In-vitro Force Assessments of an Autoclavable Instrumented Sternal

Retractor

Giovanni Saggio

, Giuseppe Tancredi

, Laura Sbernini

, Costantino Del Gaudio

Alessandra Bianco

and Jacob Zeitani

Dept. of Electronic Engineering, University of Rome “Tor Vergata”, Rome, Italy

Dept. of Enterprise Engineering, INSTM Research Unit, University of Rome “Tor Vergata”, Rome, Italy

German Hospital Tirana, Tirana, Albania

Keywords: Chronic Chest Pain, Instrumented Retractor, Median Sternotomy, Force Sensor.

Abstract: It is well known that median sternotomy might lead to rib and/or sternum micro/macro-fractures and/or

brachial plexus injuries, eventually resulting in chronic pain with significant impact on patient’s quality

life.Postoperative chronic pain is recognized as a multifactorial complex issue, it has been assessed that

excessive sternum retraction forces can be considered one of these factors. On this basis, the Authors

developed a reliable and sterilizable system potentially able to real-time monitor and control the retraction

forces along the hemisternums. A Finochietto sternal retractor was instrumented by means of ultra-thin

force sensors interfaced with ad hoc electronic circuitry. Two different sets of sensors were adopted, one of

which able to support autoclave operating conditions. In vitro tests were performed by means of a made on

purpose dummy. The instrumented retractor allows monitoring of the force exerted on both the arms during

the opening procedure. Force versus time patterns were real-time acquired and stored, distribution of forces

was determined along with the values of mean, maximum and plateau force. Results demonstrate the

reliability of the instrumented retractor in measuring forces, up to 400N. Cost-effectiveness and feasibility

can be considered further additional values of the proposed instrumented retractor.

1 INTRODUCTION

Persistent postoperative pain following sternotomy

is the Achilles’ heel of surgical procedures because

it can lead to patients’ discomfort, increased

morbidity, prolonged hospital stay, and increasing

costs (Wildgaard, 2001 and Hazelrigg, 2002).

Chronic pain has been defined as pain in the location

of surgery, different from that suffered pre-

operatively, arising post-operatively and persisting

beyond three months. Recently, in a prospective

study, Van Gulik et al. (Van Gulik, 2011) identified

a number of independent predictors for the

development of persistent thoracic pain following

sternotomy including non-elective surgery, re-

sternotomy shortly after the original surgery and

severe pain on the third postoperative day. In this

study, at one year, 42 (35%) patients reported

chronic thoracic pain. Similarly, another study

reported the prevalence of post-operative pain as

high as 39.3% at the mean time of 28 months after

surgery (Bruce, 2003). Meyerson et al. in 2001

(Meyerson, 2001) estimated a 28% overall incidence

of non-cardiac pain one year after surgery. Several

studies assessed that women are substantially more

likely to suffer early and chronic postoperative pain

than men (Van Gulik, 2011 and Ochroch, 2006) and

that the prevalence of post-sternotomy chronic pain

decreases with age (Van Gulik, 2011 and Meyerson,

2001). Chronic post-sternotomy pain can be related

to secondary sternal osteomyelitis, incomplete bone

healing, sternocostal chondritis, and surgical

technique of internal mammary(Bolotin, 2007 and

Aigner, 2013).

Indeed, Aigner et al. 2013 (Aigner, 2013)

pointed artery harvesting (required for myocardial

revascularization) and, particularly, mechanical

trauma associated to improperly applied sternal

retractors that might lead to rib and sternum

fractures (Van Gulik, 2011; Woodring, 1985; Unlu,

2007).Of course the relevance of this issue is

expected to be different for each individual patient

in terms of a number of variables such as weight,

age, osteoporosis and cartilage calcification.

Saggio G., Tancredi G., Sbernini L., Del Gaudio C., Bianco A. and Zeitani J.

In-vitro Force Assessments of an Autoclavable Instrumented Sternal Retractor.

DOI: 10.5220/0006111300250031

In Proceedings of the 10th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2017), pages 25-31

ISBN: 978-989-758-216-5

To access to the mediastinum, retractors are used

to allow adequate surgical field (Steele, 2013).

