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Transcript
Dorsal Fixation of the
Thoracic and Lumbar Spine
Michael P. Steinmetz, MD
Chairman, Department of Neurological Surgery
Case Western Reserve University
School of Medicine
MetroHealth Medical Center
Adjunct Staff
Department of Neurosurgery
Cleveland Clinic
Cleveland, OH
Dorsal Fixation of the
Thoracic and Lumbar Spine
Techniques
•
•
•
•
•
•
Thoracic and Lumbar Pedicle Fixation
Hook Placement
Sublaminar Cable/Wire
Transfacet Screws
Spinous process plate
Translaminar Screws (T1 / T2)
1
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 surface convex so
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
2
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
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
3
Lumbar Pedicle Anatomy
Thoracic Pedicle Fixation
Relevant Anatomy
Soft Tissue and Vascular Structures
T4
T5
T6
T7
T8
T9
T10
T11
T12
4
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 or
cutout by screw threads before fracture occurs)
5
Thoracic and Lumbar Pedicle
Fixation
Pre Operative Assessment
• Image guidance
– Flouroscopically
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)
Assisted free hand technique
•Flouroscopy ( AP T1 T4)
•Laminotomy (C7 and T1)
– Straight forward trajectory
(SFT)
• Straight forward trajectory allows
uniaxial or multiaxial screws
(coronal / sagittal deformity)
• 27% in pullout strength
compared to AT
Lehman et al Spine 2003
– 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
6
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
7
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
8
Thoracic Screw Placement
Free Hand Technique
• Cortical burring
– 3 mm bur 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
Entry Point
• SFT
T1, T2,and T11, T12
– T1, T2,and T11, T12
transverse line passing
through the middle of the
transverse process and
3 - 4 mm lateral to the
midpoint of the superior
facet
– T3 – T10 transverse line at
the superior border of the
transverse process (where
it joins the lamina) and 2
mm lateral to the midpoint
of the superior facet (this
point is 2 – 3 mm caudal
to the base of the superior
facet)
9
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”
• SFT
Entry Point
– T1, T2,and T11, T12 transverse
line passing through the
middle of the transverse
process and 3 - 4 mm
lateral to the midpoint of
the superior facet
– T3 – T10 transverse line at
the superior border of the
transverse process (where
it joins the lamina) and 2
mm lateral to the midpoint
of the superior facet (this
point is 2 – 3 mm caudal to
the base of the superior
facet)
• AT
– Same at each level
– 2 – 3 mm cranial to
base of superior
facet and 2 mm
lateral to midpoint
of superior facet
– Even at T1 T2 T11 T12
because of “large”
sagittal diameter of
pedicle
10
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
11
Thoracic Screw Placement
Free Hand Technique
• Gearshift probing
– Sagittal inclination (AT)
 20 – 25O
inclination from a
• Parallel to superior endplate or
perpendicular to dorsal surface of
line parallel to
superior facet (pre-op films).
the superior or
Mainly “feel” with probe
inferior endplates
– Transverse inclination (SFT / AT)
 Can utilize pre-op
• Increases from 0O – 15O from
films or intra-op
T12 – T2 with lami at T1 as big
flouro (below T4)
“jump” in inclination (pre-op films).

Mainly “feel”
Mainly “feel” with probe
– Sagittal inclination (SFT)
– Work from cranial to
caudal or caudal to
cranial to visualize
trends of entry point at
each successive level
Thoracic Screw Placement
Free Hand Technique
• Palpation
– Once probe removed
observe for CSF (the
“Trost” phenomenon)
– Palpate all four “walls” and
floor using flexible ball
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
12
What if you miss the pedicle?
• Step 1
What if you miss the pedicle?
• Step 1
SH*T!
13
What if you miss the pedicle?
• Step 1
• Step 2
– Blame someone else
What if you miss the pedicle?
