Purpose: Polyethylene wear is a recurrent problem in joint
arthroplasty. Small debris particles are also associated to
inflammation reaction of the surrounding bone, eventually
leading to the failure of the bound between the implant and
the host bone, and implant loosening. The goal of this study
was thus to estimate the volume of polyethylene wear of a
reversed prosthesis, and compare it to an anatomic
prosthesis, during one year of activities of daily living.
Material and Methods: A numerical musculoskeletal model of the glenohumeral joint
was used for this comparative study. The reversed (RP) and
anatomic (AP) Aequalis prostheses were positioned in the
numerical model. Eight levels of abduction were considered.
Their daily frequency was estimated from in-vivo recorded
data on healthy volunteers during activities of daily living.
One year of use was simulated to predict the linear and
volumetric wear. The volumetric wear was the difference of
volume between the original and worn component.
Results: With the AP, the contact pattern on the glenoid surface
moved rapidly from the inferior to the superior side during
the first 30 degrees of abduction, and then went back to the
inferior side. With the RP, the contact pattern on the humeral
cup surface remained at the inferior side. Contact pressure
was 20 times lower with the RP than with the AP. One year
of use produced a maximum linear wear of 0.2 mm with the
AP, and 0.13 mm with the RP. However, the volumetric wear
was 8.4 mm3 with the AP, but reached 44.6 mm3 with the
RP.
Conclusion: Polyethylene particles are a matter of concern with AP.
Infiltration of these particles within the bone-implant
interface can induce a implant loosening. This problem
should not be underestimated with RP. It might be associated
to a higher level of humeral stem loosening reported with RP.
The long term survival of RP might be improved by using a
highly cross-linked polyethylene, which has a better
abrasion resistance but lower plastic resistance.