Feasibility of Targeting Neutral Hip-to-Calcaneus Axis in KA-TKA for Varus OA

Overview

This study evaluates a ground kinematically aligned total knee arthroplasty (ground KA-TKA) technique targeting a neutral hip-to-calcaneus mechanical axis (ground MA) in varus osteoarthritis patients. The ground KA-TKA aims to reduce alignment outliers compared to tibia-restricted modified KA-TKA by individualizing femoral and tibial component positioning based on full-length radiographic simulation including the calcaneus.

Background

Total knee arthroplasty (TKA) alignment philosophies are shifting from mechanical alignment to personalized approaches such as kinematic alignment (KA-TKA) to better reproduce native knee kinematics and joint line orientation. Constitutional varus alignment in young healthy knees challenges the neutral mechanical axis concept, prompting consideration of the ground mechanical axis from hip to calcaneus as a more physiological target. Previous tibia-restricted modified KA-TKA techniques used a fixed 3° varus tibial cut but did not intentionally target the ground MA, resulting in occasional neutral ground MA alignment. This study proposes a ground KA-TKA technique that simulates femoral and tibial cuts to achieve neutral ground MA alignment, hypothesizing fewer alignment outliers.

Data Highlights

Radiographic simulations were performed using full-length standing coronal radiographs including the calcaneus. Femoral distal cuts were simulated 7 mm proximal to the bicondylar distal end line, accounting for cartilage thickness, and tibial proximal cuts were simulated 8 mm distal to the lateral joint line. The femoral angle (FA) was defined as the angle between the femoral mechanical axis (hip to distal cut center) and the line perpendicular to the distal cut line, typically valgus in varus OA knees. The tibial angle (TA) was set to neutralize the FA relative to the ground MA (hip to calcaneus). The navigation system referenced the ankle center, not the calcaneus, leading to a ΔTA difference representing the angle between the mechanical axis and ground MA of the tibia. The navigated tibial cut angle (nTA) was calculated as nTA = TA + ΔTA.

Key Findings

• The ground KA-TKA technique uses individualized femoral and tibial bone cuts simulated from full-length radiographs including the calcaneus to achieve neutral ground MA alignment.
• Femoral distal cuts are planned 7 mm proximal to the bicondylar distal end line, resulting in a femoral angle typically valgus relative to the mechanical axis.
• Tibial proximal cuts are adjusted to neutralize the femoral angle relative to the ground MA, differing from conventional navigation referencing the ankle center.
• The difference between the ankle center and calcaneus position (ΔTA) affects the navigated tibial cut angle, potentially increasing varus alignment if uncorrected.
• Ground KA-TKA aims to reduce alignment outliers compared to tibia-restricted modified KA-TKA by accounting for individual anatomical variations and targeting the hip-to-calcaneus axis.

Clinical Implications

Targeting the neutral hip-to-calcaneus mechanical axis in KA-TKA may improve individualized alignment accuracy and better reproduce native knee kinematics in varus OA patients. Incorporating full-length radiographs including the calcaneus into preoperative planning allows for more precise femoral and tibial component positioning, potentially reducing alignment outliers and improving clinical outcomes. Surgeons should consider the positional difference between the ankle center and calcaneus when using navigation systems to avoid unintended varus tibial cuts.

Conclusion

The ground KA-TKA technique is a feasible approach to achieve neutral hip-to-calcaneus mechanical axis alignment in varus osteoarthritis patients, potentially offering improved individualized alignment over tibia-restricted modified KA-TKA. This method may enhance the physiological restoration of knee alignment and function in TKA.

References

1. Haraguchi et al. -- True mechanical axis assessment from hip to calcaneus
2. Matsumoto et al. -- Tibia-restricted modified KA-TKA procedure
3. Kamenaga et al. -- Gait analysis after tibia-restricted modified KA-TKA
4. Hirschmann et al. -- Functional knee phenotypes classification
5. MacDessi et al. -- Coronal Plane Alignment of the Knee (CPAK) classification