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Preservation of preoperative CPAK is not associated with improved clinical outcomes one-year after functionally aligned robot-assisted TKA
, Joan Minguella,b,c, Joan Pijoana, Miguel Nuñoa, Irene Portas-Torresb,c, Marc Aguilara, Mercè Revertéa,b,c, Martí Plomera, Enrique García-Albóa, Nayana Joshia,b,c
⁎Corresponding author: Oriol Pujol. oriolp-6@hotmail.com
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Received: ,
Accepted: ,
This article was originally published by Reed Elsevier India Pvt. Ltd. and was migrated to Scientific Scholar after the change of Publisher.
Abstract
Abstract
To determine if maintaining the preoperative CPAK class influences on clinical outcomes when performing a functional aligned robotic assisted TKA (RATKA).
This prospective cohort study compared patients who maintained their preoperative CPAK classification following functionally aligned RATKA with those who did not. Clinical outcomes were assed at 6 and 12 months postoperatively. Collected data included demographics variables, pain (VAS), range of motion (ROM), KOOS scores and radiographic parameters (HKA, MPTA, LDFA). JLO and aHKA values were calculated to determine CPAK classification.
Fifty-five patients were included. The preoperative CPAK phenotype was preserved in 34.6 % of cases, the aHKA group in 55.6 % and the JLO group in 52.7 %. JLO preservation was significantly more frequent in distal-apex knees (90 %). Preservation of the preoperative CPAK phenotype, aHKA group or JLO group had no significant effect on knee ROM, pain or functional outcomes at 6 or 12 months postoperatively.
Changes in CPAK classification, aHKA or JLO during RATKA with functional alignment do not impact clinical outcomes at 6 or 12 months. Implant positioning should aim for a stable, well-balanced knee within safe positioning boundaries, rather than exact CPAK restoration.
III.
Keywords
Robotics
Total knee arthroplasty (TKA)
Total knee replacement (TKR)
Personalized
Coronal plane alignment classification (CPAK)
Knee coronal alignment
Joint line obliquity
Functional alignment
Total knee arthroplasty
Robotic assisted TKA
Visual analogue scale
Range of motion
Knee Injury and Osteoarthritis Outcome Score
Hip-knee-ankle angle
Medial proximal tibial angle
Lateral distal femoral angle
Coronal plane alignment classification
1 Introduction
Systematic alignment strategies for total knee arthroplasty (TKA) result in implants being always positioned in the same way for every patient to create a ‘biomechanically friendly’ environment, but disregarding patient-specific anatomy.1 On the other hand, functional alignment (FA) is a philosophy that balances the soft tissue laxity by modifying implant positioning. This individualized technique considers that patient constitutional (prearthritic) anatomy should be taken into account when performing a knee replacement to achieve more natural knee kinematics and to improve soft tissue balance.2 Therefore, it respects the variations in individual anatomy and aims to restore 3D constitutional alignment within set boundaries.3,4
The Coronal Plane Alignment of Knee (CPAK) classification5 aims to be a simple system for categorizing knee phenotype in the coronal plane. Knees are classified into nine phenotypes based on two variables: the arithmetic hip-knee-ankle angle (aHKA) and joint line obliquity (JLO), which can be obtained through a mathematical formula after measuring the radiological medial proximal tibial angle (MPTA) and lateral distal femoral angle (LDFA). The JLO is defined as (MPTA + LDFA). The aHKA, which theoretically predicts the pre-arthritic alignment,6–8 is defined as (MPTA – LDFA).
Studies have reported contradictory results when analysing the influence of modifying the preoperative CPAK classification during knee replacement procedure.9–16 The primary objective of this study is to determine if maintaining the preoperative CPAK class influences on clinical outcomes when performing a functional aligned robotic assisted TKA (RATKA).
2 Methods
2.1 Study design
The IRB approval was obtained and patients provided informed consent. This investigation was designed as a prospective cohort study comparing patients who maintained the preoperative CPAK classification after functional aligned RATKA surgery and those who did not.
Inclusion criteria were: a) patients with primary knee osteoarthritis, b) RATKA procedure, c) surgery performed following the FA philosophy,3,4 d) operated on between 2022 and 2023 in our institution and e) clinical and radiological data available. Exclusion criteria were: a) TKA secondary to infection, fracture, osteotomy or malignancy, b) revision surgery or c) non-robotic TKA.
