Abstract
Aims
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.
Methods
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.
Results
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°).
Conclusion
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.
Cite this article: Bone Jt Open 2024;5(12):1081–1091.
Take home message
By combining the individual's constitutional anatomy with soft-tissue balance, functional alignment minimizes changes to joint line obliquity, but may alter constitutional Coronal Plane Alignment of the Knee (CPAK) type. These changes were most pronounced in constitutional valgus knees, which often shift into neutral coronal alignment.
Functional alignment is considered a safe compromise between an unrestricted kinematic alignment and a fixed mechnical alignment approach to total knee arthroplasty.
Introduction
Functional alignment (FA) is a relatively new technique in total knee arthroplasty (TKA) that aims to achieve balanced coronal and sagittal gaps by orienting and sizing implants prior to bone resection.1-3 The implant position can be virtually modified from either a mechanical alignment (MA) or kinematic alignment (KA) start plan, most often utilizing robotic technology.4 Individualized FA starts with a KA plan, which preserves joint line obliquity (JLO), compared to a MA starting plan, which unintentionally alters JLO.4-6 FA, however, differs from KA TKA, in which the final implant position is determined by the osseous anatomy only.1 The benefits of FA include more consistent soft-tissue balance than KA, and thinner bone resections and fewer soft-tissue releases than MA.4,6-8
A final FA position is often shifted away from a knee’s KA position due to the asymmetric and patient-specific native laxities of the knee.5,8,9 Another reason for this variation is that the individualized pre-arthritic, or constitutional, alignment may have been altered, first by bone remodelling, and later, by secondary bone loss.10-12 However, this alignment change from KA to final FA position has not been examined in detail.
Therefore, the purpose of this study was to articulate the differences between KA and final FA position in TKA. The Coronal Plane Alignment of the Knee (CPAK) classification was used to define and quantify these differences.13 The primary hypothesis was that in the majority of patients, the arithmetic hip-knee-ankle angle (aHKA) and JLO after individualized FA would not be significantly different from their KA positions. The secondary hypotheses were that following FA TKA, most patients would remain within their (constitutional) CPAK phenotype, and that any differences in CPAK distribution between sexes would be minimal.
In the era of personalized surgery, the importance of restoring native alignment while reconstructing a well-balanced knee is increasingly considered vital in TKA. Defining the proportion, magnitude, and direction of alignment change, and the effect that initial alignment has on final FA position will improve our understanding of optimal implant positioning in TKA.
Methods
Study group
A retrospective CT analysis was undertaken comparing virtual intraoperative changes from KA to final implant position when performing FA TKA. Patients underwent Mako robotic arm-assisted primary TKA (Stryker, USA) by four specialist orthopaedic surgeons (SJM, DBC, GWC, DC) in two private hospitals (Centre 1: St George Private Hospital; Centre 2: St John of God Subiaco Private Hospital) in Australia between August 2018 and July 2022 (Centre 1) and between January 2018 and December 2023 (Centre 2). Ethics approval was provided by Ramsay Health Care Human Research Ethics Committee A (#2023/ETH/0072). The study was conducted in accordance with the principles of the Declaration of Helsinki.14
Patients were included if they were diagnosed with end-stage knee osteoarthritis, inflammatory arthritis, or post-traumatic osteoarthritis, and scheduled for primary robotic arm-assisted TKA with a FA strategy. Exclusion criteria included prior femoral or tibial osteotomies, prior malunions of the femur and tibia, a history of soft-tissue procedures or releases around the knee, significant preoperative ligamentous instability requiring increased constraint, and absence of signed consent. A total of 2,116 knees (1,801 patients) were included in the analysis (Figure 1). Baseline characteristics and radiological data for the cohort are presented in Table I. The Triathlon Total Knee System (Stryker) was implanted, using cruciate-retaining components in most cases. If the posterior cruciate ligament (PCL) was incompetent, a posterior-stabilized (PS) prosthesis was used instead. These PS cases were included in the analysis, as PCL resection has been shown to have minimal impact on extension balance.15
Fig. 1
Study flowchart. FA, functional alignment; TKA, total knee arthroplasty.
Table I.
Baseline characteristics.
| Variable | Value |
|---|---|
| Number of knees (patients) | 2,116 (1,801) |
| Mean age, yrs (SD) | 67.9 (8.5) |
| Mean BMI, kg/m2 (SD) | 30.8 (5.7)* |
| Female sex, n (%) | 1,101 (52.0) |
| Laterality, left, n (%) | 1,015 (48.0) |
| Mean kinematic angles, ° (SD) | |
| LDFA | 87.5 (2.0) |
| MPTA | 86.4 (2.1) |
| aHKA | -1.1 (2.8) |
| JLO | 173.9 (3.0) |
-
*
BMI was available for 97.9% (2,071 patients).
-
aHKA, arithmetic hip-knee-ankle angle; JLO, joint line obliquity; LDFA, lateral distal femoral angle; MPTA, medial proximal tibial angle.
