The Coronal Plane Alignment of the Knee Classification May be Unreliable With Advanced Knee Arthritis

Abstract

Background

Kinematic alignment (KA) seeks to restore native, prearthritic alignment, including the arithmetic hip–knee–ankle angle (aHKA) and joint line obliquity (JLO). Advanced osteoarthritis may alter these parameters through erosion, subchondral collapse, or insufficiency fracture, potentially misrepresenting the native coronal plane alignment of the knee (CPAK) phenotype. Therefore, we sought to evaluate concordance in CPAK phenotype parameters between the severely arthritic knee and the contralateral healthy knee within the same patient.

Methods

We performed a radiographic review of 578 patients with bilateral, weight-bearing, long-leg radiographs obtained prior to total knee arthroplasty. The inclusion criteria required a Kellgren–Lawrence grade 4 knee (arthritic) and a contralateral Kellgren–Lawrence grade 0 or 1 knee (healthy). Absolute side-to-side differences were calculated for aHKA, JLO, medial proximal tibial angle, and lateral distal femoral angle, with additional stratification based on the predominant compartment of arthritic degeneration (medial or lateral).

Results

Ninety patients met the inclusion criteria. The mean absolute differences between healthy and arthritic knees were 3.5° for aHKA, 2.3° for JLO, 2.4° for medial proximal tibial angle, and 1.7° for lateral distal femoral angle. CPAK concordance between healthy and arthritic knees was limited, with matching phenotypes in only 41% of patients with medial arthritis and 50% with lateral arthritis.

Conclusions

Advanced knee osteoarthritis may lead to permanent bony changes that alter the measured CPAK phenotype. Replicating this pathologic alignment during kinematic alignment total knee arthroplasty could result in unintended component malposition. Careful preoperative radiographic assessment is essential to identify and account for these deformities.

Introduction

A neutral mechanical alignment (MA) has long been considered the gold standard in total knee arthroplasty (TKA). The MA philosophy seeks a balanced load distribution, thus minimizing loosening and wear, despite very few native knees presenting with neutral alignment [1,2]. Early data supporting MA reported higher implant failure rates when alignment fell outside a defined “safe zone” of ±3° [3,4]. However, more recent 20-year survivorship data demonstrate no statistically significant difference in implant longevity between TKAs placed within or outside this ±3° window [5]. In parallel, reports of up to 10 to 30% patient dissatisfaction have fueled growing interest in alternative alignment strategies [[6], [7], [8], [9]].

One such strategy, kinematic alignment (KA), aims to restore a patient’s native, prearthritic limb alignment and joint line obliquity (JLO). By maintaining the native joint line, numerous studies have demonstrated a positive influence of patient-reported outcome measures [10,11]. Popularized by Howell et al. [12] in the early 2000s, KA often relies on caliper-measured resections, adjusting for cartilage loss. The introduction of the coronal plane alignment of the knee (CPAK) classification by MacDessi et al. [13] accelerated interest in KA by providing an objective framework to categorize native alignment patterns and JLO (Fig. 1). As recognition of anatomic variability has grown, multiple randomized controlled trials now suggest similar or improved early to mid-term outcomes with KA compared to MA [[14], [15], [16], [17], [18]].

Figure 1.

Schematic representation of the CPAK classification system. CPAK categories are defined by the intersection of arithmetic hip–knee–ankle angle (aHKA = MPTA − LDFA) on the horizontal axis and joint line obliquity (JLO = MPTA + LDFA) on the vertical axis. Thresholds of −2° and 2° define varus, neutral, and valgus alignment, while JLO values of 177° and 183° separate apex distal, neutral, and apex proximal joint line orientation, resulting in nine CPAK types (I–IX).

