MIS Cervical Pedicle Screws—Percutaneous-Assisted Technique

49 MIS Cervical Pedicle Screws—Percutaneous- Assisted Technique

Marcus Richter and Philipp Hartung

Summary

Although lateral mass screws are still the most commonly used technique for posterior instrumentation of the subaxial cervical spine, they have several significant drawbacks in terms of biomechanical stability and reduction possibilities. Pedicle screws, which are standard for thoracic and lumbar stabilizations, show significantly improved stability and reduction performance in the cervical spine. However, they are still used rarely due to difficult placement and potential risks due to misplacement. The application of a minimal invasive technique with percutaneous-assisted placement of cannulated pedicle screws through tubes allows for a significantly reduced misplacement risk, comparable to thoracic and lumbar pedicle screws, a better reduction possibilities, and a significantly reduced implant failure rate compared to lateral mass screws.

Keywords: cervical spine minimally invasive spine surgery pedicle screw fixation percutaneous-assisted technique posterior instrumentation spinal navigation

The use of spinal navigation for the placement of the cervical pedicle screws allows for further reduction of the approach size and reduces the rate of screw misplacement.2

49.1 Introduction

Pedicle screw instrumentation is the gold standard in the lumbar and thoracic spine, but it is still uncommon in the cervical spine. The small dimensions of the cervical pedicle and the proximity of vascular and neural structures may explain this. The mainly used posterior screw technique in the cervical spine, i.e., lateral mass screw fixation, results in a biomechanical stability clearly below the stability achieved with pedicle screws¹^,² and there are also complications due to the placement of lateral mass screws documented in the literature. The incidence of lateral mass fractures is 6 to 7% and injury or irritation of the posterior root is found in 4 to 10% depending on the used technique.³ Nevertheless, there are potential risks of iatrogenic damage to neural or vascular structures¹^,⁴ when using cervical pedicle screws.

Due to the advantages of cervical and high thoracic pedicle screws over other instrumentation techniques, the interest of spine surgeons in this technique was increasing rapidly.⁵^,⁶ As a result of improved biomechanical stability of pedicle screws compared to lateral mass screws, pedicle screws allow for shorter instrumentations with improved reduction capacities. Although excellent results with very low screw misplacement rates were published by Abumi et al,⁴ demonstrating 7% misplacement rate out of 669 cervical pedicle screws when using a conventional screw insertion technique without a computer-assisted surgery (CAS) system, this data may not be applicable to many other spine surgeons with significantly less experience in cervical pedicle screws.

In 1993, CAS systems were developed for the installation of pedicle screws in the lumbar spine⁷^,⁸ based on reports of misplacement rates of between 5 and 40% using conventional techniques.⁹ In vitro studies showed that misplacement rates of pedicle screws can be significantly reduced when using spinal navigation.¹⁰ In vivo studies confirmed these results.¹¹ As in vitro studies have shown that cervical pedicle screws can be safely applied using spinal navigation,¹²^,¹³ we started using spinal navigation intraoperatively for pedicle screws of the cervical and cervicothoracic spine. Meanwhile several systematic reviews have shown that the malplacement rate of pedicle screws is significantly lower when using spinal navigation, and there is no difference in the malplacement rates between cervical, thoracic, and lumbar spine.¹⁴

We decided to use a drill to prepare the pedicles due to the reduced friction of a rotating drill compared to a pedicle awl. The results of our in vitro study¹² showed that this technique is suitable for cervical pedicle instrumentation. In our clinical study,¹⁵ we had a pedicle perforation rate of 3% in the spinal navigation group without any screw-related complications. Also, the low pedicle perforation rate in the conventional group of 8.6% without any screw-related complication shows that cervical pedicles can be applied safely without spinal navigation when using the described technique with cannulated screws and percutaneous application at C3–C6. Preoperative planning and determination of the pedicle angles are mandatory for cervical pedicle screw placement without spinal navigation. The entrance point can be defined well with anatomical landmarks and the pedicle can be identified in the lateral image intensifier view but it is very difficult for the surgeon to define the exact angle of the drill guide. Especially if the angle is too low, there is a high risk of injury to the vertebral artery. Therefore, intraoperatively the use of an angle-measurement device is also mandatory. Nevertheless, the use of spinal navigation reduces the risk of screw malplacement and should be used, if available.

