Introduction: Sirona Sinius Dental Chair: Repairs and Upgrades

This instructable is only intented for a very small audience. If you're not in the dental field, it's probably not for you. If you have nothing to do with the Sirona brand, it's also very unlikely this instructable will be helpful for you.

However, since almost no DIY stuff is available for dental professionals, I hope this might be interesting for some people. I do own this chair for 3 years now and I've had several issues; actually many more than my 20 year old sirona chair it replaced. Some of the issues will be dealt below and greatly improved my life as a dentist. These are the the issues I've covered up to now. But as this is a work in progress, more might follow in the future:

  1. Dirty feet
  2. Dual side curing
  3. Broken plastic clip
  4. Broken plastic clip part 2
  5. Entangled cables
  6. My arm is too short
  7. Magically auto-refilling refilling reservoir

Step 1: Dirty Feet

Many of patients do have dirty feet and transfer this to me precious chair. Probably it might be better to ask them to remove their shoes before entering my practice; as is common in Japan. But in Belgium we're not used to do so. To protect my chair I did make a transparent, easy to clean PVC cover. I sourced some transparent PVC from the local market and cut it to the right size. I did do the free-handed, so no template to share; I'm sorry. To adhere the PVC to my chair I did stitch some velcro to the sides and glued the other part to the underside of my chair. This is a fast and easy to clean solution and once the PVC is worn it's also easy and cheap to replace.

Step 2: Dual Side Curing

Nowadays most dental curing lamps on the market are cordless, battery
powered devices. Actually I do only see disadvantages for such cordless devices in a dental practice: You have to keep the batteries charged; because of heavy use, batteries wore out relatively fast; the lamps can get lost; borrowed,... While a cable does not add any inconvenience, as we always use the lamp in the same place. When buying my chair, only one option of built-in light was provided; a Satelec mini led which would set me back 1660€ and could only be installed in the assistant unit. Apart from being expensive for a not so special lamp, I prefer to have my lamp next to my other instruments.

To make this happen we start with the power supply. Open the front compartment of the chair (located below the feet) by prying with a flat head screwdriver in the provided slot at the front of the chair until clip holding the cover releases. This provides access to the back of the main power button. By following the cables from here you'll see a terminal where you can attach a 3-way socket that can hold the transformers for your curing lights. This is more convenient than connecting it straight to the mains power, as the curing lights will power on/off together with your chair. Next cut the cable of your curing light close to the AC/DC adapter and connect it properly (by soldering or using appropriate clamps) to your electric wire. Now we have to route the wire to the dentist unit. Do not route the mains cable to the dentist unit! This might be dangerous and there is insufficient room to house the adapter. To route the wire you'll have to remove many covers. Almost all of them do pop off without using any screws; just pry a little to the sides and you'll find easily how to remove them and put them back. Once removed, you'll see how all cables to the dentist unit are routed and you can follow the same way. The most tricky place is were the cable comes out of the cable chain and enters the arm of the dentist unit. There is actually little room left for additional cables and I had to pry in a lead wire from the other side to pull my wire through it. Once this point is passed we're almost in the dentist unit. From there on its easy. Use one of the many holes to get you wire out, reattach your curing lamp taking into account the polarity (just match the colors from your previous connection). and attach the wire with a tie wrap to the correct outlet to make it look and function properly.

Because of reliability and to speed up my procedures, I prefer to have 2 curing lights at hand (Actually I have even a third battery operated one to be really sure). As you have all covers removed, you can easily repeat the previous procedure to route a second wire to the assistants' unit. Actually it is more easy than the dentist side. Unfortunately, I do not have any pictures as I did not feel removing all the covers for the sake of this instructable alone. But trust me it's not that hard.


