Introduction

Intramedullary locking nails are frequently used for long bone fracture treatment. This type of osteosynthesis requires proximal and distal nail locking in order to keep the length and lock the the bone's rotation. While proximal locking is usually performed automatically using a guide, the distal locking can be problematic as the nail undergoes some deformation inside the bone [1]. A misaligned distal locking screw can induce additional stress and damage the bone. A better understanding of this side effect could improve surgical techniques. The aim of this experimental study is to analyse the fracture propagation and to quantify the observed damage using micro-CT imaging.

Methods

Forty-five cylindrical specimens (length: 65mm, diameter: 20mm) were extracted from ewe[2] femoral diaphysis.

Micro-CT images (17.2µm) were acquired before and after the experimentation in order to explore the initial bone architecture and its damage after experiments. As most of the bony damage occurred at the metallic rod and the bone contact area and just below, regions of 2mm high below the rod were created, starting 1mm below the rod. The first 15 mm of the specimen were divided by 2mm high regions while, in order to follow the fracture line propagation the regions were expanded to 5mm for the next 10mm of the specimen and then 7.5mm and 13mm. The zones were evaluated using native transversal micro-CT slices taken from the middle of the evaluation zones.

Quasi-static compressions were performed through a 4mm cylindrical metallic rod (equivalent to a locking screw) placed on the specimen's extremity. Global mechanical behaviour such as the apparent stiffness and rupture forces were registered. Specimens were divided into three groups of 15, defined by the equivalent maximal strain applied: 1.5%, 3% and 6%.

Results

An initial linear behaviour with an associated apparent stiffness of 2648±873N/mm was observed in all cases. According to the differences observed, the mechanical behaviour was classified in 4 categories: yielding without rupture, ductile or fragile rupture, and “double spike” showing a separated cortical bone rupture.

Yielding occurred in 23 specimens (5 followed by a complete rupture) while a complete rupture occurred in 15 specimens (10 without yielding).

Specimens that underwent a fragile rupture showed longer fracture propagation compared to ductile fracture. The fragment displacement was greater in those cases. Rupture forces were measured between 2000N and 4000N.The fracture line's propagation length, the fragment displacement, the two or a single cortical bone fracture involvement were correlated to the different curve shapes.

The microstructure observation of the 3 “double spike curve” specimens confirmed a double cortical bones fracture. A similar observation was made for specimens that presented a fracture propagation line beyond 5mm.

Discussion

To our knowledge, there were no studies evaluating different forces and potential bone damage at the distal locking screw - bone interface.

Our protocol involved an animal bone cylinder and a metallic rod instead of the locking screw. The mechanical testing was quasi-static compared to a cyclical loading in a clinical setting. However, the rod displacement of about 1mm to 4mm was sufficient to cause a rupture in 1/3 of the specimens.

Our protocol allowed to evaluate different mechanical parameters and correlate them to imaging. Results let identify the limit values of rupture and study the bone damage patterns from initiation to propagation and final rupture.

References

 [1] Krettek et al. The deformation of small diameter solid tibial nails with unreamed intramedullary insertion. J Biomech. 1997Apr;30(4):391-4.

 [2] Pearce et al. Animal models for implant biomaterial research in bone: a review. Eur Cell Mater. 2007Mar2;13:1-10.