Technique to Insert  Lumbar Pedicle ScrewsDr. Ramis Huseynov

Anatomiya • Columna spinalis-in 5 bölümü var:

• Cerical – 7 vertebra;• Thoracic – 12 vertebra;• Lumbar – 5 vertebra• Sacrum – 5 vertebra• Coccyx – 3-4 vertebra

Anatomia Curves• Kifoz – 1. cervical 2. lumbarLordoz – 1. thorakal 2.sacral

Typical Vertebrae

Typical Vertebrae

Vertebral Relationships

Major Ligaments of the Spine•Anterior

Longitudinal Ligament - ALL

• Atlanto-Occipital Membrane

• Posterior Longitudinal Ligament

• Supraspinous• Interspinous• Ligamentum Flavum• Intertransverse

Special Joints of Spine• Lumbo-Sacral• Atlanto-Axial Joint

Lumbar Facets• Facet Joints (Typical)• Superior articular facets

of one vertebrae with inferior facets of vertebrae above

• Synovial gliding joints • Surrounded by joint

capsule and small capsular ligaments

• The type and amount of motion in any given part of the spine is dictated by the orientation of the articular facets as well as the fluidity, elasticity and thickness of the intervertebral discs

Pedicle

1. Pedicle: strong, cylindrical, anatomic bridge between the dorsal spinal elements and the vertebral body; consists of a strong shell of cortical bone and a core of cancellous bone.2. Tranverse pedicle angle or coronal angulation (Figure 1B) decreases as one descends caudally in the spine until the lumbar region. The angle increases as the lumbar spine is descended.3. Sagittal pedicle angle (Figure 1C) is steep throughout the midthoracic spine and in the upper lumbar spine.4. 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. Nerve root occupies the ventral and rostral one-third of the foramen. Thus, violation of medial or caudal cortex of the pedicle risks injury to the nerve root.

Deformative spinal diseases

•Scoliosis•Kyfosis•Lordosis•Osteogenesis imperfecta

Treatment

•Observation•Bracing•Surgery• Posterior approach• Anterior approach• Decompressive laminectomy• Minimally invasive surgery (MIS)

Surgical Treatment - Stabilisation

Advantages of Pedicle Screw Fixation:1. Can rigidly stabilize both the ventral and dorsal aspects2. The pedicle also represents the strongest point of attachment of the

spine and thus significant forces can be applied to the spine without failure of the bone-metal junction.

3. The rigidity of pedicle fixation allows for the incorporation of fewer normal motion segments in order to achieve stabilization of an abnormal level.

4. Can be used after a laminectomy or traumatic disruption of laminae, spinous processes and/or facets.

5.  Additional advantages include less requirements for postoperative bracing and improvements in fusion rates.

Surgical Treatment - Stabilisation

Disadvantages of Pedicle Screw Fixation:1. Steep learning curve.2. Caudal or medial penetration of the pedicle cortex can result in dural or neural injury.3. 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.4. Lengthy operative time with potential for significant blood loss and increased risk of infection.5. Postoperative imaging studies (especially MRI) are, in part, obscured by the implants.6. Rigid fixation can accelerate adjacent motion segment degeneration.7. Costly procedures.

Surgical Treatment - Stabilisation

Principal Indications for Pedicle Fixation:1. Existing painful spinal instability:* post-laminectomy spondylolisthesis* painful pseudoarthrosis2. Potential instability:* spinal stenosis* degenerative scoliosis3. Unstable fractures.4. Augmenting anterior strut grafting:* tumor* infection5. Stabilizing spinal osteotomies.

Surgical Treatment - Stabilisation

Contraindications for Pedicle Fixation:1. Recent infection.2. Laminectomies that will not cause instability3. Fusions which are normally successful without fixation.

Pedicle Screw Entry Sites and Trajectory

General Remarks:1. Preoperative planning using plain radiographs and CT scan is important in deciding the bone quality, pedicle transverse diameter and screw trajectory.2. Sagittal pedicle angle increases in the thoracic spine from an average of 0 degs at T1 to 10 degs at T8 and then decreases to 0 degs at T12.3. Usually the L4 sagittal pedicle angle is 0 degs and subsequent rostral and caudal levels are associated with progressively greater sagittal angles.4. Lordotic curve of the lumbar spine produces a rostral angulation for upper lumbar screws.5. L5 pedicle screw is 5 degs to 10 degs caudally inclined.6. Coronal plane angulation (how medial?) at T1 is 10 degs to 15 degs and at T12 is 5 degs.7. At L1 the medial angulation of 5 degs to 10 degs is satisfactory.* 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 degs per level from L1 to the sacrum 

Pedicle Screw Entry Sites and Trajectory

Schematic diagram showing that the coronal (medial) angle increase approximately 5 degs per level from L1 to the sacrum.

Pedicle Screw Entry Sites and Trajectory

Thoracic Spine:In the thoracic spine, transverse process commonly does not align with the pedicle in the axial plane. Thus, the anatomic landmarks that are used for lumbar pedicle screw insertion cannot be reliably used in the thoracic spine. The transverse process is rostral to the pedicle in the upper thoracic spine and caudal to the pedicle in the lower thoracic spine (crossover occurs at T6-7). Because of this variability, fluoroscopic guidance or direct vision and palpation of the pedicle via a laminotomy is highly recommended for insertion of thoracic pedicle screws. At T1-3, 4.5 mm diameter screws that are 25-30 mm in length are usually recommended. At T4-T10, screws are usually 4.5 mm in diameter and 30-35 mm in length.

