A computed tomography-based anatomic comparison of three different types of c7 posterior fixation techniques : pedicle, intralaminar, and lateral mass screws

Abstract

Objective: The intralaminar screw (ILS) fixation technique offers an alternative to pedicle screw (PS) and lateral mass screw (LMS) fixation in the C7 spine. Although cadaveric studies have described the anatomy of the pedicles, laminae, and lateral masses at C7, 3-dimensional computed tomography (CT) imaging is the modality of choice for pre-surgical planning. In this study, the goal was to determine the anatomical parameter and optimal screw trajectory for ILS placement at C7, and to compare this information to PS and LMS placement in the C7 spine as determined by CT evaluation.

Methods: A total of 120 patients (60 men and 60 women) with an average age of 51.7±13.6 years were selected by retrospective review of a trauma registry database over a 2-year period. Patients were included in the study if they were older than 15 years of age, had standardized axial bone-window CT imaging at C7, and had no evidence of spinal trauma. For each lamina and pedicle, width (outer cortical and inner cancellous), maximal screw length, and optimal screw trajectory were measured, and the maximal screw length of the lateral mass were measured using m-view 5.4 software. Statistical analysis was performed using Student's t-test.

Results: At C7, the maximal PS length was significantly greater than the ILS and LMS length (PS, 33.9±3.1 mm; ILS, 30.8±3.1 mm; LMS, 10.6±1.3; p<0.01). When the outer cortical and inner cancellous width was compared between the pedicle and lamina, the mean pedicle outer cortical width at C7 was wider than the lamina by an average of 0.6 mm (pedicle, 6.8±1.2 mm; lamina, 6.2±1.2 mm; p<0.01). At C7, 95.8% of the laminae measured accepted a 4.0-mm screw with a 1.0 mm of clearance, compared with 99.2% of pedicle. Of the laminae measured, 99.2% accepted a 3.5-mm screw with a 1.0 mm clearance, compared with 100% of the pedicle. When the outer cortical and inner cancellous height was compared between pedicle and lamina, the mean lamina outer cortical height at C7 was wider than the pedicle by an average of 9.9 mm (lamina, 18.6±2.0 mm; pedicle, 8.7±1.3 mm; p<0.01). The ideal screw trajectory at C7 was also measured (47.8±4.8° for ILS and 35.1±8.1° for PS).

Conclusion: Although pedicle screw fixation is the most ideal instrumentation method for C7 fixation with respect to length and cortical diameter, anatomical aspect of C7 lamina is affordable to place screw. Therefore, the C7 intralaminar screw could be an alternative fixation technique with few anatomic limitations in the cases when C7 pedicle screw fixation is not favorable. However, anatomical variations in the length and width must be considered when placing an intralaminar or pedicle screw at C7.

Keywords: Anatomic study; Intralaminar screw; Lateral mass screw; Pedicle screw.

Fig. 1

Axial CT scan at C-7 showing optimal ILS length (LLMa : lamina maximum length, LLMi : lamina minimum length) and trajectory (LA : lamina angle to sagittal plane), and determination of medialization angle in relation to the midline. CT : computed tomography, ILS : intralaminar screw.

Fig. 2

Axial CT scan at C-7 showing optimal PS length (PL) and trajectory (PA : pedicle angle to sagittal plane) and determination of medialization angle in relation to the midline. CT : computed tomography, PS : pedicle screw.

Fig. 3

Axial CT scan at C-7 showing measurement of outer cortical width (PWo : pedicle outer cortical width, LWo : lamina outer cortical width) and inner cancellous width (PWi : pedicle inner cancellous width. LWi : lamina inner cancellous width) for each lamina and pedicle. CT : computed tomography.

Fig. 4

Sagittal CT scan at C-7 showing measurement of outer cortical height (PHo : pedicle outer cortical height. LHo : lamina outer cortical height) and inner cancellous height (PHi : pedicle inner cancellous height, LHi : lamina inner cancellous height) for each lamina and pedicle. CT : computed tomography.

Fig. 5

Sagittal CT scan at C-7 showing measurement of lateral mass AP diameter (LMAP). Maximal lateral mass screw length (LM) was calculated by LMAP/cosine 30°.