Clinically, MIH enamel defects appear as well-demarcated opacities within otherwise normal-looking enamel. These opacities are generally visible to the naked eye and vary in color from creamy-white to yellow or brown and affect the cuspal areas, inclined planes and smooth surfaces. When MIH teeth, with clinically obvious opacities, are extracted and sectioned prior to any in vitro testing, the lesion usually extends from the ADJ to the surface of the tooth and the cervical enamel appears normal macroscopically (Fig. 6.3).

The term “mechanical properties” refers to the characteristics of a material that determine how it reacts when subjected to some type of force. Understanding the mechanical properties of hypomineralized enamel helps clinicians predict the behavior of MIH-affected teeth. Two properties in particular, hardness and elastic modulus, have been the focus of most research. The hardness of a material provides a measure of its resistance to permanent indentation and is considered to reflect its susceptibility to abrasive wear. Elastic modulus provides information on how a material flexes under loading/pressure such as occlusal forces. Both hardness and elastic modulus testing rely on dropping an indenter of a known size and shape into the material under test. The size or depth of the resulting indent is measured, and the resulting “hardness” is calculated as pressure (force/area). The elastic modulus depends on the shape of the loading/unloading curve of the indenter and is also measured in the units of pressure. Unfortunately, different studies use different units such as GPa or Knoop hardness number (KHN), which makes inter-study comparisons difficult.

Sound enamel has a hardness of between 3.5 and 5.0 GPa while its modulus of elasticity is 77.25 ± 3.15 GPa (see Table 6.1) [2, 3]. This confirms that sound enamel is very hard and resistant to wear. In comparison sound dentine has a low wear resistance, with a hardness of around 0.83 GP and is more flexible than enamel with a modulus of elasticity of approximately 20–25 GPa [4]. Hypomineralized enamel has been shown to have mechanical properties that are similar to those of dentine as opposed to those of sound enamel. Hardness values for enamel from hypomineralized first permanent molars have been reported as low as 0.5 GPa up to approximately 2 GPa with a modulus of elasticity of around 15 GPa [2, 3]. This 50–75 % reduction in the mechanical properties suggests that hypomineralized enamel has a low resistance to wear and will flex on loading which helps explain why MIH-affected teeth appear clinically weaker and more prone to fracture. Further work by Crombie and colleagues has shown that in general (although not always) white/cream-colored opacities are harder than yellow-brown lesions [2]. This information can be used clinically to help predict which teeth are likely to fracture or wear on eruption and which may be able to be monitored over time.

The mechanical properties of sound enamel do vary across the tooth surface [5– 7]. This has also been found in the enamel of hypomineralized first permanent molars with the surface of the lesion being harder than the body [8]. Furthermore, there is a transitional zone between the apparently sound enamel of the cervical region and the hypomineralized defect (Fig. 6.4a–c). This transitional zone is approximately 500–600 μm (0.5–0.6 mm) in width, with a linear reduction in mechanical properties from the sound to affected enamel [9].

Studies have confirmed that the hardness and modulus of elasticity of enamel in the cervical region are essentially normal. This means that clinicians can expect this region to support normal bonding for restorative materials such as resin-based composites whereas adhesion in hypomineralized regions of enamel may be compromised.

In an attempt to explain why the mechanical properties of hypomineralized enamel differ so significantly from sound enamel, microstructural studies have been completed using both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM uses the reflection of a beam of electrons that are scanned across the surface of a specimen reflected to form an image. It allows detailed three-dimensional analysis of the surface of a given specimen. TEM allows for higher resolution by transmitting a beam of electrons through an ultrathin specimen. Depending on the composition and thickness of the specimen, some of the electrons are scattered and disappear and the unscattered electrons hit a fluorescent screen, which can be photographed or visualized directly. This allows investigation to a higher resolution than light microscopy and SEM.

The microstructure of sound enamel confirms its well-organized rod architecture and well-defined borders with narrow inter-rod regions without defects or porosities [10]. This also applies to the clinically normal looking, sound cervical enamel of MIH teeth. Under SEM, the affected enamel does show some of the basic architecture of the enamel rods, but there is always a varying degree of disorganization with increased porosities [9–11]. In moderately affected areas, the boundaries between the enamel rods are somewhat blurred, and the crystals are not tightly packed. In the more severely affected areas, the rods are very thin with wide gaps in between containing what appears to be organic material. The border between the normal cervical enamel and the affected occlusal part is always distinct. This can give clinicians further confidence when extending adhesive restorations onto the clinically normal-looking cervical enamel.

TEM studies of hypomineralized enamel are difficult to do as they require very thin samples for analysis. However, a few studies have been reported in which the higher magnification associated with TEM has allowed visualization of individual crystals within the enamel. TEM imaging of sound enamel shows that the apatite crystals are well organized with elongated crystals running along the long axis of the enamel rods. The rod sheaths are barely distinguishable and the organic material in this region is minimal. In contrast, TEM images of hypomineralized enamel have shown that this enamel is more porous and the crystals more loosely packed. The rod sheaths also show increased porosity making the sheath significantly wider than in sound enamel (see Fig. 6.5) [12