Extract
We would like to thank Dr. Biedermann for addressing the problem of the
controversial ideal value for cup positioning in total hip arthroplasty.
Biedermann et al. pointed out that small variations from the desired 15°
of anteversion and 45° of abduction can increase the frequency of
dislocation and thus the cup placement should be in the optimal
position2. We concur
with that statement; however, we must ask the question: What is the
theoretical optimal cup position?We agree with Biedermann et
al.2 about the
limitations of the Lewinnek "safe
zone"1 used in
our study; however, even in studies using accurate measurement techniques such
as the EBRA (Ein Bild Roentgen Analyse) method or tridimensional computed
tomography scan methods, there are actually no data concerning the optimal cup
positioning angles that integrate individual pelvic tilt, individual bone
stock, and cup/stem combined
anteversion2-4.
Concerning the cup/stem combined anteversion, previous tridimensional
anatomical studies have shown a large range of individual femoral anteversion
angles, from 0.29° to 44.5° in patients with primary osteoarthritis
and from 2° to 80° in those with developmental dysplasia of the
hip5,6.
Concerning pelvic tilt, the values in our study ranged from —22° to
14° preoperatively, and Nishihara et al. reported that pelvic tilt values
may change postoperatively, with a mean difference of 2° ± 7.5°
(range, —26° to 15°) compared with the preoperative
value7. According to
DiGioia et al.4, the
quoted figures for optimal acetabular alignment may not be applicable to every
patient, and optimal alignment may need to be considered as a moving target
rather than a fixed pair of abduction and anteversion angles.
We would like to thank Dr. Biedermann for addressing the problem of the
controversial ideal value for cup positioning in total hip arthroplasty.
Biedermann et al. pointed out that small variations from the desired 15°
of anteversion and 45° of abduction can increase the frequency of
dislocation and thus the cup placement should be in the optimal
position2. We concur
with that statement; however, we must ask the question: What is the
theoretical optimal cup position?
We agree with Biedermann et
al.2 about the
limitations of the Lewinnek "safe
zone"1 used in
our study; however, even in studies using accurate measurement techniques such
as the EBRA (Ein Bild Roentgen Analyse) method or tridimensional computed
tomography scan methods, there are actually no data concerning the optimal cup
positioning angles that integrate individual pelvic tilt, individual bone
stock, and cup/stem combined
anteversion2-4.
Concerning the cup/stem combined anteversion, previous tridimensional
anatomical studies have shown a large range of individual femoral anteversion
angles, from 0.29° to 44.5° in patients with primary osteoarthritis
and from 2° to 80° in those with developmental dysplasia of the
hip5,6.
Concerning pelvic tilt, the values in our study ranged from —22° to
14° preoperatively, and Nishihara et al. reported that pelvic tilt values
may change postoperatively, with a mean difference of 2° ± 7.5°
(range, —26° to 15°) compared with the preoperative
value7. According to
DiGioia et al.4, the
quoted figures for optimal acetabular alignment may not be applicable to every
patient, and optimal alignment may need to be considered as a moving target
rather than a fixed pair of abduction and anteversion angles.
For these reasons, when designing our study, we defined the "safe
zone" as historically described by Lewinnek et
al.1 as the
"target value" for cup positioning with the free-hand and the
computer-assisted method. We used 40° ± 10° of abduction and
15° ± 10° of anteversion as target values on the basis of the
literature and considered these values to be more applicable than exact values
of abduction and anteversion
angles1,3.
The purpose of our study was to demonstrate that computer-assisted surgery can
be an accurate tool with which to reach target values for cup angles, and we
verified our hypothesis.
As mentioned in the Discussion of our paper, one of the next challenges in
computer-assisted cup positioning will be the definition of the optimal cup
position for each patient on the basis of accurate measurement methods and
integrating individual values for pelvic tilt, patient bone stock, and
cup/stem combined anteversion.
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