AJ Vicar

BF Morrey

BM Jolles

BR Horwitz

CA Engh Jr

CD Fackler

CF Scifert

CR Bragdon

CR Mahoney

D Fender

DE McCollum

DJ Berry

E Garcia-Rey

Einar Amlie

G Digas

GM Alberton

J Livermore

J Sanchez-Sotelo

JC Hermida

JF Williams

JM Roberts

LD Dorr

MA Ali Khan

ME Forsythe

OK Muratoglu

Olav Reikerås

P Triclot

PG Sultan

RP Robinson

RS Turner

RW Chandler

RY Woo

S Bystrom

S Glyn-Jones

SA Paterno

SS Kelley

U Hedlundh

Øystein Høvik

Journal of Orthopaedics and Traumatology

English

PubMed

Springer - Publisher Connector

ORIGINAL ARTICLE
Dislocation after total hip arthroplasty with 28 and 32-mm
femoral head
Einar Amlie • Øystein Høvik • Olav Reikera˚s
Received: 23 March 2010 / Accepted: 12 May 2010 / Published online: 27 May 2010
 The Author(s) 2010. This article is published with open access at Springerlink.com
Abstract
Background Dislocation after primary total hip arthro-
plasty (THA) is a significant complication that occurs in
2–5% of patients. It has been postulated that increasing the
femoral head diameter may reduce the risk of dislocation.
The purpose of this paper is to report our experiences with
a change from a 28 to a 32-mm femoral head.
Materials and methods The retrospective cohort study
includes 2572 primary THA performed with a 28 or 32 mm
diameter femoral head in the period February 2002 to July
2009. All patients were operated with a posterolateral
approach, and all except 18 were operated because of
osteoarthritis. Cemented stems were used in 1991 cases and
uncemented stems in 581 cases. Cemented cups were used
in 2,230 cases and uncemented cups in 342 cases. The
patients have been routinely followed for 1–8 years in
the 28-mm femoral head group and from 0.5–7.5 years
in the 32 femoral head group. We defined a dislocation as an
event in which the hip required reduction by a physician.
Results Dislocation occurred in 49 hips with a 28-mm
femoral head and in 4 hips with a 32-mm femoral head
with an odds ratio of 6.06 (95% CI = 2.05–17.8)
(P \ 0.001). Otherwise, there were no significant associa-
tions between sex, age, diagnosis and type of prosthesis.
Conclusions Multivariate analyses of patients operated at
our hospital indicate a significant association between
femoral head diameter and dislocation after THA. There
were no significant associations between dislocation and
sex, age, diagnosis, or type of prosthesis.
Keywords Dislocation  Head size  Hip arthroplasty 
Prosthesis
Introduction
Dislocation of a primary total hip arthroplasty (THA) is a
significant complication. In 1982 Woo and Morrey
reviewed 10,500 primary THAs and identified an overall
dislocation rate of 3.2% [1]. Later studies have reported a
rate of 2–5% [2, 3]. Some hips dislocate more than once,
and patients with recurrent dislocation have reported a
worse outcome than after uncomplicated THA [4, 5].
The risk factors that predispose to dislocation include
patient-related, operative, and implant design variables
[6]. It has been postulated that an increased femoral
head diameter may reduce the risk of dislocation as a result
of reduced component–component or component–bone
impingement and increased translation that is required for
hip dislocation [7–9]. Despite these theoretical advantages,
the use of larger femoral heads in THA has been limited as
a result of concerns regarding increased production of
polyethylene wear debris and osteolysis. This is based
primarily on the study of Livermore et al. [10] in which
wear of 385 cemented total hips with conventional poly-
ethylene liners articulating with 22, 28, and 32-mm femoral
heads was measured. They reported increased volumetric
wear rates, total volumetric wear, and amount of osteolysis
for 32-mm heads compared with 22 and 28-mm heads.
Highly cross-linked polyethylene (XLPE) is an alterna-
tive bearing surface that was developed to improve
E. Amlie  Ø. Høvik
Department of Orthopaedic Surgery,
Lovisenberg Deaconal Hospital, Oslo, Norway
O. Reikera˚s (&)
Faculty Division Rikshospitalet, Rikshospitalet,
University of Oslo, 0027 Oslo, Norway
e-mail: olav.reikeras@rikshospitalet.no
123
J Orthopaed Traumatol (2010) 11:111–115
DOI 10.1007/s10195-010-0097-8
polyethylene wear resistance and reduce osteolysis. XLPE
has been studied extensively in vitro [11, 12], and in a
Boston hip simulator it was shown that the linear wear rate
