dorsal fixation of the thoracic and lumbar spine workshop... · •flouroscopy ( ap t1 - t4)...

12/28/2015

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Pedicle, Facet, Cortical, and Translaminar Screw Techniques

Gregory R. Trost, MD

Professor and Vice Chair of Neurological Surgery

University of Wisconsin-Madison

Dorsal Fixation of the Thoracic and Lumbar Spine

• Thoracic and Lumbar Pedicle Fixation• Hook Placement• Sublaminar Cable/Wire• Transfacet Screws• Spinous process plate• Translaminar Screws • Cortical Screws

Techniques

Thoracic Pedicle Fixation Relevant Anatomy

• Three anatomic characteristics of the pedicle affect screw size and position– Pedicle diameter

• Transverse width• Sagittal width

– Angle of the pedicle trajectory• Transverse angle• Sagittal angle

– Length of pedicle - vertebral body complex (chord length)

• Varies for anatomic versus “straight forward”technique


• Pedicle is auricular in shape– Transverse diameter critical –

determines screw diameter• “plasticity of pedicle”

– Smallest diameter T4 – T8

– Transverse diameter T3 – T1

– Medial pedicle cortex 2-3x thicker than lateral

– Transverse diameter is often altered in deformity

• Transverse angle changes– T12 pedicles neutral or even

divergent and pedicles converge as progress cephalad with T1 pedicle trajectory approx 25 - 35O

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• Chord length generally increases as you progress caudally (body + pedicle length)– T1 – T3 26 – 34 mm– T4 – T6 34 - 44 mm– T7 – T12 36 – 50 mm

• Pedicle to “neural” distance– Distance between pedicle and

corresponding nerve root is equal along superior and inferior aspect

– Dura touches medial pedicle• Worse at concavity in deformity

• Relationship of pedicle to facet joint, lamina, and transverse process


Soft Tissue and Vascular Structures

T4

T5

T6

T7

T8

T9

T10

T11

T12

Thoracic and Lumbar Pedicle Fixation

Pre Operative Assessment• Plain X-ray

– Sagittal plane deformity• True AP view of pedicles difficult• Obtained only in the vertebral segments that are

perpendicular to the x-ray beam (beam may need to be angled above and below the apex to visualize true pedicle dimensions

• Supine / Push-prone x-rays may be helpful• Must have 36” standing films with knees/hips extended• Lying flex – ext films (lat decub)

– Coronal plane defomity• Side bending views may be helpful• Pedicle assessment often difficult• 36” films and lying flex – ext films


Pre Operative Assessment• CT scan

– Best modality to evaluate pedicle anatomy (a “must” at T4 – T8)

– Good visualization of both concave and convex pedicles in cases of coronal deformity

– Sagittal / coronal recons often helpful– CT slightly underestimates pedicle width

• Volume averaging on each window– Remember pedicles can “adapt” with “oversized

screws” especially in adolescents (expansion orcutout by screw threads before fracture occurs)

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Pre Operative Assessment• Image guidance

– Fluoroscopically assisted

– Stereotactic systems• CT (3D CAS)• Computer assisted

flouroscopy(2D CAS)

• 3D flouroscopy(3D CAS)

Thoracic Screw Placement• Two main trajectories of

screw placement (often determined by pathology)– Straight forward trajectory

(SFT)• Straight forward trajectory allows

uniaxial or multiaxial screws (coronal / sagittal deformity)

• 27% in pullout strength compared to AT

– Anatomic trajectory (AT)• Multiaxial screws much easier

(stabilization for anterior / posterior pathology such as tumor, fracture, degenerative, iatrogenic)

• Salvage (?) – 62% MIT Lehman et al Spine 2003

Lehman et al Spine 2003

Assisted free hand technique•Flouroscopy ( AP T1 -T4)•Laminotomy (C7 and T1)

Thoracic Screw Placement Free Hand Technique

• Starting points for AT and SFT for thoracic vertebrae are slightly variable and are based on posterior element anatomy that must be visualized intraop. (exposure, exposure, exposure)– Transverse process– Base of the superior

articular process– Lateral portion of the

lamina / pars

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• Exposure– Limit dissection to fusion levels

(reduce junctional kyphosis or transition syndromes)

– T-spine much easier to avoid facet disruption at termini than in LS spine

– Expose to tip of T-piece bilaterally and lateral joint / lamina / pars

• Facetectomy– Thoroughly clean facet joints– Osteotomize the inferior facet joint

and remove articular cartillage on superior facet (3-4 mm)

– Do not disrupt joint at UIV


• Facetectomy


• Cortical burring– 3 mm burr creates 3-4 mm

posterior cortical breach– Pedicle blush (cancellous

bone) may be seen– Generally use gearshift to

search for cancellous soft spot

– Entrance point • Straight forward

trajectory– Starting point varies

slightly at each level– Place screw parallel to

superior endplate. – If no lateral flouro

(T1 – T4) or pre-op films you can probe perpendicular to the dorsal cortical surface of the superior facet T1, T2, T11, T12


• Anatomic trajectory– Similar starting points

at each level– Sagittal angle

20 – 25O

inclination using the superior or inferior endplates

Can utilize pre-op films or intra-op flouro (below T4)

Mainly “feel” Transverse angulation

increases as you go cephalad (0 – 15O with a “jump” at T1) Again mainly “feel”

