PhDWith time, the rate of symptomatic acetabular component loosening accelerates and
overtakes that of the femoral component as the principal reason for the revision of
total hip replacement. In the femur extensive study has shown that cement
pressurisation and good preparation of the bone bed improves the survival rate, but
acetabular fixation requires further investigation.
Production of cement pressure in the acetabulum is anatomically difficult.
Pressurisation with conventional and novel designs of cement pressurisers has been
compared to manual techniques and component insertion. The pressurisers increased
peak and mean pressures and pressure duration. Finite element modethng of cup
insertion showed that flanges and higher insertion rates increased cement penetration
into cancellous bone. Per-operatively, one design of pressuriser produced cement
pressures comparable to those found in the laboratory.
Structural finite element modelling of the natural hip indicated that the subehondral
plate and the relatively dense cancellous bone supporting it distribute the joint contact
force into the medial and lateral pelvic cortices. A perfectly bonded cemented
polyethylene cup stiffened the acetabulum so that more load was transferred directly to
the cortices at the acetabular rim, with consequent interface stress concentrations.
However, complimentary experimental studies using a dynamic joint simulator and a
servo-hydraulic materials testing machine suggested that perfect fixation between
cement and bone at the rim was not possible, even under laboratory conditions.
Debonding of the cement bone interface at the rim, where dense bone prevents cement
interdigitation, allowed micromotion. Since the clinical mechanism of failure of the
acetabular component appears to be progressive debonding, from rim to apex, of the
cement-bone interface, these studies support the initiation of the failure mechanism by
mechanical factors, which may then allow the ingress of wear debris. The experimental
studies suggested that the use of pressurisers reduces the amount of micromotion and
thus may improve the long term stability of the interface