Robot application in orthopedic surgery: drilling control

George Boiadjiev 1, Kazimir Zagurski

2, Tony Boiadjiev

3, Kamen Delchev

1, Rumen Kastelov

4,

Vladimir Kotev 5

1 Institute of Mechanics – Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl. 4, 1113 Sofia, Bulgaria

2 Institute of system engineering and robotics - Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl. 2, 1113 Sofia

3 Institute of information and communication technologies – Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl. 2,

1113 Sofia 4 Emergency Medical Institute “Pirogov”, Orthopaedic ward, 21 Macedonia Blvd., 1606 Sofia

5 Gifu University, Faculty of Engineering,Department of Human and Information Systems, 1-1 Yanagido, Gifu 501-1193,

JAPAN

Correspondence to: George Boiadjiev, e-mail: george@imbm.bas.bg

Abstract Many orthopedic operations involve drilling and tapping

before the insertion of screws into bone. Usually the drilling is

executed by hand. It leads to lots of problems. The accuracy of

the drilling, braking the blood vessels after the rear hole,

overheating, oscillations widening the hole diameter are some

of examples for that. To avoid these problems and to reduce

the subjective factor the automation of drilling is

recommended. In the work the automatic drilling robot is

presented as well as experiments for establishment the drilling

process technical parameters and its control. The time, linear

velocity, angular velocity, resistant force, depth of penetration

and temperature are monitored during the drilling process.

The robot is user-friendly, i.e. the surgeon can operate easy.

The required working task is set simply and the information

during the process execution is received in real time. The

surgeon has no influence to the process. He must only to take

care of keeping a contact with a bone while the robot

autonomously executes the drilling and takes a decision to

stop. Conclusions are made that the automatic bone drilling

assures higher accuracy and patient safety.

Index Terms – control, experiments, orthopedic surgery,

robot

I. INTRODUCTION

Nowadays robot application in orthopedic surgery arise.

The International Society for Computer Assisted

Orthopedic Surgery was founded and Symposiums on

Automated and Computer Aided Orthopedic Surgery is

organized almost every year. That proves the actuality of

this topic and guarantees future development.

Robotic and navigation systems are one of the most

advanced areas of applications and their range is increasing

[1,2]. In order to overcome the inaccuracy of hand-

controlled positioning of the surgical tools (“...the average

projection of the drill bit beyond the far cortex was 6.31

mm” [3]) many robot system have been developed [4-8].

Electrical instruments are often applied in the orthopedic

surgery – for instance cutting and drilling machines, saws

etc. which the surgeon holds in his hands during

manipulation. The drilling devices are used approximately

in 95% of post-trauma interventions. Since orthopedic

screws are often implanted in bones it is needed to realize

the bone drilling process before. But hand-drilling leads to

some problems as getting the big outlets, breaking the

tendons or blood vessels, overheating etc. [9]. Also an

excessive growth in temperature around a drilling place

causes thermal necrosis of bone which is associated with

irreversible changes in its structure and physical properties.

The hold of the screw is decreased; reducing the stability

and strength of the fixation and the presence of necrotic

tissue delays healing and predispose to infection [10-12].

The automatic drilling could decrease the subjective factor

and avoid to some extent the existing problems.

Generally the drilling process is very complex thing – many

parameters have to be taken into account. Mostly some of

them are unknown. That makes modeling very hard and

usually experimental approaches are usually developed.

They concern parameter identification – bone structure,

forces and torques, temperature, accuracy and so on [13-

18]. As a result of our lots of experiments the following

conclusions are formulated [16]. The robot for automatic

bone drilling must be designed with components capable to

assure:

torque not less than 1.5 Nm

drilling force not less than 100N

force sensor range up to 100 N

temperature not over 52 deg, o C

accuracy: less than 0.5 mm difference of

preliminary set depth or the length of whole bone

At the same time the dimensions and masses of the robot

components must be as small as possible.

The purpose of the work is to present a robot for automatic

bone drilling in orthopedic surgery underlying on its

technical characteristics and achievements in the area of

engineering and control. Drilling algorithms are described

and experimental results are discussed.

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125 © 2012 GSTF

II. DRILLING ROBOT “ODRO”

The robot ODRO (Orthopedic Drilling RObot) is intended

to improve the patient’s safety by removing the subjective

factor and avoid the problems in hand bone drilling,

increasing manipulations accuracy and safety.

On fig.1 the kinematic structure of ODRO is shown. It has

one translation and one revolute joint with co-linear axes,

where q1 and q2 are generalized coordinates. The joints are

driven by two motors for each kind of motion.

