Introduction
Cast clasps are cast in one piece with the metal framework of a partial denture. The Ney system organizes their design around the dental surveyor: each abutment is surveyed, the survey line is drawn, and the clasp type is chosen to match that line.
The dental surveyor
A surveyor (parallelometer) holds the cast on a tilting table under a vertical arm. The first surveyor was introduced in 1918, and Ney produced the first widely used instrument. It is used to:
- Choose the path of insertion by tilting the cast.
- Draw the survey line (height of contour) on each abutment with a carbon marker.
- Measure undercut depth with gauges.
- Find guide planes and block out unwanted undercuts.
- Check parallelism of crown preparations, attachments and milled surfaces.
Undercut depth: the Ney gauge
How deep the retentive tip may go depends on how flexible the arm is:
Too deep an undercut makes the clasp overstress the tooth or deform; too shallow, and the denture won't stay in.