Effects Of Surgical Variables In Balancing Total Knee Replacements Using An Instrumented Tibial Trial Walker Et Al Trial. The

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The Knee 21 (2014) 156–161

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The Knee

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

a r t i c l e

i n f o

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

a b s t r a c t
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. Recently 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 system, tibial thickness was determined in extension using the Sag Test. Different Surgical Variables were then implemented, 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 pretension in the collateral ligaments. The pretension force averaged 145 N but there was considerable variation because 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 balancing of the knee at total knee replacement surgery.
© 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 complexities 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 emphasis 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
⁎ 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

[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 component 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 avoidance of postoperative instability and improved flexion [11,12,30–33].
However, there have been few reports of the ideal values of distraction or contact forces in order to achieve ‘correct balancing’. The first
such data was provided from a series of cases using a distractor device,
where having achieved ideal balancing empirically, average total femoral–tibial compressive forces of 120 N at both 0 and 90° flexion were reported [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 modifying bone cuts. An instrumented tibial trial can then evaluate the effects 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 provide guidelines for which surgical variable would be most effective in

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

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 carriage which advanced towards the hip along a slide rail, to flex and extend the knee with no constraints. To maintain the leg in a vertical
plane, chocks were used to prevent foot rotation. For specific tests, fixtures 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 surgical 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 thickness 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

157

this and all subsequent tests, the vastus medialis and medial arthrotomy
incision were closed with towel clips. In order to assess the rotational position 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 tibial 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 horizontally 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.

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

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.

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 tissue tension. The mean of these corrected curves was then determined (Fig. 4, dashed line).
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 measured (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. 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.

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

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

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 represents 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 corrections, which we termed ‘surgical variables’, included ligament releases, adjustments to bone cuts, or a change of component sizes. Two

159

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 determined for balanced knees in a previous study [24]. This magnitude of
force implied that the elongation of each collateral ligament producing
the pretension forces was in the range of 2–3 mm. This placed the ligaments 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 frontal 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 adjustments of only 1–2 mm.
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 flexion 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 methodically 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, whereas decreasing the tibial slope had a much larger effect. In contrast increasing the tibial slope had little effect. A previous study reported

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 reference [36].

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such effects for both posterior cruciate retaining (CR) and posterior stabilized (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 maximizing 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 alignment 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]. Also we
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 correction 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 reported 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 tissues, 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 activity 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 manufacturer of the device which is the subject of this paper. OrthoSensor Inc.
provided partial funding for the study, monitored through the Sponsored Programs Administration of New York University Medical Center. Other funding was directly from the Department of Orthopedic
Surgery.

Acknowledgments
This study was funded by OrthoSensor, Sunrise, FL, and by the Department of Orthopaedic Surgery, New York University-Hospital for
Joint Diseases. We thank Daniel Hennessy for rig design and construction, 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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Author                          : Peter S. Walker
Create Date                     : 2014:01:14 02:24:06+08:00
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Crossmark Major Version Date    : 2010-04-23
Elsevier Web PDF Specifications : 6.2
Keywords                        : Total, knee, surgery;, Knee, balancing;, Total, knee, technique;, Instrumented, tibial, trial;, Ligament, balancing
Modify Date                     : 2015:03:17 13:09:45-04:00
Doi                             : 10.1016/j.knee.2013.09.002
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Identifier                      : doi:10.1016/j.knee.2013.09.002
Title                           : Effects of surgical variables in balancing of total knee replacements using an instrumented tibial trial
Creator                         : Peter S. Walker, Patrick A. Meere, Christopher P. Bell
Subject                         : Total knee surgery, Knee balancing, Total knee technique, Instrumented tibial trial, Ligament balancing
Description                     : The Knee, 21 (2014) 156-161. doi:10.1016/j.knee.2013.09.002
Publisher                       : Elsevier B.V.
Aggregation Type                : journal
Publication Name                : The Knee
Copyright                       : Copyright (c) 2013 Elsevier B.V. All rights reserved
ISSN                            : 0968-0160
Volume                          : 21
Number                          : 1
Cover Display Date              : January 2014
Page Range                      : 156-161
Starting Page                   : 156
Ending Page                     : 161
Digital Object Identifier       : 10.1016/j.knee.2013.09.002
URL                             : http://dx.doi.org/10.1016/j.knee.2013.09.002
Major Version Date              : 2010-04-23
Cross Mark Domains              : elsevier.com, sciencedirect.com
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