  • 4-way socket with switch
  • 2 corded curing lights; I do use this one, which is actually of good quality and provides plenty of power (1600 mW/cm² as measured by a bluephase meter)
  • electric wire, with a gauge sufficient
  • some tools to make electric connections (soldering tools, electric tape, connectros,...)
  • tie wraps to cable our wires in place and keep them tidy

Step 3: Broken Plastic Clip

Only after a few months of use, the plastic clip holding the handle of my assistants' unit broke. And ever since the handle was held in place by wrapping foil. In my opinion this is clearly a design mistake; a removable handle on such an expensive chair should be made out of metal or alike. Anyway, since I do own a 3D printer, the solution is easy. I designed a new hook that slides right in the place of the old hook. You only have to cut off the old hook until the end and print the STL provided here. I did use PLA, as it's cheap, easy to print and resistant to disinfection alcohol.

Step 4: Broken Plastic Clip Part 2

The latch of the cover at the base of the main unit holding the amalgam recuperation device and some disinfecting liquid was also broken very soon upon purchasing this chair. Actually, I'm pretty sure this happened already in the showroom, as I can see some glue residue on the fractured area and I never reglued this piece. So probably they broke it in the showroom, put some superglue and prayed it would hold at least for the warranty period.

I must admit that my repair is not really very durable, but as it is so simple and takes no effort at all, it's worth doing while waiting the complete failure of the clip. All I did do was taking a 3.5 x 20 mm silicone ring and slide it under the broken part of the clip. Now the cover remains in position again. It's only a little harder to open than stock.

Step 5: Entangled Cables

Ever since I'm using this chair I'm annoyed by the foot pedal and more specifically the hook. As my pedal is usually somewhere below the assistant's unit, the suction tubes have the tendency to get entangled behind hook. A problem I did not have with my previous Sirona chair, as the pedal was equipped with a strap rather than a hook. In the beginning I thought I would get used to the hook, but after 3 years it's time to solve this issue too. The idea is to equip the current pedal with a strap-like design. For this, I did use an aluminum flat bar with rounded edges with a cross-section of about 4 x 0.5 cm. This was actually a left over from an old curtain I had laying around. I bent the bar over a solid round wood object with a 15 cm diameter to get a nice even curvature. Next, I cut the edges to the right size and connected both ends with some screws and perforated steel band.The newly created band can easily screwed with a short, flat M6 screw on the bottom of the pedal. You only have to cut away some of the rubber on the sides that interfere with the metal band.

Once finished, the aluminum fits the overall design of the chair very well. Also functional it suits much better my personal needs. The tubes do not get entangled anymore behind the hook and as a nice side benefit my feet seem to find the side buttons more easily blind-folded, as the aluminum serve as some kind of guide straight to the buttons. The only thing I might have to change in the near future is the metal band used; it's a little thin and not stiff enough; so that the strap moves a little to much to my liking and might break in the future.

12/2020 Update: After some months of use, the perforated steel band proofed to flimsy. It never actually broke, but moved a little too much to my liking. So I purchased a thicker aluminum flat bar. After some hole drilling

Step 6: My Arm Is Too Short

The last modification is the most invasive one and the one that I'm a little hesitant to recommend to others. Actually I'm a little angry that Sirona did not include this by default. Before explaining more, I need to tell you about the accident. After owning this chair for 2 years, at the end of a treatment I instruct the chair to go the home position; I turn my back and I hear a very loud cracking noise. What happened, is that the chair got under the dentist unit, lifted it for about 20 cm, resulting in the complete failure of the base plate of the dentist unit. This base plate cannot be purchased separately, so that the whole dentist unit needed replacement. This costs about 8000€ without labor and taxes and without the seat cover that was also damaged and costs almost 500€. At that moment I was so angry with Sirona for their stupid design, that I'd rather buy a new chair, than spending 10000€ fixing mine. Luckily I do have a good technician, that rather than replacing the base plate, glued it together.