Surgical technique
Preoperative CT imaging with rendering and segmentation was obtained as part of standard planning to develop a 3D bone model for each patient. This allowed determination of the lateral distal femoral angle (LDFA) and medial proximal tibial angle (MPTA). A detailed description of how the LDFA and MPTA are measured on CT has been published previously.16 The LDFA and MPTA were then used to calculate the aHKA (MPTA – LDFA), JLO (MPTA + LDFA), and CPAK types. The CPAK classification categorizes knees into nine constitutional phenotypes,8 and allows surgeons to estimate the patient’s pre-arthritic alignment. Importantly, it also serves as a universal template for comparing and evaluating alignment strategies.13,17,18
The surgical technique of Clark et al4 provides a detailed description and rationale for individualized FA.4 Intraoperatively, after verifying that the bone morphology and position were consistent with the preoperative CT plan, implants were virtually positioned with matched resections. This is the unrestricted KA start plan, which defined the KA of the knee. Next, predefined boundary restrictions were applied to the LDFA and MPTA (the restricted KA start plan), and maximum stressed medial and lateral gap laxities were measured in 10° extension and 90° flexion. These boundaries, detailed in Table II, have been shown to capture 85.4% of native alignment types.19 Virtual implant position was then adjusted within the boundaries, aiming for equal extension gaps (lateral = medial) and equal medial sagittal gaps (medial extension = medial flexion) to preserve medial collateral ligament isometry. The lateral flexion gap was maintained at its constitutional laxity, which in most cases was equal or greater than the lateral extension gap. Compartmental gap differentials ≤ 2.0 mm were accepted, but anything greater required soft-tissue release. The virtual implant angles were then recorded as the final FA position.
Table II.
Functional alignment boundaries (capturing 85.4% of native phenotypes).
| Parameters | Boundaries for FA protocol |
|---|---|
| Coronal alignment | HKA angle: 6.0° varus to 3.0° valgus Tibial coronal: 6.0° varus to 3.0° valgus Femoral coronal: 6.0° valgus to 3.0° varus |
| Femoral rotation | 6.0° internal to 6.0° external rotation from the sTEA |
| Tibial rotation | Akagi’s line |
| Femoral flexion | 0 to 7.0° to optimize sizing |
| Tibial slope | 0 to 7.0° to match LTP slope |
| Combined component flexion | Not to exceed 10.0° |
-
FA, functional alignment; HKA, hip-knee-ankle angle; LTP, lateral tibial plateau; sTEA, surgical transepicondylar axis.
Outcomes
The primary outcome compared the proportion of knees in KA to those in FA that were ≤ 2° different in aHKA and ≤ 3° in JLO, and determined the direction and magnitude of the differences per CPAK phenotype. These aHKA and JLO boundaries are based on fundamental CPAK boundary definitions, and reflect one standard deviation (SD) from the phenotype means.13 Secondary outcomes included the proportion of knees that maintained their original CPAK phenotype, and any differences in aHKA change, JLO change, and CPAK distribution between sexes with FA.
Statistical analysis
Due to the rarity of CPAK Types VII to IX, these phenotypes were excluded from the primary analysis. All continuous data were presented as means (SD) and discrete data as frequencies with percentages. Normality of data distribution was assessed using histograms, Q-Q plots, and the Shapiro-Wilks test for group sizes < 50 and Kolmogorov-Smirnov test for group sizes ≥ 50. Differences between preoperative and postoperative groups for normally distributed continuous data were analyzed with paired t-tests, and with Wilcoxon signed-rank tests for non-parametric continuous data. Differences between groups for categorical data were analyzed with chi-squared tests. Level of statistical significance was set at p ≤ 0.05. Statistical analyses were performed using SPSS Statistics v. 27 (IBM, USA).
Results
CPAK Types I (31.0% and 44.8%) and II (44.1% and 40.4%) were the most common types both preoperatively and postoperatively, respectively. The preoperative and postoperative CPAK distribution are presented in Figure 2.
Fig. 2
Frequency of Coronal Plane Alignment of the Knee (CPAK) type distribution for kinematic alignment (KA) and final functional alignment (FA) positions.
Primary outcome
Overall, 71.6% had an aHKA change≤ 2° and 87.0% had a JLO change≤ 3° from KA to FA position. The mean aHKA changed from -1.1° (SD 2.8°) varus in KA to -1.9° (SD 2.3°) varus in FA (mean difference (MD) -0.83, SD 2.0; p < 0.001, paired t-test), and the 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, paired t-test). Figure 3 and Figure 4 present the proportions and mean changes for each CPAK type. JLO was maintained in the majority of CPAK types. The aHKA was maintained to a similar degree across CPAK types with FA, except for CPAK Types III and VI.
Fig. 3
Proportion of arithmetic hip-knee-ankle angle (aHKA) change ≤ 2° and joint line obliquity (JLO) change ≤ 3° overall per Coronal Plane Alignment of the Knee (CPAK) type.
