1 Introduction

Despite advances in implant design and surgical technique, outcomes of total knee arthroplasty (TKA) continue to lag behind those of total hip arthroplasty (THA).¹ Patient satisfaction after TKA remains highly variable, with a small but clinically significant proportion of patients reporting persistent dissatisfaction with the outcome.² This has frequently been attributed to altered knee kinematics following TKA, which may contribute to dissatisfaction, functional limitations and lower outcomes.³ As a result, there has been increasing interest in restoring the patient's “normal knee” through a better understanding of native knee kinematics, aiming to overcome traditional techniques that priorities stability at the expense of physiological kinematics.⁴

Early MRI work sought to characterize knee motion using passive flexion in cadaveric specimens to identify contact points.⁵ Subsequent studies have assessed knee movement using imaging modalities such as MRI, CT, RSA, fluoroscopy, gait analysis or combinations of these techniques ^6,7. These works contributed to the understanding of the knee's functional kinematic axis and informed the development of concepts such as medial pivot (MP) motion and contemporary implant designs intended to better reproduce native knee behavior.⁸

Despite these advances, achieving reproducible and reliable intraoperative kinematic assessment remains challenging and existing techniques are often complex, technically demanding, costly, or lack reliability.⁹ Furthermore, most kinematic analyses are performed postoperatively and outside the operating theatre, limiting their utility for intraoperative decision-making. Robotic-assisted TKA platforms offer a unique opportunity to address this gap by enabling standardized intraoperative data capture within a controlled environment.

In this study, we describe a novel method of kinematic assessment using the MAKO robotic platform. Intraoperatively, the assessment was performed by capturing screenshots of the knee through its range of motion. Postoperatively, measurements were taken of the femoro-tibial contact points on both medial and lateral sides across the range of motion. These measurement were then interpreted to describe knee movement and overall kinematics.

The primary hypothesis of this study was that there would be a good-excellent inter-rater and intra-rater reliability of the in vivo knee movement screenshots measurements. The secondary hypotheses were that there would be no significant difference in the measurement when the assessment screenshots taken by two different surgeons.

2 Material and methods

After ethical approval, 30 patients were consented and enrolled into the study. Informed consent was obtained from all individual participants included in the study. All patients underwent robotic TKA using the Mako Robotic Arm Interactive Orthopaedic system (TKA2.0; Stryker) receiving a CR implant (Triathlon Total Knee implant; Stryker), an unconstrained implant with a single radius of curvature. Seven surgeons performed all robotic TKA procedures and captured the screenshots. In ten cases, screenshots were taken twice, once by the surgeon and once by their fellow. Measurements were performed independently by two doctors to assess inter-rater reliability, and the most junior doctor repeated all measurements three times, blinded to the fact they were from the same patients, to assess intra-rater reliability.

2.1

2.1 The surgical technique

On the MAKO robotic system display screen, the knee was viewed in the sagittal plane from both the medial and lateral side. To ensure correct alignment of the prosthesis the MAKO Product Specialist firstly rotates the tibial base plate in the horizontal plane until both posterior condyles of the tibia are aligned. The tibia was then rotated in the vertical plane until the slot between the polyethylene insert and the base plate became clearly visible on both sides of the joint (Fig. 1).

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Fig. 1 Before recording the screenshots, the tibial baseplate was adjusted in both planes until the posterior tibial condyles were aligned and the polyethylene slots were visible bilaterally. The top-left inset shows poor alignment with absent slots (*) and a visible medial tibial condyle (yellow shading).

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Once all the bone cuts were completed and the trial implants were seated, the knee was examined by stabilizing the foot and ankle while maintaining full flexion initially. Extension of the knee was achieved through holding the heel, without directly supporting or manipulating the knee itself. Throughout the process, care was taken to avoid tibial rotation and to preserve consistent knee alignment. Sequential screenshots of the knee were obtained in both medial and lateral views at varying degrees of flexion—120°, 105°, 90°, 60°, 45°, and 30° (Fig. 2).

