SMART COLLATERAL LIGAMENT BALANCER FOR

INTRA- AND POSTOPERATIVE MEDIOLATERAL BALANCE

Shaban Almouahed

1,2

, Chafiaa Hamitouche

1,2

, Eric Stindel

1,3,4

and Christian Roux

1,2

LATIM – INSERM U650, Brest, France

TELECOM Bretagne – Institut TELECOM, Brest, France

Université de Bretagne Occidentale, Brest, 29200, France

CHU Brest, Service d’Orthopédie-Traumatologie, Brest, France

Keywords: Total knee arthroplasty, Collateral ligament balance, Knee balancer.

Abstract: The poor ligament balance performed at the time of total knee arthroplasty (TKA) can cause postoperative

instability and consequently early loosening of the prosthetic knee. The improper intraoperative assessment

of ligamentous balance is due to the use of an accurate surgical instrument which tenses the medial and

lateral collateral ligaments in uncontrolled way. A smart ligament balancer is proposed in this paper to

assess and perform ligament balancing intra- and postoperatively. A detailed three-dimensional model of the

prototype is developed using CAD software in order to discuss the operation of this device. The

intraoperative use of this balancer could allow to accurately reestablish a rectangular tibiofemoral gap with

symmetric mediolateral load distribution across the whole range of knee flexion. On the other hand, the

implanted balancer could be used postoperatively to assess ligamentous balance and to correct it when

needed.

1 INTRODUCTION

In most arthritic knee joints undergoing total knee

arthroplasty (TKA), some degree of collateral

ligament imbalance exists (Griffin, 2000). The

ligament imbalance could be present in the form of

instability, deformity, or a combination of these two

elements. The importance of acquiring proper

ligament balance at the time of TKA is well

recognized (Insall, 1993). Many techniques have

been used to assess ligament imbalance including

knee tensioning devices (Laskin, 1989), spacer

blocks (Insall, 1984), and manual distraction

instruments. The aforementioned techniques balance

the medial and lateral collateral ligaments by loading

them with maximally or in uncontrolled way. If the

resultant gap is trapezoidal, then the ligaments are

imperfectly balanced. Moreover, the traditional

tensors are unable to accurately assess the

ligamentous balance because of the discrete

measurement of tibiofemoral forces (Attfield, 1994).

A robotized distractor (Marmignon, 2004) has been

developed to assess soft-tissue balance. This

distractor consists of a base plate which is connected

with two independent and parallel trays. The upper

trays support the condyles and can be lifted by

means of a jack and a cable or thanks to two

inflatable rubber bladders. The disadvantage of the

first approach is that the device is not powerful

enough (maximal force is equal to 100 N), while the

disfavor of the second is that the parallelism of

upper trays could not be assured, which influences

the right functioning of the device. A force-sensing

device (Crottet, 2005) has been developed to

intraoperatively enhance the ligament balancing

procedure. This device has two sensitive plates to

support the two femoral condyles, a tibial base plate

and a set of different size spacers to fit the apparatus

thickness to the patient-specific tibiofemoral space.

Each of the two sensitive plates is instrumented with

three deformable bridges. Each bridge is equipped

with thick-film piezoresistive sensor to ensure the

accurate measurement of the amplitude and location

of tibiofemoral contact forces. The soft-tissue

imbalance is then assessed by the net varus-valgus

moment. The major limitation of this device is that

the application of load is manually performed by

stressing the lower limb, thus load is difficult to be

accurately applied. Moreover, the augmentation of

tibiofemoral gap height and consequently ligament

tension must be achieved by inserting different size

spacers, which increases the time and complexity of

TKA.

139

Almouahed S., Hamitouche C., Stindel E. and Roux C. (2010).

SMART COLLATERAL LIGAMENT BALANCER FOR INTRA- AND POSTOPERATIVE MEDIOLATERAL BALANCE.

In Proceedings of the Third International Conference on Biomedical Electronics and Devices, pages 139-142

DOI: 10.5220/0002757501390142

 SciTePress

To address the aforementioned shortcomings, a

smart ligament balancer is proposed to assess and

balance the collateral ligaments intra- and

postoperatively and to correct the imbalance when

needed.

2 MATERIAL AND METHODS

A detailed three-dimensional model (Fig. 1) of the

device is designed using CAD software in order to

describe the functionality of this device. The

proposed device consists of a fixed base plate and

two mobile plates. The lower base plate is separately

connected to each of the top plates by a scissor

mechanism operated by a miniature linear actuator.

Figure 1: An Isometric view of the smart ligament

balancer: 1. the fixed tibial base plate, 2. the two mobile

plates, and 3. the two scissor mechanisms.

The actuator located at the bottom of base plate

compartment drives one sliding shaft away from the

other in order to move the upper plate downward

and vice versa (Fig. 2). The two actuators are

automatically driven by a microcontroller in

response to the command signals sent by the surgeon

and to the force and position values measured by

adequate sensors embedded into the device. The

device is equipped with force and position sensors.

Three force sensors are embedded within each

mobile plate to continuously measure the amplitude

and location of the corresponding compartmental

contact force. One position sensor is embedded into

each compartment of the base plate to accurately

measure the distance between both the upper plates

and the lower plate at any time of the balancing

procedure.

In our study, the perfect ligament balance is

defined by a rectangular tibiofemoral gap and a

symmetric distribution of compressive load between

the medial and lateral compartments of knee at both

full extension and at 90° of flexion. The purpose of

this study is to assess the soft tissue balancing per-

and postoperatively for the full range of knee motion

and to rectify the imbalance when existed by means

of smart knee balancer.

