Biomechanical study of flexible intramedullary nails

Flexible intramedullary (IM) nailing is considered a safe, minimally invasive fixation technique with relatively low complication rates for long-bone fractures in the pediatric population. At our institution, questions have arisen about stability of fixation based on the distance of the nail past the fracture site. Clinically, this question arises with proximal or distal fractures and when the nail is unable to be passed to the desired distance past the fracture site. The purpose of our study was to compare biomechanical resistance with bending forces for fixation constructs whose IM nails are at differing distances beyond the fracture site in different bones.

This study tested matched pairs of canine radii, ulnas, and tibias in 4-point bending and compared the biomechanical properties of length of nail fixation past the fracture site in relation to bone diameter.

Fixations of 1 or 2 diameters past the osteotomy yielded gross instability. There was no difference found in bending failure force, displacement, stiffness, or energy when comparing 3 versus 5 diameters of fixation past the fracture site.

Flexible IM nails act as internal splints to align the fracture ends. At 3 diameters or more beyond the fracture site, the length does not significantly affect the biomechanical properties of the construct.

Flexible IM nails act as internal splints to align the fracture ends. At 3 diameters or more past the fracture site, the length of the nail does not greatly affect the biomechanical properties of the construct. This knowledge may be helpful in clinical scenarios where there is uncertainty about the expected strength of a shorter fixation. Examples include when the nail cannot be passed completely to the distal metaphysis and in proximal or distal long-bone fractures. Further clinical studies are needed to determine implications in a patient setting.