title: The modular endoprosthesis for mandibular body replacement. Part 2: finite element analysis of endoprosthesis reconstruction of the mandible.
abstract: BACKGROUND
  Problems with loosening of the modules for the modular endoprosthesis were encountered in animal studies for mandibular body replacement. We performed a finite element analysis to look at the stress distribution and areas of stress concentration in a human sized mandible. Variations were made to the stem and defect length to look at how the forces changed. The hypothesis was: (1) reconstruction with a modular endoprosthesis did not lead to areas of stress concentration beyond the material strength of cortical bone and titanium alloy; (2) changes in dimensions of the endoprosthesis did not cause a corresponding linear increase to the stresses.
  
  
  METHODS
  The endoprosthesis was modelled to create a male, female part with stems and a connection screw (Case I). The stem length was halved (Case II) and defect length doubled (Case III). Geometric data of a human sized mandible were obtained, a continuity defect created digitally at the right molar area and the models combined. Boundary conditions were set and the model loaded to get a bite force of 300 N at the incisor region. An intact mandible was used as a control.
  
  
  RESULTS
  The right side of the reconstructed mandible became less rigid and flexed more. The highest stresses were within the endoprosthesis at two areas of stress concentration: (1) shear stress at the superior surface of the stems close to the junction of the stem and the module body; (2) compressive stresses at the bottom bevel of the dove-tailed connection. The stress distribution for Case I and II did not differ much except for the magnitude which was slightly higher for Case II. There was a tendency for outward bending at the module connection for Case III which potentially might cause loosening of the module connection. Displacements of the mandible were less than 1 mm throughout.
  
  
  CONCLUSIONS
  The endoprosthesis with its present dimensions would be expected to perform adequately at a bite force of 300 N. An increase in defect length caused a tendency for bending at the stem and the module connection. With a decrease in stem length, there were little differences except a slight increase in magnitude.
authors.affiliation: Department of Oral and Maxillofacial Surgery, National Dental Centre, Singapore. raginwcw@yahoo.com
authors.forename: Raymond C W
authors.initials: RC
authors.lastname: Wong
authors.forename: Henk
authors.initials: H
authors.lastname: Tideman
authors.forename: Matthias A W
authors.initials: MA
authors.lastname: Merkx
authors.forename: John
authors.initials: J
authors.lastname: Jansen
authors.forename: Suk Ming
authors.initials: SM
authors.lastname: Goh
mesh_terms: D000497:Alloys / Q000737:chemistry; D001696:Biomechanical Phenomena; D001732:Bite Force; D003198:Computer Simulation; D020342:Finite Element Analysis; D006801:Humans; D008334:Mandible / Q000503:physiopathology; D063175:Mandibular Reconstruction / Q000295:instrumentation*; D008410:Masticatory Muscles / Q000503:physiopathology; D008422:Materials Testing; D008954:Models, Biological; D018583:Pliability; D019736:Prostheses and Implants; D011474:Prosthesis Design; D011475:Prosthesis Failure; D019651:Plastic Surgery Procedures / Q000295:instrumentation*; D013314:Stress, Mechanical; D013499:Surface Properties; D014025:Titanium / Q000737:chemistry
chemical_list: D000497:Alloys; C031462:titanium alloy (TiAl6V4); D014025:Titanium