11 Surgical aids to prosthodontics, including osseointegrated implants
Although many advances in dental health have been made over the past few decades, it is nevertheless rare for an individual to retain a full complement of natural teeth for life. Teeth are lost for various reasons, notably periodontal disease, dental caries, pathological conditions of the jaws and trauma. Prosthodontics aims to restore not only the function and aesthetics of the dentition after tooth loss but also the aesthetics of the facial form.
A well-constructed removable prosthesis that replaces missing teeth will restore function and appearance. A removable prosthesis should be stable and have adequate retention and stability. To achieve this, the prosthesis should be seated onto well-shaped alveolar ridges with adequate basal bone and a healthy oral mucosa. There will ideally be no major vertical or horizontal skeletal discrepancy, which can compromise denture stability.
Preprosthetic surgery is a term used for surgical procedures that aim to improve the condition of the oral tissues to enable a removable denture to rest on a sound base, free from marked bony protuberances or undercuts, with no interfering muscle attachments, flabby soft-tissue excess or hyperplastic oral mucosa. To achieve the best results, the skills of the oral surgeon and the prosthodontist are combined in a team approach.
Endosseous implants are commonly placed in suitable patients to improve the stability and retention of removable dentures as well as fixed prostheses. Implants may avoid the need for more complex surgery to improve an otherwise unsatisfactory edentulous ridge.
Non-surgical options should always be considered first (e.g. remaking a technically unacceptable prosthesis, relining, adjusting the occlusal face height or extending the denture flanges to improve retention and stability) before preprosthetic surgery is undertaken.
As we age the blood supply to the jaws becomes increasingly dependent on the circulation in the periosteum, rather than from the arteries. This is largely due to age changes leading to narrowing of the lumen of vessels such as the inferior alveolar artery. Consequently, it is important to preserve the periosteum and its blood supply wherever possible, to minimize the risk of ischaemic necrosis of under-lying bone.
Physiological changes in the oral tissues are sometimes a consequence of hormonal changes. For example, oral discomfort may occur in women without overt clinical signs, and denture wearing may aggravate the symptoms in some patients. In a few cases, oral discomfort may be attributed to the menopause, and the symptoms may resolve after hormone replacement therapy. It is therefore necessary to obtain a comprehensive history from the patient in order to identify accurately the cause of any oral discomfort associated with denture wearing. Nutrition can also play a part in oral discomfort; some patients with sore mouth may be anaemic.
Changes occur in the morphology of the jaws after tooth loss (Fig. 11.1). The jaws are composed of alveolar and basal bone. The alveolar bone and periodontium support the teeth, but neither have a physiological function once the teeth are lost, and are therefore resorbed. Alveolar bone changes shape significantly with tooth loss, in both the horizontal and vertical planes, but the overall pattern of resorption is largely predictable. In the maxilla and in the anterior aspect of the mandible bone loss occurs typically in both the horizontal and vertical planes. In the posterior mandible the bone loss is mostly in the vertical plane.
Fig. 11.1 (a) A dentate mandible, illustrating the extent of supporting (alveolar) bone around the teeth. (b) An edentulous mandible, illustrating the extent of resorption of alveolar bone that occurs following loss of the teeth. Note also that the angle between the ascending ramus and body of the mandible is more obtuse than in (a), and the mental foramen is also closer to the crest of the edentulous ridge. The shaded areas in this illustration indicate areas of resorption of mandibular bone with advancing age.
After physiological resorption has occurred, the remaining jaw structure is termed the ‘residual ridge’. The bone that remains after alveolar bone has resorbed is termed ‘basal bone’. Marked resorption sometimes affects the entire mandible (Fig. 11.2). Basal bone does not change shape significantly unless it is subjected to excessive local forces, for example, in the edentulous anterior maxilla in association with retained natural lower incisors.
Other anatomical structures may become more prominent with tooth loss. The genial tubercles and their muscle attachments may become prominent in a patient with extensive resorption of the mandible, sometimes compromising denture stability. Maxillary or mandibular tori may also cause instability of a denture, or may be traumatized by it. A prominent fraenum (Fig. 11.3) can displace a denture during function, and may weaken the denture base so that it fractures through flexing.
Fig. 11.3 A prominent labial fraenum causes displacement of the denture. If the denture flange is eased to fit round the fraenum, the denture may be weakened. Excision of the fraenum (fraenectomy) is indicated.
Forces transmitted through the teeth during mastication are absorbed by the supporting structures (the periodontium and alveolar bone). In an edentulous patient, forces exerted by a denture are transmitted through the oral mucosa to the underlying bone. A denture must therefore fit well if trauma to the oral mucosa overlying an edentulous ridge is to be avoided.
Facial aesthetics are affected by tooth loss. The facial profile collapses (the nose and chin appear too close together) after tooth loss and consequent edentulousness. The loss of face height can be restored with dentures.
