Tibial Insert Micromotion Testing of Total Knee Designs

ORIGINAL ARTICLE
Tibial Insert Micromotion of Various Total
Knee Arthroplasty Devices
Safia Bhimji, M.S.,1
Aiguo Wang, Ph.D.,1
and Thomas Schmalzried, M.D.2
ABSTRACT
The objective of this study was to develop a novel method to quantify rotational
micromotion of modular tibial components that incorporates physiologic loading con-
ditions, a physiologic test environment, and constraint characteristics of the articulating
surface. The methodology is reviewed and data are presented on four total knee designs.
Results showed the design with a rotational stabilizing island to demonstrate the most
capability in resisting rotational micromotion for a given reacted torque, followed by a full
peripheral capture device, then a partial peripheral capture device, and then a full peripheral
capture device with a posterior lipped edge. Under walking and stair-climbing loads, the
full peripheral capture device imparts more torque to the insert than the other designs due
to the higher constraint of its articulating surface and thus experiences the most micro-
motion. The rotational stabilizing island device reveals the least amount of motion, due to a
combination of its locking mechanism and a less constrained articular surface.
KEYWORDS: Tibial insert micromotion, total knee arthroplasty, backside wear
Modular tibial components have become com-
monplace in total knee arthroplasties (TKAs). They
allow the surgeon the flexibility during surgery to use
any thickness of insert once the baseplate is in place and,
if necessary, allow for insert exchange during revision.
However, this modularity may allow motion to occur
between the insert and baseplate, which in turn can lead
to backside wear on the insert of some designs.1,2
The
additional source of wear adds to the total amount of
debris created within the joint, which may eventually
induce osteolysis.3
Many studies have focused on quantifying micro-
motion across various knee devices.1,4–7
Parks et al tested
nine different designs in new condition under anterior/
posterior (A/P) and medial/lateral (M/L) loads of up to
400 N.3
Testing was performed in an air environment.
All designs revealed magnitudes of shear motion large
enough to cause fretting at the insert/baseplate interface.
Wasielewski investigated shear motion of three implants
when under cyclic axial loading of 2500 N in an air
environment.7
He found micromotion magnitudes of up
to 25 mm in all designs, with the greatest amounts being
in regions of uncontained polyethylene and in the
direction the insert is engaged with the locking mech-
anism. Engh et al examined shear micromotion of a
variety of designs by testing new components, compo-
nents retrieved at revision, and components retrieved at
autopsy.4
Testing involved applying a 100-N compres-
sive load to the insert in air while cycling A/P and then
M/L loads to 100 N. Results showed micromotion of the
revision and autopsy components to be significantly
higher than that of the new components, illustrating
that locking mechanism instability increases with repet-
itive physiologic loading.
There are some limitations to these studies.
Whereas they do offer a depiction of locking mechanism
1
Stryker Orthopedics, Research and Development, Mahwah, New
Jersey; 2
Saint Vincent Medical Center, Los Angeles, California.
Address for correspondence and reprint requests: Safia Bhimji,
M.S., Stryker Orthopedics–Research and Development, 325 Corpo-
rate Drive, Mahwah, NJ 07430 (e-mail: safia.bhimji@stryker.com).
J Knee Surg 2010;23:153–162. Copyright # 2010 by Thieme
Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001,
USA. Tel: +1 (212) 584-4662.
Received: June 14, 2010. Accepted after revision: August 23, 2010.
Published online: December 6, 2010.
DOI: http://dx.doi.org/10.1055/s-0030-1268696.
ISSN 1538-8506.
153
Downloadedby:IP-ProxyStrykerOrthopaedics,StrykerOrthopaedics.Copyrightedmaterial.

