336 - The Risk of Facet Joint Breach by Cervical Pedicle Screw (CPS) Placeme...

#336 The Risk of Facet Joint Breach by Cervical Pedicle Screw (CPS) Placement in the Degenerative Spine

Cervical Therapies and Outcomes

Poster Presented by: D. Lee

Author(s):

D.-H. Lee (1)
H. Noh (1)
H.S. Lee (1)
J.J. Yang (2)
C. Hwang (1)
S.K. Cho (3)

(1) Asan Medical Center, University of Ulsan College of Medicine, Orthopedic Surgery, Seoul, Korea, Republic of
(2) Dongguk University Ilsan Hospital, Orthopedic Surgery, Ilsan, Korea, Republic of
(3) Mount Sinai School of Medicine, Orthopaedics, New York, NY, USA

Abstract

Introduction: Although there have been several studies regarding the optimal entry points and trajectories for cervical pedicle screws (CPS), it is still doubtful that those parameters could be applicable in the degenerated cervical spine. As the spine degenerates, facet joints collapse and hypertrophy, resulting in morphologic change of lateral masses. These deformations could potentially obscure the anatomical landmarks which are routinely utilized for CPS placement. The purpose of this study was to compare the entry points and trajectories of CPS in the healthy vs. degenerated spine. Also, we evaluated the risk of facet breach by CPS in the degenerated spine.

Methods: Multidetector computed tomography (MD-CT) images of cervical spine of 30 patients (15 males/15 females, mean age 64 years) were selected. All of their facet joints had degenerative changes of at least grade 1 by Walraevens et al.'s grading system. Patients with neoplasm, trauma, infection and congenital anomaly were excluded. 0.75 mm thickness axial images were reconstructed as 3-dimensional (3-D) models using a screw fixation simulation program (V-works, Cybermed Inc.). Virtual CPSs with 3.5 mm diameter were inserted passing through the coaxial axis of each pedicle from C3 to C6 (Figure 1). Horizontal and vertical offsets of entry points were measured from two different anatomical landmarks on lateral mass: lateral notch (LN) and the center of the superior ridge (SR) of lateral mass. The transverse angle and sagittal angles of the screws were also measured. The facet joint breaches were evaluated and classified into either “minor” (< 50% of screw diameter) or “major” (≥ 50% of screw diameter). The data were compared to those measured in the 30 healthy cervical spines (15 males/15 females, mean age 52 years).

Results: The mean transverse angles and the mean sagittal angles were similar between the degenerated and healthy groups. In terms of entry point, however, the degenerated group had significantly smaller vertical offsets from SRs than the healthy group at every level (p< 0.05). The entry points on the lateral masses were located more medially and inferiorly from LN, and more laterally and superiorly from SR with higher grade facet degeneration. In addition, facet joint breach was detected in 60 of 240 CPSs (25%, 44 minor and 16 major breaches) in the degenerated group, which was more common in higher grade degeneration (p< 0.05) (Table 1).

Conclusion: In the degenerative cervical spine, the anatomy of facet joint and lateral mass undergoes significant changes. This deformation is likely to make the entry point of CPS to be more superior toward the facet joint and consequently increases the risk of facet breach. This facet breach could more commonly involve the uppermost vertebra of instrumentation, which would accelerate the joint degeneration. Therefore, surgeons need to pay more attention to morphologic changes in the degenerated cervical spine to avoid facet violation during CPS placement.

Figure 1

Table 2