However, whereas MA relies on placing implants within an otherwise fixed alignment target, KA typically requires precise characterization of each patient’s unique anatomy and caliper verification. This may present challenges in the setting of advanced arthritis, where permanent bony joint line changes may occur [19,20]. Although CPAK measurements, by definition, are unaffected by loss of cartilage, they do not account for bony deformities resulting from erosion, subchondral collapse, or insufficiency fractures. Misinterpreting a patient’s “native” CPAK phenotype in the presence of such deformities could, for example, result in coronal component malposition (eg, more tibia implant varus in severe medial arthritis).

To better characterize this phenomenon, we investigated bilateral knee concordance in patients with minimal or no arthritis (healthy) in 1 knee and severe arthritis (arthritic) in the contralateral knee. We asked 2 questions:

• 1.Does advanced arthritis deviate from the contralateral healthy knee in relation to coronal alignment parameters [arithmetic hip-knee-ankle angle (aHKA), JLO, medial proximal tibial angle (MPTA), and lateral distal femoral angle (LDFA)] and therefore the CPAK classification?
• 2.Do the predominant compartmental changes (medial vs lateral arthritis) influence the degree or direction of these alterations?

Material and methods

Study design

We conducted a retrospective, institutional review board–approved review of 578 patients who underwent bilateral, weight-bearing, long-leg radiographs between January 2021 and December 2023 as part of a standardized preoperative templating protocol prior to TKA. The Institutional Review Board waived the requirement for informed consent due to the retrospective nature of this study. Inclusion criteria were adults ≥18 years with a Kellgren–Lawrence (KL) [21] grade 4 knee (arthritic) and a contralateral KL grade 0 or 1 knee (healthy). Exclusion criteria were failure to meet KL criteria, prior periarticular knee fracture, rheumatoid arthritis, prior septic arthritis, developmental deformities (eg, Blount’s disease), prior femur or tibia osteotomy, prior anterior cruciate ligament reconstruction, flexion contracture >10°, or inadequate radiographs outside the acquisition parameters as described by Chen et al. [22]. Demographic variables, including age, sex, body mass index (BMI), and laterality, were recorded.

Radiographic assessment and calculation

One reviewer assessed all radiographs to determine the CPAK classification for each knee [13]. Measurements included the LDFA and MPTA. From these, the aHKA (aHKA = MPTA – LDFA) and JLO (JLO = MPTA + LDFA) were calculated. For aHKA, negative values represent varus alignment and positive values represent valgus alignment, with values near zero reflecting neutral alignment. For JLO, values <180° indicate an apex distal joint line orientation, whereas values >180° indicate an apex proximal orientation, with values near 180° representing a neutral joint line orientation. Lower MPTA values correspond to greater proximal tibial varus, whereas higher values correspond to greater proximal tibial valgus. Conversely, larger LDFA values reflect increased distal femoral varus and smaller values reflect distal femoral valgus. Alignment thresholds were defined as follows: aHKA: neutral 0 ± 2°, varus < −2°, and valgus >2°; JLO: neutral 180 ± 3°, apex proximal >183°, and apex distal <177°.

For each radiographic parameter, the absolute side-to-side difference (delta) between healthy and arthritic knees was calculated for all patients (Fig. 2a, b). Differences were further stratified by the predominant compartment of arthritic involvement (medial vs lateral). All knees were classified according to the CPAK classification and compared between groups. Any differences between the overall CPAK class, alignment, and JLO in each patient were also recorded.

Figure 2.

Long-leg radiographs demonstrating (A) medial-based degeneration with deltas of 13.5° for aHKA, 4.8° for JLO, 9.1° for MPTA, and 4.4° for LDFA between sides, and (B) laterally based osteoarthritis with deltas of 10.1° for aHKA, 3.7° for JLO, 6.9° for MPTA, and 3.2° for LDFA.

Measurement acquisition was completed in a blinded manner with the utilization of a PACS system (Opal-Rad), which provides for accurate magnification calibration by the use of a standard 25.4 mm Orthosphere ball (OrthoMark; RolleSolutions Inc, Anaheim, California). One reviewer performed the primary radiographic measurements and determined the CPAK classification for all included knees. To assess measurement reliability, a random subset of 25 patients was remeasured by the same reviewer to evaluate intrarater reproducibility. An additional 25 patients were independently measured by a second reviewer to assess inter-rater reliability.

