Abstract
Background Cortical bone trajectory (CBT) is a relatively new technique for pedicle screw insertion
in the field of spine surgery. Previous studies have demonstrated the significantly
better pullout and toggle characteristics the new method offers, and it appears to
have certain advantages over the widely used traditional trajectory. The mechanism
of the pullout and toggle characteristics still remains unknown.
Purpose To report the medium- to long-term follow-up findings of patients who underwent posterior
lumbar interbody fusion or posterior lumbar fusion (PLIF/PLF) at our institution and
to discuss the ideal screw trajectory when using this technique.
Study Design Retrospective radiologic study.
Methods Fifty-five patients who underwent PLIF/PLF for lumbar spondylosis using the new technique
between January 2011 and January 2016 were included. Clinical outcome was assessed
using the Japanese Orthopaedic Association scores for low back pain and visual analog
scale scores. Screw loosening was evaluated via the presence of a translucent zone
surrounding the pedicle screw using plain X-radiography (Xp). The screw trajectory
was evaluated by measuring the rostral range (RR) and the lateral range score (LRS).
The RR is the angle between the line drawn along the distal end plate of the vertebra
and the line drawn along the screw on the lateral view. The LRS is the score given
depending on the position of the tip of the screw seen on the Xp taken in the anteroposterior
(AP) view. The pedicle ring is equally divided into the medial third (zone A), central
third (zone B), and the lateral third (zone C) by four vertical lines. A score of
0 to 3 points is given depending on the position of the tip of the screw: 0, outside
the pedicle ring; 1, zone A; 2, zone B; and 3, zone C.
Results Bone fusion was recorded in 49 patients (49/55 patients). The total number of screws
with a visible translucent zone on Xp was 26 (26/242 screws). When the screws are
inserted at a more acute angle in the lateral view (low RR) or when the screws are
directed more medially (low LRS), the risk of developing a translucent zone around
the pedicle screw increased.
Conclusion With CBT, the pedicle screws are inserted from the pars interarticularis that is
rich in cortical bone, strong enough to withstand the substantial forces usually encountered
in the region. Our clinical data suggest that the optimal screw trajectory is one
where the screw is inserted at an angle > 22.55 degrees (high RR) on the lateral view
Xp and where the tip of the screw is directed toward the lateral third of the pedicle
on the AP Xp. We believe the naturally occurring buttress effect that exists in the
vertebra may give CBT a mechanical and anatomical advantage over the traditional screw
trajectory.
Keywords
posterior lumber fusion - cortical bone trajectory - buttress theory - surgical technique
