Endosseous implant surgery offers considerable fl exi-bility in preprosthetic surgery, but the practitioner should gain experience in minor oral surgery proce-dures before contemplating implant surgery. Further training is required to acquire the additional diagnos-tic and surgical skills required. Implant companies will not usually sell implant components to a dentist who has not undertaken training in their implant system.
There are three main types of jaw implant:
● endosseous (literally ‘within the bone’) implant: has enhanced both the treatment options and the cost effectiveness of preprosthetic surgery. This type of implant is discussed in detail below
● subperiosteal implant: made popular in the 1970s (now rarely if ever used), this device was a custom-made metal frame inserted as an onlay directly onto the surface of cortical bone. The frame was fabricated from impressions taken of the exposed surface of the jaw. The frame was subsequently fi tted onto the jaw bone. Abutments that protrude through the oral mucosa supported a denture. Wound dehiscence and infection were common problems with this implant because of mobility between the implant and bone
Fig. 11.16 (a) A lingual torus is covered by thin oral mucosa, and considerable care is required to avoid accidental perforation of the fl ap. (b) The mucoperiosteal fl ap is raised to expose the entire bony mass. (c) A surgical bur or chisel may be used to remove the bony torus. Conscious patients may fi nd the sensation of the chisel being used unpleasant, but in experienced hands the technique is quick and safe. (d) The wound is closed with multiple resorbable sutures.
(a)
(d) (c)
(b)
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● transosseous implant: includes the Small mandibular staple implant and the Bosker transmandibular implant. A transosseous (extending all the way through the bone) implant may be indicated for severe atrophy of the mandible in which endosseous implants are contraindicated, or as an alternative to ridge augmentation.
Endosseous implants
In the past few decades the management of tooth loss has been revolutionized through endosseous implant surgery. Brånemark and colleagues pioneered research on the integration of titanium implants with direct bone contact in the 1960s. Most endosseous implants are constructed from pure titanium because of its ideal biological performance characteristics as an implant material.
Osseointegration is a clinical and histological concept that describes the natural apposition of bone to an implant with a subsequent ability of the implant to sustain loading and transfer the load to and within the adjacent bone (Fig. 11.17). Stringent clinical and technical standards are required to achieve osseointegration. Survival rates for titanium implants in excess of 86% in the mandible and 78%
in the maxilla at 15 years have been reported. Recent advances in implant design and surface treatment technology have improved the survival rates.
Factors that infl uence the success of endosseous implants
These include:
● biocompatibility of the implant material
● design of the implant
● surface characteristics—various surface treatments are applied to enhance the biological response (e.g. titanium plasma spray coating, acid etching, grit-blasting with titanium powder, machined surface fi nish to improve the gingival attachment)
● physical health of the patient
● quality of bone
● favourable anatomical conditions
● patient's cooperation, oral hygiene status, smoking habits
● operator experience
● loading of the implants after osseointegration.
Implant systems
There are many different implant systems. In selecting a suitable system, manufacturers’ claims about the superiority of a particular system should always be supported by laboratory data and published clinical trials. Most are based on a cylindrical or screw design.
Implant systems are usually placed as a single-stage fi xture (the implant or its healing abutment protrudes into the oral cavity on insertion) or two-stage (the implant is buried beneath the oral mucosa for several months to osseointegrate before it is exposed; a fi xture that protrudes into the oral cavity is then inserted). The trend now is for implants to be loaded immediately on insertion or after a relatively short period of osseointegration (a few weeks).
Treatment planning for endosseous implants
A complete history and examination will establish the patient’s suitability for implants. A standard proforma for implant surgery will ensure a comprehensive, systematic history and examination. The medical history will identify possible contraindications for implants. An appraisal of the remaining dentition, the periodontal tissues and the standard of oral hygiene is required. Surgical access to the implant site is also assessed (Fig. 11.18). The type of anaes-Fig. 11.17 Photomicrograph showing the intimate
relation-ship between newly formed bone and a titanium endosseous implant (shown in black) embedded within cancellous bone of the jaws for 12 weeks.
