Ceramic fracture in metal-ceramic restorations: the aetiology

Trauma

Physical trauma is one of the major causes of porcelain fracture.¹⁶ Low fracture toughness makes porcelain a brittle material.¹⁷ Any blow to the restoration, whether due to a fall, a fight, a road-traffic or sports accident, will result in the immediate fracture of the porcelain.

Occlusal interferences

Occlusal interferences have important implications when contemplating anterior crowns.¹⁸ Any premature contacts in centric and eccentric movements generate increased localized stresses in the porcelain. These stresses create ‘Hertzian cone cracks’ which may lead to chipping fracture of surface porcelain.¹⁶

Increased overbite

In increased overbite cases, where the patient exhibits a great amount of vertical overlap with only a moderate amount of horizontal overlap, non-axial stresses are generated.¹⁹ Excessive non-axial forces may lead to fracture of the restoration. Designing a prosthesis, such that the non-axial forces are reduced, will increase the longevity of the restoration and the restored tooth.²⁰ However, this factor affects anterior restorations only.

Parafunctional habits

Parafunctional habits, namely clenching and bruxism, are characterized by dynamic repetitive loading.²¹ Parafunctional habits expose the restorations to greater and often unfavourably directed occlusal loads, thereby increasing the risk of mechanical failure.²² The risk is significantly higher in patients who do not use a protective occlusal device.²³ Abnormal biting habits, such as nail-biting or biting a pen or pencil, exploit the brittle nature of porcelains and can lead to their fracture.

Injudicious use

If the patient, even after being instructed otherwise, uses the prosthesis injudiciously such as to crack hard nuts, to bite harder foods such as sugar cane or bones, it is likely to result in the failure of the restoration.¹⁶

Acidic beverages

Common beverages with low pH ranges have been shown to promote the breakdown of glass-containing dental restorations.^11,14 This occurs because of release of basic ions which are less stable in the glassy-phase. Such a breakdown results in surface roughening of dental ceramics, thereby decreasing strength and promoting failure.²⁴

Insufficient tooth reduction

An uneven tooth preparation may result in a porcelain layer of uneven thickness, creating areas of stress concentration and eventual fracture.¹⁴ Insufficient tooth reduction yields too little space to accommodate both the metal substructure and porcelain.²⁵ The result may be an over-contoured, bulky, opaque-looking crown,²⁶ or, if the porcelain is too thin, it will be more liable to failure.

Knife-edge margins

Knife-edge margin designs have been shown to be more susceptible to chipping and fracture, especially during the try-in and cementation.¹⁵

Inadequate impression recording

This factor affects all restorations, and not just metal–ceramic ones. An impression of the prepared tooth that has been poorly recorded, with no attention to details, will result in a restoration more likely to fail, both in aesthetics and in function.²⁵ In addition to impressions, occlusal registration may also affect the accuracy of a restoration. Dental laboratories receive a large number of unreliable and poorly recorded bite registrations.²⁷ Incorrect registration of occlusion and articulation yields premature contacts. Premature contacts, if not detected and relieved, act as stress-bearing zones on ceramic.¹¹

Use of weak material with low fracture toughness

Fracture toughness is the ability of the material to resist crack propagation when subjected to tensile stress.¹⁵ Materials with low fracture toughness are more prone to fracture. Of all the ceramics, traditional feldspathic porcelain has the lowest fracture toughness of 0.7 MPa.m^1/2.¹⁷

Elastic modulus of the metal

The support available for porcelain by the framework is directly proportional to the elastic modulus of the metal.¹¹ The higher the elastic modulus, the stiffer will be the material and better able to resist deformation under loading. An alloy with low modulus of elasticity will flex under loading, yield poor support to porcelain and increase the risk of porcelain fracture.²⁸

Presence of scratches or pits on ceramic

Scratches, pits or similar flaws present on the surface of the ceramic material behave as sharp notches with narrow tips. Tensile stresses, generated during occlusal loading, are concentrated at the tips of these defects, leading to crack propagation and fracture.^14,15

