Proceedings of BIO2006
2006 Summer Bioengineering Conference
June 21-25, Amelia Island Plantation, Amelia Island, Florida, USA
BIO2006-XXXXX
KNEE CARTILAGE CONTACT DETERMINATION USING WEIGHTBEARING MRI
Peter J Barrance, Thomas S Buchanan
Center for Biomedical Engineering Research
University of Delaware
Newark, DE 19716
INTRODUCTION
Injuries to the knee joint can alter the functional positioning of
the tibiofemoral joint, and hence the loading of the associated contact
surfaces. Such changes have been implicated in theories of the
etiology of osteoarthritis that often follows anterior cruciate ligament
(ACL) rupture. Assessment of the relative importance of such effects
requires the ability to make precise measurements of joint and
cartilage positioning. The potential for magnetic resonance imaging
(MRI) in this has been recognized in earlier studies that have used
closed and open scanners to study knee kinematics. Here, we build on
our earlier work using an open scanner that allows scanning during
weightbearing. We introduce a method to calculate the location of
cartilage contact areas relative to coordinate systems that are placed
consistently through changes in experimental conditions, and use it to
describe the joint positioning and contact changes in the knees of an
ACL-deficient subject.
NY) can be varied continuously between the horizontal position and
nearly vertical, changing the fraction of bodyweight that loads the
joint. After scan localization, sagittal plane images were acquired for
each knee in conditions of minimal weightbearing (angle of table from
horizontal = 20°) and high weightbearing (table angle =85°) (Fig. 1).
Thirty slices, spaced 3.3 mm between centers, with 3mm slice
thickness and a 25 cm field of view, were acquired for each knee at
each table angle.
Digitization of bone and cartilage
Distal femoral and proximal tibial bone surfaces, as well as
medial and lateral compartment cartilage surfaces were digitized in
each scan independently by two operators. A digitizing tablet was used
by the first operator, whereas a digitizing LCD screen was used by the
second. Each operator digitized only bone and cartilage surfaces that
could be defined clearly in the images. Custom software was used to
project the digitized points into physical space coordinates, based on
the scaling and position information in the header of each image.
MATERIALS AND METHODS
Scanning
A 46 year old male subject, who had sustained a complete tear to
one anterior cruciate ligament six months prior to examination, was
recruited for this study. Before examination, the subject read and
signed an institutionally approved statement of informed consent. All
scans were performed at 20° knee flexion - an angle that requires
active muscle contraction for weight support, yet is well tolerated by
subjects for the scan duration required (approximately 6.5 minutes).
Before the experiment, the 20° flexion position was determined using
a goniometer to measure between bony landmarks of the hip, knee,
and ankle. This position was reproduced in the scanner by alignment
of lines drawn on the medial ankle. The inclination of the patient table
of the scanner used (0.6T Upright MRI, Fonar Corporation, Melville,
Cartilage proximity calculation and display
The distance of the closest point on the femoral cartilage to each
point on the tibial cartilage was calculated using a tree-sort based
closest point algorithm implemented in Matlab (The Mathworks,
Natick, MA). Regions of contact were defined over areas of femoral
cartilage that lay within a threshold distance of 2 mm from the tibial
cartilage, and centers of contact were calculated as the centroid of all
points within this region. For display purposes, a mesh model of the
tibial cartilage surface was constructed from the digitized points, and
this was color-coded and rendered to indicate cartilage proximity (Fig.
2).
1
Copyright © 2006 by ASME
Coordinate system placement and matching
For comparison of contact point center locations between knees
and loading conditions, it was important to describe these locations in
a coordinate system that was placed consistently relative to the
anatomy. A reference tibial plateau coordinate system was first
defined by a single operator relative to a model of the tibia from one
scan set (uninjured knee, 20° flexion). This system was assigned using
a medial/lateral axis that bisected points digitized on the medial and
lateral margins of the tibial plateau, an anterior/posterior axis that was
parallel to a line connecting two points digitized on the anterior and
posterior margins in the mid-medial compartment of the tibial plateau,
and superior/inferior axis that was perpendicular to both of those axes.
A point-matching method, implemented in Matlab, was then used to
find optimal mapping transformations of the reference tibial bone
surface points to those in each of the other experimental conditions.
