The variation in medial and lateral collateral ligament strain and tibiofemoral forces following changes in the flexion and extension gaps in total knee replacement: A LABORATORY EXPERIMENT USING CADAVER KNEES

Abstract

Achieving deep flexion after total knee replacement remains a challenge. In this study we compared the soft-tissue tension and tibiofemoral force in a mobile-bearing posterior cruciate ligament-sacrificing total knee replacement, using equal flexion and extension gaps, and with the gaps increased by 2 mm each. The tests were conducted during passive movement in five cadaver knees, and measurements of strain were made simultaneously in the collateral ligaments. The tibiofemoral force was measured using a customised mini-force plate in the tibial tray. Measurements of collateral ligament strain were not very sensitive to changes in the gap ratio, but tibiofemoral force measurements were. Tibiofemoral force was decreased by a mean of 40% (SD 10.7) after 90 degrees of knee flexion when the flexion gap was increased by 2 mm. Increasing the extension gap by 2 mm affected the force only in full extension. Because increasing the range of flexion after total knee replacement beyond 110 degrees is a widely-held goal, small increases in the flexion gap warrant further investigation.

Full text

1528 THE JOURNAL OF BONE AND JOINT SURGERY
The variation in medial and lateral collateral
ligament strain and tibiofemoral forces
following changes in the flexion and
extension gaps in total knee replacement
A LABORATORY EXPERIMENT USING CADAVER KNEES
B. Jeffcote,
R. Nicholls,
A. Schirm,
M. S. Kuster
From Fremantle
Hospital, Fremantle,
Australia
B. Jeffcote, BMBS,
FRACS(Orth), Orthopaedic
Surgeon
Department of Orthopaedic
Surgery
R. Nicholls, PhD, Research
Fellow
University of Western
Australia, Fremantle
Orthopaedic Unit, Fremantle
Hospital, Alma Street,
Fremantle, Western Australia.
A. Schirm, MD, Orthopaedic
Surgeon
M. S. Kuster, MD, PhD,
FRACS(Orth), Chairman,
Professor
Klinik für Orthopädische
Chirurgie, Kantonsspital 9007,
St. Gallen, Switzerland.
Correspondence should be sent
to Professor M. S. Kuster; e-mail:
Markus.Kuster@kssg.ch
©2007 British Editorial Society
of Bone and Joint Surgery
doi:10.1302/0301-620X.89B11.
18834 $2.00
J Bone Joint Surg [Br]
2007;89-B:1528-33.
Received 30 October 2006;
Accepted after revision 27 July
2007
Achieving deep flexion after total knee replacement remains a challenge. In this study we
compared the soft-tissue tension and tibiofemoral force in a mobile-bearing posterior
cruciate ligament-sacrificing total knee replacement, using equal flexion and extension
gaps, and with the gaps increased by 2 mm each. The tests were conducted during passive
movement in five cadaver knees, and measurements of strain were made simultaneously in
the collateral ligaments. The tibiofemoral force was measured using a customised mini-
force plate in the tibial tray. Measurements of collateral ligament strain were not very
sensitive to changes in the gap ratio, but tibiofemoral force measurements were.
Tibiofemoral force was decreased by a mean of 40% (SD 10.7) after 90˚ of knee flexion when
the flexion gap was increased by 2 mm. Increasing the extension gap by 2 mm affected the
force only in full extension. Because increasing the range of flexion after total knee
replacement beyond 110˚ is a widely-held goal, small increases in the flexion gap warrant
further investigation.
Flexion often remains limited after total knee
replacement (TKR). There have been reports of
flexion ≥ 140˚ being achieved,1 but most stud-
ies describe ranges of 105˚ to 115˚ one year
after operation.2-5 This range is adequate for
most elderly patients in the Western world, but
is unlikely to satisfy younger patients, or those
for whom kneeling and squatting are impor-
tant activities.6-8 The main factors influencing
the post-operative range of flexion identified to
date are the pre-operative range, the body mass
index, correct sizing of components, patellar
tracking, the accuracy of gap balancing and
post-operative physiotherapy.9-11 This study
explores the influence the balance of the flex-
ion/extension gap has on soft-tissue tension in
deep flexion.
