cadera / hip - Active Congress.......

JUEVES / THURSDAY
168
grafting is an effective method in treating
polyethylene wear and associated osteolysis
in selected patients. Dislocation rates may
be reduced with use of the direct lateral
approach and this reduction is reported as
a signifi cant fi nding of this study. Isolated
liner exchange is becoming increasingly
common as a revision procedure following
total hip arthroplasty. A new simple computer-assisted
technique of measuring and
quantifying lesional area on standard AP and
lateral radiographs is described, and provides
a useful tool in the clinical setting.
CONSTRAINED
LINERS IN REVISION THA
W. Paprosky
Rush Arthritis & Orthopaedic Institute
St. Luke’s Medical Center
Chicago, Illinois, USA
USE OF CAGES IN
ACETABULAR REVISION
SURGERY
P. Beaulé
Associate Professor
University of Ottawa, Canada
Revision acetabular reconstructions represent
a great challenge to orthopaedic surgeons,
particularly with severe segmental
and cavitary pelvic defi ciencies. In general,
a porous-coated, noncemented cup with at
least 50% host bone coverage can be used, if
the option of cementless fi xation is chosen.1
Defi ciencies are fi lled with allograft, either
structural or particulate. However, defi ciencies
requiring greater than 50% coverage by
allograft should be supported by a metal back
ring with a cemented cup.1,2 This protects the
graft from mechanical overload during its revascularization
phase. The majority of these
defi ciencies fall into type III and IV defects as
defi ned by the American Association of Orthopaedic
Surgeons classifi cation system.2,3
The most common metal rings have been the
Muller acetabular ring4 and the Burch–Schneider
antiprotrusio cage.5,6 Another design
is the ring hook cages fi rst introduced by Kerboull7
and later adapted to the Muller ring by
Ganz,8 and also available as the Restoration
GAP Cup9 (Stryker–Howmedica–Osteonics,
Allendale, NJ). The techniques to insert
these devices are variable in terms of optimal
position and allograft use. More specifi cally,
little emphasis has been placed in restoring
normal hip anatomy, with the majority of the
surgical technique focusing on placing the
cage on intact host bone.6,9
There is clinical and biomechanical evidence
that joint reaction forces increase with
superior and lateral placement of the hip
center.10-12 In his classic article, Johnston
et al10 demonstrated that inferior and medial
placement of the hip center resulted in a
maximum reduction of the joint reaction force
and the moment-generating requirements of
the muscles. Delp et al12 showed, with a 3dimentional
computer model, that increasing
the hip center by 2 cm in the superior and
lateral direction decreases abductor moment
arms by 28%. However, increasing the hip
center in the superior direction alone, along
with prosthetic neck length compensation,
had little effect on moment arms. Joint reaction
forces similarly increase with superior and
lateral hip center placement of greater than
25 mm, as shown in an experimental model
reported by Doehring et al.13 These authors
demonstrated that with a 25-mm displacement
of the hip center, the joint reaction force