SACRAL SCREW PLACEMENT: A COMPARISON OF PLAIN RADIOGRAPHS WITH CT.

Abstract

Aim: To test the null hypothesis that plain X-rays can provide the same assessment of sacral screw placement as CT.

Introduction: Engaging the anterior cortex of the sacrum provides additional strength to fixation and is a goal of surgery. The sacrum with its unique anatomy makes it a difficult bone to assess screw placement radiologically. This study examines the positioning of sacral screws as seen on X-rays and compares the result with spiral CT “gold standard”.

Materials and methods: Inclusion criteria: Sacral fixation using Diapason (Stryker) Titanium pedicle screws by one surgeon. Spiral CT, plain AP and lateral X-rays of the sacrum. Exclusion criteria: X-rays with more than three level fixation.

There were 66 patients (132 S1 screws). Surgical technique engaged the anterior cortex to enhance fixation. Two independent observers (a musculoskeletal radiologist and spinal fellow) who were blinded to outcome, reported findings in forms with constrained fields. Assessment of plain X-ray and CT was at separate times not less than three weeks apart. Variables noted: Screw position in pedicle, screw tip position, and angle of screw (sagittal on axial CT scans).

AP X-ray was divided, for each screw, into nine zones based on the first sacral foramina. The position of the screw tip in the zones was noted. The lateral X-ray was divided into three zones to note the tip of the screw in relation to the cortex. The extent of screw protrusion was measured. X-ray technique: Supine AP centred on fusion and lateral X-ray standing, X-ray source 200 cm from the film. CT: Images acquired on Picker PQ 6000 spiral CT with collimated thickness of 3 mm, pitch 1.25 and reconstructive index of 1.Para-sagittal and coronal reconstructions. Spiral CT was used to note the position of the screw within the pedicle and the relation of the screw tip to the anterior cortex. For screws within the pelvis any structure in close proximity was noted.

Results: On CT 10% of the screws had breached the pedicle compared with 2% on the plain X-rays. Anterior cortical perforation had been achieved in 48 out of 132 screws on CT. The sensitivity of the plain X-rays to perforation was 40% with a specificity of 92%. There was an average under estimation of the extent of screw perforation by 4.4 mm (95% confidence ±1 mm). There was a correlation between the position of the screw tip on the AP X-ray and the sensitivity of the lateral X-ray to detect a perforation. The sensitivity ranged from 52% for zone 1 to 15% in zone 8. 15/31 perforations were missed in zone 1, compared with 11/13 in zone 8. For screws penetrating 5 mm or more, in zone 8, 9 out of 10 were missed on lateral X-rays.

Eighty-five screws were placed at an angle of less than or equal to 25° to the sagittal; this included 28 out of 34 screws placed in zone 8. The inter-observer variance of screw angle measurement was 1.1° and intra-observer difference 1.7°. Overall 95% confidence of a single measurement was ±3.3°.

Conclusion: Plain X-rays and CT do not provide the same assessment of sacral screw placement. This is particularly true for sagitally placed screws with screw tips in zones 7–8.