When treating trauma patients with an IEF, the surgeon is not infrequently faced with the challenge of fractures involving relatively short-length bone fragments (for instance, in the proximal or distal tibial metaphysis). In those circumstances, it may be necessary to extend the frame beyond the involved joint in order to maximize the mechanical stability of the construct. In doing so, however, this joint is unnecessarily "locked" in place together with the fractured bone fragments. The resultant immobilisation of the articular surfaces may have detrimental implications for the final outcome as it is classically known that immobilization of the articular surfaces is associated with adverse sequelae, including stiffness or even arthritis.
As an alternative, it was thought that a twin-ring module could be used for the ring located adjacent to the short bone fragment. Our surgical team has proudly observed satisfactory clinical and functional outcomes in select trauma cases where the twin-ring module was used. It remained to be proven, however, that this configuration possessed adequate mechanical features. As a proof-of-principle procedure, we decided to run a series of biomechanical tests, in order to comparatively characterize the behaviour of single- and twin-ring IEF modules.
In axial loading, curves of load vs. displacement demonstrated a trend which was close to linear at low amplitudes (± 3 mm) and less so at higher (± 5 and ± 7 mm) amplitudes. In all cases, however, trends were continuous and quite regular, enabling us to quantify values of passive stiffness as the linear slopes between the zero and maximum displacement points. In all instances, the TR module demonstrated clearly lower values of passive stiffness than the SR module. The phenomenon of wire-pretension-loss during axial loading (long recognized and still investigated for IEF [8–12], although not detrimental in any case, is expected to be more clearly manifested in TR rather SR modules; since the increased vertical distance of TR wire levels induces successive rather than simultaneous loading of wire levels and therefore a less stiff behaviour. In the current context of TR modules, a lower stiffness in axial loading is thought to be beneficial, as it allows for the necessary axial micromotion and consequent compression between bone fragments [13].
In shear loading, curves of load vs. displacement demonstrated a trend consisting of distinct loops. The loops had a longer dimension along the x- (displacement) than along the y-axis (load). This pattern can be explained by the fact that, upon application of shear load, the plastic model simulating bone first slides against the wire aligned to the direction of load and then transfers the load to the construct. Our analysis deliberately ignored intermediate sliding events and was focused on the overall effective behavior of constructs instead. Therefore, it was conducted between zero and maximum displacement points. In all instances, the TR module demonstrated clearly higher values of passive stiffness than the SR module. A higher stiffness in shear loading is thought to be beneficial because it resists motion along the axial (transverse) plane of bone segments, potentially jeopardizing, or even disorganizing, callus formation [14].
The present biomechanical study was conceived, designed and executed in order to comparatively provide an insight to the mechanical performance of single- and double-ring modules of Ilizarov constructs. By eliminating potential confounding factors, the experimental setup and methodology ensured a comparative demonstration of biomechanical events, leading to a set of reliable findings with clinical implications.
This work may be expanded (and further research is currently under way) to encompass more elaborate mechanical testing, with different wire configurations and loading conditions (e.g. torsion). Furthermore, in an effort to create a permanent numerical model of Ilizarov constructs, computational studies involving finite element modeling and analysis (FEM-FEA) on both module configurations, can be undertaken.
The results of the present study have implications in the clinical setting. In cases of knee and ankle intra- or peri-articular fractures, it is often considered necessary to extend the IEF so as to span the involved joint, for the purposes of increased stability in bending loads. By being stiffer in shear loading, use of the twin-ring configuration may achieve an equally stable fixation without the need to bridge a joint. If a surgeon still decides to span a joint, the twin-ring module allows for earlier removal of the frame. In either case, an optimal clinical outcome is more likely. Furthermore, and possibly more importantly, in the immediate postoperative period, when the limb is immobilized for a 2-3 week period, neovessel formation occurs at the fracture region [15]. After the initiation of weight-bearing, the twin-ring system is more flexible in axial loading. This increased flexibility exploits those neovessels and promotes fracture healing, while the increased shear stiffness prevents horizontal micromotion and development of a non- or mal-union.
It is accepted that pin loosening and subsequent pin track infection [16] is a problem of rather mechanical aetiology, and usually occurs at the proximal- or distal-most ends of the external fixator.
Pin track infection also appeared in some of our cases and was treated with meticulous local cleaning and dressing, antibiotics and rarely with pin exchange. In this report this IEF treatment complication is not mentioned in detail as we principally focus on the technical aspects of TR configuration.
This complication has been attributed to local bending effects, particularly those in the vicinity of joints and can be effectively addressed either by extending the construct across the joint or by locally increasing the number of wires. Both options aim at increasing mechanical stability and enhancing bony union. The latter, however, is preferable and can be implemented more easily in a TR IEF configuration.
The TR Ilizarov construct possesses favourable biomechanical properties, which enhance fracture union and at the same time reduce complications arising from joint immobilization and pin/wire track infections. Our clinical outcomes, corroborated by the biomechanical proof-of-principle, are encouraging enough for us to recommend its continued use in patients with the appropriate clinical indications.