Complications of Overdentures for the Edentulous Patient
When evaluating an edentulous patient, it is the clinician’s responsibility to explain to the patient the disadvantages that may result from edentulism. The following are consequences of edentulism that play a significant role in treatment planning (Box 15.2).
Continued Bone Loss
One of the most common issues of edentulism that is not commonly explained to patients is the continued loss of bone. Alveolar bone requires constant stimulation to maintain its form, strength and density. Roberts et al reported that a 4% strain to the skeletal system maintains bone and helps balance the resorption and formation phenomena.3 The forces applied to teeth transmit compressive and tensile forces to the surrounding bone. However, when a tooth is lost, the lack of stimulation to the residual bone causes a decrease in trabeculae and bone density in the area, with loss in external width, then height of the bone volume.4 Studies have shown a 25% decrease in width of bone during the first year after tooth loss and an overall 4-mm decrease in height during the first year after extractions for an immediate denture.5 In a longitudinal 25-year study of edentulous patients, Tallgren demonstrated continued bone loss during this time span, with a fourfold greater loss observed in the mandible.6 In 1963 Atwood introduced five different stages of bone loss in an anterior mandible after tooth loss showing the resorptive process over time.7 Even though the mandibular bone resorbs faster than the maxillary bone, the mandible has almost twice the bone initially available.
When patients wear a removable prosthesis, the bone is not stimulated in the same fashion that a tooth does. The load placed on the removable prosthesis is transferred to the bone surface, not the internal structure of the bone. This causes the blood supply to be reduced to the bone resulting in total bone volume being lost.8 The bone loss accelerates when the patient wears a poorly fitting soft tissue–borne prosthesis. The average denture wearer sees a dental professional approximately every 14.8 years after having a complete denture. Ideally, edentulous patients should be seen every 1 to 2 years and should be informed that every 3 to 5 years a reline or new denture is suggested to replace the additional bone loss by atrophy that naturally will occur.
The rate and amount of bone loss may be influenced by many factors such as gender, hormones, metabolism, parafunction, and ill-fitting dentures. Studies have shown almost 40% of denture wearers have been wearing an ill-fitting prosthesis for more than 10 years.9 Patients wearing dentures day and night place greater forces on the hard and soft tissues, which accelerates bone loss. Nonetheless, 80% of dentures are worn both day and night.9 Loss of bone in the maxilla or mandible is not limited to alveolar bone; portions of the basal bone may also be resorbed, especially in the posterior aspect of the mandible, where severe resorption may result in the mental foramen or mandibular canal becoming dehiscent. This may result in significant pain and discomfort, leading to the inability to wear a removable prosthesis. In the maxilla, the complete anterior ridge and nasal spine may be resorbed, causing pain and an increase in maxillary denture movement during function (Fig. 15.3 and Box 15.3).10
Decreased Occlusal Biting Force
The biting force of edentulous patients is a significant factor that patients must be educated about concerning various treatment plans. The difference in maximum occlusal forces recorded in a patient with natural teeth and one who is completely edentulous is dramatic. In the first molar region of a dentate person, the average force has been measured to be approximately 150 to 250 psi.11 A parafunction patient who grinds or clenches their teeth may exert a force that approaches 1000 psi. In edentulous patients the maximum occlusal force approaches less than 50 psi. This is very important when explaining various treatment plans to patients, especially with expectations. The longer patients are edentulous, the less occlusal force they are able to generate. Studies have shown that patients wearing complete dentures for more than 15 years have been shown to exhibit a maximum occlusal force of less than 6 psi.12 This is a significant disadvantage that patients need to be educated about, especially when patients decline implant treatment plans in favor of conventional removable dentures. In comparison, the maximum occlusal force may improve upwards of 300% with an implant-supported prosthesis.13
Decreased Masticatory Efficiency
As a result of decreased occlusal force and the instability of the complete denture, masticatory efficiency will decrease with tooth loss. Rissen et al showed that 90% of food chewed with natural teeth fits through a no. 12 sieve; this is reduced to 58% in the patient wearing complete dentures.12 A study of 367 denture wearers (158 men and 209 women) found that 47% exhibited a low masticatory performance.14 In patients with dentures, 29% are able to eat only soft or mashed foods, 50% avoid many foods, and 17% claim they eat more efficiently without the prosthesis.15 Lower intakes of fruits, vegetables, and vitamin A by women were noted in this group. Denture patients also take significantly more drugs (37%) compared with those with superior masticatory ability (20%), and 28% take medications for gastrointestinal disorders. The reduced consumption of high-fiber foods could induce gastrointestinal problems in edentulous patients with deficient masticatory performance. In addition, the coarser bolus may impair proper digestive and nutrient extraction functions.16
Increased Soft Tissue Discomfort
Mandibular discomfort was listed in a study by Misch and Misch with equal frequency as movement (63.5%), and, surprisingly, 16.5% of the patients stated they never wear the mandibular denture. In comparison, the maxillary denture was uncomfortable half as often (32.6%), and only 0.9% were seldom able to wear the prosthesis. Function was the fourth most common problem reported by these 104 denture wearers. Half of the patients avoided many foods, and 17% claimed they were able to masticate more effectively without the prostheses. The psychologic effects of the inability to eat in public can be correlated with these findings. Other reports agree that the major motivating factors for patients to undergo implant-related treatment were related to the difficulties with eating, denture fit, and discomfort.15
The literature includes several reports suggesting that compromised dental function causes poor masticatory performance and swallowing poorly chewed food, which in turn may influence systemic changes favoring illness, debilitation, and shortened life expectancy.17,18 In a study evaluating the ability to eat fruit, vegetables, and other dietary fiber in edentulous subjects, 10% claimed difficulty, and blood tests demonstrated reduced levels of plasma ascorbate and plasma retinol compared with dentate subjects. These two blood tests are correlated to an increased risk of dermatologic and visual problems in aging adults. In another study the masticatory performance and efficiency in denture wearers were compared with those of dentate individuals. This report noted that, when appropriate corrections were made for different performance norms and levels, the chewing efficiency of a denture wearer was less than one sixth of a person with teeth.19
Several reports in the literature correlate a patient’s health and life span to dental health. Poor chewing ability may be a cause of involuntary weight loss in the elderly population, with an increase in mortality rate.20 In contrast, persons with a substantial number of missing teeth were more likely to be obese. After conventional risk factors for strokes and heart attacks were accounted for, there was a significant relationship between dental disease and cardiovascular disease, the latter still remaining as the major cause of death.21 It is logical to assume that restoring the stomatognathic system of these patients to a more normal function may indeed enhance the quality and length of their lives.
