The coronal plane alignment of the knee (CPAK) classification system has recently emerged as a promising tool for optimizing surgical alignment strategies in TKA. CPAK categorizes individual knee joint morphology, aiming to improve implant positioning and potentially enhance long-term outcomes. Prior research has primarily investigated the application of CPAK in osteoarthritic knees [11, 18,19,20, 22]. However, there is a paucity of evidence regarding its utility in the pre-osteoarthritic stage, particularly within the Asian population [17, 18]. This knowledge gap necessitates further investigation. Therefore, the present study aims to elucidate the distribution of CPAK phenotypes in young Asian adults without knee osteoarthritis. Additionally, this study will explore potential gender-based disparities in CPAK and identify any intra-individual variations.

MacDessi first described the CPAK classification system, analyzing 500 healthy knees from young adults and 500 osteoarthritic knees undergoing knee replacement [11]. They reported a similar distribution of CPAK phenotypes across both healthy and arthritic groups. Toyooka was the first to investigate the CPAK distribution in an Asian population [20]. They found that CPAK type I (53.8%) was the most common. Notably, the distribution in this Japanese population differed significantly from ours. This discrepancy might be due to the fact that Toyooka focused on elderly osteoarthritic patients with an average age of 75 years, whereas our study included young adults without osteoarthritis. Interestingly, the most common CPAK type in our study was type II (43.3%), similar to the findings of MacDessi (39.2%).

A Systematic Review of Geographic Variation in Knee Phenotypes Based on CPAK investigated healthy and arthritic knees [16]. The review found that among healthy knees [11, 17, 18], CPAK type II was the most common in Belgium (39.2%), Taiwan (39.3%), and India (25.6%). Conversely, among arthritic knees [11, 18,19,20], CPAK type I was most prevalent in France (33.4%), India (58.8%), and Japan (53.8%), while CPAK type II remained the most common type in Australia (32.2%).

The data on gender differences in CPAK type were lacking. While some prior studies suggested an association between CPAK and gender, the data exhibited an asymmetrical distribution between males and females [11, 16, 17, 20]. MacDessi reported that CPAK type II was predominant in arthritic knees undergoing TKA, particularly in women (62%) [11]. However, the study did not explore the overall CPAK distribution between genders. Similarly, Toyooka demonstrated that CPAK type I was most common in women (81%) with osteoarthritic knees, but their study also lacked data on gender distribution [20]. Our study firstly demonstrated gender-based disparities in CPAK, with an almost symmetrical distribution (48.2% for men and 51.8% for women). Gender differences emerged: men were concentrated in zones II (45.7%), III (23.4%), and I (20.7%), while women favored zones II (41.1%), V (18.3%), and III (15.8%). Both genders had the highest rate of CPAK zone II, followed by zone III in men but zone V in women. This information is useful for reconstructive surgeons who perform TKA. Female osteoarthritis (OA) knee patients (18.3%) tended to be more suitable for TKA with neutral mechanical alignment (zone V) than male OA knee patients (5.9%).

Additionally, this study showed that there were significant differences in the overall CPAK distribution, the CPAK with apex point distal, and CPAK zone V between men and women. This study suggests that performing TKA with neutral mechanical alignment on all OA patients may be inappropriate, as evidenced by the fact that only 80% of patients reported satisfaction with the overall outcome of surgery [7,8,9,10]. On the contrary, the majority of the population (76.9%) had a distally located apex JLO (89.9% in men and 64.9% in women). The concept of a slightly varus tibial cut (approximately 2 degrees) is justified, as it effectively restores the natural JLO in most patients. This study correlates with previous study that recommended the restoration of JLO of the tibia in restricted kinematic alignment [21]. This, in turn, could facilitate restricted kinematic or functional alignment and potentially lead to a more expedited recovery due to a less extensive medial collateral ligament release.

Intra-individual participant similarity and variation in CPAK alignment is an important issue in current practices and considerations in preoperative planning for TKA. Hip and knee reconstruction surgeons typically rely on the alignment of the contralateral knee to inform potential CPAK zone determination for the affected knee undergoing TKA. However, data from this study suggests a significant discrepancy within individual participants. The concordance rate for CPAK type was only 56.4% for men and 55.4% for women. These findings necessitate a more comprehensive approach to preoperative planning for bone cuts during TKA. This approach should integrate three key factors, the CPAK alignment of the affected knee, the CPAK alignment of the contralateral knee, and the most prevalent CPAK type observed in the patient’s geographical location. By considering all three factors, surgeons can make a more informed decision regarding bone cut placement during TKA.

Several previous studies investigated the CPAK distribution at a single institution [11, 15,16,17,18,19,20, 22]. In contrast, this study examined data from two centers: a tertiary care hospital and an urban general hospital. This broader scope likely provides a more representative picture of the overall distribution. However, this study had several limitations. First, although it had a larger sample population than previous study demonstrating the existence of constitutional varus [23], the power analysis may not have been sufficient to represent the overall phenotype of CPAK in our population. Second, this study lacked a comparison group of osteoarthritic knees undergoing TKA. Additionally, the prevalence of CPAK may differ across different countries. Orthopedic surgeons should validate the applicability of this concept to osteoarthritis knee patients undergoing TKA in their own countries. Third, MacDessi found a similar distribution of CPAK in young adults and OA knees [11]. This suggests that the results of CPAK in this study could be used for planning bone cuts in TKA. However, future studies are needed to determine the exact changes in CPAK over different time intervals in individual patients.