Zirconia Implants – an overview
Zirconium Dioxide (ZrO2) – or “Zirconia” as it is more commonly known, was discovered in 1789 by the German chemist M. H. Klaproth. However, it was only introduced into dentistry a few decades ago: a product of the increasing desire for highly esthetic restorations. Zirconia became an attractive alterative material in dentistry because of its high esthetic potential and comparable strength to traditional metals. In the field of implant dentistry, Titanium has been the mainstay in implant manufacturing; however, Zirconia became a viable option because it possesses superior properties, including a higher tensile strength, compressive strength, and modulus of elasticity when compared to either Titanium alloy or commercially pure Titanium (Table 1). The Zirconia used in dentistry today is not merely the Zirconium dioxide discovered in the 18th century; the commercial grade Zirconia has several modifications that enhance its properties. In its pure phase, Zirconium dioxide has a low shear strength and is very brittle, essentially making it useless as a dental material. The addition of small amounts of Aluminum Oxide and Yttrium Oxide increase the modulus of elasticity and help to stabilize the material. This combination of oxides is mixed in the powder state and placed in a sintering oven to produce a monocline crystal structure, with equally-spaced, non-overlapping particles (Figure 1). Although the monocline crystal is a strong material, cracks can propagate easily in the structure which makes it less desirable to use as a long-term implanted prostheses. In order to eliminate this issue, today’s Zirconia is also put through a process known as “HIP” (hot isostatic pressing). The high pressure under which the monocline Zirconia is placed during HIP processing causes condensation of the particles and results in a tetragonal crystalline structure, where the particles appear to overlap (Figure 2). The significance of this innovation is that it imparts the ability to stop crack propagation. When the surface of HIP-processed Zirconia is prepared, any microcracks that may result are quickly stabilized as tetragonal particles expand to the monocline structure and fill the void. The self-repairing property is also known as the “airbag effect”. The additional stability gained by the HIP process has enabled Zirconia to be used for multiple medical prosthetic devices including auditory, finger, and hip prostheses.
Zirconia dental implants have been available on the commercial market since 2004. The current major players in the United States are Bredent white SKY, ZSystems Z-Look3, and Oral Iceberg CeraRoot. Zirconia has proven its utility in dental implants through a series of animal and human clinical studies wherein it has been shown to successfully osseointegrate into bone and be highly biocompatible.
II. Indications and Contraindications for Zirconia Implants
1. All esthetic zone cases, especially in those with scalloped, thin biotype gingival architecture and in critical gingival papilla build-up cases
2. Patients with metal allergies and chronic diseases resulting from them
3. As an alternative to Titanium dental implants in any intraoral location
1. Patients that exhibit a lack of compliance to post-operative instructions.
2. A lack of operator clinical and technical knowledge about implant surgery and prosthetic restorations
3. Any other general contraindications to implant rehabilitation with one or twopiece titanium implants, such as bruxism.
III. Bone Relationship
A. One-Piece Implant Concept
The one-piece component implant was conceived in an attempt to copy nature. Imagine an implant restoration without micro gaps, internal screws, or inner gaps. The one-piece implant allows axial forces to be applied into a solid structure without attachments, made entirely of one material with no physical interruption and excellent flexural strength. One of the major advantages of the “HIP” processed Zirconia is its ability to be prepared intraorally, as ceramics do not conduct heat like metal or natural tooth structure. Preparation of the abutment can occur immediately after insertion or after osseointegration, and allows what is essentially a custom abutment to be prepared. Despite these apparent advantages, there has been a well-documented history of unsuccessful one-piece titanium implant systems, which has hindered the ability of the Zirconia onepiece implant gain acceptance into the implant community. These Titanium one-piece systems were often used to also provide immediate loading, which has not been shown to provide predictable success. Unlike the Titanium one-piece implant, the goal of the Zirconia one-piece is not to provide immediate loading, but to provide immediate esthetics. One should also consider the differences in the cost of manufacturing and the environmental implications for one and two-piece implant systems. Certainly the engineering of the many intricate components of the two-piece implant system is fascinating, but it creates exponentially more leftover material and pollution to process the much greater number of parts. The implant industry is an ever-expanding, multi-million dollar enterprise that may see little benefit from reducing the number of parts involved in implant systems, as this may in turn reduce profit. Despite this, the need for more efficient and environmentally friendly industrial operations is critical and the push towards a more economical solution will continue. In the world we’re living in today, my opinion is that less is more and that a one-piece implant is an all-around better solution, like the one-piece tooth we all are born with.
