A New Ratio in Adolescent Hips to Classify Offset
+1Hosalkar, H; 1Glaser, D A; 1Ziebarth, K; 1Karmali, A; 1Wenger, D
+1Rady Children’s Hospital, San Diego, CA,
[email protected]
and the percentage of ratings that differed by >1 mm. In addition,
INTRODUCTION:
ANOVA testing was utilized to determine significant differences
Our understanding of adolescent hips continues to evolve. With
between the mean physeal height-to-width ratios for each rotation point.
advances in management of femoro-acetabular impingement (FAI),
This was repeated for both the pre-fabricated model and the cadaveric
diagnostic capabilities have also improved significantly. In the era of
model. Finally, regression analysis was performed to determine the
three-dimensional modalities such as magnetic resonance imaging
relationship between the physeal height-to-width ration and the degree
(MRI) and computed tomography (CT) scanning, plain radiographs
of femoral rotation. Statistical significance was defined as p ≤ 0.05 and
remain gold standard for the initial evaluation of hip disorders after
95% confidence intervals were calculated.
history and physical exam. Although some feel that plain radiographs do
not provide the detail of three-dimensional modalities such as MRI or
CT, plain radiographs still provide simplicity, availability, affordability,
and minimal risk of radiation.
Understanding and quantifying the femoral head-neck offset is
extremely important in assessment as well as treatment and
prognostication of many hip disorders including impingement and
osteoarthrosis. Adult hip offset can be reliably measured with the alphaangle that is not always identifiable in patients with open physis. We
propose a new index that is a good measurement of offset as well as
easily reproducible.
METHODS:
A bench-model of femur and a second of cadaveric hip were
equipped with radio-opaque markers over prominent locations on the
femoral head and a radiopaque wire attached to the physes. In the first
part of the study, the femur was fixed on a platform; a medical
goniometer defined the angles of rotation while AP pelvic radiographs
were obtained at various rotation angles (Figure 1).
Figure 2: Left: Representation of the rotational axis on the proximal
femur (IR/ER: internal/external rotation; ab/ad: abduction/adduction;
CFH: center of the femoral head). Right: Offset definition
RESULTS:
Both models revealed similar results though the cadaveric model
despite of variation in ab/adduction. The index is a good measure for all
clinically relevant degrees of internal/external rotations (Figure 3).
Figure 3: Proposed index as it minimally changes with varying internal
rotations.
Figure 1: Bench-top model of synthetic proximal femur (0 degrees is the
anatomical zero, negative numbers represent internal rotation, positive
numbers represent external rotation)
In the second part of the study, a human cadaver hip was prepared
similarly allowing for rotation of the femur in the acetabulum closer
representing anatomically correct hip joint movement (Figure 2). In both
cases, anteroposterior radiographs (with the help of the same
radiographer) of the cadaver hip were taken at the neutral/anatomic
position as well as 20, 40, and 60 degrees of internal and external
rotation from the neutral position for a total of 7 radiographs (figure 1).
The neutral position was defined as 20 degrees of external rotation
(normal anatomic rotation), and rotation was measured using a medical
goniometer. Physeal width and the femoral neck width were measured.
To a provide a more clinically relevant measure, rotation was
determined based upon the center of the femoral head (as determined
through the use of a vernier caliper) rather than the femoral axis; which
was utilized in phase I of the study. Amicas PACS imaging software was
utilized by the same board certified orthopaedic surgeon (HH) and
engineer (DG) to measure the physeal and neck width. Measurements
were done on two separate occasions, two weeks apart. The physeal
width-to-neck offset ratio was then calculated at different external and
internal rotations.
For both phases of the study, means and standard deviations for both
physeal height and physeal width were calculated for each rotation.
Intraobserver and interobserver agreement regarding the measurements
of was assessed with use of both the intraclass correlation coefficient
Inter- and intra-observer coefficient approached 1 (average 0.99) which
shows consistency and reproducibility of quantitative measurements
made by different observers measuring the same quantity. Since the
intraclass correlation coefficient does not take into account what is a
clinically meaningful disagreement between individual ratings, a
significant difference in intraobserver agreement was defined as 1 mm.
Only one of the measurements between the observers or between the
different readings resulted to be greater than 1mm.
DISCUSSION:
Understanding the morphology and the anatomy of the adolescent
hips is important to appropriately diagnose and subsequently treat the
hip conditions. FAI is being increasingly diagnosed in the younger
population including adolescents. Surgical treatment does involve
restoration of head-neck offset in these cases either through the
arthroscopic route or with open dislocation techniques. It is therefore
important to have good indices to measure offset accurately for surgical
planning and even for post-operative follow-up. Open physis and the
eccentricity of the capital femoral epiphysis in the adolescent and
younger age-group limits the usage of the traditional Alpha angle (Notzli
angle) measurement in this population. We have therefore proposed a
new index to accurately delineate the femoral head (base) and neck
offset in this population. Based on our biphasic study, it is possible to
identify and define the exact femoral head and neck offset with this
method. Both in saw bones and cadaver model, the index is reproducible
and reliable and mathematically remains a simple equation.
Poster No. 831 • ORS 2011 Annual Meeting