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Received June 27, 2021, accepted Februry 23, 2022, date of publication March 1, 2022

Analysis and Solution of Dental Unit Failure

By J. J. Jin1, H. Liu1, K. Li2, Y. H. Chu1 

1 Department of Clinical Medical Engineering, the Second Affiliated Hospital of Zhejiang University School of Medicine, Hangzhou 
310009, China
2 Equipment Department, Ningbo Chinese Medicine Hospital, Ningbo 315010, China

ABSTRACT

Objective: To discuss and analyze the common causes of dental unit failures and summarize maintenance experiences. 
Methods: The failures were studied through retrospective analysis in our dental clinic from January 2019 to December 2019. 
Causes for four common failures were analyzed deeply, and the corresponding improvement solution was implemented.  
Results: These solutions reduced the failure rate for dental units and improved understanding of the importance of using and 
maintaining the equipment correctly.
Conclusion: Analysing and improving proper maintenance can save costs for the hospital and effectively enhance the manage-
ment level of medical equipment maintenance.  

Keywords – dental unit, failure, solution, maintenance.

Copyright © 2021. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY): Creative Commons - Attribu-
tion 4.0 International - CC BY 4.0. The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) 
are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is 
permitted which does not comply with these terms.

INTRODUCTION

With the improvement of people’s living standards 
and the enhancement of oral health awareness, oral 
health has been paid more and more attention. The dental 
clinic of most domestic hospitals is often overcrowded, 
and the number of patients treated every day is much 
higher than in some foreign hospitals. The dental unit is 
a piece of essential medical equipment in the clinic. Since 
the establishment of BinJiang Hospital, 30 A-DEC dental 

units, including 4 A-DEC 500 units and 26 A-DEC 300 
units, have been introduced and installed. With preventive 
maintenance and emergency repair by hospital engineers’, 
they run stably. As we all know, dental units are generally 
composed of a dental chair, delivery system, cuspidor, and 
support center, dental light, etc. The schematic diagram 
is shown in Figure 1.

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51 J Global Clinical Engineering Vol.4 Issue 3: 2022

Jin, Liu, Li, Chu : Analysis and Solution of Dental Unit Failure

The structure of dental unit is complex and includes a 
waterway, electrical circuits, and gas circuits. During the 
installation of the dental units, dealers generally recom-
mend that the hospital use an independent water storage 
tank with pure water or a treated centralized water supply 
as oral treatment water. The hospital should use oil-free 
and dry air, in which the pressure is not less than 80 psi 
to drive high-speed air-powered handpieces. Considering 
the actual diagnosis and treatment needs in China, most 
hospitals will choose a centralized water supply, and the 
water quality should meet the requirements of GB 5749-
2006. After the dental units are installed, the hospital 
engineer will carefully check whether the functions of the 
equipment are normal and whether the accessories are 
consistent with the contract. However, many accessories 
may not be original but might be domestic accessories, 
so the service life cannot be accurately judged. During 
the use of the equipment, the manufacturer recommends 
preventive maintenance and regular replacement of some 
wearing parts. The hospital will also handle it according 
to the actual situation. To ensure the water quality of 
dental units, many domestic hospitals will regularly use 
sodium hypochlorite or other disinfectants to sanitize 
waterways. However, this solution will cause certain 
damage to the internal valve and pipeline of dental units 
and increase the failure rate, bringing some challenges 
to hospital engineers.1 

As a maintenance engineer, the author has been re-
sponsible for oral-related equipment maintenance work 

for many years and has accumulated rich experience in 
maintenance. In the following, the author summarized 
and analyzed the care of our hospital dental units by using 
the A-dec brand as an example in recent years. From the 
engineers’ point of view, the corresponding improvement 
measures were put forward to prevent and solve the com-
mon malfunctions for peer reference. 

Fault statistics

According to the statistics, from January 2019 to Decem-
ber 2019, our hospital’s dental unit received maintenance 
780 times. Specific failure conditions are shown in Table 
1 below. As can be seen from the table, the failure rate of 
dental units is relatively high in the waterways, electrical 
circuits, air circuits, and human factors. Particular failures 
include water pipe rupture, water valve failure, saliva 
ejector failure, and dental syringe accessories absence.2

METHODS

Cause Analysis and Solution of Four Common 
Failures
Replace worn parts and plug potential water 
leaks

The waterway is an essential part of the dental unit 
and provides the water supply during oral diagnosis and 
treatment. A water leakage problem is one of the most 
common malfunctions of the dental unit.