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Fig. 2 Screenshots obtained from both the medial and lateral compartments, captured from 120° of flexion to 30° of flexion at 15° increments.

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2.2

2.2 The analysis technique

For the analysis, two reference lines were drawn on each image. The first extended from the postero-superior tip of the femoral component to its antero-inferior flange, while the second connected the postero-superior flange to the antero-superior flange. From the intersection point—which represents the center of the femoral rotation axis in this single-radius implant¹⁰—a perpendicular line was projected to the base of the polyethylene insert. This intersection served as the reference point, from which the relative position was quantified as a percentage distance from the posterior edge (Fig. 3). This was performed independently for each of the 14 screenshots (Fig. 4).

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Fig. 3 The reference lines, points, and the measurement methodology used.

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Fig. 4 The measurements as applied across all screenshots.

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3 Results

3.1

3.1 Inter-rater reliability of the screenshots measurements

The method demonstrated excellent reliability, with measurements showing remarkable agreement between different observers. Correlation between Rater 1 and Rater 2 ranged from 0.992 to 0.994, indicating that the technique produces highly reproducible results when applied by independent assessors. This indicates that the method is reliable and remains stable regardless of subtle differences in how individual observers interpret it (Fig. 5).

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Fig. 5 Inter - observer reliability assessment.

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3.2

3.2 Intra-rater reliability of the screenshots measurements

Equally strong consistency was observed within the repeated assessments performed by the blinded Rater 2, where correlations across the three repetitions ranged from 0.996 to 0.998. All coefficients exceeded 0.99, showing excellent constancy of the method even when applied multiple times by the same examiner. Collectively, these results confirm that the technique offers extremely high reliability, both inter- and intra-rater, making it suitable for use in research and clinical settings where precise and reproducible measurements are essential (Fig. 6).

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Fig. 6 Intra - observer reliability assessment.

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3.3

3.3 Inter-rater reliability of the screenshots recording

When assessing inter-rater reliability of the screenshot recordings, with two sets of screenshots taken by different surgeons, the analysis demonstrated a correlation coefficient of 0.942, indicating excellent inter-rater reliability (Fig. 7).

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Fig. 7 Inter - rater reliability of the screenshot recordings.

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4 Discussion

This study demonstrated that a practical and reproducible method for intraoperative assessment of knee kinematics can be achieved during robotic-assisted TKA. Reliability analysis showed excellent inter-rater and intra-rater agreement, indicating a high degree of consistency even among raters with different levels of experience.

To the best of our knowledge, this is the first study to describe and validate a simple, easily adoptable intraoperative kinematic assessment technique using the MAKO robotic platform, specifically designed to support real-time evaluation of knee kinematics and potentially improve surgical technique and clinical outcomes.

Medial pivot (MP) motion is characterized by posterior translation of the lateral femoral condyle on the tibial plateau during flexion, with a relative pivot occurring in the medial compartment.¹¹ When knee kinematics were classified according to the framework described by Manara et al.,¹² a similar distribution of movement patterns was observed using the same implant and a functional alignment technique. In our cohort, a MP pattern was identified in 18 of 30 cases (60%), comparable to the 58.6% reported in their study, followed by a symmetrical rollback pattern observed in 10 cases (33.3%) versus 37.9%, respectively (Fig. 8). We also did not observe any change in the knee motion pattern when comparing screenshots obtained by two surgeons, or when the same set of screenshots was independently analyzed by two raters. Hashimoto et al. found that the intraoperative MP pattern was significantly linked to improved patient-reported outcomes one year following TKA, notably higher Forgotten Joint Score-12 (FJS-12) and Knee Society Score (KSS) when utilizing an imageless navigation system.¹³

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Fig. 8 Contact points projected onto the tibial insert illustrate the most frequently observed knee motion, medial pivot (left), and the second most common motion, symmetrical rollback (right).