2.2 Data collection
Preoperatively: Patients were visited to collect a) demographic data (age, gender, ASA classification, BMI, smoking habit and diabetes status), b) functional data (pain assessed through the visual analogue scale (VAS), range of motion (ROM) and functionality through the Knee Injury and Osteoarthritis Outcome Score (KOOS)) and c) radiological data (hip-knee-ankle angle (HKA), medial proximal tibial angle (MPTA) and lateral distal femoral angle (LDFA)). The MPTA (medial angle formed between the tibial mechanical axis and the joint line of the proximal tibia) and the LDFA (lateral angle between the femoral mechanical axis and the joint line of the distal femur) were measured using weight-bearing lower limb telemetries. These radiological parameters were measured a single time by an orthopaedic surgeon (investigator of this study) using the Raimviewer software (UDIAT, Sabadell, Spain).
Intraoperatively: Definitive implant positioning robotic plan was registered. Limb alignment (HKA), coronal femoral component alignment (LDFA) and coronal tibial component alignment (MPTA) were collected.
Postoperatively: A six-months and one-year follow-up outpatient appointment was scheduled. Each patient was examined by an investigator of the study to assess: pain (VAS scale), ROM, functionality (KOOS) and reoperations.
2.3 CPAK analysis
Using the MPTA and LDFA of each patient, the aHKA (MPTA-LDFA) and JLO (MPTA + LDFA) were calculated. These two arithmetic variables allow to categorize patients into nine knee phenotypes (I-IX) according to the CAPK classification.5 For JLO, values > 183 indicate a proximal apex obliquity, 177-183 a neutral apex and <177 a distal apex. While for aHKA, values > 182 indicates a valgus alignment, 178-182 a neutral alignment and <178 a varus alignment. Preoperative and postoperative CPAK classification of each patient were compared. Patients were divided into two cohorts based on whether the CPAK type was maintained or not.
2.4 Cohorts comparison
The two cohorts were compared to analyse if there were differences in clinical outcomes.
Functional outcomes: Pain (VAS), ROM and functionality (KOOS) were assessed. The improvement of each parameter was the difference between preoperative and postoperative (six months and one year) follow-up scores. The functional outcomes improvements were compared between the two cohorts.
Reoperation: All reoperations occurred during the follow-up were collected. Reoperation rate of the two groups were compared.
2.5 Surgical technique
The CORI robotic system (Smith & Nephew, London, Great Britain) was used in all the cases. After medial parapatellar arthrotomy, bone pins with arrays were placed. Tibial pins were positioned 10 cm below the incision and femoral pins intra-incision. Patient landmarks registration and bone mapping were performed to stablish patient anatomy. Implants were preliminary placed using a virtual representation of the patient femoral and tibial anatomy. Laxity kinematics was assessed by manual varus-valgus stress through the full arc of motion. Planning was considered a key step of RATKA. During this step, the surgeon should consider implants size, components position, limb alignment, joint line obliquity and height and joint balance. Implant positioning was adjusted and fine-tuned. Individualized planning was performed according to functional philosophy, as described by Lustig et al..3,4 Priority was given to achieve knee balance and to remain within safe limits.3,4 Then, the sclerotic articular surfaces were prudently cut using a manual saw to facilitate the use of the burr. All bone resection was performed through the surgeon-controlled CORI handheld burr. After completing the bony cuts, tibial keel, femoral box and patella were prepared. Trial implants were used to assess limb alignment, joint balance and ROM. If surgeon was satisfied with trialing results, definitive components were implanted. In all cases a cemented posterior stabilized TKA with patella replacement was used; Journey II (Smith & Nephew, London, Great Britain). After tourniquet deflation, the articulation was washed and bleeding was controlled. Finally, the wound was closed by layers and sutures were used in the skin.
2.6 Statistical analysis
Categorical variables were summarized as counts and percentages. Means were calculated for continuous variables. Normality was tested using the Kolmogorov-Smirnov test. Groups were compared using the χ2 test or Fisher's exact test for categorical variables and the student's T test for continuous variables. All p-values were two-tailed and <0.05 was considered statistically significant. The analysis was performed using Stata® v.14 software (StataCorp, College Station, USA).
3 Results
Fifty-five patients were included in the study. Mean age was 72.5 ± 8.2 years and there were 34 women (62 %) and 21 men (38 %). All patients presented an ASA-score 2 (72 %) or 3 (28 %), mean BMI was 30.9 ± 5.8 kg/m2 (50 % were obese), 8 patients had diabetes (15 %) and one was a smoker (2 %). Mean preoperative HKA was 175.4 ± 6.7°, MPTA was 87.2 ± 3.0° and LDFA was 88.1 ± 3.1°.
