Rigidity of pure lag-screw fixation as a function of screw inclination in an in vitro spiral osteotomy

The rigidity of lag-screw fixation of experimental tibial fractures was tested in 27 anatomic specimens. Spiral osteotomies with a 60 degree angle of ascent were produced using a Gigli saw. Three screws with an axial force of 100 kp were inserted at 90 degree, 80 degree, or 70 degree of longitudinal inclination. The 70 degree group was stable in axial compression up to 88 kp and the 80 degree group up to 151 kp. Rigidity in bending was 50% in the 70 degree and 80 degree groups and 27% in the 90 degree group, while rigidity in torsion was 20% as compared with the intact bone. Screws tightened with 50 kp and 150 kp showed increased rigidity as axial screw force increased. There were only small gains in torsion and bending, whereas there was a 200% gain in compression. This confirms the view that in long spiral fractures of the tibia three lag screws alone can maintain sufficient stability to permit early limb mobility. If the screws are inserted perpendicular to the bone, the high resistance to axial compression may permit early partial weight-bearing.