Fig. 4
Mean changes in a) arithmetic hip-knee-ankle angle (aHKA) and b) joint line obliquity (JLO) from kinematic alignment (KA) to final functional alignment (FA) position, per Coronal Plane Alignment of the Knee (CPAK) type. Whiskers are 95% CIs.
Secondary outcomes
Overall, 1,229 (58.1%) of knees maintained their CPAK phenotype with FA. This proportion was highest for CPAK Types I (n = 485, 73.9%), II (n = 579, 61.1%), and IV (n = 44, 51.2%). However, the majority of valgus CPAK Types III (n = 129, 67.5%) and VI (n = 33, 71.7%) were shifted to neutral phenotypes, with insignificant changes to JLO across the six CPAK types. An overview of the alignment changes for each CPAK type is presented in Table III and Figure 5. Table IV shows the postoperative CPAK distribution for each of the constitutional CPAK phenotypes. Only 0.1% of the entire cohort (n = 3) had a postoperative apex proximal JLO.
Table III.
Alignment changes from kinematic alignment to final functional alignment position.
| CPAK type | Constitutional proportion, n (%) | LDFA, ° | MPTA, ° | aHKA, ° | JLO, ° | ||||
|---|---|---|---|---|---|---|---|---|---|
| Mean KA (SD) | Mean final FA position (SD) | Mean KA (SD) | Mean final FA position (SD) | Mean KA (SD) | Mean final FA position (SD) | Mean KA (SD) | Mean final FA position (SD) | ||
| Overall | 2,116 (100) | 87.5 (2.0) | 88.1 (1.7) | 86.4 (2.1) | 86.2 (1.8) | -1.1 (2.8) | -1.9 (2.3) | 173.9 (3.0) | 174.2 (2.6) |
| I | 656 (31.0) | 88.5 (1.3) | 89.0 (1.3) | 84.6 (1.4) | 85.3 (1.3) | -3.9 (1.5) | -3.7 (1.6) | 173.1 (2.3) | 174.3 (1.9) |
| II | 947 (44.8) | 86.6 (1.3) | 87.3 (1.3) | 86.3 (1.3) | 86.0 (1.6) | -0.2 (1.2) | -1.3 (1.8) | 172.9 (2.3) | 173.3 (2.3) |
| III | 191 (9.0) | 84.7 (1.4) | 86.3 (1.4) | 88.3 (1.4) | 87.4 (1.7) | 3.6 (1.7) | 1.1 (1.5) | 173.0 (2.1) | 173.7 (2.7) |
| IV | 86 (4.1) | 91.3 (1.2) | 90.6 (1.2) | 87.2 (1.1) | 87.0 (1.5) | -4.1 (1.8) | -3.6 (1.7) | 178.5 (1.4) | 177.6 (2.2) |
| V | 178 (8.4) | 89.3 (0.9) | 89.4 (1.1) | 89.2 (0.9) | 88.1 (1.5) | -0.1 (1.2) | -1.3 (1.8) | 178.5 (1.3) | 177.5 (2.0) |
| VI | 46 (2.2) | 87.2 (1.0) | 88.0 (1.3) | 91.5 (1.7) | 88.7 (1.9) | 4.3 (2.2) | 0.6 (2.0) | 178.7 (1.8) | 176.7 (2.6) |
| VII | 1 (0.1) | 97.5 (N/A) | 92.0 (N/A) | 86.5 (N/A) | 88.0 (N/A) | -11.0 (N/A) | -4.0 (N/A) | 184.0 (N/A) | 180.0 (N/A) |
| VIII | 7 (0.3) | 92.5 (0.9) | 92.1 (1.0) | 92.8 (0.7) | 87.6 (1.2) | 0.3 (1.0) | -4.5 (1.5) | 185.3 (1.3) | 179.7 (1.7) |
| IX | 4 (0.2) | 88.8 (1.9) | 90.3 (1.7) | 95.5 (2.1) | 87.5 (2.9) | 6.7 (3.8) | -2.8 (3.9) | 184.3 (1.1) | 177.8 (2.6) |
-
aHKA, arithmetic hip knee-ankle angle; CPAK, Coronal Plane Alignment of the Knee; FA, functional alignment; JLO, joint line obliquity; KA, kinematic alignment; LDFA, lateral distal femoral angle; MPTA, medial proximal tibial angle; N/A, not applicable.
Fig. 5
Mean changes from kinematic alignment (KA) to final functional alignment (FA) position for Coronal Plane Alignment of the Knee (CPAK) Types I to VI. KA position means across all CPAK types for the full cohort (n = 2,116; 95% CI). The light red circle indicates the KA position across all CPAK types for the full cohort (n = 2,116; 95% CI). The light blue circle indicates the final FA position across all CPAK types for the full cohort (n = 2,116; 95% CI). The small red circles indicate the KA point for each CPAK type. The small blue circles indicate the final FA point for each CPAK type. The red arrows indicate the size and direction of changes from KA to final FA positions. aHKA, arithmetic ankle-hip-knee angle; JLO, joint line obliquity.
Table IV.