laxities, they do not give an accurate measure of the
functional performance of the devices. Loading param-
eters were simplified to either shear forces or compres-
sive forces and did not reproduce the complex off-axis
loading conditions seen in vivo. They only measured
A/P or M/L micromotion, despite retrieval studies by
Mikulak et al6
and Bradley et al8
that reported evidence
of rotary motion of the insert within the baseplate,
especially with posterior stabilized prostheses. These
studies also did not examine the effect of constraint of
the articulating surface of the insert on micromotion.
Lastly, although most authors presoaked their inserts, all
testing was performed in an air environment, contrary to
the natural fluid environment of the knee joint.
Conditt et al overcame some of these limitations;
translational and rotational micromotion of implants
retrieved at revision were measured in a saline solution
under walking conditions.1
Results were compared with
those of laxity testing using the methods of Engh et al.4
The findings showed that the laxity testing yielded
significantly more micromotion than the physiologic
testing, demonstrating the need for more clinically rele-
vant methodologies.
The objective of the current study was to develop
a novel method to quantify rotational micromotion of
modular tibial components that incorporates physiologic
loading conditions, a physiologic test environment, and
constraint characteristics of the articulating surface.
Specifically, the purpose of this article is to review the
methodology developed and present data generated from
it on four total knee designs.
MATERIALS AND METHODS
Test Samples
Three samples of each knee design were evaluated in this
study. All samples represented the thinnest, medium-
sized components offered by the manufacturer.
Locking mechanism designs varied among the
components (Fig. 1). Design A featured a full peripheral
capture locking mechanism with a posterior lipped edge
and a rotational stabilizing island (Triathlon PS; Stryker
Orthopaedics, Mahwah, NJ). Design B featured a full
peripheral capture locking mechanism (PFC Sigma
Stabilized; DePuy, Warsaw, IN). Design C featured a
full peripheral capture locking mechanism with a poste-
rior lipped edge (NexGen LPS; Zimmer, Warsaw, IN).
Design D featured a partial peripheral capture locking
mechanism with a posterior lipped edge and an anterior
constraint (Genesis II PS; Smith & Nephew, Mem-
phis, TN). A summary of the designs is provided in
Table 1.
Test Apparatus
All testing was performed on a multiaxis servo-hydraulic
test frame (MTS Bionix 858; MTS Corporation, Eden
Prairie, MN) consisting of two actuators: a coupled axial/
torsional actuator oriented vertically to apply compressive
loads and internal/external torques to the components,
and an axial actuator oriented parallel to the floor to apply
shear loads (referred to as the side actuator).
Test Setup
All inserts were soaked in water at 378C for a minimum
of 1 week prior to testing. Three spherical probes were
then rigidly mounted to each insert (two probes anterior
and one probe posterior) and each insert assembled to its
respective baseplate.
The femoral components and insert/baseplate as-
semblies were rigidly mounted to stainless steel holding
fixtures. Six linear variable differential transducers
(LVDTs) (Type DS40AW and DS200AW; RDP
Group, Pottstown, PA) were mounted to the baseplate
fixture. Three were placed on the anterior side, with
two in the superior/inferior (S/I) direction and one in
Figure 1 Implant locking mechanism designs.
154 THE JOURNAL OF KNEE SURGERY/VOLUME 23, NUMBER 3 2010

