Self-adapting washer system for lag screw fixation of mandibular fractures. Part II: in vitro mechanical characterization of 2.3 and 2.7mm lag screw prototypes and in vivo removal torque after healing

The aim of this study was to mechanically characterize self-tapping 2.3 and 2.7 mm titanium lag screw prototypes which are part of the newly developed 'self-adapting washer' maxillofacial lag screw osteosynthesis system. In v itro in a screw testing machine the insertion torque, maximum locking torqu e and axial force and the ultimate torsional strength were assessed. In viv o in six miniature pigs using a mandibular symphyseal fracture model, the r emoval torques after 3 and 6 months of healing were measured. Additionally the bone-metal contact (BMC) of the screws was assessed histometrically. Th e maximum insertion torque (0.185 Nm) was far below the mechanical limits o f the screws (2.3mm=0.96Nm, 2.7mm=1.6Nm). A tightening of the 2.7 mm screw with an axial force of 1000N and of the 2.3 mm screw with 500-550N leaves a safety margin of approximately one-third on the ultimate torsional strengt h. Clinically these values permit the use of two 2.3 mm lag screws or one 2 .7 mm lag screw in mandibular symphysis fractures since 1000 N tensile axia l force are required in this indication. During screw removal after 6 month s healing, torque levels close to the mechanical limits of the screws were recorded and screw failures were observed. This failure rate may have been due to the BMC of 49.8% which was in the range of titanium dental implants. Accordingly the screw heads were reinforced to prevent fractures. (C) 1999 European Association for Cranio-Maxillofacial Surgery.