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Acetabular REVISION With CemenTless Cups (Managing most Cavitary and Segmental Defects) Aaron G. Rosenberg M.D. Professor of Orthopaedic Surgery Arthritis
& Orthopaedic Institute Rush
Medical College Rush-Presbyterian
–St. Luke Medical Center Chicago,
Illinois 1) Current
technology, and several reported results of clinical series at 8-12 year
follow-up, would seem to dictate that hemispherical, porous ingrowth cups
(with adjuvant screw fixation) are appropriate for the vast majority of
acetabular revisions. Exceptions
would include: the severely deficient acetabulum (which requires structural
allograft to the extent that adequate host bone ingrowth surface contact is
not possible), the essentially intact acetabulum (where the quantity and quality
of bone, and the age of the patient, is such that one would expect a cemented
cup to last the patients lifetime), or other situations where stability of
the cup cannot be maintained by a combination of host and allograft bone and
other solutions are sought (protrusio rings, cages, saddle prosthesis, etc.) 2) Almost all acetabular revisions employ some bone graft. The vast majority however do not require any bulk or structural graft. 3) The
need for graft is dependent on the construct obtained from surgical
preparation of the acetabulum. In the
vast majority of cases the reconstruction does not require supportive
graft. In the first 604 acetabular
revisions at our institution performed with a hemispherical, ingrowth cup
with adjuvant screw fixation, only 10 required bulk structural graft (1.6%). No protrusio rings or cement (other than in the cases which
required bulk allograft) have been required to reconstruct these
sockets. At the current time under 5%
of socket revisions require some type of metal/cement or structural graft
augmentation. 4) In general, the technique employed is to: A) Completely debride the acetabulum-remove the old cup, all cement and all membrane. Purely medial cement which is intra-pelvic may be left in place if it is mobile medially and its removal would jeopardize intra-pelvic structures. Ingrown cups may require special techniques in order to remove the cup without taking most of the medial wall. B) The remaining acetabular bone stock is
then reamed into as much of a hemisphere as possible. Where there is minimal bone loss this is
essentially the equivalent of reaming for a primary cup. For those situations where there is
superior bone loss, the acetabulum is "enlarged" to place a large
cup wedged between the superior ilium above and the pubis and ischium
below. Prerequisites to placing a
"jumbo" cup in this setting include a reasonable dome of iliac
bone, an intact posterior column and some bone either medially or inferiorly
to constitute a "rim" to set the cup into. C) In general keep the reamer
"low" in the acetabulum to minimize superior migration and to
insure that the inferior aspect of the cup will be in contact with whatever
inferior ischium and pubis are remaining.
In general reaming proceeds until a rim of viable, bleeding bone is
established. As one reams to larger
sizes, when there is significant superior bone loss, the limiting factor in
getting superior- inferior contact (in terms of placing a large cup to span
the defect) is the antero-posterior dimension of the remaining acetabular
bone. I have found that loss of the anterior column does not compromise
initial cup stability, but posterior bone loss does. So, maintain the posterior column bone at the expense of the anterior column
if the anteroposterior dimension of the acetabulum becomes too small for the
superior-inferior dimension which you are seeking. D) Occasionally, (rarely) to place one of
these "jumbo" sockets, it will be necessary to ream away some of
the medial wall. This does not
prevent placement of a mechanically stable cup and the medial
"protrusio" defect is easily grafted with particulate graft. E) Once the reaming has been done, an
acetabular trial (or appropriately sized reamer head) is placed into the
reamed socket to assess: 1) the amount of host bone contact and 2) the
mechanical stability of the cup. With
the trial in position, load the cup with a ball headed pusher in the
direction of the hip resultant force vector-superior, posterior, and
medial. If the cup is stable to
loading in this direction the cup is stable enough to achieve support without
bulk grafting. F) At least 50% of the socket ingrowth
surface should be in contact with "host" bone. If less than 50% is in contact or the cup is not stable, then
alternative fixation methods are required such as bulk graft, protrusio rings, cement, etc. G) Alternatively a "high hip
center" may be attempted. This
essentially means that a smaller socket (than would be attempted as in the above)
is reamed into the remaining superior iliac wing bone stock. This
can not be done as a fall back position
if the above technique for a "jumbo socket” does not work out. It requires preservation of all
superior iliac and posterior bone stock.