Cawood and Howell (1988) classified the edentulous jaws according to the state of ridge resorption after tooth loss (Table 11.1). There are other classifications, but this one has been adopted internationally as a means of assisting communication and assessment of a patient’s edentulous state.
|III||Convex ridge form, adequate in height and width|
|IV||Knife-edge ridge form, adequate in height but inadequate in width|
|V||Flat ridge form, inadequate in height and width|
|VI||Loss of basal bone, which may be extensive but follows no predictable pattern|
A removable denture requires a firm bone support with a smooth contour, adequate to provide stability, with access to the vestibule for a peripheral seal, and without any soft-tissue excess (e.g. flabby maxillary tuberosities) likely to cause displacement or weakness of the denture.
Most patients tolerate the loss of their natural teeth and subsequent denture wearing without difficulty. However, there may be extensive loss of alveolar bone after tooth extraction, resulting in an atrophic (flat or knife-edged) edentulous ridge (Fig. 11.4). In some patients this can make denture wearing difficult or uncomfortable. The prosthodontist may be able to modify a denture design to enhance its stability and retention, but this is not always possible. Surgery may therefore be required to enhance retention and stability of the prosthesis.
The history will indicate the patient’s principal concerns and expectations of treatment. The age and health of the patient influence the treatment plan’a young person with severely resorbed alveolar ridges might tolerate more complex surgery than would a mature patient with similar jaw morphology.
The height, width and general shape of the alveolar ridges are assessed, with an emphasis on the presence of bony undercuts and the position of anatomical structures such as the mental neurovascular bundle. The depth of the buccal sulci, the position and size of fraenal and muscle attachments and the condition of the alveolar ridges are documented. The dentures should be technically acceptable before considering preprosthetic surgery.
Radiographs are taken to assess the condition of the underlying bone. A panoramic film is useful to assess the overall condition of the edentulous ridges and to identify any retained dental roots or other pathology (e.g. cysts of the jaws). A lateral cephalostat may be taken to demonstrate the anteroposterior skeletal relationship and the height of the alveolar ridges anteriorly. Periapical views are desirable if retained roots are to be removed before construction of a denture.
Articulated study casts facilitate treatment planning and are helpful when explaining the surgical procedure to a patient. A diagnostic wax-up of the prosthesis is desirable to demonstrate the anticipated final aesthetic result to the patient, and as a medicolegal record of the proposed treatment.
Hyperplastic oral mucosa under or adjacent to a removable denture usually arises in response to chronic irritation, for example, from an overextended denture flange or a deficiency in the fitting surface of a denture, trauma from a sharp cusp on an acrylic tooth or an ill-fitting denture clasp. Poor denture design may also cause mucosal hyperplasia (Figs 11.5, 11.6). Surgery may be unnecessary if the cause of the hyperplastic tissue is identified and eliminated; the hyperplastic tissue will then usually diminish in size or resolve completely. Any residual tissue that inter-feres with denture construction can be removed via an elliptical incision as for an excision biopsy (see Ch. 8, p. 109). Where possible (e.g. in the buccal sulcus or on the cheek), the incision may be closed by suturing the wound edges together (primary closure). On the edentulous ridge, the periosteum is elevated to undermine the edges of the wound, and the edges of the mucoperiosteal flaps can then be advanced to achieve wound closure. A split-thickness skin graft may be required to cover extensive areas of denuded oral mucosa. A keratinized-free mucosal graft may be harvested from the hard palate for smaller areas. It is often beneficial to place a temporary soft lining in the existing denture after surgery, to minimize the likelihood of further irritation, prior to remaking the prosthesis.
Fig. 11.5 This is an extensive ‘leaf fibroma’ of the hard palate. The lesion was attached to the hard palate by a small stalk (a peduncle) and resembled the outline of a relief chamber incorporated into the fitting surface of the denture.
The flange of a denture may traumatize a prominent labial fraenum or muscle attachment (Fig. 11.3). If the fraenum is relatively small, this may be managed by trimming back the labial or lingual denture flange. However, the denture may be weakened and it might fracture if extensive trimming is undertaken to relieve the fraenum. Excision of the fraenum (fraenectomy) may be indicated to avoid this.
For the fraenectomy procedure (also described in Ch. 12) vertical incisions are made parallel to the fraenum, extending into the sulcus from the residual ridge to form a rhomboid-shaped wound (Fig. 11.7). The incisions are widest at the base of the labial sulcus. The insertion of the fraenum into the alveolar ridge is held with either a suture or a pair of toothed tissue forceps and the fraenum is dissected, leaving periosteum covering the surface of the bone. Interrupted sutures are inserted through the mucoperiosteal flap to achieve wound closure. A modification of this procedure incorporates a Z-plasty, to preserve sulcus depth (Fig. 11.8). However, the Z-plasty can be technically more difficult than the fraenectomy technique described above.
Fig. 11.7 Conventional fraenectomy. (a, b) With the upper lip everted, a rhomboid-shaped incision is made around the fleshy fraenum, extending through the oral mucosa to the submucosal layer below, preserving muscle fibres of orbicularis oris.
Ideally, the maxillary tuberosities are firm for denture support. If they are flabby and mobile, the soft tissues of the tuberosities may displace during impression-taking for a new denture, making denture construction difficult. Fibrous enlargement of a maxillary tuberosity may be reduced (Fig. 11.9) by making two incisions along the crest of the alveolar ridge to form an ellipse, angled towards the centre of the ridge down to bone. A t/>