Disadvantages of gypsum products

Despite the many advantages, gypsum products also have several inherent limitations which affect their use in clinical and laboratory dentistry. The key disadvantages include brittleness, low tensile strength, hygroscopic expansion, moisture susceptibility, and limited shelf life. Here is an in-depth understanding:

1. Brittleness

·         Nature: Gypsum is inherently brittle due to its crystalline structure and mode of setting reaction.

·         Clinical Impact: The brittle nature makes gypsum casts and dies prone to fracture or chipping, especially in thin sections like die margins or occlusal surfaces.

·         Handling: Excessive force during cast removal from impressions or trimming can cause cracks or fractures.

·         Mitigation: Use of resin-modified stones or reinforcement additives can improve toughness but increase cost and complexity.

·         Examples: Die stones (Types IV & V) are less brittle than plaster but still require careful handling.

2. Low Tensile Strength

·         Contrast to Compressive Strength: While gypsum has moderate to high compressive strength, its tensile and flexural strengths are considerably lower due to brittleness.

·         Consequences: Tension during removal or handling may cause premature failure or fracture.

·         Clinical Relevance: Since fractures often occur in tension zones, tensile strength is a critical parameter in clinical endurance of gypsum casts.

·         Numerical Values: Tensile strength is typically less than 10% of compressive strength in dental stones; in plasters even lower.

3. Hygroscopic Expansion

·         Definition: Expansion occurring when gypsum sets or remains immersed in water or humid conditions during or after the setting.

·         Effect: Causes dimensional changes that may affect the accuracy of the cast or die.

·         Extent: Hygroscopic expansion can be about twice the normal setting expansion (0.2–0.3%) and lead to inaccuracies in final prostheses fit if not controlled.

·         Sources: Setting expansion due to crystal growth thrusts combined with absorption of free water.

·         Clinical Concerns: Dental investments use this property positively to compensate for metal shrinkage; however, uncontrolled expansion is problematic.

·         Prevention: Proper storage, use of retarders and accelerators by manufacturers, and avoiding prolonged wet storage of sets.

4. Susceptibility to Moisture

·         Moisture Sensitivity: Gypsum powder is hygroscopic, readily absorbing atmospheric moisture.

·         Consequences on Powder: Powder becomes lumpy, begins premature setting if exposed to high humidity (>70%), affecting setting time and final strength.

·         Consequences on Set Casts: Set gypsum absorbs moisture when soaked in water, causing softening, surface degradation, and loss of dimensional accuracy.

·         Storage Needs: Require airtight containers in dry environments for powder; casts need stored away from moisture or soaked only in saturated slurry water.

·         Clinical Importance: Improper storage leads to inconsistent laboratory results and possible failure of prostheses or appliances.

5. Limited Shelf Life

·         Shelf Life of Powder: Although chemically stable, the shelf life is limited mainly due to environmental factors like humidity and contamination.

·         Degradation Over Time: Powder exposed to moisture may partially set underground, reducing flowability and causing inconsistency in setting time and strength.

·         Storage Discipline: Requires that powder containers be tightly closed and stored in cool, dry places. Frequent opening exposes powder to contamination.

·         Set Material: Casts and dies do not have the same shelf-life limits but must be used or preserved properly to maintain accuracy.

·         Lab Impact: Expired or poorly stored powder affects work quality, causing loss of detail and increased porosity.

Summary Table

Disadvantage	Description	Clinical Impact	Mitigation
Brittleness	Brittle crystalline structure	Fracture, chipping of casts/dies	Careful handling, resin fortification
Low Tensile Strength	Much lower tensile vs compressive strength	Failure under tension	Choose stones with higher toughness
Hygroscopic Expansion	Expansion when exposed to moisture	Dimensional changes, inaccurate fit	Controlled setting, avoid soaking
Moisture Susceptibility	Powder and set cast absorb moisture	Premature setting, weakened casts	Airtight dry storage, slurry water soak
Limited Shelf Life	Affected by storage conditions	Inconsistent setting & strength	Proper storage, discard expired powder

These disadvantages must be considered when selecting gypsum products and in the protocols for storage, mixing, and usage in the clinical and laboratory environment. Proper technique and material choices mitigate these limitations to maximize clinical success.