157 157 thesia is discussed with the patient at an early stage;
intravenous sedation with local analgesia may be preferred in the outpatient or dental practice setting.
Contraindications for endosseous implants
Endosseous implants are not recommended for patients below the age of 16 because of the potential for further growth of the jaws; the implant is anky-losed in the bone, and therefore it will become sub-merged as the jaws grow. There is no absolute upper age limit for endosseous implants.
A patient’s medical history will determine their suitability for endosseous implants. For example, a poorly controlled diabetic is at risk of infection and soft-tissue breakdown around an implant, which may contraindicate implant surgery. The patient may have a history of psychosis, or there may be an ongoing history of drug or alcohol abuse. Such patients are unsuitable for implant surgery. Referral for a specialist opinion is recommended if there is doubt about the suitability of a patient with medical problems for implant surgery.
Poor oral hygiene will compromise implant provi-sion because of peri-implant infl ammatory disease (which is destructive in a manner similar to perio-dontitis). Patient education can effectively improve oral hygiene status in some patients. A patient must demonstrate consistency in maintaining their oral hygiene over several months; only then can im-plants become a feasible option. Smoking is a rela-tive contraindication, because some smokers are at
increased risk of losing the implant through infl am-mation of the peri-implant mucosa, and increased resorption of peri-implant bone. This manifests clini-cally as an increase in the bleeding index, the peri-implant pocket depth, the degree of peri-peri-implant mucosal infl ammation and radiographic evidence of bone resorption mesial and distal to the implant.
There is emerging evidence of an increased risk for implant loss in patients on bisphosphonates, though the risk for patients on oral bisphosphonates seems to be less than for patients having intravenous doses (see Ch. 4, p. 42 concerning osteonecrosis follow-ing tooth extraction).
Special investigations
The height, width and overall shape of the residual ridge are evaluated from study models and radio-graphs to ensure adequate bone to support the im-plant. Articulated study casts are required to evaluate the occlusion; there should be no excessive forces on the implant during lateral or protrusive excursions of the jaws. A preliminary assessment of the ideal position and angulation of implants may be made from the study casts, in conjunction with a surgical stent (Fig. 11.19). Radiographs (Fig. 11.20) will deter-mine the quality of the bone and help in assessing suitable sites for implants. The radiographs should
Fig. 11.18 Access to the implant site should always be assessed before embarking on implant surgery. At the beginning of site preparation there is little room to get the drill in a good position.
Fig. 11.19 An acrylic stent is placed in the mouth before radio-graphy to evaluate the quality and quantity of bone at the desired site of implant insertion. Gutta percha inserted into a bur hole cut into the acrylic stent will assist in radiographic planning. The dimensions of the gutta percha and the angulation of the bur hole permit accurate assessment of the proposed implant site.
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adequately demonstrate important structures (the inferior alveolar neurovascular bundle, the fl oor of the nose, the maxillary antrum and adjacent teeth).
It is necessary to establish whether an implant can be placed with the desired stability within bone and satisfactory soft-tissue aesthetics. If there is insuf-fi cient bone, then bone grafting must be considered.
Bone may be grafted from the patient, e.g. from the chin (Fig. 11.21), tuberosity region or retromolar region in the mandible. Proprietary bone scrapers facilitate the collection of autogenous bone by scrap-ing the cortical surface. Blocks of cortical bone may be taken, or cancellous bone chips may be harvested, depending on the extent of bone defi ciency at the implant site.
When the soft tissue is defi cient, a connective tissue or mucosal graft might be indicated. Such soft-tissue grafts are taken from the hard palate.
Connective tissue grafts e.g. Alloderm (an acellular dermal matrix derived from donated human skin tissue) can also be purchased for use in such situa-tions. The key objective with a soft-tissue graft is to re-establish the gingival contour for aesthetic reasons.
