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the outcome of stem fi xation. Reducing the
diameter of the neck from 12.5 to 10mm had
no effect on wear but reduced cup loosening
by over 50%. Proponents of the mechanical
theory have a number of solutions; the proponents
of the biological theory of component
loosening can either modify tissue response
or abandon the procedure. With new materials
and a better understanding of the problem
of osteolysis, it is likely that the subject will
become of academic interest only.
INTERPRETING
OSTEOLYSIS DEFECTS
C. A. Engh
Anderson Orthopaedic Research Institute
Alexandria, VA, USA
Osteolysis is a frequent radiographic fi nding
complicating cementless total hip arthroplasty.
When the acetabular cups are
poorly fi xed, osteolysis dissects around the
non-bone ingrown interface causing symptomatic
cup loosening. This type of lysis has
been termed linear osteolysis. Well-fi xed
acetabular components develop expansile
lysis. These lesions develop when joint fl uid
containing particulate debris invades soft
bone near the joint. The communication
pathway between these lesions and the joint
can be either through holes in modular cups
or around the rim with no holed non-modular
cups. When lesions develop through the
holes in the cup and remain small they are
usually not visible on AP radiographs since
they are hidden behind the cup. However
once these lesions develop a volume larger
than 10 cubic centimeters they usually become
visible on an AP radiograph. Cups
without holes develop expansile lysis around
the rim. These lesions usually develop at the
superolateral or inferomedial edges of the
cup and their progression and size can be
more easily followed on plain radiographs.
Since expansile osteolysis usually does not
result in cup loosening, these lesions are
asymptomatic.
We have found that computed tomography
(CT) is a better diagnostic tool for detecting
plain pelvic osteolysis than x-rays. We do
not recommend obtaining CT scans routinely
because it is costly and associated with a high
radiation dosage. However, once osteolysis
is suspected on x-ray we obtain a CT scan
to confi rm the diagnosis. We also use CT
scans to determine if the lesions are rapidly
expanding.
If it is determined that a patient needs revision
surgery, a CT scan is also useful for planning
the surgery. We use the CT to determine
osteolysis volume, whether or not the osteolysis
communicates with the joint space and
whether the lesion is contained by the medial
wall. The three-dimensional appreciation of
the lesion location obtained from the CT images
also help the surgeon in planning the
best surgical access route for curetting and
grafting the defect. During surgery we recommend
retaining well fi xed cups. We have four
objectives: 1) to decrease polyethylene wear
by replacing the polyethylene liner with one
that will wear less, 2) to remove the tissue
producing the lysis, 3) to fi ll the defect with
a bone graft substitute, and 4) to attempt to
block the communication pathway between
the defect and the joint space. Finally, we
found that CT is a much better method for
evaluating the results of our treatment than
plain radiographs.