The Shallow Socket
Allan E. Gross, M.D.,
F.R.C.S.(C) Head, Division of
Orthopaedic Surgery Mount Sinai Hospital Professor of Surgery,
Department of Surgery University of Toronto Toronto, Ontario The
shallow acetabulum may be considered dysplastic whether congenital or as a
result of trauma. The
issues that I shall discuss related to total hip replacement for hip
dysplasia are as follows: the surgical approach, lengthening of the extremity
and its affect on the nerve, placement of the cup, and the choice of
acetabular and femoral components. If the greater trochanter is at the right level and if the
lengthening of the leg required is less than 3 cms then a standard
transgluteal or posterior approach can be used. If the lengthening required is greater than 3 cms or if the
greater trochanter is high riding due to previous osteonecrosis then a
trochanteric osteotomy should be used.
We prefer to do an anterior slide rather than a transverse osteotomy
because of the lower incidence of trochanteric escape. Lengthening
of the leg up to 4 cms is usually safe but the nerve should be identified and
its tension monitored during the operative procedure. The patient should also be instructed with
regards to the possibility of a wake up test. It is important to know how important lengthening of the limb
is for the patient before deciding on the ideal placement for the cup. Placement
of the cup depends on bone stock and the leg length discrepancy. The cup can be placed at the correct or
near correct level with or without a bone graft as required. The other options are placing the cup high
(ref. 1 & 2), or centralization (ref. 3). Both of these techniques avoid the use of a structural
graft. The high hip centre as
advocated by Harris has the advantage that you can place a small uncemented
cup into live host bone and avoid the disadvantages of a structural autograft
(references 1 & 2). It is also
technically easier. The disadvantages
of a high hip centre is that there is a higher rate of component loosening
(references 4 & 5), a potentially higher dislocation rate because of
impingement against the ischium and a smaller cup with less poly is usually
used. Leg lengthening is possible but
limited to 2 or 3 cms. In addition,
further surgery may be difficult because bone stock has not been
restored. However, a high hip centre
is indicated if you have adequate bone stock with a minimal to moderate leg
length discrepancy of 2 to 3 cms and an acceptable compromise to the
anatomy. My own particular practice
is to place the cup at the correct or near to correct anatomical level and if
it has less than 70% coverage by host bone, then a shelf autograft is used. If I can get good coverage by elevating
the hip centre 1 or 2 cms, then I would opt for a high hip centre. Special
components should be available when operating on the dysplastic hip. The cup should have an inner diameter of
22mm or 26mm and can be cemented or uncemented. The femoral component has to be small and should have a
straight stem in order to accommodate the anteversion. Its distal diameter should be between 5mm
and 10mm. A modular femoral component
that allows adjusting the version is another option. I
shall now discuss the surgical technique is more detail. Prior to surgery an accurate measurement of the leg length is
mandatory. Decide on the level of the
cup and template it at that level.
Note how much of the cup is uncovered so that you can anticipate
whether or not a bone graft is going to be necessary. The final decision however will depend on
your intraoperative findings. Decide
on the neck cut according to your templates but usually it is at the level of
the lesser trochanter. Select your
surgical approach according to the prerequisites that I have already
outlines. After
you have decided at what level you are going to be putting the cup, start
with very small reamers usually 36mm in diameter. I use a depth gage and a drill hole to decide how deep to ream. I stop reaming when I am 1cm from the
inner cortex. A trial cup is then put
in, and if that cup is less than 70% covered then a bone graft is
performed. The femoral head is used
for the bone graft. The cartilage is
reamed off but the subchondral bone is kept intact. The graft is placed at the superior edge of the acetabulum or
just inside it and fixed with 2 cancellous screws usually 4.5mm in
diameter. The screws are placed in an
oblique to vertical direction. Do not
expose cancellous surfaces of the graft to host soft tissue. In addition, we use a flying buttress
graft, which is cancellous, morsellized, autograft bone, placed between the
top of the shelf graft and the ilium. On
the femoral side the predetermined neck cut is usually at the level of the
lesser trochanter. This is necessary
because of the extreme anteversion.
You have to use small reamers and broaches, and your component should
have a straight and narrow stem. Prior
to reducing the hip with trial components in place, it is necessary if you
are lengthening greater than 2cms or if the reduction is difficult, to
identify the nerve and check its tension both with the knee bent and with the
knee slowly extended after the trial components have been reduced. If you are still not sure about whether or
not the nerve is being stretched too much, than a wake up test must be
performed. As
of July 1996, we have performed 90 shelf autografts as part of a total hip
replacement for dysplasia. Thirty of
these have over 5 years follow-up with an average follow-up of 8.1
years. The cemented cups have an
average follow-up of 9.7 years, and the uncemented ups 6.6 years. At an average follow-up of 8.1 years
defining success as a stable cup with no further surgery, and an intact
graft, the success rate is 90%. Two
cemented cups have required revision, and 1 uncemented cup required
exploration but the graft was intact and the cup was solid (reference 6). Looking
at these graft radiographically, all grafts united and there was no cup
migration. One cup was definitely loose,
18 grafts had no resorption, 11 grafts mild resorption affecting the lateral
non weight bearing part of the graft, and 1 graft had moderate resorption of
over 30% but that was in association with a surface arthroplasty. References: 1. Russotti GM, Harris
WH. Proximal placement of the
acetabular component in total hip arthroplasty. Journal of Bone and Joint Surgery, 1991, 73‑A: 587‑592.
2. Jasty M, Anderson MJ,
and Harris WH. Total hip replacement
for developmental dysplasia of the hip.
Clinical Orthopaedics and Related Research, 1995, 311: 40‑45. 3. Hartofilakidis G, Stamos
K, Karachalios T, Ioannidis T, Zacharakis N:
Congenital hip disease in adults.
Journal of Bone and Joint Surgery, May 1996, 78-A: 683-692. 4. Pagnano MW, Hanssen AD,
Lewallen DG, and Shaughnessy WJ. The
effect of superior placement of the acetabular component on the rate of
loosening after total hip arthroplasty.
Journal of Bone and Joint Surgery, 1996, 78‑A: 1004‑1014. 5. Kelley SS. High hip center in revision
arthroplasty. Journal of
Arthroplasty, 1994, 9: 503‑510.
6. Morsi E, Garbuz D,
Stockley I, Catre M, Gross AE. Total
hip replacement in dysplastic hips using femoral head shelf autografts. Clinical Orthopaedics and Related Research,
1996, 324: 164‑168.
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