Fig. 4

Schematic of three different cannulated screws used for fixation of FNF. The initial states of FNF fixed by (a) partial threaded cannulated screws (PTS), (b) fully threaded cannulated screw (FTCS), and (c) fully threaded headless cannulated screw (FTHCS) are represented in the lower right-hand portion of each figure. a There is a definite sliding mechanism during FNF healing in PTS fixation (large downward black arrow), which results in an observable dynamic compression across the fracture site. However, the proximal fracture fragment and PTS may move lateral-distally resulting in neck shortening and lateral screw protrusion, especially in comminuted fractures. b In contrast, the FTCS may prevent the femoral head from migrating along the screws, given the lack of a sliding mechanism. However, there may be a gap present at the fracture site 2–3 weeks postoperatively as a result of bone resorption or residual malreduction, particularly in comminuted fractures. c In FTHCS fixation, there is the possibility of an asymptotical sliding mechanism (small black arrow) due to the tapered profile of the screw. However, the fully threaded length results in length control structure, which may compromise the sliding efficacy during healing