The Scaphoid Safe Zone: A Radiographic Simulation Study to Prevent Cortical Perforation Arising from Different Views

Determining the optimal radiologic wrist and forearm position to visualize screw protrusion in scaphoid fixation

BACKGROUND

Surgical fixation of scaphoid fractures may result in unrecognized screw protrusion and subsequent cartilage damage to the adjacent joints. The purpose of this study was to use a three-dimensional (3D) scaphoid model to determine the wrist and forearm positioning that will allow intra-operative fluoroscopic visualization of screw protrusions.

METHODS

Two 3D scaphoid models, with the wrist in neutral and 20° ulnar deviated, were reconstructed from a cadaveric wrist using the Mimics software. The scaphoid models were divided into three segments and further divided into four quadrants in each of the three segments along the scaphoid axes. Two virtual screws, with a 2 and 1 mm groove from the distal border, were placed so that the screws protrude from each quadrant. The wrist models were rotated along the long axis of the forearm and the angles at which the screw protrusions were visualized were recorded.

RESULTS

One-millimetre screw protrusions were visualized at a narrower range of forearm rotation angles compared to 2 mm screw protrusions. One-millimetre screw protrusions in the middle dorsal ulnar quadrant could not be detected. Visualization of the screw protrusion in each quadrant varied with forearm and wrist positioning.

CONCLUSION

In this model, all screw protrusions, except 1 mm protrusions in the middle dorsal ulnar quadrant, were visualized with the forearm in pronation, supination or in the mid-pronation position and with the wrist in neutral or 20° ulnar deviated.

3D computational anatomy of the scaphoid and its waist for use in fracture treatment

BACKGROUND

A detailed understanding of scaphoid anatomy helps anatomic fracture reduction, and optimal screw position. Therefore, we analysed (1) the size and shape variations of the cartilage and osseous surface, (2) the distribution of volumetric bone mineral density (vBMD) and (3) if the vBMD values differ between a peripheral and a central screw pathway?

METHODS

Forty-three fresh frozen hand specimens (17 females, 26 males) were analysed with high-resolution peripheral quantitative computed tomography (HR-pQCT) and dissected to compute a 3D-statistical osseous and cartilage surface model and a 3D-averaged vBMD model of the scaphoid. 3D patterns were analysed using principal component analysis (PCA). vBMD was analysed via averaging HR-pQCT grey values and virtual bone probing along a central and peripheral pathway.

RESULTS

(1) PCA displayed most notable variation in length ranging from 1.7 cm (- 2SD) to 2.6 cm (mean) and 3.7 cm (+ 2SD) associated with differences of the width and configuration of the dorsal surface (curved and narrow (4 mm) to a wider width (9 mm)). (2) High vBMD was located in the peripheral zone. Lowest vBMD was observed in the centre and waist. (3) Virtual probing along a peripheral pathway near to the cartilage surfaces for the capitate and lunate allowed the center region to be bypassed, resulting in increased vBMD compared to a central pathway.

CONCLUSION

High anatomical variations regarding the osseous and cartilage surfaces were associated with three distinct concentrically arranged zones with notable different vBMD. The complex scaphoid anatomy with its waist might alter the strategy of fracture fixation, education and research.