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 χ2-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.
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Introduction
Recently, the alignment philosophy underpinning the study of total knee arthroplasty (TKA) has tended to shift from mechanically aligned TKA as the gold standard to personalized alignment instead [10]. Hirschmann et al. created a new classification for functional knee phenotypes using a coronal lower limb alignment based on the native alignment in young individuals without osteoarthritis (OA) [12]. Based on 125 possible functional knee phenotypes, they indicated the eight most common functional phenotypes which covered two-thirds of the total population and represented which phenotypes were suitable for mechanical, anatomical, and restricted kinematic alignment. The group also confirmed the great variability of joint line orientation in osteoarthritic and non-osteoarthritic knees by assessing the femoral mechanical angle (FMA) and tibial mechanical angle (TMA), indicating the necessity of a more individualized approach in TKA [9, 11]. More recently, MacDessi et al. introduced the Coronal Plane Alignment of the Knee (CPAK) classification system which classified knee phenotypes based on constitutional limb alignment and joint line obliquity [21]. The classification system also indicated that the kinematic approach was suitable for Type I (varus, apex distal joint line) and type IV (varus, neutral joint line) out of nine classification categories. Anatomical and restricted kinematically aligned (KA)-TKA [14, 16] have gained popularity for reproducing physiological joint lines and kinematics with minimal soft tissue release, and achieve better clinical outcomes than mechanically aligned TKA. However, recent meta-analyses have shown that the advantage of KA-TKA is still controversial compared with mechanically aligned TKA; one showed better early clinical outcomes and another did not [4, 12] and MacDessi’s [21] classifications for ground KA-TKA should be investigated by widening the patient population. The radiological two-dimensional simulation of the surgery was another limitation of this study. The influence of limb rotational position on parameter changes should be validated by three-dimensional analysis. Most importantly, the clinical outcomes were not assessed. Reduced alignment outliers in ground KA-TKA may lead to good clinical outcomes without any catastrophic failures; however, its clinical relevance should be investigated further in the future.
Conclusions
In conclusion, the ground KA-TKA technique with radiological preoperative planning was easily feasible for mild-to-moderate varus OA patients. This new KA-TKA procedure, as a personalized alignment technique, may provide greater physiological alignment which is more comparable to the native knee than other alignment techniques in TKA.
Data availability
The data that support the findings of this study are available on request from the corresponding author.
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The authors thank Editage, a division of Cactus Communications, for their assistance with the preparation of this manuscript.
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Open access funding provided by Kobe University. The authors received no funding to support this study.
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The hospital’s ethics committee approved the study protocol (Kobe University, No. 290038).
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Matsumoto, T., Nakano, N., Ishida, K. et al. Targeting the neutral hip-to-calcaneus axis in kinematically aligned total knee arthroplasty is feasible with fewer alignment outliers for varus osteoarthritic patients. Knee Surg Sports Traumatol Arthrosc 31, 3880–3888 (2023). https://doi.org/10.1007/s00167-023-07306-1
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DOI: https://doi.org/10.1007/s00167-023-07306-1