Anyway this accident degraded the value of my chair on a second hand market to almost zero. And all of this could have been prevented by a simple sensor that detects when the dentist unit is lifted and stops the chair movement. This feature is present on the assistants' unit on the other side of the chair, but was probably not implemented on the dentist unit to reduce cost. The Sirona solution is to limit the movement of the dentist unit so that it can not move over the chair. While on paper this seems a good solution, in practice it is not. After the accident, my technician adjusted my unit in such a way to prevent the accident from happening again. According to him, I must have asked to remove this limitation during initial installation. I do not recall so and even if so; how would I know about this feature before owning the chair. Moreover, he should never have lifted this limitation, knowing about the damage/costs it can provoke. My guess is that, as my chair was a showroom model, they changed this limit in the showroom to hide the poor ergonomics and forgot to put it back upon installation. Anyway once the limit was properly set, I could not reach my instruments anymore when working in the 12 o'clock position. So the goal of this modification is to make a button that detects when the chair is lifting the dentist unit and stop any movement and consequential damage. Once we have this button, I can remove the limit and reach my instruments again in an ergonomic way.

To detect that the chair is lifting the unit, I designed a trigger plate that will be mounted below the dentist unit. The trigger plate was printed in white PLA, as this material is easy to print and resistant to alcohol (unlike ABS). This plate uses 2 mounting holes from the unit itself, so you only have to find 2 similar, but somewhat longer screws than the original ones. As is shown in the pictures provided. The plate also has a hole, so that the screw to open the dentist unit is still accessible.

Next, we have to attach this trigger plate to a switch that engages when the plate is lifted. For this, I used a miniature snap-action switch; you'll need one with 3 contacts as the the circuit is designed in such a way that continuity is broken upon lifting the plate. At the inner side of the left handle, I found some room for this switch, where it does not interfere with any cover, screw, mounting handle,... I mounted the switch with 2 screws in holes I drilled in the side as shown on the picture. Upon installation of the switch, the position of the trigger plate can be adjusted, as I provided some play in the screw holes, so that the screw engaging the switch is straight under the switch. Also the height of the engaging screw can be adjusted, so that the microswitch is engaged upon slight movement of the trigger plate.

Now all all that's left to do is hook this switch up to the chair in the same way as for the assistant's unit. For this connect an electric wire (gauge is not that important) to the NC (normally closed) contacts of the switch. Then you have to route your wire to the back/bottom side of the dentist unit. I did open the electronics compartment and used some tie wraps to hold the wires in position. In order to get to get the wire to the main electronic board located below the patients feet, you have to follow the same route as shown in the section about curing lamps. On this main board locate the header labelled X13. This header is connected to the switch that is engaged upon lifting the assistant's unit. Undo this header, cut one of the wires and add your new switch in series with this one and put the header back in place. All that's left i now is test your new button. The chair should move properly and when the trigger plate is touched it should stop suddenly and show an error on the main screen.

Once we have a working safety mechanism, we can lift the safety and unleash the full ergonomics of our chair. All that needs to be done is adjusting a little bit the rotating hinge mounted right on the slide at the bottom of the dentist unit. In fact it's best to do this before reassembling your chair after all wires were routed. This hinge (picture #10) comprises a big inner gear. In these teeth fits a nylon stop block. By undoing the screw holding the nylon stop block, the position of the block can be changed, so that the entire dentist unit can be rotated closer to you. While the change in position may seem minimal, for me it' a huge difference, as I do not have to bend over, any more every time I need an instrument.

Step 7: Magically Auto-refilling Refilling Reservoir

One day a little water was flooding from my chair. Upon inspection the reservoir holding the disinfecting solution (usually orotol, but I use ) for the vacuum hoses was so full, that it flooded. Of course, initially I blamed my assistant, but she claimed not to have refilled the reservoir in many months. So in short, it's a mystery; The reservoir did fill itself! After having checked that liquid was sucked during each cleaning cycle, I emptied the reservoir and added a small amount of fresh disinfectant and monitored the level for some time and indeed after three weeks the amount of disinfectant was tripled, but also clearly diluted. Using some pink dye (caries detector! I'm a dentist you know), I found that after each cycle some liquid did reflow inside the container, explaining how the reservoir got filled by itself. The cause was a valve that was not firmly attached to the base module. I solved the issue by tightening the 2 screws (circled in red on the picture), but I'm afraid that this issue will return, as the 2 screws used are flimsy and have little thread that secures them in the plastic. So be aware not to overtighten the screws, as you may damage the plastic base.