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Effects of surgical variables in balancing of total knee replacements using an

instrumented tibial trial

Peter S. Walker ⁎, Patrick A. Meere, Christopher P. Bell

Department of Orthopaedic Surgery, New York University, Hospital for Joint Diseases, United States

abstractarticle info

Article history:

Received 22 July 2013

Received in revised form 27 August 2013

Accepted 11 September 2013

Keywords:

Total knee surgery

Knee balancing

Total knee technique

Instrumented tibial trial

Ligament balancing

Background: In total knee surgery, typically the bone cuts are made first to produce the correct overall alignment.

This is followed by balancing, often using spacer blocks to obtain equal parallel gaps in flexion and extension. Re-

cently an electronically instrumented tibial trial has been introduced, which measures lateral and medial contact

forces. The goal of our study was to determine the effect of different surgical variables; changing component

sizes, modifying bone cuts, or ligament releases; on the contact forces, as a method to achieve balancing.

Methods: A special rig was designed to fit on a standard operating table, on which tests on 10 lower extremity

specimens were carried out. After making bone cuts for a posterior cruciate retaining knee using a navigation sys-

tem, tibial thickness was determined in extension using the Sag Test. Different Surgical Variables were then im-

plemented, and the changes in the condylar forces were determined throughout flexion using the Heel Push Test.

Results: condylar forces were found to consist of gravity forces due to the weight of the leg plus forces due to pre-

tension in the collateral ligaments. The pretension force averaged 145 N but there was considerable variation be-

cause of ligament stiffness properties. Balancing from an imbalanced state could be achieved with adjustments

within only 2° or 2 mm.

Conclusion: The instrumented tibial trial provided force information which indicated which surgical correction

options to carry out to achieve balancing. From an initial unbalanced state, relatively small changes could produce

balancing, indicating the sensitivity of the procedure.

Clinical Relevance: Non-clinical. This study will assist in the balancingofthekneeattotalkneereplacementsurgery.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Achieving the ideal alignment of bone cuts, together with ligament

balancing, are important goals in total knee surgery. The accuracy of

the various bone cuts is usually within 3° of target, with some studies

showing that navigation produces more consistency [1,2].However

there is still uncertainty of the basis for the alignment in terms of

achieving the optimal results [3–6] and differences between measured

resection and gap balancing have been pointed out [7,8]. The complex-

ities of soft tissue balancing have been described and evaluated by many

authors, while other balancing options have been specified including

adjusting bone cuts and component sizing [9–15]. Generally the empha-

sis is on achieving equal gaps at 0 and 90° of flexion, with the femoral

rotation playing an important role [3,10,16–20]. However, other authors

have proposed unequal lateral and medial balancing to better reproduce

the normal anatomic situation [21,22].

Various methods have been introduced to improve and quantify the

process of balancing, distractors being the most frequently used

[23–26]. The concept of measuring the tibial plateau forces and contact

locations at surgery was first demonstrated using pressure sensitive

film [27,28]. Recently an electronically instrumented tibial trial compo-

nent has been introduced, which measured and displayed in real time

the forces on the lateral and medial compartments and their locations

at all flexion angles [29]. Clinical studies have suggested that correct

balancing can improve outcomes in various ways, including the avoid-

ance of postoperative instability and improved flexion [11,12,30–33].

However, there have been few reports of the ideal values of distrac-

tion or contact forces in order to achieve ‘correct balancing’.Thefirst

such data was provided from a series of cases using a distractor device,

where having achieved ideal balancing empirically, average total femo-

ral–tibial compressive forces of 120 N at both 0 and 90° flexion were re-

ported [24]. Whatever the ideal balancing goals, the instrumented tibial

trial concept does provide the possibility of reaching a defined goal in a

methodical way. At surgery, balancing is usually attempted by ‘surgical

variables’such as ligament releases, changing component sizes, or mod-

ifying bone cuts. An instrumented tibial trial can then evaluate the ef-

fects of the different surgical variables.

Hence, the primary goal of this laboratory study was to determine

the effect of the different surgical variables on the forces on the lateral

and medial condyles over a full range of flexion. This would then pro-

vide guidelines for which surgical variable would be most effective in

The Knee 21 (2014) 156–161

⁎Corresponding author at: Laboratory for Orthopedic Implant Design, Hospital for Joint

Diseases, Suite 1500, 301 East 17th Street, New York, NY 10010, United States. Tel.: +1

212 598 6569; fax: +1 212 598 6096.

E-mail address: Peter.Walker@nyumc.org (P.S. Walker).

0968-0160/$ –see front matter © 2013 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.knee.2013.09.002

Contents lists available at ScienceDirect

The Knee

achieving balancing from an initial unbalanced state. An associated goal

was to evaluate a specific surgical test that would provide the required

force data.