Data analyses

For all alignment angles, we assessed intrarater reproducibility and inter-rater reliability using the intraclass correlation coefficient (ICC) and Kappa Statistics. The mean LDFA, MPTA, JLO, and aHKA were determined by location of arthritis. Absolute side-by-side differences between the Healthy and Arthritic knees were calculated for each parameter (LDFA, MPTA, JLO, and aHKA). These differences were summarized across the cohort using both means ± standard deviation to provide overall population-level differences.

To further characterize whether arthritic deformity represented exaggeration of the native alignment or a change in direction, we additionally evaluated signed side-to-side differences between the arthritic and healthy knees (Δ = arthritic − healthy). Negative ΔaHKA values indicated a shift toward greater varus alignment, whereas positive values indicated greater valgus alignment. Similarly, negative ΔJLO values indicated a shift toward apex distal orientation and positive values indicated apex proximal orientation. Knees were categorized according to the direction of change, and the magnitude of deviation within each category was summarized using the mean signed difference and standard deviation.

Secondary outcome measurements were analyzed using the Wilcoxon signed-rank test. Statistical analyses were performed using R version 4.5.1 (Vienna, Austria).

Results

Ninety patients met the inclusion criteria and were included in the final analysis (mean age 68.3; range, 36 to 87 years). Additional demographic characteristics are summarized in Table 1. The distribution of healthy and arthritic knee CPAK classifications is shown in Figure 3b. Inter-rater reliability was strong, with ICCs ranging from 0.84 to 0.95, and intrarater reproducibility was similarly high, with ICCs ranging from 0.84 to 0.91 (Table 2).

Table 1.

Demographics.

Demographic	N = 90a
Age	68.3 (36, 87)
BMI	30.0 (20, 40)
Sex
Men	46 (51)
Women	44 (49)
Operative laterality
Left	30 (33)
Right	60 (67)
Race
White	84 (94)
Black	2 (2)
Asian	1 (1)
Not documented	3 (3)

^aMean ± SD (min, max); n (%).

Figure 3.

(A) CPAK classification distribution based on arthritis location and healthy vs arthritic knees, knees are plotted based on aHKA and JLO, separated into nine CPAK regions (I-IX). (B) Distribution of arthritic and healthy knees across the nine CPAK classifications. (C) CPAK distribution of knees in patients with medial arthritis. (D) CPAK distribution of knees in patients with lateral arthritis.

Table 2.

Inter-rater and intrarater correlation coefficients for LDFA and MPTA measurements on long-leg radiographs.

Measurement	Inter-rater correlation coeff	Intrarater correlation coeff
LDFA (operative)	0.95	0.89
LDFA (nonoperative)	0.84	0.84
MPTA (operative)	0.95	0.89
MPTA (nonoperative)	0.85	0.91

Side-by-side alignment differences

The mean absolute difference (Δ) between healthy and arthritic knees was 3.5° (0.0 to 18.9°) for aHKA, 2.3° (0.0 to 6.8°) for JLO, 2.4° (0.0 to 10.2°) for MPTA, and 1.7° (0.0 to 8.7°) for LDFA (Table 3).

Table 3.

Absolute group and arthritis location differences (delta).

Characteristic	Overall N = 90a	Medial N = 72a	Lateral N = 18a	P-Valueb
ΔaHKA (deg)	3.52 ± 3.08 (0.00, 18.90)	3.20 ± 3.15 (0.00, 18.90)	4.80 ± 2.50 (0.40, 10.10)	.029
ΔJLO (deg)	2.25 ± 1.70 (0.00, 6.80)	2.11 ± 1.68 (0.00, 6.80)	2.79 ± 1.68 (0.20, 6.00)	.139
ΔMPTA (deg)	2.35 ± 2.06 (0.00, 10.20)	2.17 ± 2.02 (0.00, 10.20)	3.07 ± 2.11 (0.60, 6.90)	.114
ΔLDFA (deg)	1.74 ± 1.46 (0.00, 8.70)	1.64 ± 1.50 (0.00, 8.70)	2.12 ± 1.28 (0.20, 4.60)	.187

ΔaHKA, delta anatomic hip-knee-ankle axis; ΔJLO, delta joint line obliquity; ΔMPTA, delta medial proximal tibial angle; ΔLDFA, delta lateral distal femoral angle.

