1 Introduction

Total knee arthroplasty (TKA) is one of the most commonly performed orthopedic procedures. Over 790,000 TKAs are performed annually in the United States, and this number is projected to increase in the US to 1.4 million by 2030.¹

TKA is primarily indicated for patients with end-stage knee arthritis who have failed conservative treatment options. Beyond factors specific to the surgical procedure itself, improving functional capacity and decreasing pain postoperatively is essential for a successful outcome in this patient population.² Despite a very high success rate for this procedure, an estimated 15–30% of TKA patients experience continuing pain, impairment and functional limitations post-operatively.³

Evaluating a patient's recovery relies on close follow up to assess function through physical exam and functional tests performed during in-office visits, as well as patient reported outcomes (PROMs).⁴ Patient compliance with postoperative pain medication, physical therapy, and reporting to follow-up visits are important to achieve a successful outcome. The one-year follow-up rate after TKA has been reported to be as low as 35%.⁵ Given the lack of compliance with follow-up appointments, PROMs are often utilized to assess patient recovery over time without an office visit.

Previous efforts to remotely capture daily-living functional capacity have involved implantable devices which actively record gait parameters throughout the daily life of patients, or wearable activity trackers.^6,7 Previous implantable devices have required lead wires to pass through the skin, making them impractical for use. Activity trackers have had issues with variable accuracy, inability to capture TKA-specific functional capacity, and poor patient compliance.⁸ Smart implant sensors have allowed surgeons to bypass issues of patient compliance associated with external wearable devices and obtain more accurate assessments of postoperative functional recovery.^6,7

Canturio™ Tibial Extension (CTE) (Canary Medical Inc., Vancouver, British Columbia, Canada) is the first FDA-market authorized implantable device capable of wirelessly transmitting daily-living gait kinematics. This novel tibial stem extension includes an embedded inertial measurement unit. This stem extension attaches to the tibial plate of Zimmer/Biomet's Persona Knee system (Zimmer Biomet, Warsaw, IN, USA), creating Persona IQ®. This implant wirelessly transmits gait parameters including step count, distance traveled, tibial ROM, functional knee ROM, walking speed, cadence, and stride length.

The primary purpose of this study was to evaluate the accuracy of a smart knee implant in measuring absolute range of motion (ROM) after TKA compared to surgeon-measured in-office ROM.

2 Materials and methods

2.1

2.1 Participants

One hundred fifty patients (150) who received a TKA with Persona IQ® The Smart Knee® PS System (Zimmer Biomet, Warsaw, IN, USA) by a single surgeon at Arrowhead Orthopaedics (Redlands, CA, USA) from 2023 to 2025 were included in this retrospective data analysis. All TKAs were performed using a proper rapid-rehabilitation surgical technique. Inclusion criteria for the analysis were: 18-years or older, patients underwent a commercially available primary TKA with Persona IQ. Exclusion criteria were patients who had incomplete/missing data in the database, revision TKA, TKA for fracture, tumor, infection, or patients who underwent TKA with non-Persona implants (Fig. 1). Each clinic visit was assessed as an individual data point.

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Fig. 1 Flow diagram illustrating Inclusion and Exclusion data.

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3 Results

Overall, 150 patients who received Persona IQ were included for analysis, with a total of 327 clinical visits between the 2-week, 6-week, and 12-week postoperative visits. 78 (52.0%) of the patients had left TKAs, and 72 (48%) of patients had right TKAs. The mean patient age was 62.89±8.31 years, with a mean BMI of 33.25±4.87. Intraoperatively and postoperatively there were no documented complications. Additional demographic data is included in Table 1.

Of the 150 patents included in the study, the mean preoperative range of motion (ROM) was 90.85±8.15°. The mean postoperative clinic ROM at the 2, 6, and 12-week postoperative visits were 84.72±13.04, 108.39±12.94, and 114.78±8.24. (Table 2). The gait parameters at each visit (Tibial ROM, Functional Knee ROM, Step Count, Stride length, Distance, Walking Speed, and Cadence) are illustrated in Table 2.

On linear regression analysis, absolute in-office ROM was correlated to smart implant tibial ROM multiplied by 1.9 (R² = 0.97) and to smart implant functional knee ROM multiplied by 1.86 (R² = 0.98) (Fig. 2).

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Fig. 2 Linear Regression Correlation between clinical ROM and the following variables: A. Smart implant Tibial ROM B. Smart implant knee ROM.

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Multiple linear regression analysis of various gait parameters and clinical knee ROM showed statistically significant associations with stride length, distance, and walking speed (Table 3). Step count was not associated with improvement in clinical knee ROM. (Table 3). Multiple linear regression analysis is illustrated in Table 3.

Overall, 85 (57%) patients had KOOS scores and PROMIS scores available from preoperative evaluation to 3-month postoperative follow-up. Mean preoperative versus 3-month postoperative PROMs are illustrated in Fig. 3. KOOS scores for ADLs, Pain, QOL, Sportrec, Symptoms increased from 41.0 to 66.9 (63.2%), 37.4 to 66.0 (77%), 12.7 to 43.0 (238.6%), 22.8 t0 42.2 (85.1%), and 42.2 to 63.8 (51.2%) respectively, from preop to 3-months postoperatively. Patient PROMIS mental scores increased from 47.6 to 48.9 (2.73%), and PROMIS physical scores increased from 37.3 to 44.5 (19.4%) (Fig. 3) (see Fig. 4).