At the initial installation stage, the manufacturer 
equipped each unit with a water heater to heat the internal 
waterways. The structure of the heater was simple, the 
appearance of the heater was made of stainless steel, it 
was supplied by 24V power, and the material of the inlet 
and outlet water pipes was brittle. Therefore, it was com-
mon to see cracks that can lead to water leakage after 2-3 
years of use. This phenomenon had a high failure rate. 
Engineers assessed the situation and determined that 
the water leakage problem was caused by unapproved 
material supplied by the manufacturer. So the engineer 
comprehensively checked all the pipes inside all the den-
tal chairs, recorded the pipe diameter and length details, 
purchased approved water pipes, and replaced the water 
pipes at potential leakage risk one by one to prevent the 
occurrence. After monitoring over time, all dental units 
did not have a recurrence of the problem.

FIGURE 1. The schematic diagram of dental units.



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Jin, Liu, Li, Chu : Analysis and Solution of Dental Unit Failure

TABLE 1. Failure Statistical Table of Dental Units in 2019 

Fault 
Classification Fault Point Fault Phenomenon Fault Cause Number of Cases Proportion %

Waterway

Delivery system Handpiece drips water or no 
water

Handpiece failure 18 2.31
Water valve failure 38 4.87

Cuspidor and 
support center

Heater leakage water Water pipe rupture 19 2.43

Spittoon or gargle cup 
leakage water or no water

Solenoid valve failure 19 2.43
Circuit board failure 2 0.26

Communication failure 21 2.69
Cuspidor water contains air Diaphragm rupture 22 2.82

Circuit

Dental chair
Chair base can’t move

Circuit board failure 2 0.25
Communication failure 13 1.67

Chair back can’t move Sensor failure 1 0.13

Delivery system

Panel error Panel failure 22 2.82

Handpieces failure

Pipeline rupture 32 4.10
Shelf valve loose 46 5.90

Handpiece failure 15 1.92
Circuit board failure 5 0.64

Cuspidor and 
support center No water

Solenoid valve failure 6 0.77
Circuit board failure 4 0.51

Communication failure 15 1.92

Dental light Cannot work
Bulb failure 12 1.54

Switch failure 3 0.38

Gas Circuit

Delivery system Handpiece failure

Handpiece lack of 
driving gas 5 0.64

Improper regulation 21 2.69
Handpiece failure 12 1.54

Water pipe rupture 31 3.97

Cuspidor and 
support center

Small negative pressure Saliva ejector failure 68 8.72

No negative pressure
Relay failure 5 0.64

Positioning valve 
malfunction 11 1.41

Human Causes Delivery system

Plate tilt Overuse 18 2.31
Handpiece failure Incorrect setup 5 0.64

Handpiece leakage water Incorrect installation 7 0.90

Handpiece no
Water

Panel incorrect setup 5 0.64
Water master switch off 12 1.54

Dental syringe leakage water Accessories absence 74 9.49



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Jin, Liu, Li, Chu : Analysis and Solution of Dental Unit Failure

Improve the existing structure to ensure adequate 
disinfection and instrument integrity

The study showed that the water supply of the dental 
unit was seriously polluted due to multiple factors such as 
the suction effect of the treatment instruments and water 
stagnation, and regular disinfection of the water, which is 
vital in controlling nosocomial infections.3 Our hospital 
disinfects the water pipes of dental units every quarter. 
The Hospital Infection Management Department uses a 
500 mg/L sodium hypochlorite solution to disinfect the 
lines. The medical staff in the department of stomatology 
discharge water on all the effluent parts of dental units 
one by one, and the continuous discharge time shall not 
be less than 10 minutes so that the disinfectant can flow 
out of each terminal effluent point, ensuring effective 
disinfection. However, the high concentration of this 
chlorine-containing disinfectant can corrode the internal 
structure of the dental unit, mainly the valve, rubber band, 
and diaphragm. Specifically, high-speed handpieces, low-
speed handpieces, motors, and tooth cleaning machines 
hung on the valve after use will automatically leak water 
in varying degrees from spittoons and cup water spills.4 

Looking back to 2019, this kind of failure frequently oc-
curred about 3-5 days after each pipe disinfection, and 
several dental units leaked varying degrees. 

Given this phenomenon, engineers searched for rel-
evant information, consulted manufacturers, analyzed, 
and discussed the main reason for such failure. Specifi-
cally, the disinfectant had a particularly corrosive effect 
on the copper and rubber parts inside the dental unit. 
Specific damaged parts included water valve, solenoid 
valve assembly, diaphragm, etc. Considering the balance 
between disinfection effectiveness and the damage rate 

Fault 
Classification Fault Point Fault Phenomenon Fault Cause Number of Cases Proportion %

Human Causes

Dental chair Armrest failure External force 3 0.38
Cuspidor and 
support center Unreasonable Flush time Incorrect setup 81 10.38

Footswitch Handpieces work 
automatically when lift Footswitch failure 12 1.54

Doctor’s chair or 
assistant chair

The assistant chair sprang up Hydraulic failure 59 7.56
Back failure Lack of parts 36 4.61

of the dental unit component, we proposed a preventive 
strategy. After each disinfection, the nurse extended the 
discharge water time to 20 minutes on the day. Before 
starting the machine, the nurse discharged water for 10 
minutes every morning for the next 5 days to remove the 
residual disinfectant in the pipeline and reduce the corro-
sion of the disinfectant on the dental unit parts. Because 
the discharge water at all the outlet points of the existing 
dental unit cannot be controlled with one key, and the 
discharge and disinfection time cannot be controlled, the 
medical staff need to discharge water manually, which 
undoubtedly increases the workload of medical staff.

