Low temperature degradation of zirconia (3Y-TZP) oral implants and its effect on fatigue reliability is poorly documented.
The aim of this investigation was to follow the aging process occurring at the surface of implants exhibiting a porous coating and to assess its influence on their mechanical (fatigue) properties.
Tetragonal to monoclinic transformation (t–m) was evaluated during accelerated aging tests up to 100 h in autoclave (134 °C, 2 bars) by X-ray diffraction (XRD) and focused ion beam (FIB). A series of implants were steam-aged for 20 h before fatigue testing. Such temperature–time conditions would correspond roughly to 40 years in vivo . The aged specimens and a non-aged control group were step-stress fatigued until failure or survival.
The evolution of XRD surface monoclinic content was slow, i.e. 16% and 35% for 20 and 100 h respectively. However, FIB revealed a significant transformation, initiated at the interface between the porous layer and the bulk, preferentially growing towards the bulk. FIB is therefore better indicated than XRD to follow aging in such implants. Higher average fatigue strength (aged 1235 N versus non-aged 826 N) and reliability levels were observed for the 20 h aged group.
After aging for durations compatible with clinical use, 3Y-TZP with porous surface presented higher fatigue performance. This is in contrast to previous studies where loss of strength due to aging was often reported. Generalizations must therefore be avoided when considering aging of zirconia dental products and every new material/process combination should be tested before drawing conclusions.
Yttria-doped tetragonal zirconia polycrystal (Y-TZP) was introduced as an implant biomaterial (femoral heads) in orthopedics over 30 years ago, because of its high fracture toughness and strength . The orthopedic application was, however, abandoned after 15 years of use, after a series of catastrophic failures in specific batches manufactured with a new process . In contrast, zirconia is increasingly used in dental applications. It is due to its excellent strength and toughness, but also undoubtedly due to its aesthetical properties, its biocompatibility and its ability to be manufactured by CAD/CAM procedures . Dental zirconia is mainly found in the form of yttria-stabilized zirconia crowns, bridges and abutments and several companies are developing zirconia implants as an alternative to the standard biomedical grade titanium. The in vivo studies on microbial colonization on titanium and zirconia discs showed that zirconia is gathering less plaque on the surface compared to titanium . Previous in vivo and in vitro experiments on zirconia implants showed desirable osseointegration, cell metabolism and soft tissue response . Zirconia may be preferred to titanium in some cases, when the grayish color of titanium implants is perceived through a thin gingiva, causing some aesthetic drawbacks . Finally, many patients also ask for completely metal-free dental reconstructions.
However, zirconia is a complex material because it is meta-stable at room temperature . On one hand, its excellent mechanical properties (the best of oxide ceramics) are due to the transformation of meta-stable tetragonal grains to the monoclinic phase under stress (for example in the vicinity of a crack). It is accompanied by a volume expansion and induces compressive stresses that shield the crack tip from the applied stresses. This phenomenon is known as transformation toughening. On the other hand, this meta-stability leads to a possible transformation of grains in contact with water (or body fluids) with time. This phenomenon is often referred as low temperature degradation (LTD) or aging. Aging is a progressive tetragonal to monoclinic transformation at the surface triggered by water molecules, which often results in surface modification (roughening) and micro cracking and thereby potentially decreasing the device physicochemical and mechanical properties.
Tetragonal to monoclinic phase transformation proceeds most rapidly at temperatures of 200 –300 °C. However, aging may also occur in vivo , as it was clearly observed for femoral heads in orthopaedics . The experience of zirconia in this field gave some important indications on how aging may proceed, the potential impact of the transformation and the conditions by which it may be triggered. The transformation proceeds from the surface in contact with water to the bulk of the material. The kinetics by which the transformation occurs is highly dependent on process conditions and resulting microstructure. Without indicating strict limits, it is for example accepted that large grain size (due to high sintering temperatures) generally triggers the transformation, while open porosity (due to incomplete sintering or the presence of large pores) helps water to diffuse towards greater depth . Aging did occur in vivo for a large number of hip joint heads, processed under several conditions. The consequence of aging for hip joint heads was either a progressive increase of surface roughness (and grain pull out) with time after some years or even more dramatic events like fractures in the case of Prozyr ® heads processed under specific conditions (sintering in a tunnel furnace leading to open porosities) .
