Dental implants continue to grow in popularity because they are a predictable treatment to replace missing teeth. They have a high success rate; however, they are still associated with some clinical complications. This article discusses a diverse range of complications related to the restorative and mechanical aspects of dental implants and the management of such complications, as well as potential factors contributing to them.
Key points
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Dental implant restorations have a high success rate but require careful planning and management to minimize complications.
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Most prosthetic complications occur after many years of the prosthesis being in place, and it is expected that the materials will fatigue.
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Frequent mechanical complications include abutment screw loosening, abutment fracture, and prosthesis fracture of both the veneer and the framework.
Introduction
The success rates for dental implants are very high, and have historically been measured by the presence of osseointegration and lack of periimplantitis. Nevertheless, every treatment can present complications. These complications are reported to involve implant components and prostheses at higher rates compared with implant loss. The cause of these complications is multifactorial and can be biological or mechanical. These types of complications increase in incidence over time, indicating that the risk of component and material complications increase with continued use.
Prosthetic complications occur in dental implant therapies of all types, including single and multiple implants restored using fixed and removable prostheses. The cause-and-effect relationship of the complications has not been fully considered, and dynamics in the implant and component treatment result in a complex restoration that can experience mechanical complications with a tendency to become a cause of failure to the component selection, design, or manufacture. The most commonly reported implant mechanical complications include adjustment of overdentures because of loss of retention, relines, or clip/attachment fracture ; veneer fracture in fixed prostheses ; overdenture fracture ; acrylic resin fracture in hybrid prostheses ; screw loosening and screw fracture ; and metal framework fracture. Many of these complications are related to poor planning, poor case selection, or even poor implementation of the treatment plan.
Implant malposition
Implant malposition contributes to the increased risk of biomechanical complications with implants, components, and prostheses. Displacement of the implant axis from the imposed functional load creates or increases the bending moment acting on the implant restoration. This increase manifests itself in a variety of ways, the consequences of which include biological, mechanical, and esthetic complications and even implant failure.
Improper positioning of implants, whether it is in a buccolingual, mesiodistal, or occlusogingival location from the ideal position, may compromise the bony and soft tissue support necessary for long-term biological implant success. Mechanical complications arise from the increased forces being applied on the prosthesis, abutment, and/or implant body. Esthetic parameters, such as emergence profile and contours of the prosthesis, may be considerably affected by implant malposition as well. The inability to clean the prosthesis may superimpose biofilm-mediated inflammatory challenges that lead to loss of bony support. Although careful planning of the definitive restoration as the starting point for implant position is the ideal approach to ensuring proper implant position and related consequences of loading to limit biomechanical and biological risks, there are various methods to overcome misplacement-related complications. Some of those strategies include the use of customized abutment solutions and prosthetics enhancements using gingival-colored ceramics.
The ultimate goal in managing implant malposition is to create a restoration that is biologically, functionally, and esthetically synergistic with the patient’s function and expectations. One problem that is often encountered is lack of the restorative space (occlusogingival dimension) required for the creation of a robust and properly formed dental prosthesis ( Fig. 1 ). For a single-unit restoration in situations of occlusogingival dimensional limitations, a screw-retained option with crown and abutment fabricated as 1 piece reduces the required space to construct the restoration compared with a cement-retained restoration. In this situation, screw-retained restorations present less risk for prosthesis dislodgement because of lack of resistance and retention features in the implant abutment that a cement-retained restoration might have ( Fig. 2 ). This point is also true for segmental fixed dental prostheses. Restorative space for full-arch restorations is critical to their success for both esthetic and structural reasons. This prosthetic solution has minimal dimensional requirements, based on the chosen restorative material, to meet the strength requirements and minimize potential risk for mechanical failure ( Fig. 3 ). Another critical aspect for full-arch restorations is the length of the distal cantilever and how much leverage it adds to the abutments and implants.
When considering options for a multiunit prosthesis, a removable option may be fabricated instead of a fixed restoration. Although this may not meet an individual’s initial expectations, it can provide a viable solution for a prosthesis that may otherwise be unaesthetic, unhygienic, and susceptible to further complications.
