Joint Line Convergence Angle After Total Knee Arthroplasty: Comparison between Mechanical Alignment and Patient-Specific Alignment with Robotic-Assisted Surgery

Abstract

Introduction

Different alignment strategies have been used to restore the femoral and tibial surfaces to their pre-arthritic state after total knee arthroplasty (TKA), improving patient satisfaction. Due to this paradigm change, the knee joint line orientation (JLO), an essential component of knee kinematics, has recently garnered interest as a clinical parameter.

This study aimed to evaluate the JLO measured by the joint line convergence angle (JLCA) after TKA using two different alignment strategies and robotic-assisted surgery. We hypothesized that a patient-specific alignment strategy would lead to more JLCA values considered normal (0–2°).

Methods

A retrospective review of data from 98 patients (100 knees) who underwent robotic-assisted TKA was performed. Group I consisted of 50 knees that underwent TKA using a mechanical alignment strategy, and Group II consisted of 50 knees that underwent TKA using a patient-specific alignment strategy. The JLCA was measured by two different observers in pre-operative and post-operative radiographs. Statistical analysis was performed using the T-Student test, chi-square test, and linear regression for multivariate analysis.

Results

There were no statistically significant demographic differences between both groups regarding age, gender, side, body mass index, Kellgren Lawrence classification, pre-operative range of motion, and hip-knee-ankle (HKA) angle measurement. The mean age was 69 years and 61.2% of all patients were females. In Group I, the average pre-operative JLCA was 2.34° (1.49 SD), and the average post-operative JLCA was 0.35° (0.27 SD). In Group II, the average pre-operative JLCA was 2,43° (1.72 SD), and the post-operative average JLCA was 0.29° (0.24 SD). In Group I the mean difference between the pre and post-operative JLCA was 2.34 (1.48 SD), and in Group II the mean difference between the pre- and post-operative JLCA was 2.43 (1.72 SD). There was not a statistically significant difference between both groups regarding JLCA improvement (p = 0.285).

Conclusions

The JLCA improved post-operatively in both mechanical and patient-specific alignment strategies after robotic-assisted TKA without significant differences between strategies.

Keywords

total knee arthroplasty - patient-specific alignment - mechanical alignment - joint line convergence angle - joint line obliquity - robotic surgical

Introduction

The concept of making right-angle bone cuts [mechanical alignment (MA)] in both the femur and tibia, with the idea of creating parallel and equal flexion and extension spaces, was introduced in the early stages of the development of total knee arthroplasty (TKA).[1]

While the average preoperative hip-knee-ankle (HKA) angle of patients scheduled for TKA is nearly neutral, a study of 4884 patients demonstrated a large variation (from 24° varus to 25° valgus), with only 0.1% presenting neutral femoral and tibial mechanical axes,[2] even non-arthritic knees have a wide distribution of coronal alignments. MA therefore introduces significant anatomical modifications for many individuals, which can result in unequal bone resections. In the past, this led many surgeons to consider TKA as a soft tissue surgery to balance bone cuts,[3] when in reality, in most TKAs, soft tissues are rarely modified if the deformity is less than 15°.[4] While multiple soft tissue release techniques have been proposed to balance the gaps, studies have shown that these releases can be unpredictable and lead to further imbalance.[5] This imbalance can lead to abnormal kinematics, loss of range of motion, loosening, wear, and early failure.

On the other hand, it has been shown that restoring native soft tissue laxity in TKA is more easily achieved by positioning implants closer to constitutional alignment.[6] For many surgeons, this has resulted in a philosophical shift in understanding TKA, moving from MA to a more patient-specific (PS) approach. This has been supported by new methods for classifying functional phenotypes of non-arthritic knees.[7]

Restoration of constitutional limb alignment and joint line obliquity (JLO) more faithfully recreates physiological soft tissue laxities [8] [9] and normal gait patterns.[10] Due to this paradigm shift, the joint line convergence angle (JLCA), a key component of knee kinematics, has recently gained greater relevance as a clinical parameter. It is widely recognized as an indirect measure for estimating the degree of intra-articular deformity. The JLCA is influenced by both intra-articular deformity and lateral soft tissue laxity.[11] Therefore, if soft tissue laxity affects the JLCA, performing a robotic-assisted total knee arthroplasty (RA-TKA) should minimize disruption to these tissues, which could have a significant impact on the JLCA. Furthermore, if a patient-specific alignment strategy is employed, aiming to restore the pre-arthritic state with less impact on the soft tissues, the change in the JLCA should be even more pronounced.

This study aimed to evaluate the convergence angle of the joint line after RA-TKA using two different alignment strategies. Our work hypothesizes that a patient-specific alignment strategy would lead to more JLCA values considered normal (0–2°).