Determining the optimum length of an implant Most X-ray units do not give 1:1 magnifi cation of the image: this must be borne in mind when planning the optimum dimensions of an implant from the Fig. 11.20 (a) A panoramic radiograph yields much information about the teeth and jaws, but is unhelpful in determining the suitability of the bone at the proposed site of an implant in tooth 16 position. (b) A tomographic view of the bone in the tooth 16 area gives more information about the quality of the bone and the height of the maxillary sinus.
(a)
(b)
Fig. 11.21 A block cortical bone graft is being harvested from the patient’s chin to augment an atrophic upper central incisor socket. The block graft will be fi xed with a bone screw at the recipient site.
159 159 radiographs. An acrylic stent may be used to
deter-mine the planned location of an implant. The stent is fi tted over the teeth or edentulous jaw (Fig. 11.19) when the radiograph is taken.
CT scans (Fig. 11.22) are gaining popularity in the assessment of a patient before implant surgery. Cone-beam CT scan units offer improved functionality and convenience over conventional CT scanning units (http://www.imagingsciences.com/pro_iCAT_
features.htm). In the above illustration, a patient requesting implants in the right posterior maxillary sextant has undergone an I-Cat scan to assess the bone. There is a defi ciency in the buccal bone in sections 34–36, and clearly bone grafting is required before implants can be placed.
Planning the site of the implants from the radiograph
Endosseous implants are positioned within healthy bone, with an adequate height and width of bone.
Forces applied to an implant should be axial rather than lateral, to avoid forces that might result in implant failure.
A radiograph is a medicolegal document and should not be altered in any way. A copy of the radiograph is therefore made (either a digital copy or a copy made on tracing paper from the original) and
the teeth and other anatomical structures such as the inferior alveolar canal, the fl oor of the nose and the maxillary antrum are highlighted on the copy.
A transparent stencil (fi xture guide) is superimposed on the tracing to plan with accuracy the proposed position of the implant. The fi xture guide offers various magnifi cations depending on the X-ray unit used. The tracing may be used to illustrate the treat-ment plan to the patient.
The morphology of the residual ridge and the thickness of overlying mucosa are determined at the proposed site for the implants. Unless sophisticated CT scanning is available, a suitable implant cannot otherwise be selected reliably, and the procedure might have to be abandoned if there is inadequate bone. The depth of the oral mucosa cannot be assess-ed from a periapical or panoramic radiograph, because the image records the residual ridge only in two dimensions. Ridge mapping (Figs 11.23, 11.24) is simple and reliable, but may cause temporary mucosal trauma and discomfort.
Ridge mapping
Under local anaesthesia, a probe with millimetre markings along its tip is inserted into the oral mucosa overlying the alveolar ridge. The depth of the soft tissue is noted at all sites (Figs 11.23, 11.24). This information is transposed to a study model, and the shape of the residual ridge is recorded by trimming back the model stone to the corresponding depth of soft tissue.
Fig. 11.22 An example of a CT scan showing a right posterior maxillary sextant with an obvious defi ciency of bone (arrows), requiring augmentation with a sinus lift before implant placement.
Fig. 11.23 Ridge mapping. Under local anaesthesia, a graduated probe is pushed into the oral mucosa until it contacts bone.
The depth of the oral mucosa is measured.
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Diagnostic wax-up
Having planned the position of the implants, a diag-nostic wax-up (Fig. 11.25) will ensure that the desired aesthetic result can be achieved. If retained roots or other lesions are present, implant surgery must be deferred for 4–6 months after surgical intervention, to ensure satisfactory healing.
Informed consent
A complete discussion with the patient of all aspects of surgery, prior to commencing treatment is a requi-site for consent for any operative procedure (see Ch.
2). For procedures which are lengthy, expensive and irreversible, such as implant surgery and subsequent prosthodontic rehabilitation it is doubly important.
Only when the patient fully understands what is proposed can they establish realistic expectations of the treatment. Misunderstandings about implant therapy can be avoided when a written outline of the treatment proposed is given to the patient to
read and sign. An individual having high (unreal-istic) expectations may be disappointed by the out-come of surgery, and litigation may ensue. Written documented evidence of informed consent will help to avoid legal proceedings.