Transpedicular Screw Fixation: Open and Percutaneous Techniques

Overview

Although earlier reports described the use of small facet screws to facilitate spinal fusion, it was Roy-Camille, in 1970, who promoted the use of the pedicle as a point of fixation for thoracolumbar segmental instrumentation. Before pedicle screw fixation, instrumentation of the thoracic and lumbosacral regions consisted primarily of rods secured to the spinal column by hooks or sublaminar wires. Although they provided adequate segmental fixation, these systems were plagued with problems such as hook dislodgement, rod breakage, and neural injury from the sublaminar passage of wires or hooks. Such systems were also limited in their ability to attach to the sacrum.

Pedicle screw fixation has proven to be biomechanically superior to hook and sublaminar wire constructs. Steffee referred to the pedicle as the “force nucleus of the vertebral body.” Unlike hooks or sublaminar wires, a pedicle screws engages all three anatomic columns of the vertebral body: anterior, middle, and posterior. Pedicle screw fixation does not require an intact lamina for placement, and when properly placed, it has a lower risk of neurologic injury than hooks or wires placed within the spinal canal.

Pedicle fixation has evolved from earlier versions, which typically used monoaxial screws coupled with fixation plates, to the current rod-based systems that use polyaxial screws and allow the surgeon to apply a variety of corrective forces to a spinal deformity. Because of their biomechanical advantages over hook and sublaminar wire systems, they also provide the opportunity for the use of shorter constructs and for earlier postoperative recovery. More recently, techniques for placing pedicle screws percutaneously have been developed, creating the challenge of screw placement with limited exposure of the spinal anatomy.

Several methods may be used for properly and accurately inserting pedicle screws. This chapter will review the advantages and disadvantages of each method. Regardless of the method used, a thorough understanding of the pertinent spinal anatomy and the indications and techniques for pedicle screw fixation is critical. Each surgeon should carefully and accurately interpret all relevant preoperative imaging studies and use the method of screw insertion that works best for them.

Pedicle Anatomy

The vertebral body pedicle is a strong, cylindric structure composed of an outer margin of cortical bone and a core of cancellous bone. It serves as an anatomic bridge between the dorsal spinal elements and the vertebral body. Depending on the spinal level, the dimensions of the pedicle will vary with regard to sagittal pedicle width (pedicle height), transverse pedicle width, and the pedicle angle in the sagittal, transverse, and coronal planes.

The transverse width of the pedicle is the anatomic parameter that most affects the feasibility of screw placement. In the lumbosacral region, the transverse width increases gradually from L1 to S1. Most pedicles below the L1 level are at least 8 mm in width, allowing for safe placement of screws in most cases. In the thoracic spine, the transverse width of the pedicles in the midthoracic region (T4–T9) is typically narrower than that of the pedicles in the upper thoracic (T1–T3) and lower thoracic (T10–T12) regions. The transverse width of a pedicle can be easily determined with computed tomography (CT) imaging.

The transverse angulation of a pedicle determines the medial angulation required for placement of a pedicle screw. This angle gradually decreases from T1 down to T12. At the T1 level the transverse angle is 10 to 15 degrees; at T12, the angle is 5 degrees. From L1 down to S1 the transverse angle gradually increases approximately 5 degrees per level. The angle at S1 is approximately 20 to 25 degrees.

The entry point for each pedicle also varies according to the spinal level. In the thoracic spine the pedicle entry point typically lies at a point even with the upper margin of the transverse process at its junction with the facet joint. There may be minor variations of this entry site, depending on the specific thoracic vertebral level.

In the lumbar region the entry point for pedicle screws is typically at the intersection of the center of the transverse process and the facet joint. The entry point for an upper sacral pedicle screw is the intersection of the sacral ala and the inferior margin of the adjacent facet joint ( Fig. 44-1 ).

Techniques of Transpedicular Screw Insertion

Pedicle screw fixation can be used throughout the spinal column. Although screws can be placed into the pedicles of the cervical spine, in particular at the C2 and C7 levels, they are more commonly used in the thoracic and lumbosacral regions. There are several techniques for assisting the surgeon with accurate placement of pedicle screws, either through an open or a percutaneous approach. The most common technique is the use of intraoperative fluoroscopy. Other techniques include a freehand method without any intraoperative imaging; the use of electromyography (EMG) monitoring; and the use of image-guided or computer-assisted spinal navigation. The specific technique used is based on the individual surgeon’s preference, experience, and comfort level.

