Posterior Spine Fixation

Presented by

Ghazwan A. Hasan Kindi Teaching Hospital / Arab Board Council

3rd year trainee

July 2013

Introduction• For Spine fusion either Instrumental or Non-

Instrumental in both use bone graft.• Segmental pedicle screw fixation is rapidly becoming

a widely used method of spinal instrumentation.• Pedicle screw instrumentation enables a rigid

construct to promote stability and fusion for numerous spinal pathologies including: trauma, tumours, deformity and degenerative disease.

• Eliminating motion between the affected segments increases the likelihood of fusion and may reduce the degree of pain the patient experiences. Properly applied, spinal instrumentation maintains alignment and shares spinal loads until a solid, consolidated fusion is achieved.

• Spinal instrumentation systems are described in terms of where the hardware is attached:

1-Anterior.

2-Posterior.

3-Interbody.

• The anterior devices such as anterior plates and screw systems usually are classified as those systems that are designed to attach to the anterior or anterolateral aspect of the vertebral body. Typically, the plate or rod construct is transfixed to the involved vertebral segments by screws that pierce one or both cortices as well as gain purchase in the cancellous bone of the vertebral body.

• The posterior systems are affixed to the elements situated posterior to the vertebral body, the spinous processes, pedicles, facets, or laminae. These instrumentation systems use laminar hooks, pedicle screw systems, facet screws and wiring techniques.

• Interbody fusion systems promote fusion between the vertebral bodies by the incorporation of a device or graft that spans the disc space. Although allograft and autograft spacers are routinely used in combination with other anterior or posterior instrumentation, a variety of devices are now available and approved for implantation. Usually, the interbody systems are further classified by the surgical approach used during device implantation. The comingling of system and approach has given rise to such contemporary terminology as Anterior lumbar interbody fusion (ALIF), transforaminal lumbar interbody fusion (TLIF), posterior lumbar interbody fusion (PLIF), and Xtreme Lateral lumber interbody Fusion(XLIF) procedures

Definition

• Spinal stability is defined as the ability of the spine under physiologic loads to maintain normal relationships between vertebrae, in such a way that there is neither damage nor subsequent irritation to the spinal cord or nerve roots.

• Adjacent Segment Disease(ASD):Abnormal loading and increased mobility in adjacent segments may explain the development of ASD, but it is still unclear whether it is caused by fusion sequelae or is the result of natural degeneration

History• Internal fixation for the posterior aspect of the

spine was first reported in 1891 by Hadra when he documented the use of wires around the spinous processes for treating Pott’s disease.

• In 1911, Hibbs described an operation for treating progressive spinal deformities in three patients in which he proposed utilizing a “bony bridge” to prevent kyphosis and to provide a cantilever support to the deformed spine.

• In 1948, King reported the use of facet screws in the lumbo-sacral spine and indicated that he had been using them for eight years.

History• In 1949, Thompson and Ralston reported a

pseudarthrosis rate of 55.1% following spinal fusion in a group of patients in which some type of internal fixation (i.e., a stainless steel machine screw) was utilized across each of the facet joints.

• Holdsworth and Hardy reported in 1953 the use of posterior plates attached to one or more spinous processes to treat fracture dislocations in the thoraco lumbar spine.

• In the early 1980s, Dr. Steffee made the use of pedicle screws popular in the United States with his design of segmental spine plates fixed with pedicle screws

Early posterior spinal instrumentation. (A) Weiss springs. (B) Meurig-Williams plate.

Anatomy

Anatomy

• Posterior bony elements of a lumbar vertebra include the spinous process, laminae, pars interarticularis, facets, transverse processes, and pedicles.

• Posterior soft tissue structures include the supraspinous ligament, interspinous ligament, ligamentum flavum, and facet capsules.

• The pedicles are cylindrical bony projections off the posterosuperolateral aspect of the vertebral body that connect the vertebral body with the posterior bony arch.

• The transverse pedicle diameters range from 4.5 mm at T5 to 18 mm at L5, and the sagittal diameter is generally slightly larger than the transverse slant diameter.

