This study is part two of research done in France. The researchers measured bone stiffness to find safe levels before removing hardware used in limb-lengthening procedures. Removing an external fixator too soon can result in bone fracture and deformity. All patients were children with limb-length differences.
In the first part of the study, a special device called an Orthometer was used during the limb-lengthening operation. While the child was anesthetized and external fixator pins were in place, stiffness measurements were taken. This was considered the reference value.
In the second part of the study, clamps were applied to the external fixator pins. A goniometer (tool used to measure angles) was attached to the pins. A force was then placed between the two sets of pins. The amount of force and the corresponding bend of the bone were both measured. Bending stiffness was measured in two directions: anteroposterior (AP) and medial-lateral (ML).
Stiffness measurements were taken at three regular intervals between the time of the operation and the removal of the fixator. At the same time, bone mineral density (BMD) was measured using DXA scanning. Results using bone stiffness measurements were compared with findings of bone mineral density on DXA scans. The authors provided all of the mathematical formulas used to calculate bone length, density, stiffness, and strength.
Healing took place in 22 to 61 days. The device remained on the child for 141 to 284 days. After the fixator was removed, four of the 22 bones fractured without trauma. Stiffness progressed faster than the authors were expecting. A value of 75 per cent of normal bone bending stiffness (density) was determined to be the safety zone for fixator removal.
This is the first time an objective guideline to assess bone healing has been provided for children after limb lengthening. The amount of bone stiffness needed for safe removal of the external fixator is similar to adults. And like adults, stiffness increases in children as bone is regenerated.
The authors admit there are some limitations both to their study and to the method used to measure bone stiffness. First, this was a small study over a short period of time. Long-term studies with many more children are needed before the results can be used routinely. It may be possible to create a chart with end-point values for monitoring children and adults. Such a guide could be used during the early healing phase after surgery.
Second, measuring bone stiffness with an orthometer during the healing phase wasn’t easy. Pain, anxiety, and muscle spasm were common. Often the measurements had to be repeated more than once. They suggest that the Orthometer may be a good way to compare other imaging methods such as the DXA used in this study. The goal is still to assess safe levels of bone regeneration. Given the limitations, it may remain a research rather than a clinical tool.