1 Background

Total knee arthroplasty (TKA) is a common surgical treatment for patients with severe knee osteoarthritis, aimed at alleviating pain, restoring function, and improving quality of life. Between 2017 and 2019, the United Kingdom reported 312,167 primary knee replacements, constituting 24 % of the national registry.¹ In the United States, projections suggest a 673 % increase in TKA procedures by 2030, reaching around 3.48 million surgeries annually, driven by an aging population and rising rates of osteoarthritis.² Various factors can negatively impact longevity after TKA, including malalignment and instability.³ Up to one-third of patients who undergo standard TKA exhibit malalignment exceeding 3°, potentially leading to pain and instability,⁴ as the recommended alignment for limbs is within ±3°. Misalignment of components is associated with increased polyethylene wear rates, resulting in lower survival rates for TKAs compared to those with neutral alignment.⁵ Traditional methods typically employ intramedullary femoral and extramedullary tibial guides to achieve proper coronal alignment; however, attaining optimal alignment with these techniques can be challenging.^6–8 Improper bone resections and associated flexion/extension gaps may result in incorrect positioning of TKA components, contributing to malalignment.^9,10 Contemporary instruments and implants better replicate the native knee's anatomy and biomechanics, but confirming precise alignment and balance of flexion and extension spaces during surgery remains difficult with conventional manual instruments. Robotic assistance has shown effectiveness in various applications, achieving precision levels as low as 0.05 mm.¹¹ The robotic arm-assisted system, which relies on 3D computed tomography (CT), aims to reduce errors in bone resections and provide real-time guidance for intraoperative component positioning before final implantation. While it acts as a predictive tool, its effectiveness is maximized when used alongside mechanical alignment, as it does not dynamically evaluate ligamentous status in the medial and lateral compartments. Additionally, the system allows for accurate preoperative implant size predictions, potentially enhancing operational efficiency,¹² although this aspect has not been thoroughly investigated. Consequently, this study aims to evaluate the predictive accuracy of the RA-TKA system regarding bone resections and implant sizing before surgery, as well as its influence on intraoperative decision-making.

2 Methods

A prospective review was performed, single-center observational study focused on technical accuracy, including patients who underwent RA-TKA (Mako, Stryker, Mahwah, NJ) between November 2023 and May 2024. RA-TKAs with complete intraoperative resection logs was used for the primary analyses (planned versus actual resections, component sizing, immediate coronal alignment).

Ethical review was undertaken by the Health and Disability Ethics Committee of our center (approval 2013-01). All patients provided written informed consent for data usage, and no funding was obtained for the present study.

The indication for surgery was advanced knee osteoarthritis, classified as Kellgren-Lawrence stages 3 and 4, affecting at least two of the three knee compartments. Patients with coronal deformities greater than 15° on long-leg radiographs were excluded, as were those with prior ipsilateral knee surgeries or incomplete radiographic data.

2.1

2.1 RA-TKA operative technique

Patients underwent general anesthesia with inhaled and intravenous anesthetic agents, and a tourniquet was used to facilitate a bloodless surgical field. The approach the point at which the anterior margin of the tibia was approximately one third of the whole length posterior incision was then infiltrated scopic ultrasound scan was completed. Pre-operatively, the MAKO® robotic system (Stryker, MI, USA) was synched and adjusted according to a standardized procedure in place. Preoperative CT axial images of the hip, knee and ankle were acquired using the TKA application platform to establish a patient-specific model of the 3D knee. During surgery, antenna guides were secured to both the tibia and femur, and registration was performed by capturing 40 random points on each bone's surface. A precision threshold of less than 0.5 mm was required to proceed. All measurements were taken using the robotic system's interface, achieving a resolution of 0.5 mm for distances and 0.1° for angles. The surgical goal was to maintain limb alignment within 3° of the mechanical axis (see Fig. 1). The tibial and femoral resections were made according to the surgeon's plan using the robotic manipulator with haptic feedback based on CT navigation. Planned resections were limited to bone depth and excluded the cartilage. Soft tissue conditions and surgeon's expertise often require modifications of preoperative CT scan planning while always bearing in mind the patient's native alignment. The robotic interface enabled the operator to direct the oscillatory saw in defined cut planes and haptic limits designed to protect local soft tissues. The patella was routinely resurfaced with a standard oscillating saw. The operation was started with measuring width and height of the native patella after which resection was carried out to result in an equivalent 9-mm thickness, which is equal to the thickness of the polyethylene button employed. An asymmetric, medially-offset implant was then placed, intraoperative e tracking evaluated throughout the entire range of extension to flexion from 0 to 120°. A patient-specific alignment concept was used to reproduce the knee's pre-disease anatomy within a physiologic window (between 6° of varus and 3° of valgus). All patients received a posterior-stabilized (PS) implant, necessitating the sacrifice of the posterior cruciate ligament (PCL) before any gap analyses to prevent interference with the flexion gap. All patients were rehabilitated by physical therapists at our institution according to the same protocol, with recovery center-based physical therapy started immediately postoperatively with a CPM machine, early mobilization, and crutch-assisted immediate weight-bearing. A newly independent active flexion and extension program was commenced on day one post‐operation, with an average of two nights '' stay before continuing a surgery‐specific rehabilitation protocol under therapist supervision. Orthopedic canes were also prescribed for at least the first two weeks. Prophylactic oral anticoagulation was administered for 3 weeks after the operation and the first clinic follow-up was arranged at two weeks and then subsequently at 4 weeks, 2 months, 3 months, 6 months, and 1 year.

