Dental Implant Complications: What Actually Happens
Featuring the Failure of Complexity
What could go wrong? Implants are amazing! Dental prosthetic solutions are engineered by geniuses with enormous resources and delivered by specialists with unparalleled training and experience. So what could go wrong? A lot, sadly. One of the first concepts learned in prosthodontic residency is that every step in a process and every part in a prosthesis is an opportunity for error. With this in mind, and an eye toward finding the truth about what problems to expect with implants, we humbly approach two important studies regarding complications with implants and implant prostheses.
The Research
The older of the two studies is authored by Dr. Charles Goodacre, with Dr. Joseph Kan listed as fourth author. These two are titans prosthodontic practice and literature. A person could do a lot worse for a dental education than simply typing their names in to PubMed and going through their collected works. Further evidence of Dr. Goodacre’s legacy in dental education (and also in more important matters) is in the later study — astute observers will note that two younger Dr. Goodacres are the lead authors, with Charles moving to the third position. I am so grateful for all of these wonderful people and the gift of their work.
Both of these papers combined raw data from multiple studies to calculate mean incidences of complications for the several implant prosthesis types — namely implant overdentures, implant fixed complete dentures (commonly called full-arch), implant fixed partial dentures (commonly called fixed dental prostheses or FDPs), and implant single crowns. The first study covered the literature from 1981–2001, and both implant surgical and prosthetic complications. The second study covered 2001–2017, and only prosthetic complications. For the sake of brevity we’ll call the former study “2003” and latter “2018.”
Starting at the beginning, and with the nightmare: implant loss. 2003 reported 19% (!!!) implant loss in the maxillary overdentures, 4% in mandibular overdentures. For full-arch, 10% (!) implant loss in the maxilla, 3% in the mandible. Numbers that high are shocking, but we have to remember this data set from 1981–2001 includes many years of machined surface implants which had significantly lower success rates overall (as compared to roughened surface implants), especially in the maxilla. Radiation is another possible confounding factor. While implants placed in previously irradiated bone were included in this study, the raw numbers (1,513 out of 28,712 total implants, with 134 failures in irradiated bone) were not so significant as to distort the overall picture we get from the data. Notwithstanding radiation issues and the improved implant technology, one cannot ignore the relative rates of failure. In the same data set we see 3% overall failure for single crowns and 6% for FDPs in the maxilla… even the strikingly high 10% failure rate for full-arch is only about half the failure rate for overdentures. The age of the data set and the change in technology prevent us from drawing strong forward looking conclusions. Still the implant loss numbers are comparatively so bad (basically 2X) for overdentures in the maxilla that they are hard to discount completely.
Now on to the heart of the issue for restorative dentists: how do different treatments stack up in terms of complications? In other words, after we’ve done our jobs (and very well) and the patient leaves our office pleased… what happens next? Here things get a little ugly. There are a lot of incidence data here, so we’ll get it all out in the open, then go through the implications.
2003 Overdenture data:
Attachment loosening 30%
Overdenture reline 19%
Attachment fracture 17%
Overdenture fracture 12%.
2018 Overdenture data:
Adjustment 159% [not a typo]
Change of attachment 90%
Reactivation of attachment 53%
Mucosal hyperplasia 31%
Overdenture reline 26%
Loose attachment 18%
Occlusal adjustment 18%
Overdenture repair 14%
Overdenture remake 12%
Denture tooth fracture 12%
Extension bar fracture 10%
Overdenture fracture 9%
Bar screw loosening 6%
Bar fracture 6%
Attachment fracture/loss 5%
Wear of denture teeth 4%
2003 Fixed complete denture data:
Acrylic resin base fracture 7%
Prosthesis screw loosening 4%
Prosthesis screw fracture 3%
2018 Fixed complete denture data:
Denture tooth fracture 28%
Screw Access filling lost 25%
Denture tooth wear 15%
Porcelain veneer fracture 12%
Mucosal hyperplasia 10%
Prosthesis remake 9%
Framework fracture 5%
Abutment screw fracture 2%
Prosthesis screw loosening 1%
Where to begin? Let’s go from the big picture down to the individual issues. From there we’ll discuss the reasons for these issues and explore the principle of simplicity. We’ll finish by relating these data and the discussion to treatment planning and risk mitigation.
What to do with all that
So big picture — Overdentures have more complications than any other treatment in implant dentistry. The inclusion criteria for the second study had a minimum of 1 year follow up, so in no less than that time, the average overdenture required over one and a half adjustments. Nine out of ten needed attachments changed. Over half needed attachments reactivated. Nearly a third had mucosal hyperplasia. Just over one in four needed to be relined. Occlusal adjustment and loose attachments each happened in just under one in five cases. 14% of cases needed repair. Around 1 in 10 needed a complete remake.