Insall and Scott12 first recommended balanc-
ing knee ligaments by creating equal and rect-
angular flexion and extension gaps. This
recommendation has been generally accepted
and several authors have attributed poor clini-
cal results to inadequate balance of these
gaps.10,11,13,14 The flexion gap may be mea-
sured by plain radiography; MR scans are not
required.15 In practice, the exact balance of the
gaps can be difficult to measure, and various
intra-operative spreaders or distraction devices
have been used to obtain equal gaps.16 To date
there has been little biomechanical analysis of
the influence the balance of the flexion/exten-
sion gap has on the forces generated within the
tibiofemoral joint or the strains on the peri-
articular soft tissues. The aim of this study was
to quantify the compressive tibiofemoral force
and strain patterns in the collateral ligaments
in a series of cadaver knees after TKR with
equal and unequal flexion and extension gaps.
Materials and Methods
Five fresh-frozen specimens of the human knee
were tested. All had mild to moderate osteo-
arthritic changes, predominantly in the medial
compartment. The femur and tibia were
resected 250 mm from the medial joint line,
and all soft tissue except the articular capsule,
ligaments, popliteus muscle, and quadriceps
tendon was removed.
A simple system was employed to determine
the strain in the medial and lateral collateral
ligaments (MCL and LCL, respectively). Kir-
schner (K)-wires with a 1.6 mm diameter were
inserted perpendicular to the attachments of
the MCL and LCL. The anterior fibres of the
MCL were selected for measurement, with the
distal K-wire located 3 mm behind the anterior
Research

THE VARIATION IN MEDIAL AND LATERAL COLLATERAL LIGAMENT STRAIN AND TIBIOFEMORAL FORCES 1529
VOL. 89-B, No. 11, NOVEMBER 2007
edge of the insertion of the ligament on the tibia. Strain (ε)
was calculated using the engineering strain formula17 (%):
ε = (l - l0),
l0
where l represents the instantaneous length of the liga-
ment (measured with digital calipers accurate to 0.01 mm),
and l0 the reference length, which was the length at full
extension for both ligaments prior to implantation of the
prosthesis, as previously reported.18
In a pilot study, differential variable reluctance trans-
ducer (DVRT) strain gauges (Microstrain Inc., Burlington,
New England) were used on soft-fixed and fresh-frozen
cadaver knees to measure ligament strain, as suggested by
Harfe et al.19 However, the trials showed inconsistent
results beyond 120˚ of flexion. We attribute this to the
twisting and buckling of the ligaments in deep flexion.
Whereas Harfe et al19 presented the strain in the mid-
flexion range, our study interest was beyond 120˚ of flex-
ion. Measurements using the K-wire technique were more
consistent than the DVRT’s throughout the whole range of
movement and had the additional advantage of measuring
the behaviour of the ligament as a whole, rather than indi-
vidual fibre bundles.
Each knee was mounted in a customised passive move-
ment rig (Fig. 1) designed to apply a passive flexion-
extension moment to the tibia with the femur fixed. The rig
also permitted the normal rotation of the tibia during
movement. It aimed to mimic the surgical environment
where the knee is passively flexed by the surgeon with the
patient anaesthetised. A spring sutured to the quadriceps
tendon was calibrated such that it exerted no force at full
extension but gradually came into play as the knee flexed,
reaching a maximum force of 40 N at 150˚ of flexion. The
purpose of the spring was to reproduce the passive stretch-
ing of the extensor mechanism which generates force across
the tibiofemoral articulation in flexion. As there is minimal
hamstring tension during knee flexion in the anaesthetised
patient, no hamstring force was simulated.
Each joint was pre-conditioned in the rig by the applica-
tion of ten cycles of flexion from full extension to 150˚ (at
30˚ per second). The angle of flexion of the joint was deter-
mined using a calibrated rotary-angle potentiometer
attached to the rig. In each specimen, measurements of the
length of the MCL and LCL were obtained at 15˚ incre-
ments between 0˚ and 150˚ flexion. These measurements
were then repeated with two further cycles of flexion.
The knee was then removed from the rig and a medial
parapatellar arthrotomy with osteotomy of the tibial tuber-
osity was performed to gain access to the knee joint. The
osteotomy was fixed with two small fragment screws, and
the arthrotomy closed with 1-vicryl sutures. The Low Con-
tact Stress rotating platform instrumentation (LCS, DePuy,
Warsaw, Indiana) was used to prepare the bone cuts. Both
cruciate ligaments were resected. The femur was divided
using an intramedullary guide in 5˚ of valgus. Tibial resec-
tion was performed with a 7˚ posterior slope referenced
from an intramedullary guide, as the ankle was not present.
There was no significant coronal bowing of any of the tib-
iae. The width of the flexion and extension gaps was mea-
sured using digital callipers while applying a 100 N
distraction force to the distal tibia in full extension and 90˚
of flexion.