• Step 1
• Step 2
– Blame someone else
• Resident/fellow
• Rep
• Anesthesia?
14
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
15
Missing the Pedicle
• Don’t be afraid (really proud) to
perform laminotomy, fluoroscopy
• Use salvage technique
– Anatomic trajectory
– In out in
– Etc.
In Out In
16
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)
• Rod contouring
and correction
– 3D contouring –
useful to have 2 or
more rod holders
– EMG
• Useful T6 – T12
• Stimulate screws intra-op and
monitor rectus abdominis muscle
17
Remember to assign blame!
18
Lumbar Pedicle Screw Free
Hand Technique
• Entry point classically
described as intersection
of TP and infer lateral
facet margin
Trajectory
– Roy-camille: medial
entry “straight ahead”
technique
– Wienstien: lateral entry
with converging
screws
– Kraig : “up and in” to
obtain sub-chondral
purchase
19
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
20
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 xray
Application Strategies
• Triangulation
– Increases pull-out
strength
• Start low, aim high at
most rostral level to
avoid damaging facet
21
Cortical Lumbar Screws
Mobbs TJ.Orthopedic Surgery 2013
Technique
• Starting point is medial pars
22
Hook Fixation
HOOK FIXATION
• Location
– Sublaminar
• Upgoing / Downgoing
– Transverse process
• Downgoing
– Pedicle
•Strength varies with hook
type:
Lamina
Pedicle
Transverse process
• Upgoing
• Application of corrective forces
– Good for applying distraction /
compression / lateral translation
– Poor at applying rotational forces
23
Dorsal Fixation of the Lumbar and
Thoracic Spine
Hook Fixation
• Advantages
– Increased pullout resistance in osteoporotic bone
– “Easier” technical application than pedicle screws
• However much greater construct design planning
– No risk to anterior structures
• Disadvantages
– Places hardware in the canal
– Posterior elements required for fixation
– Force exerted at a distance from the IAR
• Short segment fixation less beneficial (except compression)
– Some loss of correction compared to pedicle fixation
Dorsal Fixation of the
Thoracic Spine
Hook Fixation
24
Dorsal Fixation of the
Thoracic Spine
Sublaminar Hooks
• Increased risk of neurologic injury from
canal compromise
– Hook must conform to size/shape of lamina
– Avoid bilateral hooks
– Safe if complete spinal cord injury
• Increased bone/implant interface
– Increased strength (osteoporosis) but requires
laminotomy
– Decreased surface area for bone fusion
• Since upgoing / downgoing can apply
distractive and compressive forces
Dorsal Fixation of the
Thoracic Spine
Sublaminar Hooks
• Perform laminotomy
• Insert trial feeler
– Several sizes
• Hook must conform
to size/shape of
lamina
25
Dorsal Fixation of the Thoracic
and Lumbar Spine
Offset Sublaminar Hooks
• Allows for mismatch
between implants
– pedicle hook/screw and
a lamina or T- piece
hook
• More bone / fusion
surface coverage
Dorsal Fixation of the
Thoracic Spine
Pedicle Hooks
• Pedicle is mechanically strong
– Good purchase even at T5
– “Less” hardware in canal than
sublam.