B. Importance of Proper Planning
Whereas a two-piece implant system can compensate for implant body positioning by utilization of custom-made angled abutments, onepiece implants have an inherent limited compensation ability. Proper implant positioning at the time of insertion is critical to the success of the restoration and esthetics of the final product. The abutment in a one-piece system can account for only around 20º of correction through preparation of the coronal aspect. In order to properly determine the ideal implant location, wax-ups and digital prototypes should be utilized when possible. When proper implant placement is achieved, the abutment will be in such a position that forces transmitted along the long axis will be favorable, and the unfavorable loading will be minimized. Such a relationship can then translate into a good long-term marginal bone level stability and a healthy, durable restoration.
C. Tulip-Shaped Abutment Neck
The tulip-shaped neck of the abutment is analogous to the cervical shoulder area of the implant. This area marks the transition between the implant and the abutment. It allows the implant to be inserted at a variable depth to establish the proper emergence profile with optimal gingival contour, and also enables correction of axial divergence by up to 20%. The design and material of the implant allows vertical placement in bone to vary by up to 1.5 mm. Since the Zirconia is white, there is little esthetic risk from not sinking the implant deep enough. If the crestal bone architecture is flat, the implant shoulder does not have to be countersunk, although the biologic width must be observed. In this situation, the implant should be inserted in the bone up to the coronal end of the threading. Use of the countersink is not necessary to achieve this position. For esthetic reasons such as thickness of mucosa, need for vertical adjustment of the preparation border, or with uneven crestal bone architecture it is frequently necessary to countersink the implant up to the transition of the implant tulip to a maximum of 1.5 mm. When attempting to place immediate implants in the esthetic zone, the shoulder or tulip insertion should extend to cover the edge of the extraction socket to achieve greater stability and the same results as tapered implants. After 5 years of clinical use and studies, the current recommendation is to try to avoid over-insertion of the shoulder when not needed in non-esthetic areas, as it may lead to a greater degree of bone loss over time.
D. Angled Abutments
The anatomical shape of the alveolar bone dictates the ideal position of the implant body, and maintenance of the buccal cortex is essential for the long-term success of the implant restoration. When placing implants in the anterior region, the operator often has a tendency to base the implant angulation off of the future restoration, which can consequently lead to buccal cortex violation. With the implant body at the correct angulation, the restorative components may not be properly angled for a good esthetic result; often the abutment protrudes buccally leaving little room for fabrication of a naturalappearing crown. Two-piece implant systems may use angled abutments to compensate for this discrepancy. In one-piece Zirconia implants, the issue is easily addressed by preparation of the abutment aspect to the desired angle, up to a maximum of 20 degrees. This is possible because the wide implant shoulder in combination with the large abutment allows an even force distribution, which minimizes bone loss and increases longevity of the restoration.
IV . Soft Tissue Relationship
A. Zirconia surface
The Zirconia surface of the implant is biocompatible with the oral soft tissues. As a ceramic, Zirconia inhibits formation of plaque and promotes a healthy soft tissue attachment. There has been no evidence of any inflammatory reaction or irritation to the gingiva from the Zirconia surface.
B. Scalloping the implant shoulder
The implant shoulder may be adjusted to better follow the scalloping of the gingiva to obtain the most esthetically pleasing results in the anterior region.
C. No micro gaps
Eliminating the micro gap between the implant body and abutment eliminates the possibility of bacterial attachment and inflammation. Without a micro gap, there is less long-term soft tissue irritation.
D. Gingival papillae growth
The gingival soft tissues have been found to have an affinity for the Zirconia surface, which leads to excellent esthetics. Not only can the Zirconia preserve the existing gingival papilla height, but it has even been observed to induce gingival growth. For papilla build-up cases, Zirconia therefore has a distinct advantage over traditional Titanium implants. The best results have been shown in cases with a thick and flat gingival biotype as well as a good emergence profile without violation of the biologic width.
