Free hand screw

FREE HAND TECHNIQUE FOR
PEDICLE SCREW INSTRUMENTATION
Dr Sumit Sinha MS, DNB, MNAMS, MCh
Faculty, AO Spine
Faculty, Advanced Trauma Life Support
Fellow, Fujita Health University, Japan
Associate Professor
Deptt of Neurosurgery, All India Institute of Medical Sciences
and Associated JPNA Trauma center, New Delhi

FREE HAND PEDICLE SCREW
Screw Cortical Penetration Graded on Basis of Anatomy
and Distance of Penetration
Anatomic grade
• A: lateral pedicle wall penetration
• B: superior or inferior pedicle wall penetration
• C: medial pedicle wall penetration
• D: anterolateral vertebral body penetration
Quantitative grade
• 1: >0 and r2mm
• 2: >2 and r4mm
• 3: >4mm
• 4: >4mm without pedicle or vertebral body contact
* Karapinar et al. J Spinal Disord Tech 21 (1),2008, 63-7

• Grade I- screws replacing cortex, not extending
beyond pedicular cortical margin
• Grade II- Screws < 2 mm beyond pedicular
margin
• Grade III- Screws > 2 mm beyond pedicular
margin
– False- Intentional lateral entry through CV
joint
– True anatomically significant error- Screws >
*Youkilia et al. Neurosurgery 2001;48:771-8

• Gertzbein and Robins- 4mm safe zone of
allowable medial encroachment.
• Belmont- defined the acceptable limits as 2 mm
of medial wall penetration and 6 mm of lateral
wall penetration.
• Clinically, encroachment of the epidural space up
to 2mm is conservatively considered to be safe in
most cases
• Gertzbein SD, Robbins SE. Spine 1990;15:11-5
• Belmont et al

• Pts. with violations > 6 mm- present with
neurological deficits
• T10-L4- Safe Zone- tolerates medial
violations> 4 mm
• Pedicle Transverse diameter decreases
from T1 downwards- lower in T6-T7
• T8-L5- pedicle diameter increases

• Pedicle screws that violate the medial pedicle
cortex may increase the risk of neurologic deficit.
• Pedicle screws that violate the lateral pedicle
cortex may increase the risk of vascular or
visceral complications and loss of fixation if it
occurs at the lower end of a construct.

• Pedicle screw in Th spine- difficult- Small
pedicle
• Medial error- less mobility of SC-
dangerous
• Lateral error- pleural cavity, great vessels,
esophagus- in U/ Mid Th levels
• Infr error- refractory neuropathic pain/
motor deficits- in L spine

• A total of 964 patients received 6816 free-hand placed pedicle
screws in the Th and L spine.
• A total of 115 screws (1.7%) were identified as breaching the pedicle
in 87 patients (9.0%).
• Breach occurred more frequently in the Th> L spine (2.5% and
0.9%) and was more often lateral (61.3%) than medial (32.8%) or
superior (2.5%).
• T4 (4.1%) and T6 (4.0%) experienced the highest breach rate,
whereas L5 and S1 had the lowest breach rate.
• Eight patients (0.8%) underwent revision surgery to correct
malpositioned screws.
• Image-guided assistance may be most valuable when placing
screws between T4 and T6, where breach rates are highest
*Parker SL et al. Neurosurgery. 2011; 68(1):170-8

• Accuracy of pedicle screw placement with and without the
use of operative navigation- 95.1% and 90.3%, respectively.
• More recent studies have reported an incidence of cortical
breach between 5.6% and 6.2
• Image-guided techniques may be most useful at the T4-6
segments, where the breach rate seems to be the highest.
• Screw placement without intraoperative radiological
guidance avoids the harmful effects of radiation exposure to
both the patient and surgeon.
• It also shortens the duration of surgery, which has been
correlated to a reduced incidence of surgical site infection
*Kosmopoulos V et al. Spine (Phila Pa 1976). 2007;32(3):E111-E120
**Karapinar L et al. J Spinal Disord Tech. 2008;21(1):63-67.

Correct pedicle screw placement requires
use of the correct entry point on the
posterior vertebral cortex, followed in the
correct direction to stay within the pedicle
cortex.

