Fixation - Orthopaedic Biomaterials in Research and Practice - page 317

CHaPTer THIrTeeN
Fixation
This chapter deals with the problems of fixation, that is, the coupling of
prosthetic components to the musculoskeletal system so that prosthetic
and natural elements may act together in a harmonious manner. It is no
exaggeration to say that the failure of fixation is one of the most common
causes of failure of partial and total joint replacement arthroplasty and a
major contributing element in fracture malunion and nonunion. Solving
the problem of long-term fixation is required to extend the present excel-
lent outcome of joint replacement arthroplasty to the younger patient.
Need for fixation
Charnley recognized that the major differences in mechanical proper-
ties between bone and the metals used in prostheses were a source of
problems in the use of uncemented (press-fitted) femoral endoprostheses
such as the Moore or Thompson designs. He demonstrated the problem
in a very simple experiment (Charnley and Kettlewell 1965), the results
of which are shown schematically in Figure 13.1.
After suitably preparing a cadaver femur, he installed a femoral
endoprosthesis, leaving a gap between the flange and the “calcar,” and
providing a dial indicator so that he could measure the relative motion
between prosthesis and bone. (It has become the accepted practice to
call the proximal medial femoral cortex, in the vicinity of the lesser
trochanter, the calcar, even though it may be anatomically incorrect. For
a discussion of this issue, see Harty 1957.) He performed the same pro-
cedure in the contralateral femur, but bedded the stem in poly(methyl
methacrylate) (PMMA) cement.
When the constructs were loaded, more or less axially (in a constant
deformation rate machine), he made the following observations:
The uncemented stem sank down the medullary shaft, with an occa-
sional decrease in load, until the proximal medial femoral cortex
split.