[Experimental study of compression by Herbert screw in carpal scaphoid fracture]

The purpose of this experimental work was to obtain a radiographical assessment of the effect of the compression achieved with the Herbert screw due to its different thread diameters by comparing the real and expected reduction of interfragment gap (IFG) on cadaver bones. To better understand and define the limits of this "self-compressing" effect and to study the usefulness of screw sizes smaller than the scaphoid, we measured the maximal and mean reduction of IFG without using the instrumentation.

Twelve scaphoids were obtained from fresh cadavers. The scaphoid holes were drilled and tapped in the largest axis of the scaphoid, in a central position parallel to an anti-rotation wire. A transverse osteotomy was made in the mid third of the scaphoid to simulate a Schernberg grade III fracture. The real IFG reduction was defined as the difference between the gap measured before and after screwing. The expected IFG reduction was defined by multiplying the number of screw turns by the pitch height per turn.

On the reduction average, a Herbert screw produced a 1.1 mm IFG reduction. The maximal reduction IFG measured was 1.5 mm. For 7 cases, the real IFG reduction was larger than expected, for 4 cases it was smaller than expected and for 1 case the difference was considered negligible. The Fisher test did not demonstrate any significant statistical difference between the real and expected IFG reduction for all scaphoids. The operator perceived a subjective sensation of compression after one screw turn that continued up through the last turn.

The Herbert screw develops maximal compression force when all the screw threads are totally anchored in the bone. The interfragment gap closes linearly, bringing the two bone fragments together. However, failure can result if the gap is too wide (>2 mm), the screw is poorly positioned, or there is an inappropriate correspondence between screw length and diameter and scaphoid size. Unlike spongy bone screws, the Herbert screw has a round non-conical tip thread and a shallow thread that do not bring the bone fragments together. The perception of compression corresponds to the passage of the screw through the spongy bone and not to real narrowing of the gap between the fragments. In clinical practice, because of the use of instrumentation for open surgery in carpal scaphoid fracture, it is not necessary to rely on these data, but they can be useful for percutaneous screw insertion without a guidewire and without prior compression or for another localization. Peroperative radiographs should be obtained to assess the quality of the screw position and check reduction and the reality of the compression.