Preoperative and postoperative aHKA, JLO and CPAK distributions are summarized in Table 1 and Fig. 1. The 55.6 % of the patients (30/55) maintained the preoperative aHKA group. The aHKA preservation rate was 14/23 (61 %), 12/21 (57 %) and (4/11) 36 % for preoperative varus, neutral and valgus aHKA alignments, respectively (p = 0.38). The 52.7 % of the patients (29/55) maintained the preoperative JLO group. It was preserved more often in preoperative distal apex JLO knees compared to neutral and proximal apex knees: 90 % (28/31) vs 5 % (1/20) vs 0 % (0/4); p < 0.001. Finally, the preoperative CPAK classification was preserved in the 34.6 % of the patients (19/55). The preservation rates differed significantly according to preoperative CPAK phenotypes: 73 % for type-I, 54 % for type-II, 43 % for type-III, 33 % for type VI and 0 % for other types; (p < 0.001).
| Preoperative% (N) | Postoperative% (N) | P-value | |
| aHKA | |||
| Mean | −1.6° ± 4.1° | −1.6° ± 2.5° | P = 0.97 |
| Varus (<−2) | 38 % (21/55) | 44 % (24/55) | P = 0.38 |
| Neutral (-2 - 2) | 41 % (23/55) | 47 % (26/55) | |
| Valgus (>2) | 20 % (11/55) | 09 % (05/55) | |
| JLO | |||
| Mean | 176.0° ± 4.5° | 174.0° ± 1.7° | P < 0.001a |
| Distal (<177) | 56 % (31/55) | 92 % (51/55) | P < 0.001a |
| Neutral (177–183) | 36 % (20/55) | 08 % (04/55) | |
| Proximal (>183) | 07 % (04/55) | 00 % (00/55) | |
| CAPK | |||
| I | 20 % (11/55) | 38 % (21/55) | P < 0.001a |
| II | 24 % (13/55) | 47 % (26/55) | |
| III | 13 % (07/55) | 07 % (04/55) | |
| IV | 16 % (09/55) | 05 % (03/55) | |
| V | 15 % (08/55) | 00 % (00/55) | |
| VI | 05 % (03/55) | 2 % (01/55) | |
| VII | 05 % (03/55) | 00 % (00/55) | |
| VIII | 00 % (00/55) | 00 % (00/55) | |
| IX | 2 % (01/55) | 00 % (00/55) | |
Preoperative and postoperative functional outcomes are summarized in Table 2. No significant differences in preoperative functional variables were observed among the comparison groups. There were no significant differences in knee flexion, pain or KOOS scores at 6 months or 12 months between patients who maintained the CPAK classification and those who did not. Similarly, preservation of the aHKA or JLO group had no impact on these variables. Superior postoperative knee extension was detected in patients who maintained the CPAK classification (6 months: −0.3 ± 1.2 vs −0.5 ± 1.6, p = 0.012; 12 months: −0.1 ± 0.9 vs −0.3 ± 1.1, p = 0.010) or the JLO group (6 months: −0.3 ± 1.3 vs −0.4 ± 1.4, p = 0.008; 12 months: −0.1 ± 0.9 vs −0.2 ± 1.0, p = 0.007), however, the improvement was not clinically relevant. There was one reoperation (1.8 %) during the follow-up: an acute infection that solved with DAIR procedure.
| Preoperative | Postoperative (6 months) | Postoperative (12 months) | |
| Knee Extension | −3.1 ± 5.2° | −0.4 ± 1.4° | −0.2 ± 1.0° |
| Knee Flexion | 107.9 ± 16.7° | 112.1 ± 11.1° | 114.9 ± 9.5° |
| Pain (VAS) | 7.7 ± 1.9 | 2.2 ± 2.5 | 1.8 ± 2.6 |
| KOOS-Symptoms | 50.8 ± 23.1 | 83.5 ± 19.1 | 90.6 ± 12.8 |
| KOOS-Pain | 41.8 ± 16.5 | 85.7 ± 15.0 | 90.9 ± 11.9 |
| KOOS-ADL | 42.0 ± 16.9 | 85.8 ± 15.3 | 91.0 ± 12.5 |
| KOOS-Sport | 6.4 ± 6.5 | 25.1 ± 16.6 | 27.7 ± 15.0 |
| KOOS-QoL | 20.1 ± 16.8 | 69.4 ± 26.3 | 82.9 ± 24.4 |
4 Discussion
The most important finding of this study is that preserving the preoperative CPAK phenotype, aHKA group or JLO group during functionally aligned RATKA had no significant influence on knee ROM, pain or functional outcomes at 6 or 12 months postoperatively.