Lack of Prosthesis Satisfaction
A dental survey of edentulous patients found that 66% were dissatisfied with their mandibular complete dentures. Primary reasons were discomfort and lack of retention causing pain and discomfort. Past dental health surveys indicate that only 80% of the edentulous population are able to wear both removable prostheses all the time. Some patients wear only one prosthesis, usually the maxillary; others are only able to wear their dentures for short periods. In addition, approximately 7% of patients are not able to wear their dentures at all and become “dental cripples” or “oral invalids.” They rarely leave their home environment, and when they feel forced to venture out, the thought of meeting and talking to people when not wearing their teeth is unsettling.22
A report of 104 completely edentulous patients seeking treatment was performed by Misch and Misch. Of the patients studied, 88% claimed difficulty with speech, with one fourth having great difficulty. The lower prosthesis rests upon the buccinator muscle and mylohyoid muscle when the posterior mandible resorbs. When the patient opens their mouth, the contraction of these muscles acts like a trampoline and propels the lower denture off the ridge. As a result, the teeth often click when the patient talks, not from too much of the vertical dimension restored but from the lack of stability and retention of the prosthesis. Speech problems may be associated with a concern for social activities. Awareness of movement of the mandibular denture was cited by 62.5% of these patients, although the maxillary prosthesis stayed in place most of the time at almost the same percentage.
Psychologic Impact of Edentulism
The psychologic effects of total edentulism are complex and varied and range from very minimal to a state of neuroticism. Although complete dentures are able to satisfy the esthetic needs of many patients, some believe their social lives are significantly affected. They are concerned with kissing and romantic situations, especially if a new partner in a relationship is unaware of their oral handicap. Fiske et al, in a study of interviews with edentulous subjects, found tooth loss was comparable to the death of a friend or loss of other important parts of a body in causing a reduction of self-confidence ending in a feeling of shame or bereavement.23
The psychologic needs of edentulous patients are expressed in many forms. For example, in 1970, Britons used approximately 88 tons of denture adhesive.24 In 1982, more than 5 million Americans used denture adhesives (Ruskin Denture Research Associates: AIM study, unpublished data, 1982), and a report shows that in the United States, more than $200 million is spent each year on denture adhesives, representing 55 million units sold.27 Patients are willing to accept the unpleasant taste, need for recurring application, inconsistent denture fit, embarrassing circumstances, and continued expense for the sole benefit of increased retention of the prosthesis. Clearly, the lack of retention and psychologic risk of embarrassment in the denture wearer with removable prostheses is a concern the dental profession must address.
Advantages of an Implant-Supported Prosthesis (Box 15.4)
The use of dental implants to provide support for prostheses offers many advantages compared with the use of removable soft tissue–borne restorations. A primary reason to consider dental implants to replace missing teeth is the maintenance of alveolar bone. It is well accepted that dental implants placed in the anterior mandible help retain a lower denture and are a benefit over a complete denture. However, the posterior bone loss will continue and may eventually lead to significant complications. Instead, when sufficient implants are inserted, the restoration is not only retained but it also is completely supported and stabilized off the tissue and bone. The implants also stimulate and maintain the bone of the mandible as well as serve as an anchor for the prosthetic device. As a result, dental implants are one of the better preventive maintenance procedures available in dentistry.
More Ideal Occlusion
One of the main complications of wearing a lower removable denture is the associated movement and lack of security. A mandibular denture often moves when the mylohyoid and buccinator muscles contract during speech or mastication. Occlusion is difficult to establish and stabilize with a completely soft tissue–supported prosthesis. Because the mandibular prosthesis may move as much as 10 mm or more during function, proper occlusal contacts occur by chance, not by design. In comparison, an implant-supported restoration is much more stable.25 The patient can more consistently return to centric relation occlusion rather than adopt variable positions dictated by the prosthesis’ instability. Proprioception is awareness of a structure in time and place. The receptors in the periodontal membrane of the natural tooth help determine its occlusal position. Although endosteal implants do not have a periodontal membrane, they provide greater occlusal awareness than complete dentures. Whereas patients with natural teeth can perceive a difference of 20 µm between the teeth, implant patients can determine a 50-µm difference with rigid implant bridges compared with 100 µm in those with complete dentures (either one or two).26 As a result of improved occlusal awareness, the patient functions in a more consistent range of occlusion. With an implant-supported prosthesis, the direction of the occlusal loads is controlled by the restoring dentist. Horizontal forces on removable prostheses accelerate bone loss, decrease prosthesis stability, and increase soft tissue abrasions. The decrease in horizontal forces that are applied to implant restorations improves the local parameters and helps preserve the underlying soft and hard tissues.
