Are there differences in phenotypes between Asian and Caucasian osteoarthritic knees?

Restoring the Preoperative Phenotype According to the Coronal Plane Alignment of the Knee Classification After Total Knee Arthroplasty Leads to Better Functional Results

While mechanical alignment (MA) is the traditional technique in total knee arthroplasty (TKA), its potential for altering constitutional alignment remains poorly understood. This study aimed to quantify unintentional changes to constitutional coronal alignment and joint line obliquity (JLO) resulting from MA. A retrospective cohort study was undertaken of 700 primary MA TKAs (643 patients) performed between 2014 and 2017. Lateral distal femoral and medial proximal tibial angles were measured pre- and postoperatively to calculate the arithmetic hip-knee-ankle angle (aHKA), JLO, and Coronal Plane Alignment of the Knee (CPAK) phenotypes. The primary outcome was the magnitude and direction of aHKA, JLO, and CPAK alterations. The mean aHKA and JLO increased by 0.1° (SD 3.4°) and 5.8° (SD 3.5°), respectively, from pre- to postoperatively. The most common phenotypes shifted from 76.3% CPAK Types I, II, or III (apex distal JLO) preoperatively to 85.0% IV, V, or VI (apex horizontal JLO) postoperatively. The proportion of knees with apex proximal JLO increased from 0.7% preoperatively to 11.1% postoperatively. Among all MA TKAs, 60.0% (420 knees) were changed from their constitutional alignments into CPAK Type V, while 40.0% (280 knees) either remained in constitutional Type V (5.0%, 35 knees) or were unintentionally aligned into other CPAK types (35.0%; 245 knees). Fixed MA targets in TKA lead to substantial changes from constitutional alignment, primarily a significant increase in JLO. These findings enhance our understanding of alignment alterations resulting from both unintended changes to knee phenotypes and surgical resection imprecision.

The Coronal Plane Alignment of the Knee (CPAK) is a classification that evaluates the joint line obliquity (JLO) and hip-knee-ankle angle (HKA) in osteoarthritis of the knee. The primary aim of this study was to evaluate whether changes in CPAK classification between pre- and postoperative states in patients who underwent robotic-assisted (RA) bi-cruciate stabilized (BCS) total knee arthroplasty (TKA) with a 3-degree varus inclination affected patient reported outcome measurements (PROMs). This retrospective study involved 149 patients treated with bi-cruciate stabilized (BCS) TKA between 2019 and 2023 following a physiological alignment strategy and using image-free robotic-assisted (RA) technique. Pre- and postoperative CPAK classifications were compared, along with changes in arithmetic HKA (aHKA), JLO, and PROMs including the 2011 Knee Society Score, Western Ontario and McMaster Universities Arthritis Index score, Forgotten Joint Score 12, and Patella score were collected at a mean 1.95years (range:1-5) postoperatively. CPAK classification matched pre and postoperatively in 59.1% of cases. Regardless of CPAK classification matching, there were no significant differences in any of these PROMs. aHKA matching was observed in 71.8% of cases, and JLO matching in 81.2%. Comparisons of changes in aHKA and JLO also revealed no significant differences in PROMs. Robotic-assisted "Physiological alignment" BCS TKA with 3-degree varus inclination does not affect PROMs, regardless of pre-arthritic CPAK classification matching. The pre- and postoperative CPAK classifications do not always correspond accurately.

Functional alignment (FA) in total knee arthroplasty (TKA) aims to achieve balanced gaps by adjusting implant positioning while minimizing changes to constitutional joint line obliquity (JLO). Although FA uses kinematic alignment (KA) as a starting point, the final implant positions can vary significantly between these two approaches. This study used the Coronal Plane Alignment of the Knee (CPAK) classification to compare differences between KA and final FA positions. A retrospective analysis compared pre-resection and post-implantation alignments in 2,116 robotic-assisted FA TKAs. The lateral distal femoral angle (LDFA) and medial proximal tibial angle (MPTA) were measured to determine the arithmetic hip-knee-ankle angle (aHKA = MPTA - LDFA), JLO (JLO = MPTA + LDFA), and CPAK type. The primary outcome was the proportion of knees that varied ≤ 2° for aHKA and ≤ 3° for JLO from their KA to FA positions, and direction and magnitude of those changes per CPAK phenotype. Secondary outcomes included proportion of knees that maintained their CPAK phenotype, and differences between sexes. Overall, 71.6% had an aHKA change ≤ 2°, and 87.0% a JLO change ≤ 3°. Mean aHKA changed from -1.1° (SD 2.8°) in KA to -1.9° (SD 2.3°) in FA (mean difference (MD) -0.83 (SD 2.0); p < 0.001). Mean JLO changed from 173.9° (SD 3.0°) in KA to 174.2° (SD 2.6°) in FA (MD 0.38 (SD 2.3); p < 0.001). CPAK type was maintained in 58.1% of knees, with the proportion highest for Types I (73.9%), II (61.1%), and IV (51.2%). In valgus knees, 67.5% of Type III and 71.7% of Type VI were shifted to neutral phenotypes. There was minimal change to constitutional JLO across all CPAK types (MDs -2.0° to 1.2°). Functional alignment may alter CPAK type, but does not significantly change JLO. A kinematic starting point minimizes changes to native anatomy, while final position with FA provides an optimally balanced TKA.