Fixed complete dentures had far fewer complications. Denture teeth fractured in about three out of ten cases. One in four cases lost screw access channel filling. Just under one in six showed denture tooth wear. Around one in ten would show any one of: porcelain veneer fracture, mucosal hyperplasia, or prosthesis remake.
What’s going on here? One might assume that technological design and material improvements from 1981–2017 would improve outcomes and complication rates for overdentures. Over the time studied, overdenture attachments changed from cast metal bars with metal clips (Dolder), balls with metal clips, to cast pattern bars (Hader) or ball attachments with nylon based plastic clips, or pre-manufactured titanium nitride coated Ti6Al4V ring style attachments (Locator) with nylon housings. Aside from the attachments themselves, all the ancillary materials significantly improved as well. Denture base materials became harder and smoother, manufacturing techniques from injection molding to milling and printing can create more accurate bases. Overdenture housing/clip pick up material has gone from pure PMMA like GC Pattern Resin, to various base/catalyst resin composites such as Quick Up and UFI Gel Hard C. Newer pick up material has less shrinkage and is more accurate, is less exothermic, sets faster, and has a harder and more plaque resistant final state. So everything going into the overdentures has improved, and yet the complications and complication rates for overdentures did not improve. In fact, they got worse.
Why is all this badness happening?
It’s tempting for a classically trained prosthodontist to point to a decline in the quality of complete denture training during dental education for some of these issues. But as much as these young dentists REALLY do need to get off my lawn… I don’t think that explanation can account for the data presented here. I submit that the issue at play is more fundamental: overdentures are too complex.
One might contend that they are simple enough — just a regular complete denture with some added attachments to increase support, stability, and retention. Conceptually this is correct — they are simple enough to explain. It’s in the physical world, in the engineering and execution, that complexity, and therefore chaos, reigns.
Begin with the complete denture — a hard acrylic base adapted as well as possible to some combination of keratinized gingiva and alveolar mucosa overlying bone. Gingiva and mucosa with differing degrees of thickness and mobility, as well as elastic moduli which result in differing amounts of flexure under compression due to chewing. Bone which we know will undergo differential and not precisely predictable resorption over time, necessitating periodic relines to ensure an acceptable fit. We need bone volume and area to provide support for our overdenture, but more bone volume means less available restorative space for denture base thickness and durability. Now add to that system our implants and overdenture attachments. The attachment/housing unit takes up space within the acrylic denture, leaving reduced base material to resist fracture. The implant/attachment unit has a particular elastic modulus and the attachment matrix+patrix/housing system has a particular elastic modulus. The resultant modulus of the overall attachment system is about two orders of magnitude different from the modulus of the tissues. In other words — the tissue will compress in the neighborhood of a few mm under chewing stress, whereas the overall attachment system will compress in the neighborhood of 25 microns; about 100 times less. This mismatch creates significant focal stress in the overdenture around the area of the attachment during chewing. Now we add to this the fact that the attachments go through several cycles of insertion and removal each day, adding up to tens of thousands over the lifetime of the prosthesis. Each insertion or removal wears and fatigues the attachment and the overdenture itself at some level. The parts in the attachment/housing unit are engineered for a precise fit, but each insertion carries with it the possibility of being placed slightly off, or even having a foreign body (like a sesame seed) introduced between the matrix and patrix parts of the attachment system. In either case the pressure of insertion can distort the attachment or housing.
Another way to look at it is: what are we asking an overdenture to do? We want it to fit precisely and evenly and for a long period of time over two very different substructures: oral soft tissues and implants with attachments. The tissue is much more flexible and is guaranteed to change over time, the implants and abutments are much less flexible and guaranteed to stay exactly the same. We also want this overdenture to fit these substructures under significant compressive loads in multiple directions (because chewing is not simply an up-and-down event). We want it to be durable over millions of cycles of loading with focal stress at the area of the attachment, which is also the area of greatest mechanical weakness, because we have undermined the integrity of the base material by placing the attachment housing there. We want the attachment to hold tightly over thousands of insertions and removals, but not so tightly the patient cannot remove it easily, and not so loosely that the compression and loading from chewing distorts the attachment or dislodges it. It is this set of competing and often contradictory demands that makes the complications we see with overdentures so persistent.
It is not inconceivable that some technology might be invented that would more perfectly fulfill all the demands we make on overdentures, but it does not exist now. This tech would likely be categorically different from what we currently use, and might just as likely cost quite a bit more than our current offerings, thereby eliminating overdentures single main advantage over fixed complete dentures.