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Fig. 1 Planning computer images for RTKA along with sizes of the tibial and femoral components and the tibial plateau, size of femoral implant, tibial cushion thickness, cartilage thickness, valgus, external rotation, extension, lateral, medial.

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2.1.1

2.1.1 Data extraction

Demographic data and the Knee Injury and Osteoarthritis Outcome Score Junior (KOOS JR)¹³(Fig. 2) were collected preoperatively and at 12 months postoperatively. Demographic data included age, sex, and body mass index (BMI).

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Fig. 2 Knee injury and osteoarthritis outcomes score for joint replacement (KOOS JR).

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3 Results

This study enrolled 40 patients who underwent TKA for severe osteoarthritis management (Kellgren-Lawrence grades III and IV). The mean patient age was 70.6 years (standard deviation [SD], 9.5 years) with a mean body mass index (BMI) of 29 kg/m² (SD, 3.7). The mean preoperative KOOS JR score was 45.9 (SD: 10.7), while the postoperative KOOS JR score was 78.2 (SD: 12.1), with an average total improvement of 32.2 points (SD: 16.1) (Table 1).

3.2

3.2 Bone resection

The average absolute differences between predicted and actual bone resections are outlined in Table 3. For the femur, discrepancies between predicted and performed cuts were 1.1 mm (95 % confidence interval [CI], 0.3–2.0 mm) for the distal lateral resection and 1.2 mm (95 % CI, 0.8–1.6 mm) for the distal medial resection. No significant difference was found for the medial side of posterior condyle resections (0.1 mm; 95 % CI, −0.4–0.7 mm), whereas the lateral side exhibited a significant difference (p = 0.03). On the tibial side, the medial plateau resection showed no significant discrepancy (0.5 mm; 95 % CI, −0.1–1.01 mm), but a significant difference was noted for the lateral plateau (p < 0.001). Overall, among the 240 resections performed, 210 (87.5 %) exhibited an absolute difference of less than 2 mm. The distribution of these differences is illustrated in Fig. 3.

Table 3 Summary of the actual bone resection compared to the predicted.

Location	Predicted (mm) mean (SD)	Actual (mm) mean (SD)	Difference (mm) mean (IC 95 %)	p-value
Lateral femoral	4.5 (1.8)	3.4 (2.0)	1.1 (0.3–2.0)	0.01∗
Medial femoral	8.0 (0.3)	6.8 (1.2)	1.2 (0.8–1.6)	<0.001∗
Lateral posterior condyle	5.8 (1.7)	4.9 (1.6)	0.8 (0.1–1.6)	0.03∗
Medial posterior condyle	8.1 (0.7)	8.0 (1.5)	0.1 (−0.4–0.7)	0.63
Lateral tibial plateau	6.6 (0.8)	5.6 (1.1)	1.0 (0.5–1.4)	<0.001∗
Medial tibial plateau	4.0 (1.4)	3.6 (1.2)	0.5 (−0.1–1.01)	0.09

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Fig. 3 The distribution of the absolute difference between the preoperative prediction and the actual bone resection.