Fig.1. Kinematic scheme.

The constructive scheme is presented on fig.2.

Fig.2. Constructive scheme of ODRO.

Notations: 1-liner drive; 2-force sensor; 3-BLDC motor.

Further, some details of its design will be presented – the

power and electronics blocks, the PC-terminals, the

executive mechanical module etc.

A. Components

Linear actuator 43000–17. It is a stepper motor with

embedded screw for linear motion. It has high precision at

low speeds, small by size and realizes translation of 1 mm

for 4032 micro steps.

Brushless DC motor MAXON with servo controller/driver

1-Q-CE and amplifier DEC 50-5. Such motors have many

advantages. They have better speed versus torque

characteristics, high dynamic response, high efficiency,

long operating life, noiseless operation, higher speed

ranges, rugged construction and so on. Also, torque

delivered with respect to the motor size, is higher, making it

useful in applications where the space and the weight are

critical factors. The module can give torques 1.5 – 1.7 Nm.

Controllers comprise two main components – controlling

device and power drive. They are:

-controller / Driver TMCM-110-42 for control of one-

axis stepper motor linear actuator 43000-17.

-servo Controller / Driver for BLDC MAXON. It has

built-in PID-regulators for positioning and for speed.

-force sensor MLP-25 for the bone resistant force.

-temperature sensor SP i-tec 2005D. It is an infrared non-

contact temperature measuring instrument.

B. Design

The executive module of the robot ODRO is shown on fig.

3 and its control/power block – on fig. 4.

The robot performs a good functionality. Both actuators are

mounted inside the drilling machine. All parts of the

mechanical module are made using stainless steel material

for assuring the sterility requirements. The machine allows

gas chemical sterilization before every manipulation.

Fig.3. The executive module of the robot ODRO.

Technical data:

- weight – 2.3 kg

- working zone: 0 – 105 mm

- precision – 0.5 mm

- working mode: hand and automatic

- rotation speed: 0-900 rpm

- translation speed: 0 – 9 mm/s

The following indications can be seen on the control/power

block (control unit):

- start button

- drilling mode buttons

- digital display for the depth of the drilling

- emergency indication

Control systems give information for the drilling execution,

for successful end of the task and for emergency situation.

Fig.4. Control / power block of the robot ODRO.

Terminals for connection with PC are also built-in. They

have a possibility to re-program the software, which is

recorded in the controllers; to change and update the

programs and to transfer the information between the

sensors and PC while the drilling is executed in real time.

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126 © 2012 GSTF

III. CONTROL ALGORITHM

On fig.5 the control algorithm block-scheme is shown.

The BLDC motor is controlled by MAXON controller and

the stepper motor – by TRINAMIC controller where the

force data sensor are processing in.

fig.5 Control algorithm block-scheme

The control algorithms are executed in the specific program

language TMCL-IDE. The commands can be executed

immediately after their input (direct regime) or the program

can be downloaded in the controller for autonomous

execution (stand-alone regime).

For every cycle the program recognize the current state,

executes the corresponding algorithm and takes a decision

depending on determined criteria for going to the next state.

An algorithm for current calculation of the “next target

position” for the stepper motor is realized depending on the

force sensor data. The actual force sensor data are

considered in comparison with the previous given values.

The last reflects to the safety requirements in the sense that

the exceeding of a given limit value is not allowed.

An algorithm for identification of the resistant force value

during the drilling process is executed. This value is in

relation with the objects manipulation characteristics and

the drill bit diameter. The obtained value is used for taking

the decision for completion of the bone cortex drilling.

The control algorithms work in two regimes: drilling to

reach a preliminary desired depth and through the whole

bone. In the last case the control algorithm allows breaking

the drilling process automatically when the cutting screw

reaches the marrow. Then the surgeon can make a decision

to go on or come back to the starting position. In details the

control algorithm can be described as following.

The machine first executes a “Self Test” program to report

the readiness for work. The surgeon sets the desired

working mode by pressing the corresponding button.

Looking the display information for desired task acceptance

the surgeon presses the button “Confirm”. The drilling

process starts executing the following steps:

pressing the start button

after the Ref. Pos. is found the searching for a

contact with the bone begins (speed - 6mm/s)

at the moment of contact registration a drilling

starts up to 1mm (speed 0.5 mm/s)

drilling the next 1 mm and identification of the

resistant force FR for the actual specific case; the

obtained value is used to set the limit of the

resistant force which must not be overcome further

drilling the first cortex (speed ~ 2 mm/s) up to its

end and automatically stop

when the start button is released and then pressed

again the translation starts through the marrow for

searching the second contact (speed 6 mm/s)

registration the second contact – the control is the

same as the first contact

drilling the second cortex (speed ~ 2 mm/s) up to

its end and automatically stop the translation and

rotation motion

moving back without rotation to the Ref. Pos and

checking for missed steps (speed 6mm/s)

moving to Start pos. (which is near to Ref. Pos)

and stop the motion

the machine is ready for the next setting of the

drilling mode and new working task execution

Real-time information appears on the display corresponding

to the current manipulation.

