Introduction: Posterior-cruciate sparing total knee arthroplasty (TKA) designs are claimed to reach kinematics closer to normal knees. clinical reports on this kind of TKR designs have shown excellent and goods results in the mid-term follow-up, but no relationship between kinematics and clinical results has been elicited.

Objective: Our aim in this study was to define the pathway of tibiofemoral contact from knee extension to flexion in a PCL sparing TKR design and to find a relationship between the kinematic pathway and the obtained clinical results.

Material and methods: A series of 30 cases out of 140 consecutive TKR (CKS, Biomet-Merck-IQL) by the same surgeon, were selected through astringent preop and postop exclusion criteria that allowed to identify best preserved knees and most homogeneous surgeries. Fluoroscopic lateral views were digitized at 10° intervals from 0° to maximum flexion. Still images were processed through a computerized method based on ellipse fitting (Lacoste 2003) to obtain CCAP (Contact coefficient from anterior to posterior), a percentage measurement of relative tibiofemoral contact. To compare CCAP among flexion angle intervals, ANOVA and post-hoc test (Dunnett-Tukey) were used to obtain statistically significant (P< 0.05) differences from interval to interval. Spearman correlation was calculated to establish the correlation between the CCAP and KSSS score.

Results: Twenty four cases were considered as good and excellent results and 6 cases as bad or poor results. ANOVA test showed that CCAP were significantly different among knee flexion intervals. Graphs confirmed that the sequence of tibiofemoral contact was not erratic, but followed a smooth evolution. From a mean +/– s.d CCAP of 57.8 +/–3.2 at 0° of knee flexion, the contact translated anteriorly and then posteriorly. Post-hoc tests confirmed the significant differences in CCAP among intervals. Spearman Correlation r=0.666 (P< 0.01) indicated that a higher CCAP from 90° flexion was associated with a better clinical result.

Discussion: Our method confirms anterior displacement of tibiofemoral contact through knee ROM towards mid-flexion (60°), and further posterior displacement with 90° flexion and beyond. This paradoxal displacement could not be considered as jig-saw effect but rather as a smooth evolution that required 10° to 10° analysis to be detected. This smooth pathway and not a jig-saw effect could justify the good and excellent results informed by others authors.