For this reason, based on existing dental units, our 
engineers have added an automatic discharge water 
control device.5 This design has been authorized national 
utility model patent. The specific structure is shown in 
Figure 2 below.

FIGURE 2. Structure drawing of automatic draw water control 
device.



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Jin, Liu, Li, Chu : Analysis and Solution of Dental Unit Failure

The specific working process is as follows: firstly, select 
the knob above of the time relay to 10min, and then press 
the switch, the time relay starts the timing, the exhaust 
solenoid valve opens, the backup air of dental unit will 
open four water valves in the water and air control module, 
four handpieces will drain away water at the same time, 
the solenoid valve of cup water and spittoon water will 
open, and cup water and spittoon water will also drain 
away water at the same time. After the timing is over, Pin 
1 and Pin 4 of the time relay will disconnect, Pin 1 and 
Pin 3 will pull, Pin 5 and Pin 8 will disconnect, and Pin 6 
and Pin 8 will pull. The exhaust solenoid valve, cup water 
solenoid valve, and spittoon water solenoid valve will stop 
working, The water valves in the water and air control 
module are closed, and the handpieces will not drain 
away water. Cup water solenoid valve and spittoon water 
solenoid valve are closed, cup water and spittoon water 
will stop drain away water. The clinical use of the device 
can not only realize the one-button control discharge of 
water and effective disinfection at all outlet points of the 
dental unit, but control the time of water discharge and 
disinfection accurately. However, this will also reduce the 
workload of medical staff and improve the compliance of 
medical staff in daily waterway disinfection, which is of 
great significance to clinical diagnosis and treatment in 
the department of stomatology.

Do a good job of regular maintenance to reduce 
the occurrence of suction malfunction Attract 
tube

Oral suction has a high utilization rate in daily oral 
diagnosis and treatment, and the subsequent failure 
rate is relatively high. The suction tube absorbs a large 
amount of dental debris and blood in the patient’s oral 
cavity every day, and the oral pollutants are discharged 
underground through a long and thin tube, which is prone 
to pipe obstruction or suction failure. The negative pres-
sure pump in the center of the hospital generates suction, 
and the positioning valve on the dental unit controls the 
start and stop of the negative pressure. Then the doctor 
can attract the patient’s mouth through the suction tube to 
remove the dirt in the mouth. The main fault phenomena 
in the use process are suction pipe obstruction result-
ing in reduced suction, pipe aging rupture resulting in 
insufficient suction, positioning valve failure resulting in 

no negative pressure. If the suction malfunctions, it will 
negatively impact the doctor. For this kind of problem, 
engineers analyzed: (1) the high frequency of use, and 
(2) the lack of effective maintenance of the dental unit. 
Notably, medical staff only knew the use but did not 
know the regular maintenance for the suction tube.6 For 
this reason, the engineer actively communicated with 
the users of the equipment and formulated a routine 
maintenance items list for the dental chair according to 
the infection control guidelines recommended by the 
Centers for Disease Control and Prevention of the United 
States and the Australian Dental Association,7 as well as 
the manufacturer’s maintenance manual. The guidelines 
were implemented in April 2020, and relevant records 
were made. The specific contents are shown in Table 2.

Strengthen medical education to prevent the loss 
of dental syringe accessories

Each dental unit in our hospital is equipped with a 
three-use spray gun for the doctor and a three-use spray 
gun for the assistant. Engineers often receive repair calls 
during daily use, such as leaking or unusable dental sy-
ringes. After careful observation of the use and malfunc-
tion of the three-use gun in the oral clinic, it was found 
that most malfunctions were caused by the absence of 
accessories. Engineers analyzed such problems mainly 

