The self adapting washer for lag screw fixation of mandibular fractures: finite element analysis and preclinical evaluation.
Besides rigid fixation, lag screws have distinct advantages compared with plates in appropriate indications in mandibular fractures. However, in current lag screw systems, the relatively small area of the screw head has to transfer the tensile force which can exceed 1000 N in the symphysis, to the thin cortical bone plate. Countersinking, which is obligatory in most systems, will weaken the cortical plate. Finite element analysis (FEA) revealed that load in this situation can exceed the normal tensile strength of metal and bone. Consequently, a new washer was constructed which both increased the supporting surface and did not require countersinking. The washer is self adapting (SAW) to the cortical plate in a defined position, forming a ball and socket joint with the screw head. Using the FEA model, a ten-fold reduction in load on bone and metal was observed with the new washer. In a miniature pig mandibular symphysis fracture model, the clinical applicability and a favourable histological reaction were demonstrated, compared with conventional lag screw designs.
Terheyden
H
H
Department of Oral and Maxillofacial Surgery, University of Kiel, Germany. terheyden@mkg.uni-kiel.de
Mühlendyck
C
C
Feldmann
H
H
Ludwig
K
K
Härle
F
F
D000497:Alloys / Q000737:chemistry; D000818:Animals; D016723:Bone Remodeling; D001863:Bone Screws; D004867:Equipment Design; D020342:Finite Element Analysis; D017102:Fracture Healing; D006253:Haversian System / Q000648:ultrastructure; D008334:Mandible / Q000473:pathology / Q000503:physiopathology / Q000601:surgery; D008337:Mandibular Fractures / Q000601:surgery*; D009984:Orthopedic Fixation Devices; D013314:Stress, Mechanical; D013499:Surface Properties; D013552:Swine; D013556:Swine, Miniature; D013718:Tensile Strength; D014025:Titanium / Q000737:chemistry
D000497:Alloys; C031462:titanium alloy (TiAl6V4); D014025:Titanium