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Fig. 3 Patient KOOS scores at preoperative clinic visit and 3-months postoperatively for: a) KOOS ADL b) KOOS Pain c) KOOS QOL d) KOOS Sportrec e) KOOS Symptoms.

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Fig. 4 Patient PROMIS scores at preoperative clinic visit and 3-months postoperatively for a) PROMIS-10 Mental and b) PROMIS-10 Physical.

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4 Discussion

As patient expectations and demand for TKA increases with time, surgeons will need to leverage technology to facilitate practice efficiency. Smart implant technology can allow surgical care teams to create accurate, comprehensive, and objective assessments of functional recovery. The current study showed the smart knee implant accurately measured postoperative TKA absolute ROM.

The evaluation and management of a patient's recovery after total knee arthroplasty relies on assessing their function through physical exam and functional tests typically performed during in-office visits. However, compliance rates for these assessments can be as low as 35% following surgery, highlighting the need for remote monitoring methods with high compliance rates for postoperative surveillance.⁵ Prior remote monitoring efforts employed implantable devices requiring lead wires or wearable activity trackers, requiring significant patient compliance for success.^6–8 The smart implant device employed in our study offers a solution by enabling wireless transmission of daily gait kinematics directly to a database which can be tracked and assessed by both the surgical care team and patient. This can benefit both patient and surgeon, by reducing postoperative clinic visits and the associated costs of time and transportation for the patient (i.e. changing the 6-week in-person visit to telemedicine), while still allowing the surgeon to closely follow patient progress.

Smart implant gait data demonstrated both similarities and differences to data reported in other studies. Patient clinical ROM improved from a mean of 91° pre-operatively to 115° at 12 weeks postoperatively. This is similar to the findings of Mutsuzaki et al. who observed a mean ROM of 110° at 12 weeks post-operatively in their prospective study of 39 patients who underwent TKA.⁹ In our linear regression analysis, the smart implant tibia ROM and functional knee ROM were highly correlated with a patient's clinical knee range of motion. The surgical care team could accurately predict a patient's clinical ROM directly from smart implant data.

All collected smart knee gait parameters (cadence, walking speed, step count, distance, stride length, knee ROM, tibial ROM) improved from 2 weeks post-operatively to 12 weeks post-operatively. This is to be expected during this period as patients begin to see a reduction in postsurgical pain and swelling as well as begin physical therapy to improve ROM. Although there is no study at present which has compared these gait parameters at different time points, the gait parameters we observed at the 6-week time point were similar to the findings of a study by Yocum et al., who reported a mean step count of 2800 steps, walking speed of 0.583 m/s, stride length of 0.698, tibia ROM of 47.3°, and knee ROM of 52.3 at 6 weeks post-operatively in 130 patients who underwent TKA with a smart knee implant device.¹⁰ The improvement in cadence, walking speed, step count, distance, and stride length was associated with improvement in clinical ROM. This may offer a valuable surrogate to measuring a patient's recovery and improvement after TKA. In a study conducted by Bonnefoy-Mazure et al. investigating gait biomechanics and clinical outcomes in patients after TKA, they reported an association between improvement in gait velocity and increased knee ROM in patients after TKA.¹¹ Additionally, they observed increases in knee ROM were associated with decreased pain and increased PROMS.¹¹

Our study, like several studies reported in the literature investigating conventional total knee arthroplasty outcomes, show an improvement in PROMS at 3-months postoperatively using the smart knee implant. Although there was a non-statistically significant increase in the PROMIS-10 Mental score, the PROMIS-10 Physical and all KOOS scores were significantly improved at 3-months post-operatively.

There were several limitations in our study. All patients were operated on by a single surgeon and one implant design, which could limit generalizability. However, the same surgeon also measured postoperative ROM at every follow-up visit—limiting variability in ROM collection. All data reported in this study are reflective of patients who received a specific implant, and while this is reflective of many of the patients who underwent TKA at the site, it may not reflect outcomes for all TKA patients, or patients who undergo TKA with a different smart knee implant.

Overall, Persona IQ® The Smart Knee® accurately predicts absolute knee ROM after TKA and provides surgeons with objective data to monitor patients’ functional recovery. Further research is needed to evaluate the efficacy of remote therapeutic monitoring in reducing the number of in-person postoperative visits within the 90-day global period.

Guardian/patient's consent

Given this was a retrospective review using patient Chart checking, patient consent was not obtained. However, patient consent was obtained for all procedures performed which were retrospectively reviewed in this study.

Ethical statement

This study was a retrospective review. All authors listed contributed to the creation of this manuscript. Please refer to the author credit statement for specific contributions. Additionally, there was no use of AI in the creation of this work. This is original work, and not plagiarized or copied from prior work. Lastly, all procedures which were retrospectively chart reviewed were performed in compliance with relevant laws and institutional guidelines.

Authors

Akash Shah, MD: Conceptualization, Methodology, Validation, Investigation, Data Curation, Supervision, Project administration, writing review and editing.

Damien Cannon, MD: Software, Validation, Formal Analysis, Investigation, data curation, Writing original draft, Writing review and editing, visualization.

Kishan Talati: Software, Validation, Formal Analysis, investigation, data curation.

Brittany McPhee: Writing original draft, writing review and editing, visualization by making graphs and figures.

JOA funding statement

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.