occlusally along the axiolingual and axiofacial line angles, allowing the lock to become shallower and to terminate at the axio-linguo-pulpal or axio-facio-pulpal point angle. Jose Mondelli et al suggested three retention designs at axio-buccal and axio-lingual line angles. 1)
An angular area from the axio-gingivo-buccal and axiogingivo-lingual point angles to a narrow vertex just apical to the axiopulpal line angle.
2)
A cone shaped vertical groove which diverges towards the occlusal surface. The greater diameter is located at the level of occlusal dentinoenamal junction, while the minor diameter is located close to the axio-gingivo-buccal and axio-gingivo-lingual point angles.
3) A cylindrical groove made with straight fissure bur for deciduous teeth. Regardless of the method used in placing the locks, extreme care should be taken to prevent the removal of dentin that immediately supports the proximal enamel. Also, it is essential not to prepare locks entirely in the axial wall because no effective retention is obtained and there is risk of pulpal involvement.
Advantage: A relatively conservative method for obtaining auxiliary retention in class II dental amalgam restoration. Disadvantage: In case of wrong translation, there is risk of pulpal involvement if lock is placed too far axially.
Proximal retention locks in Box-only preparations: One of the concepts in class II dental amalgam, as suggested by Markley, is to eliminate the occlusal portion of the preparation if no caries is present. The Box-only preparation considered to be ideal for teeth in which there is no evidence of any caries in occlusal portion. 4