of electron beam XLPE was extremely low and indepen-
dent of femoral head size for standard size femoral heads
(22–32 mm) [13]. Preliminary clinical studies of XLPE in
small series of patients undergoing THA have confirmed
these promising findings with a 45–99% reduction in wear
compared with conventional polyethylene at 3–5 years
[14–20]. Based on these results we have increased the size
of the femoral head gradually from 28 to 32 mm in an
effort to reduce the incidence of dislocation after THA. The
purpose of this paper is to report our experiences with this
change to a larger femoral head.
Materials and methods
The study was performed in accordance with the ethical
standards of the 1964 Declaration of Helsinki as revised in
2000, and all patients gave informed consent. The study
cohort includes all 2,572 primary THA performed with a
28 or 32 mm diameter femoral head in 2,257 patients
(1,673 females and 584 men) at our clinic from February
2002 to July 2009 (Fig. 1). With the introduction of highly
cross-linked polyethylene we gradually increased use of
the 32-mm femoral head, and 88 patients had bilateral
operations with different head size.
The mean age at the time of the operation was 68 years
(range, 24–93 years) in the cohort with a 28 mm diameter
femoral head, and 69 years (range, 24–91 years) in the
cohort with a 32 mm diameter femoral head. There were
1,047 women (1,182 hips) and 365 men (395 hips) in the
28 mm cohort and 691 women (739 hips) and 242 men
(256 hips) in the 32 mm cohort. All patients were operated
with a posterolateral approach, and all except 18 were
operated for osteoarthritis of the hip, either primary (2,296
cases) or secondary (258 cases). They were operated with a
cemented stem in 1991 cases and an uncemented stem in
581 cases, and with a cemented cup in 2,230 cases and an
uncemented cup in 342 cases.
The patients have been routinely followed for 1–8 years
in the 28 mm group and for 0.5–7.5 years in the 32 mm
group, and we defined dislocation as an event in which the
hip required reduction by a physician. During follow-up,
five hips have been revised because of infection and eight
because of mechanical failures. Altogether 126 patients
have died of unrelated causes. These patients have been
censored at time point of revision or death. Survival free of
dislocation after arthroplasty was estimated by use of the
Kaplan–Meier survival method. The Cox proportional
hazards model was used in a multiple variable model to
assess the risk of dislocation in association with femoral
head diameter, with adjustment for sex, age, diagnosis, and
type of prosthesis (SPSS, Chicago, IL, USA). An alpha
level of 0.05 was set for significance.
Results
Dislocation occurred in 49 hips (3.1%) with a 28-mm
femoral head and in four hips (0.4%) with a 32-mm fem-
oral head (P \ 0.001). The Kaplan–Meier survival from
dislocation is presented in Table 1 and in Fig. 2. Multi-
variate analysis shows an odds ratio of 6.06 (95%
CI = 2.05–17.8) for the 28-mm femoral head compared
with the 32-mm femoral head (P \ 0.001). In the whole
cohort, there were no significant associations with sex
Fig. 1 Use of 28 and 32-mm
femoral heads during
2002–2009
112 J Orthopaed Traumatol (2010) 11:111–115
123
(P = 0.987), age (P = 0.272), diagnosis (P = 0.856), cup
(P = 0.555), or stem (P = 0.523).
Discussion
In this study we evaluated the effect of femoral head
diameter on the risk of dislocation. We found that the
dislocation risk was significantly higher for the 28 mm
diameter heads than for the 32 mm diameter heads, with
a relative risk of 6. In the whole cohort there were no
significant associations with sex, age, diagnosis, or type of
prosthesis.
The major weakness of our study is that it was not ran-
domized. The strength of our study include a large study
group of patients with strict inclusion criteria, all with diag-
nosis of osteoarthritis, undergoing surgery in the same hos-
pital by the same surgeons, with the same surgical technique.
Most reports on femoral head diameters and dislocation
have used historical controls, and only a few clinical studies
have demonstrated that an increased femoral head diameter
may reduce the risk of dislocation [7, 21–24], and most
clinical studies have not demonstrated this effect [25–28].
One main reason for these discrepancies may be that in most