E. Surgical Considerations
For the best esthetic results, one should start the process of contouring the soft tissues at the time of tooth extraction during immediate placement, and when the provisional is first made during the traditional protocol. When planning an immediate placement case, a conservative, atraumatic extraction will aid tremendously in maintaining the best gingival architecture. The provisional should have a smooth and well-contoured finish line to facilitate the best gingival health. Often the tissues will be inflamed at the time of surgery, especially with immediate implant placement, because of pre-existing infection in the tooth. It is quite common, therefore, to have what appears to be recession of the tissues during the healing process. As the Zirconia surface is biocompatible and does not trap plaque, the tissue inflammation will subside in 1-2 weeks after placement. Flapless surgery is a good alternative to help with soft tissue maintenance. Flapless procedures are usually done with the help of a tissue punch, and require good case planning for proper implant placement. There is a range of approximately 1.5 to 2 mm of acceptable variation in the vertical insertion depth of the implant shoulder. The soft tissue biotype, height of the bony crest, and soft tissue thickness must all be taken into account to obtain the best implant position and subsequently the correct emergence profile.
V. Intraoral Adjustments
A. Implant selection
As with any treatment planning for any type of dental implants, several factors must be taken into consideration when planning for one-piece Zirconia implants. The total number of implants, diameter, length, and position should all be based on the available space, quantity, and quality of bone. The minimum height required for one-piece Zirconia is thought to be 7 mm. Bone grafting procedures should be undertaken when necessary to achieve this minimum height. If the crestal bone architecture is flat, the implant does not need to be countersunk; however, if the soft tissue esthetics dictate that the implant must be countersunk, it may be placed up to 1.5 mm deeper than the last thread. All Zirconia one-piece implants should be surrounded by at least 1.5 mm of bone, with 3 mm of bone between two implants. The implant diameter should be based on the tooth being replaced, anticipated occlusal forces, and the available space between the roots of neighboring teeth. The minimum distance of the implant shoulder to the adjacent teeth is 0.5 mm, measured from the greatest curvature of the neighboring teeth, keeping in mind that the implant shoulder can be adjusted up to 1 mm when necessary.
B. Abutment preparation
After insertion of the one-piece implant, it may be necessary to prepare the abutment to meet the anatomical demands of the site. Ideally all biting forces should be directed along the long axis of the implant, but the abutment aspect of the implant may be prepared to compensate for angulations up to 20º. When available, wax-ups should be used to aid in treatment planning. When adjusting the abutment immediately after implant placement, red-striped ultra fine grain (46 μm) diamond burs should be used to a maximum bur speed of 160,000 rpm. A minimum of 50 ml/min of irrigation should be utilized during the procedure, and excessive forces should be minimized on the newly placed implant. The abutment should only be prepared enough to allow for adaptation of the provisional restoration, as more definitive adjustments will be made following soft tissue healing. If the shoulder needs to be lowered in the mesial or distal aspects of the site, this should be completed prior to closure of the soft tissues. As the provisional restoration will need to be out of occlusion, the abutment should be a minimum of 1.5 mm below the plane of occlusion, but no less than 3 mm in height. After the healing phase and implant osseointegration, the definitive preparation of the implant shoulder can be completed. At this time, one should ensure proper scalloping of the shoulder to match the soft tissue contours and the abutment should be properly rounded prior to the final impressio.
C. Inserting the implant and increase the bone to implant contact
One of the key factors in dental implantology is a good primary stability. What we considered in our learning curve is that we increased the bone to implant contact by condensing the spongy bone. Depending on the bone we did drill with the final drill only through the corticalis and not the spongiosa anymore. By inserting the implant with a higher torque (up to 45-50 Ncm) we compressed the spongy bone with the implant and increased in the spongy bone a higher bone to implant contact. Please make sure using this technique only for the spongiosa.
VI. Guidelines for an ideal emergence profile
A. Gingival Biotype
The thick and flat gingival biotype offers the best overall esthetic results, including the best coverage of the margin and papillae preservation. The thin and scalloped biotype makes it more challenging to adjust and maintain the best cervical margin; however, using Zirconia implants eliminates the problem of the grey gingival shadow associated with Titanium implants. If recession occurs and exposes the crown margin, although less esthetically pleasing, it will not be as undesirable as an exposed Titanium surface.