Anatomy• The angle at which the pedicle emerges from

the vertebral body in the transverse plane also varies with craniocaudal location; being less than 10" in the thoracic spine (with a slight anterolateral angulation at T 12), it progressively increases in the lumbar spine to a maximum of almost 30"medial angulation from posterolaterally to anteromedially at L5

Anatomy • The pedicles also exhibit varying angles in the

sagittal plane. The pedicles are directed approximately 15"-17" cephalad for the majority of the thoracic spine, and neutral (90") for the majority of the lumbar spine with the exception of L5, which angles caudally an average of 18"

Anatomy

• The distance to the anterior vertebral cortex as measured from the posterior aspect of the pedicle, through the pedicle, is approximately 40 to 45 mm in the thoracic spine and 50 mm in the lumbar spine.

• A pedicle screw should ideally be placed along the axis of the pedicle, incorporating the largest available transverse and sagittal pedicle diameter.

• Biomechanical in vitro studies of axial pull-out strength and load to failure have helped in determining the optimal depth of pedicle screw penetration, approximately 60% of the fixation strength of the thoracic and lumbar pedicles is in the pedicle itself.

Anatomy

• The cancellous bone in the vertebral body adds another 15 to 20% of strength, whereas purchase in the anterior cortex offers another 20 to 25% increase, Thus, it is generally thought that it is not necessary to routinely engage the anterior cortex, which also avoids the potential danger of injuring the anterior vascular structures, including the aorta.’

• Usually, the risks associated with penetration of the anterior cortex exceed the benefits gained, unless the pedicle and body are extremely osteoporotic, weakened, or fractured, or maximum available fixation strength (eg., reducing a spondylolisthesis) is required.

Paravertebral muscles

CLINICAL ANATOMYO OF SURROUNDINGNEURAL AND VASCULAR STRUCTURES

Coroneal section through the pedicles of L 1 and L2 at the level of the ventral cauda equina roots and the anterior portion of the conus medullaris.

Axial section of L4 through the midportion of the pedicle.

New Technology in Spine instrumentation

New Technology in Spine instrumentation

• Silver Oxide Coated Implant as antibacterial.• Hydroxiapetide Coated screws for

Osteoparosis.• Canulated scews for cement injection in

Osteoparosis.• Facet Joint Replacement.• Expandable cages ( Anatomical).• Balloon Kyphoplasty.

Properties of Graft Materials• Graft Osteogenic Oseto-

Osteo- Materials Potential inductionconduction

• Autogenous bone o o o

• Bone marrow cells o ? x

• Allograft Bone x ? o

• Xenograft bone x x o

• DBM x o o

• BMPs x o x

• Ceramics x x o

• DBM = Demineralized bone matrix; BMP = Bone morphogenetic proteins

Indication of Spine fixation

• Spine deformities.• Unstable Vertebral fractures.• Spondylolisthesis.• Spinal Stenosis.• Spine instabilities.• Revison decompression.• Degenerative disc disease.• Spine tumors.

Preoperative Planning• A fundamental objective of preoperative planning is to

confirm the correct vertebral level or levels of fusion and to localize the incision. Anteroposterior (AP) and lateral radiographs, as well as axial computerized tomography (CT) or magnetic resonance imaging are used for the purpose of identifying the injured vertebra or vertebrae and to estimate the position, orientation, and width of the vertebral pedicle; the latter pedicle information is important for later instrumentation of the vertebrae.

• Using flexion and extension radiographs, translational motion of 3.5 mm or angular motion of 11 degrees of one vertebra over another is indicative of segmental instability.

•

Operative Techniques• Patient Positioning( Prone or Knee chest)

• The intra-abdominal pressure must be minimized to avoid venous congestion and excess intraoperative bleeding, while allowing adequate ventilation of the anesthetized patient.

• Position the hips in neutral or slight extension.This helps maintain lumbar lordosis, which is imperative.

• All pressure points should be carefully padded. The abdomen and genitalia (in males) should hang free. The arms must be placed in a 90/90 position to avoid brachial plexus traction injury. The ulnar nerve should be protected.

• The lower extremities are placed in anti-embolic stockings and compression boots.

• Spinal cord monitoring may be used.• X-rays and fluoroscopy should be available for verification of the

appropriate level and in the case of instrumentation, to verify appropriate instrumentation placement

• Surgical exposure of the lumbar spine:• Midline incision extended to an additional level

Screw Hole Preparation• Exposure of the junction

between the pars interarticularis and transeverse processes

• Pedicle entrance point is at the crossing of two lines• Vertical line: 2-3 mm lateral from

the pars and slants slightly from L4 to S1.