Hemisternums separation might lead to rib fracture,

eventually associated to brachial plexus injury (BPI)

(Baisden, 1984; Greenwald, 1983; Gumbs, 1991;

Healey, 2013; Suzuki, 1991).

Median sternotomy provides a wide access to the

thoracic cavity. It is considered the standard

approach for open heart surgical procedures, but it is

also a useful incision for a number of other

operations. It is well known that a certain risk of

chronic pain is associated to the extent of the force

impressed during sternum opening due to rib

fractured and/or BPI. Thus, there is an actual clinical

need to provide to the surgeons suitable

instrumented retractors able to monitor in real time

the forces exerted on the two halves during the

sternum opening procedure. Furthermore, with the

increasing interest of shifting the cardiac surgery

procedures from full to partial sternotomy, including

the “J” and “T” incisions, the proposed study might

be useful to evaluate and compare the forces applied

on the sternum in the various surgical approaches to

determine the best access to that supposed to be the

optimal sternum separation allowing at the same

time the optimal surgical view.

Only few data are available for the actual value

of the forces exerted by a retractor on the skeletal

cage and all reported studies have been conducted

on corpses or animal models. Data obtained from

human patients are not presently available in the

literature probably due to the lack of an

instrumented sternal retractor readily suitable for the

translation to surgery.

For this purpose, we designed and realized a

sterilizable system based on a commonly adopted

straight sternal retractor (Finochietto) equipped with

ultrathin force sensors and conditioning electronic

circuitry. The forces experienced during the

retraction were monitored in real-time by means of a

home-made dummy.

The idea is to acquire data on the intensity and

distribution of exerted retraction forces during

hemisternums separation, in view of future

challenging clinical studies aimed at reducing the

risk of chronic post-sternotomy pain.

2 MATERIALS

A commonly adopted straight sternal

retractor,Finochietto type (Figure 1a) was equipped

with ultra-thin force sensors and conditioning

electronic circuitry. This instrument was tested by

means of a home-made dummy.

Figure 1: (a) The Finochietto retractor equipped with the

four sensors placed in positions designed from 1 to 4

according to the figure. (b) The ultra-thin force sensors,

HT201 (top) and A201 (bottom) types. (c) Aluminum

sensors’ housings: front, back and cover.

2.1 Force Sensors

We considered two different types of commercial

piezo-resistive flexible ultra-thin (0.203mm,

0.008in.) off-the-shelf force sensors, the

FlexiForce® A201 (these according to Aigner et al.,

2013) and the FlexiForce® HT201 (both types by

Tekscan, Boston, USA), having a circular sensing

area of 9.53mm (0.375in.) in diameter (Figure 1b).

The A201 type, with a polyester substrate, can

measure forces up to 440N, within a temperature

operating range of -9°C to +60°C (15°F to 140°F).

The HT201 type, with a polyimide substrate, can

measure forces up to 445N, within -9°C to +204°C

(15°F to 400°F).

BIODEVICES 2017 - 10th International Conference on Biomedical Electronics and Devices

2.2 Electronic Circuitry

The electronic circuitry was developed on the basis

of a previous one, which was made to interface flex

and electromyography sensors (Saggio,2016). In

particular, the electrical resistance values (outputs of

the sensors) were converted into voltages by means

of voltage dividers. Those voltage signals fed an

electronic circuitry, based on Luigino328 (an

Arduino-compatible microcontroller board based on

an ATMega328 MCU), which operated 10bit digital

conversions and sent data to a personal computer via

USB port at a sampling rate of 175Hz. The

following data process was handled by ad-hoc

home-made LabVIEW routines (National

Instruments, Austin, TX, USA).

2.3 Sternal Retractor

An aluminum straight Finochietto retractor (by

Tekno-Medical Optik-Chirurgie GmbH Tuttlingen,

Germany) was equipped with an array of four force

sensors. Two sensors were placed on the blade of the

mobile arm and two on the blade of the fixed arm

(Figure 1a), the size of the blade being 44.4mm

(1.75in.) in length and 30.9mm (1.22in.) in width.

The sum of the single detected forces on each blade

yielded the total force for both the fixed and the

mobile arm. The ultra-thin force sensors were placed

in ad-hoc smooth aluminum housings (Figure 1c).