Improper implant placement resulting in inadequate volume and contour of the bony and/or gingival architecture has several options for management. Preemptively, hard or soft tissue grafting may be used to restore the appropriate tissue dimensions with varying degrees of success. Creative prosthesis design incorporating gingival-colored ceramic or acrylic may also be used to replace absent or inadequate gingival architecture. This approach must be performed with caution, because an overcontoured or bulky prosthesis can lead to further biological consequences because of lack of cleanability.
Another complication commonly seen is a screw access channel exiting through a nonideal location, such as the incisal edge, an embrasure space, or even through the buccal surface. A viable option for correction is using an angled abutment, which can correct up to 25° of the misangulation. Most recently, the use of angulated screw access channels has permitted equal correction of the misangulation within the abutment and crown ( Fig. 4 ). This method is a remarkably effective way to overcome screw access channel limitations of esthetics and design and requires only specific screwdrivers to accommodate this design. If the restoration was initially planned to be screw retained, another solution can be found using computer-assisted design/computer-assisted manufacturing (CAD/CAM) custom abutments and cement-retained restorations (see Fig. 4 ).
Not all options to overcome implant malposition may be adequate to facilitate construction of prostheses with acceptable biological, functional, and esthetic parameters. Under such circumstances, implant removal may be considered. Although the time and cost to remove the implant, perform a bone graft, and place a new implant may be substantial, the long-term benefits are superior.
Management of dental implant component complications
A variety of implant and abutment designs and materials have been introduced to overcome perceived and real limitations found clinically. The mechanical integrity of these components is determined by the dimension, type of connection, type of material, and type of restoration. The general categorization of abutments includes titanium versus zirconium and internal conus versus internal sliding fit or external hex connections. When internal connection abutments were compared with external connection abutments by systematic review, the abutment did not influence implant survival or complication rates, but internal conus connections showed less marginal bone loss. Regarding the materials, no differences regarding the survival or failure of ceramic or metal abutments were observed in a review of single-tooth implants. However, complications emerge in clinical practice.
Screw loosening or fracture is more common with prosthetic screws compared with abutment screws, and this is the most common component complication. Screw complications are also more common in single-tooth implant restorations compared with multiple units. Consistently, one of the most frequently reported complications is loosening of both the abutment screw and the prosthetic screw, with an incidence of 5.6% after 5 years, and it could be as high as 59.6% within 15 years. The outcome is affected by design of the implant connection, with implants with external connections having a higher incidence, with a mean of 18.3% after 5 years, whereas screw loosening associated with internal connection was 2.7% after 4.5 years. It has always been proposed that abutment and prosthetic screws must be tightened with a torque instrument to achieve the required clamping force derived from the recommended torque.
Occlusion and cantilevers are considered important risk factors in the outcome of the implant restoration. Fracture of implants is a terminal failure for implant therapy and is associated with several factors, including material, implant diameter and length, presence of a cantilever, and bruxism. Bruxism may significantly reduce implant survival. The presence of a cantilever in implant prostheses is associated with increased incidence of complications such as veneer fracture and screw loosening. These two factors of increased force and magnification of imposed force should be paramount concerns in treatment planning. Encountering bruxism or increased loading should encourage the use of larger diameter and additional implants as well as the absolute avoidance of cantilevers. Placement of implants using short implants or tilted implants can avoid the requirement of distal cantilevers in many implant prostheses.
Retrieval of the fractured screw depends on whether the screw is loose within the implant or abutment or tightly bound within the screw threads. Many methods have been proposed to remove a fractured screw, including a dental instrument (explorer, hand scaler, ultrasonic scalers) rotated counterclockwise as well as a fork remover in a drill running in reverse. If the head of the screw has been stripped and a driver cannot grasp the screw, drilling a horizontal groove cut into the screw head has been advocated to engage the instrument with a flat-head driver or instrument. There are also many companies that provide screw-retrieval kits; no matter the solution, extra attention must be paid when drilling so as to not damage the internal bore threads, because then the implant might need to be removed.
The prefabricated titanium-bonding base (Ti Base) is used to create a hybrid abutment or a screw-retained implant crown. Although this system offers several advantages, including low component cost, chair-side manufacture, titanium-titanium implant connection, and ability to lute extraorally, there are reports of fracture and Ti Base debonding. These complications may be caused by the small surface area and cross-sectional area of these components. However, clinical data are lacking. Management of the Ti Base debonding consists of using an abutment with a more robust base to support the restoration, including a cast-to abutment system or a CAD/CAM custom abutment ( Fig. 5 ).
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