In this study two femoral components were used. The first
was the standard LCS femoral trial, which was press-fitted to
the divided distal femur. This component was used to obtain
measurements of baseline force and ligament strain for a
standard implantation technique with balanced flexion and
extension gaps (balanced gap (BG) series). The second com-
ponent was a custom-designed LCS femoral component
mounted on an intramedullary rod (Fig. 2). The geometry of
the articular surface was identical to that of the trial
Fig. 1
Photograph of a cadaver knee joint mounted in a customised passive
movement rig.
Fig. 2
Photograph of the adjustable low contact stress femoral component
in situ.

1530 B. JEFFCOTE, R. NICHOLLS, A. SCHIRM, M. S. KUSTER
THE JOURNAL OF BONE AND JOINT SURGERY
prosthesis. This device allowed the articular surface to be
translated proximally or distally in known increments, thus
allowing accurate adjustment of the extension gap. The
adjustable femoral component was implanted aligning the
component with the previous femoral cuts. In order to
increase the extension gap (EG) by 2 mm (EG+2 series) the
adjustable femoral component was translated 2 mm proxi-
mally. In order to increase the flexion gap (FG) by 2 mm
(FG+2 series) the adjustable femoral component was trans-
lated 2 mm distally while the tibial component was also
simultaneously translated 2 mm distally, with the effect of
increasing the flexion gap by 2 mm relative to the extension
gap.
In place of the standard LCS tibial tray, a miniature force
platform was implanted (Fig. 3). This was designed, cali-
brated and validated in our laboratory, and was con-
structed to allow the trial LCS rotating platform insert to
rest on the platform and rotate a central hub. A brass base-
plate was constructed to house six load sensors arranged in
triangular arrays in the medial and lateral compartments.
These sensors allow the measurement of compressive loads
(tibiofemoral force) in each compartment individually. The
system was linked to a PC notebook computer via a
National Instruments SC-2345 connector block and 6024E
12-bit data acquisition card (National Instruments, Austin,
Texas). The force platform was implanted by mounting it
on an intramedullary rod which could be adjusted to trans-
late the platform proximally or distally. The rod was fixed
within the tibial shaft using three screws and a wire stirrup.
The force platform was stabilised on the cut tibial surface
using metal shims, small metal wedges 10 mm in length and
2 mm maximum height, manufactured at the Fremantle
Hospital Biomedical Services Department.
After implanting the components, an identical loading
regimen was undertaken for each condition (BG, EG+2,
FG+2). After pre-conditioning, two cycles of flexion were
performed with measurements of tibiofemoral force and
ligament strain obtained at 15˚ increments between 0˚ and
150˚ knee flexion.
Results
Tibiofemoral force. The mean tibiofemoral force measure-
ments obtained from the force platform for the three meas-
urement series are shown in Figure 4. The main variations
of force occurred in full extension and beyond 90˚ of knee
flexion.
The mean tibiofemoral force in the balanced gap group
was 50 N (SD 26.6 for five specimens) at full extension. This
was a starting point chosen to reproduce the force that
might be expected in a TKR during implantation. The mean
force between 15˚ and 75˚ of flexion in the BG series was
15.5 N (SD 9.6), before rising in an exponential manner to
a peak at a mean of 175 N (SD 104) at 150˚ of flexion.
For the FG+2 group the mean force at full extension was
slightly less at 37 N (SD 18.9) and remained less than 15 N
(SD 9.7) until 90˚ of flexion. Thereafter, the force again
rose exponentially but less steeply than in the BG series, peak-
ing at a mean of 107 N (SD 65.1) at full flexion. This amounted
to a mean force reduction of 40% beyond 90˚ of knee flexion.
The mean force at full extension was also lower for the
EG+2 group at 27 N (SD 18.6), but the force from 15˚ of
flexion to 150˚ of flexion was similar to the BG series.
Collateral ligament strain. The measurements of collateral
ligament length prior to implantation of a prosthesis were
used as a baseline. The mean strains in the anterior MCL
and LCL are shown in Figures 5 and 6.
The anterior portion of the MCL recorded very little
change in length throughout the flexion range in the intact
knee. The mean peak strain was 1.7% (SD 2.4) at 90˚ flex-
ion, with a mean minimum strain of -0.8% (SD 3.9) at 150˚.
The LCL showed a very different strain pattern, with little
change in length over the first 60˚ of flexion followed by
progressive loosening to a mean minimum strain of -15.3%
(SD 4.5) at 150˚.
Fig. 3
Photograph of a customised miniature force platform designed for
measurement of uniaxial compressive tibiofemoral force in vitro.