TABLE 2. Routine Maintenance Items List of Dental Units in 
Dental Clinics

Project Content Maintenance 
Frequency

Suction tube

After use, draw clean water 
and rinse for 1 min Once per person

After treatment, detergent 
was attracted for 3 min Once per day

Clean suction tube solid 
strainer Two times per week

Handpieces

Flush the pipeline for 2-3 
min Daily before use

Flush waterway for 20-30 sec Between each 
patient

Spittoon and 
mouthwash 
cup stand

Rinse and wipe Between each 
patient



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Jin, Liu, Li, Chu : Analysis and Solution of Dental Unit Failure

due to improper operation of medical staff and insufficient 
understanding of the spray gun structural components. 
Prevention measures could be taken from two aspects: 
First, strengthen the education and training of medical 
staff. The engineer communicated with the director of 
the department using the equipment and organized 
training on daily use and other matters of attention with 
the dental unit. This ensures that the medical staff can 
understand the structure of the dental unit, be familiar 
with the structure of the three-use gun, master the daily 
disassembly and assembly, and put the three-use gun 
into the daily inventory list. The nurse at each position 
was responsible for checking the related accessories of 
each tooth chair and reported if anything was missing.8 
Second, engineers should strengthen regular inspection 
and prepare relevant accessories as needed.

RESULTS

Since the beginning of 2020, engineers have purchased 
and replaced all the internal pipes of our dental units to 
prevent water leakage from the heater. Figure 3 (a) shows 
the heater failure trend, and the number of failures is re-
duced to zero. After the dental unit pipes are disinfected, 
the failure rate of water valves are significantly reduced, 
and water valves replacement costs are saved through the 
installation of automatic discharge water control device 
and the implementation of relevant measures. Figure 3 
(b) is the failure trend diagram of the water valve. Since 
the routine maintenance items list of the dental unit was 
implemented in April 2020, the saliva ejector’s failure 
rate has decreased significantly. Figure 3 (c) shows the 
trend chart of the failure rate of the saliva ejector. In ad-
dition, since the management of the dental syringe was 

3(a). Heater fault trend chart 3(b). Water valve failure trend chart

3(c). Saliva ejector fault trend chart 3(d). Dental syringe fault trend chart

FIGURE 3. Chart showing the different kinds of fault trends before and after improvement measures.



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Jin, Liu, Li, Chu : Analysis and Solution of Dental Unit Failure

strengthened in May 2020, the failure rate of the dental 
syringe decreased significantly. Figure 3 (d) shows the 
failure trend of the dental syringe.

Although the daily maintenance will increase a certain 
amount of work and maintenance time, it can effectively 
reduce the downtime of dental unit failure, reduce the 
number of repairs, reduce the repair expenditure, pro-
long the service life of the equipment, and improve the 
quality of clinical diagnosis and treatment. Comparison 
of items before and after maintenance are shown in Table 
3. Among them, the average downtime was based on the 
time of failure to treat patients caused by each repair, 
and the average maintenance time was based on the time 
spent to complete the maintenance project. The average 
number of repairs was based on the number of repairs 
per dental unit in a year. The repair cost mainly includes 
replacing the water valve, saliva ejector, and position valve. 
The average repair expenditure was based on the repair 
expenditure per dental unit in one year.

DISCUSSION

In this paper, by sorting out the common failure cases 
of the dental unit and selecting several typical problems 
with a high failure rate, the causes of the failures are 
deeply analyzed, and the corrective measures are put 
forward.9 The reasons for each failure type are analyzed 
from different perspectives in these cases, such as failing 
to replace wearing parts in time, inadequate equipment 
maintenance, incorrect operation, etc. There is also the 
question of balancing the requirements of hospital dis-
infection with the damage of dental unit components. 
Finally, engineers put forward the improvement schemes 
from their own point of view. The practice has proved that 
our dental units are running well, and the failure rate of 
several typical malfunctions is obviously reduced. 

CONCLUSIONS

Maintenance engineers not only need to deal with 
daily failures but also need to deeply analyze the causes 
of failures and how to prevent similar failures. Medical 
engineers and technicians should use their professional 
knowledge to make appropriate innovations and feasible 
improvements to the existing equipment to solve the cur-
rent problems.10 In the context of the current advanced 
management of medical equipment, engineers should 
improve their maintenance concepts, transform their 
experience into practical maintenance practices, and use 
information technology and quality management methods 
to improve medical equipment maintenance.11

ACKNOWLEDGMENT

We want to extend our sincere gratitude to Dr. Yadin 
David for his revising this paper. We are also deeply 
indebted to our other clinical engineering staff that sup-
ported our work.

CONFLICT OF INTEREST

The authors declare that they have no conflict of interest.

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TABLE 3. Comparison of Items Before and After Maintenance

Items Before 
Maintenance

After 
Maintenance

Average downtime/ min 20±5 5±2
Average maintenance time/ min 3±1 30±3
Average number of repairs/time 26±1 5±1
Average repair expenditure/ ¥ 353±3 13±3



57 J Global Clinical Engineering Vol.4 Issue 3: 2022

Jin, Liu, Li, Chu : Analysis and Solution of Dental Unit Failure

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