the M/L direction. The remaining three were placed on
the posterior side, with one in the A/P direction, one in
the M/L direction, and one in the S/I direction. The
LVDTs were positioned such that the plunger contacted
the spherical probes on the inserts.
The insert/baseplate/LVDT assembly was
mounted at a 0-degree slope to a bath and the bath
then mounted to a bearing table connected to the side
actuator of the test frame. The assembly was mounted in
two different orientations (Fig. 2); one such that the side
actuator applied a load in the A/P direction of the
component assembly while allowing M/L motion to be
unrestricted, and the other such that the side actuator
applied a load in the M/L direction while allowing A/P
motion to be unrestricted.
The femoral component was mounted to the
axial/torsional actuator via fixturing that allowed the
flexion angle to be adjusted to the desired setting and
varus/valgus rotation to be unrestricted.
To mimic the in vivo lubrication conditions of the
knee, serum (Alpha Calf Fraction Serum; Hyclone
Laboratories, Logan UT), diluted with water (1 part
serum:6 parts water) was added to the bath such that the
fluid level completely covered the level of tibiofemoral
articulation. The temperature was heated to 378C using
a heater (time to heat was $30 minutes), and circulation
was maintained using a pump.
To initially align the femoral component to the
insert at each flexion angle, a 200-N compressive force
was applied while the femoral component was rotated
through Æ 10 degrees. The insert was allowed to float in
the A/P and M/L directions to find its natural resting
position.
Test Procedure
Resultant joint loads of 2600 N and 3800 N were applied
to the insert using the vector sum of the axial and side
actuators. These loads were chosen to represent max-
imum peak loads throughout the motion cycle of walk-
ing and stair-climbing activities.9
The loads were reacted
through the femur-insert interface. The orientation of
the loads were varied both in the A/P and M/L direc-
tions (Fig. 3). In the A/P direction, loading varied from
17 degrees anterior to 1.5 degrees posterior, with the
anterior orientations simulating the resultant knee load
vectors of various activities including gait, stair climbing,
and rising from a seated position. The posterior orien-
tation was included to simulate resultant load vectors
that could occur in hyperextension due to anterior
impingement of the femoral component on the tibial
post. The M/L orientations ranged from 5 degrees
lateral to 5 degrees medial, representing resultant load
vectors that could occur due to off-axis loading as well as
varus/valgus malalignments at the knee.10
A summary of
the load vectors tested can be found in Table 2.
Each insert was tested in all A/P and M/L loading
conditions at five flexion angles: 0, 15, 60, 90 degrees, and
a hyperextension angle. The first four were chosen to
encompass the range of flexion angles normally achieved
throughout various activities of daily living.11
A hyper-
extension angle was also included because total knees
may be implanted with femoral flexion and/or tibial
posterior slope, resulting in relative hyperextension of
Table 1 Implant/Locking Mechanism Design Summary
Design
Implant and
Manufacturer Locking Mechanism
A Triathlon PS, Stryker Orthopaedics Full peripheral capture locking mechanism with a posterior lipped
edge and a rotational stabilizing island
B PFC Sigma Stabilized, DePuy Full peripheral capture locking mechanism
C NexGen LPS, Zimmer Full peripheral capture locking mechanism with a posterior lipped edge
D Genesis II PS, Smith & Nephew Partial peripheral capture locking mechanism with a posterior lipped
edge and an anterior constraint
Figure 2 A/P loading setup. Figure 3 Load orientations.
TIBIAL INSERT MICROMOTION OF VARIOUS TKA DEVICES/BHIMJI ET AL 155

the components during some phases of the walking
cycle.12
The hyperextension angle tested was design
dependent and represented the angle at which the fem-
oral component first contacted the anterior side of the
tibial insert post when in its natural A/P-M/L resting
position, so-called hyperextension impingement.
Once the resultant load had been applied, the
femoral component was rotated between Æ 5 degrees of
internal/external rotation at 5 deg/s for 5 cycles, and then
incrementally increased to Æ 10, Æ 15, and Æ 20 degrees
for 5 cycles each. A torque limit of 35 NÁm was used to
prevent damage to the inserts. Axial load and displace-
ment, shear load and displacement, torsional torque and
angle, and the six LVDT signals were recorded at 12.5
Hz throughout the loading process.
Analysis of Results
ARTICULAR SURFACE CONSTRAINT
Articular surface constraint was measured at each flexion
angle as the amount of torque required to rotate the
femoral component on the insert from –20 to þ 20
degrees. Comparisons between designs were made by
plotting rotational torque data as a function of femoral
rotation in the 0-degree load orientation condition.
INSERT/BASEPLATE MICROMOTION
In this study, micromotion was defined as the amount of
rotation the insert undergoes within the plane of the
baseplate as a result of torques applied at the articulating
surface. Details on calculation of this motion for the
LVDT data can be found in Appendix A.
MICROMOTION VERSUS REACTIVE TORQUE
For each test condition, insert rotation was calculated
from the LVDT data and graphed as a function of
reactive torque. Figure 4 displays a sample plot for one
test condition (i.e., data for one flexion angle/load
magnitude/load orientation combination). From this
plot, the peak-to-peak magnitudes of insert rotation
and reactive torque were determined for each level of
femoral rotation ( Æ 5, Æ 10, Æ 15, Æ 20 degrees). Scat-
terplots, with best-fit linear regression lines, of these
peak-to-peak magnitudes across all load orientations and
femoral rotations were then generated for each flexion
angle under the 3800-N load condition. A two-factor
ANCOVA with a ¼ 0.05 was used to test for significant
differences between designs across the range of torques
obtained at each flexion angle.
INSERT/BASEPLATE MICROMOTION: EFFECT OF WALKING
AND STAIR-CLIMBING CONDITIONS ACROSS DESIGNS
Because insert/baseplate micromotion causes backside
wear, an investigation of this motion under repetitive
clinical activities that could lead to wear is useful. Two
activities were chosen for analysis: walking, which rep-
resents a high-cycle/low-load activity, and stair climb-
ing, which represents a low-cycle/high-load activity. A
combination of loading conditions that most closely
Table 2 Load Vector Magnitudes and Orientations for A/P Loading
Load Magnitude A/P Load Orientations M/L Load Orientations
2600 N 1.5 degrees posterior, 0, 1.5 degrees
anterior, 10 degrees anterior, 17 degrees
anterior
5 degrees medial, 1.5 degrees medial, 0, 1.5 degrees
lateral, 5 degrees lateral
3800 N 1.5 degrees posterior, 0, 1.5 degrees
anterior, 10 degrees anterior, 17 degrees
anterior
5 degrees medial, 1.5 degrees medial, 0, 1.5 degrees
lateral, 5 degrees lateral
Figure 4 Sample insert rotation versus torque curve. Abbreviations: Pk-pk, peak to peak.

