Lawrence L Buchan1,2, Honglin Zhang1,2, Mark Harmon3, Elaine Ni Mhurchu3, Morgan Barber4, Shannon Jennifer Patterson5, Jacek Kopec4, Hubert Wong6, John Esdaile4, Jolanda Cibere4, Charles R Ratzlaff4, Bruce B Forster3, and David R Wilson1
1Orthopaedics, University of British Columbia, Vancouver, BC, Canada, 2Centre for Hip Health and Mobility, Vancouver, BC, Canada, 3Radiology, University of British Columbia, Vancouver, BC, Canada, 4Arthritis Research Canada, Richmond, BC, Canada, 5Vancouver Coastal Health, Vancouver, BC, Canada, 6University of British Columbia, Vancouver, BC, Canada
Synopsis
In this study, we describe a novel open MRI method for evaluating femoroacetabular
impingement (FAI) in subjects with and without symptomatic FAI. First, we acquired images of subjects in postures suspected of inducing impingement using a T1-weighted gradient echo MR sequence. Second, readers measured β-angle to quantify clearance
between the femur and acetabulum. Reader agreement for β-angle was equivalent
or better to agreement when measuring α-angle, a similar structural measure. The open MRI method has the potential to assess functional relationships between
the femur and acetabulum that otherwise cannot be assessed with closed-bore MRI
scanners and conventional imaging protocols. This approach may be
useful in addressing fundamental questions in FAI.Purpose
Femoroacetabular impingement (FAI) is a pathomechanical
process in which bony deformities cause hip pain and potentially osteoarthritis
(OA).1 However, a high proportion of hips
with FAI deformities do not develop pain or OA,2–6 indicating that the
biomechanics and pathophysiology of FAI are not fully understood.7
Open MRI represents a promising direction for exploring the
FAI mechanism because the open configuration allows imaging of hips in the postures
suspected of inducing impingement.8 Also, recent work in cadavers
established that open MRI measures of impingement relate to contact mechanics,9 demonstrating the potential
for direct imaging to explore the biomechanics of FAI.
Further study of FAI requires a validated, quantitative method
for assessing the relationship between the femur and acetabulum in postures
suspected of inducing impingement. Our objectives were: a) to describe a
non-invasive, quantitative method for imaging the FAI mechanism in vivo; and b) to evaluate the
reliability of this method.
Methods
We included 63 unilateral hips for this study (Table 1) from
a random population-based sample of 700 subjects and grouped each hip as
symptomatic FAI, asymptomatic FAI, or control. We defined FAI using α-angle
above 55° on Dunn view x-ray, centre-edge angle above 40° on A-P x-ray, or
cross-over sign.10 Pain was defined using physical
examination by 9 clinicians.11
We
assessed impingement using a 0.5T upright open MRI scanner (MROpen, Paramed;
Genoa, Italy), that allowed imaging of participants in functional postures. We
placed a 1-channel RF send-receive coil around each subject’s study hip and
then positioned the hip in an anterior impingement posture of 90° flexion,
internal rotation, and adduction (Figure 1). With hips positioned in an
anterior impingement posture, we acquired images using a T1-weighted gradient
echo sequence (256x256 matrix; 220x220mm FOV; 12 slices; 5mm thickness, 1mm
gap; 1 excitation; 12ms/333ms TE/TR; 60° flip angle; 2min 13sec scan time)
through the femoral neck axis, orthogonal to the femur’s coronal plane.
We quantified clearance between the femoral head-neck
junction and the acetabular rim for each hip using Wyss’ β-angle12 (Figure 2), a measure shown
to be related to force on the acetabulum in hips in an impingement posture.9
We evaluated inter-reader agreement of the
β-angle measurement protocol for two readers (MSK radiology fellows). Images were
read in a randomized order with the reader blinded to all subject data. To
establish a baseline, we conducted a training session where the two readers and
an experienced MSK radiologist read 15 pilot images by consensus. We then evaluated
intra-reader agreement of one reader who repeated the readings for a random
subset (21 images) after four weeks. We reported inter- and intra-reader agreement
using both root-mean-square (RMS) β-angle error between trials and intraclass
correlation coefficient (ICC, two-way random single measure) for each group. Readers also measured the α‑angle (widely used in studies of
FAI) to establish baseline angle measurement errors.
Results
For β-angle measurements, both the inter- and intra-reader
RMS errors were less than 3° and ICCs were greater than 0.9 for all hips (Table
2) as well as for each subgroup (FAI+pain, FAI-pain, control) (Table 3). Agreement
was not affected by the presence of FAI or pain. In comparison, for α‑angle
measurement, the inter- and intra-reader RMS errors were 1.9-2.5° higher than
for β-angles, while ICCs were less than 0.9.
Discussion
This study established that reader agreement for β-angle in
an impingement posture is at least equivalent to the α-angle reader agreement
on the same images, and equal or better than α- and β-angle agreement reported
in FAI literature13,14. The key advantage of this open MRI method is that
β-angle is a functional measure that depends on both anatomical structure and
joint position, allowing readers to identify how close the femur is to
impingement against acetabular chondrolabral structures.
One application of the β-angle in the
impingement posture is to identify if hips have an 'impingement sign' (defined as β < 0°; found to be associated with abnormal joint mechanics9). Given
that the mean β-angle in the impingement posture was -9.9° for cadaver cam hips,9 our finding of an RMS error of less than 3° for β-angle in vivo suggests that the impingement sign definition
will be robust.
Conclusion
The β-angle
can be measured in an impingement posture in an open MR scanner with
comparable reliability to measurements of α-angle. Our findings support use of the open MRI protocol to assess functional
relationships between the femur and acetabulum that cannot be assessed with
current conventional imaging protocols. This open MRI approach has the potential to address fundamental questions in FAI.
Acknowledgements
Canadian Institutes for Health Research, Centre for Hip Health and Mobility, Arthritis Research Centre of Canada, and the Investigations of Mobility, Physical Activity, and Knowledge in Hip Pain (IMPAKT-HiP) team.References
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