1 Introduction

Total knee arthroplasty (TKA) surgeries are projected to increase in the US by 143% by 2050. Data suggests this trend in the US will continue over the next 30 years with TKA annual projections of 1,272,000 in 2030 and 1,921,000 by the year 2040.^1,2 This common surgical procedure is prescribed to patients with end-stage knee arthritis that have failed conservative treatment options. Improving functional capacity and decreasing pain of these patients post-operatively is essential to their quality of life.^3,4 Unfortunately, an estimated 15–30% of TKA patients experience continuing pain, impairment and functional limitations post-operatively.^5,6 These functional limitations may indicate the need for an additional intervention, such as manipulation under anesthesia (MUA), to improve patient recovery.⁷ Evaluating a patient's recovery relies on assessing their physical function through physical exam and functional tests performed during in-office visits, and by patient reported outcomes (PROMs).^8–10 However, this requires patients to come into the office, which has compliance rates as low as 35% at one year.¹¹ Given the lack of compliance with follow-up appointments, PROMs are often utilized to assess patient recovery over time without an office visit. Unfortunately, their use is restricted due to patient bias and effectiveness that result in inaccurate assessments.^12,13 Further, previous research indicates conflicting evidence on the correlation between PROMS and the functional capacities.^14–16 Consequently, remote methods of monitoring the functional recovery of a patient, especially those with high compliance rates, should be considered for post-operative surveillance of short- and long-term recovery.

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.^17–21 Previous implantable devices have required lead wires to pass through the skin, making them impractical for use. Activity trackers, while being more patient and clinician friendly, have had variable results including limited accuracy, inability to capture TKA-specific functional capacity, and poor compliance.^12,22 With recent advancements in wireless technology and implantable sensors, it is now possible to remotely monitor post-operative functional recovery in TKA patients and ensure compliance, potentially aiding the clinician in determining recovery progress and potential need for additional intervention.

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 provides consistent functional capacity data that is not reliant on patient compliance or perception of recovery. The primary purpose of this analysis was to assess the relationship of the gait parameters collected by Persona IQ® to PROMs and clinically measured knee ROM.

2 Materials and methods

2.1

2.1 Participants

One hundred and thirty patients (130) who received a TKA with Persona IQ® The Smart Knee® System (Zimmer Biomet, Warsaw, IN, USA) by a single surgeon at South Bend Orthopaedics (South Bend, IN, USA) between October 27th, 2021, and July 22nd, 2022, were included in this retrospective data analysis. All TKAs were performed using a proper rapid-rehabilitation surgical technique.²³ Inclusion criteria for the analysis were: 18-years or older, patient underwent a commercially available primary TKA with Persona IQ. Exclusion criteria were patients undergoing simultaneous bilateral TKA, documented current alcohol or drug abuser, severe instability secondary to the absence of collateral ligament integrity, and patients with a second TKA within 6 weeks of the primary TKA. For patients with a second TKA longer than 6-weeks following their initial TKA, only the first limb was included for analysis. The process for patient exclusion and enrollment is described in Fig. 1.

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Fig. 1 Patient inclusion and exclusion process map.

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3 Results

Overall, 130 patients who received Persona IQ were included for analysis of the six-week post-operative period. Seventy (53.8%) of the patients had right TKAs. Thirty-eight (29.2%) of these patients had a second Persona IQ TKA more than six-weeks after the primary TKA (data from the second TKA was not included in the analysis). The average patient age was 64.20 ± 9.8 years, 50% of patients were over 65 years, with a mean BMI of 34.7 ± 8.4 kg/m². Additional demographic and relevant medical history summaries are given in Table 1.

In total, five patients required additional interventions following care for their knee. These additional interventions included three (3.1%) cortisone injections for pain and soft-tissue inflammation about the operative knee, at 74-, 137- and 169-days following surgery. These three patients (one male) were all over 65 years in age. Further, two (1.5%) patients were indicated for MUA due to poor ROM identified during post-operative clinical evaluation. These patients were an over 65-year-old female and an over 65-year-old male, who received a MUA 42 days and 63 days post-operation, respectively.

Overall, 98 (75.4%) of patients had VR-12 and/or KOOS Jr. data available at both baseline and 6-weeks. KOOS Jr. scores improved significantly from baseline (48 ± 13.3) to 6-weeks (66.5 ± 12.1) following surgery (p < 0.001). Post-operative VR-12 physical health scores (38.7 ± 8) were significantly (p = 0.003) higher compared to baseline (35.7 ± 9). VR-12 mental health scores were significantly decreased six-weeks after surgery (53.8 ± 10.2), compared to baseline (56.1 ± 9.2, p = 0.023). Baseline preoperative extension (6.1° ± 8.1°) and flexion (94.7° ± 15.6°) ROM improved significantly to 0.5° ± 1.6° and 122.7° ± 9.4°, respectively, at six-weeks following surgery (both p < 0.001).

Of the 130 patients, six (4.6%) did not have qualified step count data and three (2.3%) of patients did not have walking speed, cadence, stride length, tibial ROM, or knee ROM data available during the gait analysis period (visit date ±2 days). The average qualified step count during this time was 2800 ± 2380 steps/day, with an average walking speed of 0.58 ± 0.14 m/s. Further, our patients had an average cadence of 87.5 ± 9.7 steps/min and stride length of 0.70 ± 0.17 m. Average tibial ROM was 47.3° ± 7.3°, with an average total knee ROM during gait of 52.3° ± 4.8° (Table 2).