Why do fixed complete dentures have fewer complications? Simplicity. All we ask of them is to fit the implants/abutments and to be aesthetic, functional, and durable. These demands are not so numerous, and not so directly and necessarily contradictory (yes, yes, maximizing durability might compromise aesthetics and function, and vice-versa) as they are for overdentures. This means that improvements in materials and manufacturing processes have predictable corresponding improvements in prosthetic performance. For example: the most common reported complication for these prostheses was denture tooth fracture. Most of the full arch prostheses made today do not even use denture teeth. Either the entire prosthesis is a zirconia, or the full arch of teeth are a nanoceramic over a glass fiber reinforced resin framework. In any case, using acrylic denture teeth processed to an acrylic base is far less common. By eliminating the complexity of multiple teeth processed to a second material, and then improving the mechanical properties of the tooth material used, we can (and have) predictably improve the incidence of this type of complication.
A note about the appeal of overdentures: there is something inherently interesting about complexity. Think of the fascination with Rube Goldberg machines, or the enduring market for mechanical watches when a quartz watch will in fact keep better time. The human mind likes fancy solutions for whatever reason. We find these things dazzling. Yet we began this post by mentioning that in prosthetic rehabilitation, we know every step, every layer of interaction, every change of material, every movement in a system is an opportunity for something to go wrong. Over large groups of patients, and over any significant period of time, these things will go wrong. Cliches are ignored because they’ve been repeated until they become background noise. But wisdom becomes cliche because it is enduringly true. So let’s repeat some cliches now! In any system, the fewer moving parts, the more reliable the system will be. Sydney Dekker (and Tim Ferriss) says “complexity fails.” Leonardo da Vinci (who knew a thing or two about making things work) said “simplicity is the ultimate sophistication.”
“complexity fails” — Sydney Dekker
What to do
So now we have all these data, and we have thought through the reasons and implications. We understand the principles governing these outcomes to some degree. How then, shall we live? Ok… too broad, perhaps. How then, shall we treatment plan? As we reviewed in a previous post about systemic medical conditions and implants, there is no getting around the responsibility of being a clinician. We’ll call it DeRossi’s rule. We have to weigh the options and give our patients real informed choices. We know that overdentures will cost less up front, but is this a better investment than fixed full arch treatment? This question has to be answered from both patient and practice perspectives.
Many patients are likely attracted to the idea of paying less for overdentures vs. fixed. But what is the patient paying for? What job are they hiring our treatment to do? The answer is aesthetics and function, in one way or another. The function may be eating better food, however the patient defines better. It may be allowing a patient to have confidence for a romantic life. Whatever the case, the overdenture may deliver these on day one. In the days after that, the high rate of complications will invariably erode patient satisfaction. More time repairing things means more time making and then going to dental appointments, more opportunity cost. Every minute spent on treatment after the final delivery (also before and during the delivery, if we’re honest with ourselves) is one that the prosthesis is not doing the job the patient hired it to do, and the patient is not doing what he/she wants to be doing. This will build resentment in even the most stoic patient, whether or not they are paying for each fix with cash as well as time. Even outside the 12% of overdentures that we can expect will require a remake, the treatment may be a failure for the patient due to the ongoing costs and time needed to maintain the prosthesis.
From the practice perspective, all of this is bad. Doing precise actuarial calculation on the expected cost of goods and labor of an overdenture vs. fixed treatment would be difficult and would certainly vary depending on the practice. Some of the issues described can be mitigated, but even that mitigation has a cost. A practice can increase the upfront price in order to cover some of the back end chair time and materials costs associated with the relines, attachment switches, repairs, and remakes that they know are coming. This can reduce the frustrations the patient would have if they pay a second (and third and fourth…) time to fix complications. In the same way, a thorough informed consent and treatment plan that includes all the anticipated post-delivery complications (“maintenance” may be a better word to use in this situation) can help patients know what to expect. More cliches: if you tell them before it goes bad it’s an explanation, tell them after and it’s an excuse. Even the best patient management may fall short. You cannot talk your way out of persistent complications after any treatment.
So there we have it. Overdentures historically have had the most complications in implant dentistry (which is also the most complications in all of restorative dentistry). Until the invention of a revolutionary new overdenture system that drastically reduces complexity and thereby drastically reduces complications, we are left with a highly compromised option for patients who are unable to get fixed full-arch treatment. We can choose to offer this option, but we ought to do so with our eyes open, and be sure our patients can see their future clearly as well.
