Anterior screw placement in occipital clivus for craniovertebral fusion: an imaging study

English Keywords:Craniovertebral fusion  CT scans  Screw placement  Anterior approach surgery

English Abstract:

  【Abstract】 Objectives: To study the imaging parameters related to anterior screw placement in occipital clivus for craniovertebral fusion. Methods: CT scans on craniovertebral region between May 2006 and April 2011 in our institution were collected for analysis. Exclusion criteria included: (1)patients aged <20 years; (2)after operation; (3)craniovertebral deformities, such as congenital atlantooccipital assimilation, and (4)patients with tumor, trauma, or infection. Finally, 30 healthy East Asian patients(19 males, 11 females) with an average age of 38.8 years(range, 20-64 years) were included. The original data were imput into the MXV work station, and 2-dimensional(2-D) image reconstruction was performed using multiplanar reconstruction(MPR) methods. And then, angles and lines were drawn and measured using Star PACS. A2 was the optimal entry point on the extracranial clivus, C2 was just at the top portion of the middle line of extracranial clivus, and B2 was determined as the midpoint between B2 and C2. The line A, B, C was the horizontal line that went through the point A2, B2 and C2 respectively. The distance between adjacent points on a line was 3.5mm(the pitch diameter of Zephir plate). The points on line A were regarded as the optimal entry points, the points on line B as the candidate points, and the points on line C as the limit entry points. In the reconstructed midsagittal images, the safe scope for the screw length and angle for every entry point at the horizontal angle(HA), vertical angle(VA) and limit angle(LA) were measured. At the same time, the bending angle that the plate or net cage required was measured. Results: At the optimal entry angle(VA), the total mean screw length was （7.57±1.38）mm, （10.13±2.46）mm and （15.60±3.12）mm for the optimal entry points, candidate points and limit entry points respectively, and there was significant difference among the three points(P<0.05). The measurement of clivus screw placement in 2-D CT reconstruction images showed that 29 cases had line B settled to VA, and 1 case did not reach it at the points of B1, B2 and B3. And yet, only 13 cases met the vertical angle at C1 and C2. 12 cases met it at C3, of them, 1 case reached the vertical angle at C1 other than C2 and C3, and 1 case settled to it at C1 and C2 rather than C3. The maximum uptroversion angle for the optimal entry points, the candidate points, the limit entry points was approximately 130.19°±8.00°, 125.23°±13.24° and 85.72°±24.33° respectively, which showed significant difference among the three points(P<0.05). The bending angle that the plate or net cage required was 130.19°±8.00°(121.36°-140.18°). Conclusions: Anterior screw placement in occipital clivus for craniovertebral fusion is feasible. There exists a safe zone and screw length requirements for screw placement.