The Morphology of a Kinematically Aligned Distal Femoral Osteotomy Is Different from That Obtained with Mechanical Alignment and Could Have Implications for the Design of Total Knee Arthroplasty

Abstract

Background: Kinematically aligned total knee arthroplasty (KA-TKA) may lead to a different pattern of osteotomy from mechanically aligned total knee arthroplasty (MA-TKA). This paper aims to analyze the effects of KA and MA on the morphology of the distal femoral osteotomy surface. Methods: Computed tomography scans of 80 TKA candidates were reconstructed into 3D models. The measurement of bone morphology was performed after the distal femur cut according to two different alignment techniques. The aspect ratio, trapezoidicity ratio, and asymmetry ratio of the distal femur were assessed. Results: The aspect ratio and the asymmetry ratio in the KA group was significantly lower than that in the MA group in the general population (p < 0.001). The trapezoidicity ratio in the KA group was significantly higher than that in the MA group in the general population (p < 0.001). Conclusions: It was found that KA-TKA and MA-TKA presented different morphologies of the distal femoral osteotomy surface, and this difference was also influenced by gender. The surgery pattern of KA-TKA and MA-TKA and gender should be considered when surgeons choose femoral prostheses.

Keywords: kinematic alignment; mechanical alignment; morphotype; total knee arthroplasty.

Figure 1

(A) Distal femoral cutting of kinematically aligned total knee arthroplasty (KA-TKA). (B) Distal femoral cutting of mechanically aligned total knee arthroplasty (MA-TKA). The cutting thickness was 7 mm.

Figure 2

Three geometric ratios describe the morphology of bone after distal femur cutting: (A) aspect ratio (ML/AP), (B) asymmetry ratio (LAP/MAP), and (C) trapezoidicity ratio (PML/AML). The maximum medial AP perpendicular to the line connecting the medial condyle and the posterior margin of the medial and lateral condyles was taken as medial anteroposterior (MAP). The maximum lateral AP was lateral anteroposterior (LAP). The larger of the two values was set to the AP value. The posterior medial-lateral (PML) width was the posterior femoral condylar cutting width and the anterior medial-lateral (AML) width was measured at 75% AP.

Figure 3

Transverse CT slice at the level of the distal cutting of the femur, indicating the femoral cortical contours (blue). The posterior medial-lateral (PML) width was the posterior femoral condylar cutting width, the central medial-lateral (CML) width was measured at 50% AP, and the anterior medial-lateral (AML) width was measured at 75% AP. Lateral narrowing: αL and βL; medial narrowing: αM and βM.