B. The Zirconia Implant surface characteristics
Zirconia has been shown through a number of clinical studies to have great tissue biocompatibility and long-term stability. When in contact with tissue fluids, the implant surface carries a neutral polarity which disables bacterial aggregation. This, in combination with the lack of a micro gap, makes the one-piece Zirconia implant a great tool for management of the soft tissues. These characteristics allow for excellent gingival health and even spontaneous growth of soft tissues, which is an advantage for the long-term esthetics of dental implants.
C. Bone and soft tissue level
Just as with any dental implant, the best esthetics will be achieved when the implant has good bony support on all four walls. Clearly this is best accomplished with an atraumatic extraction and ideal placement of the implant, but when this is not possible, bone grafting may be necessary. If a significant amount of marginal bone is lost during extraction or there is a vertical discrepancy in ridge height compared to adjacent teeth, an implant restoration will require a longer crown to compensate. This situation should be avoided in the esthetic zone, particularly in patients with a high smile line. If a one-wall or small volume defect is present and the patient is planned for immediate implant placement, bone grafting material may be used, which is very wellaccepted by Zirconia implants. For larger defects where a significant volume of bone is missing, a two-stage procedure should be undertaken and implant placement delayed until completion of grafting.
D. Implant positioning
The ideal emergence profile of an implant will be created by placing the implant in its ideal position. Selecting the proper implant diameter is a vital part of this process. By using a small diameter implant to support a large restoration, we create an unnatural and unesthetic emergence profile. Implant diameters must be properly matched with the size of the interdental space to be restored. Implants must also be placed in their ideal vertical position to achieve proper emergence. For the one-piece Zirconia implants, there is a range of 1.5 mm in vertical positioning for which ideal esthetics can be maintained. Necessity of countersinking is situation-specific and depends on operator preference, but in general is necessary when the crestal bone is thin or irregular or soft tissues are very thin. Implants can be countersunk so that the implant neck is partially embedded in crestal bone and the shoulder remains subgingival; they may be countersunk from the last coronal thread up to half the height of the implant neck.
E. Implant Preparation
Customizing the abutment portion of the one-piece implant is a technically simple procedure that is generally very quick. After implant placement, gross discrepancies can be adjusted if necessary, bearing in mind that tissues will likely undergo some degree or recession. One must also take into account that adjustments made will be irreversible, and it is always preferable to defer adjustment until after healing has occurred. Ideally, implants are prepared after osseointegration and tissue remodeling has been completed. The implant shoulder should be scalloped to match the gingival contour of the tissues and allow for subgingival placement of the crown shoulder. The recommended shoulder design is a chamfer, which can be easily created with a Torpedo ISO 016. The maximum speed of rotary instruments used on Zirconia implants is 160,000 rpm with copious irrigation. Other important adjustments include angulation of the abutment portion to match adjacent teeth and creating a common path of insertion for multi-unit prostheses. Narrow neck implants, which are designed without a clear marginal line, may require less or even no intraoral adjustments. When necessary, they can be prepared with the flame ISO 012 for a knife-edge type shoulder design.
Most Zirconia implants are temporarily restored after placement with provisional restorations. Provisionals should be well-adapted and polished so as not to irritate the tissues and hinder the healing process. Since the implant shoulder will be slightly subgingival, so must the provisional. It should have good circumferential contact with the shoulder and be wide enough to allow the tissues to heal with the proper contour for emergence and to maintain papillae architecture. The operator should consider changing or rebasing the provisional restoration after approximately 3 weeks of healing to aid in soft tissue management. After this time, the tissues wi l l be approaching their final conformation, and additional contouring of the provisional wi l l allow for any necessary adjustments to soft tissue shape.
VII. Common Mistakes
A. Incorrect Implant Positioning
One-piece implants demand accuracy in placement due to our limited ability to compensate for mistakes as compared to twopiece implant systems. It is important to plan properly and use advanced planning techniques such as cone-beam CT, digitally guided implant placement, and surgical guides whenever possible. Improper placement can lead to nonrestorable implants, apical exposure, proximity to adjacent roots, or unfavorable forces on the restored implant.
