Here are the things that we need to know in design:

characteristics of the proximal and distal ends of these devices

Intramedullary nail systems at the proximal, middle, and distal regions need to have their strength and flexural rigidity (stiffness) evaluated

Test strength and stiffness between the middle and the proximal or distal aspects of rods.

Strength and rigidity which increases with rod diameter

Static strength

Mechanical properties (bending rigidity)

You need to know the characteristics of strength and rigidity for the particular devices available and how these might impact fracture healing.

Dear professor carpenter ;

thank you so much for your help. could you introduce me some books that write about design of nails and biomechanic charecters of screws?

I really have problem to discuss about them .

I need basical sources to discuss nail from view of biomechanics.

As resources on tap for biomechanical principles, history, technique and more I suggest you to visit AO foundation site. They have lots of shared lessons and PDFs you could browse or download.

Hope this helps.

You basically need optimum level of relative stability to achieve union for which nails are largely designed.

Axial and rotational stability both are needed in the design.

Rotational stability can be achieved by nail surface characteristics including longitudinal compressible slots, triangular cross section, interlocking screws, Expandable nail design.

Axial stability is largely provided by Interlocking screws through the nail and also provide scope for dynamisation.

Metal characteristics should be close to elastic modulus of bone. Titanium alloys are close to that requirement.

Nail bending strength can be increased by either making the nail solid which makes it more rigid,heavy and difficult to negotiate in the canal across fracture without guide wire.

The other way is to increase the diameter of the nail which allows cannulation for use of guide wire and increased bending strength as well.

Ultimately besides mechanical properties of nail bone quality and fracture anatomy determines the correct application.

Dear Michele francesco surace

I really appreciate your help.