Biomechanics of Intramedullary Nailing in Fracture Treatment

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Interlocking Nails

Intramedullary nailing of the bone involves use of a rod or intramedullary nail to fix the fractured bone. By forcing a metal rod into the medullary cavity of a bone to fix the fractures.

It is commonly used to fix the long bone fractures. Today, there are several types of nails available. Siora is one the successfully used best orthopaedic implant and instruments manufacturer, who offer many other implants like bone screws, bone plates, etc.

An intramedullary nail works as a form of internal splint, which stabilizes long bone fracture with minimal damage to the surrounding soft tissues. Since, they are sturdy and generally of approximately round shape, they withstand the heavy load of the body itself. Limbs with nailed fractures may be mobilized early after surgery with weight bearing, before union of fractured bone only on advice of the treating Surgeon.

Kuntscher nail has been used commonly but now is more of historical importance. It was a nail with triangular or cloverleaf cut-section without any locking mechanism. Gerhard Kuntscher is credited with the first use of this medical device in 1939, during World war II.

Most of the modern nails come with mechanism of locking. Locking is a procedure of fixing the nail to the bone using predrilled holes in the nail on both distal and proximal end for a stable fixation.

Nails that can be locked are called interlocking nails. Nails are made of titanium and stainless steel. Interlocking intramedullary nailing system is the standard today.

Modern nailing is a technique whereby the nail is inserted into the human bone from one end under an x-ray image intensifier, whilst not disturbing the fracture site at all.

Biomechanics of Intramedullary Nailing

The nail length that transmits load from one main fragment of a fractured bone to the other is known as the working length. Stiffness of a nail in both bending and rotation is inversely proportional to its working length.

The longer the working length, the greater the relative movement between the primary bone fragments.

Nails may be either hollow or solid. Solid nails are stronger than hollow ones of the same diameter simply because they have more metal for their volume.

Hollow nails are less stiff in bending than solid ones, although their stiffness may be altered by making the walls thinner or thickness. The thicker the wall the stiffer and stronger the nail.

The benefit of having a somewhat flexible nail is that it will have a little “give” in it on insertion which will make it easier to put in and will allow the nail to deform slightly to conform to the natural shape of the bone. this slight flexibility doesn’t affect the nail rigidity to support the broken bone.

Very stiff nails may damage the bone if there is any discrepancy between the shape of nail and the bone shape.

Note: Orthopedic nails are of a standard shape, but people are not, even after reaming.

One way of reducing stiffness is to put a longitudinal slot in the nail wall. This makes it much more flexible but does so at the cost of it losing overall torsional strength and bending strength.

Nail design, like that of any engineering structure, is always a compromise between including as many desirable properties as possible whilst trying to keep unwanted properties to a minimum.

Nails are curved according to the shape of the bone involved and slotted along their nail length. The wall thickness of 1.2 mm and the slot give an optimal balance between flexibility and strength and allow good contact between nail and femur when the nail is not locked.