B. Premature Loading
The design of the one-piece implant dictates that the restorative abutment be present and exposed to the oral cavity from the time of placement. This leaves the implant vulnerable to premature forces during the healing period which can potentially compromise osseointegration. Chewing, cheek pressure, and tongue pressure can all cause implant micro-movements that may lead to failure of the integration process. In order to adequately protect the implant, there are a variety of provisional restorations that can be employed, including an Essix appliance, eggshell temporary, reworked denture, Maryland bridge, posterior adhesive bridge, or thermoplastic clasp denture. The success of the implant is highly dependent on adequate protection during the integration period and a proper protective device should be fabricated within the first 24 hours. When possible, a premade provisional that can be adjusted at the time of implant placement is preferable. The device should provide 1 to 1.5 mm of free space circumferentially around the abutment and be out of occlusion during all functional and parafunctional movements.
C. Improper Abutment Preparation
Poor abutment preparation may lead discrepancies in spacing or angulation. If the implant is prepared in such a way that one side of the abutment is trimmed much more than the other, the resulting crown may not be balanced over the implant and deleterious forces may be transmitted.
D. Incorrect Implant Width
As with traditional dental implants, the mesiodistal width of the site for implant placement should provide at least 1 mm of bone between the implant and adjacent teeth. In order for the one-piece implants to be placed, including the wider shoulder area, the important area to measure is between the height of curvature of the adjacent teeth. There should be a minimum of 0.5 mm on either side of the implant to allow placement. With less than 0.5 mm of space, esthetics will be compromised and the patient may have difficulty appropriately cleaning the area. In addition, ingrowth of papillae may be truncated, which would also negatively impact the esthetic outcome.
E. Lack of Consideration for Soft Tissue Biotype
When treatment planning for implants, the soft tissue biotype must be taken into consideration. Failure to address this issue can lead to less than ideal esthetic outcomes that may have been easily avoided. For one-piece Zirconia implants, one important consideration is that implants should be countersunk in those with a thin and scalloped gingival biotype. The implant shoulder should be inserted into the bone as deep as possible to attain a suitable cervical emergence profile. By misjudging or neglecting to consider the gingival biotype, one may end up excessively grinding the implant shoulder to attempt to place the finish line in a subgingival location. Often the result is an unesthetic supragingival finish line and poor papillae ingrowth.
VIII. One–piece implants Vs. Two-piece implants with Zirconia Abutments
A. Clinical benefits of One-piece Zirconia Systems
1. Having a single stage procedure
2. Decreased chair time – preparing the abutment is similar to preparing a traditional crown, and intuitive for the experienced operator.
3. Less complex armamentatium, fewer parts required for restorative procedures
4. Elimination of lab time for abutment fabr i cat i on; No need for healing abutments, screws, analogs, or transfer copings.
5. No internal screws, no internal gaps, no micro gaps, fewer locations for hardware failures.
6. Excellent soft tissue integration.
7. Less consequences from gingival recession. As minor recession may occur over time when chewing forces are extreme, and exposure of the white implant surface will be less noticeable than Titanium.
8. No gray gingival showthrough. Zirconia implants will never cause the unesthetic gray gingival tint which attracts the eye and makes Titanium implant restorations in the esthetic zone very challenging.
9. Flexural Strength. Zirconia is bone friendly and has good flexural strength for supporting implant prostheses.
10. Improved gingival health. Due to the lack of micro gaps and the neutral polarity of the implant surface, there is much less tendency to have an erythematous, inflamed implant neck as is often seen after removal of the healing abutment with Titanium implants.
11. Force distribution. The one-piece design allows force to be transmitted equally throughout the structure.
12. No metal parts. Zirconia is the only material outside of Titanium being used for dental implants. This is advantageous for those with metal allergies or with an interest in holistic dentistry.
B. Clinical Disadvantages of One-piece Zirconia Systems
1. The implant must be protected during healing.
2. Less ability to compensate for incorrect implant angulation.
3. Necessity for good patient compliance. Not every patient may be able to be compliant enough to allow for the necessary healing phase without force being applied to the implant.
4. The healing process may last from 3 to 6 months, depending on bone quality.