• Horizontal line passes through the middle of the insertion of the transverse processes or 1-2 mm below the joint line.

• 1-2 mm lateral from the center of the pedicle to insert the screw without disturbing the facet joint above and to medialize the screw for better fixation

Screw Hole Preparation

Preparation of Fusion Bed and Grafting

• Decortication

• Marking screw holes

• Grafting

Screw Selection and Insertion

• Screw Diameter: • Perforation of the pedicle into the medial or inferior side

has higher chance of nerve root injury.

• Screw Length:• Long enough to pass the half of the vertebral body but • Short enough not to penetrate the anterior cortex

Rod-Connector-Screw Assembly

Rod Length:- Rod length must not be too long so that the proximal tip of the rod do not touch the inferior facet of the upper vertebra.

Rod Bending. Connector Selection. Rod-Connector Assembly. Screw-Connector-Rod Assembly. Tightening the nuts and set screws.

Consideration Factors in Spinal Instrumentation

• Materials:• Bio-compatibility and Imaging compatibility• Stiffness (or elasticity) and strength• Corrosion

• Implant Strength:• Component (screw, rod, plate, wire, etc.) strength• Metal-metal interface strength• Construct strength• Bone-metal interface strength: Bone–wire, -hook, and -

screws

• Construct Stability:• Segmental stiffness or flexibility

• Profile:• Ease of Use:

Spinal Implant Materials• 316L Stainless steel:

• Biocompatible• Strong and stiff• Poor imaging compatibility: artifact to CT and

MRI.

• Titanium Alloy (Ti6Al4V ELI):• Biocompatible• No artifacts during CT and MRI• Excellent fatigue strength, high strength, high

elasticity• High resistance to fretting corrosion and wear

(surface treatments)

Bone-Metal Interface Strength

• Pedicle screws are known to provide the strongest bone purchase compared to wires, hooks, and vertebral screws.

• Screw Pullout Strength:• Affected by major diameter and bone quality, but

not by minor diameter, thread type, and thread size.

• Insertion depth is not critical.• Screw insertion torque was known to have

relationship with screw pullout strength. • Conical screws showed similar pullout strength

to that of the cylindrical screws.

Surgical Construct Stability• Construct stability varies depending on the size of the

screws and rods (plates).• Recommended rod diameter is 6 mm or ¼ inch in adult spine surgery.

• Preservation of more than 70% of the disc or meticulous

anterior grafting is critical to obtain stable construct with no

hardware failure (screw or rod breakage).

• Modern spinal fixation systems, regardless of anterior or

posterior fixation, similarly significant stability in flexion,

extension, and lateral bending, but not effective in preventing

axial rotational (AR) motion. • Use of a crosslink (DDT) is recommended to improve the AR stability,

particularly in the fixation of long segments (more than 2 levels).

Implant Assembly Profile• Anterior Instrumentation:

• Critical in anterior plating of the cervical spine, and the profile must be less than 3 mm.

• Lower profile is recommended in the anterior fixation of the thoracolumbar spine.

• Posterior Instrumentation:• Assembly profile is not as critical as in anterior

fixation, but lower profile is recommended because a high profile may cause a surgery for implant removal due to patients’ uncomfortness

Ease of System Assembly• Screw Insertion:

• Screw insertion according to the best possible anatomic orientation and location

• Adjustment in Screw-Rod Assembly:• Rod bending• Angular adjustment• Medial-lateral adjustment• Polyaxial screw head vs. Connector

• Top-tightening• All assembly procedures can be made from the top.

Complication & Difficulties • ADJACET SEGMENT DISEASE.• ANATOMICAL VARIATION: may preclude

placing a screw in a particular pedicle, or necessitate using a smaller screw than originally planned.

• OSTEOPOROSIS: may prevent adequate screw fixation In such situations, the options are either to remove the screw or to place it with polymethylmethacrylate (PMMA) augmentation or Hydroxiapetide coated.

Complication & Difficulties • TECHNIQUE-RELATED FACTORS: Poor

fixation could be the result of inadequate surgical technique, such as placing the screw outside the pedicle, fracturing of the pedicle, or placing screws of inadequate circumference, thread size, or length.