Constitutional versus final Coronal Plane Alignment of the Knee distribution after functional alignment total knee arthroplasty.
| Constitutional CPAK type | Final CPAK type after FA implant positioning, % | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| I | II | III | IV | V | VI | VII | VIII | IX | |
|
I
n = 656 |
73.9 | 15.4 | 0 | 7.0 | 3.7 | 0 | 0 | 0 | 0 |
|
II
n = 947 |
31.8 | 61.1 | 1.1 | 0.6 | 4.3 | 1.1 | 0 | 0 | 0 |
|
III
n = 191 |
3.1 | 61.3 | 17.8 | 0 | 6.3 | 11.5 | 0 | 0 | 0 |
|
IV
n = 86 |
30.2 | 3.5 | 1.2 | 51.2 | 12.8 | 0 | 0 | 1.2 | 0 |
|
V
n = 178 |
19.1 | 20.2 | 0.6 | 14.0 | 44.4 | 1.1 | 0 | 0 | 0.6 |
|
VI
n = 46 |
4.3 | 39.1 | 2.2 | 4.3 | 30.4 | 17.4 | 0 | 2.2 | 0 |
-
Blue cells indicate the proportion of knees that maintained their constitutional CPAK phenotypes after FA.
-
CPAK Types VII to IX have been excluded from the first column of this table due to the low constitutional frequency (n = 1, n = 7, n = 4, respectively).
-
CPAK, Coronal Plane Alignment of the Knee; FA, functional alignment.
The mean aHKA in females changed from -0.68° (SD 2.8°) varus in KA to -1.6° (SD 2.3°) varus in FA (MD -0.93, SD 2.1; p < 0.001, paired t-test), and in males from -1.5° (SD 2.8°) varus in KA to -2.2° (SD 2.2°) varus in FA (MD -0.73, SD 2.0; p < 0.001, paired t-test). The mean JLO in females changed from 173.7° (SD 3.0°) to 174.1° (SD 2.6°) (MD 0.45, SD 2.3; p < 0.001, paired t-test), and in males from 174.1° (SD 3.1°) to 174.4° (2.7°) (MD 0.31, SD 2.2; p < 0.001, paired t-test). Preoperative and postoperative distribution of CPAK types across sexes is presented in Figure 6.
Fig. 6
Coronal Plane Alignment of the Knee (CPAK) distribution for kinematic alignment (KA) and final functional alignment (FA) position, by sex.
Discussion
This is the first study to evaluate phenotype changes in knee alignment in patients undergoing robotic FA TKA. Overall, a FA strategy resulted in minimal changes to JLO (87.0% had≤ 3° change) and aHKA (71.6% had≤ 2° change), and 1,229 (58.1%) of knees maintained their CPAK type. Importantly, across the primary six CPAK types, JLO was not significantly altered, which is a key objective of FA. Maintenance of CPAK type was most pronounced in constitutional varus (CPAK Types I and IV) and neutral (CPAK Types II and V) coronal alignments. However, constitutional valgus alignments (CPAK Types III and VI) shifted horizontally into neutral coronal alignment.
Previously, conflicting results have been reported when comparing personalized and fixed alignment strategies in terms of clinical outcomes.20-24 However, when looking more closely at alignment subgroups, several differences have been demonstrated. Restoration of JLO and varus alignment are associated with a positive effect on clinical outcomes in patients with constitutional varus.25-28 Also, a significant positive correlation has been reported between postoperative neutral limb alignment and patient-reported outcomes in patients with constitutional neutral and valgus alignment.29 In the present study, patients with constitutional varus and neutral coronal alignment were more commonly restored to their CPAK type with FA, while patients with valgus alignment were more often realigned to a more neutral coronal alignment (while still maintaining their JLO). It remains unclear whether a preference for achieving balanced gaps (as in FA) versus retaining native laxities (as in unrestricted KA) will result in different outcomes.
Knees with a constitutional apex proximal JLO (CPAK Types VII to IX) are extremely rare (< 1%),13,30,31 a statistic that was confirmed in this study cohort (< 1%). Corban et al32 reported that a MA strategy resulted in a substantial proportion of patients (11.1%) with an unintentional postoperative apex proximal JLO. Because restoration of joint line is so sensitively tied to satisfaction,25 it is possible that this increase in JLO may contribute to a higher dissatisfaction rate in TKA,33 making the knee feel ‘unnatural’. In the present study, only 0.1% (n = 3) had a postoperative apex proximal JLO (CPAK Types VII to IX), reflecting expected proportions among native phenotypes and highlighting both the precision and accuracy of robotic-assisted TKA with FA in preventing significant changes to constitutional JLO.