Pedicle Screw Entry Sites and Trajectory

Lumbar Spine:Conventional entry site for pedicle screw placement in the lumbar spine is at the junction of the lateral facet and the transverse process (Figure 4A) or bisection of a vertical line through the facet joints and a horizontal line through the transverse process 

Pedicle Screw Entry Sites and Trajectory

Nuances in pedicle screw insertion in lumbar spine:1. 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. At L5, it is an average of 1.5 mm caudal to the pedicles.2. Muscle dissection is performed as lateral as possible to allow palpation of the transverse process. You have to be persistent and patient- have some good retractors.3. Lateral aspect of the pedicles is palpated with a nerve hook over the transverse process.4. Small laminotomy may be performed (if no decompression is being performed) to palpate the  medial aspect of the pedicle and its rostral/ caudal borders.5. Palpation of the pedicle helps to guide accurate placement of the screw into the pedicle.6. Some of the superior facet at the transverse process has to be decorticated for a good entry site  of the screw.

Pedicle Screw Entry Sites and Trajectory

Step 1: Entry site is decorticated using a burr and a high-speed drill or a rongeur.Step 2: Burr or awl is used to penetrate the dorsal cortex of the pedicle.Step 3: 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.* advancement of the probe should be smooth and consistent.* sudden plunge suggests breaking out of the pedicle laterally.* an increase in resistance indicates abutment against the pedicle or vertebral bodycortex.Step 4: After cannulation, the pedicle sounding probe is placed into the pedicle that is then palpated from within to make sure there is not a medial, lateral, rostral or caudal disruption in the  cortex of the pedicles.* sound 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 hasnot occurred.

Pedicle Screw Entry Sites and Trajectory

Step 5: After pedicles have been probed, one can either:a. place Steinman pins or K-wires bilaterally or unilaterally into the pedicles to confirm the trajectory and entry site and then go to (b) below:b. tap the pedicle screw path if non-self tapping screws are used; then go to c. below:c. place the permanent screws with the longest diameter that will not fracture the pedicle.* length of the screw can be determined by measuring the length of he Steinman pin/K-wire/pedicle probe from the pedicle entry site to a depth of 50-80% of the vertebral body.* the screws in the lumbar spine usually have a 4.5 to 7 mm diameter and a 35-50 mm length.Step 6: After pedicle screw placement, the transverse process and the lateral aspects of the facet joints are decorticated, screws are connected to a longitudinal construct, usually a rod (can be a plate)- these may need to be bent to conform to the proper curvature of the spine; screws are secured (top-loading or side-loading); bone graf is then placed on the previously fusion bed.

Surgical Treatment

• Fixation methods • Roy-Camille – midpoint An

- 1 mm medial • Magerl - 2 mm medial to

the midpoint of lateral mass

Treatment• The recommended lateral

angulations for Roy-Camille, Magerl, and An are 10°, 25°,and 30°

• Magerl and An techniques have a wide margin of safety. Caution should be practised with Roy-Camille's technique at C3, C4, and C6 levels to avoid vertebral vessels injury

Treatment• Screw projections were

determined based on . A line parallel to the vertical axis (A-B) is drawn over the entry point (line C-D). The axis of screw will be represented by a straight line connecting the entry point of lateral mass and the point 1.75 mm (based on the 3.5 mm diameter screw is commonly used screw) lateral to the border of vertebral artery canal (line C-E). The angulation between the line C-D and the line C-E will be the minimum lateral angulation of the lateral mass screw. These measurements were repeated from C3 to C6 vertebra. 

Intraoperative Verification of the Screw Trajectory and Placement

1. Lateral and an AP radiograph or fluoroscopic image* this does not guarantee accurate screw placement.* accuracy can be improved with a slightly oblique AP view- 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 AP films raises the concern of medialpenetration of the pedicle by the screw.* lateral imaging is useful to view the depth of penetration into the vertebral body and sagittalangulation of the trajectory (Figure 6).- ventral screw penetration is usually between 50 and 80% of the AP diameter of the vertebralbody; penetration >80% of the vertebral body on lateral plain x-ray raises the concernof ventral penetration of the vertebral body cortex.

Intraoperative Verification of the Screw Trajectory and Placement

2. Screw placement can be checked electrophysiologically with direct stimulation of the pedicle probe or screw producing an electromyogram (EMG) response peripherally; if this response occurs below the threshold expected for intact cortical bone, the screws may be redirected or  removed.* For more information on this topic please refer to Clements et al in Spine 21:600-604, 1996.3. Interactive frameless stereotaxy systems (including fluoronavigation) can be help guide in he proper placement of the screws. In the future, such technology will lead to percutaneous pedicle screw placement- Stay tuned!!

Complication

Screw misplacement Coupling failure Superficial wound infection Deep wound infection Nonunion Permanent neural injuryScrew looseningScrew bendingScrew breakage

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