Design Consideration for an Implant Bar

I must admit, when it comes to designing and fabricating an implant bar, I get a little lazy.  I do all the initial planning, making sure I take a good impression capturing both the implant information and the soft tissue information.  I go through my usual steps for taking jaw relation records, determining where I want my teeth to be; do the teeth try in to confirm aesthetics and jaw relation.  Index the teeth arrangement on the model to evaluate that I have the prosthetic space for both the implant bar and the prosthesis….then I ship it off to the lab and tell the lab to make me an implant bar.

Now with so many attachment systems, materials and technology available, there is really no limit what you can do with an implant bar.  Your imagination is really the limit.  I say I get a little lazy because I often would ask the lab what would you like to do….thinking the lab knows more than I do because the lab has seen and done more implant bars.

Recently, I delivered an implant bar overdenture that caught me by surprise.  I was quite happy with the surgical placement of the implants in the mandible.  I knew I had plenty of prosthetic space for the bar and the denture.  When I delivered it, I secretly thought that the patient was going to love it….after going through five failed implants, then refusing to have any more implants done, then trying to get by with conventional full dentures…then admitting that he cannot function with conventional dentures and finally  agreeing to having more implants.  So I planned and placed four implants for him.  I made an implant bar supported overdenture for him.  This implant bar was a titanium milled bar with 4 locator abutments screwed onto the bar.  The distribution of the abutments are favourable and I thought the denture will be very stable retentive and well supported.

Few months later, he came back and said the denture was loose.  I really didn’t expect it.  I couldn’t see him at the time but my associate did the evaluation and after changing some male components for the locators and a reline, the implant overdenture finally settled in and he was fine after that.

This experience got me thinking about some of the implant bars I have made in the past twenty years.  I know the usual wear and tear of an implant denture will result in the need to change the components of the attachments.  But with an implant bar with four retentive elements distributed like a four legged table, I didn’t expect the denture to come loose so quickly.  Then I recalled there are some bars I have changed components frequently and then there are those that I rarely changed.  I started thinking about what are some critical features in a bar that has less wear and tear…and less maintenance need for the patient.

So let’s go back to the basics: what do we know about an implant bar so far.  I know that by splinting the implants with a bar, the path of insertion for the denture is not influenced by the position nor the angulation of the individual implants but rather it is built in based on the design of your bar.  You can have a very precise path of insertion by having a friction fit milled bar that is highly retentive.  You can have an implant bar waxed and casted to any shape you desire and the overdenture be fitted over by way of a metal sleeve or by just acrylic.  Any bar you design will improve the support and stability for your prosthesis. But the retention will be variable based on the attachments systems selected and the how fitting your denture is to the bar.

One of the things I never thought about before was the path of insertion for the bar vs the path of the attachments that need to travel to obtain adequate retention.  Ideally, they should be similar.  If they are, they work well together without necessarily wearing out the attachments quickly.  For example, the bar I have made with two precis vertix attachments on the distal end of the bar are made to be parallel to one another and to the shape of the bar.  The vertical distance of the attachment works like long guiding plane. Essentially there is a “longer” distance or surface area before the attachment is worn off and become non-retentive.   Personally, I noticed that when I design my implant bar using these attachments systems (precis vertix), the attachments rarely need replacement.  I have had patients with the same attachments for over 5-10 years and never need to change the nylon sleeve.

 

Now in recent years, I decide not to use precis-vertix attachments for my bars and switch to using locator abutments to simplify my inventory stock of attachments in the office.  Locator abutments got popular because of its lower profile height allowing more prosthetic space.  The other feature is that they allow implant angulation correction up to a certain degree.  It is this feature that can be your best friend or your enemy when it comes to implant overdenture.  If it is used as individual abutment, it can work quite well even if the implants are not quite parallel to one another.  But the retentive components will wear out faster.  This is because the components are designed to flex over the abutment in many direction to engage the “undercut”.  The vertical distance it needs to travel before it engages the undercut is quite short ~1.5mm.

But if you have a milled bar, the only direction the denture can be seated over the bar is influenced by the opposing buccal lingual walls of the bar.  In fact, if you have a denture that is precisely fitted over the implant bar, it will not have “room” to flex over the locator abutment but in only one direction.  Instead of flexing over the locator abutment buccally lingually or mesially distally, the denture is travelling in one occlusal gingival direction, dictated by the walls of the implant bar.  Given the “short” distance the male cap needs to travel before engaging the “undercut”, these locator abutments can be very retentive when engaged, but are bound to wear out very quickly in a short time, leading to a situation either with great retention at the beginning to losing to almost no retention very quickly.  This has certainly been my experience when I changed my bar design to using locator abutments to previously using precis vertix attachments.

There are many other elements you have to consider when you want to make an implant bar that I have not covered here.  But I have learned that not all attachments are created equally for your clinical needs.  Instead of jumping on to the next best attachments and following what others are doing, it is important understand the mechanics of how your removable prosthesis works.  I hope you find my anecdotal experience helpful in designing your implant bar next time.

Thank you for reading!

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