# Roughness Digital Characterization and Influence on Wear of Retrieved Knee Components
## 4 Discussion  
TKA is, nowadays, a well-known and widespread surgical procedure. Given the long-term problem of TKA tribological issues, surgeons are more and more interested in studies which improve in vitro tests of implants. Nevertheless, replication of in vivo wear behavior on in vitro tests remains a challenge. In this scenario, the identification of damage patterns and surface roughness distribution evaluation of retrieved components remains a crucial step to enhance the knowledge of tribological phenomena associated to the TKR exercise.  
In this study, 18 retrieved TKA femoral components were selected choosing the same prosthesis design, but different sizes, to investigate surface characteristics of medial and lateral compartments. Results are given in terms of three roughness parameters (Ra, Rt, Rsk) and of 3D topographical maps of Ra, representing the worn surfaces.  
Results show a non-uniform wear intensity on the two condyles. This is due to the mechanism of action of misalignments and its effect on load distribution. In fact, a varus knee alignment shifts the load-bearing axis medial to the knee center, creating a moment arm that increases forces across the medial compartment and reduces lateral load. Equally, the lateral shift of the load-bearing axis due to valgus alignment increases forces across the lateral compartment and reduces medial load. In other words, considering a standardized gait, i.e., Figure 5, it is possible to say that the increase in the valgus movement causes a varus compression and this also affects the mode of synovial lubrication of the contact surfaces [21; 22].  
Figure 4: Topographical picture of the medial and lateral condyle with the highest Ra value.  
It has been evidenced that surface roughness affects the wear and lubrication mechanism in total artificial joints [23; 24].  
In this paper, although the cause of the failure was not related to wear, the surface analysis was performed to obtain information on the surface damage mode for early implanted inserts to evaluate the early effects of in vivo exercise and provide information to understand.  
This article has, obviously, some limitations. The first one is related to the low number of specimens taken into consideration in this study. The second limitation is the lack of information about measurements performed in different directions than the A/P. Further studies will be planned considering more specimens with similar characteristics (i.e., F.U., body mass index, etc.) and to measure the roughness along the medial-lateral direction at different angles.  
Moreover, the measurement of contact surfaces topography plays a key role in the synovial lubrication regime acting in the artificial joints. In fact, according to [PERSON] et al. [25], the k parameter, calculated as the ratio between the minimum synovial meatus height and the roughness of the surfaces, could assume values lesser than 1, which is characteristic of boundary lubrication phenomena, while for 1 < k < 3, mixed lubrication is expected.  
Figure 5: Tibial based TKR reference system.  
Only in the case of k > 3, a continued film of synovial lubricant is present, which allows the complete separation of the contact surfaces with consequent low wear phenomena.