Anatomic landmark technique versus open
laminectomy technique
• Performing screw insertion under direct vision
after laminectomy- rational alternative where
anatomy not clear
• No evidence of superiority of one vs amother
technique

Adjuvant techniques- Intraoperative
Imaging/ navigation/ Neurophysiological
monitoring
• Routine use- added costs and more
operative time
• Gross intersegmental instability may
preclude accurate intraop guidance

• Rampersaud et al- evaluated the amount of radiation exposure
to the surgeon’s neck, torso, and dominant hand during the
placement of pedicle screws from T11-S1 using lateral
fluoroscopy only in a cadaveric model.
• Average fluoroscopy exposure time was 9.3 seconds per screw.
• The average hand dose rate was 58.2 mrem/min.
• The internationally recommended maximum limit for annual
hand radiation exposure is 50,000 mrem
• A surgeon would exceed occupational exposure limit for the
eyes and extremities by placing 4854 and 6396 screws
percutaneously, respectively
*Rampersaud YR et al. Spine (Phila Pa 1976). 2000;25:2637–2645.
**Mroz TE et al. J Spinal Disord Tech 2011;24(4):264-7

Anatomic factors favoring wide use of free hand
technique in TL/LS spine and disadvantages in U/
Mid Th spine
Th/LS spine U/Mid Th spine
Gold standard Challenging
Antr violation less dangerous Adherence of Th viscerae to ALL
Large pedicle size Small pedicle size
Safety zone between T10-L4 Small dia of spinal canal
Several anatomic and morphological
studies
Interindividual variability in pedicle body
angle and pedicle axis
Lat violations often asymptomatic Lateral violations dangerous to pleura, grt
vs, oesophagus

• Free hand technique relies on
identification of postr elements bony
landmarks- lateral border of pars
interarticularis, entire TP and cephalad
and caudad facet joints

Lumbar spine-
Partial facetectomy of inferolateral 1/3 of infr
art process of supr vertebrae done to-
• Identify exact limit between supr and infr art
processes
• Identifies ideal initial perforation site
• Decreases amount of cortical bone
• Provides a smooth surface for final screwhead

L spine- entry point
• Intersection between a line passing lateral
to infr art process and line bisecting TP
At S1- entry point
• At the inferolateral margin of the base of
supr art process of sacrum

L spine- direction of screws
• Follows axis of pedicle
• Slightly oblique towards midline- 10-15
degree
• Preop determined by CT
Screw length-
• L1-L4- 40-45 mm
• L5- 45-50 mm
• S1- 35-40 mm

free-hand pedicle screw placement
technique in the lumbar spine with the
entry point occurring at the site of the
accessory process. Screw trajectory is
directed from lateral to medial and
parallel with the superior endplate.

Entry site for S1 screw placement at the
bony rectangle below the S1 superior
(Sup.) articular process and above the S1
dorsal foremen. Mediolateral trajectory is
parallel to the S1 superior articular
process.
Inferolateral trajectory parallels the
superior endplate toward the sacral
promontory.

Entry site is determined by
identification of the triangular bony
confluence formed by the superior
(Sup. articular process, the
transverse process, and the pars
interarticularis. The center of this
triangle is selected as the entry site.

Th spine-
• Variation in individual anatomy
• Clinical and cadaveric studies- screw
violation- 15-50% with free hand technique

Th Spine- Starting point-
• T10-T12- junction of vertical line passing
along lat boundary of pars and line
bisecting TP into half
• T7-T9- most medial- along a vertical line
lat to midpoint of sup art process and a
transverse line along the supr border of TP

Starting point (Contd.)-
• T5-T8- proximal edge of TP/ lamina, just
lateral to midpoint of base of supr art process
• Moving proximally from mid Th- U Th- screw
entry point moves back and lateral
• T1-T2- at intersection of a vertical line along
lat border of pars and transverse line bisecting
TP

Th Spine- Diameter of screws-
• T10-T12- 6.3-7.8 mm
• T4-T9- 4.7-6.1 mm
• T4 and T6- smallest
• T12- Largest
• T1-T4- 5.6-7.9 mm

Angles
for free
hand
pedicle
screw
insertion-
Cranio- Caudal Medio- lateral
-14 T1 22
-13 T2 18
-12 T3 15
-11 T4 14
-11 T5 13
-10 T6 11
-8 T7 10
-6 T8 9
-4 T9 8
-1 T10 8
1 T11 11
4 T12 14
9 L1 14
9 L2 16
11 L3 17
13 L4 19
14 L5 22
20 S1 35

Conclusions
• Routine use of image guided systems- a
luxury that many spine surgeons do not need
• Evidence suggests- free hand technique of
TL/ LS screw palcement- reliable and safe
even in deformity and difficult surgeries
• In Th spine- image guidance and
neurophysiology is a useful armementarium

Free hand screw

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