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The most commonly used method to calculate the anteroposterior tibial contact location in each compartment is identifying the lowest point of the femoral component relative to the tibial baseplate. However, this approach does not account for the curvature of the tibial insert.¹⁴ Simileysky et al. argued that the lowest point method is fundamentally flawed for determining the anteroposterior tibial insert contact location and should not be employed for this purpose. They emphasized that referring to the lowest points as indicators of contact locations in TKA is misleading, and this terminology should be avoided.¹⁵ To overcome this, we used a fixed point at the center of the single-radius design axis and measured the perpendicular distance to the tibial insert surface, with the tibia maintained in a fixed, static position and rotation.

In addition, prior fluoroscopic and radiographic studies were limited to assessing the relative motion of the radiopaque femoral and tibial components, without the ability to visualize the radiolucent polyethylene insert. As a result, accurate determination of the true contact interface between the femoral component and the tibial insert was not possible.¹⁶ In robotic systems, a virtual polyethylene insert is already visualized, which represents an advantage by allowing a more accurate assessment of the true femorotibial contact points.

Commonly utilized in in vivo studies of knee kinematics, intraoperative sensor technology measures medial–lateral compartment pressures, while the “Kinetic Tracking” function visualizes knee motion across the full range of motion to evaluate joint kinetics.¹⁷ Nevertheless, these system requires additional accessories for operation, including the software application, link station, evaluation kit, and display unit, which increase cost, operative time, and space requirements in the operating room. Also, they cover only a limited triangular sensing area, so when the femorotibial contact points fall outside this region (e.g., in deep flexion), force measurement errors increase substantially and the device may not detect true load distribution across the full articular surface. Additionally, the sensors are calibrated only for passive forces up to ∼310 N per compartment, meaning higher physiological loads may exceed their reliable measurement range and compromise accuracy.¹⁸ Finally, sensors may not be applicable across all knee implant designs. In contrast, our technique is not tied to a specific implant and can be used across different robotic systems.

This technique has the potential to be integrated directly into the robotic TKA workflow. Because the MAKO system already provides the necessary intraoperative visualization, the process required no additional hardware, adding minimal operative time, and offering potential for real-time kinematic feedback to guide fine intraoperative adjustments. Further application of this method earlier in the procedure, before bone cuts and soft-tissue releases, and comparison with implantation measurements may enhance understanding of how best to restore normal knee kinematics.

The key limitation of this study is that it was conducted at a single center using one robotic platform and a single implant design, with measurements obtained during passive range of motion, consistent with most intraoperative studies, meaning that muscle forces and weight-bearing effects were not accounted for. However, previous studies have demonstrated a correlation between intraoperative and postoperative kinematics.¹⁹ Finally, there was no gold-standard reference available for comparison. However, multiple studies have shown that MAKO provides greater accuracy and reproducibility in prosthetic position planning, osteotomy volume control, gap balancing, and restoration of the lower limb mechanical axis. ^20,21.

5 Conclusions

In conclusion, this method provides a reproducible, accessible, and efficient tool for intraoperative kinematic assessment. Its potential for integration into the robotic system makes it a practical tool for generating larger datasets, refining surgical techniques, and supporting future efforts to restore more physiological kinematics in total knee arthroplasty.

Patient consent statement

After ethical approval, all the patients were consented and enrolled into the study.

Ethics declarations

The study was approved by the local ethics committee and performed according to the guidelines of the Declaration of Helsinki. All patients were consented.

Ethics statement

The study was approved by the St Vincent's Hospital Melbourne (SVHM) Human Research Ethics Committee (HREC).

Authors’ contribution

Yusuf Omran Hasan: Conceptualization, Methodology, Data Analysis, Statistical Analysis, Writing & Figures Preparation.

Jackson Ellis: Data Analysis.

Sina Babazadeh: Conceptualization, Methodology, Review and Statistical Analysis.

Dirk van Bavel: Conceptualization, Methodology, Review & Supervision.

Funding statement

This study received no external funding, and the authors report no financial support from any organization for the submitted work.