2.4 In Vitro Test

In vitro tests of the instrumented retractor were

performed by means of a made on purposedummy

built up with four gas pistons (manufactured by

Team Pro), two for each side, laterally anchored to a

wooden shell (Figure 2a). Different set of gas

pistons were evaluated, i.e.150N, 100N and 80N.On

the basis of several opening/closing cycles

performed by three different surgeons, the dummy

equipped with the 80N pistons offered the most

realistic feeling with respect to the clinical practice.

The Authors are aware that the mechanics of the

proposed dummy is very simple with respect to the

complex biomechanics of the rib cage. Anyway, the

idea was to realize a dummy able to support the test

of the device and not meant to be taken as a

biomechanical model of the rib cage.

Figure 2: (a) The home made dummy built up using four

gas pistons fixed to a wooden skeleton, the compressible

parts positioned outward in a face-to-face configuration. In

vitro tests: (b) the instrumented Finochietto retractor

positioned into the dummy.

3 METHODS

Beforehand, eight sensors of each type were

characterized in terms of electrical resistance versus

applied force (R vs F), by means of an universal

tensile test machine (LRX, by Lloyd Instruments,

Berwyn, PA, US). In order to investigate if HT201

sensors can effectively support autoclaving

conditions, these sensors were also characterized

following the same procedure after five cycles of

autoclave treatment (VaporMatic 770, AsalSrl,

Milan, Italy).

Test procedure consisted in four opening/closing

cycles of the dummy by means of the instrumented

retractor up to two different fixed widths, i.e.5cm

(1.97in.) and 10cm (3.94in.). On the basis of the

feeling/practice of the surgeons, each

opening/closing cycle was performed at a roughly

constant rate of 2s/cm, that is 10s for 5cm (1.97in.)

and 20s for 10cm (3.94in.). The two final positions

(5cm, 1.97in. and 10cm, 3.94in.) were held for 60s

so to evidence response decay, if any. The response

of all the sensors in term of force (F) versus time (t)

was real-time acquired. Then, mean force (F

mean

maximum force (F

max

) and plateau force (F

plateau

)

were evaluated, the latter as the mean value of the

force recorded during 60s in the final rest position.

In-vitro Force Assessments of an Autoclavable Instrumented Sternal Retractor

The distribution of the forces exerted along the two

halves of the dummy was also determined.

4 RESULTS AND DISCUSSION

Ultrathin flexible force sensors HT201 and A201

both showed exponential resistance decay with the

impressed force (Figure 3a-b).

Figure 3: Measured resistance versus force (R vs. F) for

different sensor (a) HT201 type, (b) A201 type and (c)

H201 type in comparison before and after autoclave

treatments.

Moreover, HT201 sensors did not show a

significantly different behaviour after five cycles of

autoclave conditioning (Figure 3c), which is

reasonable result since these sensors have been

specifically designed for high temperature

applications (up to 400°F, approximately 200°C). In

any case, in the occurrence of degradation in

performances, those sensors can be easily and

conveniently replaced.

The investigated range of force (i.e. 5-400N)

includes the values reported by Bolotin et al.

(Bolotin, 2007) and by Aigner et al. (Aigner, 2013).

In more details, Bolotin et al. in 2007 reported the

first known successfully attempt to employ an

instrumented retractor to monitor forces during

cardiothoracic surgery. They equipped stainless steel

curved profile retractor blades with strain gauges to

measure applied forces during retraction, and

reported results for lateral thoracotomy and median

sternotomy on cadavers and sheep. The average

force applied during force-controlled retraction was

(77.88±38.85N) and the maximum force displayed

during force-controlled retraction

(323.99±127.79N).

Aigner et al. equipped a straight (SSR) (MTEZ

424 735; Heintel GmbH, Vienna, Austria) and a

curved retractor (CSR) (Dubost Thoracic Retractor

DC30000-00; Delacroix-Chevalier, Paris, France),

with FlexiForce sensors, A201 type (Tekscan Inc).

