Effect of Intramedullary Rod Diameter on the Bending Behavior of Sop-rod Constructs

Introduction
While conventional plate-rod constructs (PRC) are often used to spare a plate from deleterious bending stresses, the effect of an IMR on PRCs using locking plates (e.g. String of Pearls – SOP), is unknown. Our objective was to evaluate the effect of IMR diameter on bending compliance (BC) and angular deformation (AD) of SOP-rod constructs. We hypothesized BC and AD would decrease with increasing IMR diameter.

Materials and Methods
SOP groups with an IMR 24%, 32%, or 40% the medullary cavity (MC) diameter were compared to an SOP with bicortical fixation and conventional PRC. Specimens were tested in mediolateral bending; BC and AD were statistically compared (p<0.05).

Results
Construct BC and AD decreased with increasing IMR diameter (p<0.001). The SOP-24 and bicortical SOP control were statistically similar. The SOP-32 was statistically similar to the conventional PRC control. The SOP-40 was the least compliant construct (p<0.001). AD followed an identical pattern of significance.

Discussion/conclusion
Biological osteosynthesis relies on more compliant constructs to promote beneficial micromotion at the fracture site. This study suggests an SOP plate and thinner IMR, 32% the MC diameter, is comparable to a conventional PRC. Should a more compliant construct be desired (e.g. immature patients), a yet thinner IMR with a locking SOP is as compliant, and as stable as an accepted bridging construct with bicortical screws only. Previous studies have shown the SOP to be less compliant than conventional plates. Therefore, SOP constructs augmented with an IMR 40% the MC diameter may be unnecessary.