• Should not be used bilateral at
the same level due to
pseudoarthrosis risk
• Cannot be at the caudal end of a
construct (joint disruption)
• Only provides distractive force
– Unless combined with
laminar / T-piece hook / pedicle
screw
26
Dorsal Fixation of the
Thoracic Spine
Pedicle Hooks
• Trial introduced into facet joint
• Remove inferior facet with ¼ inch
osteotome
• Hook blade engages pedicle
• Hook throat engages inferior facet
• Superior facet sandwiches hook
• Place hook + screw
Dorsal Fixation of the
Thoracic Spine
Transverse Process Hooks
• Not as strong purchase as laminar or
pedicle hook so limited correction
– However T2 - T5 can withstand some
corrective forces applied (holding power)
• Easy to isolate and prepare
• Placed in down going direction
– Compressive forces
• Blades typically short/stout
–
risk of pullout / dislodgement
• Blades angulated for medial attachment
of rod
27
Dorsal Fixation of the
Thoracic Spine
Transverse Process Hooks
• Must strip
costotransverse ligament
• Placed in down going
direction
• Blades typically
short/stout
• Blades angulated for
medial attachment of rod
Application Strategies
• Laminar claw
• Pedicle screw –
Laminar hook
– provide additional
fixation for
osteoperotic bone
28
Wire/Cable
Technique
• Requires laminotomy-avoid excessive
bone removal
• Ligamentum flavum removed with curetteall rough spots removed
• Bend wire to approximate the shape of the
lamina
• Pass from caudal to rostral
29
30
FACET SCREWS
31
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
32
33
Percutaneous Transfacet Fusion
34
35
ILIF™ CASE
• L4-5 ILIF with 45mm plate and 14mm H2
36
Conclusions
• Pedicle screw fixation is work horse
– Different trajectories depending on anatomy
and situation
– Key to safe placement is a clear
understanding of the relevant anatomy
– Bail-out options available
• Many fixation options available
– May be better in certain circumstances
• Laminar hooks and osteoporosis
THANK YOU
37
Dorsal Fixation of the
Thoracic Spine
Construct Design Strategies
Dorsal Fixation of the
Thoracic Spine
Construct Design Strategies
• Pre-op plan Application Strategies
– Goals
• Neurologic
– Decompression
• Restore stability /
maintain stability
• Alignment
– Sagittal balance
(kyphosis begets
kyphosis)
– Coronal balance
• Fusion
38
Construct Planning
• Most may be figured
out preoperatively
• Use map or white
board in OR
• Fixation points and
types
– Anatomic variants
– Construct length
– Anterior column
support
General Prinicples
Construct Design Strategies
• Construct length – How long is long enough ?
– Always depends on availability of intermediate points
of fixation, level of spine, anterior column support
– 2A – 2B
– Rostral failure at cervico-thoracic thoraco-lumbar
junction especially when sagittal balance not fully
restored has led to 3A – 2B constructs
• Don’t do this
– When adequate anterior column load bearing is not
fully restored or multilevel instability exists consider 4
point bending constructs
• 4A – 3B (must prevent both translational deformation and
provide axial load bearing)
39
General Principles
• Ensure anterior column
support
– Allows for shorter dorsal
construct
– Improved stability
– Add it if required
• With poor anterior
column support
– Need long dorsal
construct
General Principles
Construct Design Strategies
Application Strategies
• Avoid ending long
constructs at junctional
regions
• End construct at most
neutral VB as possible
– Never apex of curves (sag
and coronal)
– Minimal to no rotation
40
General Principles
Construct Design Strategies
Application Strategies
• Cross – fixation
– Cross-links resist torsional deformation
– This especially helps to minimize the
chance of hook – bone interface failure
– Terminal cross-links are not as
effective as more intermediately
placed cross members
– However often “positional” placement
to prevent hook dislodgement
• Below but abutting up-going laminar
or T-piece hook
General Principles
Construct Design Strategies
Application Strategies
• Osteoporosis
– Hooks (wires) resist pullout
better than screws
– Use multiple hooks for load
sharing because they apply
forces to the spine at a
considerable distance from the
IAR ( bending moments)
• Pedicle Hook-TP Hook
• “Laminar claw” – alternate levels
– Utilize hooks to help prevent
screw pullout
• Construct terminus :
pedicle screw – laminar hook
41
Thoracic Screw Placement
Remember some people have anatomy
incompatible with placement of thoracic pedicle
screws
• Salvage and other techniques
available
– Hooks
– Sublaminar wires/cables
– Alternative screw techniques
• In out in
• Rib head
• costotransverse
42
THANK YOU
43