The variable pitch thread and the distinctive non-linear shape of the core diameter (minor diameter) improves the compression and fixing effect of the small bones of the hand and the midsection of the foot, in all those circumstances where a headless screw is required.
The use of CSS-HH (Hidden Head) screws provides instant compression due to pitch variation, which permits a primary stabilisation of bone fragments but includes additional functionalities in the secondary compression, achieved when the CSS-HL screw reaches its final seat. Such effect is due to the particular shape of the thread core diameter (see fig. 1), making the CSS-HH screw, not just a headless screw, but a hidden inner head screw. The advantages of this compression are crucial to prevent micro-movements in the fracture site, which can lead to the formation of fibrous tissue and imperfect or delayed bone healing.
The CSS-HH screw ensures a perfect compression of the fracture area, which manifests differently depending on the section of screw concerned.