Limb Lengthening for Bone Tumors
The Clinical Challenge
The scenario of the growing child with a tumor in the limb is one of the most difficult challenges in musculoskeletal oncology. When tumors are close to a center of a bone responsible for growth, removing them can result in stunted growth in the limb as the other limbs grow normally. In specific areas furthermore, these challenges can result in dysfunction around adjacent joints (eg. hip dislocations). A number of solutions have been proposed for this situation. In some centers amputations are resorted to fearing for the crippling effect misplaced solutions are apt to cause. In the very young child amputations and their variations do remain a very viable solution and should be seriously considered.
We have covered elsewhere techniques to preserve growth. Neverthless, sometimes the growth plate cannot be preserved and an obligatory stunting of the growth of the bone results either from the tumor itself, surgery to ensure clearance or radiation damage or a combination of these factors. In these instances some form of bone lengthening is employed.
How much of a discrepancy should be corrected?
It may be surprising to note that many seemingly normal people walk around with marginal limb inequalities. Therefore while it seems intuitive that limb length should be corrected until both limbs are perfectly equal the reality is this is not actually necessary. Considering that these factors significantly impact on management decisions, it has been determined that a limb length discrepancy of up to 2 cm is quite readily compensable. More than 2 cm up to 4 cm discrepancy can be corrected by shoe raises or with contralateral limb growth retardation (eg. epiphysiodesis) and anything more than 4 cm requires active surgical correction (ie. lengthening).
Therefore working backwords from the growth norms given above for example, a distal femur tumor would really only require correction in a boy younger than 12 years old because one would anticipate 2 cm to be compensable and another 2 cm to be correctable. In practice however, if a device or intiative has been incorporated to correct for length then one would aim for complete correction. For example a 10 year old boy with a growing prosthesis would be aimed for correction within 2 cm of normal although a correction to within 4 cm would suffice.
Traditionally there have been two methods of lengthening the bone - a growing or extensible prosthesis or bone transport (Figure 1).
Figure 1.Traditionally there were two methods of limb lengthening prosthesis with an extendable segment or bone transport with an external fixators. Both methods had very high infection rates.
In the course of a standard resection with endoprosthetic joint replacement, the growth issues can be incorporated into the prosthetic design by including an extendable shaft. This has been the traditional approach to the problem for nealr half a century. Unfortunately more recent reviews of this technology show and unacceptably high failure rate later in life. This occurs to due the metal reaction around the prosthesis and the effect of essentially stretching out an area of tissue that has poor blood supply. This results in long segments of scarring and a tendency to infection. In addition some models require repeat operations to open the knee and turn a screw in order to stretch the limb. Often such mechanisms will break under the sheer tensile stiffness of the scarred limb a year after surgery.
Bone transport and distraction osteogenesis
There are semantic differences between these two terms but essentially the method involves using an external frame (external fixator) that has a rail mechanism. This is pinned onto the bone and the bone itself stretched out. It was originally conceived of by Ilizarov in Poland to correct deformities caused by the second world war in children. Unfortunately patients with bone tumors are distinctly different from patients with war injuries. Bone tumor patients may have to undergo chemotherapy and radiation and may have implants next to where the tumor was removed. All this makes the risk of infection very high from the pins that go through the skin.
Figure 2. The method proposed by us is to use a completely embedded bone distractor that is powered by electromagnetic induction
We have been monitoring these technologies for a number of years and have noted the failure of the aforementioned techniques. Extensible nails as a form of internal bone lengthening (Figure 2) has become available over the last few years. Earlier models of these were simply too large to be used in Asians let alone children. More recently we have developed a method using smaller calibre nails in children especially of Asian descent. In this method a nail is put into the bone. This means that it is completely closed (unlike external fixators). Also the bed of the area to be lengthened is completely biologically viable (unlike extensible prosthesis). Our initial experience with this technique had been highly encouraging (Figure 3).
Figure 3 (Video). We present three cases here that we've done in the past. Compared to the previous methods this technique has much lower downtime and infection risk