To The Editor:
In their recent article, "Validation and Usefulness of a
Computer-Assisted Cup-Positioning System in Total Hip Arthroplasty. A
Prospective, Randomized, Controlled Study" (2007;89:494-9), Parratte and
Argenson found that 57% (seventeen) of thirty cups placed freehand and 20%
(six) of thirty cups placed with a computer-assisted positioning system were
placed outside a so-called safe zone (outliers) (p = 0.002), as defined by
Lewinnek et al. in 19781. They concluded that the use of an
imageless navigation system can improve cup positioning in total hip
arthroplasty by reducing the percentage of outliers.
Lewinnek et al.1 proposed a radiographic safe range for the
position of the cup as anteversion of 15° ± 10° and abduction
of 40° ± 10°, although this was based on only nine
dislocations. (In the study by Lewinnek et al.1, the rate of
dislocation was 1.5% for cups placed within the so-called safe zone and 6.1%
for outliers.)
In contrast, in a more recent study of 137 dislocations, we2
demonstrated that there is no "safe" range for the position of the
acetabular component, although the lowest risk of dislocation was found in
patients with a cup placed in 15° of anteversion and 45° of abduction.
Furthermore, there was a constant increase in the relative risk (odds ratio)
of anterior dislocation as anteversion increased and a constant increase in
the relative risk of posterior dislocation as anteversion decreased. Patients
with anteversion of <10° had a sixfold higher relative risk (odds
ratio) of posterior dislocation than did those with anteversion of 15°
± 5°. Patients with anteversion of >20° had a 6.3 times
higher relative risk of anterior dislocation. In our study, 79% of all cups in
the stable control group were positioned inside the safe zone as defined by
Lewinnek et al.1. The percentage of dislocated hips in the group in
which the cup was within the safe zone was significantly lower, but it was
still 60% (chisquare test, p < 0.01). Altering the safe zone to 45°
± 10° of abduction and 15° ± 10° of anteversion
would result in a 93% prevalence of stable hips and a 67% prevalence of
unstable hips (p < 0.01)2.
Thus, cup placement must be in the optimal position. Any deviation from the
optimal position will increase the risk of dislocation.