FreeHand Technique

The freehand technique relies solely on the surgeon’s knowledge of the anatomy and spatial conception of the appropriate entry point and trajectory angle for screw insertion. Little or no intraoperative imaging is used; it can only be used with open, as opposed to percutaneous, spinal surgery. Performing this technique after decompression of the spinal canal can improve the accuracy of screw insertion by allowing the surgeon to directly visualize and palpate the medial wall of each pedicle. The accuracy of this approach will vary significantly depending on the individual surgeon’s experience and skill level.

This freehand technique requires the surgeon to use anatomic surface landmarks in the surgical field to correctly identify the appropriate entry point for each pedicle. Once the entry point is identified, it is decorticated with a drill or rongeurs. A curved or straight pedicle probe, or “gearshift,” is positioned over the site. The surgeon must then estimate the appropriate trajectory in both the transverse and sagittal planes as the probe is gently advanced into the pedicle. The probe is advanced slowly, using a constant twisting motion of the surgeon’s dominant hand with gentle pressure. The blunt tip of the probe is designed to minimize the potential for the probe to break through the cortical walls of the pedicle. Care must be taken not to apply a significant downward force that may plunge the probe through the vertebrae or fracture the pedicle.

After the pilot tract has been created, the probe is removed, and a smaller sounding probe is inserted to assess for bony integrity of the pedicle and to determine the appropriate screw length needed. If no pedicle breach is detected, the hole can be tapped, and the appropriate screw can be inserted. If a breach of the pedicle is detected, the “gearshift” is reinserted, and an attempt is made to correct the trajectory. After each pedicle to be fixated has been cannulated, K-wires or pedicle markers can be placed into each tract, and intraoperative imaging can be obtained to confirm appropriate placement ( Fig. 44-2 ).

The freehand technique is best used by experienced spine surgeons. In addition to requiring a precise conceptualization of the nonvisualized spinal anatomy and its relationship to the adjacent neural elements and soft-tissue structures, it also requires a feel for the correct amount of pressure to apply to the pedicle probe as it is advanced into each pedicle. If an incorrect entry point or trajectory is selected, it may irreversibly damage the pedicle, thereby preventing its use for screw fixation.

Karapinar reviewed a series of 640 pedicle screws placed in the thoracolumbar spine using a freehand technique; postoperative CT imaging confirmed a pedicle breach by 37 inserted screws (5.8%). This breach rate was confirmed by Amato in a review of 424 consecutive freehand lumbosacral pedicle screws in which the overall screw misplacement rate was 5%. The most common direction of screw misplacement was lateral; the most common level of misplacement was L3 (11%).

Parker and colleagues recently published a review of 6816 consecutive pedicle screws placed into the thoracic and lumbosacral regions using a freehand insertion technique. This study found that the most common screw breach of a pedicle occurred laterally. The overall incidence of pedicle breach in the lumbar spine was 0.9% as opposed to a rate of 2.5% in the thoracic spine. The lowest rate of screw breach occurred at the L5 and S1 levels. Although these rates of misplacement are relatively low, they demonstrate that even with routine use, consistently accurate placement may be difficult to achieve.

Fluoroscopically Assisted Screw Placement

The most common technique for placing pedicle screws is with the assistance of intraoperative fluoroscopy. As with the freehand technique, the surgeon must understand and recognize the surface landmarks and appropriate trajectories for screw placement. Unlike the freehand technique, intraoperative fluoroscopy adds an additional level of screw insertion accuracy by providing the surgeon with real-time imaging during creation of the pilot tracts and insertion of the screws.

Before screw insertion, a C-arm fluoroscopic unit is positioned so as to provide a lateral view of the surgical anatomy; this will assist the surgeon in determining the appropriate trajectory in the sagittal plane. It is also preferable to allow for rotation of the C-arm during instrumentation to provide an anteroposterior (AP) or oblique image. This will help guide the appropriate screw trajectory in the axial plane.

The entry site for a pedicle is identified using surface landmarks, and a pedicle probe is positioned over this point. A lateral fluoroscopic image is obtained to confirm the appropriate sagittal trajectory for the pilot hole. The probe is gently advanced into the upper part of the pedicle with additional spot imaging obtained. The C-arm is then rotated to provide an AP image to confirm that the probe is not being directed too far medially. With satisfactory fluoroscopic imaging, the probe is advanced to its final depth and is removed; bony integrity is confirmed with a sounding probe. When all pilot holes have been created, pedicle markers or K-wires can be placed within them, and final lateral and AP images are obtained before screw placement.

