Submerged or Transmucosal
As a rule, the majority of implant manufacturers support the concept that implants should be submerged below the mucosa during the healing or osseointegration phase and that this is an essential requirement for predictable implant success.'-4 The soft tissues are tightly sutured over the implants, thus isolating them from the intra-oral microbial environment and protecting them from the loading of a temporary overlying prosthesis.
Consequently, it is necessary to locate these implants, identifying their position with a probe (Fig 3-1), and perhaps, utilising the original surgical template to expose them through the overlying soft tissues, prior to prosthodontic procedures. It would therefore seem correct to consider this a second surgical procedure, but with the advent of the temporary heaiing abutment (transmucosal post), any surgery required is often minimal and ensures that the selection of the final abutment can be left to the prosthodontist. It must also be recognised that the ITI Implant
System today advocates a single stage or transmucosal approach5 7 which leaves implants exposed from the day of insertion. This has important ramifications for the team approach, removing the burden of abutment selection from the surgeon, who simply places a standard temporary healing abutment with minimal trauma to the patient. This then leaves the critical decision making to the prosthodontist, who will be more aware of the requirements of the final abutments.
At this stage it is now possible to ascertain the surgical success of implant osseointegration. The criteria used to denote implant success are for the greater part subjective. In 1986 Albrektsson et al., proposed an update8 of minimal criteria previously proposed.9 and which are still used today as the guide for assessing the success of new implant systems. These criteria are reproduced in Table 3-1. However it should be noted that these criteria are for the most part, time dependent, leaving little scope for assessing implant success at exposure.
Table 3-1 Criteria for Implant Success
Albrektsson. Zarb. Worthington and Eriksson. 1986
1 That an individual, unattached implant is immobile when tested clinically
2 That a radiograph does not demonstrate any evidence of peri-implant radiolucency.
3 That verlical bone loss be less than 0.2mm annually following the implant's first year of service.
A Thai individual implant performance be characterised by an absence of persistent and/or irreversible signs and symptoms such as pain, infections, neuropathies, paresthesia, or violation of the mandibular canal.
5 Thai, in the context of the above, a successful rate of 85% at the end of a five-year observation period and 80% at the end of a ten-year period be a minimum criterion for success.
Some effort has been made in using the Periotest® (Siemens AG, Bensheim, Germany) to provide an objective, measurable, clinical diagnosis of bone-implant apposition10 for early assessment, but for the majority, a reliance on radiographic evaluation, the absence of mobility, pain and/or infection and the presence of a metallic ringing tone on implant percussion, remain the only sources for determining implant success.
Whether using the Periotest® or simply the handle of a dental mirror for percussion, the result will be influenced by the implant/abutment interface. If not accurately apposed, as a result of inaccurate seating, or tissue trapping, this will give either a false positive or a dull percussion note. Reflecting on the criteria above, it is clear that an indication of early success, should only be considered in the context of long term functional success and is therefore only the first of many opportunities that should be taken to measure or assess implant viability.
The healing abutment (Fig 3-2) which allows for a period of soft tissue maturation, is placed through a punch or slit incision (Fig 3-3) to locate the implant exposed below (Fig 3-4). Locating abutments correctly on those implants with a hex top external Interface can be time consuming and protocol recommends that all abutments, particularly the final abutments, are checked for correct seating by means of an mtra-oral radiograph. With the internal conical interface all abutments are self guiding and it is not possible for the abutment to be seated incorrectly.
Fig 3-1 Implants are identified first on a radiograph and then a probe is used to locate fhe domed cover screw below. Reuse of the surgica! stent might help in Iheir location.
Fig 3-2 Healing abutments are widely used as lemporary trans-mucosal components, allowing peri-implant soft tissues to mature, prior to final abutment seating The presence of grooves, which act as in situ soft tissue depth markers, enables the clinician to accurately select the correct length of final abutment
Fig 3-3 Cover screws are exposed through a slit or punch incision, avoiding the need to strip periosteum. The punch is not indicated where only a narrow band of keratinised tissue exists.
Fig 3-4 Healing abutments are inserted directly to engage the internal conical interface. All three depth grooves are clearly visible, which would indicate that the shortest Uni-abutments™ will be selected.
It is advisable to irrigate the interface and flush out any debris with chlor-hexidine prior to seating the healing abutments. The reader is directed to manufacturer recommendations with respect to the handling and seating of abutments, as this will vary between systems.
