Intramedullary Pins

Previous

Intramedullary pins are the simplest implant used in the management of long bone fractures. The medullary cavity is another name for marrow cavity in the centre of the tube like bone. As the name suggests, an Intramedullary pin is placed in the cavity. Being made of solid surgical stainless steel, these pins are strong. By placing them at the centre of the bone, they resist bending forces very well. This position however and there smooth circular cross section means that they are not good at resisting rotational forces. Sometimes the fracture line is angled or irregular and this itself will lock the top and the bottom sections, limiting rotation. This would be an ideal and simple way to manage this type of fracture. If the fracture line is Transverse (straight across) as in figure 2, then, since  the fracture and the pin are poor at resisting rotation, failure is highly likely.

Intramedullary Pins can be used in the Femur, Tibia, Metatarsal bones, Humerus, Ulna and Metacarpal Bones. Although their use is described in the radius, they are rarely used as they have to be introduced from the carpus (wrist) and this can cause Degenerative Joint Disease of this joint.

Fractures appropriately managed by Intramedullary Pins heal very quickly as the micro instability that they permit results in a large stabilising callus (scar) which then transforms into bone. Healing will generally take 6 to 8 weeks. This process of healing is called "Healing by Secondary Intention".

Problems seen with the sole use of Intramedullary Pins are:

1. Failure to eliminate rotation resulting in Non Union or Malunion.

2. Migration of the pin. This is generally due to persistent rotation or at least excessive motion at the fracture site. This can result in penetration of a joint or result in problem 3 below.

3. In Femoral fractures this may result in sciatic nerve injury causing pain and sometimes Nerve Injury and paralysis of the hindlimb.

4. Collapse of the fracture. This device is inappropriate fro fractures with multiple unreconstructable fragments in the central section as the pin cannot resist the "telescoping" effect of the muscle contraction (fig 3).

Most problems can be avoided by supplementing an Intramedullary pin with another device that resists rotation (Plate and Screws, Screws, External Fixator).

Torrington Orthopaedics, IES House, Mission Street, Brighouse, West Yorkshire, HD6 1NQ
Telephone 0844 880 8051 Fax 01484 404771
Contact me: andy@torvet.co.uk