Fig. 4
Graph showing the mean values for tibiofemoral force during flexion for
the balanced gap (BG), extension gap +2 mm (EG + 2) and flexion gap +
2 mm (FG + 2). The error bars show the range of results at each flexion
angle across five specimens.
Force (N)
250
200
150
100
50
0
0153045607590105120 135 150
Flexion angle (o)
BG
EG+2
FG+2

THE VARIATION IN MEDIAL AND LATERAL COLLATERAL LIGAMENT STRAIN AND TIBIOFEMORAL FORCES 1531
VOL. 89-B, No. 11, NOVEMBER 2007
After implantation of the components with BG, the mean
anterior MCL strain at full extension and full flexion was
slightly increased compared with the measurements in the
intact knee (1.6% strain, SD 1.8). The LCL was slightly
looser than the pre-implantation measurements at full
extension with a mean of -1.5% (SD 2.1) strain, but as the
knee flexed the slackening of the LCL was delayed to 90˚ of
knee flexion and less marked than in the intact knee. The
mean minimum strain was -7.0% (SD 3.8) at full flexion
after implantation of the components.
In the FG+2 group, the ligament strains were very similar
to those in the BG group. In the EG+2 group the ligament
strains were approximately 2% lower near extension than
in the balanced knee. This was the comparison of the mean
strain at extension in the BG group compared with the
mean strain in the EG+2 group. Otherwise no obvious
differences were detected.
Discussion
We believe this is the first study to examine the tibio-
femoral forces and collateral ligament strain for variations
in flexion and extension gaps. The behaviour of the collat-
eral ligament strain was not greatly different with the var-
iations, but the changes in tibiofemoral force were. The
non-linear stress-strain behaviour and complex anatomy
of the knee ligaments, combined with the limitations of
the strain measuring device, made collateral ligament
strain measurement less sensitive than with force trans-
ducers to changes in soft-tissue tension. The force trans-
ducers were able to detect force generated not only from
the collaterals but also from tension in the extensor mech-
anism, retinaculum and capsule, allowing a more compre-
hensive measurement of the soft-tissue balance. The effect
of the mobile-bearing articulation on the collateral liga-
ment strain in this study is difficult to identify clearly, but
we believe that the relative insensitivity of the measure-
ments of ligament strain compared with the force trans-
ducers would apply equally to fixed- as well as mobile-
bearing prostheses.
Precise balancing of flexion and extension gaps is at
times difficult to achieve at operation.20 Surgeons may have
to accept slight discrepancies in balance and it is, therefore,
important to know the effects of small variations in gap
ratio on the soft-tissue tension.
The results of the balanced gap group show that the com-
prehensive force within the cadaver knees decreased in
early flexion, remained low throughout mid-flexion, and
then increased exponentially after 90˚. When the extension
gap was increased by 2 mm relative to the flexion gap, there
was a decrease in the tibiofemoral force in full extension
only. Beyond 15˚ of flexion the force and ligament strains
were essentially the same as in the balanced gap group.
When the flexion gap was increased by 2 mm relative to the
extension gap the results were markedly different. The
tibiofemoral force in full extension was hardly influenced
(mean 37 N vs 50 N), and during mid-flexion the tibio-
femoral force was similar to the other two groups. Beyond
90˚ of flexion, however, the force was reduced by a mean of
40%. This finding suggests that small variations in the flex-
ion or extension gaps have little effect on the soft-tissue ten-
sion between 15˚ and 100˚ of flexion, which is the range of
movement achieved by most patients after TKR. However
in deeper flexion, ligament tension is sensitive to changes in
the flexion gap. Increasing the flexion gap by as little as
2 mm may have benefits in terms of reduced generation of
tibiofemoral force beyond 90˚ of flexion.
0153045607590105120 135 150
Strain (%)
4
6
2
0
-2
-4
-6
Flexion angle (o)
Intact
BG
EG+2
FG+2
Fig. 5
Graph showing the mean values for medial collateral ligament strain dur-
ing passive flexion for three total knee replacements compared with the
baseline (intact knee strain) (BG, balanced gap; EG + 2, extension gap + 2
mm; FG + 2, flexion gap + 2 mm). The error bars show the range of results
at each flexion angle across five specimens.
0153045607590105120 135 150
Strain (%)
5
0
-5
-10
-15
-20
Flexion angle (o)
Intact
BG
EG+2
FG+2
Fig. 6
Graph showing the mean values for lateral collateral ligament strain dur-
ing passive flexion for the total knee replacements compared with the
baseline (intact knee strain) (BG, balanced gap; EG + 2, extension gap +
2 mm; FG + 2, flexion gap + 2 mm). The error bars show the range of
results at each flexion angle across five specimens.

