The third article in a series of six by Dr. John Moorhouse, Dr. Frances Trainer, and Colin Dean offering experience-based, practical guidance on this challenging technique



Osteotomy preparation
We discussed during the planning phase the importance of placing the head of the All-on-4™ (Nobel Biocare™) posterior implant(s) as far distally as possible. Ideally, this will be in the second premolar position, but can be as far mesially as the distal of the canine site. The distal-limiting anatomical factors are the antrum in the maxilla, the mental foramen in the mandible, and the volume of overlying bone. In addition, the osteotomy should not exceed a 45-degree angle from the midline sagittal plane, must not perforate what is often a narrow alveolar ridge, must maintain as much buccal bone as possible to avoid later soft-tissue complications, and still afford adequate intermaxillary space for the prosthetic components. The combination of these requirements presents a difficult three-dimensional exercise and is why the procedure is not recommended for the inexperienced. Although the process can be greatly simplified by establishing reference points, these will be different for each jaw.

Mandible
The inferior mandible reference point (IRP) is made using a No. 3 round bur in the cortex 5 mm anterior to the anterior lip of the mental foramen. (Figure 1) This needs to be increased if the mental loop was found to be longer during the planning stage.
The superior reference point is determined by making a mark 5 mm perpendicularly above the mental foramen. The point of insertion of the drill needs to be marked with a bur on the alveolar crest as far distally as possible. This point is located where a line between the inferior and superior reference points, made using the Nobel Biocare™ depth gauge as a guide, crosses the alveolar crest. An osteotomy in this line will be at the maximum 45 degrees (Figure 2), will suit a 30-degree multiunit abutment, and the angulation will avoid the IDB and anterior loop.

Very often, there will be less than 5 mm of bone above the mental foramen, and in these cases, the line of insertion increasingly rotates about the inferior reference point, steepening the angle to suit a 17-degree multi-abutment (Figure 3) until finally, the angle approximates 90 degrees, indicating a straight multiunit abutment. In such cases, it may be necessary to rotate about a point 1-2 mm anterior to the IRP, subject to the length of the loop on the x-ray (Figure 4). However, the path of the osteotomy must then not violate the area of bone directly underneath or immediately distal to the inferior reference point to prevent damage to the IDB.
Checking the orientation of the drill for the distal osteotomies with the All-on-4™ alignment guide is completed by fitting the guide into a median pilot hole made by a 2-mm twist drill. Either visually, or using the depth gauge as a guide, the line joining the point of insertion with the two anterior-point marks on the guide is then extended. The resulting angle with the guide’s perpendiculars can help the surgeon decide whether a 17- or a 30-degree abutment will be required, or whether the angulation of the osteotomy needs to be changed (Figure 5).

Osteotomy preparation is usually relatively simple in the lower lateral incisor area. However, the narrow ridge often found in patients with a Class 3 skeletal relationship and/or the bone loss associated with extensive periapical lesions of lower anterior teeth can cause complications with the placement site.

 

Maxilla
The All-on-4™ protocol describes making small, horizontal access holes into the sinus cavity and probing anteriorly to define the anterior wall (Figure 6). This surgical approach to the antrum has medicolegal implications. It is clear that the All-on-4™ procedure falls into the “complex treatment” category as defined in the General Dental Council (GDC)-supported “Training Standards in Implant Dentistry,” published by the Faculty of General Dental Practice in the United Kingdom. In addition, indemnity companies such as the Medical Protection Society regard any form of sinus surgery as introducing a higher level of risk, which attracts an increased annual premium. All surgeons should therefore ensure they have received the appropriate training and have medical indemnity insurance in place before accessing the sinus cavity.

We prefer to create a wider access cavity parallel with the anterior wall of the sinus to allow clear visualization and instrumentation without the risk of tearing the lining. A line of 3 to 4 marks with a No. 3 rose-head bur is then made in the buccal cortex 5 mm anterior to the anterior wall of the sinus. This line is then extended to the newly recontoured alveolar surface, and a mark is made at the intersection to mark the provisional point of insertion of the drill. A second, anterior reference point is made on the line at a height midway between the alveolar surface and the level of the floor of the nose. The point of insertion and the anterior reference point are joined using the depth gauge as a guide, and if the resultant angle exceeds 45 degrees, the line of insertion should be increasingly rotated about the anterior reference point, steepening the angle to suit first the 30-degree (Figure 7) and then the 17-degree multi-abutment (Figure 8). The path of the osteotomy must not violate the area of bone directly above or distal to the anterior reference point to prevent damage to the sinus lining.

