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Abstract
Post-operative
radiographs of 111 patients who had undergone cemented Total Hip
Replacement performed or supervised a single surgeon (GCF) were
analysed. All surgeries were performed via an anterolateral
approach with patients in the lateral position. Patients were
stratified according to Body Mass Index (BMI) into three groups
(healthy, overweight and obese) and acetabular cup angles
measured.
Mean values for cup
abduction angle were similar for all three groups (44.5°, 46.8°,
44.0° respectively, p=0.335). Furthermore there were no
differences in mean cup anteversion between groups (11.6°,
12.2°, 10.7 respectively, p=0.665). We conclude that obesity
does not influence the accuracy of cup positioning in Total Hip
Replacement.
J.Orthopaedics 2007;4(1)e6
Introduction:
Total hip
replacement is one of the most commonly performed adult
reconstructive procedures that provides pain relief, functional
improvement and improved quality of life for thousands of
patients each year worldwide 1.
The accuracy of implantation of total hip replacement (THR)
components is fundamental to the function, stability and the
longevity of the implants 2,3,4.
Whilst the components are inserted as accurately as possible
with reference to anatomical landmarks and various
intra-operative guides, their position is subject to variation
and surgical error 5,6.
Total hip
arthroplasty is performed with the patient in a supine or
lateral decubitus position. When performed with the patient in
the lateral position the pelvis can rotate making positioning of
acetabular component more difficult 7
with some studies citing a tendency to position the cup in less
than ideal anteversion 8 and a more abducted or open
position 3. Some surgeons believe that obese patients
are more likely to be positioned incorrectly on the operating
table and therefore the accuracy of implantation of their
acetabular components is subjected to greater error.
The purpose of
present study is to determine if obesity affects cup anteversion
and abduction angle in THR.
Material and Methods :
Between 1996 and 2005 two hundred and one Total Hip Replacements
using the ZCA® cemented acetabular cup (Zimmer,
Warsaw) were reviewed retrospectively. All surgeries were
performed or supervised by a single surgeon (GCF). All cases
were performed with the patient in the lateral position via an
antero-lateral approach and the same bone cement (Palacos®,
Zimmer, Warsaw) was used in each case. Of these patients 111
were considered to have sufficiently good quality antero-posterior
pelvic or hip postoperative X-Rays available for analysis of cup
anteversion and 72 of these patients had antero-posterior pelvic
radiographs for measurement of cup abduction. Abduction angle of
acetabular cup is the angle between a horizontal reference line
(line joining inferior teardrop points or inferior pubic rami or
inferior points on sacroiliac joint) and the axis of opening of
acetabular cup. On the radiographs of one hip joint the
reference line cannot be drawn and abduction angle cannot be
measured.
All radiographs were scanned to provide high definition TIFF
images and the abduction and
anteversion angles were measured using MATLABâ
software (The MathWorks, Inc., USA). Cup
anteversion angles were measured on either an AP radiograph
centered on the hip (111 cases) or on an AP pelvic X-ray (72
cases). An ellipse of best fit was
plotted onto the radio opaque wire marker of the acetabular cup
and the anteversion angle was then calculated using the software 10,11,12. A correction factor of 4 degrees was
added for anteversion angles measured when pelvic radiographs
centred on the pubis symphysis were used as described by Widmer 13.. For the cases with
pelvic radiographs abduction angle was calculated by plotting a
horizontal reference line between the most inferior point of the
acetabular teardrops. The cup inclination or abduction
angle was then calculated as the angle between a line through
the long axis of the ellipse and the horizontal reference line
(Figure 1).
Patients were stratified into groups for Body Mass Index
according to the World Health Organisation Classification of
obesity 9. All measurements of BMI were taken less
than two weeks pre-operatively. Patients were categorised as:
healthy weight (BMI 18.5-24.9), overweight (BMI 25.0-29.9),
grade I obese (BMI 30.0–34.9), grade II obese (BMI 35.0–39.9) or
grade III obese (BMI≥40.0). The number of patients who were
grade II or grade III obese were small and considered too small
for statistical analysis therefore they were grouped with the
grade one obese group leaving three study groups – healthy (BMI 18.5-24.9), overweight and obese (BMI≥30.0).
Of the 111 patients analysed thirty-nine patients were in the
healthy weight category, 49 were overweight and 23
were obese .(16 grade I obese, 6 grade II obese and 1
grade III obese) Of the 72 patients who had AP pelvic X-Rays to
determine cup abduction angle 26 patients were in the healthy
weight category, 30 were overweight and 16 were obese
(9 grade I obese, 6 grade II obese and 1 grade III obese).