represent these two activities were chosen from the
matrix of conditions tested.
For walking, loading conditions were chosen that
most closely matched the loading profiles specified in the
International Organization for Standardization (ISO)
specification for wear testing of total knee prosthe-
ses.13,14
Table 3 provides a summary of the loads
suggested in this specification for each of the flexion
angles tested. Table 4 provides a summary of the loading
combinations chosen from the test matrix to most closely
represent the parameters of Table 3. A hyperextension
angle was added to Table 4, as many knees are implanted
with a posterior slope, placing the knee into hyper-
extension during some phases of the walking cycle.12
For stair climbing, loading conditions were
chosen that most closely matched those found in the
literature. Table 5 provides a summary of the loads
specified in literature for each of the flexion angles
tested.15
Table 6 provides a summary of the loading
combinations chosen from the test matrix to most closely
represent the parameters of Table 5. Again, the reacted
torques and resulting insert rotations of these loading
conditions were compared across designs.
RESULTS
Articular Surface Constraint
Rotational constraint of each design has been previously
published by the authors.16
To summarize, be-
tween Æ 10 degrees of rotation, results show designs A
(Triathlon) and C (NexGen) to have similar rotational
constraint characteristics at 0, 15, and 90 degrees of
flexion and to be the least constrained of the five designs.
In hyperextension and 60 degrees, NexGen shows lower
constraint in internal rotation. Across all designs, the
design B (PFC) demonstrates the most constraint.
Beyond Æ 10 degrees of rotation, PFC continues
to show high levels of constraint, while Triathlon’s begins
to rapidly increase. In internal rotation, NexGen is the
least constrained, and PFC and design D (Genesis II) are
the most constrained. In external rotation, NexGen and
Table 3 Loading Parameters from ISO Standard for Wear Testing of Total Knee Arthroplasties, Walking
Flexion
Angle (deg)
Compressive
Force (N)
A/P Force
(N)
Load Orientation
(deg)
Internal/External
Rotation (deg)
0 167.6 0 0 1.6 external
15 2600 109.6 anterior 2 anterior 1.1 internal
60 167.6 47.0 anterior 16 anterior 3.9 internal
Table 4 Loading Combinations Chosen from Test Matrix to Represent ISO Walking Profile
Flexion Angle (deg) Resultant Load (N) Load Orientation (deg)
Internal/External
Rotation (deg)
Hyperextension 2600 1.5 posterior Æ 5
0 2600 0 Æ 5
15 2600 1.5 anterior Æ 5
60 2600 17 anterior Æ 5
Table 5 Loading Parameters from Literature for Stair Climbing
Flexion
Angle (deg)
Compressive
Force (N) Assuming
BW ¼ 91 kg
A/P Force (N)
Assuming
BW ¼ 91 kg
Load Orientation
(deg)
Internal/External
Rotation (deg)
15 3640 346 anterior 5.4 anterior 2.5 external
60 3185 346 posterior 6.2 posterior 2.5 external
90 0 182 anterior 90 anterior 3 external
Abbreviations: BW, body weight.
Table 6 Loading Combinations Chosen from Test Matrix to Represent Stair Climbing
Flexion
Angle (deg)
Resultant
Load (N)
Load Orientation
(deg)
Internal/External
Rotation (deg)
15 3800 10 anterior Æ 5
60 3800 1.5 posterior Æ 5
90 3800 17 anterior Æ 5