Increased Masticatory Function
In a randomized clinical trial comparing conventional vs. implant-supported prostheses by Kapur et al, the implant group of patients demonstrated a higher level of eating enjoyment and improvement of speech, chewing ability, comfort, denture security, and overall satisfaction.27 The ability to eat several different foods among complete denture vs. mandibular overdenture patients was evaluated by Awad and Feine. The implant overdenture was superior for eating not only harder foods, such as carrots and apples, but also softer foods, such as bread and cheese. Geertman et al evaluated complete denture wearers with severely resorbed mandibles before and after mandibular implant overdentures. The ability to eat hard or tough foods significantly improved.28
Researchers at McGill University evaluated blood levels of patients who had complete dentures and 30 maxillary dentures and mandibular implant prostheses 6 months after treatment. Within this rather short period, implant patients had higher vitamin B12 hemoglobin (related to iron increase) and albumin levels (related to nutrition). These patients also had greater body fat in their shoulders and arms, with decreased body fat in their waists.29
Less Morbidity in Comparison to Teeth
The success rate of implant prostheses varies depending on a host of factors that change for each patient. However, compared with traditional methods of tooth replacement, the implant prosthesis offers increased longevity, improved function, bone preservation, and better psychologic results. According to 10-year survival surveys of fixed prostheses on natural teeth, decay is indicated as the most frequent reason for replacement; and survival rates are approximately 75%.30 In a partially edentulous patient, independent tooth replacement with implants may preserve intact adjacent natural teeth as abutments, further limiting complications such as decay or endodontic therapy, which are the most common causes of fixed prosthesis failure. A major advantage of the implant-supported prosthesis is that the abutments cannot decay and never will require endodontics. The implant and related prosthesis have been shown to attain a 10-year survival rate of more than 90%.
Increased Biting Force
Patients with an implant-supported fixed prosthesis may increase their maximum bite force by 85% within 2 months after the completion of treatment. After 3 years the mean force may reach more than 300% compared with pretreatment values. As a result, an implant prosthesis wearer may demonstrate a force similar to that of a patient with a fixed restoration supported by natural teeth. Chewing efficiency with an implant prosthesis is greatly improved compared with that of a soft tissue–borne restoration. The masticatory performance of dentures, overdentures, and natural dentition were evaluated by Rissin et al. The traditional denture showed a 30% decrease in chewing efficiency; other reports indicate a denture wearer has less than 60% of the function of people with natural teeth. The tooth-supported overdenture loses only 10% of chewing efficiency compared with natural teeth. These findings are similar with implant-supported overdentures. In addition, rigid, implant-supported fixed bridges may function the same as natural teeth.31
Improvement of Retention and Stability
Stability and retention of an implant-supported prosthesis are great improvements over soft tissue–borne dentures. Mechanical means of implant retention are far superior to the soft tissue retention provided by dentures or adhesives and cause fewer associated problems. The implant support of the final prosthesis is variable, depending on the number and position of implants, yet all treatment options demonstrate significant improvement over conventional prosthesis treatment options.
Phonetics may be impaired by the instability of a conventional denture. The buccinator and mylohyoid muscles may flex and propel the posterior portion of the denture upward, causing clicking, regardless of the vertical dimension. As a result, a patient in whom the vertical dimension already has collapsed 10 to 20 mm may still produce clicking sounds during speech. Often the tongue of the denture wearer is flattened in the posterior areas to hold the denture in position. The anterior mandibular muscles of facial expression may be tightened to prevent the mandibular prosthesis from sliding forward. The implant prosthesis is stable and retentive and does not require these oral manipulations. The implant restoration allows reduced flanges or palates of the prostheses. This is of special benefit to new denture wearers, who often report discomfort with the bulk of the restoration. The extended soft tissue coverage also affects the taste of food, and the soft tissue may be tender in the extended regions. The palate of a maxillary prosthesis may cause gagging in some patients, which can be eliminated in an implant-supported overdenture.
Improved Psychologic Health
Patients treated with implant-supported prostheses judge their overall psychologic health as improved by 80% compared with their previous state while wearing traditional, removable prosthodontic devices. They perceived the implant-supported prosthesis as an integral part of their body.32 For example, Raghoebar et al evaluated 90 edentulous patients in a randomized multicenter study. Five years after treatment, a validated questionnaire targeted patient esthetic satisfaction, retention, comfort, and the ability to speak and eat with either a complete mandibular denture, complete mandibular denture with vestibuloplasty, or mandibular two-implant overdenture. Implant overdentures had significantly higher ratings, but no significant difference was found between the two complete-denture groups.33 Geertman et al reported similar results comparing chewing ability of conventional complete dentures with mandibular implant overdentures.34
Not Understanding the Disadvantages of a Removable Implant Prosthesis (Overdenture)
Although dental implant overdentures are successful in many situations, patients should be informed of the inherent disadvantages and complications that may occur. Many studies have shown a rather high incidence of complications. Goodacre et al have shown overdenture complications to include clip/attachment loosening (30%), clip/attachment fracture (17%), and overdenture fracture (12%). In a study by Bilhan et al on 59 patients, two thirds of implant-supported overdenture (IOD) patients had prosthetic-related complications the first year. For example, relines were necessary in 16%, loss of retention in 10.2%, fracture of the IOD in 8.5%, pressure spots in 8.5%, dislodged attachment in 6.8%, and screw loosening in 3.4%.35 Other studies have shown up to 18% of overdentures requiring a reline of the prosthesis within the first year.36 It is imperative that patients understand fully the basis and long-term issues that may be associated with implant-retained overdentures.