The All-on-4™ alignment guide can then be fitted into a midline hole made with a 2-mm twist drill to check the orientation of the drill for the distal osteotomies. This is again achieved by visually (or with the depth gauge) extending the line joining the point of insertion with the anterior reference point on the guide. The resulting angle with the guide’s perpendiculars will reveal whether a 17- or a 30-degree abutment will be needed—or whether the angulation needs to be changed by moving the point of insertion. Where the sinus is large, it can be helpful to plan for a 17-degree abutment and utilize the available bone volume lateral to the nasal cavity for a longer implant (Figure 8).

When treating the terminal dentition, it is very often possible to use the premolar and canine sockets as reference points and therefore avoid the need to access the sinus. The osteotomy site is pre-planned clinically and radiographically in the usual way but with special attention to its position relative to the available socket depths. Marks are made in the buccal cortex at the depth of the sockets using a depth gauge or narrow forceps (Figure 9). Two points are then made, with reference to the relation of the socket and sinus outlines on the panoral radiograph, 5 mm anterior to the anterior wall of the sinus; one on the alveolar surface (point of insertion) and the other (anterior reference point) at a height halfway to the level of the plane of the floor of the nose measured relative to the marks at the socket depths. The All-on-4™ alignment guide is used as before, and the angle of insertion is adjusted to suit the appropriate multiunit abutment.

The canine socket always has a thin labial wall that must be preserved if at all possible. There is often a useful volume of bone palatal to the canine socket, which will accommodate a distal implant with a point of insertion through the premolar region. However, its presence should not be relied on, and it is rarely found in patients with a Class 2 div 2 skeletal relationship.
Although the anterior maxillary ridge can be narrow, especially in the lateral incisor area, adequate bone volume for implant placement can often be found in two other areas: immediately lateral to the incisive canal, and at the base of the nasal cavity. The latter can involve considerable reduction of thin crestal alveolar bone and creates problems with abutment angulation and excess soft tissue at closure. It can be helpful to angle the osteotomy mesiodistally across the floor of the nose to gain more surface area for integration and primary stability. The use of a 30-degree abutment is necessary for alignment, and its 4-mm height will make it easier to manage the excess soft tissue and take the impression (Figure 10).

Point of insertion, osteotomy, and implant insertion
It is helpful to mark the point of insertion of the drill using a No. 5 round bur. This mark should be deep enough to stop the drill slipping, and placed centrally or toward the palatal or lingual side of the ridge if the ridge is wide enough (Figure 5). This point also marks the planned position of the head of the implant, and the multiunit abutment requires space to be fitted passively. It is often possible to use an adjacent distal socket to accommodate the abutment and avoid further bone removal close to the implant head.
Recontouring the alveolar bone has already provided the surgeon with an indication of the bone density, and Table 1 shows the available Branemark® system regular-platform NobelSpeedy™ Groovy drills, and the Nobel Biocare™ recommended insertion sequence.



The surgeon must now adapt the drilling sequence to improve primary stability by omitting the larger diameter drills in less dense bone and/or by drilling short of the proposed full implant depth. We include each diameter of drill for medium and hard bone, as we believe it improves irrigation and encourages a more sensitive tactile appreciation of the bone density, which allows us to adapt the depth of the final drill accordingly—or omit it completely. We also have a regular platform 7-13 mm bone tap (33072) available for the very dense bone often found in the mandibular symphysis.

Abutment choice and fitting
After insertion, it is usually necessary to relieve the surrounding bone because of the angulation of the implant. This can be carried out with rongeurs or the special bone-milling tool (28978). In extremis dense bone can be removed with burs, taking great care not to touch the implant head (Figure 11).

A 17-degree (2-mm height) abutment will require a wider margin of superficial bone clearance as it overlaps the implant head. We use 30-degree abutments with 4-mm height and sometimes find it necessary to deepen the implant head to ensure adequate intermaxillary clearance.

The area around the implant heads must be thoroughly debrided and irrigated with normal saline to remove bone particles. It is good practice to irrigate under the flaps at this stage.