Genesis II are the least constrained, and PFC is the most
constrained. These trends vary somewhat across flexion
angles. Note that NexGen did not reach Æ 20 degrees of
rotation at 90 degrees due to deformation of the posterior
lip of the insert condyles at the extreme range of rotation.
Micromotion Versus Reactive Torque
Figure 5 displays an insert rotation versus torque re-
gression plot for hyperextension. Plots for the remaining
flexion angles are also shown; however, for purposes of
clarity, only the regression lines are shown. R2
values for
each of these regressions can be found in Table 7.
Results demonstrated that increasing torques re-
acted at the articulating surface of the insert are trans-
lated to the baseplate interface, resulting in increased
rotations of the insert. When comparing across designs,
NexGen demonstrated the largest amount of insert
rotation for a given torque at the lower flexion angles,
followed by Genesis II, and then by PFC. At 60 and 90
degrees, Genesis II showed the most motion. Triathlon
demonstrated the least amount of insert rotation of the
Figure 5 Insert rotation versus torque scatterplot: hyperextension, 0, 15, 60, and 90 degrees of flexion.
Table 7 Linear Regression R2
Values
Design
R2
Hyperextension 0- 15- 60- 90-
Design A 0.96 0.95 0.84 0.74 0.54
Design B 0.86 0.78 0.78 0.73 0.51
Design C 0.51 0.72 0.69 0.57 0.70
Design D 0.73 0.85 0.84 0.67 0.23

five designs across all flexion angles for the same torque.
In general, these differences tend to become more
pronounced as a function of torque, with the designs
being similar at levels less than $30 NÁm, and differing
at higher levels. Results of the ANCOVA showed
statistically significant differences between most of the
designs at each angle (Table 8).
Insert/Baseplate Micromotion: Effect of
Walking and Stair-Climbing Conditions across
Designs
Figure 6 displays the reacted torques and resulting
insert rotations of the test conditions that most closely
simulate walking. The graphs display the average and
standard deviation over the three trials of each knee
design. Results showed the range of applied torques to
be lowest for Triathlon, similar for NexGen and
Genesis II, and highest for PFC. Resulting insert
rotations were lowest for Triathlon and highest for
PFC. NexGen demonstrates particularly high motion
when in hyperextension. Similar results were seen for
the test conditions simulating a stair-climb activity, as
shown in Fig.7.
DISCUSSION
The objective of this study was to investigate insert/
baseplate micromotion under physiologic conditions
across four total knee designs, focusing on the effect
of articular surface constraint and locking mechanism
design.
Results showed that induced insert/baseplate mi-
cromotion has a direct linear relationship with the
magnitude of torque reacted at the articulating surface
of the insert. The constraint characteristics of a knee
design thus play an important role in induced micro-
motion. The more constrained a knee is, the more torque
is transferred to the insert/baseplate interface, and hence
the more stress induced in the locking mechanism. This
finding is supported by Bradley et al8
in a study that
Table 8 Two-Factor ANCOVA p Values at Each Flexion Angle
Design A Design B Design C
Hyperextension
Design B 0.980
Design C < 0.005 < 0.005
Design D 0.265 0.554 < 0.005
0 degrees
Design B 0.266
Design C < 0.005 < 0.005
Design D 0.024 < 0.005 < 0.005
15 degrees
Design B 0.551
Design C < 0.005 < 0.005
Design D < 0.005 < 0.005 < 0.005
60 degrees
Design B < 0.005
Design C < 0.005 0.681
Design D < 0.005 < 0.005 0.037
90 degrees
Design B < 0.005
Design C < 0.005 1.000
Design D < 0.005 < 0.005 < 0.005
Figure 6 Reacted torque and micromotion during walking
conditions.