Not a Fixed Prosthesis
Many patients are not educated in the inherent mechanics of overdentures and the various levels of support that exist, which are dependent on the position, number, and types of retentive mechanisms. The most common misconception that may lead to patient disapproval is an overdenture is not a “fixed prosthesis.” With that, the patient must understand the decreased occlusal force in comparison to fixed prosthesis. An overdenture is considered a prosthesis, whereas a fixed prosthesis is considered a body part. Fixed implant prostheses are often considered by patients to be similar to their natural teeth in esthetics and function. With an overdenture, movement of the prosthesis is usually noted by the patient along with decreased masticatory function (Fig. 15.4).
Need of Adequate Crown Height Space (CHS)
A greater crown height space (CHS) is required for an implant-retained overdenture in comparison to other types of implant prostheses. With an overdenture prosthesis, there is often a lack of space that results in a prosthesis fracture or loss of denture teeth. When space is compromised, inadequate room results in modification of the materials (reduction of acrylic from denture base or hollowing out of denture teeth) to obtain room for attachment mechanisms. In these situations, the appropriate attachment (e.g., Locator 6–low profile) must be selected to minimize these complications. When abundant bone is present and implants are already inserted, a compromised CHS may indicate a change in treatment planning to a fixed prosthesis if adequate number and position of implants are present (Fig. 15.5).
An increased maintenance protocol is often required for an overdenture. Denture teeth wear more rapidly on an IOD in comparison to a denture (i.e., increased occlusal force). As a result, a new IOD may need to be fabricated on a more continuous basis depending on the amount of wear. The patient should be informed at the beginning of treatment of the need for maintenance to avoid patient complications. Although the initial cost of treatment may be less for an IOD, overdenture wearers often incur greater long-term expenses than those with fixed restorations. Attachments (i.e., O-rings or clips) regularly wear and must be replaced, which in some cases is very time consuming. Walton and McEntee noted there were three times more maintenance and adjustments issues for overdentures compared with fixed restorations.37
A common complaint with IODs is food impaction under the prosthesis. When the denture is fabricated, border molding captures the muscles in their contracted position. In the relaxed muscle state, food will sometimes become displaced under the denture border. For example, when the patient swallows, the food is pushed under the denture. Because the IOD moves less than a denture, the food remains under the IOD and becomes lodged, usually requiring the patient to physically remove the irritants (Fig. 15.6).
Loss of Posterior Bone
Most mandibular IODs used by the profession are supported by implants anterior to the mental foramina and soft tissue support in the posterior regions. The anterior implants allow improved anterior bone maintenance, and the prosthesis benefits from improved function, retention, and stability. Dental implants placed in the anterior mandible help retain a lower denture and are certainly a benefit over a complete denture. The posterior bone loss will continue and may eventually lead to significant complications with respect to lack of soft tissue support (i.e., RP-5 prosthesis). Studies have shown posterior bone loss to occur four times faster than anterior bone loss.38 The lack of posterior support in two- and three-implant overdentures allows continued posterior bone loss. A primary concern for RP-5 overdentures (soft tissue support in the posterior regions) compared with RP-4 or fixed restorations (restorations completely supported, retained, and stabilized) should be the continued bone loss in the posterior regions.
Wright et al evaluated the change in the posterior mandibular residual ridge over time comparing patients wearing mandibular IOD with bar attachments or mandibular fixed cantilever prostheses stabilized on five or six implants. Bone measurements were made by digitizing tracings of panoramic radiographs. The mandibular fixed cantilever prostheses demonstrated bone growth in the posterior mandibular residual ridge while IOD showed bone resorption in the same area (Fig. 15.7).39
Reddy et al also confirmed this finding with the measurement of bone height under a fixed detachable cantilever restoration supported by five or six endosseous implants. A computer-enhanced method was used to measure mandibular height distal to the last implant, which showed a significant bone growth in the mandible the first year of function.40
Not Understanding the Wide Range of Overdenture Retention
A common complication that arises with overdenture treatment planning is utilizing the same or “favorite” treatment plan (i.e., implant number and position) for all patients. The clinician must understand there exists a significant difference between overdenture treatment plans including number of implants, implant positioning, opposing dentition, and patient expectations.
To develop a mandibular IOD with reduced complications, the final prosthesis should be predetermined and related to three factors:
2. Support: the support of a prosthesis is related to the amount of vertical movement of the prosthesis toward the tissue. This will vary greatly depending upon the patient’s ridge form. Support is primarily related to implant number, independent attachments or presence of a bar design in the posterior region.
