Assessment of influences of posterior rotation of the tibial condyles on the Insall-Salvati ratio

Abstract

The positional relationship between patellar and femoral articular surfaces may vary according to the degree of posterior rotation of the tibial condyle, which may influence the patellar configuration. We hypothesized that the configuration of the patella has a rhomboid transformation similar to that of the tibial condyle. This cohort study included 313 patients with knee pain who underwent lateral-view knee digital radiography. The length of the long axis, short axis of the patella, and patellar tendon length of the patellofemoral joint were measured. The patella axis ratio (length of long/short axis) as patellar configuration and Insall-Salvati ratio were calculated. Correlations between the configuration of the tibial condyle and the three length parameters and the Insall-Salvati ratio were assessed. Posterior rotation and the rhomboid transformation of the tibial condyle were positively correlated with the length of the long axis of the patella and negatively correlated with the Insall-Salvati ratio. The more the tibial articular surface shifted posteriorly due to posterior rotation and rhomboid transformation of the tibial condyle, the longer the long axis of the patella was, and the smaller the Insall-Salvati ratio was. The long axis of the patella became longer due to rhomboid transformation, similar to the tibial condyle.

Figure 1

Schema of the tibial condyle with posterior rotation and rhomboid transformation during bone growth. (a) The tibial condyle moves superiorly with bone growth of the physis; however, body weight loading is applied. Loading has two directions (vertical loading and shear stress). (b) Vertical loading disturbs bone growth in the posterior area of the physis, and the tibial condyle rotates posteriorly with bone growth. (c) Shear strain by the shear stress induces rhomboid transformation of the tibial condyle.

Figure 2

Influence of posterior rotation of the tibial condyle on the locations of the patella and the femoral condyle. (a) A tibia with a larger posterior slope, (b) A tibia with a minimum posterior slope of the tibial articular surface, (c) Contours of both tibiae were superimposed. With posterior rotation of the tibial condyle, the femoral condyle and patella shift posteriorly and inferiorly.

Figure 3

Parameters on lateral view radiograph of the knee. (a) Dist. LA: length of the long axis of the patella, Dist. SA: length of the short axis of the patella, Dist. PT: length of the patellar tendon, (b) Angle AW represents the angle of posterior rotation of the tibial condyle relative to the anterior cortex of the tibial shaft. Angle TS represents the posterior slope angle relative to the anterior cortex of the tibial shaft. Angle CS represents the degree of rhomboid transformation of the tibial condyle.

Figure 4

Schema of the long and short axes of the patella. (a) The long axis and the short axes are drawn, and the patella axis ratio is calculated. (b) With the simple elongation of the patella, both the long and short axes are getting longer and the patella axis ratio is getting smaller. (c) With rhomboid transformation of the patella, only the long axis is getting longer. The patella axis ratio is getting larger.

Figure 5

Correlation between Angle AW and length of the long axis of the patella. The more the tibial condyle rotates posteriorly, the longer the long axis of the patella becomes.

Figure 6

Correlation between Angle AW and the Insall-Salvati ratio. The more the tibial condyle rotates posteriorly, the smaller the Insall-Salvati ratio becomes.