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• Gearshift probing– 2 mm blunt-tipped slightly

curved probe– Ventral pressure as “search”

for pedicle– Gearshift pointed laterally and

insert to 15 – 20 mm– Remove probe and turn tip

medially and place tip down to base of prior hole

– Then continue path down medial into the body (sudden advancement suggests penetration into ST)

• 35 – 40mm T7 – T12• 30 – 35mm T4 – T6• 20 – 25mm T1 – T3


• Gearshift probing– 2 mm blunt-tipped slightly

curved probe– Ventral pressure as “search”

for pedicle– Gearshift pointed laterally and

insert to 15 – 20 mm– Remove probe and turn tip

medially and place tip down to base of prior hole

– Then continue path down medial into the body (sudden advancement suggests penetration into ST)

• 35 – 40mm T7 – T12• 30 – 35mm T4 – T6• 20 – 25mm T1 – T3


• Gearshift probing– Sagittal inclination (SFT)

• Parallel to superior endplate or perpendicular to dorsal surface of superior facet (pre-op films). Mainly “feel” with probe

– Transverse inclination (SFT / AT)• Increases from 0O – 15O from

T12 – T2 with lami at T1 as big “jump” in inclination (pre-op films). Mainly “feel” with probe

– Work from cranial to caudal or caudal to cranial to visualize trends of entry point at each successive level

– Sagittal inclination (AT) 20 – 25O

inclination from a line parallel to the superior or inferior endplates

Can utilize pre-op films or intra-op flouro (below T4)

Mainly “feel”


• Palpation– Once probe removed

observe for CSF – Palpate all four “walls”

and floor using flexibleball tipped probe

• Majority of wall perforations are lateral

• Can determine chord length with probe

• If wall breach occurs can redirect screw with tap (utilize AT)

– “Undertap” pedicle tract• 4.2 tap for 5.2 scew• 4.0 tap for 5.0 screw

– Repalpate

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• Confirmation of screw placement– Imaging

• True AP and lateral flouroscopy(T1 – T4 AP)

• Screw crossing midline of body (? medial wall breach)

• Screw not crossing medial cortical wall of the pedicle (? lateral wall breach)

• Screws that intersect an endplate (SFT) and should not extend beyond 75-80% of vertebral body sagittal distance (T1 – T4)

– EMG• Useful T6 – T12• Stimulate screws intra-op and

monitor rectus abdominis muscle

• Rod contouring and correction– 3D contouring –

useful to have 2 or more rod holders

Missing the Pedicle

• Most often too lateral• Look at other successful

holes/screws– Importance for moving in a uniform

fashion• Assess landmarks

–Move starting point more medial–Aim medial

Missing the Pedicle

• If successfully locate–Make sure utilize correct pedicle hole–Use a k-wire, cannulated tap and/or

screw• If can’t easily locate pedicle

–Most often skip unless at ends of construct

Missing the Pedicle

• Don’t be afraid (really proud) to perform laminotomy, fluoroscopy

• Use salvage technique–Anatomic trajectory– In out in–Etc.

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In Out In

Lumbar Pedicle Anatomy

• Less variability compared to thoracic spine

• L1-L5– Steady increase in transverse width

– Slight decrease but fairly stable sag width

– Significant increase in transverse angle

– Only small changes in sagittal angle, neutral at L1

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Lumbar Pedicle Screw Free Hand Technique

• Entry point classically described as intersection of TP and inferolateralfacet margin

Trajectory– Roy-camille: medial

entry “straight ahead”technique

– Wienstien: lateral entry with converging screws

– Kraig : “up and in” to obtain sub-chondral purchase

Placement• Decorticate entry

point with burr

• Use pedicle probe or curette to advance down pedicle into body.

Placement• Use ball-tipped probe

to feel for cortical breech

• Place screw +/-tapping

• Adjuncts: image guidance, fluoroscopy, direct visualization of pedicle

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Placement

• Screw size?– Pedicle diameter

measured at isthmus

– Pick largest diameter screw that will fit inner cortical diameter (C)

– Length 5 mm short of ant cortex on lateral x-ray

Cortical Screws

Typically 4.5 X 25-35 mm screws

Cortical Lumbar Screws

Mobbs TJ.Orthopedic Surgery 2013

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FACET SCREWS

History

• Boucher first described true transfacet fixation in 1959

• Magerl described translaminar facet screw fixation in 1984

Clinical Data

Lumbar cadavers tested in short term and long term cyclic loading conditions

Results

Short term

NO DIFFERENCE between fixation except in

flexion-STIFFER with facet screws

Long term

NO DIFFERNCE, no decrease in fixation

FACET SCREWS PROVIDE EQUAL FIXATION TO PSF-BETTER IN FLEXION

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Percutaneous Transfacet Fusion

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Translaminar facet screws

• 1 or two level fixation when no reduction needed

• Most commonly used for dorsal fixation w/ ALIF in DDD

Facet Screw fixation

• Contraindications:– Isthmic spondylolisthesis

– Prior laminectomy

– Lamina fracture

– Severe osteoperosis

– Instability

– Deformity

– Anterior column deficiency

Facet Screws

• Goal is to insert perpendicular to orientation of facet

Stiffness approximately equivalent to pedicle screw fixation in 360 degree fusion (Sasso)

Advantages: Small exposure

Cost

Low profile

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Entry point Trajectory

dorsal fixation of the thoracic and lumbar spine workshop... · •flouroscopy ( ap t1 - t4)...

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