This is also the first study that has compared the alignment alterations between sexes after FA TKA. The absolute change in both aHKA and JLO in this study was found to be similar in females and males (although males started with 0.8° more varus than females). Therefore, alignment alterations from a kinematic position do not differ between sexes with FA. Several studies have examined alignment differences between sexes. All, including ours, have found males to have greater constitutional varus than females,34,35 with CPAK Type II being the most common type among both males and females. The overall proportion of varus knees reported in the literature (33.7% to 68.5% in males and 19.7% to 50.8% in females) was similar to our results (40.4% in males and 30.2% in females). However, the proportion of valgus knees (11.8% to 17.2% in males and 25.8% to 34.1% in females) is higher in the literature than in our results (8.9% in males and 10.4% in females).31,36 It is important to note that this study was CT-based, whereas most previous studies used long-leg radiographs. Plain radiological analysis has been shown to underestimate the degree of constitutional JLO compared to CT, and this needs to be considered in the context of these new findings.16
Understanding the valgus-to-neutral change in limb alignment
The present study showed that patients with valgus phenotypes (CPAK Types III and VI) shifted horizontally into a more neutral alignment, with a final aHKA of 1.1° and 0.6°, for CPAK Types III and VI, respectively. Similar changes with FA have been reported by Clark et al,25 with fewer CPAK Type III knees with FA. There are several possible explanations for this finding.
First, after accounting for a patient’s bony alignment, final implant orientation is defined by the patient’s soft-tissue profile. This alignment shift is contingent upon the lateral soft-tissue laxity being equivalent to, or greater than, the medial laxity in near-extension. This has been substantiated by several in vivo and in vitro studies,37-41 all reporting a more pronounced lateral compared to medial joint opening when a load is applied in near-extension. Near-extension, as opposed to full extension, alleviates tension from the posterior capsule and mitigates the effect of posterior osteophytes. Despite this, constitutional coronal laxity in normal individuals remains poorly understood, and to date, consensus on normative values is absent. This truism is an example of the complex and highly variable nature of knee alignment and soft-tissue balance.
Second, the unrestricted KA position, considered by many to represent constitutional alignment of the knee once chondral loss is accounted for, is unlikely to represent that actual pre-arthritic state. Bone remodelling occurs in moderate degrees of OA, while secondary bone loss eventuates in later stages. Although we consider CPAK-defined alignment as the best current method to estimate the pre-arthritic state, the precision of this estimate reduces as the arthritic process advances, a fact that may also contribute to differences between KA and final FA position. Ultimately, future modelling techniques that can account for morphological bone changes may improve our understanding in this area.
Third, with the restricted boundaries used in the present study, a small alteration to aHKA will convert these knees into neutral CPAK types, as the boundary for these is 2°. It is therefore essential to further define normative laxity values and address potential variations among different patient characteristics (e.g. knee phenotype, sex, age) for a more individualized approach to TKA.
This study has limitations, primarily the fact that CPAK does not consider sagittal or axial alignment. A recent study was unable to demonstrate a relationship between axial or sagittal alignment to CPAK type, and we therefore believe CPAK is at present an appropriate framework to report alignment changes.42 Although CT imaging was used for the measurement of the alignment parameters, which is more reliable compared to long-leg radiographs,16 certain conditions, such as bone remodelling and bone loss, can still affect measurement precision and determination of the individualized constitutional alignment. Furthermore, this was a radiological analysis without the associated gait analysis, patient-reported outcomes, and survival data that may further inform the effectiveness of this alignment strategy over other techniques. Future research should focus on validating these CT findings with gait studies, patient-reported outcomes, and survival analyses. Finally, we analyzed two populations originating in the same country. These results may not be generalizable to other regions, as geographical differences in alignment have been well documented.30
This study provides valuable insight into the ways in which soft-tissue laxities alter implant position when performing FA TKA, specifically differences between CPAK types. Additionally, functional alignment may alter CPAK type, but it does not significantly change JLO. Patients with constitutional varus phenotypes (CPAK Types I and IV) and neutral phenotypes (CPAK Types II and V) maintained their CPAK category, while patients with constitutional valgus (CPAK Types III and VI) were aligned to a more neutral coronal alignment, but again without significant changes to JLO.
The advantage of a tailored approach to both the patient’s constitutional anatomy and laxity profile is avoidance of soft-tissue releases and JLO alterations that pre-resection balancing affords. KA should be considered as a baseline reference to minimizing changes to native anatomy, while a final position achieved by FA provides an optimally balanced TKA. This combined strategy may be considered a safe compromise between an unrestricted KA and a fixed MA approach.
