134S Proceedings of the NASS 23rd Annual Meeting / The Spine Journal 8 (2008) 1S–191S

Gyungki-do, South Korea; 2Washington University in St. Louis,

St. Louis, MO, USA

BACKGROUND CONTEXT: One potential complication of C1 lateral

mass screw placement is inadvertent perforation of the atlanto-occipital

(C0-C1) and/or atlanto-axial (C1-C2) joint(s).

PURPOSE: The purpose of this study was to identify a simple lateral fluo-

roscopic landmark to help prevent C0-C1 and C1-C2 joint violations dur-

ing C1 lateral mass screw insertion.

STUDY DESIGN/ SETTING: Radiographic analysis.

PATIENT SAMPLE: 154 consecutive patients 18 years of age or older

who had thin-sliced (1.0 mm) CT scans and three-dimensional reconstruc-

tions from the occiput to C3.

OUTCOME MEASURES: Trajectory which would allow safe placement

of C1 lateral mass screws while avoiding perforation of the C0-C1 and/or

C1-C2 joint(s).

METHODS: We used 1.0 mm-sliced CT scans and three-dimensional re-

construction and screw trajectory software to simulate insertion of 4.0 mm

screws. The entry point was set at the middle of the junction of the poste-

rior arch and the posterior inferior part of the lateral mass. Two medially

angulated screw trajectories were evaluated: 0 � and 15 �. For both trajec-

tories, we determined the maximum cranial and caudal angulation that

avoided injury to the C0-C1 and C1-C2 joints. We then determined how

high and low the screw could safely be directed in the anterior arch of

C1 on a lateral view with these maximum cranial and caudal angulations.

We expressed these targeting points as a percentage of the total height of

the anterior atlas arch such that 100% represented a screw touching the

cranial border of the arch, 50% the center and 0% the caudal border.

RESULTS: Screw trajectories in 154 patients (308 screws) were evalu-

ated. With a 15 � medial angulation, the C0-C1 joint was safe in all cases

when the trajectory was below the 40% point of the anterior arch. The C1-

C2 joint was safe when the trajectory was above the 20% point. With

0 � medial angulation, the safety margin was slightly wider. Since it may

be difficult to differentiate between 0 � and 15 � of medial angulation in-

tra-operatively, we suggest aiming the screw tip between the 20% and

40% points for either trajectory. We call this the ‘‘safe zone of C1.’’

Figure. With the medial angulation of the screw between 0 � and 15 �,both C0-C1 and C1-C2 joints are safe in all patients as long as the AAT

is between 20–40% (green area), which we call the ‘‘safe zone of C1."

CONCLUSIONS: Inadvertent perforation of the C0-C1 and/or C1-C2

joint(s) is a potential complication of C1 lateral mass screw insertion. When

the screw is directed between 0 � and 15 � medially, it can be inserted without

C0-C1 and C1-C2 joint violation if the screw tip trajectory lies between the

20–40% points of the anterior atlas arch (i.e. the ‘‘safe zone of C1’’).

FDA DEVICE/DRUG STATUS: Insertion of C1 lateral mass screws: Not

approved for this indication.

doi:10.1016/j.spinee.2008.06.311

P68. Navigation-assisted Fluoroscopy in Minimally Invasive Direct

Lateral Interbody Fusion: A Cadaveric Study

Jonathan Webb, BS1, Gilad J. Regev, MD2, Choll Kim, MD, PhD2;1University of California, San Diego, School of Medicine, San

Diego, CA, USA; 2University of California, San Diego, San Diego,

CA, USA

BACKGROUND CONTEXT: MIS surgery is increasing in popularity.

Unfortunately, MIS is heavily dependent on intraoperative fluoroscopy

for visualization and implant insertion. Increased use of radiation in the op-

erating room significantly increases the surgeon’s exposure to radiation in

comparison to other non-spinal procedures. Computer-assisted navigation

(NAV) is a potential method of decreasing radiation exposure and improv-

ing operating room ergonomics. The direct lateral interbody fusion (DLIF)

technique is a new MIS method for MIS anterior lumbar interbody fusion.

PURPOSE: This study assesses the use of navigation for the DLIF proce-

dure (NAV DLIF). Comparisons of radiation exposure and procedure time

using navigation versus standard fluoroscopy were assessed. Accuracy of

NAV DLIF using a reference frame mounted in the anterior superior iliac

spine (ASIS) is also assessed.

STUDY DESIGN/ SETTING: Cadaveric specimens are used to evaluate

the efficacy and accuracy of computer-assisted navigation compared to

standard fluoroscopy for the direct lateral interbody fusion technique.

PATIENT SAMPLE: No patients used in this study.

OUTCOME MEASURES: Radiation exposure to the surgeon was mea-

sured using dosimeters for each procedure and times for specific surgical

steps were recorded and compared between each group. Accuracy was

evaluated by measuring intraoperative deviation from a known marker

placed in the vertebral bodies of the lumbar spine and comparing the error

found at each level as the surgeon works further from the ASIS tracker.

METHODS: Three fresh whole body cadavers underwent DLIF from

T10-L5 using either navigation-assisted fluoroscopy (NAV) or standard

fluoroscopy (FLUORO). Various surgical times were recorded at each

level. One fresh whole body cadaver was used to evaluate the accuracy

of the NAV DLIF procedure from L2-L5 by comparing measurements of

navigation pointer deviation from a known intraoperative marker.

RESULTS: In comparing navigation with standard fluoroscopy for the

DLIF procedure, statistically significant differences were obtained for the

set-up, approach, diskectomy and total fluoroscopy times. Approach time

for the FLUORO group (p50.024). Diskectomy time was also significantly

longer for the FLUORO group when compared to the NAV group (p50.009).

Total fluoroscopy times for the FLUORO group was nearly double times for

the NAV group (p50.004). In contrast, the set-up time for the NAV group was

higher than the FLUORO group (p50.005). There was no statistical signif-

icance obtained for cage insertion or total operating times. Radiation expo-

sure of the surgeon for the NAV group was virtually undetectable

(0.8260.6 mREM per level), unlike the radiation exposure for the FLUORO

group (4.8063.08 mREM per level, p50.0005). The accuracy of the NAV

DLIF technique were: L2-3 (0.8660.08 mm), L3-4 (0.9760.12 mm), L4-5

(0.7860.33 mm).

CONCLUSIONS: The use of navigation-assisted fluoroscopy for the min-

imally invasive DLIF procedure is feasible. Accuracy for this procedure is

less than 1mm over the most common levels (L2-L5) which is likely to be

sufficient for safe clinical application. Although initial set-up time is longer

with NAV, simultaneous AP and lateral imaging with NAV decreases the time

for the approach and diskectomy, making overall surgery time similar to that

of standard fluoroscopy. Navigation also minimizes radiation exposure to the

surgical team.

FDA DEVICE/DRUG STATUS: StealthStation Treon: Approved for this

indication.

doi:10.1016/j.spinee.2008.06.312

P69. Five-fold Increased Risk of Pseudoarthrosis or Delayed

Infection Following Early Wound Infection in the Neuromuscular

Scoliosis Patient

Burt Yaszay, MD1, Jeff Pawelek2, Shunji Tsutsui3, Tracey Bastrom1,

Peter Newton, MD1; 1Rady’s Children’s Hospital, San Diego, San Diego,

CA, USA; 2University of California, San Diego, San Diego, CA, USA;