The coronal plane alignment of the knee (CPAK) classification is a nine-phenotype matrix based on limb alignment and joint line obliquity. This study aimed to provide a global overview of CPAK distribution, hypothesising significant geographic, osteoarthritic and sex-related variations. A systematic literature search (Embase, Medline/PubMed and Cochrane Library) following PRISMA guidelines was conducted, utilising the search terms "Coronal Plane Alignment of the Knee" OR "CPAK". Studies considering image modalities other than long-leg radiographs were excluded. A random-effects meta-analysis of proportions was performed, and statistical significance was defined as p < 0.05. A total of 38 studies comprising 46,966 knees were analysed. The most common phenotypes worldwide were CPAK I (33.1%), II (25.9%) and III (14.4%) in the osteoarthritic population and CPAK II (34.9%), I (21.5%) and III (19.3%) in the healthy population. Among osteoarthritic populations, CPAK type I was predominant in Europe (29.2%), Asia (41.9%) and America (33.6%), type II in Australia (32.6%) and type III in Africa (28.6%). In healthy populations, type II was predominant in Europe (42.8%) and Asia (35.3%), whereas type I was most common in South America (44.8%). Significant regional differences were observed among both osteoarthritic and healthy knees, and between osteoarthritic and healthy knees in individual countries. In Europe, significant sex differences were observed in the distribution of types I (39.1% M; 23.5% F) and III (11.4% M; 24.6% F) in the osteoarthritic population, and in the distribution of types I (26.7% M; 9.4% F), II (43.9% M; 34.4% F) and III (11.3% M; 20.6% F) in the healthy population. In Asia, significant sex differences were found for type III in osteoarthritic knees (6.3% M; 11.4% F). CPAK distribution varies significantly as a function of geographic, osteoarthritic, and sex-related factors. A personalised approach to TKA may be desirable to better accommodate these differences. Level IV.

BackgroundThe Coronal Plane Alignment of the Knee (CPAK) classification describes knee phenotypes based on limb alignment and joint-line obliquity (JLO). This study evaluates CPAK distribution in the Hong Kong population and assesses how prosthesis design affects CPAK reproducibility and clinical outcomes.MethodsThis retrospective study included patients who underwent navigated total knee arthroplasty (TKA) with adjusted mechanical alignment in a single institution in 2021. First, we analyzed the epidemiology of CPAK phenotypes in our population. Second, patients were divided into 2 propensity score-matched groups based on implant design: Journey II BCS TKA (JLO prothesis) vs Persona PS TKA (joint line neutral prothesis). Radiological outcomes (mechanical hip–knee–ankle angle, lateral distal femoral angle, medial proximal tibial angle) and clinical outcomes (range of motion, Knee Society Knee Score, Western Ontario and McMaster Universities Osteoarthritis Index score, Forgotten Joint Score) assessed preoperatively and at 6 and 12 months postoperatively.ResultsOur CPAK distribution was comparable to that of other East Asian populations. The Journey II BCS TKA more effectively restored JLO and CPAK phenotypes compared to the Persona PS TKA. However, there was no statistically significant clinical correlation between the restoration of JLO, arithmetic HKA, and CPAK.ConclusionsAlthough radiological restoration improves with JLO prostheses, short-term clinical outcomes remain unaffected.

Maintaining Joint Line Obliquity Optimizes Outcomes of Functional Alignment in Total Knee Arthroplasty in Patients With Constitutionally Varus Knees

