2D-finite element analyses and histomorphology of lag screws with and without a biconcave washer

For osteosynthesis and for bone transplant fixation in particular, a lag sc rew with a biconcave washer, the so called "Anchor Screw" (AS) has been int roduced in maxillo-facial surgery. Using 2D-finite element analysis (FEA), the v.Mises and the circumferential stresses induced in underlying bone by this AS are analysed and compared to those under a conventional lag screw. The stress distributions below the biconcave washer of the AS were correlat ed with histomorphological bone reactions after AS osteosynthesis in two tu mor patients. retrieved 12 weeks and 19 months after tumor surgery, respect ively. Depending on the thickness of cortical bone, the v.Mises stress conc entrations below the biconcave washer were lower than under the head of the conventional lag screw (CLS), but with a higher stress maximum concentrate d around the rim of the washer. The circumferential stresses were only half as high around the AS, and thus the deformation of bone was reduced. As pr edicted by FEA, histology showed microcrack formation, but then after minim al resorptionl remodelling of bone below the biconcave washer. Stable osteo synthesis could be demonstrated by bony union already after 12 weeks, and, while bone remodelling continued in the healed osteotomy, it had decreased around the screws after 19 months. It can be concluded from the biomechanic al principles and the histomorphological findings that the AS appears super ior to the CLS. (C) 1999 Elsevier Science Ltd. All rights reserved.