Principles

An impression is necessary to transfer the position and design of an implant or abutment to a master cast. Additionally, this captures the soft-tissue contours that frame the restoration, providing the pink aesthetic frame around the tooth. An impression is usually made at the implant level or abutment level with an elastomeric impression material such as polyether or polyvinyl siloxane. A poor impression may lead to an inaccurately fitting prosthesis or compromised result, which may add to the cost incurred as well as extend treatment times and inconvenience for the patient.

The implant impression will provide:

• Position
• Depth
• Axis/angulation
• Rotational position
• Soft-tissue contour (emergence profile).

The requirements of impression materials include:

• Accuracy
• Rigidity yet resilience
• Ability to be removed from undercuts
• Dimensional stability.

The use of polyether or additional polyvinyl siloxane has been recommended. Silicone impression materials have better biomechanical stability than polyether, which is susceptible to moisture and sunlight. Silicone tends to have a favourable modulus of elasticity that allows simpler removal from the mouth, especially in soft-tissue undercuts.

The fit of implant prostheses requires extreme accuracy due to the precise machine fit and the rigid connection to bone. Implants do not have a periodontal ligament that may allow for minor inaccuracies such as in teeth. Impressions for multiple implant restorations are even more critical. A passive fit is the objective, as a misfit may lead to stress placed on the implants, resulting in bone loss and even loss of integration.

Impression techniques used in implant dentistry include abutment-level impressions, both direct and indirect; and implant-level impressions, both pick-up (open-tray) and transfer (closed-tray). See Table 11.3.1 for a comparison of the latter types.