Thermal incompatibility of materials

A large difference in the coefficient of thermal contraction of metal and ceramic, where ceramic contracts more than the metal, can generate excessive tensile stresses in the ceramic layer, thus promoting fracture.¹⁴ Moreover, if the veneering porcelain has a coefficient of thermal contraction lower than that of the core porcelain, tensile stresses will be generated at the framework surface, making the material more prone to fracture.^7,11 Such a thermal mismatch between the core porcelain and the veneer porcelain may lead to increased failure of metal-ceramic systems.

Fatigue failure

All intra-oral restorations are exposed to small alternating forces during mastication. Such repeated loading may lead to fatigue failure of the restoration.^11,14

Low thermal conductivity

Low thermal conductivity of core porcelain, as compared to that of veneering porcelain, creates a temperature difference between the core and the veneering porcelain.¹⁵ Tensile stresses arise in the deeper layers of the material and facilitate crack propagation.

Ageing

Premature failure of restorations may ensue in the humid oral cavity. The oral environment expedites the ageing of dental ceramics, reducing flexural strength and lowering fracture toughness. Studies have shown that silicate bonds present in ceramics are susceptible to hydrolysis by moisture present in the oral environment.¹⁷ The phenomenon, termed as ‘static fatigue’, is further exaggerated in the presence of mechanical loading.¹¹ Ceramics undergo ‘stress corrosion cracking’ in the presence of water.²⁹ This results in a reduced metal-ceramic bond strength, leading to crack propagation and eventual failure of the restoration.

Type of prosthesis

Metal-ceramic restorations on implant-supported prostheses are more prone to fracture as compared to the ones on tooth-supported prostheses.³⁰ This is probably because implants lack the resilient periodontal ligaments and their associated neurosensory mechanisms that help in the detection of excessive occlusal loads or occlusal interferences.³¹

Coping design

Metal coping design is paramount to the success of metal-ceramic restorations and yet it is often overlooked. The coping must be designed so as to allow the porcelain to remain in compression by supporting the incisal region, the occlusal table and the marginal ridges³² or the unsupported porcelain will fracture.

A metal–ceramic restoration is likely to fail if the coping does not meet six important design features including:

Porcelain thickness: Relatively thin porcelain, of uniform thickness and supported by rigid metal, is strongest. The absolute minimum thickness of porcelain is 0.7 mm, and the desirable thickness is 1.0–1.5 mm. Extensions of porcelain beyond 2.0 mm are prone to fracture, even if these thick areas of porcelain are not in areas of force concentration.¹¹ Stresses generate in the thick bulk of porcelain during initial firing and cooling.

Support of porcelain: Porcelain veneer that is supported by an evenly contoured metal framework is better able to withstand stress. The metal should be contoured so that the overlying veneering porcelain will be subject to compressive forces when a load is applied. Examples of this consideration include avoiding the extension of lingual metal to the incisal edge of a maxillary anterior restoration (Figure 4) and establishing a supporting ledge under the facial cusp of a maxillary premolar or molar metal-ceramic restoration (Figure 5).³² Failing to meet these criteria will expose the brittle porcelain veneer to shearing forces and may lead to premature porcelain fracture.

Metal thickness: Copings should be rigid to avoid flexing during seating or under occlusal loads. Rigidity of the coping is directly proportional to the thickness of the metal used. A noble metal coping should be at least 0.3–0.5 mm thick, while a base metal alloy with a higher yield strength and higher melting temperature may be as thin as 0.2 mm.¹⁴ A metal coping with lesser thickness is likely to flex and cause porcelain fracture.

