Biomechanical Study On The Interlocking Nails Of Femoral Interlocking Intramedullary Nail

Objective:The interlocking fixation has been widely used in clinics,especially in treating the limbs fractures. Though as an effective method,it also has many postoperative complications. Nail breakage is the most serious one. The study aims at analyzing the causes of interlocking nails breakage,which one tend towards failure fragmentation and how many nails should be put in distal fracture segment.Methods:Six antiseptic adult femora were used,clearing all soft tissue and coating the distal tip with dental base acrylic resin powder. All femoral were length measured and X-ray photographed to evacuate osteopathies and measure the diameters of medullary cavities. Specimen were fixed and installed on MTS machine,put on migration sensor on each inter-locking nail,axial compression was 0-500N,average 50N/sec,auto compressed by MTS machine,migration (or strain ) auto receded from sensors and data processed in computer. Each specimen tested under six conditions at (a)-(f) sequence:(a),normal;(b),install the interlocking intramedullary nail to simulate cured fracture;(c),saw an across or slope crack to simulate the stable fracture;(d),as(c) and remove a distal interlocking nail;(e),as (c)and saw asunder a 1.5cm fragment to simulate unstable fracture;(f),as(e)and remove one distal nail. Measured the stress and strain changes on every interlocking nail and did the statistical analysis.Results:All six conditions showed no notable stress and strain changes at proximal interlocking nails (p>0.05);which were significant between distal and proximal (p<0.05);stress and strain had the outstanding changes in distal ones except the normal and stable fracture groups (p<0.05);those groups removed one distal interlocking nail,another one should endure much stressand had more strain. The stress and strain changes on the first and second interlocking nail were not significant at those unstable fractures and with one of their distal nails removed,but significant at other conditions. The third nail happened marked changes at every conditions. The forth one had the significant changes except the normal and cured fractures. Conclusions:Interlocking nails under the crack tend towards broken especially very near it,as far as possible apply two nails interlocking the distal fracture and apart from the crack. Internal fixation instruments should be removed at time when the fractures had cured or removed the interlocking nails when had the sufficient bony callus,which turn the quiescenced fixation into mobilized fixation.