Most of the research on zirconia dental ceramics today focused on mechanical properties of the devices , their fatigue resistance and surface modifications that could enhance bone in-growth . This is particularly the case of oral implants, where a direct contact with bone is present. The search for better implant-bone integration has led researchers and companies to develop methods to increase surface roughness and/or to create micro-porosity. Among them, we may cite sandblasting , chemical-etching , spray drying of a bioactive phase or coating by a porous zirconia layer . All these surface modifications may have a positive effect on bone apposition and bone in-growth, but also could lead to a modification of the stability of the tetragonal phase under humid atmosphere. One should also keep in mind that the zirconia used today in dentistry (3Y-TZP) is the same as that used for orthopaedics years ago, therefore exhibiting the same dependency against process variations. The only variation is the addition of 0.25 wt% of alumina, which is believed to limit the kinetics of aging ( www.tosoh.com/Products/basic_grades.htm ).
There is no report available so far on the coupling between aging and fatigue resistance of oral implants, while this is the case in vivo : oral implants are indeed in contact with body fluids and constantly subjected to cyclic loading conditions during mastication. Thus, the aim of the present work was study in detail how aging proceeds in oral implants exhibiting a porous coating and how aging could affect their fatigue resistance.
Materials and methods
Nobel Biocare (Göteborg, Sweden) provided the experimental implants, with 4.3 mm in diameter and 17 mm in length. The implants were processed from a bio-medical grade 3% mol yttria-stabilized zirconia spray-dried powder (Tosoh TZ-3Y SB-E, Tokyo, Japan), by cold isostatic pressing and sintering-hot isostatic pressing technologies. The ‘E’ grade in ‘SB-E’ means that a small amount (0.25 wt%) of alumina is added to the powder in order to decrease the sensitivity of zirconia to aging. The ‘E’ grade is the one commonly used today for the process of dental products. A porous surface was then achieved by coating the endosseous part of the implants with slurry containing zirconia powder and a pore former (patent application SE03022539-2). Further sintering of the sample yielded to the burn off of the pore former and to a porous surface. Sintering of the porous surface was performed on an already dense, sintered implant cylinder. According to the producer, the presence of the coating resulted in a rough and porous surface of 10–15 μm thickness. The Sa was measured in a prior work, being of 1.24 μm .
Accelerated aging tests and aging kinetics
Aging kinetics was evaluated by performing accelerated aging tests on a series of 3 specimens in water steam at 134 °C, under 2 bars pressure for 5 h runs, up to 100 h. It has been reported from our previous records that 1 h at 134 °C would roughly correspond to 2 years at 37 °C .
Follow up of t–m transformation of the porous coating by X-ray diffraction
Monoclinic content was measured at the surface of the porous part of the implants by an X-ray diffraction (XRD) technique (CuKα radiation) in a –2 mode (2 ∈[27–33°]) on a Bruker D8 Advance (Bruker, Karlsruhe, Germany) instrument (scan speed of 0.2°/min and a step size of 0.02°). Monoclinic content was then calculated by using the formalism given by Garvie and Nicholson :
X m = I m ( 1 ¯ 1 1 ) + I m ( 1 1 1 ) I m ( 1 ¯ 1 1 ) + I m ( 1 1 1 ) + I t ( 1 0 1 )
where X m is the integrated intensity ratio, I m( h k l ) is the area of the ( h k l ) peak of the monoclinic phase and I t( h k kl) is the area of the ( h k l ) peak of the tetragonal phase. The experimental volume content of monoclinic phase f was then determined with:
f = 1.311 ⋅ X m 1 + 0.311 ⋅ X m