<title>Abstract</title> <bold>Background</bold> Accurate assessment of coronal alignment is essential for total knee arthroplasty (TKA) planning. The Coronal Plane Alignment of the Knee (CPAK) classification integrates mechanical alignment and joint line obliquity into nine phenotypes, but its reliability depends on measurement accuracy. This study aimed to compare the accuracy and reliability of CPAK classification and coronal alignment parameters obtained from computed tomography (CT) and long-leg standing radiographs. <bold>Methods</bold> A prospective comparative study was conducted on 100 patients undergoing primary TKA for degenerative arthritis. Each patient underwent standardized long-leg standing radiographs and full-limb CT scans using MAKO robotic planning software. Measurements included the hip–knee–ankle (HKA) angle, lateral distal femoral angle (LDFA), medial proximal tibial angle (MPTA), and joint line obliquity (JLO). CPAK classification was determined from HKA and JLO values. Two independent observers recorded all parameters. Inter-modality differences were analyzed using paired t-tests, and reliability was assessed using intraclass correlation coefficients (ICC) and Cohen’s kappa. <bold>Results</bold> A total of 100 patients were analyzed, with complete datasets for 84–86 knees. CT consistently produced higher values for HKA, LDFA, and MPTA compared with radiographs (p &lt; 0.001), while JLO and CPAK classifications showed no significant difference. Both modalities demonstrated excellent interobserver reliability (ICC &gt; 0.88) and near-perfect CPAK agreement (κ = 0.86–0.88). These results indicate that CT offers greater precision for coronal alignment <bold>Level of Evidence</bold> Level II – Prospective comparative study.

The Coronal Plane Alignment of the Knee (CPAK) classification has been developed to predict individual variations in inherent knee alignment. The impact of preoperative and postoperative CPAK classification phenotype on the postoperative clinical outcomes of total knee arthroplasty (TKA) remains elusive. This study aimed to examine the effect of postoperative CPAK classification phenotypes (I to IX), and their pre- to postoperative changes on patient-reported outcome measures (PROMs). A questionnaire was administered to 340 patients (422 knees) who underwent primary TKA for osteoarthritis (OA) between September 2013 and June 2019. A total of 231 patients (284 knees) responded. The -Knee Society Score 2011 (KSS 2011), Knee injury and Osteoarthritis Outcome Score-12 (KOOS-12), and Forgotten Joint Score-12 (FJS-12) were used to assess clinical outcomes. Using preoperative and postoperative anteroposterior full-leg radiographs, the arithmetic hip-knee-ankle angle (aHKA) and joint line obliquity (JLO) were calculated and classified based on the CPAK classification. To investigate the impact on PROMs, multivariable regression analyses using stepwise selection were conducted, considering factors such as age at surgery, time since surgery, BMI, sex, implant use, postoperative aHKA classification, JLO classification, and changes in aHKA and JLO classifications from preoperative to postoperative. The preoperative and postoperative CPAK classifications were predominantly phenotype I (155 knees; 55%) and phenotype V (73 knees; 26%), respectively. The change in the preoperative to postoperative aHKA classification was a significant negative predictive factor for KOOS-12 and FJS-12, while postoperative apex proximal JLO was a significant negative predictive factor for KSS 2011 and KOOS-12. In primary TKA for OA, preoperative and postoperative CPAK phenotypes were associated with PROMs. Alteration in varus/valgus alignment from preoperative to postoperative was recognized as a negative predictive factor for both KOOS-12 and FJS-12. Moreover, the postoperative apex proximal JLO was identified as a negative factor for KSS 2011 and KOOS-12. Determining the target alignment for each preoperative phenotype with reproducibility could improve PROMs.

Classical neutral mechanical alignment in total knee arthroplasty (TKA) has been a standard paradigm, while more recently, other alignment schemas, such as kinematic, individualized, and functional, have been explored. This study aimed to investigate the effect of three-dimensional (3D) computed tomography (CT)-based surgical robotics inputs on a classically trained surgeon's TKA component positions and alignment targets over time. Data from 1,394 consecutive robotically-assisted TKAs by a single surgeon from 2016 to 2020 were analyzed. Metrics collected included pre-balance planned implant component positions, final planned implant component positions after soft tissue balancing, and constitutional alignment from CT scans. Joint line obliquity was plotted against the arithmetic hip-knee angle (aHKA) using coronal plane alignment of the knee (CPAK). Three categories of alignment strategy were defined: true mechanical alignment (tMA), adjusted mechanical alignment (aMA), and no mechanical alignment (noMA). A shift to overall varus component positioning was observed over the years. Joint line obliquity according to CPAK showed a wider spread in later years, and the distribution of tibial and femoral coronal alignment angles expanded over time. The study revealed a change in alignment targets and final positioning of components away from neutral biomechanical axes in a large volume of TKAs by a single, classically trained surgeon over five years of using a robotic arm-assisted TKA system with CT-based planning. The most dominant factor for this change was the use of 3D CT planning, allowing the surgeon to assess patient-specific anatomy and plan accordingly. Outcome data is needed to determine if this change in behavior and surgical technique was beneficial. In summary, using a CT scan-based robotically assisted technique led to a gradual and complete shift from tMA to predominantly a non-mechanically aligned philosophy in TKA.

