Development and testing of a new self-locking intramedullary nail system: testing of handling aspects and mechanical properties
Introduction
Based on a procedure for distal interlocking, a new system was developed in which the interlocking bolts are inserted transverse to the bone axis, prior to the intramedullary nail [1]. During insertion, the intramedullary nail tip is designed to engage one or more of these bolts. The idea of this author was to place the bolts offset to one another and to profile the nail tip such that the previously set guiding bolts are automatically engaged, creating an alignment effect on the distal fracture fragment, in the rotational and axial directions (Fig. 1). An alternative mechanism would be to use a combined procedure that takes advantage of the natural landmarks of the bone; for example, if one could use the edge of the ventral cortex (tibial or femoral, respectively) as an additional fixation point, it would be possible to set only one bolt temporarily as a guide bolt. A specially produced drill guide would serve to aim and set the interlocking screw holes with respect to the edge of the ventral cortex. However, the advantageous orientation or guiding effect of the system (Fig. 1) would no longer apply (Fig. 2).
A first prototype model (Fig. 3) was made in order to demonstrate and test the self-guiding interlocking system. A plastic distal femoral segment (Synbone®, Filisur, Switzerland) was reamed (14 mm) and cut along the sagittal plane. Schanz screws (4.5 mm, Stratec®, Switzerland), designed to simulate interlocking bolts, were inserted offset to one another and glued to one half of the plastic bone. A model of the intramedullary nail tip was filed from a round aluminium cylinder (14 mm diameter). Based on the first prototype model, the author developed the idea further, with the goal of producing a functional prototype that could be tested and compared with commercially available systems. Two prototypes were produced, the main difference between the designs being the position and number of guiding bolts, which are placed offset to one another and correspond to the geometry of the intramedullary nail tip.
Several advantages over the current, commercially available system are intrinsic to the new design; the newly developed self-locking intramedullary nailing system is easy to use and produces an aligning effect on the distal fracture fragment. Thus, image intensification is not required for the interlocking procedure, reducing radiation exposure for the surgeon as well as the patient and shortening the length of time of the operation as a whole. The theoretical application and handling advantages of the new system are evident. In order to test both the handling aspects as well as the mechanical properties of the prototype designs in comparison with a standard, currently used nail design, an extensive biomechanical study was carried out.
The aim of this study was to test both the handling aspects as well as the mechanical properties of two prototype self-locking intramedullary nail designs in comparison with a standard, currently used nail. The Unreamed Femoral Nail (UFN) System of the AO Group was chosen as a comparative standard for this study. The nail–bone systems were evaluated experimentally in axial compression and four-point-bending modes, in a 4 cm distal defect fracture model and a 10 cm middiaphyseal defect fracture model, respectively, in 12 matched pairs of human cadaver femora.
Section snippets
Prototype preparation
In order to construct prototypes with material properties identical to those of the commercially available nail, a prototype of each new design was machined to a final length of 380 cm from three extra long titanium unreamed femoral intramedullary nails (AO/ASIF “Unreamed” Femoral Nail — UFN, Titanium-6 Aluminium-7 Niobium–TAN, 12×400 mm, Synthes®, USA). In addition, “standard” control nails were machined from the same type of extra long nails, reproducing exactly the standard nail tip and bolt
Handling aspects of prototype intramedullary nail insertion
With regard to handling aspects, for both prototypes, the interlocking screw holes were drilled prior to the insertion of the medullary nail. The ventral cortex of the distal femur served as a references for setting of the holes. A custom drill template was produced for each nail. Using this template as well as the ventral cortex as a reference point, the hole position was achieved with relative ease. The two distal bolts were inserted, followed by the nail. During nail insertion it was
Discussion
Interlocking systems offer distinct advantages over conventional intramedullary fixation techniques, particularly with regard to enhanced mechanical stability of the fractured bone–implant construct which is achieved by preventing rotation, distraction and telescoping of the fracture fragments. Furthermore, interlocking per se is a prerequisite for nonreamed nailing procedures. However, the difficulties associated with the distal interlocking procedure detract from the inherent advantages of
Conclusions
In summary, the compliance behaviour of a standard, commercially available intramedullary implant was examined in order to develop a new distal interlocking intramedullary nail system. Two prototypes were developed based on the results of these preliminary studies. In order to test both the handling aspects as well as the mechanical properties of the prototype designs in comparison with a standard, currently used nail design, an extensive biomechanical study was carried out. Based on the
Acknowledgements
The authors extend thanks to Mr Jan Piet Imken for creating the schematic drawings of the nailing mechanism, Mr Dieter Wahl for his assistance with the mechanical testing procedures, and Professor Dr Frank Baumgart for sharing his literature database.
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Observational prospective unblinded case-control study to evaluate the effect of the Gamma3® distal targeting system for long nails on radiation exposure and time for distal screw placement
2023, InjuryCitation Excerpt :While these systems have been effective in decreasing radiation exposure and distal locking time, some studies have reported difficulty using surgical navigation systems, including device malfunction, significant set-up time, and precision-related issues [15]. Self-locking nails have also been proposed as a way to decrease radiation exposure [16–18]. While earlier prototypes had inferior torsional qualities and potentially poor distal locking quality, [3] newer models have demonstrated good results in small trials [17–19].
The effect of loading in mechanical response predictions of bone lengthening
2012, Medical Engineering and PhysicsA new intramedullary nailing device for the treatment of femoral shaft fractures: A biomechanical study
2008, Clinical BiomechanicsCitation Excerpt :Excessive motion of implant bone interfaces, stress concentrations within the implant, and stress shielding of bone have been implicated in implant failures (Harris, 1979). Intramedullary nails are load-sharing devices, allowing the bone to transmit compressive forces while maintaining axial alignment (Knothe et al., 2000). Dynamic interlocking (in which screws are located at only one end of the nail) relies on interference friction of fracture fragments and muscle action to prevent rotation of the unlocked fragment.
Influence of muscular contractions on the stress analysis of distal femoral interlocking nailing
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