The appropriate abutment was chosen during the planning stage, but surgical complications in relation to the implant site, depth, and alignment may have introduced changes to the choice of the abutment angulation. Each abutment has six different positions of rotation on the NobelSpeedy™ Groovy implant head. The aim is to have the abutment alignment guide-pin emerge through the occlusal table for the posterior implants and the cingula of the appropriate incisors for the anterior implants (Figure 12).
To check the alignment of the abutments with the guides in situ, we use a clear plastic blow-down guide stent with the cingula and occlusal surfaces removed (Figure 13). This works very well in the upper jaw where the stent can be held firmly against the palate. However, it is easy to displace the stent in the lower jaw, and here, a more accurate assessment can be made by removing the abutment guides and bringing the lower jaw into the planned occlusion, keeping the stent in place until the correct vertical dimension is obtained. The stent is then held firmly in the same relation to the lower jaw, which is opened slowly to reveal the alignment of the abutments. In the lower anterior region, it is often necessary to angle the abutments forward to reduce the bulk of lingual acrylic. Finally, the abutments should be tightened in place (torque 35 Ncm for straight and 15 Ncm for 17- and 30-degree abutments) and the guides removed.

Flap closure and impression taking
Before closing the flap, the technician should be invited to note the level of the bony ridge relative to the abutments and be made aware of the depth of soft tissue over the ridge once suturing is complete. This is to ensure that the model will be correctly trimmed and the fitting surface of the bridge will seat accurately and not crush soft tissues during or after fitting.  
Closure should be achieved with single interrupted sutures distally and between the implants to allow modification of the flap around the abutments. Excess soft tissue is judiciously removed from the palatal or lingual aspects with a scalpel to preserve as much buccal attached gingivae as possible. The aim is for the impression copings (and temporary bridgework) to be able to locate easily and be largely free of soft-tissue coverage to ensure they are firmly embedded in impression material.



Once the abutment position and angulation have been established, the technician will prepare corresponding holes in the special tray. Impression copings are best brought to the abutment with the fingers, tightened lightly, and then unscrewed about half a turn (180 degrees). A flat plastic instrument is then passed around the perimeter of the junction of the impression coping and abutment to ensure no soft tissue is trapped, before the coping is tightened more firmly in place with a screwdriver (Figure 14).
Pressure should be applied to the soft tissue around the copings until hemostasis is established and the copings are clear. The special tray should then be tried in to make sure the copings pass easily through the prepared holes. At the same time, it is important to retain as much support from the tray as possible without making it too difficult to locate the copings in the tray holes when fully loaded.

A monophase crown-and-bridge material should be employed. Our own choice is Impregum™ Penta™ H DuoSoft Quick (3M ESPE), and we syringe material around the copings before fitting the loaded special tray. We use a tray with a broad handle to facilitate positioning over the copings. Two fingers from each hand are then placed on the tops of the copings on each side and upward pressure maintained on the handle with both thumbs to prevent the tray dropping (Figure 15). This simple measure maintains a bulk of impression material around the copings, and the fingers can be peeled away when the impression has set to reveal the top of each coping (Figure 16). When taking upper jaw impressions, the assistant monitors the airway and wipes away with the mirror head any excess impression material that may flow towards the back of the mouth.

Care should be taken to stabilize the impression while the last screws are removed to avoid stress on the implant or the coping in the impression. Access to unscrew the posterior copings is restricted, and it can be helpful if the nurse maintains locating pressure on the screwdriver to prevent slippage (Figure 17). The assistant should keep flat tweezers instantly at hand during impression-making and the subsequent stages in case any item should drop to the back of the mouth.

The Lymm technique of registration
The All-on-4™ protocol describes the use of wax registration blocks. However, this technique introduces a potential margin for error into a critical stage of the procedure, and in 2007, we devised the “Lymm” method of registration using modified Nobel Biocare™ components. We have found this method to be consistently accurate and reliable.



These registration aids are simply impression copings that have been milled flat (parallel to the base) at the neck and at 3 mm above the base (Figure 18). It is essential to have four short and four long identical aids, with a “spare” available in case of a spill from the sterile area.

A registration aid of the approp-riate height is fixed to each abutment, with care taken to ensure no soft tissue becomes trapped on seating. The aid must protrude sufficiently from the soft tissue to enable surface detail to be recorded, but not so far as to lead to excessive trimming of the temporary bridge. A note needs to be made and passed to the technician of the height of the aid on each abutment so that the same height of aid is attached to the appropriate analog in the newly cast model (Figure 19).
While the abutments are being fitted, the technician will grind out corresponding preliminary depressions in the fitting surface of the temporary bridge to accommodate the registration aids (Figure 20).