3. Stability: stability of a prosthesis is evaluated with horizontal or cantilevered forces applied to the restoration. The stability of the IOD is more related to implant (attachment and bar) position. This is often a misconception of patients because they believe there should exist no movement of the prosthesis.
Treat According to Patient’s Desires.
The patient’s initial complaints, hard and soft tissue anatomy, prosthetic desires, and financial commitment determine the amount of implant support, retention, and stability. Because different anatomic conditions and patient force factors influence these factors for an IOD, not all prostheses should be treated in the same manner. In other words, the two-implant overdenture should not be the only treatment plan offered to a patient. One should emphasize that most mandibular overdentures should be designed to eventually result in a RP-4 prosthesis (e.g., totally implant supported). If the patient wishes for a less retentive prosthesis, a prosthesis with less support, and with minimal stability, the patient needs to understand the inherent disadvantages. It is highly recommended that the clinician furnish all patients with ALL possible overdenture treatment plans. This is important for two reasons. First, the patient will be well aware of the type of prosthesis they have selected, and, secondly, this allows for the patient to understand they can “upgrade” to a prosthesis at a later time that will have increased retention, support, and stability. This will prevent unacceptable expectations from the patient following the insertion of the prosthesis.
Understand Prosthesis Movement.
The most common complications found with mandibular IODs are related to the prosthetics and a misunderstanding of retention, support, and stability of the prosthesis. When a fixed restoration is fabricated on implants, it is rigid, and cantilevers or offset loads are clearly identified. For example, rarely will a practitioner place a full-arch fixed restoration on three implants, especially with excessive cantilevers because of implant positioning. However, three anterior implants with a connecting bar may support a completely fixed overdenture, solely because of attachment design or placement. The restoring clinician believes the three-implant overdenture has less prosthetic occlusal load but does not realize that a three-implant overdenture may become “fixed” just by the design of the attachments. A complete understanding of the concept of prosthesis movement (PM) is needed by the clinician (Fig. 15.8).
Not Understanding the Concept of Prosthesis Movement
An overdenture is by definition a removable prosthesis; however, in function or parafunction the prosthesis may not move (e.g., because of the retentive mechanism, it may become fixed). Clinicians need to understand that an attachment may be classified as having movement in all directions; however, using it incorrectly or in combination with other attachments may actually change the range of motion. This may result in damaging forces to the implant or the prosthesis.
Many precision attachments with varying ranges of motion are used in IODs. The motion may occur in zero (rigid) to six directions or planes: occlusal, gingival, facial, lingual, mesial, and distal.41 For example, a type 2 attachment moves in two planes and a type 4 attachment in four planes. Attachment and the prosthesis movement are independent from each other and should be evaluated as such. An important item for the IOD treatment plan is to consider how much prosthesis movement the patient can adapt to or tolerate with the final restoration. It was to address this need that Misch has formulated the concept of prosthesis movement instead of the classification of the individual attachment. The PM classification encompasses movement from PM-0 to PM-6 (Fig. 15.9).
A PM-0 attachment exhibits no movement in any direction. For example, if the prosthesis is rigid (i.e., movement in no direction) when in place but can be removed, the PM is labeled PM-0 regardless of the attachments used. If an O-ring is used individually, they may provide motion in six different directions. However, if four O-rings are placed along a complete arch bar, the prosthesis rests directly on the bar and result in a PM-0 restoration. Because of the design of the attachments and prosthesis, the end result would be a fixed prosthesis (Fig. 15.10).
A hingelike PM permits movement in two planes (PM-2) and most often uses attachments that have the capability to hinge. The most common examples of PM-2 attachments are the Dolder bar and clip without a spacer or Hader bar and clip. A Dolder bar is egg shaped in cross section, and a Hader bar is round. A clip attachment may rotate directly on the Dolder bar. A Hader bar is more flexible because round bars flex (x4) related to the distance between the abutments and other bar shapes flex (x3). As a result, an apron often is added to the tissue side of the Hader bar to limit metal flexure, which might contribute to unretained abutments or bar fracture.42 A cross section of the Hader bar and clip system reveals that the apron, by which the system gains strength compared with a round bar design, also limits the amplitude of rotation of the clip (and prosthesis) around the fulcrum to 20 degrees, transforming the prosthesis and bar into a more rigid assembly. The Hader bar and clip system may be used for a PM-2 when posterior ridge shapes are favorable and soft tissue is stable enough to limit prosthesis rotation (Fig. 15.11).
It should be noted that for these systems to function as designed, the hinge attachment needs to be perpendicular to the axis of prosthesis rotation so the PM also will be in two planes (i.e., PM-2). If the Hader or Dolder bar is at an angle or parallel to the direction of desired rotation, the prosthesis is more rigid and may resemble a PM-0 system. As a consequence, the implant system may be overloaded and cause complications such as prosthetic screw loosening or fracture, implant crestal bone loss, and even implant failure. A Hader bar-clip system is an ideal low-profile attachment for a RP-4 prosthesis with PM-0. Usually, these clips are placed on the bar in different planes of rotation around the arch.
A prosthesis with an apical and hinge motion is defined as a PM-3. An example is a Dolder bar with a space provided over the bar. As a result, the prosthesis moves toward the tissue and then rotates.
PM-4 allows movement in four directions.