cases hip dislocation has a multifactorial etiology, and the
effect of a single variable requires a larger study population
with control of operative and implant design variables. We
isolated the effect of femoral head diameter as the single
variable. Our study confirms and expands on previously
published evidence that the dislocation rate is reduced by use
of larger heads. Kelley et al. [21] found the rate of dislocation
to be higher with a 22-mm-diameter head than with a 28-mm
head when a posterior operative approach was used.
Hedlundh et al. [23] demonstrated a higher risk of recurrent
dislocation, but not overall dislocation, for 2,875 hips treated
with a Lubinus implant with a 32-mm head compared with
3,192 treated with a Charnley implant with a 22-mm head,
but there was no difference in overall dislocation. Also
Bystrom et al. [22] found that the rate of revisions because of
dislocation was lower for 32-mm heads than for 28-mm
heads. However, as only reoperation because of dislocation
was used as the end point, and because many patients with a
dislocation never have a reoperation, the relationship
between head diameter and overall risk of dislocation could
not be assessed by this study.
The literature suggests that more than half of all dislo-
cations occur within the first three months after surgery and
more than three quarters within one year [1], and one series
demonstrated that 70% of dislocations after THA occurred
within the first month after surgery [29]. Our observations
are in agreement with these reports, because 37 of 53
dislocations occurred within 100 days after THA.
Table 1 Kaplan–Meier survival from dislocation for 28 and 32-mm
femoral heads
Time (days) Cumulative survival
28-mm head 32-mm head
10 0.994 (0.990–0.998) 0.998 (0.996–1.0)
50 0.986 (0.980–0.992) 0.997 (0.993–1.0)
250 0.980 (0.972–0.988) 0.996 (0.992–1.0)
500 0.976 (0.968–0.984)
1000 0.970 (0.962–0.978)
Fig. 2 Kaplan–Meier survival
from dislocation for 28 and
32-mm femoral heads
J Orthopaed Traumatol (2010) 11:111–115 113
123
In a previous study multivariate analysis showed a
moderate effect in favor of a 32-mm head compared with a
28-mm head [30]. The strongest effect was, however, the
operative approach with a significantly higher risk of dislo-
cation with the posterolateral approach than with the anter-
olateral and transtrochanteric approaches. Most series have
reported that the dislocation rate after a posterior surgical
approach to the hip is 2–3 times greater than that seen after
an anterior approach [31–34]. At our clinic we have routinely
used the posterolateral approach, and in the choice of
changing operative or implant variable to reduce the risk of
dislocation, we chose a larger femoral head because this was
simpler. Also, the availability of bearing surfaces that have
been predicted to have low wear rates even with larger head
diameters led us to the use of 32 mm diameter femoral heads.
This attitude was based on biomechanical and clinical
studies that have suggested substantially reduced wear rates
with XLPE compared with conventional polyethylene
sterilized in an inert environment [14–17].
We could not find any difference in risk of dislocation
between men and women. In one study dislocation was
reported to occur 2.5 times as often in women as in men
[1], but this was not verified in other studies [35–37]. Also
it has been noted that patients’ age is not a risk factor for
dislocation [1, 36], but Ekelund et al. [38] found a nearly
twofold risk of dislocation among very old patients (above
80 years). In our series, there were no associations between
dislocation and sex and age.
In conclusion, our analyses of patients operated at our
hospital indicate a significant association between femoral
head diameter and rate of dislocation after THA. Otherwise
we could not find any significant associations between
dislocation and sex, age, diagnosis, or type of prosthesis.
Conflict of interest None.
Open Access This article is distributed under the terms of the
Creative Commons Attribution Noncommercial License which per-
mits any noncommercial use, distribution, and reproduction in any
medium, provided the original author(s) and source are credited.
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Dislocation after total hip arthroplasty with 28 and 32-mm femoral head

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