23: THE EVOLUTION OF THE ANGLED IMPLANT

CHAPTER 23 THE EVOLUTION OF THE ANGLED IMPLANT

The use of angled implants has evolved in recent years to the point where several of the variations available can be considered standard practice. A significant benefit of this approach has been the elimination of grafting procedures, including sinus and onlay grafts in both arches, for many patients who previously would have required an augmentation approach. This not only simplifies treatment and reduces time needed to reach a final result, it also eliminates the necessity to engage a donor site to produce the bone necessary for grafting.

A final potential benefit of the use of angled implants is the increased ability to immediately load a provisional prosthesis in specifically indicated situations. Certainly other factors such as roughened implant surface treatments and improved implant geometry for higher initial torque values play an important role in immediate loading capabilities. However, creating an improved anterior/posterior spread with longer distal implants via the tilted, or angled approach is a critical aspect of a successful immediate loading protocol.

image The Pterygoid/Tuberosity Implant

The first use of an implant placed in the pterygo-maxillary region was documented by Dr. Tulasne at the suggestion of Dr. Paul Tessier in 1985,1 only 3 years after the introduction of the concept of osseointegration to North American practitioners at the Toronto Conference in 1982. By necessity, these implants needed to be angled toward the anterior part of the mouth for access and to create the greatest possible length of implant-bone contact.

Successful placement of an implant in this area requires a volume of bone in the tuberosity region that will accommodate an implant without fenestration laterally. Due to the angulation of placement, however, it is not uncommon to have an implant length capability of 13-15 mm. There are even variants of this procedure using engagement of the sphenoid bone, through a guided approach, that allow placement of implants 20 mm or longer.2

The emergence of this implant into the oral cavity may appear to be problematic for access due to the distalized position of the fixture. In clinical practice, however, there is seldom a limitation for engaging the screw access hole with the use of angled abutments and contra-angle drivers. Occasionally, the tilt of the pterygoid implant is such that straight drivers can be used for direct screw access.

Indications

The pterygoid implant generally is used where posterior support for a full-arch prosthesis beyond the first molar is required. The placement of dentition in the area of emergence of this implant is seldom required, but the additional support gained with a beam extension can eliminate cantilever stresses that can be difficult to counter when opposing second molar occlusion is present (Figure 23-1).

This is generally a niche concept, but one that can be very valuable in prosthesis design and construction when additional cantilever support is necessary. As seen in the existing literature specific to the pterygoid approach, there are reported rates of implant success varying from the 92.7% initially seen by Tulasne to between 67% and 93% success in more recent papers.38 Although probably not considered a mainstream approach due to a lack of universal application, it is still valuable in specifically indicated situations. When tuberosity bone is available, the use of the pterygoid implant may eliminate the necessity for sinus grafting, which is often a desirable option if the preference of the patient is to avoid a graft approach.

image The Zygoma Implant

The implant was first created and used by Professor P.I. Brånemark in the late 1980s as a means of creating prosthesis stability in maxillary arches compromised by trauma or the ravages of oral cancer. It remained a specialty product for nearly 10 years, and was introduced commercially only after other more conventional applications were discovered.

The original design was a stepped, straight, hex-head implant that varied in available lengths up to 50 mm (Figure 23-2). The zygoma implant, as currently used, has an oxide enhanced surface and a premachined 45 degree angled head for improved screw access intraorally (Figure 23-3). The concept is to engage the zygomatic buttress with 11-13 mm of osseointegrated surface apically, and, after passing through the sinus, engage the alveolar and palatal bone in the first molar area coronally. Once integrated, the zygoma implant can provide significant distal support for a variety of clinical applications due to the length and strength of the fixture itself. As with the pterygoid approach, the need for sinus grafting to gain osseous volume and distal implant positioning is often eliminated.916

Indications

Cancer

In situations of severe injury or ablative cancer surgery, the zygoma implant can play a critical role in creating a retentive base for prosthesis construction. In many presentations, these patients have minimal or no available alveolar bone, and thus are severely compromised in normal eating or speaking activities. Originally, restorations created before implant availability were only marginally successful, and were dramatically challenged in terms of retention and defect obturation.1719

Many oral cancer defects involve the removal of large portions of the alveolus and basal bone, creating a large fistula or communication with the maxillary sinus and associated structures (Figure 23-4, A and B). In such cases, the only available bone in the defect area is in the nasal rim, which is usually very thin, and remotely in the zygomatic buttress. The zygoma implant, in combination with conventional implants placed in any available bone sites, can often provide the basis for a retentive bar structure at the alveolar level (Figure 23-4, C and D). The obturating prosthesis can then be based entirely on the bar structure or on the bar in combination with residual dentition. The resultant restoration will then be resistant to vertical dislodgement in both the superior and inferior directions, and can provide the stability and seal to obturate the defect space effectively (Figure 23-4, E and F).