Geographic Variation in Knee Phenotypes Based on the Coronal Plane Alignment of the Knee Classification: A Systematic Review

The Coronal Plane Alignment of the Knee (CPAK) and Functional Knee Phenotype (FKP) classification system categorize knee phenotypes into subtypes. It has been proposed that maintaining constitutional knee alignment in knee arthroplasty may enhance outcomes. Therefore, this study aimed to examine the phenotypic distribution in a Thai population using the CPAK and FKP classifications and its correlation with other Asian populations. This retrospective cohort study included 509 patients who underwent knee arthroplasty. Long-leg standing weight-bearing radiographs were used to categorize patients according to the CPAK (arithmetic hip-knee-ankle angle [aHKA], joint-line obliquity [JLO]) and FKP (hip-knee-ankle angle [HKA], femoral mechanical angle [FMA], and tibial mechanical angle [TMA]) classification system. The most common CPAK distribution was type I (49.3%), followed by type II (20.6%) and type IV (11.0%). In males, CPAK Types I and II were the most common, similar to females; however, males had a significantly higher rate of Type I than females (58.8% vs. 46.5%, p = 0.049). The most frequent FKP was VARHKA6°VARFMA3°VARTMA3° (10%), VARHKA3°VARFMA3°NEUTMA0° (7.7%), and NEUHKA0°NEUFMA0°NEUTMA0° (7.1%). Thai arthritic knees predominantly exhibited apex distal JLO with constitutional varus alignment, consistent with findings from other Asian populations. There were no significant differences in CPAK distribution between TKA and UKA patients; however, aHKA and lateral distal femoral angle were significantly different between groups. The Thai arthritic population demonstrates a significant trend toward apex distal JLO and varus alignment, aligning with findings from other Asian populations, which aids in the personalization of knee arthroplasty approaches.

ABSTRACTPurposePronounced apex‐distal joint line obliquity (JLO) complicates total knee arthroplasty (TKA) by challenging patellofemoral tracking and medial tibial bone support. Joint line obliquity–modified kinematic alignment (JLO‐KA)—a selective modification of kinematic alignment (KA) that omits femoral cartilage‐wear compensation and reallocates correction to the tibial side—was developed. This study quantified postoperative component and limb alignment with JLO‐KA versus true KA.MethodsRetrospective comparison of 20 JLO‐KA knees and 15 true‐KA knees with preoperative apex‐distal JLO (CPAK I–III). Pre‐/postoperative computed tomography (CT) measured lateral distal femoral angle (LDFA), medial proximal tibial angle (MPTA), femoral component rotation (FCR), arithmetic hip–knee–ankle angle (aHKA), and JLO; postoperative Coronal Plane Alignment of the Knee (CPAK) distribution was analysed (Δ = JLO‐KA minus true KA).ResultsGroups were similar at baseline: preoperative LDFA 87.7° versus 87.6°, MPTA 83.5° versus 83.5°, aHKA −4.3° versus −4.1°, JLO 171.2° versus 171.2° (all p > 0.05). Postoperatively, JLO‐KA increased LDFA to 90.4° ± 2.3° versus 87.0° ± 1.9° (Δ = +3.4°, 95% confidence interval [CI]: 1.9–4.8; p < 0.0001), MPTA to 88.0° ± 1.4° versus 85.6° ± 2.0° (Δ = +2.4°, 1.1–3.7; p = 0.0015), and FCR to 3.1° ± 2.0° versus 0.1° ± 2.0° (Δ = +2.9°, 1.5–4.3; p = 0.0002), while aHKA was similar (−2.4° ± 3.1° vs. −1.4° ± 2.8°; Δ = −1.0°; p = 0.324). JLO was closer to neutral with JLO‐KA (178.4° ± 2.2° vs. 172.7° ± 2.8°; Δ = +5.8°; p < 0.001). Neutral‐JLO CPAK types (IV–VI) occurred in 16/20 (80%) versus 2/15 (13%) (p = 0.00013). The restricted KA 90° ± 5° range for LDFA and MPTA was met by 19/20 (95%) versus 7/15 (47%) (p = 0.0019).ConclusionReallocating cartilage‐wear compensation from the medial femur to the medial tibia within the same calliper‐verified workflow reduced femoral valgus, limited tibial varus, and increased femoral external rotation by ≈3° while maintaining aHKA. Shifts were consistent with lateralizing the prosthetic trochlear groove and preserving medial tibial bone support, positioning JLO‐KA as a targeted option for apex‐distal knees (CPAK I–III).Level of EvidenceLevel III, retrospective comparative study.

Coronal Knee Alignment in Adult Spinal Deformity Patients: Implications for Total Knee Arthroplasty Planning and Coronal Plane Alignment of the Knee Classification Reliability.

The Role of TKR Design on Knee Stability and Kinematics