[A three-dimensional finite element analysis on relationship between abutment undercut and retentive arm width of...

To analyze the stress distribution on cast retentive clasp arms in dislodging denture, and to discuss the deepest undercuts of the second mandibular premolar (abutment) for cobalt-chromium alloy cast clasps with different widths.

Three-dimensional finite element models of the abutment with different depths of undercuts and retentive arms with different widths were set up. Dynamic displacement load (3 mm/s) was exerted on the middle of the retentive arms to analyze the stress in retentive arms while they were being removed from the abutment.

The peak stress in retentive arms was positively correlated to the undercuts displaced by clasp tips, and those were not obviously related to the undercuts displaced by the middle of retentive arms. When width/thickness of retentive arms was 3, the increase of peak stress of retentive arms with similar locations of clasp tips was significantly related to the increase of the arm width. The deepest undercuts of the second mandibular premolar for cobalt-chromium alloy cast retentive arms with different widths of 1.8 mm, 1.6 mm, and 1.4 mm were 0.25 mm, 0.30 mm, and 0.35 mm respectively.

When width/thickness of the retentive clasp arm is fixed, the wider the arm is, the smaller depth it should be placed on the undercut of abutment. Retentive clasp arms with different widths should be placed on different depths of undercuts in order to prevent their permanent deformation.