Mechanical study of the safe distance between humerus shaft fracture and distal locking screws in antegrade nailing.

BACKGROUND

Optimal positioning of distal locking screws in intramedullary humeral nailing remains uncertain, particularly the influence of the distance between the fracture plane and the proximal distal locking screw on construct stability. This study aims to evaluate the mechanical stability of humeral nailing under different fracture-to-screw distances and numbers of distal locking screws using finite element analysis and mechanical testing on bone models.

METHODS

A finite element model and mechanical testing on six sawbones models were performed under traction (500 N), compression (500 N), and torsion (3 Nm). Models were tested with two osteotomy distances from the proximal distal locking screw (2 cm and 5 cm) and with either one or two distal locking screws. Axial and torsional stiffness and fracture displacement were recorded and analyzed statistically.

RESULTS

Finite element analysis showed higher stress concentrations near the distal fracture fragment. Mechanical testing demonstrated that traction and torsional stability were significantly affected by fracture-to-screw distance (p = 0.006 and p = 0.015), while compression stability was influenced by the number of distal screws (p = 0.035).

CONCLUSION

A fracture-to-screw distance of 5 cm was associated with improved axial and torsional stability, while double distal screws enhanced compressive stiffness. These biomechanical results, although very promising, should be confirmed with clinical studies.