Targeting the neutral hip‐to‐calcaneus axis in kinematically aligned total knee arthroplasty is feasible with fewer alignment outliers for varus osteoarthritic patients
Tomoyuki Matsumoto, Naoki Nakano, Kazunari Ishida, Toshihisa Maeda, Shotaro Tachibana, Yuichi Kuroda, Shinya Hayashi, Takehiko Matsushita, Ryosuke Kuroda
Abstract
Abstract Purpose Assessment of the conventional mechanical axis (MA) (hip‐to‐talus axis) is reported to result in constitutional varus in the native knee. However, the ground MA (hip‐to‐calcaneus axis), which is the line from the hip center to the bottom of the calcaneus, passes through the center of the knee joint in the native knee and is a possible alternative target for total knee arthroplasty (TKA) assessments. Therefore, this study aimed to present a “ground kinematically aligned (KA)‐TKA.” In this technique, the femoral component is placed on the cylindrical axis using the calipered technique and the tibial component is placed to give a neutral ground MA. Radiographical investigation was used to determine whether physiological alignment can be individually achieved with ground KA‐TKA; this was compared with that of a tibia‐restricted modified KA‐TKA, referring to conventional MA (hip‐to‐talus axis) results. Methods As the primary endpoint, this prospective cohort study compared the ground MA ratios of the knee joints in 40 ground KA‐TKAs (G group: Coronal Plain Alignment of the Knee (CPAK) 28 type I, 7 II, 1 IV, and 4 V) with those of the preceding 60 modified KA‐TKAs (M group: CPAK 46 type I, 12 II, and 2 V) performed for patients with varus osteoarthritis (OA). The number of outliers differing over ± 5% from the neutral were compared between groups using the χ 2 ‐test. The Hip–knee–ankle (HKA) angle, coronal femoral/tibial component alignment (FCA/TCA), and joint line orientation angle (JLOA) were compared between the groups using non‐paired t‐ tests. Statistical significance was set at p < 0.05. Results The G group had a higher ratio of the ground MA passing through the knee center than the M group did; outliers differing over ± 5% from the neutral of the ground MA were 2/40 cases in the G group and 20/60 cases in the M group, which was a significant difference ( p = 0.001). The HKA angle, FCA/TCA, and JLOA were not significantly different between the groups. Conclusions Targeting the ground MA in KA‐TKA for patients with varus OA was feasible and has the potential to provide a physiological alignment more similar to the native knee in TKA than other kinematic alignment techniques. Level of evidence Level III.