2. Methods and materials

A test rig was developed for mounting lower body specimens to a

standard operating table (Fig. 1). The pelvis was fixed to the base of

the rig and a surgical boot was firmly strapped to the foot. A spherical

bearing fixed to the heel of the boot was attached to a low friction car-

riage which advanced towards the hip along a slide rail, to flex and ex-

tend the knee with no constraints. To maintain the leg in a vertical

plane, chocks were used to prevent foot rotation. For specifictests,fix-

tures on the rig allowed the leg segments to be supported at a required

flexion angle, to maintain the thigh and lower leg in a vertical plane, to

prevent axial rotation of the femur, and to be correctly positioned

medio-laterally. A total of 10 leg specimens were tested, of which two

were used for experimental development.

Once the specimen was mounted, navigation trackers were fixed to

the femur and tibia. A subvastus medial approach was then used for sur-

gical exposure. The bone cuts were made for the insertion of a posterior

cruciate retaining total knee (Triathlon, Stryker Orthopaedics, Mahwah,

NJ) using an optical navigation system (Stryker Navigation, Kalamazoo,

MI). The frontal plane cuts were perpendicular to the mechanical axes

of the respective bones, while the tibia was cut at 5° posterior slope in

the sagittal plane. The femoral rotation was 3 to 4° external to the

epicondylar axis, verified by Whiteside's line and the posterior condylar

line. The trial femoral component and tibial baseplate were inserted, the

latter rotationally aligned initially to the medial third of the tibial tubercle.

The wireless instrumented tibial trial was then introduced

(OrthoSensor Knee Balancer, OrthoSensor, Inc., Sunrise, FL), of a thick-

ness so that the knee just reached full extension when the foot was

lifted up from the table. This procedure was called the Sag Test. For

this and all subsequent tests, the vastus medialis and medial arthrotomy

incision were closed with towel clips. In order to assess the rotational po-

sition of the tibial component, the foot was oriented vertically, and was

manually pushed along the horizontal rail such that the knee flexed

from 0 to 120°, with the leg moving in a vertical plane. This procedure

was termed the Heel Push Test. The anterior–posterior (AP) locations

of the contact points were observed and the rotational position of the tib-

ial component was adjusted if necessary to produce uniform locations of

the contact points on the lateral and medial sides. The tibial baseplate

was then pinned in place to define its position for all subsequent tests.

A surgical variable (Fig. 2) was selected based on the initial output

data. For example if there was a consistently higher medial than lateral

force during flexion, a two degree tibial varus angle was applied by

stuffing the lateral side with a two millimeter wedge. The Heel Push

Test was then repeated. The difference in the output data caused by

the surgical variable (in this case, the two degree tibial varus) was

then determined. This principle of differences was used throughout

the sequential testing to determine the effect of each surgical variable.

The order of applying the surgical variables was based mainly on the

output data from the preceding test. The method was to move towards

and away from a balanced state with each variable. All variables could

not be applied to every knee due to situations when the contact forces

became zero, or were excessive in certain flexion ranges.

3. Results

3.1. Analysis of Heel Push Test

The leg is represented in Fig. 3, where the hip is a fixed pivot, the foot slides horizon-

tally along the slide rail, and a heel push force is applied. The forces between the femur and

tibia are shown as a force JF down the axis of the tibia, which would be measured by the

instrumented tibial trial, and a shear force. JF is the sum of the lateral and medial forces.

The weights of the thigh and lower leg are WT and WS, acting at the distances shown.

This equation for the total joint force JF (Fig. 3) predicts that the forces will be small at

higher flexion angles, but increase rapidly as the flexion angle becomes about 15°. The

Fig. 1. The test set-up for carrying out the experiments on the lower limb. The custom-made rig was fixed to a standard operating table.

157P.S. Walker et al. / The Knee 21 (2014) 156–161

analysis breaks down at low flexion because soft tissue forces are not accounted for. The

results for the total joint force JF plotted against angle of flexion F from 15 to 120° flexion

are shown for the average male leg size and weight (Fig. 4, full black line) [34]. The curve

(full black line) represents the force on the tibial surface due entirely to the weight of the

leg, without accounting for soft tissue forces. Any forces in excess of those values would be

due to soft tissue tensions. The curve will vary for each individual leg depending on its

weight.

During the testing of the 10 knees using the Heel Push Test, ranges of force–flexion

curves were obtained before and after the implementation of different surgical variables.

The colored curves in Fig. 4 were selected based on an average lateral:medial or medial:

lateral force ratio of less than 2.5:1, over the flexion range. For each curve, the calculated

force due to the weight of the leg was subtracted, leaving only the force due to the soft tis-

sue tension. The mean of these corrected curves was then determined (Fig. 4,dashedline).

