Why Do Implant-Supported Fixed Dental Prostheses (ISFDP) Sometimes Fail?

The short answer is: inadequate bone reduction.

The greatest bulk of complications in full-arch rehabilitation cases when the restorative treatment of choice is implant-supported fixed dental prostheses (ISFDP), are due to poorly-designed restorative space. Biomechanical and esthetic failure of the definitive prosthesis can largely be attributed to improper clearance created by non-ideal reduction of the maxillary or mandibular bony ridge. 

Introduction

The prosthesis-tissue interface is of great importance, and care should be taken in order to obtain a seamless transition at the junction, particularly in cases where the gingival display is excessive. For that purpose, the role of bone reduction in full-arch ISFDP cases is done to prepare the restorative space to receive the implants as well as to make for a natural transition between the soft tissue and the prosthesis. This results in a surface that can be cleansed easily using simple oral hygiene routines.

However, more often than not, bone reduction done with conventional CAD/CAM surgical guides is immoderate and the resulting implant-recipient area is unnaturally flush. This excessive bony tissue removal is the main culprit in compromising the final prosthesis functionally, esthetically and biomechanically. The manifestations of said failure can range from prosthetic breakage to patients’ dissatisfaction with the inferior esthetics of poorly-concealed prosthesis-tissue intersection in some instances. Moreover, such inadequate prosthetic contours make it difficult for some patients to properly clean the area, which might lead to plaque formation and subsequent peri-implantitis. As a result, the chances of implant loss are increased, affecting the overall success rate.

What’s the purpose of bone reduction?

Bone reduction or alveoplasty is usually indicated in cases of terminal dentition where dental implants are to be placed after the surgical extraction of the remaining teeth that have unfavorable prognosis. The main objectives of the alveoplasty are:

1. Creating an adequate restorative space 

To achieve functional clearance, the planned ISFDP needs an interarch restorative space that’s large enough to accommodate implant placement as well as the future prosthesis. Interarch restorative space can be defined as the space measured between the crest of the ridge of one arch to the occlusal surface of the other arch.  

2. Enhancing the implant-placement area

Flat plane reduction is used to widen the thin (i.e. knife-edge) bony ridge and thereby improve the implant-recipient site architecturally. Too much or too little bone reduction will compromise the alveolar bone, resulting in inaccurate placement of implants as well as inadequate restorative space. 

How to achieve ideal bone reduction?

Computer-aided design/computer-aided manufacturing (CAD/CAM) technology with digitally integrated workflow is used to 3D print highly accurate and anatomically stable surgical (i.e. bone reduction) guides. Dental practitioners utilize said guides to:

• Reduce the bone adequately to the desired contours required for each case
• Acquire the ideal position for implant placement according to the prosthetic requirements
• Immediately load polymethyl methacrylate (PMMA) temporary denture on the same day of surgery

Like many other fields, implant dentistry is becoming an increasingly digitized field. Over the last decade alone, the improvements in CAD/CAM technology have made anatomically-sound alveoplasty a reality. 

Many types of surgical guides currently exist, including tooth-supported, mucosa-supported and bone-supported. Although tooth-supported CAD/CAM guides have been reported to have the most stability, they can’t be used in completely edentulous patients. Therefore, to increase the mechanical stability of mucosa- and bone-supported surgical guides, the use of anchor pins to stabilize the guides during surgery is indicated. 

To further eliminate cantilever or rocking effects, fixation screws are used to secure bone reduction guides in place (e.g. 3Sixty Anatomic Guide™)

Case selection for implant-supported fixed dental prostheses (ISFDP)

Evidently, bone reduction will depend entirely on the amount of remaining bone already available to be reduced. 

Cawood and Howell Class I (i.e. terminal dentition patients), Class II (i.e. post-extraction, completely edentulous patients), and Class III (i.e. completely edentulous patients with rounded ridges of adequate height and width) have minimal bone resorption. Such patients are good candidates for guided-bone reduction and implant-placement surgery needed in ISFDP. 

Conversely, patients classified as Cawood and Howell Class IV, V and VI have markedly resorbed ridges, and would benefit more from alternative restorative treatments such as implant-retained overdentures. Inadequate height and width of the remaining bone in such cases require relatively large prostheses to compensate for vertical and horizontal tissue loss, as well as support the cheeks and lips. Fixed prostheses, therefore, are contraindicated for patients with extensive bone resorption as there’s an increased chance of plaque-related peri-implantitis and subsequent implant failure. 

Cawood & Howell Classification of the bony ridge

Cawood & Howell Classification of Edentulous Ridge (1988)

Summary

• Acquiring the ideal bone reduction is essential for the success of prosthetically driven implant placement procedure. 
• Under- or over-reduction can result in the failure of the final implant-supported fixed prosthesis. 
• CAD/CAM surgical guides are used to customize the bony ridge according to each patient’s restorative space requirements. 
• However, most conventional bone reduction guides create an aggressively-flattened bony ridge which may not be desirable in some cases. 
• Therefore, there’s an increased need for anatomically-accurate guides that are small, stable and strong. 
• The field can benefit from customizable and versatile surgical guides that could be used to acquire the desired amount of bone removal while preserving as much tissue as possible. 
• Conservative alveoplasty ensures a proper restorative space is obtained, creates an architecturally-sound implant-recipient site, conceals prosthesis-tissue line, and improves cleansability.
• Aggressive flattening of the bony ridge can result in poor esthetics, decreased functionality, implant loss and prosthetic failure (e.g. material fracture).