A guide to implant dentistry part 2: surgical and prosthodontic considerations

Surgical considerations

Successful implant rehabilitation requires careful surgical manipulation which will take into consideration anatomic structures and will follow a strict surgical protocol regarding drill sequence and an aseptic technique.⁴ Each case will have a different degree of surgical complexity depending on the site (aesthetic versus non-aesthetic), the number of missing teeth and the soft and hard tissue deficiencies (Table 1).²

A surgical guide is advisable to facilitate a restoratively-driven implant placement; fixtures should be positioned in the correct three-dimensional position to achieve an optimum emergence profile (Figure 1).⁴ The correct type of implant should also be selected for a favourable emergence profile; narrow neck implants are used for maxillary lateral incisors and mandibular incisors, standard-neck implants are used for maxillary central incisors, canines and premolar teeth and wide-neck configuration for replacement of molar teeth.^{1, 5}

The timing of implant placement following tooth extraction has been investigated in various studies in the literature (Table 2).^{6, 7, 8} Similar survival rates have been reported for all types of implant placement but outcomes might be adversely affected by timing of implant placement.^{6, 7, 8}

Type 4 implant placement

This shows additional soft tissue volume and bone healing but greatest chance of increased bone resorption, thereby limiting the volume of bone for implant placement.^{11, 13} Type 4 implant placement should be considered in cases of excessive local pathology.^{11, 13}

Peri-implant defects with gaps of less than 2 mm following type 1 and type 2 implant placement may heal spontaneously.¹¹ However, peri-implant defects of 2 mm or more in the oro-facial dimension show reduced predictability for spontaneous bone regeneration.¹¹ Guided Bone Regeneration (GBR) is a bone augmentation technique in localized alveolar ridge defects using semi-permeable membranes with different particulate bone filler materials.¹⁴

Bone augmentation procedures show high survival rates of 95.7%, and dehiscence and fenestration resolution of 54% and 97%, respectively. GBR is more effective with type 1 and 2 implant placement (Figure 2).¹² The technique offers bone fill and defect resolution in peri-implant defects, improved soft tissue contour and hence aesthetics, and minimizes the risk of gingival recession (Figure 3).^{15, 16}

Survival rates of implants placed in guided tissue regenerated bone after treatment of localized defects in an alveolar ridge are comparable to survival rates of implants placed in native bone.¹⁷ Furthermore, the superiority of one augmentation technique over another, based on implant survival rates, was not demonstrated.¹⁷

Prosthodontic considerations

Fixed or removable implant-supported prostheses can be used to replace missing teeth as part of implant rehabilitation. Fixed implant-supported prostheses can be classified based on the number of teeth they are replacing: single crowns, short span, long span or full arch bridges, or classified based on the type of retention: screw- or cement-retained.

The choice between screw- and cement-retained, implant-supported prostheses remains controversial in the literature.^{18, 19} Screw-retained prostheses show more technical problems, such as screw loosening or ceramic fracture compared to cemented-retained restorations that can present with greater biological complications, such as peri-implant bone loss associated with marginal cement extrusion.^{20, 21, 22, 23} Ideally, screw-retained prostheses may be more desirable since they offer restoration retrievability, which facilitates long-term maintenance provision.²⁴ The provision of a screw-retained prosthesis requires accurate restoratively-driven implant placement, so that the implants are parallel and the screw emergence is not visible. It can be technique-sensitive, as optimal implant position and passive fit of the prosthesis are a pre-requisite to avoid problems.¹

The choice of material for the abutment of the definitive implant-supported reconstruction is generally between zirconium and titanium. Which material is more appropriate remains highly debatable in the available literature.^{24, 25, 26} The 5-year survival rate, technical and biological complications are similar for zirconium and titanium abutments supporting implant restorations.^{25, 26} Moreover, no differences have been found in soft tissue response in the peri-implant area between zirconium and titanium surfaces.²⁷ For prostheses in the aesthetic zone, zirconia abutments may be indicated to enhance the aesthetic outcome, but the clinical use of ceramic abutments in posterior sites or bridges should be used with caution (Figures 4–6).^{25, 28} Although zirconium and titanium abutments show similar survival rates, zirconium abutments require careful manipulation since adjustment of zirconium can adversely affect its mechanical properties.²⁹ Careful polishing is recommended after adjusting the abutments to keep surface roughness and phase transformation (commonly referred to as ‘ageing’) low.²⁹