The mean soft tissue tension value was close to 145 N (32.5 lbf) for the whole flexion

range.

3.2. The effect of the Surgical Variables

From an initial status, implementing a surgical variable led to a change in the Output

Data. Several surgical variables were tested on each knee. The mean values of the force

changes on the lateral and medial condyles after different surgical variables were mea-

sured (Fig. 5). For a 2 millimeter distal femoral cut, a 2 millimeter thicker tibial spacer

was used to preserve extension. As a result, the condyle forces in low flexion were little

changed, but the forces in high flexion were increased. For 2 millimeter lateral stuffing,

the lateral forces increased and the medial forces decreased. The opposite was the case

for medial stuffing, but with a large medial increase. Increasing the tibial slope had little

effect, possibly due to the contact points not displacing posteriorly a large amount. On

the other hand a decreased slope achieved with a 2 millimeter wedge caused large force

increases in both flexion ranges. An increase in the femoral component size with the

same anterior and distal contours but 3 mm of additional posterior condylar offset,

showed a large increase in the total condylar force.

Fig. 2. The surgical variables which were tested to determine the changes in the condylar force values. Spacers and wedges were used for the distal femur and proximal tibia. For the LCL

and PCL, the variables were unaltered (0) and released (R). The numerical values are the changes relative to the initial status.

Fig. 3. Analysis of the Heel Push Test. The joint compressive force (JF) due to the weight of the thigh (WT) and lower leg (WS) is calculated, from the equation.

158 P.S. Walker et al. / The Knee 21 (2014) 156–161

To put this data into perspective, the force increases due to a 1 millimeter increase in

length of the lateral or medial collateral are shown (Fig. 5, right), taking data from force

elongation tests of the collaterals [35]. Two force ranges are shown: the soft region up to

the toe of the force–elongation curve and the subsequent stiff region. The former repre-

sents an elongation in the range of 0–2 mm, the latter an elongation of 3–5 mm. This

data explains the relatively large changes in either the lateral or medial condyle force

when the frontal plane slope of the tibia was modified by only 2°, equivalent to about

2 mm of collateral length change. A decrease in sagittal tibial slope had a similar effect,

in this case on both of the condyles together.

4. Discussion

In this study we investigated the use of an instrumented tibial trial

which measured and displayed the lateral and medial condyle forces

as an indicator of balancing. An important feature of the system is that

the forces are measured with the actual implant trial components [36]

which differentiate it from the distractor approach. In principle the

force values obtained and their locations, can be interpreted to indicate

what corrections would be needed to obtain a balanced state. These cor-

rections, which we termed ‘surgical variables’, included ligament re-

leases, adjustments to bone cuts, or a change of component sizes. Two

surgical tests were used in the experiments. The Sag Test determined

the required tibial insert thickness, which would then determine the

overall magnitude of the forces during flexion due to the tensions in

the soft tissues, primarily the collaterals. The Heel Push Test determined

the forces during the full flexion range and indicated whether a lateral–

medial imbalance was uniform throughout flexion, and whether there

was an increase or decrease in the forces from extension to flexion.

The combined condyle force in the Heel Push Test, subtracting the

calculated force due to the weight of the leg, averaged 145 N,

representing soft tissue tension. The value is similar to the 120 N deter-

mined for balanced knees in a previous study [24].Thismagnitudeof

force implied that the elongation of each collateral ligament producing

the pretension forces was in the range of 2–3 mm. This placed the liga-

ments at the start of the stiff part of their force–elongation curves. This

explains the important finding in our study, that a change in tibial fron-

tal plane angle of only 2°, equivalent to an increase of approximately

2 mm at the lateral or medial side, produced relative medial–lateral

force changes which would compensate for any initial imbalance. This

would also indicate the small amount of ligament release that would

be needed to achieve balancing, consistent with a progressive ‘pie

crusting’approach [37]. Hence, balancing is very sensitive to adjust-

ments of only 1–2mm.

Regarding the Surgical Tests, the Heel Push Test proved to be simple

to carry out, and covered the whole flexion range. It was necessary to

slide the heel along a track to maintain the leg in a vertical plane,

which was achieved by attaching a simple fixture to the operating

table. The condyle forces recorded in this test were shown to include

the weight of the thigh and lower leg, with a greater effect in lower flex-

ion angles. In order to isolate the soft tissue forces only, the gravity

forces from the weight of the leg had to be subtracted. A similar test

but supporting the weight of the leg with a hand at the posterior of

the knee may result in only soft tissue forces, but this was not method-

ically tested in the present study.

