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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
Thoracic Pedicle Fixation Relevant Anatomy
• 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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Thoracic Pedicle Fixation Relevant Anatomy
• 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
Thoracic Pedicle Fixation Relevant Anatomy
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
Thoracic and Lumbar Pedicle Fixation
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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Thoracic and Lumbar Pedicle Fixation
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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Thoracic Screw Placement Free Hand Technique
• 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
Thoracic Screw Placement Free Hand Technique
• Facetectomy
Thoracic Screw Placement Free Hand Technique
• 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
Thoracic Screw Placement Free Hand Technique
• 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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Thoracic Screw Placement Free Hand Technique
• 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
Thoracic Screw Placement Free Hand Technique
• 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
Thoracic Screw Placement Free Hand Technique
• 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”
Thoracic Screw Placement Free Hand Technique
• 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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Thoracic Screw Placement Free Hand Technique
• 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
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