Effect of Femoral Component Flexion Implantation on the Mediolateral Bone-prosthetic Fit in Total Knee Arthroplasty

Abstract

Objective: Femoral component overhang in total knee arthroplasty (TKA) has been reported in previous studies. The purpose of this study was to evaluate the effect of femoral component flexion implantation on mediolateral bone-prosthetic fit in TKA.

Methods: Virtual prosthesis implantations were performed on computed tomographic models of 10 Chinese knees with femoral prostheses of the Advance Medial-Pivot knee system (MicroPort Orthopedics, Arlington, TN, USA), with the femoral component positioned at 0°, 3°, or 6° of flexion in the sagittal plane. For each degree of flexion implantation, the differences between the knee and femoral component models on the lateral and medial sides at trochlea (zone 1), anterior-distal condyle (zone 2), posterior-distal condyle (zone 3), and posterior condyle (zone 4) were measured. Positive difference values indicate component overhang, and negative difference values indicate component underhang. The values of component overhang (underhang) in each zone were statistically analyzed across the 3° of flexion implantation.

Results: With a greater degree of flexion implantation, overhang was reduced and even changed to underhang. With 0° of flexion implantation, an overhang exceeding 3 mm existed mainly on the medial side of zone 1 (5.81 mm) and the lateral side of zone 2 (3.39 mm). With 3° of flexion, overhang exceeding 3 mm was observed only on the medial side of zone 1 (3.10 mm), and underhang was observed only on the medial side of zone 4 (-0.32 mm). No overhang exceeding 3 mm was observed for 6° of flexion, while underhang was observed except on the lateral sides of zone 2 (1.32 mm) and zone 4 (1.10 mm) and on the medial side of zone 1 (1.54 mm). A significant difference in overhang values on the lateral and medial sides of zone 1 was observed between 0 and 6° of flexion (P < 0.05).

Conclusion: The present study demonstrated that femoral component flexion implantation by 3° can reduce excessive overhang, although 3.10 mm of overhang remained at the medial side of zone 1. Conversely, 6° of flexion implantation can avoid 3 mm of overhang for any zone, but increases the risk of underhang. Slight flexion implantation may be an effective alternative technique to prevent excessive component overhang, especially in the trochlea and anterior region of the distal condyle, in Chinese patients with standard TKA prostheses.

Keywords: Component overhang; Femoral component; Flexion implantation; Morphometry; Total knee arthroplasty.

Figure 1

Measurement of APk dimensions of the knee models at different degrees of flexion implantation and measurement of APc dimensions of the femoral component models. Transparent parts represent bones resected after implantation of the femoral component in different simulations. APk, distance between the anterior femoral cortex and posterior condyle of the knee model. APc, distance between the internal surface of the proximal anterior flange and posterior condyle of the femoral component.

Figure 2

Lateral and axial views of the measurement zones for the knee and femoral component models. (A) Lateral view showing the four measurement zones of the knee and femoral component models. Zones 1 to 4 correspond to the trochlea, anterior‐distal condyle, posterior‐distal condyle, and posterior condyle, respectively. In the knee model, the transparent part indicates bones resected after implantation of the femoral component. (B) Axial view showing measurement of transverse distances on the lateral (a, b, c, and d) and medial (a′, b′, c′, and d′) sides of the four zones in the knee and femoral component models. The component model was made transparent in order for back to be seen.

Figure 3

Variation in the overhang (underhang) values on the lateral and medial sides of the four zones for the 3° of flexion. *P < 0.05 between 0° and 6° of flexion.

Figure 4

Variation in the gross overhang (underhang) values of each zone for the 3° of flexion. *P < 0.05 between 0° and 6° of flexion.