Why an overdenture instead of full crowns on implants?
An overdenture is a compromise: a removable prosthesis (patient removes it) + stabilization on an implant/implants.
When it makes sense:
- The patient has insufficient bone for implants (implants = expensive, due to necessary augmentation)
- The patient has a limited budget — an overdenture on 2 implants (mandibular) costs less than 4–6 crowns on implants
- The patient wants the option of removal (older patients, difficulty with hygiene around crowns)
- Bone continues to resorb — an overdenture is easier to reline (vs. all crowns needing to be redone)
Overdenture geometry — what changes in the design?
Compared to a regular removable partial denture, an overdenture has added complexity:
- Recess for the attachment — where the implant anchor will seat
- Geographical position of implants — must be in pre-defined positions (usually along the premolar line)
- Angulation of implant axes — must be almost vertical (if the implant has a 30° angulation, the attachment will lose function)
Retention systems — ball, locator, bar
Ball Attachment
A metal device on the implant (like a small ball). The denture has a recess — the ball "snaps" into the recess and holds the denture.
Advantages:
- Simple, easy to repair (just replace the rubber O-ring / matrix)
- Inexpensive (less than locator)
- Well-known to technicians
Disadvantages:
- Less precise (O-ring wears out in 1–2 years)
- Requires regular replacement
Locator Attachment (precise)
A newer system. A device on the implant + a corresponding insert in the denture. More precise than a ball.
Advantages:
- High precision (tolerances up to 0.2 mm)
- O-ring lasts 2–3 years (vs. 1–2 in ball)
- Less "play" between the denture and implants
Disadvantages:
- More expensive
- Requires precise implant positioning
Bar Attachment
A bar-shaped device connecting 2+ implants. The denture rests on the bar.
Advantages:
- Highest stability (even with lateral forces)
- Less load on a single implant
- Ideal for patients with bruxism
Disadvantages:
- Most expensive
- Requires precise alignment of 2+ implants
- More difficult to repair in the clinic
Number of implants — how many are needed for stability
Mandible (intermaxillary space):
- 2 implants = minimum. An overdenture on 2 mandibular implants is de facto standard (supports 80–90% of chewing force)
- 3 implants = optimal (distributes load, more stable)
- 4+ implants = if the patient has bruxism or wants maximum certainty
Maxilla:
- 3–4 implants (cranium is larger, greater dispersed forces)
- Harder to be minimal — better to install more
Implant placement and overdenture design
If there are 2 implants — they should be in the premolar area (so that loads are even, not tilting the denture).
If 3 — they can be arranged trianglarly (stable in all directions).
An error: Implants too close together (< 8 mm) → bone between them resorbs → implants become mobile.