In general this technique may be (should be?) attempted if the
acetabular bone stock remaining after debridement does not appear capable of
supporting a large socket because of loss of posterior column bone at the
level of the proposed "jumbo" socket. The major problem with this type of reconstruction is that it
truly does raise the center of rotation of the acetabulum. This leads to two major problems with the
hip reconstruction. One is that the
femoral reconstruction must be performed (in order to maintain leg length,
abductor tension, etc.) with the elevated center in mind. Extra long neck or calcar replacement
type reconstructions may be required.
In addition the elevation of the hip center of rotation provides a
whole new environment for femoral impingement on the now
peri-articular pelvic structures that were previously far removed from the
femur. Both of these problems may
need to be managed in the "high hip center" reconstruction. H) Particulate graft is then packed into
any contained defects as well as medially to fill in any “protrusio” type defects. To the best of my knowledge, particulate graft cannot be used
to support a cup that does not have intrinsic mechanical stability on viable
bone. The particulate graft can then
be densely packed by using the acetabular reamer on reverse to prepare the
socket bed for component implantation. I) While press fitting of the socket has
become popular with primary cementless replacement, it is only in the
essentially "virgin" revision socket that this should be
attempted. The less one has a
circumferential 180 degree bony coverage, the less confidence one can have in
an under reamed cavity firmly gripping an oversized impacted cup! Consequently I believe (and I think
retrieval studies support the concept) that, as many screws as will obtain
good purchase should be used to fix the revision cup. Some degree of press fit may be possible,
but of course the greater the bone loss, the more careful you will have to be
regarding impacting the cup and avoiding further damage to the underlying
bone stock. 5) We reviewed our first 138 revisions of failed cemented acetabular components revised to a cementless hemispherical porous ingrowth acetabulum in 134 patients performed between 1983 and 1986 utilizing the techniques described above. This series (please see Padgett, et. al., JBJS:75-A, #5, 663-673, 1993 for details on evaluation of these reconstructions) was reviewed for a follow-up note for the Journal and the results published in 1996 and rereviewed for the Hp Society '99 and are little changed at what is now 10-12 year follow-up. A) Of 176 hips in 167 patients, 35 patients
died prior to 9 years (none requiring revision) leaving 139 components in
which 33 had less than 9 year follow-up (none requiring revision) leaving 106
hips for radiographic and clinical evaluation at an average of 130 (102-169)
months. B) Acetabular bone loss
staging 1)
minimal bone loss............................................ 17% 2)
<10 mm superior migration............................. 23% a)
+ <7 mm medial migration......................... 11% b)
+ >7 mm medial migration......................... 12% 3)
Superior migration 10-25 mm.......................... 41% 4)
Superior migration >25mm or column defect.... 17% 5)
Pelvic disassociation........................................... 2% C) 108 hips required some
type of bone graft (78.3%) 1)
particulate autograft........................................
62% 2)
particulate allograft.........................................
14% 3)
combined particulate auto and allograft............
32% 4)
bulk structural allograft......................................
9% 5)
83% of grafts were "contained" and 17% uncontained (mainly
medial to fill in "protrusio" defects.) D) Host bone contact with
the cup averaged 81% (30- 1 00%) E) Of the original 176 revised sockets,
bulk structural graft was required in 9 cases (5%). All grafts healed radiographically. At final follow up, 87% of cups were radiographically stable
and not revised, however, 7% were radiographically and clinically stable but
required revision: 5 for recurrent dislocation, 5 for late sepsis and 2 were
revised at the time of loose stem revision at 71 and 75 months. No revisions for aseptic loosening were
required. F) No non-septic cup demonstrated migration or progressive radiolucency. However partial radiolucent lines were extremely common (80%) and almost always supero-lateral and/or medial inferior. An additional 4% (7) were noted to have 4/5 modified Charnley zones with radiolucent lines (RLL) of < 2 mm and were considered possibly unstable, while 4 cups (2%) were unstable demonstrating migration or RLL of > 2mm in 4/5 zones. G) Osteolysis was minimal at average 7 year
follow up and was seen in less than 5% of cases and in no cases required
revision. No screw breakage was seen,
but at this longer follow up period we noted at the cup periphery, separation
of the fiber metal pad from the underlying shell in 2 cases, with pad
fragmentation in 11 additional cases (noted at an average of 113.6 months). 2
cases had lucencies about a single screw while 12 additional cases
demonstrated peri-acetabular lytic lesions at the component periphery. These were noted at an average of 118 (59
-166) months. Pad Fragmentation was
statistically associated with lytic lesions.
All were less than 2 c. in max diameter and none have required
treatment to date.  Previous Lecture |
