Single Tooth Replacement STR

Whilst STR is perhaps the most exciting advance in implant technology, with the genera! practitioner expressing a great deal of interest in this particular area, it is nonetheless a most demanding and difficult restoration.

The unique requirements of STR have been considered by the experts to be complex, though it is recognised that STR may well become the most sought after treatment referred by the general practitioner. It therefore rightly takes precedent in this prosthodontic section Prior to tackling STR it is necessary to have an appreciation of the causes of tooth loss or absence.1 It is useful to consider the surgical sieve and to determine if tooth loss is developmental or acquired and furthermore whether the acquired condition is the result of trauma, infection, or pathology. As a rule it is worth remembering that the ideal restoration should not be compromised by the cause of tooth loss; and should itself not compromise general dental health.

Requirements for STR have been listed in Table 4.1 and for con venience are split into surgical and prosthodontic sections. The loss of an anterior single tooth can have a profound effect on the appearance of an individual (Fig 4-1). often resulting in social embarrassment. Concerted efforts have recently been made in the implant field to accommodate STR within the remit of implant therapy and already a number of articles have been published discussing this treatment concept,2"7

An essential technical requirement has been the construction of an abutment that provides dual antirotation, not only between fixture and abutment (already available for the hex top implants) but also between the abutment and the crown, which being unsplinted would be subject to loosening under torquing forces. Furthermore it was recognised that the average implant width would not lend itself to supporting a crown with an aesthetic optimal profile as it emerged through the soft tissues. New abutment designs were required, that would incorporate dual anti-rotation features and help improve the crown emergence profile.

Table 4-1 Local Indications and Requirements for Use of a Single Tooth Implant

Surgical

Absence of local pathology or infection Adequate bone volume Suitable socket status or ridge morphology Acceptable bone quality Healthy soft tissue status Absence of vital structures in the surgical field

Prosthodontic

Available saddle width (with respect to component specifications) Available interocclusal space Occlusal factors, ie Anterior/canine guidance.

group function and conlact scheme Absence of paraiunction Crown: implant ratio = 1:2 Minimally restored adjacent teeth

A number of publications have presented various options available for use with a variety of hex top implant designs.611

The system demonstrated in this book so far, has presented an internal conical design that does not incorporate any anti-rotational mechanism. It was therefore necessary for the manufacturers to design a single tooth system which should incorporate dual anti-rotation features and an improved emergence profile to accommodate the needs of STR. This chapter will introduce the concept of STR using this system. Having determined that an implant is the optimum treatment of choice (according to those guidelines set out in chapter 1) and that it will promote and not compromise general dental health (Fig 4-2), the basic surgical protocol is unchanged, with incision, flap elevation (Fig 4-3) and bone preparation (Fig 4-4) following similar lines to those described in chapter 2.

Additional care is required in the handling of soft tissues to ensure that at least 1 mm of interdental tissue is left intact (Fig4 -3).This is necessary in order to maintain a functional and aesthetic interdental papilla which will otherwise recede if it is stripped along with the mucoperiosteai flap. Furthermore it is also necessary to adapt the bone site to the design of the fixture. In this case the use of a conical drill (Fig 4-5) flares the coronal aspect of the bony site (Fig 4 - 6) to meet the geometry of the single tooth fixture (See Fig 2-16). The surface texture of this single tooth implant is roughened by a cleaning process that blasts titanium dioxide particles at the implant (TiOblast'") thus roughening the surface available for osseointegra-tion by a pitting of the titanium. This has been shown to result in an

Fig 4 -1 Loss or absence of a tooth can be emotionally upsetting and socially embarrassing as well as compromising function and the ¡deal dental state. A glance at the completed restoration (Fig 4-23, p78) will show the difference that can be made.

Fig 4-2 An ideal case for implant replacement. All adjacent tissues demonstrate health and vitality and the provision of an implant would prove more conservative than other more conventional restorations.

