SLR - May 2012 - Sara Karamloo
Reference: Gessman, J., Citak, M., Jettkant, B., Schildhauer, T.A., and Seybold, D. (2011). The Influence of a Weight-Bearing Platform on the Mechanical Behavior of Two Ilizarov Ring Fixators: Tensioned Wires vs. Half-Pins. Journal of Orthopaedic Surgery and Research, 6 (61).
Scientific Literature Review
Reviewed By: Sara Karamloo, DPM
Residency Program: Yale / VACT
Podiatric Relevance:
As the scope of practice for our profession continues to grow in many states, Ilizarov external fixators are being utilized more frequently throughout the podiatric community. Weight-bearing platforms are often used to ease the burden for patients who would otherwise have to be non-weightbearing for months at a time. The configuration of the external fixator combined with this platform, however, has not been well studied and may affect the final outcome.
Methods:
External fixators with two common yet different configurations were mounted onto composite tibia sawbones. After complete assembly of the frame, with respect to anatomical conditions, and tensioning, a mid-diaphyseal osteotomy was made in the same location based on the design. One design consisted of a standard four-ring frame and two 1.8-mm wires per ring, while the other was built with two ring frames and three 6-mm half pins per ring. All rings were of the same diameter and were connected by four threaded rods equidistant from one another. Next, both direct loading, in which both ends of the bone were fixed to the test machine, and indirect loading, where only the proximal aspect was fixed, were analyzed. Continuous axial loading and unloading at a velocity of 5mm/min and up to 700 N, which is equal to full weight bearing, were applied while a transducer at the osteotomy site measured interfragmentary motion, relative motion of the bone fragments to the rings and motion between the rings. The test was repeated 10 times for each frame configuration, each time using new wires and half pins. The data was then analyzed using an analysis of variance (ANOVA) and the student’s t-test. Statistical significance was considered at p < 0.05.
Results:
With respect to direct loading, an axial load of up to 200 N, equal to that of partial weight bearing, resulted in a mean compressive load equal to 189.76(+/- 3.73) in the wire frame and 186.52(+/- 7.72) in the half pin frame, at the osteotomy site. With the same conditions, indirect loading produced results reduced by 67 percent in the wire frame and 57 percent in the half pin frame. These results were statistically significant. In addition, axial loading in the wire frame resulted in purely axial fracture site displacement under both conditions, while in the half pin frame angular displacement also occurred, especially with indirect loading. Greater stiffness, equal to 25 percent, was also noted in the half pin frame under both direct and indirect loading, which was again statistically significant.
Conclusions:
When choosing an Ilizarov fixator configuration with a weight-bearing platform, is it crucial to consider the influence direct and indirect loading will have on the osteotomy site. Although half pin frames may be stiffer and easier to apply, shear forces should be taken into consideration as they can lead to an undesirable outcome.