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					76 The Dentist October 2007


Screw it
Stewart Harding discusses two-piece implant systems.


wo-piece implant systems have a small space at the implant abutment interface (IAI) called a microgap, which can give rise to an area of local inflammation, facilitating a predisposition to bone resorption. Bone loss will continue until a biological width of two to three millimetres is achieved above the bone crest, sufficient to establish a soft tissue barrier. Experience has found that the biological width need not be a vertical dimension but can have a horizontal component. Platform switching provides this horizontal distance and so preserves the crestal bone (fig 1). For the effect of platform switching to be consistent, all restorative components must be platform switched.

lFig 1: The influence of platform switching on crestal bone level.

lFig 2: Two possible routes of bacterial penetration.

Experience has found that the biological width need not be a vertical dimension but can have a horizontal component.
For two-piece implant systems there are two potential pathways for bacterial penetration resulting in crestal bone loss. One route is via the inside of the abutment, along the screw threads eventually exiting at the IAI (microgap). Alternatively, bacteria may migrate along the outer surface of the abutment. If a cold weld is formed at the IAI,

lFig 3: Friction fit abutment. Note the lack of a screw.

lFig 4: Minimal drilling required to cut the osteotomy, preserving bone and improving primary fixation.

lFig 5: Bur harvests bone from the site during preparation.

lFig 6: Implant screwed into place using a ratchet.

Stewart Harding
is an honorary associate clinical professor at the University of Warwick.

creating a hermetic seal, then the bacteria travelling through the inside of the abutment cannot reach the bone, eliminating a potential cause of bone resorption (fig 2). The implant platform combines the benefits of a taper lock IAI and platform switching with the known restorative stability of the internal hex. An internal hex is

more stable than an external hex and helps resist abutment loosening, especially in friction fit abutments (fig 3). A hex or similar anti-rotation device is necessary, otherwise occlusal force may cause a friction fit abutment to rotate on top of the implant. This may break the cold weld and taper lock, causing the‹

78 The Dentist October 2007


€abutment to dislodge. This situation is particularly likely to occur in the upper anterior region of the mouth if angled friction fit abutments have been used. The shape of the implant also plays an important role in bone preservation during insertion. It is self-cutting and self-tapping, so the implant acts as its own sizing drill, a further time saving design feature, meaning that minimal site preparation is needed (figs 4 and 5). When placing, the implant will create its own threads, it compacts the bone and drills itself into position (fig 6).

Combi system The Combi system has a range of specifically designed healing abutments to manage soft tissue healing that help establish and

The shape of the implant also plays an important role in bone preservation during insertion.
maintain contour enhancing tissue aesthetics (fig 7). Lock-on bases are healing abutments that incorporate a friction lock that securely retains the lock-on base on the implant without the need for a screw (fig 8). This screwless friction

design eliminates micro-leakage. Lock-on bases are made from a specially formulated radiopaque polymer that permits radiographic confirmation of accurate component seating (fig 9). Directly following placement or at the exposure stage, a lock-on base is inserted into the implant to manipulate or form the soft tissues into the correct profile (fig 10). These healing abutments also act as a base from which a transfer impression may be taken, thus avoiding undue disturbance of healing soft tissue. A quick transfer is pushed firmly into the‹

lFig 8: Lock-on base pushed into position on the implant and held in place without the use of a screw. lFig 7: Lock-on base being carried to the implant.

lFig 9: Radiopaque lock-on base correctly seated on the implant.

lFig 10: Soft tissue healing after two months.

lFig 11: Quick transfer pushed into position in the lock-on base.

lFig 12: Section though the implant and lock-on base showing the transfer system.

lFigs 13 –15: Transfer impression.

80 The Dentist October 2007


lFig 18: Radiograph of final restoration. Note the maintained marginal bone level.

lFig 16: Friction fit post. Note the hex at the end of the taper. This improves restorative stability and means a locating jig is not required.

lFig 17: Final crown built up in composite type material applied directly to the post to produce a single unit.

lFig 19: Completed crown pressed firmly into position on the implant.

€lock-on base until the friction lock engages, giving a stable fit (figs 11 and 12). The impression material is syringed down through the central channel of the quick transfer and around it (figs 13 and 14). The filled impression tray is positioned and, once set, the quick transfer will be locked in place within the hardened impression material (fig 15). This type of impression

increasing stability and reducing the incidence of abutment loosening. Friction and screw fit abutments The friction fit range of abutments share the same features as the lock-on screw fit abutments, but rely on friction for retention and are screwless (fig 16). The lack of a screw in this design means that the IAI is held together by the cold weld formed when the abutment and implant are pushed together. This design may prove useful in the posterior jaw where space is at a premium and where it may be difficult to gain access to the abutment screw for tightening. Also, this type of abutment is ideal in the posterior jaw where screw loosening has been reported as a problem. The abutment hex has the added advantage that, when fitting the final restoration, the abutment locates in the internal implant hex (fig 17). This avoids the need for a positioning jig, as it is obvious if the abutment is in the correct position. Complete abutment seating is achieved by

Friction fit abutments may prove useful in the posterior jaw where space is at a premium.
means that posts can be prepared at the laboratory and not in the patient’s mouth, a time consuming and laborious task for patient and dentist alike. A screw fit or friction fit lock-on abutment will fit the entire implant range and is selected depending on the position in the mouth and the anticipated occlusal force. These restorative components all incorporate a taper lock and hex connection forming a biologic seal, eliminating odour,

having the patient bite together on a cotton wool roll (figs 18 and 19). The cold weld is then formed and the abutment is secure. In the case evidenced in this article, although the implant could equally as well have been restored using a screw retained abutment, a decision was made to use a screwless abutment and integral composite post crown. This decision was made on the basis of implant location and the direction of occlusal load in the posterior region. Had the implant been placed in the anterior maxilla, a screw retained abutment would have been the abutment of choice. Both types of abutment incorporate a hex taper lock seal and are platform switched, eliminating microleakage so marginal bone level need not be compromised.n

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