A computed tomography-based feasibility study of translaminar screw fixation in the upper thoracic spine.
OBJECTIVE
Translaminar screws (TLSs) offer an alternative to pedicle screw (PS) fixation in the upper thoracic spine. Although cadaveric studies have described the anatomy of the laminae and pedicles at T1-2, CT imaging is the modality of choice for presurgical planning. In this study, the goal was to determine the diameter, maximal screw length, and optimal screw trajectory for TLS placement at T1-2, and to compare this information to PS placement in the upper thoracic spine as determined by CT evaluation.


METHODS
One hundred patients (50 men and 50 women), whose average age was 41.7 +/- 19.6 years, were selected by retrospective review of a trauma registry database over a 6-month period. Patients were included in the study if they were over the age of 18, had standardized axial bone-window CT imaging at T1-2, and had no evidence of spinal trauma. For each lamina and pedicle, width (outer cortical and cancellous), maximal screw length, and optimal screw trajectory were measured using eFilm Lite software. Statistical analysis was performed using the Student t-test.


RESULTS
The T-1 lamina was estimated to accommodate, on average, a 5.8-mm longer screw than the T-2 lamina (p < 0.001). At T-1, the maximal TLS length was similar to PS length (TLS: 33.4 +/- 3.6 mm, PS: 33.9 +/- 3.3 mm [p = 0.148]), whereas at T-2, the maximal PS length was significantly greater than the TLS length (TLS: 27.6 +/- 3.1 mm, PS: 35.3 +/- 3.5 mm [p < 0.001]). When the lamina outer cortical and cancellous width was compared between T-1 and T-2, the lamina at T-2 was, on average, 0.3 mm wider than at T-1 (p = 0.007 and p = 0.003, respectively). In comparison with the corresponding pedicle, the mean outer cortical pedicle width at T-1 was wider than the lamina by an average of 1.0 mm (lamina: 6.6 +/- 1.1 mm, pedicle: 7.6 +/- 1.3 mm [p < 0.001]). At T-2, however, outer cortical lamina width was wider than the corresponding pedicle by an average of 0.6 mm (lamina: 6.9 +/- 1.1 mm, pedicle: 6.3 +/- 1.2 mm [p < 0.001]). At T-1, 97.5% of laminae measured could accept a 4.0-mm screw with 1.0 mm of clearance, compared with 99.5% of T-1 pedicles; whereas at T-2, 99% of laminae met this requirement, compared with 94.5% of pedicles. The ideal screw trajectory was also measured (T-1: 49.2 +/- 3.7 degrees for TLS and 32.8 +/- 3.8 degrees for PS; T-2: 51.1 +/- 3.5 degrees for TLS and 20.5 +/- 4.4 degrees for PS).


CONCLUSIONS
Based on CT evaluation, there are no anatomical limitations to the placement of TLSs compared with PSs at T1-2. Differences were noted, however, in lamina length and width between T-1 and T-2 that must be considered when placing TLS at these levels.
Kretzer
Ryan M
RM
Department of Neurosurgery, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21287, USA. rkretzer@jhmi.edu
Chaput
Christopher
C
Sciubba
Daniel M
DM
Garonzik
Ira M
IM
Jallo
George I
GI
McAfee
Paul C
PC
Cunningham
Bryan W
BW
Tortolani
P Justin
PJ
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001863:Bone Screws; D016208:Databases, Factual; D005240:Feasibility Studies; D005260:Female; D005592:Fracture Fixation / Q000295:instrumentation* / Q000379:methods*; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008297:Male; D008875:Middle Aged; D012042:Registries; D012189:Retrospective Studies; D012737:Sex Factors; D012984:Software; D013904:Thoracic Vertebrae / Q000000981:diagnostic imaging* / Q000293:injuries / Q000601:surgery*; D014057:Tomography, X-Ray Computed; D055815:Young Adult