Relationship between the Apex of Flexible Nail and the Level of Fracture: A Biomechanical Study
Abstract
Background:The use of flexible nails in paediatric femoral fractures is widely accepted. Flexible nails are uniformly bent in a C-shaped curve and apex (maximum curve of the nail) is matched to the fracture site. This biomechanical study analyzes how the relationship of the apex of the elastic nails to the level of the fracture influences the stiffness in a synthetic bone model.
Materials Methods: Twelve synthetic femurs were divided into three groups (n=4/group) depending on the level of fracture. Retrograde elastic nailing was performed with a pair of pre-bent C-shaped nails and inserted with the apex of the nail in middle of the shaft. We compared Group A (fracture 2.5cms distal to the middle of the shaft) group B (fracture in the middle of the shaft) and group C (fracture 2.5cms proximal to the middle of the shaft) using 4-point testing 90º (antero-posterior) and 0º (medio-lateral) to the plane of the nails.
Results: Group A showed significantly higher stiffness in antero-posterior and medio-lateral planes.
Conclusion: This study demonstrates that when the cortical entry point and the apex of the nails are on either side of fracture has highest stiffness characteristics.
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Introduction
Fixation using elastic stable intra-medullary nails (ESIN) has established itself to be a reliable mode of stabilising long bone fractures in children [1-3]. Mechanical stability of a nail-bone construct is essential for satisfactory clinical outcome.
Stability of retrograde inserted elastic stable nails of femoral fractures is created by pre-tensioning of two elastic nails by curving them in a C- shaped fashion. To get maximum stability at the fracture site it is recommended to curve the nails at the level of the fracture [4-5]. To get the maximum curve (apex) of the nails at the level of fracture is technically difficult, especially if the fracture is proximal or distal to the mid-shaft. Little has been published in comparing positions of the apex of the nails to the level of fracture in terms of bending stiffness of the construct when the fracture is not in the mid-shaft.
This study aims to understand the biomechanical relationship of the apex of the nail to the level of fracture by studying two C shaped nails inserted in a synthetic femur when the level of fracture is changed from proximal, midshaft and distal to the apex of the nails. The apex of the nails is kept constant in the middle of the bone. The change in stiffness characteristics of a flexible nail-bone construct is studied using four point bending test.
Conclusion
This study demonstrates that in a synthetic femoral model stabilised with retrograde inserted elastic stable nails curved in a C shaped fashion providing three point contact, the highest stiffness characteristics of the nail-bone construct in antero posterior and medio lateral planes is on having the cortical entry point is on one side and apex & tip of the nail are on other side of the fracture.