Although the use of fluoroscopy is optional during open surgery for pedicle fixation, it is necessary when using percutaneous techniques for screw placement. The lack of any visualization of the spinal surface anatomy during percutaneous procedures creates this dependence on fluoroscopic assistance. Percutaneous pedicle fixation is best performed with two C-arm units: one is positioned to provide a lateral image, the other is positioned to provide an AP image. The unit providing the AP image is tilted toward the head of the patient to allow for better surgeon access to the surgical field. Images are obtained from both units before beginning the procedure to ensure adequate positioning. A metallic instrument is placed on the skin overlying the levels to be instrumented, and images are obtained to select the appropriate sites for the stab incisions to be used.

Several different percutaneous pedicle screw systems are currently available, and each system has design features that distinguish it from the others. In general, the percutaneous method typically involves placing a Jamshidi needle through a stab incision and advancing it to the level of the pedicle entry point. AP and lateral fluoroscopy are obtained to confirm the appropriate entry point and the axial and sagittal trajectories. The pedicle is cannulated by advancing the Jamshidi needle through it and into the vertebral body. Serial imaging is obtained as the needle is advanced to confirm a satisfactory trajectory.

When satisfactory positioning has been obtained, the core of the needle is removed, and a K-wire is placed into the pedicle. If desired, a tap with a soft tissue protector sleeve can be advanced over the guidewire to tap the pedicle. The tap is removed, and the appropriate pedicle screw is inserted over the K-wire. The screw is advanced into the pedicle with fluoroscopic imaging to monitor its depth and trajectory. During tapping of the pedicle and insertion of the screw, care must be taken to ensure that the K-wire remains in the vertebral body until the pedicle screw has passed the limit of the pedicle. Any manipulation of the K-wire should be performed under fluoroscopic guidance; the K-wire can be removed once the pedicle screw has entered the vertebral body. Following satisfactory placement of all screws, the appropriate-length rods are placed according to the manufacturer’s guidelines.

Regardless of the surgical approach, several earlier studies have demonstrated the inaccuracies of fluoroscopy in guiding pedicle screw placement in the lumbosacral spine. The rate of disruption of the pedicle cortex by an inserted screw ranges from 15% to 31% in these studies. The disadvantage of fluoroscopy in orienting the spinal surgeon to the unexposed spinal anatomy is that it displays at most only two planar images. Although the lateral view can be relatively easy to assess, the AP or oblique view can be difficult to interpret. For most screw fixation procedures, the position of the screw in the axial plane is most important; this plane best demonstrates the position of the screw relative to the neural canal. Conventional fluoroscopic imaging cannot provide this view.

In some cases it may be difficult to acquire satisfactory imaging with fluoroscopy. Because of the need to penetrate the shoulders with the imaging beam, pedicle fixation in the upper thoracic region can be difficult to monitor with fluoroscopy. Optimal imaging may also be difficult to obtain in the lumbosacral region in obese patients.

A significant concern with the use of intraoperative fluoroscopy is the radiation exposure experienced by the surgical team and the patient, particularly with percutaneous pedicle screw fixation techniques. Rampersaud demonstrated that, compared with other orthopedic procedures that use intraoperative fluoroscopy, spinal procedures potentially result in a tenfold to twelvefold increase in radiation exposure as a result of factors such as backscatter radiation and the increased energy levels needed to image the lumbar spine. This creates a potentially significant hazard to those individuals who perform a high volume of complex spinal surgeries. If fluoroscopic imaging is used, it is important that exposure times be kept to a minimum and that all appropriate safety precautions for intraoperative imaging be implemented.

Image-Guided Navigation-Assisted Screw Placement

Image-guided, or computer-assisted, spinal navigation is a computer-based surgical technology designed to improve intraoperative orientation to the nonvisualized anatomy during fixation screw placement. It gives the spinal surgeon the ability to manipulate multiplanar CT and fluoroscopic images during the procedure in order to gain a greater degree of orientation to the surgical anatomy, thus optimizing the precision and accuracy of the surgery. An additional advantage is that, compared with conventional intraoperative imaging, it eliminates or significantly reduces radiation exposure to the surgical team.

Spinal navigation systems consist of an image-processing computer workstation interfaced with a two-camera optical localizer device ( Fig. 44-3 ). A handheld navigational probe mounted with a fixed array of passive reflective spheres serves as the link between the surgeon and the computer workstation ( Fig 44-4 ). Passive reflectors can also be attached to standard surgical instruments. The spacing and positioning of the passive reflectors on each navigational probe or customized, trackable surgical instrument are monitored by the computer workstation. During navigation the optical localizer emits infrared light toward the operative field. The infrared light is then reflected back to the optical localizer by the passive reflectors. This information is relayed to the computer workstation, which can then calculate the precise location of the instrument tip in the surgical field and can also calculate the location of the anatomic point on which the instrument tip is resting.