A power calculation was performed which calculated that 7
patients were required in each group to detect a difference of
10 degrees based on a power of 95% and a probability of 0.05 of
a Type I error
Statistical analysis was performed using SPSSâ
(version 13). Differences in mean cup abduction and anteversion
angles between groups were sought using analysis of variance
(ANOVA) with the Bonferroni correction for multiple comparisons.
The significance level was set at 0.05.
Table 1:
Characteristics of the study groups.
|
|
Healthy
(n=39) |
Overweight
(n=49) |
Obese
(n=23) |
|
Mean age (range) |
72.6 (55–87) |
71.1 (55–85) |
68.1 (59 -87) |
|
Male: Female |
17:22 |
26:23 |
7:16 |
|
Left: Right side |
20:19 |
24:25 |
10:13 |
|
Mean BMI (range) |
22.6 (18.9 – 24.9) |
27.1 (25.0– 29.9) |
33.4 (30.0 – 48.1) |
Table 2. Mean cup
abduction and anteversion angles according to BMI
|
Grade of obesity |
Cup abduction angle (sd) |
Cup anteversion angle (sd) |
|
Healthy |
44.5° (7.7°) |
11.6° (6.6°) |
|
Overweight |
46.8° (6.7°) |
12.2° (6.9°) |
|
Obese |
44.0° (7.3°) |
10.7° (6.8°) |
|
p |
0.335 |
0.665 |
Figure 1.
Measurement of inclination and anteversion angles
Results :
Characteristics of
the study groups are outlined in Table 1.
For all 111 patients in the study the mean
cup anteversion angle was 11.7 ± 6.7° sd (range 1.8° to
34.2°) For the 72 patients who had pelvic radiographs centered
on the symphysis mean cup abduction angle was 45.4 ± 7.2° sd
(range 27.1° to 66.0°).
Neither
the cup abduction angle nor the anteversion angle was influenced
by BMI (Table 2). Mean cup anteversion angle was 11.6° for the healthy group, 12.2° for the overweight group and
10.7° for the obese group (p=0.665). Mean cup abduction
angle was 44.5° for the healthy group, 46.8° for the
overweight group and 44° for the obese group
(p=0.335).
Discussion :
The accuracy of
implantation of components in Total Hip Replacement is paramount
to the success of the procedure. The orientation of the
acetabular component affects the longevity of the prosthesis;
wear of the bearing surfaces and dislocation rate 2,3.
Ideally the acetabular component should be implanted with an
inclination angle of 35 - 45°
and an anteversion angle of 0 - 15°
14.
Acetabular cups are implanted with the aid of
intra-operative alignment guides and the accuracy of
implantation depends on the proper positioning of the patient
and anatomical landmarks. The pelvis rotates intra-operatively
with retraction of soft tissues and femur 7.
It has been postulated that in obese patients proper positioning
of the patient upon the operation table is difficult and more
retraction of soft tissues and the femur is required for
visualisation of the acetabulum.
The aim of the
present study was to determine if the accuracy of implantation
of cemented acetabular component in total hip replacement is
influenced by obesity. This study
suggests that obesity has no effect on the accuracy of
acetabular component implantation in total hip replacement.
All 111 patients in
the present study were operated by or under direct supervision
of a single surgeon in the lateral position through
anterolateral approach to the hip joint. The same acetabular
component and bone cement was used in all the patients. Single
observer took all the observations. The observer was blinded
while measuring the angles from the radiographs using software.
The present study
demonstrates that the obesity does not affect implantation of
acetabular component in total hip replacement. Although patient
positioning and adequate exposure of the acetabulum may be more
difficult in the obese patient it is likely that the operating
surgeon takes greater care in positioning the obese patient and
in implanting the cup thus maintaining the accuracy of cup
implantation.
Of the 23 patients
in the Obese group 16 patients were grade one obese, 6
were grade two obese and 1 was grade three obese. The number of
patients who were grade 2 and 3 obese were too small for
statistical analysis and these patients were therefore grouped
with grouped with grade 1 obese patients. This study has not
addressed whether higher grades of obesity affect the accuracy
of cup implantation. We would suggest that this could be an area
for further work.
The present study
shows that obesity does not affect orientation of acetabular
component in total hip replacement however extra care may be
required during patient set-up, exposure and cup implantation
during Total Hip Replacement.
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