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

VIERNES / FRIDAY
292
for proximal seating of the tibial component if
the rest of the tibial architecture is severely
destroyed. In the worst cases, all cancellous
bone is gone, leaving a large cavitary
defect and substantial defi ciency of the tibial
rim. Long-stem fi xation is advisable in these
cases regardless of whether block allografting
or morselized allografting technique is used.
When morselized graft is used, the tibial tray
should seat on the intact portion of the tibial
rim, and the stem should engage the isthmus
of the tibia. As with the femoral component,
the tightly fi t diaphyseal stem maintains stability
and prevents tilting of the component,
so that massive defects may be fi lled with
allograft and protected until healing and bone
formation occur in the grafted area.
GRAFTING TECHNIQUE
Block allografts traditionally have been used
for massive bone defi ciencies, but their complication
rates are high, and the destructive
effects of allograft rejection can limit their
long-term success. Large segments of allograft
also heal slowly, are never replaced by
new bone, and weaken as the ossifi cation and
vascularization front proceeds. In contrast,
morselized allograft has proven structurally
reliable for both small and large defects while
supporting new bone formation. Morsels that
are 1 cm in diameter maintain their integrity
long enough to act as a substrate for new
bone formation. Morsels less than 0.5 to
1 cm in diameter tend to be resorbed while
those larger than 1 cm incorporate slowly, if
ever, and tend to collapse.
Rejection can be a major problem with allograft
because marrow is immunogenic.
However, marrow elements can be thoroughly
removed from morselized allograft to
prevent the infl ammatory response and loss
of graft and to capitalize on the osteoconductive
potential of the allograft. The allograft
acts as scaffolding for new bone growth, and
although it is not osteoinductive, demineralized
bone (mildly osteoinductive) and bone
marrow aspirate (highly osteoinductive) can
be added to the allograft to enhance bone
formation. The surrounding bone structure
supplies most of the osteoinductive activity
because metaphyseal bone has a rich blood
supply and maintains the capacity to heal
even after repeated failed arthroplasty.
Grafting preparation and placement
Fresh-frozen cancellous allograft in morsels
measuring 0.5 to 1 cm in diameter is soaked
for fi ve to ten minutes in normal saline solution
that contains polymyxin 500,000 units,
bacitracin 50,000 units, and cephazolin 1 g
of per liter. The fl uid is removed and 10 cc
of powdered demineralized cancellous bone
is added to each 30 cc of the cancellous
morsels. Bone fragments and diaphyseal
reamings are added to improve the osteoinductive
potential. This mixture is packed
into the bone defects, then the implants are
impacted so as to seat on the remnant of viable
bone while compacting the morselized
bone graft.
MANAGEMENT OF
PERIPROSTHETIC
FRACTURES IN TKA
R. “Dickey” Jones, M.D.
U.T. Southwestern Medical Center,
Dallas, TX, USA