1532 B. JEFFCOTE, R. NICHOLLS, A. SCHIRM, M. S. KUSTER
THE JOURNAL OF BONE AND JOINT SURGERY
Although the benefits of a balanced flexion and extension
gap are based on sound theory, there are no published bio-
mechanical data to confirm this recommendation. There
are good reasons to question this traditional approach.
Several studies, including this one, have shown a non-iso-
metric behaviour of the LCL,21 and to a lesser extent the
MCL.19,22 Hence, balancing the gaps at 0˚ and 90˚ of flex-
ion will not necessarily provide ideal soft-tissue tension in
deeper flexion. Indeed, inferring from the present data, an
increased flexion gap of 2 mm reduces soft-tissue tension by
approximately 40% in deep flexion. Although equal flexion
and extension gaps will obtain knee flexion up to approxi-
mately 110˚, it seems advantageous to increase the flexion
gap slightly when deep knee flexion is a goal. There is also
clinical evidence in the literature to support this hypothesis.
In a clinical trial with bilateral TKRs it was shown that
patients consistently preferred the laxer knee, which tended
to provide an increased range of movement.23 Several other
authors have suggested that an increased flexion gap might
improve the post-operative range of movement.24-26
Our study showed that a looser flexion gap does decrease
soft-tissue tension beyond 120˚ of knee flexion. This is in
contrast to the work of Bellemans et al,27 who emphasised
the importance of a sufficient posterior condylar offset.
Whereas reduced condylar offset can induce posterior
tibiofemoral impingement, an increased offset tightens the
flexion gap and increases soft-tissue tension in flexion. As
both mechanisms seem to restrict movement, a balance
between condylar offset and soft-tissue tension in flexion
must be achieved. Furthermore, femoral rollback during
flexion can reduce the influence of the posterior condylar
offset on tibiofemoral impingement. Indeed, in the study by
Bellemans et al27 the kinematic analysis demonstrated a
parodoxical roll forward of the femur in flexion. Hence, the
patients in their series depended on a high condylar offset
to avoid impingement, whereas patients with sufficient fem-
oral rollback are less reliant. In most current designs of
TKR the tibial insert is concave. In order to allow the femur
to roll back the ligaments must loosen in flexion to accom-
modate the necessary superior translation. This can be
obtained by an increased tibial slope and a slightly looser
flexion gap. Kinematic in vitro and in vivo analyses are
necessary to give further insight into the complex and at
times conflicting interactions between the amount of
condylar offset, tibial slope, soft-tissue tension, and their
influence on femoral rollback and tibiofemoral impinge-
ment. The lack of kinematic data is therefore a limitation of
the current study.
The potential disadvantage of increasing the flexion gap
is that of instability in flexion, which in some cases can
require revision surgery.14 However, we do not believe that
symptomatic instability in flexion is likely when increasing
the gap by 2 mm, but to consistently obtain a 2 mm
increased flexion gap and to avoid instability, optimal ten-
sion is needed.
We chose to use a mobile-bearing posterior cruciate liga-
ment (PCL) sacrificing prosthesis in this study. Although
this is not representative of all TKR designs, it is a com-
monly-used implant with a long and successful track
record.28 The effects of a relative increase in the flexion gap
are unlikely to be specific to this individual design of pros-
thesis. We anticipate that TKR designs with a concave tibial
insert, such as most PCL-sacrificing, PCL-substituting and
medial pivot designs, may demonstrate an increased range
of flexion with a looser flexion gap. The effect of an
increased flexion gap on PCL-retaining designs, which gen-
erally have flatter tibial inserts, is difficult to anticipate
from this study. A recent study showed results that support
a relatively loose flexion gap also for PCL-retaining
knees.29
This study was only able to evaluate compressive tibio-
femoral force generation in the unloaded cadaver knees. It
does not take account of the generation of shear forces
within the knee, which could be significant, especially in a
lax or unstable implantation. Also, the compressive forces
are low compared with the forces expected within the knee
during normal walking. However, the generation of tibio-
femoral force in deep flexion is a reflection of the soft-tissue
tension around the knee. This is likely to be a restriction to
flexion in the knee replacement in vivo. Given that increas-
ing the flexion range of TKRs is a widely-held goal, small
increases in the flexion gap relative to the extension gap
may be a method worth investigating further.
No benefits in any form have been received or will be received from a commer-
cial party related directly or indirectly to the subject of this article.
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