Materials and Methods

Patients

The authors reviewed the records of a consecutive series of patients who underwent RA-TKA (Stryker Triathlon® CR knee) with mechanical alignment (MA) (n = 50, group 1) and with patient-specific alignment (PSA) (n = 50, group 2) ([Figure 1]). The indication for which alignment strategy to use for each patient was random. To account for the learning curve in the use of robotic-assisted TKA, the first ten cases from each group were excluded. The indication for surgery was advanced knee osteoarthritis, stages 3 and 4 according to the Kellgren-Lawrence classification, in at least two of the three knee compartments.

Fig. 1 Study flowchart.

A standard radiographic evaluation was performed on preoperative and postoperative weight-bearing X-rays: anteroposterior, Rosenberg, lateral, axial patella (skyline), and lower extremity tele-radiography.

Postoperative clinical follow-up was scheduled for 2 weeks, 1 month, 2 months, 3 months, and 6 months after surgery for both groups. All patients had provided written informed consent for the use of their data and images for research and publication purposes, and the institutional review board approved the study.

Surgical Technique

All patients underwent general anesthesia (inhalational combined with intravenous). In both patient groups, ischemia was achieved using a tourniquet, which was inflated at the start of the surgery. The approach was through a medial parapatellar incision, with care taken during the release of soft tissues. The anteromedial soft tissue release of the tibia was performed carefully, ensuring that the patellar tendon was not injured. The robotic system (MAKO®, Stryker, MI, USA) was calibrated and configured following a standard protocol before surgery. A preoperative CT scan of the hip, knee, and ankle was requested for the patient on the TKA platform, and a patient-specific three-dimensional model was created. This information was uploaded to the robotic system, and preoperative planning for the implant sizes to be used was done.

Intraoperatively, the guides for the robotic system antennas were fixed on both the tibia and femur, and registration was performed by capturing 40 random points on the bone surface of each bone.

When registering the 40 points on the bone surface, a level of precision of less than 0.5 mm had to be achieved to proceed with the procedure. All measurements were taken intraoperatively using the robotic system's user interface, which has a resolution of 0.5 mm for distances and 0.1° for angles. The positioning of the femoral and tibial components was then virtually planned according to each alignment technique, aiming to balance the flexion and extension gaps based on either mechanical alignment (MA) or patient-specific alignment (PSA) using the MAKO 2.0 software, depending on the corresponding group.

Mechanical Alignment (MA)

The mechanical alignment (MA) technique aims to restore axial alignment to 0° or at least correct the coronal deformity within a ± 3° margin. To achieve this, the femoral resection is adjusted to preserve a mild constitutional deformity (<3°) or reduce a more pronounced deformity, while the tibial component remains mechanically aligned. The tibial component is positioned perpendicular (90°) to the tibial mechanical axis. Regarding the flexion and extension spaces, both components are strategically positioned to achieve a residual laxity of 1–2 mm in both compartments. This ensures that the CRT angle remains within a safe range of 177–183°.

Patient Specific Alignment (PSA)

In the PSA technique, the tibial component was first positioned by planning a resection of equal amounts of medial and lateral bone on the tibia, considering bone wear. The goal was to restore the pre-arthritic medial proximal tibial angle (MPTA) within a safe zone of 84° (varus) to 92° (valgus), which represents the native alignment of the knee. The tibial tilt was set equal to the native medial tibial tilt. On the femoral side, the femoral component is positioned to restore medial joint line height in both extension and flexion. The flexion and extension gaps are balanced by adjusting the distal lateral and posterior lateral resection levels on the femur. For the flexion gap, the goal was to achieve a residual laxity of 1–2 mm in the medial compartment and a greater residual laxity of 1–3 mm in the lateral compartment. For the extension space, the goal was to achieve a residual laxity of 1–2 mm in both compartments, staying within a safe zone of HKA angle of 174–183°.

Bone Resection

The tibial and femoral resections were performed according to the surgeon's intraoperative plan using the haptic feedback-assisted robotic system. Because a computed tomography (CT)-based navigation tool was used, all planned resection thicknesses were bone, ignoring cartilage. The robotic system allowed the surgeon to move the oscillating saw in the defined cutting plane within the haptic limits, protecting the soft tissues. The patella was routinely resurfaced using a conventional oscillating saw and installing an asymmetric patellar insert.

Rehabilitation

The rehabilitation of the patients was carried out by physiotherapists from our center, following a set protocol. This included immediate postoperative rehabilitation, which involved the use of a continuous passive motion knee device starting in recovery, direct mobilization, and immediate full weight-bearing supported by crutches. Under the supervision of the physiotherapist, patients were encouraged to perform active flexion and extension exercises starting on day 1. On average, patients stayed three nights at our center, undergoing two physiotherapy sessions per day.

Daily physiotherapy sessions continued at home. The use of walking canes was recommended for at least the first two weeks. All patients received routine prophylaxis with oral anticoagulation for three weeks after surgery. The first postoperative appointment at the outpatient clinic was at two weeks.

