greater sagittal angles. The lordotic curvature of the lumbar spine produces a rostral angulation for upper lumbar screws. The L5 pedicle is 5 to 10 degrees caudally inclined. Coronal plane angulation decreases from the cervical spine to the thoracolumbar region and then increases as the lumbar spine is descended. The coronal plane angulation at T1 is 10 to 15 degrees and at T12 is 5 degrees.²⁸ A 10-degree medial angulation is satisfactory at L1. A wider angle in the coronal plane is necessary to avoid lateral penetration of the pedicle in the lower lumbar spine. The coronal plane angle increases approximately 5 degrees per level from L1 to the sacrum.²⁴

In the thoracic spine, the transverse process commonly does not align directly with the pedicle in the axial plane. For this reason, the anatomic landmarks that are used for lumbar pedicle-screw insertion cannot be used in the thoracic spine.²⁸ The transverse process is rostral to the pedicle in the upper thoracic spine and caudal to the pedicle in the lower thoracic spine. The crossover occurs at T6-7.²⁸ Because of the variability of the relationship of the pedicle to the transverse process, fluoroscopic guidance or direct vision and palpation of the pedicle via a laminotomy is recommended for insertion of thoracic pedicles. At the T1 to T3 levels, Louis suggests the use of 4.5-mm diameter screws that are 25 to 30 mm in length.²⁹ At the T4 to T10 levels, screws are usually 4.5 mm in diameter and 30 to 35 mm in length.²⁹

The conventional entry site for pedicle-screw placement in the lumbar region is at the junction of the lateral facet and the transverse process (Fig. 148-3). Although the midline of the transverse process corresponds to the location of the pedicle at L4, this relationship does vary at different lumbar levels. Above L4, the midline of the transverse process is rostral to the pedicle, and at L5, it is an average of 1.5 mm caudal to the pedicle.³⁰

FIGURE 148-3 The junction of the lateral facet and the transverse process is the conventional pedicle-screw entry site (arrow) in the lumbar spine.

Several nuances should be considered for accurate cannulation of the pedicle, as well as to enable proper screw trajectory. The muscle dissection is performed as lateral as possible to allow palpation of the transverse processes. The lateral aspect of the pedicle is palpated with a nerve hook over the transverse process. If a decompressive laminectomy is not being performed as part of the operative procedure, a small laminotomy may be performed to palpate the medial aspect of the pedicle and its rostral and caudal borders. Palpation of the pedicle helps guide accurate placement of the screw into the pedicle and increases the safety of the placement. It is strongly recommended for placement of pedicle screws at the level of the conus medullaris and above.

Screw Insertion

The entry site is decorticated by using a bur and a high-speed drill or a rongeur. A bur or awl is also used to penetrate the dorsal cortex of the pedicle. A curved or straight pedicle probe is used to develop a path for the screw through the cancellous bone of the pedicle into the vertebral body. The advancement of the pedicle probe should be smooth and consistent. A sudden plunge suggests breaking out of the pedicle laterally. An increase in resistance indicates abutment against the pedicle or vertebral body cortex. After cannulation of the pedicle and vertebral body with the pedicle probe, the pedicle sounding probe is placed into the pedicle, which is then palpated from within to make sure there is not a medial, lateral, rostral, or caudal disruption in the cortex of the pedicle. The sounding probe should also be used to determine that there is bone at the bottom of the pilot hole, verifying that penetration of the ventral cortex of the vertebral body has not occurred.

After the pedicles have been probed, different-length Steinmann pins or K-wires are placed bilaterally into the pedicular hole at each level. Using different-length pins on each side helps distinguish right and left on the lateral plain radiograph in case the pedicle hole trajectory or depth needs to be corrected. A lateral and an AP radiograph or fluoroscopic image is used to verify the trajectory of the pins. This, however, does not guarantee accurate screw placement.³¹ With slightly oblique AP views, a pin located in the middle of the pedicle has a characteristic “target sign.” Direct AP views demonstrate the lateral-to-medial orientation of the screws. Excessive medial orientation of the screws seen on an AP film raises the concern of medial penetration of the pedicle by the screw. Lateral imaging is useful to view the depth of penetration into the vertebral body and the sagittal angulation of the trajectory. Ventral screw penetration is usually between 50% and 80% of the AP diameter of the vertebral body. Ventral screw penetration greater than 80% of the vertebral body on a lateral plain radiograph raises the concern of ventral penetration of the vertebral body cortex because of the convex shape of the vertebral bodies. To avoid vascular and visceral injury, the ventral cortex of the vertebral body is not penetrated, although this would enhance the overall strength of the construct.

After confirmation of correct placement of the K-wires or Steinmann pins, the pedicle screw path is tapped if non–self-tapping screws are used. Some tapping screws are hollow and are slid over K-wires into the pedicles. This helps guide the tap along the same trajectory that the pedicle was previously probed. The screw is then guided into the pedicle by its purchase of the threads tapped into the pedicle. However, tapping is less desirable in cancellous bone because tapping weakens the implant-bone junction.²⁵ It is of questionable value in the insertion of pedicle screws because pedicle screws rarely obtain cortical purchase within the pedicle.³² To maximize the bone purchase of the permanent screws, a tap that is smaller in diameter than the screw is used. Tapping is usually not performed at a greater depth than the pedicle.