Thoracic Screw Placement in Deformity: Technique Pitfalls, Complications, Results

Technique - Free Hand Placement of Thoracic Pedicle Screws

1. Meticulous exposure - all bony landmarks exposed:
• pars
• facet
• t.p. - out to tip

2. Begin most caudal level (neutral rotation and largest pedicle)
remember that LI and L2 pedicles are smaller diameter than TIO-TI2!

3. Starting point (mark with drill bit)
lower thoracic (TIO-TI2) - down slope of bisected t.p. at junction of t.p. and lamina at same level as lateral pars

• mid-thoracic (T4-T9) -junction of down slope of proximal t.p. and lamina at base of superior facet, medical to lateral pars

• proximal thoracic (T I -T3) -junction of proximal t.p. and lamina medial to lateral pars

4. Burr small (~ 5 mm) defect in dorsal cortex and search for "pedicle blush" of bleeding cancellous bone that indicates entrance to pedicle; may not be seen in very small apical thoracic pedicles

5. Blunt slightly curved gear shift used with ~ 2 mm rounded tip:
Orientation -
• frontal: perpendicular to lamina (superior facet)
• sagittal: cephalad (lower thoracic) orientation versus caudad (upper thoracic) orientation
• axial: based on degree of rotational deformity obviously maximal at apex of scoliosis

6. Advanced gear shift first pointing slightly lateral and then once engaged at base of pedicle, turn tip 180Ú to point slightly medial to advance down pedicle into the vertebral body

7. Should advance smoothly and snug without any jump/catches. If in doubt, head more lateral! Okay to poke out lateral, no structures at risk (first 1-2 cm), than this provides orientation to more medial pedicle.

8.  Probe advancement should be snug, if loose - probably out lateral. If cannot advance, probably aiming too medial!

9. Palpate five walls of pathway:
• bony floor (vertebral body) and four pedicle walls (medial, lateral, superior, inferior)

10. If completely interosseous, place TAP/smaller screw (.5 mm) less diameter than anticipated

11.  Remove tap and palpate once more the five bony borders

12. Place final screw - only if completely intraosseous!

II. Confirmation Of Intraosseous Screw

1.  Palpation! - Delicate sounding probe - palpate floor and four walls (medial, lateral, superior, inferior) entire pedicle path - palpate twice, once after the pedicle is located with the gear shift and once after it is tapped.

2. Screw orientation - concave versus convex; sagittal plane angulation; compared to adjacent screws

3.  Intraoperative x-lvs - done after all screws placed but before the rod is placed. Need AP and lateral films; place arms forward to see the proximal thoracic region!

4.  Pedicle screw stimulation with EMG recordings
• rectus abdominus (~ T7/T8-TI2)
• intercostal muscles - unable to create a known tidemark for completely intraosseous screws in an animal model (Lewis, Lenke, et al, SRS 2000)

III. How To Judge If The Pedicle Is Large Enough To Instrument?

1. Plain radiogaphs - check pedicles in proximal thoracic, lower thoracic, and upper lumbar
• Scoliosis - check all convex apical pedicles if they are at least 5 mm in diameter, the concave ones are probably large enough to place screws

• Kyphosis - check x-rays of proximal thoracic and lower thoracic spine, in particular, the frontal plane of the vertebra. Often, the true AP radiograph only profiles the apical 2 or 3 segments.

It appears that there is definitely some plasticity to the pedicles such that one can often get a 1-2 mm larger screw diameter completely intraosseous than one could measure on x-ray or CT scan.

2. CT scans - not routinely done preop
• need to get perpendicular to the plane of vertebra
• O'Brien, Lenke et al study showed that main pedicle diameter is between 4-6 mm in thoracic scoliosis

3. MRI scan - Liljenqvist et al found the mean concave pedicle lift to be between 2.4-5.5 mm on the concavity, and between 2.3-6.0 mm on the convexity.

IV. Pitfalls

1.  Pedicle not large enough (medial--lateral dimension) to accept screw
• Extrapedicular screw placement (in-out-in technique)
• Use hooks!

2. Unable to locate pedicle entrance
• Skip that level; may return if adjacent levels provide additional information
• Use hooks!

3. Violate medial wall
• If pedicle is large enough, may reconstitute medial wall with a more laterally placed screw
• Use hooks!

4. Screws do not line up well to accept rod
• Check for aberrent screw placement
• Use polyaxial screws

5. If any step problematic - Use hooks!

V. Complications/Results

1.  Personal series (reviewed by Yongjung Kim, MD)
• 172 consecutive patients
• 1118 screws
• screws per level

TI-6       T4-75        T7-65        TIO-126

T2-21    T5-75        T8-80         T11-199

T3-59    T6-76        T9-83         T12-253

2.  Screw/Diagnosis:
• Pediatric Scoliosis       n = 585
• Pediatric Kyphosis      n = 73
• Adult Scoliosis            n = l 19
• Adult Kyphosis           n = 67
• Pediatric Tumor/Fx     n = 15
• Adult Tumor/Fx          n = 259

3.  No screws removed postop due to poor placement/neurologic/vascular complications, or revision surgery needed up to 9 year follow-up.

4.  400 screws placed in pediatric scoliosis evaluated by postop CT scans:
• 363 (90.7%) entirely intraosseous
• 30 (7.5%) with 0-4 mm cortical perforations (7 screws violated medial wall)
• 7 (1.8%) inserted between lateral pedicle and ribs

5.  AIS Correction

Technique Years % Thoracic Coronal Correction
psf with hooks 1992-93

 

48%

psf-Hybrid

(hooks-T spine; screws-L spine)

1994-99

 

57%
psf-all screws 2000-01

 

77%

 

VI. Benefits Of Thoracic Pedicle Screws In Deformity
Segmental fixation possible
• Secure fixation
• No implant in epidural space
• Ease of rod attachments to either fixed or polyaxial screw heads
• Increased cantilever possible with less risk of implant pull-off - both short and long term
• In situ translation possible, best with screw at every level of correcting rod
• Improves apical translation and coronal correction
• Improved apical derotation???
• Improved sagittal plane alignment? (Yes for hyperkyphosis correction,? for hypokyphosis/lordosis correction)

 

Updated on: 10/05/12
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