PM-6 has a range of PM in all six directions. The most common overdenture attachments for a PM-6 are independent O-rings or Locator attachments (Fig. 15.12).
Height of the Attachment
An important aspect of overdenture PM is also related to the height of the attachment connection. There are two crown height dimensions for overdentures: (1) the occlusal plane to the height of the attachment rotation and (2) the height of the attachment to the level of the bone. The occlusal plane to attachment height is a force magnifier to the overdenture with any lateral or cantilevered force. When an attachment is connected to an implant directly, the crown height above the attachment is greater than when the attachment is placed on a bar. If you double the crown height, the force is increased 200%. Hence, the individual implant attachment has a greater crown height above the attachment and greater lateral force to the prosthesis. Therefore, the overdenture is less stable (Fig. 15.13).
When the attachment is placed on a bar, the stability of the prosthesis is improved because less lateral force is applied to it (i.e., the crown height above the attachment is reduced). Whenever possible, the implant should be connected with a bar and an attachment placed on top of the bar. Rotation of the prosthesis should be as far off the bone as practical. However, there should be a minimum of 2 mm or more of acrylic space between the attachment and the denture teeth. This allows adequate bulk of material to decrease prosthesis fracture or dislodgement of teeth.
The second CHS is the height of the attachment to the bone level. The greater the attachment-to-bone level, the greater the force placed on the implant abutment screw, marginal bone, and implant-bone interface with any lateral load. When the attachment-to–bone height distance is greater than 7 mm, the implants should be splinted together to decrease the risk of complication of the implant system.
Too Large of a Cantilever (“Hidden Cantilever”)
The hidden cantilever was postulated by Misch in 1993 and describes the portion of the removable prosthesis that extends beyond the last implant or connecting bar.43 When hidden cantilevers exist, they may result in lack of support for the prosthesis, excessive wearing of attachments, increased stress to the bar-implant interface, and possible fracture of the prosthesis. In many of these cases, the attachment system does not wear because the removable prosthesis is PM-0 (i.e., no prosthesis movement); however, the excessive resultant forces place the prosthesis, abutment screws, and marginal bone at risk.
If the bar/attachment is not designed correctly, the removable prosthesis does not rotate at the end of the implant or bar to load the soft tissue. This results in a “hidden cantilever.” For example, if a cantilevered bar extends to the second premolar but forces on the second molar of the restoration do not result in movement of the restoration (i.e., posterior-inferior movement, anterior-superior movement), the cantilever extends to the second molar position. The cantilever length is measured to the point of PM, not to the end of the bar and attachment system.
With most overdentures, the final restoration should not include extension beyond the first molar. This will minimize the hidden cantilever and will decrease the possibility of excessive force. The design of the overdenture system should have the proper movement, and care should be exercised to not induce a PM movement, especially when minimal number of implants exist. (Fig. 15.14).
Mandibuar Overdenture Complications
In 1985 Misch presented five organized treatment options for implant-supported mandibular overdentures in completely edentulous patients. With these mandibular overdenture treatment options, studies have shown less than 1% implant failure and no prosthesis failure over a 7-year period with 147 mandibular overdentures (IOD) when using the organized treatment options and prosthetic guidelines presented in this chapter.43 Kline et al reported on 266 implants for mandibular splinted implant-supported overdentures for 51 patients with the Misch protocol. An implant survival rate of 99.6% and a prosthesis survival rate of 100% were reported.44
When evaluating a patient for a mandibular overdenture, ideal treatment planning and overdenture mechanics must be considered. A determination of the final prosthesis being totally implant supported or soft tissue supported must be ascertained. Misch has determined the IOD treatment options to range from primarily soft tissue support and secondary implant retention (RP-5) to a completely implant-supported prosthesis (RP-4) with rigid stability and retention gained primarily from overdenture attachments (no soft tissue support). The prostheses are usually supported by two to five anterior implants for these five treatment options between the mental foraminal area. There are four RP-5 options that have a range of retention, support, and stability. The RP-4 restoration is defined as a rigid cantilevered bar that completely supports, stabilizes, and retains the restoration (Fig. 15.15).
Complications arise when patients are treated with the same protocol without regard to the patient’s needs and expectations. For example, a two-implant overdenture may work very well with some patients; however, for others it tends to work very poorly. There are many factors that need to be evaluated prior to implementing an overdenture treatment plan. A thorough understanding of the positioning, number, and type of implants along with patient expectations should be ascertained according to various treatment plans. In addition, reports indicate that RP-5 mandibular IODs may cause a combination-like syndrome, with increased looseness, subjective loss of fit, and midline fracture of the upper denture.45
Inadequate Crown Height Space
Implant overdentures may exhibit significant complications when reduced CHS (i.e., interocclusal space) exists. When sufficient CHS is lacking, the prosthesis is more prone to component fatigue and fracture and has more complications in comparison to porcelain-to-metal fixed prostheses.1
Removable prostheses have space requirements for elements such as a connecting bar, type, and position of attachments and restorative material (metal vs. resin). According to English, the minimum CHS for individual attachments is 10 mm of CHS for Locator-type attachments and between 12 and 15 mm for a bar and O-rings.14 The ideal CHS for removable prostheses is greater than 14 mm, and the minimum height is 10.5 mm.46 The lowest possible profile attachment should be used in situations of reduced CHS to fit within the contours of the restoration, provide greater bulk of acrylic resin to decrease fracture, and allow proper denture tooth position without the need to weaken the retention and strength of the resin base.16
The minimum CHS provides adequate bulk of acrylic to resist fracture; space to set denture teeth without modification; and room for attachments, bars, soft tissue, and hygiene. In the mandible the soft tissue is often 1 to 3 mm thick above the bone, so the occlusal plane to soft tissue should be at least 12 mm in height (Fig. 15.16).