Occlusal/proximal contacts: Contact near the metal–ceramic junction can lead to metal flow and subsequent porcelain fracture. The porcelain-metal junction should be placed 1.0 mm from occlusal contacts at the position of maximal intercuspation.^25,33 Proximal contacts for anterior teeth should be on porcelain and the metal should be placed more lingually. Placing the porcelain-metal junction lingual to the proximal contact areas leads to improved stress distribution.^11,32

Extent of area to be veneered: The porcelain on the facial surface usually extends over the cusp tip and about halfway down the palatal incline of the facial cusp on posterior teeth (Figure 6a). There must be a rounded ledge of metal under the facial cusp to support the porcelain. Without a supporting ledge, the ceramic will fracture.¹¹ This design is more resistant to fracture than those in which the porcelain extends to the central groove or covers the entire occlusal surface (Figure 6b).²⁵

Design of facial margin: Porcelain-covered metal margin design requires either a heavy chamfer or a beveled shoulder finish line with the metal coping extending to the cavo-surface margin and thinned to the minimum thickness possible.²⁵ Porcelain is extended to cover this metal. This design may lead to metal distortion during firing and metal flexure with resulting porcelain fracture as a result of excessive thinning of the coping.³²

Lack of aesthetics with the conventional metal collar led to the use of the all-porcelain facial margins.³⁴ They utilize special shoulder porcelains that are stronger in flexure than conventional porcelains,¹⁵ making the margin more resistant to fracture.

Antero-posterior span of fixed partial denture

Long span fixed partial dentures flex more under heavy occlusal loads, leading to the fracture of porcelain. If all other parameters are kept constant, a four-unit fixed partial denture replacing two teeth will bend eight times more as compared to a three-unit fixed partial denture with a single pontic (Figure 7).^11,15,25 Where a large number of missing teeth need to be replaced, considering an implant-supported prosthesis may help decrease the span length and yield better results.¹¹

Thickness of pontics

Occluso-gingival thickness of the pontic affects the deflection of framework. Deflection varies inversely with the cube of the occluso-gingival thickness of the pontic. If the pontic thickness is halved, the framework deflection will be increased eight times.^24,32

Design of connectors

For clinical longevity, connectors of a fixed partial denture should be thick enough to resist the occlusal loads.^25,32 However, for optimal aesthetics, occlusal and gingival embrasures must be created.¹¹

Poor porcelain adaptation

One of the most common fabrication flaws is the incorporation of air in the ceramic mix.³⁵ If air is entrapped within the ceramic particles, porosities occur in the final restoration, reducing its strength and increasing the chances of fracture.^25,33 Porosities may be introduced in the dental porcelain due to faulty condensation, incorrect powder/liquid ratio or firing time and temperature disparities.³⁵-³⁷ This factor is applicable to all restorations that use feldspathic porcelain.

Method of adding the veneering layer

Hot isostatically pressed (HIP) glass ceramic materials are less prone to chipping and fracture as compared to hand layered veneering porcelains.^11,25

Poor metal-ceramic bond

Clinical success of a metal-ceramic restoration largely depends upon the bond formed between the metal and the porcelain. A metal-ceramic bond results from the interplay of a number of different factors including mechanical bonding, chemical bonding, Van der Waals forces and compression fit due to a difference in coefficient of thermal expansion.³⁸ A poor metal-ceramic bond can be due to poor choice of metal, eg one that does not form oxides, or by inadequate preparation of metal to be bonded to porcelain.¹⁵ When the metal-ceramic bond fails, it leads to delamination of porcelain from the metal or adhesive failure.

Firing protocols

During firing procedures, a material with low thermal conductivity will be incompletely baked, hence becoming more prone to chipping fracture.¹⁷ Also, any mismatch between the coefficient of thermal expansion of the core porcelain and the veneering porcelain will generate residual stresses and promote fracture. During the cooling phase, the difference in thermal conductivity of core and veneer porcelain results in residual stresses. These stresses bring about adhesive failure of the restoration.¹⁵

Faulty polishing and glazing

Glazing helps reduce the depth and width of flaws present in the ceramic surface,¹⁵ hence is considered a ceramic-strengthening method.³⁵ Any fault in glazing or polishing would hamper the strength of ceramic.