Apart from the abutment material, the design of the prosthetic superstructure is of paramount importance. Designs should be smooth and convex to allow oral hygiene access and hence minimize biological complications.^{19, 28} Although CAD/CAM fabricated prostheses demonstrate similar survival rates to conventionally fabricated prostheses, reproducibility, passive fit and reduced stress at the implant-fixture interface can be more easily achieved with digital technology.¹⁹

Removable implant-supported prostheses can be classified as bar-retained or retained on individual attachments (balls, studs or magnets) (Figures 7, 8). For mandibular overdentures, two individual attachments are preferable to bars since they offer adequate retention and stability, similar patient satisfaction, easier fabrication, maintenance and are cost-effective.³⁰ For maxillary overdentures, bars supported by four or more implants show better outcomes regarding survival of both the implants and the prosthesis compared to four or less implants and individual attachments.^{31, 32} Implant-supported prostheses offer improved outcomes compared to conventional dentures and some clinicians suggest that two-implant supported mandibular overdentures should be the minimum offered to edentulous patients as a first choice of treatment.^{33, 34}

Maintenance

Although clinical and technological advances have led to an evolution in implant therapy, technical and biological complications are commonly encountered both in fixed and removable prostheses.^{3, 35, 36} Systemic conditions and treatments that might affect healing or susceptibility to disease, history of treated periodontitis and smoking are related to increased incidences of biological complications. Mechanical factors such as excessive cantilever extension, the vertical height of the superstructure, parafunctional activity or a patient with a history of previous complications, may increase the incidence of technical complications.^{37, 38, 39}

Implant-supported overdentures are a favourable solution for edentulous patients with 80% survival rate over 10 years.³⁵ Overdentures require regular maintenance, such as replacement of worn or deformed female retainers, repair of fractured bars, fractured acrylic or replacement of worn abutments. These maintenance episodes are reported to be approximately 85% for such prostheses.³⁵ Other common complications are repeated adjustment, loose abutments or attachments, denture fracture, denture reline, soft tissue hypertrophy around bars, peri-implant bone loss and implant failure.³⁶ Bar-retained overdentures require less maintenance compared to those dentures retained on individual attachments, but the former complications are usually more complex and financially costly.³⁷ Bar design and configuration may influence prosthetic maintenance. For example, round cross-sectional bars require more maintenance (1.2% complications per year) compared to milled bars (0.3% complications per year) due to reduced movement and rotation of the prosthesis.⁴⁰ Also, the presence of cantilever extension(s) over 15 mm are related to increased incidence of technical complications.^{38, 39}

Implant rehabilitation with fixed implant-supported prostheses demonstrate high survival rates; the 10-year survival rate of implant-supported crowns and implant-supported bridges are 89.4% and 86.7%, respectively.^{3, 41} Although survival rates are high, the complication rate is also high; 38.7% over a 5-year period, with implant-supported bridges showing higher incidences of technical complications compared to implant-supported crowns (Table 3).^{3, 42} The most frequent technical complications were fractures of the veneer material (ceramic fractures or chipping), abutment or screw loosening and loss of retention.³ Biological complications were encountered with similar incidences between the two treatment modalities.^{3, 42}

Summary

Implant rehabilitation is considered a predictable treatment modality to replace single and multiple missing units with high survival rates. Technical and biological complications are commonly encountered and careful treatment planning, restorative-driven implant placement and long-term maintenance are prerequisites of a successful implant rehabilitation minimizing these complications. The buccal bone thickness and interproximal bone levels for soft tissue stability, the correct implant dimensions for a favourable emergence profile, careful surgical technique, pink and white aesthetic harmony, design of the prosthesis to permit adequate oral hygiene, and planning for long-term maintenance care are the most important elements of treatment planning for implant-supported reconstructions. Attention paid to these factors will result in successful reconstructions with high patient satisfaction levels.