GeneralG08BiomechanicsEditedwithQuestions幻灯片.pptVIP

* 5 Here is the initial stress in the bone due to tightening (roughly 0). When the bone is loaded along the axes of the bone, the bone around the screw is compressed on the load side of the fracture. * 7 If we compare the stresses in the bone immediately after implantation, the bone around the bi-cortical screws is already stressed from tightening whereas the bone around the locked screw is not subjected to any shear stresses. When the patient starts to weight bear, the bone in the bi-cortical screws is more highly stressed than the locked screws. * 5 Here is the initial stress in the bone due to tightening. When the bone is loaded along the axes of the bone, the bone around the screw is compressed on the load side of the fracture. As the load is increased, and the angle between the screw and the plate starts to change, the resultant loading vector on the screw changes from pure compression to a compressive and a tensile component. It is this additional tensile component which, when the elastic limit of the bone is exceeded, will cause toggling and eventual collapse across the fracture gap. * Once the angular stability of one screw is lost [by pulling thru 2nd cortex], the cycle begins and the inherent angular stability of the construct is compromised. * Resistance of larger area decreases this form of pullout. Rather the next form is necessary [see next slide]. * Through experience with failure the recommended strength of plate and number of screws in each fragment has been determined. These are general guidelines and more may be required with poor quality bone. Anatomic constraints may limit also the construct. Also it is important to note that the increased spread of screws is more important than the number of screws (provided each screw has equal purchase) for construct stability. * * * * * * * * * * * * * * * * Combined bending axial loads result in oblique fractures or those with a butterfly fragment. Figure from: Tencer A., Johnson K., Biomechanic