An osteoplastic procedure to increase CHS before implant placement is often indicated, especially when abundant bone height and width are present. The crown height space should be evaluated and properly treatment planned prior to implant placement. Otherwise, a fixed porcelain-metal restoration should be considered (Fig. 15.17).
The most common method of bar retention by almost the same percentage for overdentures is screw retention. In minimum CHS situations, the screw-retained bar has a clear advantage, but in ideal to excessive CHS situations, the cemented bar may be considered. A combination of screw retention on some abutments and cement retention on others offers an advantage for many RP-4 prostheses, especially when implants are placed in the posterior region.
Poor Osseous Angulation (C-A)
With increased bone resorption, the mandible resorbs from a division A to division C, eventually being compromised in angulation (division C−a). The division C−a is defined as an anterior mandible that is angled more than 30 degrees. If the clinician is unaware of this angulation, the implants may perforate the lingual plate and irritate the tissues of the floor of the mouth. If the implants are within the confines of the bone, they may enter the crest of the ridge at the floor of the mouth and make it almost impossible to restore (Fig. 15.18). In a study by Quirynen et al of 210 computer tomogram images, 28% of the anterior mandibles were lingually tilted −67.6 degrees ± 5.5 degrees.47 The mandibles with less than −60 degree tilt represent about 5% of the cases (Fig. 15.19).
Almost 10% of patients may have a greater than 30-degree angle to the occlusal plane. When present, this condition is most often found in the anterior mandible. Root form implants placed in this bone category (i.e., within the cortical plates) may have the abutments positioned within the floor of the mouth and compromise prosthetic reconstruction, speech, and comfort.
In patients with division C−a, implants may be contraindicated to prevent surgical and prosthetic complications. In some cases, conventional removable prosthetics may be indicated or extensive bone grafting may be warranted to transform the division C−a into a division A.
If implants have been placed prior to restoration, it may be the implants are nonrestorable. If the implants are nonrestorable, treatment options include:
Nonideal Implant Positioning
Usually, the greatest available bone height in an edentulous mandible is located in the anterior region between the mental foraminae. This location most likely exhibits optimal density of bone for implant support and ease of implant placement. The implant overdenture treatment options presented are designed for anterior implant placement between the mental foramina because the prostheses movement will be more limited and available bone volume and density is more favorable than posterior implants.
Nonideal implant positioning leads to many complications, which include hard and soft tissue complications, prosthesis dissatisfaction or fracture, and questionable long-term implant health.
The available bone in the anterior mandible (between the mental foramen) is divided into five equal columns of bone serving as potential implant sites, labeled A, B, C, D, and E, starting from the patient’s right side (Fig. 15.20).48 The mean interforaminal distance has been shown to be approximately 53 mm.49 Five implants of 4 to 5 mm diameter can usually be placed with general placement guidelines of 3 mm between implants and 2 to 3 mm from the mental foramen.
When placing implants, regardless of the treatment option being executed, all five implant sites should be ideally located at the time of treatment planning and surgery. There are three reasons for this treatment approach:
1. The patient always has the option to obtain additional implants and prosthesis support and stability in the future if all five sites were not initially used for implant support. For example, a patient may receive adequate retention, stability, and support for an IOD with four implants. However, if the patient desires a fixed prosthesis in the future, the four implants most likely will be inadequate for the new prosthetic requirements. If the clinician does not plan an additional implant site during the initial surgery, but instead placed the four implants an equal distance apart, the additional interimplant space may not be available without removing one of the preexisting implants.
2. A patient may desire a completely implant-supported restoration (e.g., RP-4 implant supported overdenture or FP-3—fixed prosthesis), but for financial reasons may not able to enter into the treatment at that time. Three implants in the A, C, and E positions and an IOD may be provided first. This is the initial or first phase of the treatment, which has many advantages over a conventional complete denture. Two more implants may be added in the B and D locations at a later date, and a completely implant-supported overdenture or fixed restoration may then be fabricated (Fig. 15.21).
3. If an implant complication occurs, the preselected option sites permit repeatable corrective procedures. For example, if implants were placed in the A, B, D, and E positions and an implant fails to achieve rigid fixation, the failed implant may be removed and an additional implant placed in the C position at the same time. This saves an additional surgery and eliminates the time required for bone grafting and healing before another implant could be reinserted.
Not Understanding Mandibular Overdenture Treatment Plans
The five treatment options proposed for mandibular implant–supported overdentures provide an organized approach to solving a patient’s complaints that takes into consideration various anatomic limitations. The treatment options initially proposed are designed for completely edentulous patients with division A or B anterior bone who desire an overdenture. These options are modified when the CHS is excessive (as when the anterior bone is division C−h) and eliminated for division D. The implants are placed between the foramens in the A through E positions (Fig. 15.22).
Overdenture Option 1 (OD-1)
The overdenture option 1 (OD-1) must be a RP-5 (soft tissue supported with secondary implant support), which means it must be able to rotate and load the posterior soft tissue regions of the mandible. The stability and support of the prosthesis are gained primarily from the anatomy of the mandible and prosthesis design, which is similar to a complete denture.