The blade of the mobile arm of the SSR (length 6.5

cm and width 4.5cm) was equipped with two arrays

of 4 sensors, and the mobile arm of the CSR (length

9.7cm, width 4.8cm, curvature radius 21cm) was

equipped with two arrays of 5 sensors. The sum of

the single sensor forces yielded the total force. Force

distribution, total force and displacement were

recorded to a spread width of 10cm in 18 corpses

(11 males and 7 females). For every corpse, 4

measurement iterations were performed for both

retractors; each retraction was performed in

14.3±6.2s to reach 10cm widespread. The Authors

concluded that the shape of sternal retractors

considerably influences the force distribution on the

sternal incision. On the other side, it is reported that

the total mean retraction force was not significant

different between SSR and CSR (222.8±52.9N

versus 226.4±71.9N). Nevertheless, the recorded

mean total force was remarkably dependent on the

gender. For the first retraction, it was 256.2±43.3N

for males and only 174.9±52.9N for females.

Moreover, in the case of SSR the forces on the

cranial and caudal sternum are significantly higher

than in the median section. For SSR the maximum

total force for full retraction was 349.4±77.9N,

while force distribution during the first retraction for

the cranial/median/caudal part of the sternum was

101.5±43.9/29.1 ±33.9/63.0 ±31.4N.

Aigner et al assessed that the force distribution

did not change significantly for the other 3

retractions, for the different investigated spread

widths (i.e. 5, 7.5, and 10cm) and not gender-

dependent. The maximum force for full retraction

was 493.6N, whereas the smallest maximum force

was 159.0 N.

Results obtained for HT201 sensors are resumed

in Table I and the typical force (N) versus time (s)

patterns are presented in Figure 4-5.

BIODEVICES 2017 - 10th International Conference on Biomedical Electronics and Devices

Figure 4: Response of the four sensors (housed as shown

in Figure 1a) in terms of force [N] versus time [s] during

the 5cm opening procedure.

In all cases, a high stability of the response to a fixed

exerted force was evidenced. In fact, the value of

plateau

showed a mean standard deviation as low as

0.33±0.16N. Some valuable information can be

obtained from the acquired data. For example, the

total average force for the mobile blade ranged

between 60.1±6.0N for 5cm spread and 98.0±36.5N

for 10cm spread, as expected for a dummy built-up

with 80N gas pistons. The deviation with respect to

this value is also expected and has to be attributed to

the uneven pressure distribution onto the circular

sensors due to the rough surface finishing of the

contact area i.e. wood in the dummy.

It is interesting to observe during the retraction,

the Finochietto experienced along the mobile arm a

total F

max

(sensor#4 + sensor#3) that exceeded 200N,

ranging from 219.1±9.7N for 5cm spread and

266.6±25.4N for 10cm spread.

The force distribution along the retractor blade is

also particularly interesting. In fact, in all cases, the

highest maximum force (F

max

) was detected by

sensor #4 positioned on the mobile arm in proximal

(cranial) position (Figure 4-5), the value ranged

between 156.4±12.5N for 5cm spread and

199.7±21.2N for 10cm spread. The lowest F

max

Table 1: Values of the mean, maximum and plateau forces

(expressed in N) measured by HT201sensors positioned

according to Figure 1a (i.e. S1, S2, S3, S4). The related

standard deviation values are reported in parentheses.

sprea

Force

[N]

S1 S2 S3 S4

S1+S2

fixedblad

S3+S4

mobile

blade

5cm

mean

60.8

(5.7)

39.2

(4.4)

18.3

(2.1)

41.8

(7.7)

100.1

(8.5)

60.1

(6.1)

max

97.3

(10.6)

115.0

(19.5)

62.7

(5.4)

156.4

(12.5)

212.2 (14.8)

219.1

(9.7)

plateau

63.9

(5.8)

39.4

(4.9)

17.9

(1.8)

38.3

(8.3)

103.3

(9.0)

56.3

(7.2)

max-

mean

36.5

(8.7)

75.7

(21.1)

44.4

(4.0)

114.6

(12.9)

112.2 (12.5)

159.0

(12.6)

10cm

mean

37.6

(8.8)

82.5

(5.60)

15.2

(10.4)

82.8

(26.8)

120.1 (12.8)

98.0

(36.5)

max

79.6

(17.5)

126.3

(6.62)

66.9

(4.3)

199.7

(21.3)

205.9 (20.8)

266.6

(25.4)

plateau

41.2

(6.7)

89.1

(7.02)

13.1

(12.9)

84.7

(32.0)

130.2 (11.7)

97.8

(44.4)

max-

mean

42.0

(14.1)

43.8

(6.98)

54.7

(9.4)

116.9

(16.9)

85.8

(13.6)

168.6

(26.3)

values were 62.7±5.4N for 5cm and 66.9±4.3N for

10cm, registered in correspondence of sensor #3 of

the mobile arm in distal (caudal) position.

