Estimation of the Extent of Residual Osteonecrosis in the Resurfaced Femoral Head using 3D-MRI
+1Nakasone S, 1Takao M, 1Nishii T, 1Sakai T, 2Nakamura N, 1Yoshikawa H, 1Sugano N
+1Department of Orthopaedic Surgery, Osaka University Graduate School of Medicine, Osaka, Japan
2
Center of Arthroplasty, Kyowakai Hospital, Suita, Osaka, Japan
[email protected]
INTRODUCTION:
Indication of hip resurfacing for patients with osteonecrosis (ON)
remains controversial. Some authors have reported no significant
difference in survivorship using the current-generation hip resurfacing
implants between ON and osteoarthritis (OA). Others have reported
worse results of hip resurfacing for ON than those for OA. The extent of
residual ON in the resurfaced femoral head is considered to be a critical
factor for the success of hip resurfacing. However, there is no reliable
method which could assess the extent of residual ON in the resurfaced
femoral head, preoperatively. Thus we have developed 3D MRI
planning system for resurfacing arthroplasty for ON.
The purpose of this study was to establish a simple method to
evaluate residual ON volume using 3D-MRI based planning system.
METHODS:
31 patients of 36 hips affected with ON were included in this
study. There were 22 men and 9 women with a mean age of 39 years
(range, 20 to 59). ON was related to steroid use in 20 hips and related to
alcohol abuse in 16 hips. Of the 36 hips, 31 were stage 3 and 5 were
stage 4 according to the Association of Research Circulation Osseous
international classification.
3D-MRI data of bilateral hips was acquired using a 1.0-Tesla MRI
system (SIGNA Horizon LX 1.0T; GE Medical Systems, Milwaukee,
WI, USA). 3D T1-weighted images were obtained using a 3D spoiled
gradient recalled echo pulse sequence.
Preoperative planning for hip resurfacing was performed with a
original 3D templating software that can show multiple planar
reconstructed images through any orthogonal planes. Oblique coronal
and sagittal planes of the femoral neck were reconstructed through the
femoral neck axis as follows. First, the center of the femoral head was
defined as the center of the sphere fitting to the subchondral bone of
non-collapsed part of the femoral head. The center of the femoral neck
was defined as the center of the sphere fitting to the inner cortex of the
femoral neck at its isthmus. The line passing through the center of the
femoral head and the center of the femoral neck was defined as the
femoral neck axis. Then, the center of the proximal femoral canal was
defined as the center of the sphere fitting to the femoral canal at 15cm
distally from the femoral head center. The plane consisting of the
femoral neck axis and the center of the proximal femur was defined as
the oblique coronal plane of the proximal femur, and the plane
perpendicular to the oblique coronal plane though the femoral neck axis
was defined as the oblique sagittal plane of the femoral neck (Fig 1).
CAD models of implants were placed on these planes and the head
centers were matched. The stem of the femoral component was tilted so
that the femoral stem axis aligned to the femoral medial cortex on the
coronal plane without notching. The anteversion of the femoral
component was determined to be parallel to the femoral neck axis on the
oblique sagittal plane.
Fig.1
Reconstruction of two
orthogonal planes along the
femoral neck axis.
The residual ON volume in the resurfaced femoral head was
calculated by multiplying the residual ON lesion areas on serial oblique
coronal planes. The volume of the resurfaced femoral head was
calculated by multiplying the bony areas within the femoral component
on serial oblique coronal planes. The volume percentage of residual ON
in the resurfaced femoral head was calculated as follow (Fig. 2).
Volume percentage of residual ON
= (residual ON volume/ resurfaced femoral head volume) ×100
The two simple methods were assessed. One is the areas
percentage of ON on two orthogonal planes through the femoral neck
axis (Method 1). The areas of the femoral head were determined by
segmenting the femoral head proximally to the head-neck junction (A
and C). And the areas of ON were measured by segmenting areas of low
signal intensity or areas of normal fat signal intensity surrounded by low
signal intensity bands (B and D). The areas percentage of ON was
calculated as follows (Fig 3):
Areas percentage of ON = (B/A+D/C) / 2×100
The other is the areas percentage of residual ON on the two
orthogonal planes through the femoral stem axis (Method 2). The areas
of the resurfaced femoral head were measured on the two orthogonal
planes through the femoral stem axis. (E and G). And the areas of
residual ON were measured on the two orthogonal planes by segmenting
residual ON areas (F and H). The areas percentage of residual ON was
calculated as follows (Fig 4):
Areas percentage of residual ON = (F/E+H/G) / 2×100
We evaluated the correlation of the two simple parameters with
the residual ON volume in the resurfaced femoral head using
Spearman’s correlation test.
Fig. 2
Volume percentage of
residual ON of the
resurfaced femoral head
Fig. 3
Areas percentage of ON
of the femoral head
RESULTS:
Fig. 4
Areas percentage of residual ON
of the resurfaced femoral head
The mean areas percentage of ON was 37.1% (range, 12.0 to 78.3).
The mean areas percentage of residual ON was 27.4% (range, 7.3 to
61.0). The mean volume percentage of residual ON was 26.8% (range,
7.9 to 77.2). Both areas parameters correlate well with the volume
percentage of residua ON, while the areas percentage of residual ON
showed higher correlation coefficient (Fig 5).
Fig. 5 Correlation with the residual ON volume in the resurfaced femoral head
DISCUSSION:
Our results showed that volume percentage of residual ON in the
resurfaced femoral head can be assessed by the areas percentage of
residual ON in the resurfaced femoral head on the two orthogonal planes.
The areas percentage of ON in the femoral head showed lower
correlation with the volume percentage of residual ON compared with
the areas percentage of residual ON. This is because the substantial
amounts of the ON and the femoral head were removed by reaming.
Thus, it is necessary to assess the extent of residual ON by simulating
femoral head resurfacing using 3D-MRI.
Poster No. 1195 • ORS 2011 Annual Meeting