Figure 2: Exploded view of the smart ligament balancer: 1.

the scissor arms, 2. the sliding shafts, 3. the pivot shaft,

and 4. the distraction and contraction of the actuator.

The immobile lower base plate of smart ligament

balancer is positioned onto the proximal cut of the

tibia while the two mobile upper plates are in contact

with the corresponding femoral condyles. The

balancer must be introduced into the tibiofemoral

space after the tibial osteotomy is performed and

before the femoral cuts are made. Measurement

must be made at full extension with the smart

balancer fixed onto the proximal tibial cut and acting

against the distal femoral condyles and at 90° of

flexion using the posterior femoral condyles and the

proximal tibial surface.

After the tibial cut is made, the smart ligament

balancer must be positioned within the knee. The

surgeon must send a command signal in order to

expand the balancer with a predetermined tension on

both medial and lateral sides until both mobile plates

are in full contact with distal femoral condyles and

the predetermined tension of collateral ligaments is

sensed by the force transducers embedded within the

upper plates. If the medial and lateral gaps are not

equal, a ligament release needs to be carried out in

order to rectify the mediolateral imbalance until the

flexion and extension spaces seem to be symmetric

and the collateral ligaments are once again well-

balanced (Fig. 3).

The relationship between the force measured on

the upper surface of mobile plate and the force

exerted by the actuator to expand or collapse the

scissor mechanism and consequently the balancer is

given by the following equation:

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140

Figure 3: A schematic diagram of ligament balancing

procedure using the smart knee balancer.

Where F

Actuator

is the force provided by the actuator

arm, F

Condyle

is the force applied to the upper plate by

the corresponding femoral condyle, W is the weight

of the mobile plate and W

Arm

is the combined weight

of the two scissor arms (Fig. 4). The collapsed

height of the smart ligament balancer is 5 mm while

the expanded height is given by the next equation:

Where H

Expanded

is the expanded height of the

balancer, H

Collapsed

is the collapsed height of the

balancer, L is the length of scissor arm and θ is the

angle between the horizontal and scissor arm.

Figure 4: Scissor lift jack.

The starting angle is equal to 0° to allow the

balancer to be completely collapsed with initial

height of 5 mm (Fig. 5) and the ending angle must

not exceed 30° in order to maintain the parallelism

of the upper plates and to ensure their stability in the

transversal plane. Since the length of the scissor arm

equals 25 mm. Consequently and according to the

equation (2), the balancer can expanded from 0 mm

to 25 mm in a continuous movement.

Figure 5: The collapsed height of the balancer (H

Collapsed

5 mm) and the expanded height (H

Expanded

= 30 mm).

3 CONCLUSIONS

Tibiofemoral mechanical malalignment and

collateral ligament imbalance can result in a

postoperative instability of the prosthetic knee which

is a major complication after total knee arthroplasty

(Fehring, 1994). Since surgeons strive for perfection

in ligament balance, an accurate ligament balancer

(Fig. 6) is indispensable to prevent postoperative

complications such as instability. The importance of

postoperative assessment of ligament imbalance is

due to the fact that the mediolateral laxity of

prosthetic knee can change and increase after total

knee arthroplasty without resulting in postoperative

tibiofemoral mechanical malalignment but

increasing coronal ligament imbalance and

consequently knee instability. The primary cause of

postoperative ligament imbalance is that the

intraoperative ligamentous balance couldn’t be

perfectly achieved at the time of surgery, even with

considerable release of one of the two collateral

ligaments. This might be due to the absence of

adequate assessment of ligament balance during

surgery. Furthermore, the mediolateral ligament

balance varies postoperatively even if proper

balance is performed intraoperatively.

Figure 6: In vivo ligament balancer embedded onto the

tibial component of knee prosthesis.

SMART COLLATERAL LIGAMENT BALANCER FOR INTRA- AND POSTOPERATIVE MEDIOLATERAL

BALANCE

141

To the best of our knowledge, the smart ligament

balancer proposed in this study is the first one that

could quantitatively assess the mediolateral balance

of collateral ligament intra- and postoperatively in

both extension and flexion. In addition, this balancer

could allow a continuous assessment of ligament

imbalance over the whole range of flexion while

most balancing instruments assess this balance at

full extension and 90° of flexion and don’t allow

measurements at other positions of flexion (Attfield,

1996).

Since the medial and lateral collateral ligaments

of knee are of different cross-section, length and

shape and do not represent extensile strings, but

viscoelastic, extendible structures, the ligament

imbalance could not be constant at different

separation gaps and depends on the compressive

tension used to distract the bones. Therefore,

ligament imbalance must be quantified at different

distraction gaps by tensing the knee by equal forces

both medially and laterally but with different force

at each time. This is completely possible by means

of our smart knee balancer given that the ligament

imbalance is quantified by measuring the difference

in height between the medial and lateral sides of a

trapezoidal tibiofemoral gap produced when

identical tensions are applied to both medial and

lateral ligaments of the knee. In addition, the

parallelism of the upper trays is perfectly ensured by

the use of scissor mechanism instead of inflatable

rubber bladders (Marmignon, 2004). Furthermore,

the continuous movement of the upper tray ensures a

continuous augmentation of the gap height rather

than the discrete movement achieved by inserting

spacer blocks (Crottet, 2005), which decreases the

time and complexity of knee surgery.

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