image

Figure 23-4. A, Postoperative panorex from an ablative tumor surgery on a 23-year-old man. The residual dentition in the molar region was inadequate to support a conventional obturating prosthesis, and the teeth themselves were being manipulated dramatically by the cantilever effect. B, Intraoral subtotal bilateral maxillectomy visualized with exposure of the nasal and maxillary sinus on one side gives an impression of the magnitude of the defect in this compromised maxilla. C, Conventional and zygoma implants were placed in areas with the necessary available bone. The ability to utilize parallel or converging placement is overridden by the fact that available bone for this process is remote from the oral cavity and implants have to be placed in whatever angle the bone dictates. D, Five implants were used to create the framework pictured here with connection points around the periphery of the defect site and minimal connecting bar material interference. Two large Samarium Cobalt magnets were placed in the middle to support vertical stress and create retention against opposing magnets in the surface of the obturator. E, The obturating removable partial denture clasp attached to the residual dentition posteriorly and used the large retentive magnets in the center of the defect area for vertical and horizontal displacement resistance. F, The final prosthesis in place obturates the defect area adequately and provides a level of functional capability that would have been unobtainable without the use of implants in the remote bone areas.

Trauma

Defects of the maxilla from trauma may be similarly challenging with regard to creating prosthesis retention and may vary dramatically in topographical presentation, depending on the nature and severity of the injury. The relative capacity provided by the zygoma implant in these situations is equally valuable for prosthesis construction, even in the near absence of normal tissue position or relationships (Figure 23-5).

image

Figure 23-5. A, Postoperative radiograph of a self-inflicted gunshot wound to the mandible and anterior maxilla that created a tremendous amount of osseous destruction and anatomical disruption. B, Following healing and numerous graft attempts for reconstruction, the patient presented with this highly distorted maxilla. C, Implants were placed in the zygoma bone and wherever possible in the residual fragments of maxillary bone. As can be seen in this radiograph, the maxillary sinus on the patient’s right side was obliterated and the implant literally passes from the zygoma anchorage point into the oral cavity with virtually no osseous connection at its alveolar entrance point. D, Mounted casts illustrate the amount of displacement of normal anatomic architecture, and the amount of implant divergence experienced through this approach in the maxillary arch. E, The final restoration in position in the maxilla was accomplished with unilateral occlusion on molar teeth and bilateral premolar occlusion on the right side. F, Final radiographs of the reconstructed maxilla following trauma reconstruction and implant placement show that anchorage and fixation can be accomplished even in the event of massive disruptive bone loss. G, Occlusion, although not ideal or classic, is functional enough to allow the patient to experience normal eating and speech production, contributing to a relatively normal lifestyle.

(From Miloro M, editor: Peterson’s principles of oral and maxillofacial surgery, ed 2, Hamilton, Ontario, 2004, BC Decker.)

Immediate Loading

One of the factors that most influences the ability to immediately load a maxillary full-arch implant restoration is the strength of the two distal fixtures. Before the advent of a tilted, or angled, approach, almost all maxillary implants were vertically aligned. As the most distal implants approached the maxillary sinus area, the amount of available bone diminished dramatically, thus reducing the vertical space available for this pair of critical supporting elements. Given the documented poor success of short vertical distal implants (7 and 10 mm) adjacent to the anterior sinus wall, many clinicians were hesitant to attempt loading immediately.2225 The use of a long and stronger implant in these sites, as the zygoma, created an improved support potential at the cantilever junction and an increased potential for implant integration during the provisional phase (Figure 23-6). Favorable reports on the immediate loading of zygoma/standard implant combinations have shown that this can be a viable approach.

Modifications

The original trans-sinus approach to implant placement described by Brånemark created an exit point in the maxilla that was usually palatal to the residual ridge crest. This intrusion of eventual components was not objectionable to most patients, but it did require a buccal cantilever to get the denture teeth positioned over the ridge crest.

A modification in surgical technique suggested by Stella and Warner allowed the zygoma implant to realign against the vertical maxillary buttress, and exit more centrally on the ridge crest of the maxilla. This approach is also less invasive to the sinus and, therefore, allows more implant body contact with bone (Figure 23-7).26

Jan 7, 2015 | Posted by in Implantology | Comments Off on 23: THE EVOLUTION OF THE ANGLED IMPLANT
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