^aMean ± SD (min, max).

^bWelch’s t-test (unequal variances). Assessed between medial and lateral cohorts.

Effects of medial vs lateral compartment degeneration

Knees with predominantly medial vs lateral compartment degeneration demonstrated differences across all alignment parameters: aHKA was −2.8° vs 7.0° (P < .001), LDFA was 88.1° vs 84.5° (P < .001), and MPTA was 85.4° vs 91.5° (P < .001), respectively (Table 4).

Table 4.

Variation by predominant arthritis location.

Characteristic	Medial N = 72a	Lateral N = 18a	P-Valueb
aHKA (operative)	−2.8 ± 3.8 (−16.5, 6.3)	7.0 ± 3.0 (1.1, 12.2)	<.001
JLO (operative)	173.5 ± 3.4 (162.3, 180.1)	175.9 ± 3.5 (167.5, 182.5)	.013
LDFA (operative)	88.1 ± 2.1 (82.1, 94.9)	84.5 ± 1.7 (81.4, 88.5)	<.001
MPTA (operative)	85.4 ± 2.9 (77.8, 91.8)	91.5 ± 2.8 (84.3, 95.8)	<.001

^aMean ± SD (min, max).

^bWelch’s t-test (unequal variances).

Lateral compartment disease was associated with a greater absolute change in aHKA than medial disease (4.8° vs 3.2°, P = .029). Parameter-specific absolute differences based on the site of degeneration are detailed in Table 3. When compared with the healthy knee, medial arthritis produced a smaller mean aHKA and MPTA and a greater LDFA. In contrast, lateral arthritis produced the opposite pattern, with increases in aHKA and MPTA and a corresponding decrease in LDFA (Table 5).

Table 5.

CPAK distribution between groups.

CPAK classification	Medial—nonoperativea	Medial—operativea	Lateral—nonoperativea	Lateral—operativea	Total
I	19 (26)	38 (53)	2 (11)	0 (0)	59 (33)
II	24 (33)	18 (25)	3 (17)	1 (5.6)	46 (26)
III	14 (19)	4 (5.6)	8 (44)	12 (67)	38 (21)
IV	3 (4.2)	5 (6.9)	0 (0)	0 (0)	8 (4.4)
V	8 (11)	5 (6.9)	1 (5.6)	0 (0)	14 (7.8)
VI	3 (4.2)	2 (2.8)	4 (22)	5 (28)	14 (7.8)
XI	1 (1.4)	0 (0)	0 (0)	0 (0)	1 (0.6)
Total	72 (100)	72 (100)	18 (100)	18 (100)	180 (100)

^aN (%).

CPAK classification patterns

The most common CPAK type in medial arthritic knees was type 1 (53%), whereas the corresponding healthy knees most commonly demonstrated type 2 (33%) (Table 5; Fig. 3a-d). In lateral arthritic knees, type 3 remained the most frequent category and increased in prevalence compared with healthy knees (44% vs 67%). Across the full cohort, CPAK type 1 was nearly twice as common in arthritic compared with healthy knees [42.2% (n = 38) vs 23.3% (n = 21)]. Concordance between healthy and arthritic knee CPAK classifications was limited, where only 41% (31/72) of medial arthritis patients and 50% (9/18) of lateral arthritis patients demonstrated matching CPAK types (Fig. 4). In medial arthritis, aHKA matched in 50.0% of cases and JLO in 83.3%. In lateral arthritis, aHKA matched in 66.7% of cases, with JLO matching in 77.8%.