Statistical Analysis

Descriptive statistics were used to summarize the data. The JLCA was measured by two different observers on preoperative and postoperative radiographs. Statistical analysis was performed using Student's t-test, the chi-square test, and linear regression for multivariate analysis ([Table 1]).

Table 1

Results

There were no statistically significant demographic differences between the two groups regarding age, sex, side, body mass index, Kellgren Lawrence classification, preoperative range of motion, and hip-knee-ankle (HKA) angle measurement. The mean age was 69 years, and 61.2% of all patients were women.

In Group I, the mean preoperative JLCA was 2.34° (1.49 SD), and the mean postoperative JLCA was 0.35° (0.27 SD). In Group II, the mean preoperative JLCA was 2.43° (1.72 SD), and the mean postoperative JLCA was 0.29° (0.24 SD). In Group I, the mean difference between preoperative and postoperative JLCA was 2.34 (1.48 SD), and in Group II, the mean difference between preoperative and postoperative JLCA was 2.43 (1.72 SD). There was no statistically significant difference between the two groups in terms of JLCA improvement (p = 0.285). ([Table 1]) ([Charts 1] and [2]).

Chart 1 Statistical result: comparison between both groups of the change in preoperative versus postoperative JLCA.

Chart 2 Statistical result: comparison between both alignments regarding the percentage change of each.

Discussion

The most important finding of our study is that both the mechanical alignment strategy and the patient-specific one optimized by robotics improved the postoperative joint convergence angle, bringing it close to 0° in both cases. The homogeneity between both groups is noteworthy, making them comparable, and avoiding classic confounding factors. Both pre-surgical deformities and average patient age are similar.

According to Mabrouk et al.,[11] the joint convergence angle is directly related to the degree of osteoarthritis in patients undergoing joint replacement surgery and is often synonymous with knee cartilage wear. Therefore, restoring an angle close to 0° (JLCA 0°-2°) better reproduces the knee's original state.[12] [13] [14]

Mechanical alignment has been the gold standard in prosthetic alignment for over 30 decades. It essentially seeks to distribute the load on the prosthetic joint evenly, thereby reducing the potential risk of loosening. At the beginning of our robotic experience, as a group, we performed this type of alignment in all our joint replacement surgeries, with few outliers. By adopting a more patient-specific alignment approach, our planning limits became more flexible, allowing us to consider implants with greater degrees of varus in some cases. However, it is striking that this variation in alignment did not generate a significant change in the postoperative value of the JLCA.

As studied by the group of Macdessi et al.,[15] if limits of ± 3° are applied to the mechanical axis (HKA), the lateral distal femoral angle (LDFA), and the medial proximal tibial angle (MPTA), the constitutional alignment and the joint line obliquity (JLO) will be restored in only 36.2% of knees. Therefore, it is likely that more than 60% of patients undergoing TKA with this surgical approach will present some degree of soft tissue imbalance that will require ligament release to restore the native parameters of that knee. The percentage of patients in whom the constitutional alignment of the limb is restored increases significantly to 74% if restrictions on the JLO are removed while maintaining the ± 3° limit for the HKA.

When orthopedic surgeons aim for mechanical alignment in the coronal plane, soft tissue releases may be necessary to achieve proper knee balance. In contrast, when restoring the constitutional alignment of the knee, fewer soft tissue releases are required, allowing for better soft tissue balance without causing ligament injuries or requiring extensive soft tissue releases.[9]

Regarding what has been reported in the literature, there is a study by Sappey-Marinier et al.[16] with more than 900 patients studied, who underwent knee prosthetic surgery with mechanical alignment and in whom they attempted to see if there was any clinical difference when the previous JLCA was restored in addition to the phenotype of the native operated knee, finding no differences were found between those in whom the native phenotype was restored (approximately 18%) and those who ended up with a phenotype different from the native one. It is striking that although they advocated a neutral mechanical alignment (HKA = 180°), more than 60% of the patients obtained an alignment outside the range of 178° - 182°, demonstrating that many times what was planned is not what is obtained about the bone cuts.

It is interesting to note how similar results were obtained without intentionally seeking them, suggesting that certain mechanical alignment concepts are reproducible with MAKO 2.0 (patient-specific alignment) software. This system emphasizes prosthetic balance based on ligament status and medial and lateral laxity throughout the flexion-extension range. However, we consider it important to highlight that there are safety margins that we have identified as we have gained experience in robotic-assisted surgery, a field that is constantly evolving. These margins are closely related to the native condition of the knees we operate on. That said, we can affirm that the JLCA is a reliable indicator for obtaining reproducible results in this context.

Conclusion

The JLCA improved postoperatively in both mechanical and patient-specific alignment strategies after robotic-assisted TKA, with no significant differences between strategies. Studies with larger sample sizes and functional outcomes are needed.

Conflicto de Interés

DF, WG, JI, FF, LF y RG no tienen nada que declarar en relación con este manuscrito.

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Publication History

Received: 20 November 2024

Accepted: 02 April 2025

Article published online:
20 May 2025