Limbs can be lengthened by creating new bone. An artificial break in the bone allows new tissue to form. If this new tissue is very slowly pulled apart, it stretches and allows lengthening of the bone. The new tissue gradually becomes solid over time. Typically this takes over a month for every centimetre of lengthening.
The bone is stretched 1mm each day. This can be achieved by one of two methods. One involves having a circular external fixator being applied to the leg. The other involves a telescopic metal rod being inserted within the bone.
Bones may need to be lengthened for a number of reasons. If the bone is short due to disease, injury or surgery, then this can have knock on effects such as back pain, hip pain, knee pain, ankle pain or wear and tear arthritis. The correction of a difference in the lengths in the bones can help reduce these problems, although it depends on how long they have been a problem whether the pain is completely resolved.
External Fixator Technique
The bone can be gradually pulled apart using an external fixator called a circular frame or Ilizarov frame. The metal rings in a fixator are connected to the bone underneath with wires and thicker metal pins. If the rings are pulled apart, then the bone they are connected to is pulled apart too. The bone has to be broken in order to allow the pulling apart. The mechanism to pull the rings apart can be through special rods that directly join the rings, or through a series of pistons which criss cross the space between the rings.
There are advantages and disadvantages to their use
Full weight bearing throughout the lengthening period
Working parts easy to adjust
Externally visible how much lengthening has been achieved
Less pain from application
Wires and pins lead to irritation of the skin or infection anywhere on the frame
Skin is dragged by the wires, and can lead to bunching up of skin which requires release
More pain from lengthening
Frame prevents normal clothes from being worn
Can be cumbersome and impractical
Expanding Nail Technique
The bone can be pulled apart from the inside with a metal rod within the bone. The metal rod is inserted within the bone, and is connected to the bone at both ends. A large magnetic device is applied over the top of the leg and this results in the motor in the metal rod extending the metal rod by a set amount. The bone has to be broken to allow the ends to be pulled apart. The number of times that the magnet is applied to the outside and for how long determines how much the metal rod expands. Xrays can show how much the bone has been pulled apart. Unless there are specific reasons, this is the method that we use nowadays to stretch the bone for correcting differences in leg length.
Completely within the bone
Can be worn with normal clothes
No dragging or bunching up of the skin
Less pain from lengthening
No weight bearing while the bone is lengthening and while it is calcifying
Can sometimes create an angulation of the bone
The motor can fail or the bone can fail to lengthen
More pain from application