• NEURAL DAMAGE: Risks of neural damage can be minimized by thorough knowledge of the spinal anatomy as well as practicing on anatomic specimens.

• VASCULAR INJURY: Perforation of the anterior cortex of the vertebral bodies or sacrum can cause significant injury to the adjacent anterior structures, most notably the aorta, at levels at or above L4.

Complication & Difficulties • POSTOPERATIV EPIDURAL HEMATOMA: Because

of the increased dissection and exposure necessitated by pedicle screw placement, postoperative epidural hematoma is a potential risk.

• OVERCORRECTION: With increased fixation strength, afforded by pedicle screws, overcorrection of a deformity, especially in cases of degenerative scoliosis in which distraction is used, is a potential problem.

• PSEUDARTHROSIS: It is a potential concern that increased fixation of the lumbosacral spine may lead to stress shielding and, therefore, an increased incidence of pseudarthrosis.

Complication & Difficulties • Loss OF CORRECTION/STABILIZATION: Loss

of correction or stabilization or both can be the result of poor fixation attributable to a number of factors, including osteoporosis or improperly placed pedicle screws.

• INFECTION: There are several sources for infection, such as an extensive dissection, a large number of instruments, and the use of an image intensifier, which involves more operating room traffic

Tips and Pearls 1. Preoperative planning is paramount in determining the

extent of fusion and the site for bone graft harvesting.

2. If instrumentation is to be used, preoperative planning allows determination of the levels of instrumentation and the site for purchase (pedicle, lamina, facet). If pedicle screws are planned, the sagittal and transverse orientation of the pedicles and their width, length, and height should be determined.

3. Imaging studies should be evaluated for the presence of any spinal anomalies.

4. The patient should be positioned in spinal lordosis. This is particularly important when long lumbar fusions are planned; it diminishes the possibility of postoperative flat back syndrome and spinal dysfunction.

5. Intravenous antibiotics are administered preoperatively. In the case of instrumentation, broad gram positive and gram-negative spectrum antibiotics are administered.

Tips and Pearls 6. Paravertebral soft tissue dissection should proceed in a

caudal-cephalad direction due to the caudalcephalad orientation of the paravertebral muscle attachments.

7. The facet joint immediately cephalad to the fusion must be preserved to avoid iatrogenic instability.

8. Dissection should extend to the tips of the spinous processes bilaterally and to the lateral aspect of the facet joints. All soft tissue should be denuded to allow maximal surface area for bony fusion.

9. The majority of the decortication is carried out using sharp instruments such as osteotomes, curettes, and rongeurs.

10. Facet joints should be thoroughly cleared of all soft tissues and the intra-articular portion of the joint denuded of cartilage.

11. If a lumbar decompression has been performed prior to the fusion, the exposed dura should be protected with cottonoids to diminish the likelihood of an iatrogenic dural laceration.

What To Avoid1. Avoid prolonged muscle retraction. Self-retaining retractors

should be relaxed every hour and the wound re-irrigated. This allows re-perfusion of the paravertebral muscles and diminishes the possibility of infection. If there is excessive tension on the muscles, the incision can be lengthened.

2. Avoid using an inadequate amount of bone graft; occasionally harvesting of both iliac crests may be necessary and/or augmentation with allograft bone may be needed.

3. Avoid blood pooling, which occurs if the lordotic cephalad segments are decorticated first.

4. Avoid bony fragment extrusion into the spinal canal.

5. Avoid injury to the facet immediately cephalad to the fusion.

6. Avoid postoperative administration of nonsteroidal anti-inflammatories for 3 months(increase the use of non-union)

References

1. Orthopedic Key Review Concepts 2007.

2. Atlas of Orthopaedic Surgery 2004.

3. Chapmans Orthopedic Surgery 3rd edition.

4. Key Technique in Orthopedic Surgery

5. OKA 9th edition

6. Spine Technology Handbook 2006.

7. Fracture of Cervical, Thoracic & Lumber spine 2002

8. Anatomic and Technical Considerations of Pedicle Screw Fixation (JAMES N. WEINSTEIND).

9. Technique, challenges and indications for percutaneous pedicle screw fixation(Ralph J. Mobbs).

10. Lumbar pedicle screw placement: Using only AP plane imaging (Anil Sethi, Adrienne Lee, Rahul Vaidya) 2012