Implant Placement

The ideal siting and orientation of the implant is dictated by the restorative requirements,but this may have to be modified by the existing ridge morphology and adjacent anatomical structures. Following elevation of the flaps the surgical stent, should be tried in. In partially dentate cases the stent should be stabilised on adjacent teeth and provide guidance of where the planned labial faces, occlusal surfaces or cingulae of the teeth to be replaced are to be located. An example of a suitable stent is shown in figure. In edentulous cases it is far more difficult to provide a stable stent as it will have to rely upon a mucosal fit in areas where the mucoperiosteum has not been raised. Ideally an implant should be placed such that: 

• It is within bone along its entire length. Exposure of limited areas of implant surface associated with bone defects such as dehiscences or fenestrations may be acceptable, but larger ones may require augmentation.

•  It does not damage adjacent structures such as teeth, nerves, nasal or sinus cavities.

•  It is acceptable to engage the nasal or sinus floor with a small degree of penetration (eg 1 to 2 mm). An adequate safety margin of about 2 mm above the inferior dental canal is recommended.

Diagrammatic representation of Implant placement.

•  It is located directly apical to the tooth it is replacing and not in an embrasure space.

•  The angulation of the implant is consistent with the design of the restoration.This is particularly important with screw retained restorations where it is desirable to have the screw access hole in the middle of the occlusal surface or cingulum of the final restoration. Multiple implants are placed in a fairly parallel arrangement, to facilitate seating of the restoration. However most systems allow convergence/divergence of up to 30° without the use of angled/ customized abutments.

•  The top of the implant is placed sufficiently far under the mucosa to allow a good emergence profile of the prosthesis. This is often achieved by countersinking the head of the implant. For example, it is suggested that the top of a standard diameter implant (about 4 mm) when used to replace a single upper incisor tooth, should be 2 to 3 mm apical to the labial cemento enamel junction of the adjacent natural tooth.

•  There is sufficient vertical space above the implant head for the restorative components.

•  The implant should be immobile at placement. A loose implant at this stagewill fail to osseointegrate.

•  Adequate bone is present between adjacent implants, and between implants and adjacent teeth. This should preferably be about 3 mm and never less than 1 mm. In some cases 1 mm of bone may be acceptable for implant spacing, but the abutments may have a larger diameter and therefore prevent proper abutment seating, thereby complicating the restorative procedure. A distance of 3 mm will also allow better soft tissue adaptation and may allow the maintenance of an 'interdental papilla'. The above requirements are not always easy to achieve and in many circumstances would be impossible without a properly designed stent produced from the diagnostic set up. Inevitably there are situations where a careful balance is needed between the ideal 'set up' and possible implant placement.

Fig.: Relationship of the implant to the mental foramen. Most anterior extent of the mental foramen is usually 3 mm from the anterior most extent of the mental nerve ("the loop") before it exits from the bone. The implant should be place at least 2 mm from the mental nerve; which means the implant should be at a minimum of 5 mm from the mental foramen.

Anatomical limitations to implant placement

Structure             Minimum required distance

Buccal plate           0.5 mm

Lingual Plate          1 mm

Maxillary sinus        1 mm

Nasal cavity           1 mm

Incisive canal         avoid midline maxilla

Interimplant distance   3 mm between outer edges of implants

Inferior alveolar canal  2 mm from superior aspect of bony canal

Mental nerve             5 mm from anterior or bony foramen

Inferior border          1 mm

Adjacent natural tooth    0.5 mm