Figure 4.

Alluvial diagram showing transitions in CPAK classification from the contralateral healthy knee to the arthritic operative knee. The left strata represent healthy knees and the right strata represent arthritic knees. Flow width is proportional to the number of patients within each healthy-to-arthritic CPAK transition.

Directional deviation from the contralateral healthy knee

To determine whether arthritic deformity represented exaggeration of the native alignment or directional change, we evaluated the signed difference between the arthritic and healthy knees. In medial arthritis, the arthritic knee most commonly demonstrated a shift toward greater varus alignment (79.2%), whereas 18.1% demonstrated a shift toward greater valgus alignment and 2.8% showed no change. The mean magnitude of the varus shift in medial arthritis was −3.6° ± 3.4°. In contrast, lateral compartment arthritis demonstrated a strong shift toward valgus alignment, with 94.4% of knees demonstrating greater valgus relative to the contralateral healthy knee. The mean valgus shift in this group was 4.9° ± 2.6°.

Joint line orientation demonstrated a similar directional pattern. In medial arthritis, the arthritic knee most commonly shifted toward a more apex distal joint line orientation (56.9%), with a mean deviation of −2.24° ± 1.84°, whereas 40.3% shifted toward apex proximal orientation. In lateral compartment disease, the arthritic knee most commonly shifted toward a more apex proximal orientation (61.1%), with a mean deviation of 2.85° ± 1.67°. These directional changes are illustrated in Figure 4.

Discussion

The rise of alternative alignment strategies, most notably KA, has been driven by persistent dissatisfaction rates associated with traditional mechanically aligned TKA and an improved understanding of individualized patient anatomy [1,6,9,23]. Numerous studies have suggested that constitutional alignment, including a natural “slight varus” in many patients, is both common and physiologic [24,25]. The introduction of the CPAK classification further advanced this concept by providing surgeons with an objective framework to categorize patients according to their unique alignment and JLO [13].

KA seeks to restore a patient’s prearthritic anatomy, thereby producing a more individualized reconstruction while minimizing soft tissue releases [26]. Restoration of the joint line has important implications beyond radiographic alignment, particularly with respect to the surrounding soft tissue envelope. The medial collateral ligament (MCL), for example, serves as a primary stabilizer of the knee while functioning in a near-isometric fashion in the native joint. When the joint line is appropriately restored, physiologic MCL tension is maintained, allowing for balanced flexion and extension gaps and likely more normal knee kinematics. In contrast, elevation of the joint line may lead to relative laxity of the MCL in mid-flexion, contributing to mid-flexion instability and paradoxical motion [27]. Growing level I evidence suggests improved functional outcomes with KA compared to MA [14,15,[28], [29], [30], [31], [32], [33], [34], [35]]. Moreover, long-term survivorship data have challenged early concerns regarding implant durability in KA, despite the nontraditional component positioning inherent to this technique [33,34].

However, the presence of severe arthritic changes, as demonstrated in this study, may result in substantial alterations to the joint line orientation (eg, 3.5° aHKA) when compared with the contralateral “healthy” knee. These degenerative changes can affect the CPAK classification and its associated parameters, including aHKA, JLO, MPTA, and LDFA. Therefore, relying on the radiographics and caliper assessment alone may lead to misinterpretation of a patient’s native, prearthritic phenotype and potential component malposition. For example, knees with predominantly medial compartment disease demonstrated decreased aHKA and MPTA relative to the contralateral healthy knee. Attempting to “match” these arthritic angles with or without caliper verification, accounting only for cartilage wear, would inadvertently position the tibia component and limb into pathologic varus. The same issue may occur for those with lateral compartment arthritis, which could, in part, explain the altered patellofemoral mechanics reported with KA [35,36].