Fig 4-3 For single tooth replacement, the incision line and muco-periosteal flap should leave a 1 mm margin of sound interdental tissue unstripped from the underlying bone. This will ensure the maintenance of aesthetically pleasing papillae, enhancing Ihe contour and shade of the porcelain restoration.

Fig 4-4 Bone preparation is carried out in the manner described in chapter 2. The need for a drill extension connector is often indicated when placing an implant between adjacent nalural teelh.

Fig 4-5 The need to customise bone preparation, according to implant design, is highlighted here by the use of a conical drill for the flared Astra single tooth implant.

Fig 4-6 Ideally the host bone site should comfortably accommodate the preparation, without resulting in fenestrations or perforations. The use of accurate ridge mapping techniques will encourage predictable results.

Increased resistance to torqulng forces12 and may have an Impact on the rate and degree of osseointegra-tIon.13 Similar results have been claimed for titanium plasma spray and hydroxyapatlte coatings,14"17 Additionally Microthreads™ characterise the flared coronal part (Fig 47), ensuring that osseointegration is achieved even at the most coronal aspect of the fixture. Healing periods follow standard protocol, as does the exposure of the implant through a slit or punch incision (Fig 4-8). Having ascertained mucosal thickness (Fig 4-9), it is then possible to insert the single tooth abutment (Fig 4-1 0), securing It to the fixture with a central abutment screw. The single tooth abutment is characterised by an internal hex at the base of the conical interface, with an external Octagonal Star Design w that provides anti-rotation between crown and abutment (Fig 4-11), The concave surfaces of the octagon also act as cement venting reservoirs, preventing excess cement from Interfering with crown seating (Fig 4-12).

Impression techniques are made simple by utilising a prefabricated plastic Impression coping which is similar for a number of systems (Fig 4-1 3). The ideal impression material of choice Is Impregum® (ESPE, Oberbay, W. Germany) being a relatively mobile material on inser-

Fig 4-7 The Astra single tooth implant is characterised by a TiOblast™ surface with coronal Microthreading1" which optimises interfacial surface area and encourages predictable osseointegration.

Fig 4 -8 The underlying cover screw is exposed through a slit or punch incision,

Fig 4-9 Sofî tissue thickness is measured with a depth gauge thai locates directly into the fixture. Readings correspond with available abutment lengths.

Fig 4-10 The abutment locates the internal hexagon of the fixture and is secured to it by means of an abutment screw, which passes through the centre of the hollow abutment.

Implantes Tbr

Fig 4-11 The Astra single tooth abutment is characterised by a locating hex at the base of (he conical interface; an external Octagonal Slar Design™ for abutment/crown anti-rotation; and a cylindrical body for support of the restoration.

Fig 4-12 The Octagonal Star Design™ incorporates concavities that act as venting reservoirs, to allow the free flow and accommodation of excess cement, thus preventing mis-seating of the crown due to hydrostatic pressure caused by excess cement in a confined space.

Fig 4-13 Plastic friction fit copings are secured to the abutment for the impression. Retention tags ensure that the coping is withdrawn with the impression.

tion, yet withdrawable whilst being rigid at final set.

II is essential that the Impression coping is accurately seated on the abutment. On inspection of the impression (Fig 4-14), any material apparent on the inside of the coping is indicative of inaccurate seating and obligates a retake of the impression. The use of a stock tray is acceptable for single implant impressions; however for multiple implant impressions it may be preferable to arrange for a customised tray to be fabricated. The patient will require temporisation while the impression is sent to the laboratory for casting and crown fabrication. In the system demon strated an impression coping can be cut to size and cold cured acrylic bonded to it for aesthetics (Fig 4-15), Though some systems have historically advocated the manufacture of implant borne prostheses on stone die replicas of the abutments,'8 the majority of implant manufacturers today recommend the use of laboratory abutment replicas {or analogues), which are inexpensive stee! dummies that are incorporated into the master cast (Fig 4-16). In keeping with the concept of accuracy, most single tooth systems provide a prefabricated gold alloy cylinder to act as the fitting surface of the crown. Alternatively the BrSne-mark system also provides a pre-

Fig 4-14 Impregum (ESPE, Oberbay, W, Germany) impression material with coping in situ. Any material on the fitting surface of the coping would denote inaccurate sealing, requiring the impression to be retaken.