The temporary bridge is positioned over the aids and brought into occlusion. This is checked against the planned occlusion and vertical dimension. The depressions are then adapted by the surgeon to remove interferences to the passive seating of the aids. The aim is to achieve the planned occlusion in RCP at the pre-determined closed vertical dimension using the marks on the nose and chin and the setting on the Willis gauge. This stage can take some time to complete satisfactorily.

When there are few teeth in the opposing arch, it is helpful to use the midline and interpupillary line as reference points to guard against the occlusal plane of the temporary bridge tilting in lateral and anteroposterior planes. Time spent on spatial orientation at the start of the procedure is always well spent. The surgeon should also refer to the pre-operative photograph for clarification of the original overbite and overjet.

The temporary bridge is then removed, cleaned, dried, and adhesive is applied to the fitting surface. Gauze swabs are used to maintain pressure on the soft tissue while the bridge is being prepared. A layer of impression material is then placed on the fitting surface, and we normally allow half the setting time to elapse before seating it in the mouth and re-establishing the occlusion and vertical dimension as detailed above. Once it has set, a registration is taken of the bite using a snap-setting mousse, such as Aquasil Bite from Dentsply, and the bridge and mousse are removed together after a final check with the Willis gauge (Figure 21).
The impression surface should be checked to ensure a clear imprint of the surfaces of the registration aids, (Figure 22), and the impression is then placed in a bowl of disinfectant. The registration aids are removed, placed in the disinfectant, and replaced with the temporary caps (Figure 23). It is useful to apply pressure with gauze at this stage to ensure hemostasis and minimize any potential hematoma formation.

The first surgical phase is now complete. Following discharge by the anesthesiologist, the patient is taken to the recovery room, re-united with his/her guardian, and regularly monitored while the bridge is prepared for fitting.

Figures 2-4 and 7-9 were drawn by Zac Davidson.

For further information, please visit www.nobelbiocare.com. The authors can be contacted by email at lymm@oasis-healthcare.

Bios
John Moorhouse, BDS, DDH, MFGDP, DPDS, Dip Implant Dent RCS Eng, was awarded his BDS at Manchester University in 1978, winning the prize for periodontology. He has spent many years in the NHS and private practice providing the full range of skills required of the modern dental practitioner. He has worked extensively abroad establishing dental clinics in Zaire, Thailand, and Malawi. He was the lecturer in oral health for the school of Medicine in the University of Malawi and has published several research papers on oral health and health education. He was awarded a Diploma in Dental Health for Educators, DDH, in Dublin in 1989, winning the Presidents prize. He was awarded the Diploma of Membership in the Faculty of General Dental Practitioners of the Royal College of Surgeons (MFGDP), in 2001. He also gained the Diploma in Postgraduate Dental Studies (DPDS) in 2001 involving children’s dentistry, denture design and provision, and the management of tooth wear. Dr. Moorhouse was in one of the first groups of dentists to be awarded a Diploma in Implant Dentistry and has been a tutor of the course since.

Frances Trainer, BDS, FDS, RCS (Eng), qualified in Dundee and after postgraduate posts in London and Edinburgh, gained her fellowship from the Royal College of Surgeons, England. She has spent the last 20 years in general dental practice, the last 10 years in Lymm with John Moorhouse, where restoration of dental implants has become a major part of her clinical work. She has been involved with undergraduate teaching at The Manchester Dental School and is currently a postgraduate tutor for the Manchester and Stockport areas. Dr. Trainer is also a clinical-support advisor for Oasis Healthcare, and a regional clinical-services advisor at Oasis.

Colin J. Dean, FBIDST, Dip RCS (Eng), qualified as a dental technician in 1966 and has spent many years completing both NHS and private prosthetic and crown-and-bridge work. He worked for a time at Birmingham Dental Hospital before becoming Senior Lecturer in Dental Technology at Manchester Polytechnic. He became co-owner of Bolton Dental Laboratory in 1979 and was awarded Fellowship of the British Institute of Dental and Surgical Technologists in the same year. Colin further qualified as a Clinical Dental Technician in 2003 while working as a Senior Dental Technician at Manchester Dental Hospital. In 2005, together with his son Jonathan, Colin opened C&J Dental Technologists Ltd, where they specialize in implant-retained prostheses. The business also incorporates a surgery where they see patients directly for the provision of dentures. Colin is an occasional Lecturer in Dental Technology at Manchester Metropolitan University.