In this series, 34.6 % of patients preserved their preoperative CPAK phenotype following the TKA procedure. It is important to note that CPAK preservation was not our primary objective during implant positioning. According to functional knee positioning principles, priority was given to achieving knee balance while remaining within safe boundaries.3,4 Femoral and tibial component positioning in all three planes was adjusted to balance flexion and extension medio-lateral gaps.17 Similarly, Bertugli et al. reported a CPAK preservation rate of approximately 50 % when performing FA-TKA.10
There were no significant differences between preoperative vs postoperative mean aHKA values, nor in the distribution of aHKA groups (Table 1). However, only 55.6 % of patients maintained the preoperative aHKA group. This finding may be explained by the fact that minor variations in aHKA among patients near the classification cutoff can lead to a shift in CPAK category.18,19 On the other hand, 52.7 % of patients preserved their preoperative JLO group, particularly those with a distal apex JLO (90 %), (Table 1). Although the differences between preoperative and postoperative mean JLO values were statistically significant, the absolute change was relatively small (174.0° ± 1.7° vs 176.0° ± 4.5°). Moreover, according to the CPAK-JLO modification (90 - JLO/2) proposed by Hsu et al.,18 the actual mean JLO variation would be only 1°. The original CPAK-JLO may be misleading, as it is an arithmetic formula that indicates the direction of the obliquity but does not quantify its real magnitude.19–21 Finally, it is noteworthy that 92 % of patients presented a postoperative distal JLO (<177), which may have been influenced by the implant design used in this series. The Journey II (Smith & Nephew, London, Great Britain) features a built-in distal JLO of three degrees due to asymmetry in the medial-lateral compartments thicknesses, producing an effect similar to an anatomical alignment.22
Similar to our investigation, Bertugli et al. analysed the impact of postoperative CPAK change on functional outcomes following RATKA performed with FA.10 They also reported that satisfactory functional outcomes can be achieved regardless of changes in CPAK type, if the knee is stable and within a “functional safe-zone”. Agarwal et al. and Sappey-Marinier et al. demonstrated that altering the preoperative CPAK phenotype to mechanical alignment (type-V) does not significantly contribute to patient dissatisfaction or increased postoperative pain.9,14 They also advocated that achieving a well-balanced knee should be the target of modern TKA. Streck et al. specifically investigated the effect of CPAK modification in valgus knees and concluded that the preoperative phenotype did not influence outcomes when mechanical aligned TKA was perfromed with standardized soft-tissue release.15
Therefore, probably, the target for implant positioning should be a stable and well-balanced knee (recreating the joint plane and obliquity as dictated by the soft tissues) and within safe positioning boundaries, rather than an exact restoration of the preoperative CPAK phenotype.
Nonetheless, other authors have argued that preserving the preoperative CPAK phenotype does influence postoperative clinical outcomes. Pangaud et al. analysed 178 mechanically aligned TKAs and found that functional results at two-years of follow-up may be improved by restoring the CPAK class, especially in varus patients.16 Similarly, Franceschetti et al. reported that TKA with mechanical alignment did not yield uniform outcomes across all CPAK phenotypes; varus aHKA categories converted to neutral demonstrated significantly inferior results. Konishi et al. concluded that changes in aHKA category and a postoperative apex-proximal JLO were negative predictive factors for clinical outcomes.12 They suggested that modifications in aHKA may increase soft-tissue imbalance, potentially leading to pain and discomfort.
This study is not without limitations. First, the relatively small sample size may limit the study statistical power. Second, a longer follow-up would be necessary to determine whether CPAK preservation influences survival rates; however, this was not the aim of the present study. Third, radiological parameters were measured a single time by an orthopaedic surgeon. However, the parameters of the study (HKA, MPTA and LDFA) are simple to measure and routinely used in our knee unit.
5 Conclusions
Modification of the preoperative CPAK classification, aHKA class or JLO class during robot-assisted TKA using functional alignment does not affect knee range of motion, pain or functional outcomes at 6 or 12 months postoperatively. The target for implant positioning should be a stable and well-balanced knee within safe positioning boundaries, rather than an exact restoration of the preoperative CPAK phenotype.
CRediT authorship contribution statement
Oriol Pujol: All authors contributed equally to this work, All authors contributed to the study conception and design, material preparation, data collection, analysis, The first draft of the manuscript was written by.
Consent to participate
Informed consent was obtained from all individual participants included in the study.
Data availability
Data is not openly available, but it can be available on reasonable request to the corresponding author.
Ethics approval
The study was approved by our Center's Ethics Committee (PR(AT)406/2022). The study was performed in accordance with the ethical standards as laid down in the 1964 Declaration of Helsinki.
Consent to publication
Patients were informed that data concerning their cases might be submitted for publication, and they gave their consent.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial or non-profit sectors.
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