The Design of a Novel Hip Resurfacing Prosthesis
Abstract
Total hip replacement (THR) is one of the most successful and most frequently
performed operations. For most implants the published rate of revision at 10 years is
less than 10%. However the revision rates are higher for younger and more active
patients who are likely to outlive their implants.
The most frequent cause of THR failure is aseptic loosening, commonly accompanied
by bone loss at the implant site. THR revisions give worse functional results and fail
sooner than primary THR and are complicated by this loss of bone stock.
A resurfacing hip prosthesis replaces the diseased surface layer of bone and cartilage
and retains the majority of the femoral head. The stress distribution in the proximal
femur is closer to that in an intact hip. A conservative resurfacing prosthesis will
present the surgeon with no greater problems at revision than encountered at primary
conventional 11-JR.
Early designs of resurfacing prosthesis conserved femoral bone stock at the expense
of acetabular bone. Revision rates were high and while some failures were caused by
avascular necrosis and femoral neck fracture the predominant cause was acetabular
loosening.
The design of a bone conserving prosthesis requires knowledge of the shape of the
bony surfaces of the hip joint. A survey of the morphology of the acetabulum showed
a wide variation in shape. While early resurfacing designs had hemispherical
acetabular cups the bony surface is less than hemispherical. The morphology and
desired range of hip motion constrain prosthesis thickness and shape.
A novel resurfacing design using a polyacetal femoral component and an UHMWPE
acetabular component is proposed. This bearing combination has a lower volumetric
wear rate than an equivalent Co-Cr on UHWMPE bearing. Computer modelling of the
resurfacing concept showed that lower moduli materials reduced stress shielding and
distributed implant-bone interface stresses more evenly. Mechanical testing of
polyacetal following immersion in Ringer's solution showed substantial decreases in
Young's modulus while strength was unaffected.
Authors
Thompson, Mark SCollections
- Theses [4186]