Abstract
Background
Several studies have reported that tibial component in varus alignment can worsen the survivorship of medial unicompartmental knee arthroplasty (UKA). On the other hand, Varus/valgus inclination of the tibial component can affect the location of the contact point between femoral and tibial component especially in round on flat bearing surface design. Along with the tibial component inclination, changes in the contact point may also alter the tibial condylar bone stress, which would affect the longevity or complications after UKA.
Method
We constructed a validated three-dimensional finite element model of the tibia with a medial component and assessed stress concentrations by changing the tibial component coronal inclinations (squale inclination, 3° and 6° varus, 3° and 6° valgus inclination). We evaluated the Von Mises stress on the medial tibial metaphyseal cortex and the proximal resected surface when a load of 900N was applied on the tibial component surface by two conditions in each inclination models; one is that the loading site is fixed at the mediolateral center of the tibial component (fixed model), and the other is that the loading site is variable depending on the tibial component inclination (variable model) (Fig.1).
Result
In variable models, the loading site moved medially 22.8% of the tibial component width as the tibial component inclination changed from 6°varus to 6°valgus. The Von Mises stress concentrations were observed on the medial tibial metaphyseal cortices and on the anterior and posterior corner of the resected surface in all models (Fig.2). Stress concentration was also observed along the medial cortical rim of the resected surface in valgus tibial component inclination of the fixed model and varus inclination of the variable model (Fig.2). The stress on the medial tibial metaphyseal cortices did not markedly change in any inclination of fixed models, but increased in variable models as the tibial component inclination changed from varus to valgus (Fig.3A). The stress on the medial cortical rim of the resected surface increased with varus inclination in the fixed model and decreased with varus inclination in the variable model (Fig.3B). Changes in the Von Mises stress on the anterior and posterior corner of the resected surfaces did not differ between the fixed and variable model.
Discussion
Varus inclination of the tibial component has been considered to increase the bone stress in previous studies. However, in the current study, bone stress on the medial metaphyseal cortex and the medial cortical rim of the resected surface conversely decreased in varus inclination when the change of the femorotibial contact point was taken into consideration. Recent opinion has advocated that restoring the constitutive patient's anatomy by compensating cartilage wear is critical in producing the excellent clinical outcome after UKA. Therefore, three to five degrees of anatomical varus inclination of the tibial component would reduce the tibial condylar bone stress and protective against complications such as unknown postoperative pain or tibial component migration.
For figures/tables, please contact authors directly.
