Torsional strength reduction due to cortical defects in bone

Abstract

This study correlated torsional strength reduction with circular defect size in cortical bone, to define the "stress riser" and "open-section" effect of the defects. The experimental model was developed and verified. Circular defects from 10 to 60% of bone diameter were then created in paired sheep femora and the bones loaded to failure. Contrary to theory, this experimental study suggests that small defects (10%) of bone diameter cause no significant torsional strength reduction. A 20% defect caused a 34% decrease in strength, representing the "stress riser" dimension. Defects between 20 and 60% of bone diameter decreased strength linearly as a function of defect size, and thus no discrete "open section" dimension was identified. For circular defects, we were unable to demonstrate a discrete "open-section" effect at which dramatic strength reduction is observed. These data may prove to be helpful when planning surgery that involves placing defects in bone such as for infection, biopsy, and prosthesis removal. The accepted guideline to avoid defects of greater than 50% of the bone diameter may be too great. Our data reveal this 62% reduction in torque strength and 88% energy to failure exist with a 50% circular defect.

Torsional strength estimates of femoral diaphyses with endosteal lytic lesions: Dual‐energy x‐ray absorptiometry study

“…Several previous studies have analyzed the effect of transcortical holes. 26,44,[53][54][55][56][57][58][59] We suspect that inclusion of a stress concentration factor would improve our strength estimates. However, the clinical setting was more closely replicated by not correcting for this.…”

Evaluation of the Effects of Holes of Various Sizes on Fracture Rates in Sheep Femurs

“…Such procedures prevent the formation of regions of stress concentration in their in vitro study of sheep femurs demonstrated that transcortical drill holes decreased endurance against torsional forces 16,19 . It is known that in humans, the thickness of the cortical wall decreases with age 4 .…”

“…Various studies have shown that fractures do not generally occur with small holes, indicating that the effect of stress concentration increases with an increase in the size of the hole. In addition, when the geometry of round holes is compared with that of non-round holes, a decrease in stress concentration is observed 19 .…”

Femoral nailing with transverse locking screws through the femoral neck

“…While there are concerns that the integrity of the femoral neck might be compromised, there were no femoral neck fractures or clinically significant screw failures in our series. Biomechanical tests have demonstrated that cortical defects less than 20% of the diameter of a long bone do not incur a significant stress riser or weaken the bone, predisposing it to fracture [6,7]. The average femoral neck diameter is 40-50 mm and insertion of 4-mm cancellous cross-locking screws is well within the 20% margin of safety.…”

“…When late removal is performed, the bone has had a chance to develop adaptive strain patterns, and the strength reduction is less than that of acute removal. Furthermore, within 6 weeks of screw removal, the strength approaches that of normal bone [6]. Recently, the biomechanical performance of cephalad-positioned intramedullary nails with screws placed into the medial femoral neck was reported to be equal to that of reconstructiontype nails under both axial and torsional loads.…”