In addition to absolute differences between the healthy and arthritic knees, our directional analysis demonstrated that these changes generally followed predictable biomechanical patterns rather than occurring randomly. In medial arthritis, the arthritic knee most commonly demonstrated a shift toward greater varus alignment relative to the contralateral healthy knee (79.2%), while lateral arthritis demonstrated a strong shift (94.4%) toward greater valgus alignment. Joint line orientation demonstrated a similar directional pattern, with medial arthritis more frequently shifting toward an apex distal orientation and lateral arthritis more commonly shifting toward apex proximal orientation. These findings suggest that advanced osteoarthritis (OA) exaggerates compartment-specific collapse patterns, altering the measured CPAK phenotype relative to the patient’s native anatomy and indicating that reliance on the arthritic knee alone may misrepresent the prearthritic alignment.

The development of bony joint line changes in advanced OA has been well described [19,20,[37], [38], [39], [40], [41]]. Steele et al., using deep learning analysis of 1946 full-length radiographs, demonstrated that KL 4 knees were mostly CPAK type I, whereas less arthritic knees were more frequently type II [20]. Importantly, nearly half of the KL 4 knees were categorized as varus aHKA. The authors emphasized that recognizing osseous bone loss is essential to accurately understand true prearthritic alignment. Similarly, Kim et al. [19] evaluated 17,365 radiographs using a comparable modeling approach and identified a leftward shift in CPAK distribution (type II → type I) with advancing OA, attributing these changes to progressive osseous attrition or bone remodeling. Jenny et al. [37] likewise reported normal alignment in only 12.7% of arthritic knees compared with 24.7% in nonarthritic knees, with a substantially wider distribution of phenotypes in the OA population.

Although it is reasonable to argue that some deformity and asymmetry may precede the onset of radiographic arthritis, several longitudinal studies suggest that bony joint line changes evolve with disease progression [38,39,42]. Nomoto et al. [38] evaluated patients 8 years apart with documented progression from KL 0–2 to KL 3–4. Despite a similar CPAK distribution (type I most common in both groups), they observed decreasing MPTA (83.7° → 82.3°) and aHKA (−3.6° → −4.9°). Colyn et al. [39] reported similar findings over a shorter interval (1–4.5 years), noting decreasing MPTA and increasing LDFA, with the largest changes occurring in KL 3–4 knees, which they attributed to progressive subchondral erosion.

Our data suggest that KL 4 knees, beyond the expected cartilage loss, infrequently maintain their native joint line orientation. As a result, the KA surgeon faces an added challenge in estimating the prearthritic anatomy. Several strategies may help guide this determination. The most reliable, though seldom available, is the review of prior imaging (eg, radiographs or computed tomography scans) obtained before advanced degenerative changes occurred. Alternatively, current imaging in a lesser arthritic knee (KL 0 to 3) may still reasonably reflect the original joint line, as osseous remodeling is unlikely to be clinically meaningful at these stages. Howell et al. advocated referencing the base of the tibial spines during caliper evaluation, though this method may overlook variability in cartilage thickness and the true dwell point of the tibial plateau [27,43]. Finally, an additional approach may be to reference the contralateral knee as performed in this study, provided it is KL 3 or less. While bilateral knee anatomic concordance remains debated, recent data suggest a lack of clinically significant asymmetry between sides [44,45]. Interestingly, Beckers et al concluded the 2 sides were unreliable regarding HKA concordance based on radiographic evaluation. However, this study did not assess CPAK classification. Importantly, when assessing for the tibia mechanical axis and femoral mechanical axis within a ±3° window, there was a 95% concordance [44]. A more recent computed tomography–based study by Jacquet et al. [45] demonstrated no statistically considerable asymmetry and that any present asymmetry would likely induce minimal to no clinical differences. Ultimately, radiographs should be used as a guide and correlated with intraoperative findings, including caliper verification and ligamentous balancing, to more accurately approximate the patient’s native alignment.