The main control program is realized as a set of separate

states – “Searching for Contact State”, “Drilling State”,

“Checking for Missing Steps State”, “Contact not Found

State”, “Ready State” etc. They are relevant to

corresponding sub-operations that have to be taken during

the whole drilling process. While in “Stand-alone mode”,

the program recognizes the current state, executes the

corresponding algorithm and takes a decision, depending on

determined criteria, for going to the next appropriate state.

IV. RESULTS

As it was said, before drilling start the robot is performing

“self-test” to specify its state and confirm a readiness for

work. The checking includes execution of preliminary set

rotation and translation speeds, working zone; force sensor

and start button functioning, and reference position. At the

beginning the resistant force is identified specifically for

the concrete object – its age, sex and medical status. Such a

“tuning” is important for safe execution of drilling.

On fig. 5 the information in real time of drilling execution

about the size of bone walls and marrow is shown on LCD

display. It gives the surgeon a possibility to preset a

“maximal-border limitation” which improve the safety.

Fig.5. Bone structure identification result after the whole

bone drilling: first cortex (20 mm), marrow (17 mm) and

the last cortex (18 mm). The sum (bone width) is 55 mm.

GSTF International Journal of Engineering Technology (JET) Vol.1 No.1, 2012

127 © 2012 GSTF

By means of the designed setup various experiments have

been executed. The drilling mode includes the following

monitored parameters:

Time [s]

Linear velocity [mm/s]

Angular velocity [rpm]

Resistant force [N]

Depth of penetration [mm]

Temperature [deg., o C]

The graphics for resistant force and “stop-decision”

function are shown on fig.6. On horizontal scale (x) is the

time – one discrete sample corresponds to 100 ms, i.e. the

whole process is 13.7 sec. On vertical scale (y) is the force

[N]. The first contact is realized at 20 samples and the

second – at 60 on x-axes. The stop-decision takes place

when its value repeats being less than zero during 5 sample

times. On fig.6 it occurs at 55 samples for the first cortex -

the value is less than zero but it is not repeated 5 times and

the drilling goes on. For the second cortex at 122 samples

the value is less than zero more than 5 times and the drilling

stops.

Fig.6. Graphics of resistant force during full bone drilling

(2mm/ s; drill bit Ø4 mm) and “stop-decision” function.

During the drilling process the resistant force FAct is tracked

all the time. A component ε=FAct-FR and an integral

component Ierr=4ε are formed, i.e. a “window” is obtained

with dimension 4 samples, which gives an information for

resistant force deviation. The stop decision is realized as a

function Fstop=f(F0, ε, Ierr), where F0 is the machine

resistance during free motion depending on its orientation

to the bone (including gravity effect). In the same time the

speed is reduced twice aiming to assure minimal

penetration and not to allow big outlet and breaking some

pieces of the bone.

The last figure illustrates the resistant force when the

cutting screw reaches the marrow. During “decision time” it

is zero and appears again while the cutting screw goes to

drill off the rear part of the bone.

V. CONCLUSIONS AND DISCUSSIONS

The automatic bone drilling can entirely solve the problems

arising during usually executed hand drilling. The proposed

experimental setup is designed to identify some parameters

of bone drilling process as resistant force due to variable

bone density, appropriate mechanical torque during the

manipulation, linear speed as well as motors and drives

electromechanical parameters.

Generally the accuracy problem at the automatic bone

drilling can be formulated as: to determine the right

moment of time to stop the drilling assuring not to widen

the rear whole and to realize minimal penetration in soft

tissues behind the bone wall. Also the automatic drilling

must guarantee the patient safety, i.e. to exclude the bone

damage for the sake of overheating and the big drilling

force stress.