Interestingly, median sternotomy in corpses

performed by means of a straight sternal retractor

gave a comparable force distribution (Aigner, 2013).

This result suggests that the made-on-purpose

dummy enable to perform reliable test and thus it

might also be employed by surgeons in order to

assess their own learning curve for each specific

instrumented retractor.

Furthermore, sensor #4 detected also the highest

value of (F

max

-F

mean

), i.e. 114.6±12.9N and

116.9±16.9N, respectively for 10cm and 5cm

opening. For all the other sensors, this value does

not exceed 75.7±21.1N, independently from the

position on the retractor.

On the basis of these results, the presented

implementation system can be considered a valuable

tool to evaluate intensity and distribution of

retraction forces in human patients for conventional

sternotomy procedures. On the basis of our

knowledge, these data are not yet available in the

Literature. As already previously suggested by

Bolotin (Bolotin, 2007), the final goal is to develop

clinical studies aimed at coherently correlating the

biomechanical information obtained for a specific

surgical procedure with the incidence of post-

sternotomy chronic pain. In this respect, for

example, the actual outcomes of cranial versus

caudal positioning of the sternal retractor could be

assessed. On the basis of our knowledge, in the past

decade such kinds of studies have not yet been

performed probably due to the lack of an

In-vitro Force Assessments of an Autoclavable Instrumented Sternal Retractor

implemented user-friendly retractor suitable for

conventional clinical sterilization process.

Figure 5: Response of the four sensors (housed as showed

in Figure 1a) in terms of force [N] versus time [s] during

the 10cm opening procedure.

Moreover, the performance of this versatile

design might also contribute to estimate the actual

impact of minimally invasive cardiac surgery

techniques. In fact, since the 1990s, these procedures

are receiving an increasing interest due to a number

of potential advantages with respect to traditional

sternotomy, including reduced operative trauma, less

perioperative morbidity along with improved

aesthetic outcomes, shorter hospital stay and

accelerated rehabilitation (Ward, 2013). According

to recent studies, the overall outcomes and costs are

believed to be comparable with those of

conventional sternotomy (Reser, 2015; Alturi,

2015). It has to be considered that partial

sternotomy, in minimally invasive cardiac surgery

procedures, allows the displacement of only a part of

the hemithorax, the latter might be subject to

increased exerted forces eventually leading to

excessive stress on the “dynamic” chest wall. The

proposed study might be useful in the clinical setting

to determine the optimal balance between surgical

field and sternum separation.

The system can be considered cost-effective and

potentially adaptable to different surgical retractors

simply providing the appropriate housings.

5 CONCLUSIONS

This study demonstrates that the proposed system

allows performing measurements of retraction forces

in the range 5-400N using different models of

flexible force sensors; in particular Flexiforce A201

and HT201, the latter being suitable to operate in a

temperature range compatible with conventional

autoclave procedures. The implemented system was

thus demonstrated to be able to support autoclave

sterilization either removing or keeping in place the

force sensors, thus eventually allowing the reuse of

the HT201 sensors, which is more cost-effective

than a disposable use. In this perspective, we plan in

future work to investigate the maximum number of

autoclaving cycles that preserve the performances of

HT201 and other ultrathin force sensors available on

the market.

The user-friendly and low cost developed system

allowed at instantaneously measuring, displaying

and storing the force versus time pattern for each

sensor, during and after the opening phase. Accurate

and reliable data were obtained, in terms of

maximum force, mean force, total force and force

distribution. Measurements were acquired in real

time and readily available on a computer monitor.

ACKNOWLEDGEMENTS

The Authors wish to thank Antonella Camaioni for

help in autoclaving procedures, Andrea Iovino for

technical support in the early stage of device design.

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In-vitro Force Assessments of an Autoclavable Instrumented Sternal Retractor