This is the most fundamental factor.
If the material itself lacks sufficient fatigue resistance, the restoration is more likely to fracture after prolonged stress in the mouth.
The key points to consider are:
For example, using high-translucency zirconia (5Y) in high-occlusal areas, posterior teeth, or long bridges
may not cause issues in the short term, but it is more prone to fatigue fracture over the long term.
It is not that the material cannot be used, but rather that the material selection does not match the indications.
Some materials have decent initial strength,
but once microcracks form, they propagate rapidly.
A common manifestation of such restorations is:
They appear normal initially, but suddenly fracture after six months to two years.
In many cases, this failure is not a “strength issue,” but rather “poor resistance to crack propagation.”
For example:
Unstable sintering curve
Insufficient holding time
Too rapid cooling
Poor batch-to-batch consistency
These factors lead to unstable crystal phases and high residual stresses, making the material more prone to cracking over time.
This is the most common—and most easily overlooked—source of fracture in the laboratory.
Many fractures aren’t caused by poor materials, but by design flaws that create hidden risks from the start.
This is one of the most common causes in clinical practice.
When restorations are too thin:
Bending stress increases
Tensile stress rises
Microcracks form more easily
Common high-risk areas:
Occlusal surfaces that are too thin
Anterior incisal edges that are too thin
Bridge pontics that are too narrow
Margins that are too thin
Many so-called “material issues” are essentially due to insufficient thickness in the design.
These areas create stress concentrations.
Stress is not distributed evenly;
instead, it concentrates primarily at sharp angles and transition zones, where cracks often originate.
Common locations:
Bottoms of deep fissures
Sharp cusp angles
Transition zones of bridge connectors
Lingual grooves of anterior teeth
The sharper these areas are, the more prone they are to cracking.
The most common fracture site in bridges is the connector.
If connectors are too small, too narrow, or too sharp,
they are most likely to fracture after prolonged stress.
Many bridge fractures do not occur at the bridge surface;
rather, the connectors fail first.
This is the aspect that laboratories have the most control over.
Many cracks in dentures are not actually caused by wear during use,
but are already present from the milling process.
For example:
Worn cutting tools
Mismatched rotational speed
Over-cutting
Localized vibration
These factors leave microcracks on the surface that are invisible to the naked eye.
Fractures in the mouth are simply the result of these cracks propagating over time.
Microcracks are most likely to form during occlusal adjustment, edge trimming, and reshaping.
In particular:
Dry grinding
Coarse-grit burs
Localized overheating
Failure to polish after trimming
These are very common sources of hidden cracks.
Many cases of “sudden fracture after two years of wear” actually have cracks that began on the day of trimming.
An unstable sintering process can leave residual internal stress.
For example:
Too rapid heating
Too rapid cooling
Excessive thickness variations
Improper placement
These factors can result in restorations that appear “intact on the surface but damaged internally.”
A rough surface increases the risk of crack propagation.
If the restoration is not thoroughly polished after adjustment,
micro-cracks and scratches on the surface become natural starting points for fractures.
No matter how well the restoration is fabricated in the lab, it will still fracture if the clinical usage environment is unsuitable.
High points cause long-term localized overload.
In such cases, “single-point stress concentration” is common, leading to rapid fracture.
This is one of the most common causes of fatigue fractures in dentures.
Even with normal daytime occlusion,
continuous high-frequency fatigue at night significantly accelerates crack propagation.
If the inner crown does not fit tightly, cementation is uneven, or support is poor,
the restoration will experience abnormal stress, making it prone to localized fractures.
For example:
Biting hard objects
Chewing on bones
Opening bottle caps
Chewing on one side only
All of these can accelerate fatigue failure.
The most effective strategy for the laboratory is not simply to increase strength,
but rather:
Reduce crack initiation points + Reduce stress concentration + Improve fatigue life
Focus on the following:
Posterior teeth / long bridges / high occlusion → Prioritize high-strength, high-toughness materials (3Y / 4Y)
Anesthetic zone of anterior teeth → Then consider high-translucency materials (5Y)
Do not use high-translucency materials in high-load areas where they will be subjected to excessive stress.
This is one of the most effective ways to prevent cracking.
Sufficient thickness mitigates many issues.
The lab should prioritize structural integrity over slight loss of translucency;
never compromise on thickness.
Design with smooth transitions:
No sharp cusps
No deep pits or grooves
No sharp corners
Smooth transitions at the abutments
The better the rounding, the better the stress distribution.
For bridge structures, focus on the abutments:
Sufficient surface area
Reasonable height-to-width ratio
Smooth transitions
Many bridge fractures occur at this location.
Replace cutting tools promptly
Avoid tool dullness
Use appropriate cutting paths
Minimize machining vibrations
Do not allow cracks to form during the machining stage.
Trimming is not the problem;
failing to polish after trimming is the real issue.
This is one of the points laboratories most easily overlook, yet it has the greatest impact on the prosthesis’s lifespan.
Strictly follow the sintering curve
Do not sinter too quickly
Avoid rapid cooling
Avoid extreme thickness variations
Reduce residual stress.
Many fractures are not caused by the laboratory,
but by high points in the patient’s bite.
The lab should remind the dentist:
Check for high points
Check for bruxism
Recommend occlusal relievers for high-risk cases
The most crucial point:
When a denture breaks in the mouth, it is usually not because the “material suddenly fails,”
but rather:
A crack forms first, then slowly propagates over time during occlusion, eventually leading to failure.
Therefore, the lab’s primary focus should not merely be on making the material harder,
but rather:
To prevent cracks from forming in the first place.
