All-on-4 Planning Calculator
Cantilever and angulation planning index.
About this calculator
This calculator estimates the anterior-posterior (A-P) spread and cantilever ratio for a full-arch implant-supported prosthesis built on the All-on-4 concept, where two axial anterior implants and two distally tilted posterior implants support a fixed arch. A-P spread is the horizontal distance between the most anterior and most posterior implant contact points; tilting the posterior implants 30-45 degrees (rather than placing them vertically) lets them engage more available bone anterior to the maxillary sinus or mental foramen while pushing the effective A-P spread wider, which is the whole biomechanical premise of the technique. Arch length is the dominant driver of A-P spread here because it sets the anterior-posterior envelope the implants are distributed across; posterior implant angle contributes a smaller secondary effect through the tilt geometry.
The cantilever ratio (distal cantilever length divided by A-P spread) is the standard chairside proxy for how much unsupported lever arm the prosthesis carries beyond the implants — a ratio at or below 1.5 is the commonly cited biomechanical ceiling, with 1.0 or less preferred for safety margin. This tool does not account for bone density or quality, individual implant diameter and length selection, screw- versus cement-retained design, occlusal scheme, parafunction, or the CBCT-based 3D planning a real surgical case requires — it is a planning-stage index, not a substitute for guided surgical planning or a prosthodontist's case-specific workup.
Medical Disclaimer
This calculator is for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about your health. Never disregard professional medical advice or delay seeking it because of results from this tool.
Inputs
Results
A-P spread (mm)
29
How to Use This Calculator
- Enter Arch length (mm), Posterior cantilever (mm), and Anterior implant angle (deg).
- Set Posterior implant angle (deg).
- Review the A-P spread (mm) result.
- Use Cantilever ratio and Cantilever acceptable (0/1) to inform your decision.
How the result changes with Arch length (mm)
| Arch length (mm) | A-P spread (mm) |
|---|---|
| 30 | 16.5 |
| 41 | 22 |
| 80 | 41.5 |
What each input means
- Arch length (mm)
- Anterior-posterior distance of the edentulous arch.
- Posterior cantilever (mm)
- Length of distal cantilever extension beyond last implant.
- Anterior implant angle (deg)
- Angulation of anterior implants (typically near-vertical).
- Posterior implant angle (deg)
- Angulation of posterior implants (typically 30-45 degrees).
What each result means
- A-P spread (mm)
- Anterior-posterior spread between outermost implants.
- Cantilever ratio
- Cantilever length relative to A-P spread (should be <=1.5).
- Cantilever acceptable (0/1)
- 1 = cantilever within acceptable biomechanical limits.
- Max cantilever (mm)
- Maximum recommended cantilever for this A-P spread.
- Teeth in prosthesis
- Recommended number of teeth in the full-arch prosthesis.
- Tilted implant length (mm)
- Effective bone engagement length of the tilted posterior implants.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersArch length (mm) = 55, Posterior cantilever (mm) = 12, Anterior implant angle (deg) = 0, Posterior implant angle (deg) = 30 = 4 input(s) provided
- Calculate A-P spreadA-P spread29 = 29
- Calculate Cantilever ratioCantilever ratio0.41 = 0.41
- Calculate Cantilever acceptableCantilever acceptable1 = 1
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
How is A-P spread calculated for an All-on-4 arch?
A-P spread is derived from half the arch length minus half the fixed 10mm anterior-implant spacing (the anterior implants sit near, not exactly at, the midline), plus the horizontal offset the posterior implants gain from being tilted, calculated as 13mm of implant length times the sine of the tilt angle. A wider arch or a steeper posterior tilt angle both increase the resulting A-P spread.
What cantilever ratio is considered safe on an All-on-4 prosthesis?
A cantilever ratio of 1.5 or less (cantilever length divided by A-P spread) is the widely cited upper limit before distal cantilever loading is considered biomechanically risky; many clinicians target 1.0 or below for extra safety margin, especially in patients with heavier occlusal forces or bruxism.
Why are the posterior implants tilted instead of placed vertically in All-on-4?
Tilting the posterior implants 30-45 degrees lets them anchor in denser bone anterior to the maxillary sinus or mental foramen while avoiding those anatomic structures, and the tilt itself increases the effective A-P spread compared to a vertical implant of the same length, which shortens the effective cantilever and improves load distribution across the arch.
How many teeth does a typical All-on-4 prosthesis replace?
A full-arch All-on-4 prosthesis typically replaces 10 to 14 teeth, scaled to the total prosthetic span (arch length plus both distal cantilevers) at roughly one tooth per 7mm of span, since a longer supported arch accommodates more pontics within the same four-implant framework.
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