All-on-4: bone resorption?

This will be a great discussion it's the part they don't tell you about in the ads. I have had some discussions with very experienced doctors on long term maintenance due to the soft tissue management and removal of the prosthesis for hygiene. I have seen the typical circumferential bone loss consistent with peri implantitis. Also fixtures fracturing due to inadequate intra arch space and alveolar reduction. Once this bridge is burned things get dicey.

In my experience lower All-on-4's have worked out very well. All of my patients have tolerated a small space 1-2 mm's under the prosthesis without a complaint. No major problems of any kind. Upper All-on-4 cases are a different matter. Adequate bone reduction and careful pre-planning are essential. If the prosthesis is too thin cracking and breaking teeth off is a real problem. One patient, who is a clencher, broke the stem of one of the abutments off inside of the implants rendering it useless. Dr. Ken Parrish (periodontist) who lectures for Nobel does an excellent job of describing proper bone reduction. He works with RDL dental lab, also from Kentucky, who will fabricate the surgical splint. Dr. Sia Abai, a prosthodontist, who lectures for Glidewell Lab does a great job on the stained zirconia prosthesis that Glidewell is promoting. It is heavy, but very strong and will work well if adequate bone reduction is provided. Until you've done at least a dozen locator overdentures (half in the maxilla and half in the mandible) don't even think of taking on one of these cases. Remember the wise words of Dr. Carl Misch: "With this implant I thee wed!"

I have been doing All on 4's for 7-8 years now and have done lots of them. I have NOT had any implant, abutment or abutment screw fractures. Yes I have had occasional circumferential bone loss, but no more than round any other implants. If you follow the protocols, remove enough height of bone, get a wide enough ridge for good implant placement and basically do it right, it works very well. For those who have persistent problems with this system I suggest re-training and using a well tried and trusted system as there are many systems that claim to do All on 4's but do not have the correct bits and pieces to get it right. Remember with dental implants there are no compromises, it is either right or it is wrong

The main problem I have seen is a lot of Dr's saying they do "all-on-4" but do not follow the clinical protocols set out by Malo and others. As stated above, bone reduction is not carried out by a lot of Dr's (as a prosthodontist restoring several of these cases I have waded thru a few surgeons just for that reason), secondly just becuase the implant is "tilted" does not make the case "all-on-4". In the early years Malo took a lot of stick "all on 4 none on 3" etc but now everyone seems to be getting in on the act. Would I use all-on-4 on a case where I had extracted multiple teeth and had sufficient bone to house say 8 implants upper jaw and +5 in the lower - simple answer is "no". I would go conventional. All on 4 has indications but again being abused and "sold" as a quick fix answer

Dear collegues, with all respect - it seems almost nobody is reading questions. 2 very concrete questions asked. Only CRS replied about 1st question properly. others..... I'm sorry.

I don't know what happens in the posterior mandible since the bone is no longer loaded sometimes it is already down to basal bone or it would be removed in the surgical procedure to allow space. I guess you could look at serial radiographs and compare. It is an excellent question. There would be no pressure on it since the denture is implant supported but there would be no stresses on it either. The issue in the maxilla is also dependent on where the finish line is in relation to the upper lip.

Because I don't do mandibular all on 4 I can't answer the questions posted but I have another related question. We know cantilevers work because patients do not chew on them. Bone is a load carrying organ and it is excellent at sensing load and overload. When overload is sensed chewing forces stop. My question is what happens to the masseter muscles? Masseter muscle atrophy is common in our patients with compromised occlusions and I wonder if the same thing is going on with mandibular all on 4. Greg Steiner Steiner Biotechnology

CRS, It's OK that the answers deviate from the original questions. There are often side issues and words of wisdom that might otherwise be missed. I often enjoy the side bar discussion more than the answers to the original questions.

Well, I think this is an important point, for the first question, resorption is observed mostly at the distal side of the posterior (angled) implants.. simply the cantilever will always deliver larger crestal forces... for the second point, disuse atrophy is expected!

