Osteochondromas in the long bones often result in limb length discrepancies. A limb length discrepancy (LLD) less than 2 cm will usually not require surgical treatment. LLD greater than 2 cm will require surgery to correct.
Surgery to correct LLD can either be done with shortening the long leg or lengthening the short leg. In children, shortening of the long leg can be accomplished through a procedure called epiphysiodesis. This is a minimally-invasive surgery where we will tether the growth plate with small metal plates, inhibiting growth. This allows the shorter leg to catch up during growth. Once skeletal maturity has been reached, epiphysiodesis is no longer an option. Shortening in adults is carried out by removing a segment of the bone and fixing the bone in place with a metal rod inserted into the intramedullary canal. The femur can be safely shortened by 5 cm (~2 inches) and the tibia by 3 cm (~1.25 inches).
The alternative to shortening is lengthening. Limb lengthening works by separating the bone into two segments via an osteotomy. A fixator (either internal or external) is applied which gradually separates the two bone segments. New bone forms in the gap resulting in increased length. Once the desired length is achieved the bone is held in place until it joints together. The new bone will grow stronger, a process called consolidation.
For more information, see Limb Lengthening Center
There are a variety of devices that are used for limb lengthening. They are can be divided into two categories: external and internal devices. The most common external fixator that we use to treat MHE is the circular Taylor spatial frame (TSF). The TSF is attached to the bone by means of wires that go from one side of the limb to the other, passing through the skin and bone, and exiting to the other side. These wires are tensioned across the rings of the TSF to strengthen them. The TSF manually lengthens the bone using a screw mechanism. The major advantage of the TSF is that it can correct angular deformities simultaneous with lengthening. The major disadvantages of the TSF are the risk of developing pin site infections and discomfort associated with the bulky external fixator.
Recently, Dr. Paley, together with Ellipse Technologies, developed a new internal lengthening nail: the PRECICE. The most recent device, the PRECICE 2 was developed in 2013. The PRECICE implants inside the intramedullary canal of the bone and lengthens similar to the way an antenna extends. An external remote control (ERC) device is used to rotate a magnet inside the nail. This, in turn, rotates gears which turn a drive screw, which extends the telescopic nail, lengthening the bone. The PRECICE is the most advanced lengthening device available on the market today. The Paley Institute has implanted more PRECICE nails than any other center in the world. The major advantages of the PRECICE are the minimal pain and discomfort associated with an internal nail, no risk for pin site infections, and safe and reliable rate control. The disadvantages are that the PRECICE cannot be implanted in small children whose bones are too thin, and it cannot correct angular deformities during lengthening. We have successfully used the PRECICE nail to correct limb length discrepancies in patients with MHE.
For more information, see PRECICE
Deciding between lengthening and shortening is based on a few factors. Shortening is only applicable for discrepancies less than 5 cm. Shortening cannot simultaneously correct deformities while lengthening can. Shortening will decrease the patient’s height, while lengthening will not. Shortening is a much smaller procedure, while lengthening is more significant and requires longer treatment time. Therefore, for a patient with less than 5 cm of LLD and no deformity who is not short or is not concerned about their height, epiphysiodesis or shortening are good alternatives. Most cases of MHE do have associated deformities, however, and therefore our preference is to perform lengthening to simultaneously correct the deformity and the LLD.