The advantage of a submerged implant is that the marginal metal collar of the implant is placed well below the mucosal margin. In recognition of the much higher aesthetic standards which are demanded today, even the transmucosal implants are pseudo-submerged, with an extension healing cap that acts in a similar manner to the healing abutment Healing abutments are usually left in situ for approximately two weeks post insertion, during which time soft tissues mature to form a tight cuff around the abutment. This has been shown to adhere to the surface via hemidesmosonai attachments, with an organised inner implant epithelium of non-keratinised, flattened squamous cells covering a collagenous stroma.11 Fibres within the stroma generally run either parallel to the abutment surface or as a circular network, as recently shown around the transmucosal variety.12 On removing the healing abutments, or extended healing caps, a firm soft tissue cuff should demonstrate tone, that is it should not collapse and there should be an absence of bleeding (Fig 3-5). No local anaesthetic is generally required on removing these temporary transmucosal components.
It is now possible to measure soft tissue thickness using a periodontal probe or a purpose designed soft tissue depth gauge (Fig 3-6) giving direct readings for the final abutment length. In figure 3-2 the healing abutment is seen to be graded with grooves that correspond to the available final abutment lengths, thus acting directly as an in situ soft tissue depth gauge.
The selection of final abutments will depend on:
1 Technical considerations (Fixed bridge or overdenture treatment)
2 Functional considerations (Occlusion, guidance and parafunc-tion)
3 Aesthetic considerations (Implant position and patient perception)
4 Hygiene considerations (Access and manual dexterity)
Inter-occlusal space, access for instrumentation, implant position and inclination, may all further influence the decision of final abutment selection.
The variety of abutment designs even outweighs the plethora of implants, potentially making life for the novice even more confusing. It is not the intention of this chapter to offer an inventory for different designs. Only standard abutments will be described, drawing particular attention to the difference between one and two piece abutment designs. For those implants that present an
Fig 3-5 After approximately two weeks of soft tissue maturation, healing abutments are removed to reveal a healthy peri-implant cuff, that should demonstrate tone and be absent ol frank bleeding
Fig 3-6 If necessary soft tissue thickness can be assessed using a soft tissue depth gauge, which is indicated when healing abutments are not used, or for those systems where graded healing abutments are not available.
hexagonal anti-rotation device on top of the fixture, it is necessary for the abutment to be constructed in two pieces (Fig 3 - 7). The first piece is the transmucosal collar which engages the hexagonal part of the fixture head. The second piece, the abutment screw, passes through the middle of the collar screwing into the fixture, thus uniting the fixture and abutment collar with a butt joint interface.
For those implants with an internal conical interface, abutments are designed as one piece units (Fig 38) that screw directly into the fixture. Though no claims are offered for anti-rotation, the concept of a Conical Seal Design™ (Astra Tech AB, Molndai. Sweden} has been shown to impart superior strength and support at the implant abutment interface.13
For single tooth restorations it is essential to have an anti-rotation feature between the fixture and abutment. The next chaper will deal with this specific restoration in greater detail.
For the system being described in this text, there are two types of standard, Uni-Abutment™, with a 20° or 45° tapered top (Fig 3-9). These permanent abutments are selected for fixed bridgework or overdenture treatment respectively.The45° abutment is also useful for those situ ations where inter-occlusal space precludes the use of the 20° abutment for fixed bridgework; conversely the 20° abutment can be used for bar type overdenture attachments to provide additional support against lateral loads. The details of abutment selection for individual prostheses will be discussed in subsequent chapters. However the insertion of abutments is considered below. Having removed the healing abutment and measured soft tissue depth, the relevant final abutment is selected and mounted on the abutment adapter (Fig 3-10). Care should be taken not to damage the delicate threading of the bridge screw hole. The conical interface is flushed with chlorhexidine and the permanent abutment is secured either with light finger pressure or using a torque controller as dictated by manufacturer recommendations, to ensure complete seating (Fig 311), If a hex top implant is being utilised it is essential to take an intraoral radiograph to confirm accurate seating of the abutment. With the final abutments in place, subsequent restorative procedures can begin. Between visits, small plastic protective caps can be secured to the abutments to prevent food impaction into the bridge screw holes and to avoid any damage,
Fig 3-7 Two piece abutments comprise a hollow cylinder of various lengths with a central abutment screw that secures the cylinder to the implant. In the system shown (BrSnemark) a plastic nng is used to seal the communicating channel between intra-oral and submucosal environments.
Fig 3-8 One piece abutments screw directly in to the fixture and are solid components. There is no internal communication between intra-oral and submucosal environments.
Fig 3-9 The Astra abutment is presented with the option of a 20° tapered top, or a 45° tapered top. These are used for fixed bridge and/or overdenture treatment respectively.
Fig 3-10 Permanent Uni-abulments™ are located using an abutment adapter. Due to the precise fit and guidance of the internal Conical Seal Design™, only light finger pressure is required for accurate seating. In this case no radiographs are required to confirm correct abutment seating
Fig 3-11 Final abutments are shown above in situ It is now possible for the prosthodontist to take impressions and embark on the prosthetic rehabilitation (See chapter 5),
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