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4 Discussion

The key finding of this study emphasises the accuracy of the RA-TKA setup in aiding pre-operative planning of bone cuts, implant sizes, and the capacity to achieve desirable postoperative alignment based on the surgeon's preferences. RA-TKA is a new trend in development toward a wise system in orthopedic surgery, increasing the confidence and accuracy of the surgeons. This is in line with previous findings that stress the benefits of RA-TKA in terms of optimal alignment of the coronal plane as well as improved clinical and functional outcomes and the preservation of soft tissue integrity.^15–21 Learning curves for this system are relatively short, as illustrated in a 2022 study by Young et al., where three surgeons achieved consistent performance in line with robotic system capabilities after only 16 cases, concurrently reducing operative time.²⁸ The RA-TKA system demonstrated high alignment precision, with post-TKA alignment averaging 1.85° from the neutral mechanical axis (SD = 0.89°), aligning with earlier findings. A systematic review by Mannan et al.²² reported superior post-TKA alignment accuracy with RA-TKA compared to traditional TKA. The improved limb alignment obtained with RA-TKA can be attributed to its high degree of accuracy in preoperative planning, bone resections during surgery, measurements of the flexion and extension gaps, and the angulation of the implant components.²⁹ This should be examined carefully, as the principle of patient-specific alignment has also increased in popularity and is reporting good results in certain cases. The focus now should be on preoperative planning rather than only on the result, as success used to be described by deviation of ±3° from the neutral axis about.³⁰ In this study, the difference between predicted and actual femoral and tibial bone resections was less than 2 mm in 210 (87.5 %) cases, with precision being slightly higher for tibial resections compared to femoral ones, albeit marginally. Bone resections exhibited high precision across all six sites, with an overall standard deviation of less than 1.0 mm. The larger discrepancies noted in lateral cuts may relate to hypoplasia of the lateral femoral condyle and rotational issues associated with valgus alignment, along with the accelerated wear of the lateral tibial plateau observed in these cases. This results are consistent with those reported by Sires et al.,²³ who noted high accuracy in both femoral and tibial bone resections, with 94 % of actual cuts deviating less than 1 mm from the predicted cuts. This 1-mm difference may be contextualized by Sires' study, which analyzed only 105 bony cuts across 37 patients, whereas we evaluated 210 cuts in 40 patients, potentially introducing greater variability in measurements. But this discrepancy hardly influences the ligamentous and soft tissue condition), the remainder of the laxities of 2 mm – in the lateral compartment during flexion – are usually regarded acceptable.

Our second important finding was the correct prediction of the size of the implant, which was achieved due to the correct resected bone and the balanced flexion and extension spaces. These factors had been demonstrated in earlier studies to affect TKA alignment.^24–26 This predictability of implant size in terms of being able to rely on it, is especially of use in academic and community medical settings by aiding in quality control and the reduction of inventory by making sure the right implant is available pre surgery. In the operating room, preoperative awareness of implant size reduces the number of required surgical trays, enhancing setup efficiency and resource allocation.²⁷ Furthermore, it contributes to decreased operative time and costs.¹² The tibial insert size is significantly impacted by the surgeon's intraoperative perception, as individual patients may present varying degrees of laxity. In cases of greater laxity, a thicker polyethylene insert may provide a perceived sense of enhanced stability, although quantifying such perceptions objectively is challenging.

Third, beyond millimetric accuracy, our patient-reported outcomes provide clinical context. KOOS-JR increased markedly after surgery, and the distribution of postoperative scores clustered in a range generally considered compatible with acceptable symptoms for many patients. This aligns with reports that TKA—regardless of technique—can yield large functional gains, while the incremental clinical benefit of robotic assistance versus conventional instrumentation remains mixed across studies.^21,29 Our data therefore complement the technical findings by showing that patients improved meaningfully on a validated PROM, without implying comparative superiority.

A limitation of this study is the relatively small sample size; however, low standard deviation values suggest a high repeatability of results. A larger cohort study would be necessary to draw more definitive conclusions. Moreover, this study only consisted of the patients who experienced the RA-TKA procedure, and the lack of comparison group hinders its generalization. Further comparison of the outcomes of RA-TKA and other operation methods should be explored in the future for a more comprehensive knowledge. Furthermore, examination of patient-reported clinical outcomes would be necessary to evaluate the effect of RA-TKA on postoperative convalescence and future functional recovery.

5 Conclusion

In this prospective single-arm cohort, RA-TKA showed high agreement between CT-based planning and intraoperative execution for bone resections and component sizing, with immediate coronal alignment close to neutral. These results indicate technical accuracy; they do not establish comparative clinical effectiveness versus conventional techniques. Future controlled studies with radiographic follow-up and PROMs are warranted.

Patient consent statement

Informed consent was taken from all patients for use of data.

Data availability statement

Data available on request from the authors.

Authors’ contributions

All the authors contributed to the design, analyses and reporting for this manuscript. Both authors read and approved the final submitted manuscript.

Ethics statement

N/A.

Funding statement

No sources of funding were used to assist in the preparation of this article.