This study directly measures the ideal point of insertion and direction of sacral screws in the Jackson intrasacral fixation.Objectives.
To design a template for safe positioning of transpedicular endplate screws that does not require fluoroscopy.Summary of Background Data.
Screw and rod insertion into the sacrum as described by Jackson requires careful intraoperative fluoroscopic or radiographic control. The technique can *be difficult and demanding, especially in the case of severe pelvic obliquity. No anatomic data based on cadaveric studies are available to aid in the described technique for screw insertion.Methods.
Fifty dry sacrums were used. On each bone, the length of the intrasacral portion and the ideal direction of transpedicular endplate pins introduced at the originally described point of insertion were measured in reference to the plane formed by the two posterior sacral foramen and the posterior arch of S1. The limit angles in each direction, where the pin perforated the cortex outside of the S1 endplate, were noted. Then, the point of insertion of a pin with the ideal angles was progressively modified to determine the limits of the safe insertion area. The 95% confidence interval was reduced to the smallest safe angle.Results.
The safe angles were 54.3° to 57.9° for the sagittal plane, 47° to 50° for the coronal plane, and between 46.9° and 49.7° for the horizontal plane.Conclusions.
A "cylinder of safety" for the transpedicular endplate screws was determined. Its coordinates are related to the sacrum and not to the sacral position. This allowed for a template to be designed, the efficiency of which will have to be confirmed by anatomic and clinical studies.