Over 70% of the occlusal forces will be borne by the cantilever segments, Malo advises that the heads of the tilted implants line up so that angled abutment is as close as possible to the central fossa of the 1st molar so reducing the cantilever length (easily done in the maxialla providing sinus expansion is not too great). If you are going to hang +10mm cantilevers in any design then you will have some degree of bone loss. I don't think dis-use atrophy (commomly found in natural dentitions) is an issue in implants (the implant closest to the point of load application will experience the bulk of the forces with very little force distribution to adjacent implants even if they are rigidly connected together with a bar etc)

also comment above regarding bone being excellent at sensing loads - studies show an implant needs to experience a load approxiamtley 7-9 times greater that that experienced by a natural tooth before the bone fires any responses. teeth had a pdl bone relies on pressure sensors so ideally if you can hold on to a few posterior molars you will have a better long term prognosis

Tuss Can you give me references on the 70% and the 7 to 9 times. Thanks Greg Steiner Steiner Biotechnology

I will try to answer some of the questions posed above from a prosthodontic viewpoint. 1) in the mandible we can see bone apposition on the superior portion of the posterior edentulous ridge even though there's no implants. Wolf's law is in effect meaning that the periosteum of the superior border is place under tension when a mandibular hybrid is loaded. This stimulates bone apposition. Compression of the PDL or periosteum stimulates resorption (hence a tooth can move orthodontically and one of the reasons we can see accelerated bone loss under tissue borne dentures). Therefore 4-5 implants placed in the severely resorbed mandible will not cause mandibular fracture at the body or continued loss of bone. My graduate research looked at spontaneous mandibular fracture after implant placement and we found no body fractures but several fractures through osteotomy sites in the first two months if the anterior mandible was less than 6mm wide. 2) To G.S., there IS load on the cantilevers. We routinely cantilever the posterior teeth in both the maxilla and mandible. These are functional teeth. If we are not careful in our cantilevers then retaining screws can fracture; one of the first symptoms of excess cantilever length. Also, bone reacts to loads placed on it (see above) but does NOT sense "overload". The PDL is the only organ to register occlusal force. Williamson wrote a paper describing the cuspids as a sensor that moderates lateral forces and decreases masseter and temporalis forces but simply restoring a full arch implant patient with cuspid guidance does not introduce the same safeguards as natural cuspid guidance. That is why bruxers can kill our restorations and deintegrate our implants. I agree with you that our fully edentulous patients or compromised dental patients may have decreased function but when you replace the majority of the teeth and retain them on implants function can return with a vengence! In regards to the maxilla it's a totally different animal than the mandible. It does not flex quite like the posterior mandible so less chance of bone apposition. Also, Parel describes the maxilla as more of an all on six situation due to the softer, more type III or type IV bone found in the maxilla. There is the possibility of crestal bone loss if overloaded so, when engineering, overengineer don't underengineer. I also like platform switched type implants for the maxilla so less chance of crestal bone loss. Hope this long winded explanation helps. After 25 years I've seen a lot of "train wrecks"!

mwjohnson Much of what you say makes very good sense but you state that severely resorbed mandibles never fracture. That contradicts your following statement. Also, bone reacts to loads placed on it (see above) but does NOT sense “overload”. I am also waiting for Tuss's references. Thanks Greg Steiner Steiner Biotechnology

After going through all the expense and effort - why not place at east 6 impacts? "All on 4 - none on 3". It saves some cost up front putting fewer implants. But if you have a problem that can affect your reputation. Throw in a couple extra implants or so and but yourself insurance and peace of mind.

I have done over 25 cases of All on 4 in Mandible & around 20 cases of All on 6 in Maxilla in last 4 years. This concept is essentially for Atrophic Maxilla & Mandible where augmentation procedure is avoided. There is no particular resorption pattern seen with angulated implants. Few of the cases are Extraction + Implants + same day interim prosthesis for eshtetic purpose - no occlusal load. Again no bone resorption seen. For all lower prosthesis there is no need to remove prosthesis for hygiene purpose if 3 mm space between convex undersurface of prosthesis & crest of alveolar bone is maintained. For maxilla the gap is kept on palatal side where patient can brush well.If patient is given a choice between the gap to keep it clean or no gap with stinking mouth - all patients have opted for cleaning gap. I do not dare to give any cantilever in Maxilla. There has to be distal implant in Pterigopalatine area. It avoids all the maxillary prosthetic complications & give full occlusion up to 2nd molar.Mandible can tolerate cantilever depending on AP- spread. As far as All on 4 in mandible is concerned with angulated implants , what difference it makes even if there is resorption in posterior mandible post prosthesis ? Its implant supported & no soft tissue comes in contact. I would like to post some cases , to have critical evaluation so that patient care can be improved.