It is also important to acknowledge that some degree of bilateral alignment asymmetry may exist prior to the development of OA. Certain individuals may naturally demonstrate differences in alignment between knees, including patterns of varus–valgus asymmetry [[44], [45], [46]]. In such cases, some of the side-to-side differences observed in this study could reflect native anatomical variation rather than changes attributable solely to degenerative remodeling. However, the directional patterns observed in our cohort–namely medial compartment disease shifting toward greater varus alignment and lateral disease toward greater valgus alignment—are consistent with known biomechanical patterns of compartment-specific collapse in OA. This suggests that although native asymmetry may contribute to some variability, progressive osseous remodeling likely accounts for a substantial portion of the observed deviations in CPAK phenotype.

This study has several limitations. First, the retrospective, cross-sectional design from a single institution inherently restricts generalizability, as the findings may not fully reflect geographic variations in anatomy, arthritic severity, and radiographic technique. Additionally, the study population was predominantly White, which may further limit the generalizability of these findings to other racial populations. Radiographic-based measurements rely on proper patient positioning, and although the use of standardized long-leg imaging protocols helps minimize rotational error and flexion contracture, subtle variations cannot be entirely excluded. The study population was restricted to patients with severe unilateral KL 4 arthritis and a contralateral KL 0 or 1 knee, which may limit applicability to intermediate KL grades. Importantly, the CPAK classification assesses only the coronal plane and represents a two-dimensional radiographic descriptor, thereby neglecting critical anatomical considerations involving the sagittal and axial planes, as well as individual ligamentous asymmetry. Additionally, the CPAK classification does not account for severe arthritis with severe coronal plane subluxation, which may further distort measurements due to femoral-tibial mismatch. For example, a markedly varus knee with medial translation of the femur relative to the tibia may artificially alter the calculated alignment parameters and CPAK classification. Furthermore, the concordance of right and left knee radiographs remains controversial. Some individuals may naturally demonstrate different CPAK phenotypes between knees prior to the onset of OA. As a result, a portion of the side-to-side differences observed in this study may reflect native anatomical variation rather than degenerative change alone. However, our results demonstrate a clear trend toward collapse in the predominantly arthritic compartment, suggesting a deviation from an original joint line orientation likely closer to that of the contralateral knee. Finally, the clinical implications of the radiographic differences identified are unknown, as this study did not evaluate associated functional outcomes, implant positioning, or survivorship following TKA.

Conclusions

The results of our study demonstrate a notable difference in joint line orientation and CPAK classification between the healthy and arthritic knee. We hope these findings serve as a foundation to further investigate the impact of progressive OA on native bony anatomy, as well as the resulting clinical implications following TKA. Emerging and existing technologies, including robotics, computer navigation, and augmented reality, may provide surgeons with increasingly objective methods to restore a patient’s prearthritic morphology. Until those solutions become more universally available and validated, careful scrutiny of preoperative radiographs, paired with thoughtful intraoperative assessment, remains the most reliable strategy for approximating a patient’s native alignment.

Conflicts of interest

B.H. Naylor is a paid consultant for Link and Smith & Nephew, a member of the speakers’ bureau/paid presentations for Beckton Dickinson, and owns stock in ForCast; J.M. Schwab is a paid consultant for Mizuho OSI, receives research support from Canary Medical, Inc., and serves as a board member of the Anterior Hip Foundation; all other authors declare no potential conflicts of interest.

For full disclosure statements, refer to https://doi.org/10.1016/j.artd.2026.102102.

CRediT authorship contribution statement

Brandon H. Naylor: Writing – review & editing, Writing – original draft, Supervision, Project administration, Investigation, Conceptualization. Charlotte C. Baker: Writing – review & editing, Validation, Investigation, Formal analysis, Data curation. Anita Alex Bradham: Methodology, Investigation, Data curation. Natalie L. Gresham: Methodology, Investigation, Data curation. Zachary M. Ricciardelli: Writing – review & editing, Validation, Investigation. Joseph M. Schwab: Writing – review & editing, Visualization, Supervision, Project administration, Formal analysis, Data curation, Conceptualization.