1 Introduction

In the pursuit of enhancing longevity, patient-reported outcomes, and the forgotten joint score, surgeons and researchers have increasingly focused on the intricacies of alignment, knee biomechanics, and the adoption of advanced technologies like robotics. Personalized alignment strategies during total knee arthroplasty are increasingly favored for restoring a patient's pre-arthritis anatomical alignment.¹ In the past, the "mechanical alignment (MA) method" was thought to be the best approach for TKA, owing to its excellent long-term survival rates.² In order to form a horizontal joint line—which is thought to offer the best mechanical conditions for prosthesis longevity—The approach involves sectioning the proximal tibia and the femur at the distal end perpendicular to the mechanical axis.³

However, while MA has demonstrated effectiveness in prolonging implant survival, it does not account for the significant individual variations in normal knee anatomy. Critics argue that an exclusive focus on achieving perfect mechanical alignment can lead to soft tissue imbalance along with deviations from the natural varus alignment of the limb, potentially resulting in lower patient satisfaction post-surgery. In this context, Bellemans et al. introduced the ‘constitutional varus’ concept to describe the inherent varus alignment patterns in non-arthritic knees.⁴

This resulted in the development of alternative alignment procedures, notably kinematic alignment (KA) and its variants, which seek to maintain the knee's inherent kinematics and minimize the necessity for soft tissue releases, enhancing patient satisfaction.⁵ As the debate between alignment strategies continued, researchers sought a scientific framework that could encompass both approaches and guide surgeons in selecting the most appropriate technique for their patients.

MacDessi et al.-introduced CPAK classification, which aims to improve the comprehension of coronal plane abnormalities and develop a therapeutically pertinent nomenclature for knee alignment.⁶ Based on Joint line obliquity (apex: neutral, proximal, or distal) & coronal plane alignment (neutral, varus, or valgus), lower extremity alignment is categorized into nine groups. Their work highlighted the importance of understanding the variability in Joint Line Obliquity (JLO) to achieve optimal surgical outcomes. The restoration of both alignment and joint obliquity to their original anatomy might substantially influence the satisfaction of patients post-surgery, as per the classification.

Under the context of robotic-assisted TKA, the possibility of "personalized arthroplasty" that restores JLO & accommodates constitutional varus or valgus has materialized. Comprehending population-level variations in knee phenotypes is crucial, as previous anthropometric studies on implant sizing and tibiofemoral condylar shape have demonstrated the existence of racial variations, which could influence personalized implant choices.⁷

This study aims to classify Indian arthritic knees undergoing TKA based on the CPAK classification system and to explore whether significant racial differences exist in the distribution of the nine described CPAK phenotypes.

2 Materials and methods

The Orthopaedics Department at Sir H.N. Reliance Foundation Hospital & Research Centre conducted this retrospective cross-sectional survey from July 2017 to August 2021. The study encompassed consecutive patients attending the arthroplasty unit with knee discomfort and radiographic confirmation of osteoarthritis, defined as Kellgren-Lawrence grade three or four, who had been slated with TKA. To ensure a homogeneous study population, patients with secondary osteoarthritis, traumatic arthritis, previous knee surgeries, metabolic diseases, inflammatory arthritis, congenital malformations, and post-poliomyelitis sequelae were eliminated. Informed consent had been attained from all participants, and the procedure and planned intervention were thoroughly explained to each patient.

3 Results

Table 1 includes details on the research participants' radiological results and their demographic characteristics. All 378 participants in the current research (500 knees) had TKA to treat advanced osteoarthritis (Kellgren-Lawrence grades 3 & 4). The average age of the patients was 66.58 ± 8.19 years, with a gender distribution of 117 males (30.9 %) and 261 females (69.1 %).

The mean aHKA was −4.70° ± 6.31°, and the mean JLO was 175.89° ± 5.31°. These findings indicate that the majority of arthritic knees in our cohort exhibited a varus alignment with an apex distal JLO. Further, supported by the mean MPTA of 85.58° ± 4.56° and the mean mLDFA of 90.29° ± 3.62°.

The most widespread CPAK alignment phenotype was Type I, which was observed in 44 % of the knees, followed by Type IV, which was seen in 22.2 %. This distribution highlights that most patients in our research had a varus limb alignment combined with either an apex distal or apex neutral joint line configuration. Table 2 provides a detailed summary of the distribution of knees as per the classification of CPAK, and the corresponding scatter plot for arthritic knees is shown in Fig. 1.

Close

Fig. 1 Scatter plot of CPAK classification in the arthritic patient group.

Export to PPT

Additionally, the mean mHKA was 9.11° ± 8.67°. When plotted against the JLO, the prevalent CPAK alignment phenotype was Type-I, accounting for 51 % of the knees, followed by Type IV, which accounted for 25.8 %. The scatter plot for this analysis is depicted in Fig. 2.

Close

Fig. 2 Scatter plot of CPAK classification in the arthritic patient group with mHKA vs JLO that shows a nearly identical distribution.

Export to PPT

Comparisons with similar studies conducted globally revealed notable differences in the distribution of various CPAK phenotypes among different populations. These critical variations are summarized in Table 3.

