To The Editor: What appears most evident in the paper
"Ceramic Failure After Total Hip Arthroplasty with an Alumina-on-Alumina
Bearing" (2006;88:780-7), by Park et al., is the high percentage of
fractures of ceramic components (four liners and two heads out of a total of
357 implants, or 1.7%). The same figures were presented in a poster by Park et
al. at the 2006 AAOS annual meeting in Chicago1. Also, on the same
occasion, other Korean surgeons presented a similar poster2, in
which five of 157 sandwich ceramic liners were reported to have fractured,
giving a 3.2% fracture rate. Summing up the experiences of the two groups of
surgeons, we calculated a percentage of fractures of 2.1%. The sandwich type
of acetabular liner considered in these works has been in use since 1994, and
to date more than 20,000 liners have been implanted in Europe, Asia, and
Oceania. Excluding those in Korea, twenty-eight fractures of these implants
have occurred (a rate of about 0.14%), to our knowledge. In all of the cases
examined, the cause of the failure was a subluxation of the head, which often
can be traced back to malpositioning of the acetabular cup.
It is therefore plausible, as Park et al. affirmed, that the high rate of
fracture reported in Korea (fifteen times greater than rates reported from
elsewhere) may be ascribed to the particular posture habits of Asian
populations (squatting) as, under conditions of hyperflexion, there is
impingement between the neck and cup with consequent subluxations of the head.
The fractures occurred with 28-mm couplings, so the range of motion is of
limited amplitude. We therefore agree fully with Park et al. that the
subluxation and neck-cup impingement could be avoided, or at least reduced, by
using large-diameter (36-mm) ceramic-on-ceramic couplings. There are no known
cases of breakage with 36-mm ceramic-on-ceramic couplings (after follow-up of
about 5000 implants for a maximum of six years), excluding those due to
incorrect positioning of the liner in the metal shell. Furthermore, as
underlined by Park et al., it is certainly an advantage if the neck of the
prosthetic stem is adequately shaped to increase the range of motion. All
fractures reported in the article were of old versions of the prosthetic stems
that nowadays present anteroposteriorly lowered necks to reduce the risk of
impingement.
I would also point out that the paper by Park et al. contains several
inaccuracies; I believe it might be of use to clarify some issues.
With regard to the use of ceramic-polyethylene (sandwich) liners, the
authors stated that the only benefit lies in the reduction of the risk of
chipping during introduction of the liner in the cup and cited, in this
connection, the report by Ravasi and Sansone3. Curiously, Park did
not quote a previous report that he and colleagues had written about the same
concepts a year before4. As for the risk of fracture associated
with liners with a 28-mm diameter, it must be borne in mind that the main
difference between sandwich liners and ceramic-metal back liners with direct
connection lies in the fact that, after a fracture, sandwich liners come out
of the polyethylene shell (as seen in Figure 4 in the JBJS article by Park et
al.) while in liners with direct connection the breakage of the rim is
invisible on radiographs, is often asymptomatic, and can be perceived only
from the presence of articular noise. In other words, while with sandwich
liners the breakage of the rim is always visible and can therefore be
diagnosed, the same is not true for directly fixed liners. Finally, it must be
remembered that the presence of the polyethylene shell protects the internal
metal-back seat and a new liner can therefore always be used during
revision.
With respect to the way that the ceramic liner comes out of the
polyethylene shell, the above-mentioned Figure 4 and its legend in the article
by Park et al. describe an action that is not possible. According to the
explanation given by Park et al., the neck of the stem produces the fracture
of the ceramic by impingement (Fig. 4, I and II).
Subsequently, the neck comes in contact with the ceramic rim opposite it, and
the thrust of the neck itself (Fig. 4, III) causes the liner to come
out of the polyethylene shell on the side where the liner is fractured (Fig.
4, IV). I believe that the process that leads to the liner breaking
out of the shell is completely different. With reference to Fig. 4,
I, the contact between the neck and the ceramic does not produce a
severe breakage of the liner; instead it causes a subluxation of the head on
the opposite side. The high contact pressure that is generated by the contact
between the head and the rim of the liner causes some ceramic grains to break
off. This mechanism has also been explained by Willmann5. What is
known as an "avalanche effect" then takes place, so the fracture
advances ever more rapidly until the head manages to wedge itself between the
fractured rim and the polyethylene shell, generating the eccentric motion that
allows the liner to come out.
Park et al. believe that, in sandwich liners, the reduction of the
thickness of the ceramic articular core favors the fracture of the rim. The
thickness of ceramic liners is, according to many surgeons, a potential cause
of fracture, and this problem is particularly seen in Korea. Undoubtedly, the
thickness of the ceramic liner has an impact on the mechanical resistance, but
it has no influence whatsoever in the cases of fracture of the rim. Although
it may seem strange, it has been proven by Lima-Lto with the finite element
method that, in the event of a subluxation of the head, the greater the
thickness of the ceramic, the greater the contact stress. In the past, rim
fractures have been reported even in liners thicker than those mentioned by
Park et al. The thickness of the ceramic, on the other hand, might have more
importance in axial load conditions, although, to the best of our knowledge
and that of CeramTec, no cases of fracture under axial load have ever
occurred.
Park et al. reported two cases of fracture of the femoral head in the first
year after implantation. The authors commented on these failures, questioning
the data in the literature and the "proof test" carried out by
CeramTec on 100% of the femoral heads during production. It is odd that Park
and colleagues did not realize that the fractures of the two femoral heads
were consequences of the failure of the rim of the ceramic liner. In Figures
E1-A and E1-B published in the Supplementary Material of the article, it can
be observed that the non-fractured head of one of the explants is coarsely
abraded and riddled with microfractures. These deteriorations of the head are
due to the presence of ceramic particles that have detached themselves from
the fractured rim and interposed themselves between the head and the liner.
Since these particles are made of the same material and therefore have the
same hardness as the head, they produced on it abrasions and cracks. In fact,
if one examines Figure E1-A, it is possible to see that the polar abrasion of
the head has a circular profile (these are not traces of the processing of the
piece, as the paper affirms), precisely because it was caused during the
rotational movement in the area close to the coupling taper hole. The
fractures of the heads, therefore, took place because of the presence of
cracks. This allows us to understand Figure 2 as well, where at the center of
the image it is possible to observe the polar fragment, which is similar in
shape and size to the deteriorated polar area in Figure E1-A. This fracture
mechanism has been known to have occurred with non-sandwich and thicker
couplings. With reference again to Figure 2, we agree with the authors that
the curvilinear shape of the fracture of the liner was due to the explosion of
the head, but the liner (as can be seen on the opposite side of the
curvilinear fracture) was already fractured, and this was the cause of the
fracture of the head.