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. With transverse connection to the other side, stability is achieved in all six planes of motion. 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 for 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. Finally, fusion rates are thought to be improved with the pedicle-screw systems compared with earlier devices, as well as with noninstrumented fusions.

Disadvantages of Pedicle-Screw Fixation

A prolonged 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 and subsequent fusion are difficult to achieve. Postoperative imaging techniques (especially magnetic resonance imaging [MRI]) are, in part, obscured by the implant. Rigid fixation can accelerate adjacent motion segment degeneration.

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 less than the sagittal pedicle height except in the lower lumbar spine. Pedicle width is more important than pedicle height for pedicle-screw placement ( Fig. 84-4A ). The transverse pedicle width increases from L1 to S1. 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. In a study measuring the pedicle diameters with computed tomography (CT), Bernard and Seibert found that 20% of pedicles were less than 7 mm at L2, 15.6% at L3, and 1.9% at L4. No pedicles were 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.

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. 84-4B ) 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. 84-4C ) is steep throughout the midthoracic spine and in the upper lumbar spine.

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. 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. 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. 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 or rod allow toggling of the screws ( Fig. 84-5A ). These systems constitute nonfixed moment arm cantilever beam fixation. 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 pullout ( Fig. 84-5B ). For this reason, a fixed moment arm system more effectively resists sagittal translation.

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 pullout ( 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. 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.

The most frequently encountered problems with pedicle-screw fixation are hardware failure or failure at the screw-bone junction. Screw pullout, 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 typically 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. The larger the minor diameter, the greater the resistance to screw bending or breakage. The outside (major) diameter is an important factor in screw pullout resistance. Other important components of pullout 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. Pullout 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 pullout resistance. 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 fixed head (monoaxial) pedicle screws, and they are vulnerable to fatigue failure or fracture. Despite inherent weakness compared to monoaxial screws, polyaxial screws are much more frequently used due to the improved ease of rod placement at various head angles. 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.

Screw Insertion

The entry site should be 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 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, typically 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.

One possible technique, after the pedicles have been probed, is that different-length Steinmann pins or K-wires may be 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. An alternative technique would be, after probing of the pilot hole ensured no cortical breach on all sides and bottom, to then tap the holes, reprobe, and then place the pedicle screws without the use of Steinmann pins. Using this technique, intraoperative x-rays are obtained after the pedicle screws are placed. 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 must not be 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. It is of questionable value in the insertion of pedicle screws because pedicle screws rarely obtain cortical purchase within the pedicle. 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.

After tapping of the pedicles, the permanent screws with the largest diameter that will fit and not fracture the pedicle are placed. The length of the screw is determined by measuring the length of the Steinmann pin or K-wire from the pedicle entry site to a depth of 50% to 80% of the vertebral body. The screw length should also be estimated based on measurement from preoperative imaging, usually CT or MRI. Screws in the lumbar spine usually have a 4.5- to 7-mm diameter and a 35- to 50-mm length. Next, the transverse processes and lateral aspect of the facet joints are decorticated to increase the likelihood of subsequent bone fusion. The screws are connected to a longitudinal member, usually a rod. A bone graft is then placed on the previously prepared fusion bed; the bone graft can be autograft from bone harvested during decompression or allograft.