The orthopedic robots and integrated computer systems as

ROBODOC, CASPAR etc. are very complex which reflects

to a high cost. Operating with so high-technology apparatus

needs a specialized training and education not so for the

surgeons but for whole teams. The maintenance also takes

additional resources. In this sense it is good not to neglect

the simple systems specialized just for one specific

manipulation. Such an understanding becomes more

popular recently. In comparison with the other systems

ODRO has differences as low cost, user friendly (no need

of special engineering knowledge for working), small size

and simple construction. In the same time it assures the

needed accuracy, precision, reliability and safety of the

manipulation. Moreover, the presented robot is user-

friendly, i.e. the surgeon can operate very easy and the

required task is set very simply and the information during

the process execution is received in real time. The most

important thing is that the surgeon can not influent the

process; he has only to take care of keeping the contact with

the bone. The drilling accuracy of the depth of the hole is

0.2 mm.

Finally, the automatic drilling eliminates whole surgical

manipulation – manually measurement of the penetration in

the bone. The last is one more proof of the automatic

drilling advantages.

The bone resistant force was experimentally obtained to

vary round 30-40 N and its maximal value does not exceed

50 N. The presented experiments estimate the value of 50 N

as an average; some small peaks appear but not over 100 N.

The resistant force function, which can be obtained in

experimental way only, is much helpful for the robot

control. The last means maintaining speeds (linear and

angular) assuring execution of the preliminary determined

control law.

Summarizing, drilling robot application in surgery is an

interesting practical task itself which is still under

development. Its successful solution will have very positive

effect on bone drilling manipulations and screw

implantations, their precision executions, reliability and

safety.

ACKNOWLEDGEMENT

This work was supported under FY2012 Researcher

Exchange Program between the Japan Society for the

Promotion of Science and the Institute of Mechanics –

Bulgarian Academy of Sciences.

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George Boiadjirv is an associate

professor with the Department of

Mechanics of Multibody Systems at

the Institute of Mechanics,

Bulgarian Academy of Sciences. He

received the M.Sc. degree in

mathematics from the Faculty of

Mathematics and Mechanics at Sofia

University (1984), and the Ph.D.

(1993) in electromechanical systems

modelling from the Institute of Mechanics, Bulgarian

Academy of Sciences. He is currently working in the areas

of dynamics and control of multibody systems and robot-

manipulators.

Kazimir Zagurski is a Senior

Research Assistant at the Institute of

System Engineering and Robotics –

Bulgarian Academy of Sciences. He

received the M.Sc. degree in

Electrical Engineering from the

GSTF International Journal of Engineering Technology (JET) Vol.1 No.1, 2012

129 © 2012 GSTF

Technical University of Sofia. His main activities and

responsibilities are:

R&D in the field of medical robotics.

Participation in a Group for Development of

Automatic bone drilling robot for orthopaedic

surgery.

Participation in number of collaboration activities

for CERN (European Organization For Nuclear

Research) and ETHZ, Institute for Particle Physics

(Swiss Federal Institute of Technology - Zurich)

Microcontroller control systems – hardware and

software design.

Tony Boiadjiev is an associate

professor with the Institute of

information and communication

technologies – Bulgarian Academy

of Sciences. He received the M.Sc.

degree in electronics and

automation from the Technical

University of Sofia (1988), and the

Ph.D. (1997) from the Central

Laboratory of Mechatronics and Instrumentation –

Bulgarian Academy of Sciences. He is currently

working in the areas of modelling, simulation and control

of robotic manipulators.

Kamen Delchev is an associate

professor with the Department of

Mechanics of Multibody Systems at

the Institute of Mechanics,

Bulgarian Academy of Sciences. He

received the M.Sc. degree in

mechanics from the Faculty of

Mathematics and Mechanics at

Sofia University (1980), and the

Ph.D. (1994) in robot control from the Keldysh Institute of

Applied Mathematics at the Russian Academy of Sciences.

He is currently working in the areas of dynamics and

identification of multibody systems, point-to-point motion

tasks and Iterative learning control of robotic manipulators.

Rumen Castelov is an associate

professor on Orthopaedics and

Traumatology, (2003), and Head of

the Department of Orthopaedic

Trauma Surgery, University

Emergency Hospital “Pirogov”,

Sofia. He received his M.Sc. degree

in Medicine, (1986), and the Ph.D.

degree, (1998), from the Medical

University, Sofia, Bulgaria. His major fields of scientific

research are: Reduction of intraarticular knee fractures,

arthroscopy, artificial joints.

Vladimir Kotev is an assistant

professor at Institute of Mechanics

at the Bulgarian Academy of

Science. He is a JSPS Postdoctoral

fellow at Kawasaki and Mouri

Laboratory, Faculty of Engineering,

Gifu University. He has been

working on design and dynamics of

mechanisms for mechatronics

systems and robots. He received the

PhD in Biomechanics at Institute of Mechanics (2008), and

M.Sc. degree in Mechanical engineering from Technical

University Sofia (2002).

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