|Abstract / Summary
The hip prosthesis is designed as a ball and socket joint between the femoral head on
the one side and the acetabular insert on the other. This design was challenged in the
seventies by the introduction of a second articulation within the same joint (dual
mobility). In the late seventies, G. Bousquet and his colleagues laid the groundwork
for today’s dual mobility design, which has been further developed over the years.
Although the dual mobility design has shown to effectively reduce the risk of hip
dislocation, we still have much to learn about the kinematics, wear and implant
migration of this design. Radiostereometric analysis (RSA) holds the ability to measure
kinematics, wear and implant migration with high precision and accuracy, and was
used as the main method in the present study.
In Study I, we developed a method for measuring polyethylene movement with
dynamic RSA. Firstly, a method was developed for uniquely marking the liner for
measuring liner orientation with RSA. Secondly, a method was developed for
including as many markers as possible in the analysis. Lastly, the methods were tested
in a clinical setup with a patient from Study II.
In Study II, 20 patients with marked anatomic dual mobility liners were analysed
postoperatively and one year after surgery for liner movement, and the movements
were tested for correlation with the biomechanics of the hip prosthesis and clinical
outcomes. It was shown that the liners could still move 1 year after surgery and that
they changed orientation over time.
In Study III, a cohort of patients with the anatomic dual mobility cup was investigated
for risk factors 5 years after surgery. Due to the large joint surface of the dual mobility
hip, wear is a particular concern and excessive polyethylene wear has been shown to
contribute to implant loosening. Likewise, cup migration has been shown to correlate
with the risk of implant loosening. Low polyethylene wear and cup migration are
therefore critical to dual mobility hip implant safety. In this study, we found low
polyethylene wear and acceptable cup migration in a mixed population.
In conclusion, the thesis studies show that dynamic RSA can be used to evaluate
polyethylene liner movement. The dual mobility liners in ADM cups were shown to
move 1 year after surgery. Liner movement may protect patients from hip dislocation
and excessive polyethylene wear, especially when the cup position is suboptimal and
has a high inclination. These findings may also apply to other dual mobility hip
implant designs and support the general use of dual mobility prostheses.