References
1. MacDessi SJ , Oussedik S , Abdel MP , Victor J , Pagnano MW , Haddad FS . The language of knee alignment: updated definitions and considerations for reporting outcomes in total knee arthroplasty . Bone Joint J . 2023 ; 105-B ( 2 ): 102 – 108 . Crossref PubMed Google Scholar
2. O’Callaghan WB , Gouk C , Wilkinson MPR , Haztratwala K . Computer-aided surgery-navigated, functional alignment total knee arthroplasty: a surgical technique . Arthroplast Today . 2022 ; 14 : 121 – 127 . Crossref PubMed Google Scholar
3. Oussedik S , Abdel MP , Victor J , Pagnano MW , Haddad FS . Alignment in total knee arthroplasty . Bone Joint J . 2020 ; 102-B ( 3 ): 276 – 279 . Crossref PubMed Google Scholar
4. Clark GW , Esposito CI , Wood D . Individualized functional knee alignment in total knee arthroplasty: a robotic-assisted technique . Tech Orthop . 2022 ; 37 ( 3 ): 185 – 191 . Crossref Google Scholar
5. Clark G , Steer R , Wood D . Functional alignment achieves a more balanced total knee arthroplasty than either mechanical alignment or kinematic alignment prior to soft tissue releases . Knee Surg Sports Traumatol Arthrosc . 2023 ; 31 ( 4 ): 1420 – 1426 . Crossref PubMed Google Scholar
6. Shatrov J , Battelier C , Sappey-Marinier E , Gunst S , Servien E , Lustig S . Functional alignment philosophy in total knee arthroplasty - rationale and technique for the varus morphotype using a CT based robotic platform and individualized planning . SICOT J . 2022 ; 8 : 11 . Crossref PubMed Google Scholar
7. Chang JS , Kayani B , Wallace C , Haddad FS . Functional alignment achieves soft-tissue balance in total knee arthroplasty as measured with quantitative sensor-guided technology . Bone Joint J . 2021 ; 103-B ( 3 ): 507 – 514 . Crossref PubMed Google Scholar
8. Van de Graaf VA , Chen DB , Allom RJ , Wood JA , MacDessi SJ . Functional alignment in total knee arthroplasty best achieves balanced gaps and minimal bone resections: an analysis comparing mechanical, kinematic and functional alignment strategies . Knee Surg Sports Traumatol Arthrosc . 2023 ; 31 ( 11 ): 5118 – 5127 . Crossref PubMed Google Scholar
9. Shatrov J , Batailler C , Sappey-Marinier E , Gunst S , Servien E , Lustig S . Kinematic alignment fails to achieve balancing in 50% of varus knees and resects more bone compared to functional alignment . Knee Surg Sports Traumatol Arthrosc . 2022 ; 30 ( 9 ): 2991 – 2999 . Crossref PubMed Google Scholar
10. Vandekerckhove P-JTK , Matlovich N , Teeter MG , MacDonald SJ , Howard JL , Lanting BA . The relationship between constitutional alignment and varus osteoarthritis of the knee . Knee Surg Sports Traumatol Arthrosc . 2017 ; 25 ( 9 ): 2873 – 2879 . Crossref PubMed Google Scholar
11. Felson DT , Goggins J , Niu J , Zhang Y , Hunter DJ . The effect of body weight on progression of knee osteoarthritis is dependent on alignment . Arthritis Rheum . 2004 ; 50 ( 12 ): 3904 – 3909 . Crossref PubMed Google Scholar
12. Brouwer GM , van Tol AW , Bergink AP , et al. Association between valgus and varus alignment and the development and progression of radiographic osteoarthritis of the knee . Arthritis Rheum . 2007 ; 56 ( 4 ): 1204 – 1211 . Crossref PubMed Google Scholar
13. MacDessi SJ , Griffiths-Jones W , Harris IA , Bellemans J , Chen DB . Coronal Plane Alignment of the Knee (CPAK) classification . Bone Joint J . 2021 ; 103-B ( 2 ): 329 – 337 . Crossref PubMed Google Scholar
14. World Medical Association . World Medical Association Declaration of Helsinki: ethical principles for medical research involving human subjects . JAMA . 2013 ; 310 ( 20 ): 2191 – 2194 . Crossref PubMed Google Scholar
15. Kayani B , Konan S , Horriat S , Ibrahim MS , Haddad FS . Posterior cruciate ligament resection in total knee arthroplasty: the effect on flexion-extension gaps, mediolateral laxity, and fixed flexion deformity . Bone Joint J . 2019 ; 101-B ( 10 ): 1230 . Crossref PubMed Google Scholar
16. Tarassoli P , Corban LE , Wood JA , Sergis A , Chen DB , MacDessi SJ . Long leg radiographs underestimate the degree of constitutional varus limb alignment and joint line obliquity in comparison with computed tomography: a radiographic study . Knee Surg Sports Traumatol Arthrosc . 2023 ; 31 ( 11 ): 4755 – 4765 . Crossref PubMed Google Scholar
17. Hsu C-E , Chen C-P , Wang S-P , Huang J-T , Tong K-M , Huang K-C . Validation and modification of the coronal plane alignment of the knee classification in the Asian population . Bone Jt Open . 2022 ; 3 ( 3 ): 211 – 217 . Crossref PubMed Google Scholar
18. Mulpur P , Desai KB , Mahajan A , Masilamani ABS , Hippalgaonkar K , Reddy AVG . Radiological evaluation of the phenotype of indian osteoarthritic knees based on the coronal plane alignment of the knee classification (CPAK) . Indian J Orthop . 2022 ; 56 ( 12 ): 2066 – 2076 . Crossref PubMed Google Scholar
