Download ACL Inclination Angle on MRI and Femoral Tunnel Angle on AP X

ACL Inclination Angle on MRI and Femoral Tunnel Angle on AP X-ray: A Comparison Study with Tunnel Positions
on 3D CT in Single Bundle ACL Reconstruction.
1
Illingworth, KD, 1Hensler, D, 1Working, ZM, 2Casagranda, B, 1Tashman, S, +1Fu, FH
+1University of Pittsburgh, Pittsburgh, PA. Department of Orthopaedic Surgery
2
University of Pittsburgh, Pittsburgh, PA. Department of Radiology
[email protected]
INTRODUCTION
Anatomical placement of grafts during anterior cruciate ligament (ACL)
reconstruction may lead to better control of rotational stability and help
to preserve long term knee health. However, anatomical tunnel position
can be difficult to validate from magnetic resonance imaging (MRI) or
plane x-rays. The purpose of this study was to investigate whether the
ACL inclination angle and femoral tunnel angle on clinically available
2D images can be used to evaluate anatomic positioning after single
bundle (SB) ACL reconstruction.
MATERIALS AND METHODS
With IRB approval, 50 patients were identified that had SB ACL
reconstructions and post-operative AP x-ray, MRI with intact ACL graft
and CT scans. 50 patients that had native intact ACL on MRI were
identified for a comparison group. Tunnel Position on 3D CT: The CTscans of each patient were imported into Mimics (Materialise, Leuven,
Belgium) for segmentation and then Geomagic Studio (Geomagic,
Research Triangle Park, North Carolina) for 3D model development and
analysis. Tunnel positions of single bundle reconstructions were
analyzed on both the femur (figure 1a) and tibia (figure 1b) using
anatomical coordinate systems and normalized to the AP and ML
dimensions of the tibia or femur, as described by Forsythe et al [1].
and our approximation of the long axis of the femur is used for femoral
tunnel angle measurement as shown in fig 3b.
Figure 3- X-ray Femoral Tunnel Angle
RESULTS
The native ACL inclination angle average was 49.9° ± 2.8°, with a range
from 43-57°. ACL inclination angle on MRI for SB reconstructions
ranged from 46.4° to 79.4° and femoral tunnel angle ranged from 2.1° to
46.1°. There was a strong correlation between anterior tunnel position on
the femur and an increase in ACL MRI inclination angle (p<0.001, r =
0.74) and decrease in x-ray femoral tunnel angle (p<0.001, r = 0.78), as
well as a good correlation between ACL inclination angle and femoral
tunnel angle (p<0.001, r = 0.63).
Figure 1- CT Anatomic Coordinate System and tunnel position.
ACL Inclination angle on MRI: A single sagittal proton density image
best showing the ACL was selected. An estimation of the long axis of
the tibia was determined by selecting the most distal portion of the tibia
and measuring the width of the diaphysis (line a) on a line parallel to the
physeal scar. Another line was measured proximal on the diaphysis half
the distance of the most distal line measured. The midpoint of both lines
was determined and a line bisecting these points was drawn. The tibial
horizontal line (THL) was drawn perpendicular to this line at the most
distal portion of the ACL. The angle between a line drawn parallel to the
anterior most fibers of the ACL (Line d) and the THL was designated as
the ACL inclination angle.
Figure 2- ACL Inclination Angle
Femoral Tunnel Angle on AP radiographs: A line is drawn from the
most proximal points of the femoral epidondyles, which we refer to as
the trans-epicondylar axis (DTE), as shown in figure 3a. Next, the most
proximal portion of the femur is located and the width of the diaphysis is
measured in a parallel fashion to the DTE. Another width of the
diaphysis is measured distally half the distance of the DTE. The
midpoint of both line a and line b is determined. A line is drawn down
the center of the femoral shaft connecting the midpoints. An axis
bisecting the femoral tunnel is drawn and the angle between this axis
DISCUSSION
Forsythe et al. described the normalized femoral footprint of the native
ACL to be between 0.089 and 0.364, from posterior to anterior [1]. The
SB reconstructions that fell within this range had an MRI inclination
angle range of 46.4° - 54.8°, which is consistent with the native MRI
inclination angles measured in this study. Also, these same
reconstructions had an x-ray femoral tunnel angle range of 33.9° - 46.1°.
Although it is possible to have a non-anatomic SB ACL reconstruction
that falls within these ranges of MRI inclination and femoral tunnel xray angle, these simple measurements on routine clinical imaging allow
the surgeon to have a quick an easy measurement that can aid in
determining whether a graft is placed anatomically.
1.
Forsythe et al. (2010). J Bone Joint Surg Am
Poster No. 871 • ORS 2011 Annual Meeting