PEDICLE SCREWS FIXATION OF DORSAL-LUMBAR-SACRAL SPINE

DR. BEN TUNGOECENTRAL INSTITUTE OF ORTHOPEDICS, SAFDARJUNG HOSPITAL; NEW DELHIPEDICLE SCREWS FIXATION OF THORACO-LUMBAR SPINE

LEARNING OBJECTIVESBRIEF ANATOMY OF THORACIC AND LUMBAR VERTEBRAPEDICLE ANATOMYPEDICLE SCREW AND ITS PROPERTIESENTRY POINTS FOR THORACIC AND LUMBAR SPINEPECULARITIES IN UPPER THORACIC VERTEBRA FIXATIONDIFFERENT TECHNIQUESCOMPLICATIONSTIPS FOR GOOD FIXATION

THORACIC VERTEBRA

THORACIC VERTEBRATYPICAL VERTEBRA:medium-sized, heart shaped vertebral bodiesmedium-sized vertebral canalsprominent transverse processes with costal facetslong spinous processes angulating downwards

ATYPICAL VERTEBRA:T1 and T9 to T12 are considered atypical vertebrae.T1 bears some resemblance to low cervical vertebrae.T9 has no inferior demifacet. T10 often, but not always, shares features withT11 andT12.

LUMBAR VERTEBRA

LUMBAR VERTEBRAbody:kidney shaped, convex anteriorly, flattened posteriorly; resulting vertebral canal is triangular in cross-sectionpedicles:project directly posteriorly,attached to the upper half of the bodytransverse processes:spatulate, project laterally on both sides;L3 most often has the longest transverse processes of the lumbar spine, a fact that can be used to number the vertebraelamina:broad, thick, overlap minimallyarticular processes(superior and inferior): lie at the lateral angle of the laminae (junction with pedicle), connected via the pars interarticularisspinous process:single, short, thick, roughly horizontal,hatchet-shaped (upper border is straight, lower border curves down)L1 to L4 are consideredtypical lumbar vertebrae, whereas due to various and distinctive differences thefifth lumbar vertebrais considered atypical.

The widest pedicles are at L5 and narrowest at T5 in the horizontal plane. The widest pedicles in the sagittal plane were at T11, and the narrowest are at T1.In the sagittal plane, the pedicles angle caudad at L5 and cephalad at L3-T1

Some noteworthy points to rememberThe respective facet joint space and the middle of the transverse process are the most important reference points.The pedicles of the thoracic and lumbar vertebrae are tubelike bony structures that connect the anterior and posterior columns of the spine. Medial to the medial wall of the pedicle lies the dural sac. Inferior to the medial wall of the pedicle is the nerve root in the neural foramen. The lumbar roots usually are situated in the upper third of the foramen; it is more dangerous to penetrate the pedicle medially or inferiorly as opposed to laterally or superiorly.

PRE-OP ASSESSMENTAssess individual spinal anatomy by:

1.high-quality antero-posterior and lateral radiographs of the lumbar and thoracic spine.

2. Axial CT at the level of the pedicle.

Commonly used techniques(1) the intersection technique,

(2) the pars interarticularis technique,

(3) the mammillary process technique

INTERSECTION TECHNIQUEThe intersection technique is perhaps the most commonly used method of localizing the pedicle. It involves dropping a line from the lateral aspect of the facet joint, which intersects a line that bisects the transverse process at a spot overlying the pedicle

Note: The pars interarticularis is the area of bone where the pedicle connects to the lamina. Because the laminae and the pars interarticularis can be identified easily at surgery, they provide landmarks by which a pedicular drill starting point can be made.The mammillary process technique is based on a small prominence of bone at the base of the transverse process. This mammillary process can be used as a starting point for transpedicular drilling.Mamillary process starting point is lateral than the intersection technique starting point, which also is more lateral than the pars interarticularis starting point.

LUMBAR SPINE: ENTRY POINTS

Lumbar pedicle screw entry points

THORACIC SPINE: ENTRY POINTS

STEPS: OPENING THE CORTEX

CRANIO CAUDAL ANGULATION

MEDIO LATERAL INCLINATION

SCREW INSERTION

T1-T3 PEDICLE SCREW FIXATION

GENERAL CONSIDERATIONS

PITFALLS

ENTRY POINTS

OPENING OF THE CORTEX

MEDIO-LATERAL INCLINATION

CRANIO-CAUDAL ANGULATION

CRANIO-CAUDAL ANGULATION

PREPARATION OF THE PEDICLE

SCREW INSERTION

BREACH OF THE PEDICLE

INCREASING PULL OUT STRENGTH OF SCREWS

OTHER TECHNIQUES

PEARLS FOR GOOD FIXATION

PEARLS FOR GOOD FIXATIONThere are three basic concepts that are important to the biomechanics of pedicle screw-based instrumentation. First, the outer diameter of the screw determines pullout strength, while the inner diameter determines fatigue strength. Secondly, when inserting a pedicle screw, the dorsal cortex of the spine should not be violated and the screws on each side should converge and be of good length. Thirdly, fixation can be augmented in cases of severe osteoporosis or revision. A trajectory parallel or caudal to the superior endplate can minimize breakage of the screw from repeated axial loading. Straight insertion of the pedicle screw in the mid-sagittal plane provides the strongest stability. Rotational stability can be improved by adding transverse connectors.

References:Campells Operative Orthopedics 12th Edition

AO Surgical Reference (https://www2.aofoundation.org/wps/portal/surgery)

Spine Surgery-Tricks of the Trade; 3rd edition (Alexander R Vaccaro)