The problem of identifying how to correct an imbalanced state was

addressed by applying a surgical variable and determining the force

changes from an initial state. This would provide the ‘signature’for

that particular variable. Looseness in flexion could be addressed by an

additional distal femoral resection, but the effect was not major, where-

as decreasing the tibial slope had a much larger effect. In contrast in-

creasing the tibial slope had little effect. A previous study reported

Fig. 4. Force–flexion curves (colored) in the Heel Push Test where the medial:lateral or later-

al:medial force ratio averaged less than 2.5:1 for the flexion range. The theoretical curve for

the weight of the average male leg is shown (full black line). The mean soft tissue tension

forces for the knees, corrected for the weights of the legs, are shown (dashed black line).

Fig. 5. The mean differences in the condyle forces in the Heel Push Test caused by implementing different surgical variables (Fig. 2). Lo = low flexion range 0–60°. Hi = high flexion range

60–120°. Lat + Med = combined lateral and medial forces. Collateral ligament stiffness in the initial soft region (0–2 mm) and the subsequent stiff region (3–5 mm) is taken from ref-

erence [36].

159P.S. Walker et al. / The Knee 21 (2014) 156–161

such effects for both posterior cruciate retaining (CR) and posterior sta-

bilized (PS) knees [38]. Interestingly, increasing the AP dimensions of

the femoral component caused an increase in condyle forces throughout

flexion. Although increasing the femoral component size is impractical

in a surgical setting, it did emphasize that from the point of view of max-

imizing the flexion range, a larger posterior femoral condyle offset is an

advantage [39,40].

Regarding the significance of balancing, in gait and other activities,

the axial compressive forces are in the range of 2.5–4.0 times body

weight [41,42]. The frontal plane moments are affected by the align-

ment of the leg, muscle actions, and the individual's gait pattern. The

magnitude of these moments and functional forces is around an order

of magnitude greater than the soft tissue pretensions of approximately

145 N. Hence, it is likely that passive imbalance within certain limits is

unlikely to affect load-bearing function itself, at least in terms of

varus–valgus and AP stability. However, in the swing phase, the axial

compressive forces have been measured at only a fraction of body

weight, comparable to the soft tissue pre-tension forces. Hence for an

unbalanced knee the relative position of the femur on the tibia in the

swing phase, and at the time of heel strike, could result in an unstable

phase until the femur and tibia reach equilibrium determined largely

by the geometry of the total knee components. Such instability has

been detected in the varus–valgus and anterior–posterior directions

using accelerometers [43,44].

Our study was limited in several ways. Due to using only 10 knee

specimens and the few separate tests that could be performed on

each, the data cannot be used for a statistically valid analysis of force

changes for each surgical variable, which would be better carried out

in a clinical study with a large number of arthritic patients. Nevertheless

we were able to identify overall trends, and to determine the sensitivity

of the condylar forces to small gap or angular changes. Our models

oversimplified the soft tissue structures and the changes in balancing

which could be achieved by various releasing strategies [37].Alsowe

did not specifically account for the effect of the posterior cruciate, for

which an AP drawer test would be applicable [45]. Studies have

shown the variation in the force–elongation properties of different

parts of the collaterals [35,46,47] and in the variation of the actual

varus and valgus angles on lift-off [20]. These factors emphasize the

patient-specific nature of balancing. This highlighted the sensitivity of

the balancing process but also indicated the range within which the cor-

rection procedures need to be carried out. Finally, we have reported

here only results from the Heel Push Test. We did obtain data from

varus–valgus, anterior–posterior, and internal–external rotation tests.

The data obtained supplemented the Heel Push Test and will be report-

ed separately.

In conclusion, we studied the application of an instrumented tibial

trial for achieving balancing of a total knee replacement. The variations

in condylar forces between different knees emphasized that there was

no universal set of values that would apply to all, but that a systematic

approach could lead to a balanced knee within acceptable limits. The

balancing procedures appeared to be sensitive to changes within two

millimeters and two degrees, whether from corrections made to soft tis-

sues, bone cuts, or component sizing. The condylar forces of the passive

knee were an order of magnitude less than those in function and hence

balancing is expected to have a larger effect in the swing phases of activ-

ity rather than in the stance phases.

5. Conflict of interest

Dr. Walker and Dr. Meere have received consulting payments for

participation in workshops organized by OrthoSensor Inc., the manufac-

turer of the device which is the subject of this paper. OrthoSensor Inc.

provided partial funding for the study, monitored through the Spon-

sored Programs Administration of New York University Medical Cen-

ter. Other funding was directly from the Department of Orthopedic

Surgery.

Acknowledgments

This study was funded by OrthoSensor, Sunrise, FL, and by the De-

partment of Orthopaedic Surgery, New York University-Hospital for

Joint Diseases. We thank Daniel Hennessy for rig design and construc-

tion, and Mathew Hamula and Dana Kjolner for technical assistance.

The experiments were carried out in the Surgical Skills Laboratory of

the Hospital for Joint Diseases.

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