A simple question focused on edentulous treatment options, for what might be an entry level dental implant related prosthesis, results in several case dependent scenarios. A literature search results in a large variation in recommended space requirements from 8.5mm to 12mm — what does that mean exactly?
Not many outcome-oriented treatment results in dentistry would accept 3.5mm or more of variation. What information currently available assists in the treatment planning for an edentulous patient looking for a Locator assisted or supported removable prosthesis?
Vertical height considerations
The lingering thought is this: If 8.5mm of restorative space is the minimum requirement, is it reasonable to assume that is enough, and move forward with treatment? How does more restorative space help us?
Let's say 12mm is at the top of the range for the minimum space needed. What about 15mm? What are the risks associated with 8.5mm, and the risks associated with 15mm?
The first consideration relates to what it is that we are measuring—the starting and ending points of the measurement. Let's assume that we are working with a Straumann Bone Level implant where the restorative platform is level with the alveolar bone. We will begin the measurement at this point.
- The thickness of soft tissue in a hypothetical example is 2mm
- The recommended thickness of acrylic around the metal Locator housing is 2mm
- The combination of the Locator abutment (1mm above tissue) the locator housing and attachment (2.25mm) along with a space between the attachment & abutment to accommodate for movement (1mm) equals 4.25mm
- The restorative space measurement required to embed the Locator components in the acrylic denture base adds up to 8.25mm
The 8.5mm measurement as a “minimum” for restorative space works well for a 2-implant retained overdenture where the attachments are embedded in acrylic and positioned lingual to the denture teeth in the anterior segment.
What about a scenario where the denture tooth is positioned over the attachment components?
It's possible for the visible labial aspect of the tooth to overlap the attachment components while maintaining 2mm of acrylic horizontally. The lingual height of the denture tooth that accommodates for some overlay of acrylic resin onto the surface, for example, is 3mm.
When the denture tooth is included in the vertical space measurements, now the distance is 11.25mm to the crest of the alveolar bone or in this scenario the restorative platform of the dental implant.
Think about the cross section of a denture tooth positioned over the attachment components. A posterior tooth, premolar or molar, works well with a 3mm lingual tooth height providing surface area for acrylic to overlap the denture tooth approximately 1mm—leaving 2mm if the tooth exposed relative to the denture base.
Contrast this with an anterior tooth, where 2mm of exposed tooth on the lingual or palatal aspect will result in a bulky contour leading up to the incisal edge. When the goal is to simulate a more natural contour, additional restorative space is needed in the anterior segment where 3mm of lingual tooth height may not be enough.
All the above scenarios relate to Locator abutments connected directly to the dental implant fixture. Adding a connecting bar, making use of the Locator attachment system, will increase the vertical space requirements. The height of the bar must also include the desired space between the bar and the soft tissues to facilitate cleansability.
Horizontal space considerations
Overdenture design incorporates the support of the extra oral soft tissues while at rest as well as during facial expression. The extra oral landmarks that help determine tooth position are dependent on an understanding of the soft tissue support provided by the overdenture.
This is the concept described in the LTR Classification, where the L focused on lip support, the T on tooth position and the R as the third phase focused on the ridge relative to the desired tooth position that was based on the lip support. The focus of the LTR Classification is aimed toward the horizontal component of the removable overdenture prosthesis.
The horizontal space requirements of a Locator attachment system include 5.45mm for the metal cap and 2mm of acrylic surrounding the cap. When evaluating the space in cross section, 9.45mm of space is required where the 2mm of acrylic is on the facial and lingual of the 5mm wide metal housing.
Here is where the denture tooth comes into view once again. As mentioned above, a denture tooth can overlap the Locator components if enough acrylic remains (2mm) to surround the metal Locator cap or housing.
What happens when the denture tooth is adjusted?
Layered composite denture tooth designed specifically with a layer that bonds well to acrylic and a layer that responds well to the functional forces involved in chewing. When a modern denture tooth is modified to accommodate a Locator cap, for example, it is certainly possible to compromise the bond to the denture base. The modification of the denture tooth may increase the chances of repair in the future.
What happens clinically?
Dental implants improve the retention and stability of a removable prosthesis while exposing weak points in the design that would otherwise be unnoticed.
Advertisements for direct to implant attachment systems often highlight the degree of accommodation available for angled dental implants. The angled Locator abutments add stress to the system that shows up as nylon attachments that quickly wear out, increased wear of the abutment itself, and the need for additional restorative space to accommodate the components and the path of insertion/removal. Angle correction can be accomplished using a connecting bar with adequate restorative space.
Acrylic is less than 2mm resulting in increased flexibility where either the Locator cap comes out of the denture base or the denture base fractures over the Locator housing. The material properties of acrylic processed under heat and pressure improve with increased thickness.
The goal of at least 2mm thickness of acrylic is to improve flexural strength and resistance to bending that occurs during function as well as on insertion and removal of the overdenture.
A metal intaglio framework is recommended to increase the stiffness of the overdenture with the goal of minimizing material fracture. The framework is typically made of a cobalt chrome alloy designed to limit bending and torsional challenges that could break the overdenture. The volume of the metal framework adds to the amount of restorative space required vertically and horizontally.
It is important to understand that the recommended 2mm for acrylic thickness is not reduced, but rather in addition, to the metal framework thickness and remains as part of the restorative space calculation.
Inevitably, complications arise when pushing the envelope on the material requirements. Sometimes teeth break simply due to dropping the prosthesis in the sink while cleaning it.
As teeth functionally wear over time and as the patient inserts and removes the implant retained/assisted overdenture over time, the weak point of the prosthesis will come to light providing an opportunity to discuss the next steps.
Having the discussion with the patient in advance to set the expectations is an important component of care related to implant retained or assisted overdentures.
Douglas G. Benting, D.D.S., M.S., F.A.C.P. is a member of Spear Resident Faculty.