The primary advantage with treatment option OD-1 is reduced cost to the patient. The two implants are usually the fewest implant number, and no connecting bar reduces the prosthetic appointments and the laboratory costs.
The first treatment option for mandibular overdentures (OD-1) is indicated primarily when cost is the most significant patient factor. However, it is important to note the patient’s desires should also be minimal (i.e., patient must understand the limitations of the treatment plan), and the bone volume in both the anterior and posterior regions should be abundant (division A or B). The posterior ridge form should be an inverted U shape, with high parallel walls for good to excellent anatomic conditions for conventional denture support and stability. The problem associated with the existing denture should relate primarily to the amount of retention, not stability or support. In addition, the opposing arch should ideally be completely edentulous and restored with a traditional complete denture (Box 15.5).
Usually, two implants are placed in the B and D positions. The implants remain independent of each other and are not connected with a superstructure. The overdenture attachment primarily improves retention and gives little additional support or stability to the prosthesis. The stability of the restoration is slightly improved in the anterior section by the implants, and the posterior inverted U shape regions from the ridge form are required to improve this factor (Fig. 15.23).
In the past, most two-implant overdentures positioned the implants immediately anterior to the mental foramen in the A and E positions. Positioning of the implants in the B and D position is a much better prosthetic option in OD-1 than positioning in the A and E regions. Independent implants in the A and E positions are usually implant locations in the first premolar region, which is more posterior to the anterior fulcrum line of the anterior teeth and allows a greater amplitude of rocking of the restoration. When using B and D implants (with positions similar to the natural canines), the anterior movement of the prosthesis is reduced (Fig. 15.24).
The support of the OD-1 restoration is provided primarily from the buccal shelf in the posterior and the ridge in the anterior, similar to a traditional denture. The IOD must be RP-5 with preferably a PM-3 or more, which allows for rotation and loading of the posterior soft tissue regions of the mandible (Fig. 15.25).
The most common type of attachment used in OD-1 is an O-ring or Locator design. The implant support mechanism is poor because stress relief of the attachment is permitted in any plane. In other words, the stability and support of the prosthesis are gained primarily from the anatomy of the mandible and prosthesis design, which is similar to a complete denture.
Complications of OD-1
If the posterior area is not supported (buccal shelf) adequately, the prosthesis will be loaded mainly from the implants. This will lead to excessive forces being applied to the implants, which may result in bone loss. As the posterior ridge resorbs, the patient will gradually lose posterior occlusion, leading to a posterior open bite.
The two independent implant retention systems often have more prosthetic-related complications in comparison to other treatment plans. There are several reasons the complication risk is increased (Fig. 15.26A).
Perpendicular to occlusal plane.
The two implants should be perpendicular to the occlusal plane to allow the posterior regions of the overdenture to move downward and load the soft tissue over the mandibular buccal shelves for support. The hinge rotation should be at 90 degrees to the rotation path; otherwise, one side is loaded in a different manner than the other. In addition, because only two implants sustain the occlusal load during function or parafunction, minimization of the forces to the implant components and crestal bone is achieved by placing the implants in the long axis of the implant body and perpendicular to the occlusal plane (Fig. 15.26B).
Height parallel to occlusal plane.
The two independent implants should be positioned at the same occlusal height parallel to the occlusal plane. If one implant is higher than the other, the prosthesis will disengage from the lower implant during function and rotate primarily on the higher implant. This situation will accelerate the wear of the O-rings or Locator attachments. In addition, because the higher-positioned implant receives the majority of the occlusal load, an increased risk of complications may occur, including abutment screw loosening, marginal bone loss around the implant, and implant failure (Fig. 15.27). If the implant bodies are positioned at different heights, the use of different size abutments should be used to make them as level as possible.
Distance from midline.
The implants should be equal distance off the midline. If one implant is more distal (farther from the midline), it will serve as the primary rotation point or fulcrum when the patient occludes in the posterior segments. In such a case, the more medial implant attachment will wear faster, and the more distal implant will receive a greater occlusal load when the patient occludes in the anterior region (Fig. 15.27).
The two implants in this treatment option should be parallel to each other. The path of insertion of the prosthesis should also be similar to the path of insertion of the attachments. When the implants are not parallel, the first attachment to engage wears less, and the second attachment rubs along the side of the male and increases the wear rate. When the path of insertion of the restoration is different than the attachments (as when a facial undercut below the crest exists), the attachments will wear prematurely (Fig. 15.27).
If a facial undercut exists, the path of insertion of the restoration may not coincide because of the implant position. This may lead to chronic sore spots and the need for multiple adjustments. This most often occurs when there has been extensive atrophy (buccal resorption) from a division A to a division B or C (Fig. 15.28).
There are two types of arch forms: the tooth position arch form and the edentulous ridge arch form. It should be noted that the edentulous residual ridge may be square, ovoid, or tapering. The dentate arch form is also divided into square, ovoid, and tapering categories and may be different than the ridge form. When a tapered tooth position arch form is supported by two independent implants in a square residual ridge form, the anterior teeth are cantilevered anteriorly from the implant retentive system. More implants are required in this type of tooth-supported ridge form combination to help stabilize the prosthesis, and the OD-1 option will have a considerable disadvantage.