4 Discussion

The field of arthroplasty is increasingly shifting towards a personalized approach, with the aim of optimizing patient-reported outcomes without compromising implant survivorship. Central to this evolution is the debate among arthroplasty surgeons regarding the ideal coronal plane alignment for individual patients. Historically, mechanical alignment (MA) with a horizontal joint line was the gold standard, regarded as essential for long-term success.² However, the introduction of the CPAK classification and the rapid adoption of newer technologies like robotics have added layers of complexity and nuance to this discussion.

Our study yielded several significant findings pertinent to the Indian population, particularly the predominance of CPAK Type I and Type IV phenotypes. This contrasts with the original research by MacDessi et al., where CPAK Types I and II were the most common, accounting for nearly 50 % of cases.⁶ Interestingly, our findings align closely with the only other reported study from India by Mulpur et al., which also demonstrated a similar distribution pattern.¹² This suggests that traditional mechanical alignment may not be appropriate for most Indian patients with typical osteoarthritis characterized by varus deformity. However, the notable presence of Type IV phenotypes involving varus alignment with a neutral joint line obliquity (JLO) warrants careful preoperative planning. This phenotype may indicate the presence of extra-articular or extra-ligamentous factors contributing to the varus deformity, as previously explained by Bagaria et al., who found that approx. 1 in 7 varus deformities had an extra-articular component.¹⁴

The varus alignment with a neutral JLO should prompt surgeons to carefully assess whether the deformity is purely intra-articular or involves extra-articular components such as metaphyseal varus or compensatory valgus at the distal femur. Understanding these nuances allows for selecting an appropriate alignment and balancing strategy that maximizes patient satisfaction and long-term outcomes.

The relatively lower prevalence of Type II phenotypes in our study, as well as in the previous study by Mulpur et al., indicates a strong tendency towards varus morphometry in the Indian population.¹² This trend has been corroborated by various anthropometric and race-specific studies, which have informed the design of implants better suited to the Asian population. This finding is reminiscent of the gender differences noted by Cooke et al., where varus alignment was more prevalent in males than females.¹⁵ Similarly, Hirschmann et al. reported significant intra- and inter-gender variations in lower limb and knee joint alignments, further emphasizing the need for personalized implant solutions.¹⁶

While the CPAK classification system has been praised for its innovation, it has also confronted criticism for its complexity and potential challenges in recall. The introduction of the aHKA angle as a novel concept contrasts with the more commonly used mechanical HKA (mHKA). Our study utilized both aHKA and mHKA to enhance its practical applicability, demonstrating that similar alignment patterns can be observed with either approach. Before CPAK's formal introduction, many surgeons advocated restoring pre-arthritic anatomy during total knee arthroplasty (TKA), a concept that Howell et al. developed through kinematic alignment (KA), which respects the native knee's kinematics and yielded superior outcomes compared to mechanical alignment.^5,17

This study is one of the few large-scale analyses from a country with a vast and diverse population exceeding 1.4 billion. Our findings highlight the significant prevalence of varus alignments across all ages and genders in the Indian population and the potential presence of extra-articular deformities. This observation raises whether leaving residual varus might improve patient-reported outcome measures. Notably, the prevalence and distribution of valgus phenotypes in our study were comparable to those in Western populations. Given the surgical challenges associated with valgus deformities, which often require complex soft tissue balancing and alignment strategies, advanced technologies like robotics may offer a pathway to more consistent and favorable outcomes.^18,19 Additionally, cultural factors and activities relevant to specific geographical regions, races, and genders should be considered when planning surgeries.²⁰

The study does have limitations, including the exclusive focus on arthritic knees scheduled for TKA and the exclusion of non-arthritic knees. However, MacDessi et al.'s original work indicated that the CPAK-type distribution in arthritic and non-arthritic knees is similar.⁶ Therefore, our study can serve as a surrogate for both populations. Although the sample size may seem small given India's vast population, our findings are consistent with similar studies from other countries, where sample sizes of 500 knees have provided robust data for meaningful interpretation.

Despite its limitations, the CPAK classification system offers a reliable, pragmatic, and comprehensive framework for understanding coronal plane deformities, among the most common and critical issues in knee arthroplasty. By incorporating functional classification systems like CPAK into surgical planning, surgeons can offer a more personalized approach to knee replacement, particularly relevant for the Indian population.

5 Conclusion

This research verifies that most Indian patients receiving total knee arthroplasty have had varus alignment, characterized by either a distal apex or a neutral apex joint line, corresponding to CPAK Types I and IV. Comprehending these traits facilitates a more customized strategy for each patient, perhaps enhancing surgical results and patient satisfaction.

CRediT authorship contribution statement

Akshay Ks: Conceptualization, Formal analysis, Investigation, Methodology, Project administration, Resources, Validation, Visualization, Writing – original draft. Pathik Shah: Data curation. Rahul Ragate: Data curation, Resources. Aashiket Sable: Writing – review & editing. Vaibhav Bagaria: Conceptualization, Supervision, Visualization, Writing – review & editing.

Informed consent (patient/guardian)

Not applicable.

Institutional ethical committee approval (for all human studies)

This research was approved by the Institute Ethics Committee (IEC/2020/DNB/ORTH/03).

Funding/sponsorship

This research did not receive any specific grant from funding agencies in public, commercial or not-for-profit sectors.