Two statistically significant correlations were found between PROM scores and the average gait kinematics collected 6-weeks following surgery. VR-12 physical demonstrated a statistically significant but weak positive correlation (r = 0.30, p = 0.004) to average daily step count and to knee ROM (r = 0.21, p = 0.041) 6-weeks after surgery. All correlation statistics are summarized in Table 3.

4 Discussion

Previous research on functional recovery indicates an early post-operative decline in functional capacity followed by significant improvements through the first nine weeks after the procedure, after which, the recovery begins to slow down.^26,27 Gait data gathered via Persona IQ demonstrated both similarities and differences to data previously reported in biomechanical studies. Patients in our analysis averaged 2800 qualified steps/day by six-weeks after surgery, which is lower than previously reported by Cooper et al., who reported an average of 4305 steps/day for 62 TKA participants wearing an accelerometer.²⁸ Differences in the step count reported in our analysis and those previously reported may be due to our utilization of qualified steps that meet the minimal requirements for continuous steps at an acceleration adequate to trigger the embedded accelerometer. While this may lead to an underestimation of daily steps, qualified steps count allows us to separate gait data from non-gait activity and may represent a better indicator of recovery than non-purposeful steps. A large study population-based prospective cohort study has shown that purposeful steps are associated with greater risk reductions than incidental steps.²⁹ Further, Senden et al. reported an average gait speed of 1.09 m/s at six weeks post-operation, which is similar to the gait speed of 1.10 m/s reported by Amemiya at six-week, both higher than the gait speed (0.58 m/s) reported in our analysis.^30,31 Senden et al. reported an average walking cadence of 104 steps/min, which is similar to the step frequency of 106–118 steps/min reported by Amemiya et al., and also higher than the cadence of 87 steps/min reported in our analysis.^30,31 These differences in speed and cadence may be due to the location on the body where the accelerometer was placed, as Amemiya et al. and Senden et al. utilized an accelerometer affixed to the lower back, or to the manner the data collection occurred, as our patients were not assessed in a laboratory, but were monitored during daily activities. Further, our patients had a longer stride length on average (0.70 m) compared to both publications, 0.43 m and 0.56 m, respectively.^30,31 These longer stride lengths may be attributable to differences in data collection protocols, average patient height, and may contribute to a decreased cadence in our patient population.

Due to the well documented importance of passive knee ROM following TKA for improved clinical outcomes, patient satisfaction, and quality of life, we measured passive ROM during patient visits, as well as knee and tibia ROM during qualified steps.^14,32–35 Our patients exhibited low preoperative passive ROM and exhibited similar functional ROM at six-weeks post-operative compared to those at 12-months following surgery reported previously.³² Further, the average knee ROM of 52° and tibial ROM of 47° collected during qualified steps were different than previously reported ROM during gait collected in a laboratory setting.^36,37 Rahman et al. reported an average shank ROM during swing phase of 63° and an average knee ROM during swing phase of 41° in TKA patients eight weeks following surgery.³⁷ The difference could be due to the way the data was collected. We collected data during daily living activities, whereas Rahman's lab data was generated by a 20-m walk in a corridor.

In addition to the comparisons of our functional data, we found that our patients had similar VR-12 mental, VR-12 Physical, and KOOS Jr. scores at baseline and 6-weeks following surgery as previously reported by Feng et al.³⁸ Although our patients exhibited PROMs and gait kinematics similar to those previously reported in literature, we did not find strong correlations between the two. We did not find any significant correlations at the p ≤ 0.05 level between PROMS and passive knee ROM. Other studies have shown weak and statistically significant correlations between PROMs and passive knee ROM.^{14–16,33,39} These findings may indicate that the patient's perception of clinical improvement and improvement of functional capacity during gait are not interchangeable and should both be considered when determining patient outcome quality. The potential for a lack of correlation between PROMs and gait kinematics, and PROMs and passive knee ROM was anticipated, as we often found in clinic that patients with poor PROM scores had clinical evaluations and kinematic data that bespeak a good clinical outcome, and vice versa.

5 Conclusion

In conclusion, the purpose of this analysis was to determine the correlation between functional gait parameters, PROMs and passive ROMs. Similar to previous reports of maximal ROM, we found that neither passive knee ROM nor gait ROM during daily living correlate strongly with the PROMs, despite our patients exhibiting similar scores as previously reported in literature. Therefore, our analysis suggests that PROMs and functional knee capacity are measuring two separate phenomena and that patient perception of a good outcome may not be equivalent to a good outcome for the joint itself. Persona IQ® The Smart Knee® has the capacity to improve the surgeon's ability to monitor how well the implant is functioning and acts as an excellent supplement to the PROMs typically collected for monitoring patient outcomes following TKA.

Ethical statement

This study was approved by an IRB with approval for a waiver of consent due to the retrospective nature of the study.

Funding

Research support was provided by Canary Medical Inc for this project.

Author contributions

Conceptualization – All.

Data curation – Derek Yocum, Barbara Elashoff.

Formal Analysis – Derek Yocum, Barbara Elashoff.

Funding Acquisition – Jeffrey Yergler, Patrick Verta.

Investigation – Jeffrey Yergler (surgery, clinical visits), Gary Armock (surgery, clinical visits).

Project Administration – Derek Yocum.

Writing (Original Draft) – Derek Yocum, Barbara Elashoff.

Writing (Review & Editing) – All.