19. MacDessi SJ , Allom RJ , Griffiths-Jones W , Chen DB , Wood JA , Bellemans J . The importance of joint line obliquity: a radiological analysis of restricted boundaries in normal knee phenotypes to inform surgical decision making in kinematically aligned total knee arthroplasty . Knee Surg Sports Traumatol Arthrosc . 2022 ; 30 ( 9 ): 2931 – 2940 . Crossref PubMed Google Scholar
20. Young SW , Sullivan NPT , Walker ML , Holland S , Bayan A , Farrington B . No difference in 5-year clinical or radiographic outcomes between kinematic and mechanical alignment in TKA: a randomized controlled trial . Clin Orthop Relat Res . 2020 ; 478 ( 6 ): 1271 – 1279 . Crossref PubMed Google Scholar
21. Waterson HB . The early outcome of kinematic versus mechanical alignment in total knee arthroplasty: a prospective randomised control trial . Bone Joint J . 2016 ; 98-B ( 10 ): 1360 . Crossref PubMed Google Scholar
22. McEwen PJ , Dlaska CE , Jovanovic IA , Doma K , Brandon BJ . Computer-assisted kinematic and mechanical axis total knee arthroplasty: a prospective randomized controlled trial of bilateral simultaneous surgery . J Arthroplasty . 2020 ; 35 ( 2 ): 443 – 450 . Crossref PubMed Google Scholar
23. Dossett HG , Estrada NA , Swartz GJ , LeFevre GW , Kwasman BG . A randomised controlled trial of kinematically and mechanically aligned total knee replacements: two-year clinical results . Bone Joint J . 2014 ; 96-B ( 7 ): 907 – 913 . Crossref PubMed Google Scholar
24. Dossett HG , Arthur JR , Makovicka JL , et al. A randomized controlled trial of kinematically and mechanically aligned total knee arthroplasties: long-term follow-up . J Arthroplasty . 2023 ; 38 ( 6 ): S209 – S214 . Crossref PubMed Google Scholar
25. Clark GW , Steer RA , Khan RN , Collopy DM , Wood D . MAintaining joint line obliquity optimizes outcomes of functional alignment in total knee arthroplasty in patients with constitutionally varus knees . J Arthroplasty . 2023 ; 38 ( 7 Suppl 2 ): S239 – S244 . Crossref PubMed Google Scholar
26. Vanlommel L , Vanlommel J , Claes S , Bellemans J . Slight undercorrection following total knee arthroplasty results in superior clinical outcomes in varus knees . Knee Surg Sports Traumatol Arthrosc . 2013 ; 21 ( 10 ): 2325 – 2330 . Crossref PubMed Google Scholar
27. Winnock de Grave P , Luyckx T , Claeys K , et al. Higher satisfaction after total knee arthroplasty using restricted inverse kinematic alignment compared to adjusted mechanical alignment . Knee Surg Sports Traumatol Arthrosc . 2022 ; 30 ( 2 ): 488 – 499 . Crossref PubMed Google Scholar
28. Young SW , Zeng N , Tay ML , et al. A prospective randomised controlled trial of mechanical axis with soft tissue release balancing vs functional alignment with bony resection balancing in total knee replacement-a study using stryker mako robotic arm-assisted technology . Trials . 2022 ; 23 ( 1 ): 580 . Crossref PubMed Google Scholar
29. Slevin O , Hirschmann A , Schiapparelli FF , Amsler F , Huegli RW , Hirschmann MT . Neutral alignment leads to higher knee society scores after total knee arthroplasty in preoperatively non-varus patients: a prospective clinical study using 3D-CT . Knee Surg Sports Traumatol Arthrosc . 2018 ; 26 ( 6 ): 1602 – 1609 . Crossref PubMed Google Scholar
30. Pagan CA , Karasavvidis T , Lebrun DG , Jang SJ , MacDessi SJ , Vigdorchik JM . Geographic variation in knee phenotypes based on the Coronal Plane Alignment of the Knee classification: a systematic review . J Arthroplasty . 2023 ; 38 ( 9 ): 1892 – 1899 . Crossref PubMed Google Scholar
31. Steele JR , Jang SJ , Brilliant ZR , et al. Deep learning phenotype automation and cohort analyses of 1,946 knees using the coronal plane alignment of the knee classification . J Arthroplasty . 2023 ; 38 ( 6 ): S215 – S221 . Crossref PubMed Google Scholar
32. Corban LE , van de Graaf VA , Chen DB , Wood JA , Diwan AD , MacDessi SJ . How often do we alter constitutional limb alignment, joint line obliquity, and Coronal Plane Alignment of the Knee (CPAK) phenotype when performing mechanically aligned TKA? Bone Jt Open . 2024 ; 5 ( 2 ): 109 – 116 . Crossref PubMed Google Scholar
33. Gunaratne R , Pratt DN , Banda J , Fick DP , Khan RJK , Robertson BW . Patient dissatisfaction following total knee arthroplasty: a systematic review of the literature . J Arthroplasty . 2017 ; 32 ( 12 ): 3854 – 3860 . Crossref PubMed Google Scholar
34. Deep K , Eachempati KK , Apsingi S . The dynamic nature of alignment and variations in normal knees . Bone Joint J . 2015 ; 97-B ( 4 ): 498 – 502 . Crossref PubMed Google Scholar