Fig 4-15 A separate coping can be cut down and used for temporisation. between visits. Cold cured acrylic can be bonded to the coping to restore tooth form.

Fig 4-16 Steel or brass laboratory replicas are secured in to the coping, prior to casting. The set master cast will then incorporate an accurate analogue, on which the final restoration can be fabricated. In this case a securing screw will help to keep the casting cylinder firmly on the replica.

Fig 4-17 In order to achieve an accurate fit, the metalwork for the crown is cast around a prefabricated gold alloy cylinder which locates on to the abutment replica. The press fit plastic sheath is helpful during the waxing procedure.

Fig 4 18 Metalwork can be cast in precious or semi precious bonding alloys. However different systems produce cylinders in different alloys and it is therefore necessary to confirm with the manufacturer, which casting metals are suitable.

Fig 4-19 Slurrying and firing porcelain follows standard procedures. In the case shown, a narrow cervical margin will aid an aesthetic result.

fabricated sintered ceramic cylinder. In the system demonstrated the cylinder is manufactured from a gold alloy with a press fit waxing sheath (Fig 4 -1 7), that is suitable for casting with the majority of precious and semi-precious bonding alloys (Fig 4-18), and a design which encourages a gradual increase in tooth bulk from the cervical margin (Fig 4-19). On the articulator it Is necessary to check that the crown is in light contact (Shimstock^ Hanel-GMH-Dental, W. Germany) in occlusion and that it is free of premature or non-working side contacts. If the tooth being replaced is a canine, it is preferable to place the associated quadrant Into group function, thus preventing overload of the implant in lateral excursions. If however the tooth is in any other position it is wise to opt for natural canine and/or anterior guidance so that the implant borne restoration is in discclusion during these excursive movements. An assessment of crown: implant ratio will also influence occlusal contact, since an unavoidably long crown, the result of excessive crestal bone loss, will lead to unfavourable moment arm forces, which may result in crown loosening or implant overload. In these cases it may be preferable to place the restoration out of contact in working excursions. Insertion of the crown is technically simple, using a standard cementation procedure (Fig 4-20). Prior to cementation it is necessary to protect the head of the abutment screw with some cotton wool or Fermit® (Ivoclar-Vivadent UK Ltd, England). Cement should be sparingly applied to both the inside of the crown and the abutment, preferably using a brush. The cement of choice should be Zinc Phosphate or Glass lonomer for permanent cementation and a modified Temp Bond®(Kerr UK Ltd, England) for provisional cementation, which may be necessary if there are doubts about shade or crown contour. In all cases it is essential that an even pressure is applied and that the crown is correctly seated, fully engaging the external hexagon or octagon. The use of intra oral radiographs will confirm whether or not a crown is accurately seated (Fig 4-21 and 4-22).

There is little doubt, that the single tooth implant will do much to restore the dental imbalance of tooth loss and social embarrassment caused by anterior single tooth loss, without relying on the support or destruction of adjacent tissues (Fig 4-23 and 4-24).

Fig 4-20 The crown is inserted and checked for aesthetics, occlusion and contact schemes. Permanent cementation is effected by the use of zinc phosphate, glass ionomer, or composite cements.

Fig 4-21 Fig 4-22

Fig 4-21 (left) The abutment design allows for an appreciation of accurate seating but post insertion radiographs are advisable to assess accuracy of fit. This canine is incorrectly positioned and will require removal and reseating.

Fig 4-22 (right) This radiograph of a central incisor shows the crown correctly cemented in place.

Fig 4-23 The overall effect of permanent tooth replacement, especially where there may have been social embarrassment, is profound and extends well beyond simple dental rehabilitation With care and attention to soft tissues and the aesthetics of the ceramometal restoration (see fig 4-24), an implant relained crown can prove a most natural tooth replacement

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