Retrograde (volar) scaphoid screw insertion - A quantitative computed tomographic analysis

Purpose: The benefit of placing the screw tip in the center of the proximal pole of the scaphoid is supported by clinical and biomechanical data. In this investigation we attempted to quantify guidelines for optimal screw insertion into the scaphoid through a volar percutaneous approach using measurements from computed tomography images. Methods: The parameters of safe insertion of a 3.0-mm cannulated-headed screw (Synthes, Paoli, PA) using a volar (retrograde) insertion technique were measured using quantitative computer analysis of computed tomography images of 15 unfractured scaphoids. In the coronal plane the average screw length for the most radial, most ulnar, and intermediate paths for safe screw insertion and the radial clearance of the trapezium were measured. In the sagittal planes defined by these screw paths the length of the screw, the length and depth of the concavity on the volar surface of the scaphoid, and the distance between a line corresponding to the center of the screw-path and the volar surface of the trapezium were measured. Results: Significant differences (1-way analysis of variance) were measured for the average screw lengths for different screw insertion paths in both the coronal and sagittal planes, (with the shortest screw lengths observed for the most ulnar starting points) and for the volar clearance of the trapezium, which was greater for a radial screw path. The intermediate screw path-a measure of optimal screw insertion-passed radial to the radial edge of the trapezium in 10 of 15 patients (67%) with an average clearance of 3.9 mm (range, 1.1-7.9 mm) in those patients. The depth of the concavity on the volar surface of the scaphoid averaged 1.6, 2.0, and 2.5 mm in the 3 sagittal planes. Conclusions: Screw fixation of the scaphoid through a volar approach is hindered by the trapezium, risks cutout through the concavity in the volar surface of the scaphoid, and is most likely to violate the dorsoulnar aspect of the radiocarpal articular surface of the scaphoid if the screw is too long. A relatively radial starting point facilitates placement of the screw tip in the center of the proximal pole and helps avoid the trapezium but drilling or partial excision of the trapezium often may be necessary for optimal screw placement.