Lateral symmetry of bone surfaces was assumed, in order to allow
mapping between sides by reversal of the medial/lateral coordinate.
These mapping transformations were used to transform the reference
coordinate system into correspondence with the bone in the remaining
three experimental conditions, after which the coordinates of the
contact positions were calculated were calculated relative to this
system. Additionally, joint positioning parameters were calculated [1],
after using a similar method of coordinate placement and matching for
the femur.
a
b
Figure 1: Sample images from the ACL-injured knee. (a)
Table set at 20° from horizontal (minimal weightbearing)
(image has been rotated to vertical orientation for
comparison). (b) Image acquired with the table near
vertical (high weightbearing). Anterior subluxation of the
tibia is evident and highlighted by the white arrow.
a
b
RESULTS
Table 1: Positioning parameters and centers of tibial
cartilage contact. UN: Uninjured side, IN: ACL-injured side.
20/90°: table angle; ∆: change from 20° to 90°. Flexion,
Varus, Int. rot, A/P: joint positioning parameters; ‘Med/Lat
C’: A/P coordinate of center of contact on medial/lateral
compartment. ‘X/Y’: X – avg. of operators’ results, Y difference between operators’ results.
UN
20°
90°
IN
20°
90°
∆
∆
Flexion
(°)
16.3
14.5/2.4
-1.8/2.4
12.1/2.3
12.7/1.0
0.6/1.3
Varus
(°)
-2.6
-2.3/0.5
0.3/0.5
-3.5/0.6
-5.1/0.3
-1.6/0.4
Int.rot
(°)
2.0
6.2/0.4
4.2/0.4
-0.8/0.9
-0.7/0.2
0.1/0.7
A/P
(mm)
-1.8
3.4/0.4
5.2/0.4
0.7/0.1
9.4/2.1
8.8/2.1
Med C
(mm)
3.0/0.1
0.5/0.1
-2.5/0.0
-0.6/0.0
-7.2/1.2
-6.6/1.2
Figure 2: Tibial plateau models, coordinate systems, and
cartilage proximity distributions for the ACL-deficient
knee in (a) minimal weightbearing, and (b) high
weightbearing conditions. Cartilage proximity (mm) is
indicated by color coding according to scale at right.
White dots denote locations of estimated centers of
cartilage contact in each compartment.
Lat C
(mm)
-1.2/0.5
-3.7/0.5
-2.5/1.0
-0.2/0.1
-4.4/0.5
-4.2/0.6
with tibial anterior translation, posterior migration of the medial and
femoral contact centers of the tibial cartilage were recorded (Fig. 2);
however, these displacements were in all cases smaller than the same
knee’s anterior translation. Significant internal rotation was recorded
in the transition to weightbearing in only the uninjured knee.
Technique repeatability
The differences in parameter values between operators, as
indicated by the second number in each pair in Table 1, are below 2.5°
for all angular variables, and below 2.5 mm for all positional variables.
The changes in transition from low weightbearing to high
weightbearing in the descriptors of tibial displacement and contact
position (i.e. A/P, Med C, Lat C) are several times higher than the
inter-operator differences, and the technique appeared sufficiently
robust to these differences to allow comparison of measurements of
tibial contact position.
DISCUSSION
These early results demonstrate the potential of this technique to
provide measures of tibiofemoral joint contact conditions in the
presence of true physiologic loading. Operator-related effects on the
results were small compared with the consistent changes seen. More
extensive validation and development work on the technique is in
progress. A potentially deleterious increase in anterior tibial translation
and posterior migration of contact regions on the tibial cartilage were
measured in the injured knee of this ACL-deficient subject.
Tibiofemoral positioning and contact
The flexion angles recorded from the analysis were lower than
the 20° flexion intended. This discrepancy may be a result of the
somewhat imprecise nature of goniometer-based positioning, as well
as an effect of bone coordinate system choice. Little change in flexion
angle was seen in each knee in the transition to higher weightbearing.
Anterior translation after the application of weightbearing was seen in
both knees, and was larger in the ACL-deficient knee. Concomitant
REFERENCES
[1] Grood, E. S., and W. J. Suntay, 1983, "A Joint Coordinate
System for the Clinical Description of Three-Dimensional
Motions: Application to the Knee," J Biomech Eng, 105(2), pp.
136-44.
2
Copyright © 2006 by ASME