Polyethylene acetabular liner rim damage in total hip replacements

Total hip replacement surgery is a successful orthopaedic intervention with over
90,000 procedures carried out annually; however, some implants still fail. The failure of
THRs in the first year have been related to mechanical failure, such as dislocation and/or
implant fracture. The mechanical failure of polyethylene liners has been mainly
associated with damage at the rim of liners, due to edge loading and/or impingement.
Implant failure typically leads to the revision of the implant, which is more costly and
predominately less successful for the patient; therefore, the occurrence and
consequence of impingement and dislocation are required to be understood to reduce
mechanical failure of future THRs. The aim of this project was to investigate and
characterise rim damage of polyethylene liners of THRs.
To understand the occurrence of impingement, a geometric model was
developed that incorporated THR components into a bone model and investigated
impingement during different dislocation-prone activities. The model was applied to
investigate the effect that varying the surgical positioning of the acetabular component
and the component design has on the likelihood of impingement. In terms of the
positioning of the acetabular component, increasing the inclination and anteversion of
the acetabular cup reduced the risk of impingement. Varying the design of the liner from
a neutral to a lipped liner caused more frequent impingement, and reducing the neckshaft
angle reduced the frequency of impingement.
To understand what rim damage is caused by impingement, methods were
developed to quantitatively assess rim damage of polyethylene liners and to
experimentally replicate clinically relevant rim damage due to impingement. The method
applied motion and load inputs to simulate a clinically relevant activity, and was applied
to investigate the effect of varying the severity of impingement and medial-lateral load
has on the severity of damage. Increasing the severity of impingement was found to
increase the severity of rim damage and the amount of separation of the head.
To check the clinical relevance of the results from the developed geometric model
and the experimental method, a series of retrieved polyethylene liners of the same
design were reviewed; damage was visually assessed, rim damage quantified, and invivo
orientation of components were measured.
Overall the series of developed methods have been used to assess and
characterise rim damage of polyethylene liners, and the methods have the potential to
assist with the pre-operative planning of THR surgeries, pre-caution post THR surgery
and aid with the design process of future designs of implants.