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Principles of clasp retention: a review.

A review is provided of various theories of the ways in which clasps act to retain removable partial dentures. The claimed roles of each component of a clasp-unit are examined. The use of undercuts, paths of withdrawal, guide planes and the relationship between retentive-force and work needed to deform the retentive arm are reviewed. Oversimplifications related to ‘classic theories’ are highlighted and the need for reciprocation is questioned.