Pedicle widths of C3 averages 4.9 mm in males and 4.5 mm in females, the minimum reported width being 3.0 mm. C4 averages 4.7 mm in males and 4.6 mm in females, with the minimum reported width of 3.1 mm.¹⁴,¹⁶ The pedicle widths in C5 and C6 are slightly larger. These anatomic data show that some pedicles may not be suitable for pedicle screws. Therefore, pedicle width should be measured preoperatively using computed tomography (CT). Although the risk associated with pedicle screws C3–C6 is obviously higher compared to C7 and high thoracic pedicle screws, the use of spinal navigation for C7 and high thoracic pedicle screws may also be beneficial, as the quality of the intraoperative image intensifier picture in this region is usually rather poor due to the shoulders of the patient.

Although spinal navigation is a useful and fascinating tool for spine surgery, anyone using such a system should be aware of possible errors. As the system can crash at any time during the operation due to hardware, software, or human failure, every surgeon using the system should be experienced in the operation without the use of spinal navigation. Furthermore, the surgeon should never rely on the virtual information from spinal navigation without his or her own verification if the surgical reality matches the virtual reality shown on the screen of the spinal navigation system. The surgeon must ensure that the correct vertebrae are instrumented via image intensifier control or other techniques. Although registration of the instrumented vertebrae is possible in all cases, it can be difficult especially in C3 and C4, as these vertebrae have very small spinous processes and in many patients, the posterior surface of the vertebrae, which is used for the surface matching, is quite similar. Therefore, it is possible to achieve an acceptable registration on C3 with the surface data of C4 or vice versa. This problem underlines the mandatory need for the surgeon to verify that he/she is working on the correct vertebra. Another error may result from the reference clamp, which is attached to the vertebra on which the surgeon will be working. In the middle of the cervical spine, the small dimensions of the spinous processes make it difficult to achieve a stable fixation of the reference clamp, especially as most of the reference clamps available for the different spinal navigation systems were initially designed for the lumbar and thoracic spine.

49.2 Indications

In general, this technique can be used for all patients requiring a posterior cervical stabilization. Indications are instabilities, degenerative changes, and tumor involvement. The following indications have to be taken into consideration:

a)Instabilities due to rheumatoid arthritis.

b)Instabilities due to neoplasia with/without spinal cord compression.

c)Spondylodiscitis of the cervical spine.

d)Traumatic instabilities, especially in patients with ancylosing spondylitis.

e)Cervical spondylotic myelopathy, especially multisegmental or significant posterior stenosis, also in mono- and bisegmental cases.

f)Pathologies of the sagittal profile of the cervical spine requiring corrective surgery.

g)Congenital instabilities.

h)Posterior stabilization of the cervical spine in patients with severe osteoporosis.

49.3 Contraindications

a)Poor health prohibiting general anesthesia.

b)Small pedicle dimensions with a pedicle width below the screw diameter or anatomical variations of the vertebral artery.

49.4 Preoperative Planning

Clinical assessment including neurological status should be done.

The following imaging studies are needed:

a)X-ray in anteroposterior (AP) and lateral projection.

b)Extension/flexion in lateral projection X-rays if instability is suspected.

c)CT/angio-CT with multiplanar reconstructions.

d)Magnetic resonance imaging (MRI)/angio-MRI if spinal cord compression is suspected or in tumor cases.

Preoperative screw planning is done based on the CT and multiplanar reconstructions in the axis of the pedicles with determination of the vertebral artery course and planning of angulation and length of the screws (Fig. 49.1a–c). If pedicle screws are not possible due to anatomical reasons, lateral mass screws are the alternative in C3–C7. In C2, translaminar screws are a viable alternative to pedicle screws.