compared wear patterns of 14 retrieved posterior stabi-
lized (PS) components to 13 retrieved cruciate retaining
(CR) components, both of the PFC design. Seventy-one
percent of the PS trays displayed existence of rotary
motion, whereas only 31% of the CR components
displayed the same. Evidence of rotary motion has also
been reported by Mikulak et al6
in an examination of 16
retrieved PS PFC devices. Across the four designs
evaluated in the current study, Triathlon and NexGen,
in general, demonstrated similar constraint character-
istics between Æ 10 degrees of rotation and were the least
constrained of the designs, and PFC was the most
constrained.
To directly compare the performance of each
design’s locking mechanism, the amount of induced
micromotion was evaluated as a function of the magni-
tude of torque reacted at the insert, regardless of the
motions being applied at the articulating surface. Results
demonstrated NexGen and Genesis II had the largest
amount of insert rotation for a given torque, depending
on flexion angle, followed by PFC. Triathlon demon-
strated the least amount of insert rotation across the five
designs for the same torque. This finding is consistent
with the design of Triathlon’s locking mechanism,
which has a rotational stabilizing island not found in
the other devices. The magnitudes of the observed
motions tended to decrease with increasing flexion
angles. A possible explanation could be that because
cam/post engagement occurs at these angles, the inserts
may become pushed up against the anterior lip of the
baseplates, minimizing the amount of potential micro-
motion.
To compare the functional performance of each
design, the effect of loading conditions during two
physiologically relevant activities was also investigated:
walking and stair climbing. Results showed Triathlon
to have the least amount of micromotion, due to a
combination of its rotational stabilizing locking mech-
anism and a less constrained articular surface. NexGen
and Genesis II demonstrated similar amounts of re-
acted torque during both activities, with NexGen
displaying more motion during walking and Genesis
II more motion during stair climbing. PFC, due to its
highly constrained articular surface, revealed the high-
est amounts of reacted torque for each activity across
all the designs and also some of the highest motions. It
should be noted that, because the load magnitudes
investigated in this study represented peak loads for
each activity, an exact match to the referenced loading
conditions at each flexion angle was not possible. In
particular, the load magnitudes used at 0 and 60
degrees for walking and 90 degrees for stair climbing
are higher than the load magnitudes referenced in the
literature at those flexion angles. The relative compar-
isons between the designs, however, are expected to
remain the same.
Comparison of the current results with those of
previous studies is difficult as the majority did not
evaluate micromotion under physiologically relevant
loading conditions, nor did they examine rotation of
the insert. One exception is a study performed by
Conditt et al1
that measured insert rotation under
walking conditions. Although the loading scenarios
varied somewhat from the current study, their results
of 153 Æ 109 millidegrees of motion are comparable with
the results reported here. Conditt et al also compared
their physiologic loading results with testing that meas-
ured locking mechanism laxity in an unloaded condition,
a method used by many other investigators. Their find-
ings indicated micromotion to be 8 times larger in the
unloaded condition than in the physiologically loaded
one, stressing the need for more clinically relevant
studies such as the current one.
This study investigated micromotion between the
insert and baseplate of four modular total knee designs.
This motion has been shown to potentially lead to
backside wear on the insert, which may eventually induce
osteolysis.1–3
Correlations between the magnitudes of
motion measured in this study and backside wear need to
be established. Also, results represent micromotion of
brand-new devices and do not account for any break-
down of the locking mechanisms that may occur over the
in situ duration.
CONCLUSION
The locking mechanism of Triathlon demonstrated the
most capability in resisting rotational micromotion for a
Figure 7 Reacted torque and micromotion during stair-
climbing conditions.

given reacted torque, followed by PFC, and then Nex-
Gen and Genesis II. Under walking and stair-climbing
loads, PFC imparted more torque to the insert than that
of the other designs due to the higher constraint of its
articulating surface and thus experiences the most micro-
motion. Triathlon revealed the least amount of motion
due to a combination of its locking mechanism, which
features a rotational stabilizing island not found in the
other designs, and a less constrained articular surface.
Correlations between the magnitudes of motion meas-
ured in this study and backside wear need to be estab-
lished.
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APPENDIX A
In this study, micromotion was defined as the amount of
rotation the insert undergoes within the plane of the
baseplate as a result of torques applied at the articulating
surface. To calculate this motion, a coordinate system
was established on the insert with the x-axis correspond-
ing with the A/P axis, the y-axis with the M/L axis, and
the z-axis with the S/I axis (Fig. A-1). Insert rotation
was calculated about the z-axis using the equations
below and LVDT displacement data of the three spheres
shown (AL, AM, and P):
where are initial positions of
spheres AL, P, and AM along the y- and z-axes before
testing is started; are posi-
tions of those spheres along the y- and z-axes at every
time interval collected; and are distances
between the spheres as illustrated in Fig. A-1.

Then,
where wz is deﬁned as rotation of the insert within the
plane of the baseplate.
Figure A-1 Sphere locations and coordinate system deﬁ-
nition.

Tibial Insert Micromotion Testing of Total Knee Designs

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