35. Bellemans J , Colyn W , Vandenneucker H , Victor J . The chitranjan ranawat award: Is neutral mechanical alignment normal for all patients? The concept of constitutional varus . Clin Orthop Relat Res . 2012 ; 470 ( 1 ): 45 – 53 . Crossref PubMed Google Scholar
36. Huber S , Mitterer JA , Vallant SM , et al. Gender-specific distribution of knee morphology according to CPAK and functional phenotype classification: analysis of 8739 osteoarthritic knees prior to total knee arthroplasty using artificial intelligence . Knee Surg Sports Traumatol Arthrosc . 2023 ; 31 ( 10 ): 4220 – 4230 . Crossref PubMed Google Scholar
37. Yoo JC , Ahn JH , Sung K-S , et al. Measurement and comparison of the difference in normal medial and lateral knee joint opening . Knee Surg Sports Traumatol Arthrosc . 2006 ; 14 ( 12 ): 1238 – 1244 . Crossref PubMed Google Scholar
38. Te Molder MEM , Wymenga AB , Heesterbeek PJC . Mid-flexion laxity in the asymptomatic native knee is predominantly present on the lateral side . Knee Surg Sports Traumatol Arthrosc . 2019 ; 27 ( 11 ): 3614 – 3625 . Crossref PubMed Google Scholar
39. Okazaki K , Miura H , Matsuda S , et al. Asymmetry of mediolateral laxity of the normal knee . J Orthop Sci . 2006 ; 11 ( 3 ): 264 – 266 . Crossref PubMed Google Scholar
40. Heesterbeek PJC , Verdonschot N , Wymenga AB . In vivo knee laxity in flexion and extension: a radiographic study in 30 older healthy subjects . Knee . 2008 ; 15 ( 1 ): 45 – 49 . Crossref PubMed Google Scholar
41. Mueller JK , Parratte S , Blatter I , et al. How does the normal knee behave? Results of a robotic cadaveric study on 85 human specimens . Orthopaedic Research Society Annual Meeting . 2021 . Google Scholar
42. Corbett J , Sinha P , Esposito CI , Wood JA , Chen DB , MacDessi SJ . Multi-planar expansion of the coronal plane alignment of the knee classification? A computed tomographic study indicates no significant correlation with alignment parameters in other planes . J Arthroplasty . 2024 ; 39 ( 2 ): 336 – 342 . Crossref PubMed Google Scholar
Author contributions
V. A. van de Graaf: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Writing – original draft
G. W. Clark: Investigation, Writing – review & editing
D. Collopy: Investigation, Writing – review & editing
J. A. Wood: Data curation, Project administration, Visualization, Writing – review & editing
D. B. Chen: Conceptualization, Investigation, Methodology, Supervision, Writing – review & editing
S. J. MacDessi: Conceptualization, Formal analysis, Investigation, Methodology, Supervision, Validation, Writing – original draft, Writing – review & editing
Funding statement
The authors disclose receipt of the following financial or material support for the research, authorship, and/or publication of this article: fellowship funding from Stryker.
ICMJE COI statement
S. J. MacDessi and D. B. Chen report receiving research support (Stryker – research fellowship funding, Ramsay Hospital Research Fund – support for an unrelated study), reimbursement for presentations (Stryker), and paid consultations (Stryker, Amplitude SAS). S. J. MacDessi has a patent application lodged on IP pertaining to this study, and is also on the Editorial Board of The Bone & Joint Journal. G. W. Clark reports receiving research support, reimbursement for presentations, and paid consultations from Stryker. D. M. Collopy reports receiving research support (Stryker), reimbursement for presentations (Stryker, Zimmer, AO Recon, Corin), paid consultations (Stryker, Corin), and research fellowship funding (Stryker). D. M. Collopy is also an executive board member of AOA. None of the COIs are in relation to the current manuscript. The other authors have no conflict of interest to report.
Data sharing
The data that support the findings for this study are available to other researchers from the corresponding author upon reasonable request.
Acknowledgements
The authors would like to thank Stryker employees Andrew Sergis, Tom Donaldson, Alec Becvarovski, and Alec Nethery for assisting with the data collection for this study.
Ethical review statement
Ethics approval was provided by Ramsay Health Care Research Ethics and Governance (2023/ETH/0072) and St John of God Health Care (SJGHC) Human Research Ethics Committee (#1388). Informed consent was obtained from all participants included in the study.
Open access funding
The open access fee for this article was self-funded.
Social media
Follow S. J. MacDessi on X @samuelmacdessi
© 2024 van de Graaf et al. This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives (CC BY-NC-ND 4.0) licence, which permits the copying and redistribution of the work only, and provided the original author and source are credited. See https://creativecommons.org/licenses/by-nc-nd/4.0/
