Effect of Intramedullary Rod Diameter on a String of Pearls Plate-Rod Construct in Mediolateral Bending: An In Vitro Mechanical Study.

Objective: To evaluate the effect of intramedullary rod (IMR) diameter on the mechanical behavior of string of pearls (SOP) plate-rod constructs.

Study Design: In vitro mechanical study.

Sample Population: Synthetic bone models (n = 24).

Methods: Locking 3.5 mm SOP plates were fixed to a tibial bone model with a 50 mm fracture gap. Four experimental groups (n = 4) were tested: monocortical SOP construct alone and monocortical SOP constructs augmented with a 2.4, 3.2, or 4.0 mm IMR corresponding to 24, 32, or 40% filling of the medullary cavity diameter (SOP-24, SOP-32, SOP-40). Control groups (n = 4) were stabilized with either a bicortical SOP plate (SOP-B) or a 3.5 mm limited contact dynamic compression plate (LC-DCP) with a 4.0 mm IMR filling 40% of the medullary cavity diameter (LC-DCP-40). Specimens were tested in mediolateral bending. Construct compliance (CC) and angular deformation (AD) were compared between construct types (P < .05).

Results: CC and AD incrementally decreased with increasing IMR diameter (P < .001). There were no statistical differences between SOP-24 and SOP-B (P = .806) or between SOP-32 and LC-DCP-40 (P = .773), which was also the least compliant of all constructs (P < .001). AD followed an identical pattern of significance.

Conclusions: Biological osteosynthesis often relies on more compliant bridging constructs to promote beneficial micromotion at the fracture. Our study suggests use of a smaller diameter IMR (SOP-32) is comparable to a conventional plate-rod construct (LC-DCP-40). Should greater compliance be desired, an even smaller diameter IMR (SOP-24) may prove beneficial while as stable as an accepted bicortical construct (SOP-B).