Examination of knee sagittal plane movements with wearable sensors andtheir application to the clinic: a scoping review

Abstract

AimThis study aimed to provide information about knee function by examining sagittal plane movements of the knee with wearable sensors, to summarize the growing literature to determine its clinical applications, and to provide an up-to-date overview.MethodsWe performed a comprehensive search of PubMed, Google Scholar, IEEE, and Scopus databases using various combination of the keywords “knee”, “knee joint”, “wearable technology” and “wearable devices”. We have thoroughly searched the included studies and reference lists to get more devices and references and took the device name and manufacturer for each reference.ResultsIt is important for knee-related wearable devices to capture the diversity of knee movements in daily activities. Misalignment of anatomical landmarks and axis of rotation, fast or slow movement, soft tissue artifact, and sensor flexibility can lead to inaccurate measurements.ConcclusionWhen the studies are examined, there is a rapid growth in this field and accurate and reliable measurements can be made with less error rates. Future studies should determine how to make the most valuable and accurate measurements for patients, reduce sensor complexity, and develop cost-effective models.

Keywords

Introduction

The knee is a frequently injured joint and the frequency of injury is increasing.¹ Clinically significant information obtained by monitoring the kinematic changes of the knee for clinical and research purposes is important for understanding the prognosis of diseases and also for evaluating rehabilitation practice.² Evaluation of sagittal plane movements of the knee joint is widely used to evaluate patient function and recovery.³ This kinematic information is usually obtained with protractors or electro-goniometers and three-dimensional (3D) motion capture cameras. However, electrogoniometers analyze motion in two planes. Three-dimensional motion capture cameras, on the other hand, are considered the gold standard for motion capture, but require a laboratory environment, experienced personnel and cost.⁴ Also, it is used only in a limited area, its evaluations are limited to short periods (1-2 hours) and often couldn’t be done because of its high costs.⁵ Considering the last ten years, significant progress has been made in the field of wearable technology.⁶ Many researchers have developed technologies to assess knee health using wearable sensors, and approaches have focused primarily on sensor detection, knee kinematics or gait assessment.^5,7-8-9
Wearable devices, often using magnetometers, accelerometers, and gyroscopes, can provide a higher level of information about joint motion, potentially related to the underlying pathophysiology or rehabilitation condition.⁵ Accelerometers are common sensors found in wearable devices and can give acceleration as well as top speed.¹⁰ Gyroscopes are another common sensor in wearable devices that detect angular accelerations.¹⁰ Magnetometers are often combined with and complement accelerometers and gyroscopes to filter the direction of movements.^10,11 However, Inertial Measurement Unit-based (IMU) approaches must use additional sensing units with accurate computational capability for signal processing and require regular calibration and modeling to reduce the effects of directional drift.^12,13
The examination of sagittal plane movements of the knee joint seems to be useful for evaluating functionality and medical condition. Studies on sagittal plane movements of the knee have been done,[5,7–9] but the accuracy and reliability of existing devices have not been mentioned. This review aims to provide an up-to-date overview of “Examination of Sagittal Plane Movements of the Knee with Wearable Sensors and Application to the Clinic.” Using the information in this review, informed and accurate device selections can be made for specific research objectives.

Materials and Methods

We conducted a study to examine the movements of the knee in the sagittal plane with wearable sensors and to determine its application to the clinic. Between January 1, 2018 and August 20, 2021, we searched the following databases: PubMed, Google Scholar, IEEE, and Scopus. Due to advances in technology, we did not include older articles in the study.
We have thoroughly searched the included studies and reference lists to get more devices and references. The search strategy included medical topics (i.e. MeSH), terms and text words related to “knee”, “knee joint”, “wearables” and “wearable technology”. We took the device name and manufacturer for each reference.Ethics ApprovalThis study is a scoping review of the literature and does not require ethical approval.

Results

The initial search resulted in 2,294 articles and we removed similar ones. We extracted 2,128 more articles based on inclusion and exclusion criteria. Finally, we reviewed the remaining 13 articles to examine the sagittal plane motion of the knee with wearable sensors and to determine their clinical application. A flow chart explaining the identified and included articles is given in articles. Inclusion and exclusion criteria are given below Figure 1.Inclusion Criteria• Studies involving the use of any combination of accelerometer, pedometer, or inertial measurement unit for the study of sagittal plane movements of the knee.
• Studies involving a least of n=10 knee patients.
• Works written in English
• Studies whose full text accessed
• Studies that provide accuracy and validity dataExclusion Criteria• Conference summaries.
• Book chapters
• Systematic reviews.
• Articles featuring animals, robotic assistive devices, orthoses, exoskeletons, or virtual reality environments
• Primary outcome measures; studies on heart rate, sleep or cognitive/emotional conditions, physical activity level
• Studies that only determine gait parameters and are used as an assistive device during surgery
• Studies describing published study protocols or abstracts.
The purpose, model and manufacturer, technology (sensor) and location characteristics of the studies examined are given in Table 1.
Accuracy and reliability and results of the measurement characteristics of the studies examined are given in Table 2. Limitations of the reviewed studies are given in Table 3.

Discussion

This review provides a comprehensive overview of 13 articles currently available for the study of sagittal plane movements of the knee with wearable sensors, along with hardware features and measurement capabilities (accuracy and reliability), and to determine clinical application. Continuous and inconspicuously monitoring of kinematic parameters (e.g., range of motion or peak knee flexion/extension angle during the stance phase) can provide important information in determining changes in individuals’ health status and in rehabilitation and/or pharmacological treatments.²⁶ The use of these systems provides clinicians with the opportunity to closely track the joint healing process and develop personalized, optimal rehabilitation programs to maximize the quality of life of individuals, the main goal of rehabilitation.²² In addition, the systems provide feedback to the person about their situation, and it is an important support for the participant to reach the predicted goal regarding the quality and completion of the exercises and can also be used to monitor therapy dosage and range of motion throughout the inpatient intervention. At this point, combining sensors with mobile health systems is an important step; however, there is no standard clinical system for continuous knee angle measurement with mobile health devices.
Sensor-based wearable systems should be able to evaluate the individual’s performance not only in rehabilitation outcome and progression, but also during activities of daily living. Because knee movement in daily activities can be highly variable, a sensor system needs to capture the rich variety of knee movements produced during an individual’s daily movement.¹⁴ Büttner et al.¹⁴ showed in their study that the knee angle outputs of the sensor during various activities of daily living were consistent with those of the reference system, and the results were comparable between short- and long-term measurements. Also, non-physiological data outputs were few and sensor failure was not evident. This is promising in that wearable technologies provide accurate results about performance in daily life.
Complex tasks such as jumping are difficult to accurately analyze with wearable sensors due to the large ground impact force. For example, decreased knee flexion during landing from a jump is associated with higher peak moments at the knee joint.²⁷ It has been stated that MoJoXlab²⁸ could accurately calculate joint angles for such complex tasks, so it is recommended to be developed to calculate potentially other complex tasks and exercises. Islam et al.¹⁶ also demonstrated that MoJoXlab²⁸ could use in a clinical setting for complex tasks such as walking, squatting, and jumping, and among a variety of participants, both healthy and participants with anterior cruciate ligament reconstruction.
There are accelerometers, gyroscopes, magnetometers and potentiometers as sensors in wearable devices related to the knee. Allseits et al. stated that the gyroscope is an adequate tool to measure the range of flexion and extension of the knee.²⁰ On the other hand, Joukov et al.²⁹ stated that for a complete motion evaluation and effective tracking, an accelerometer, a gyroscope, and a motion capture tracker should be combined in a single tracking unit.
Sensor systems could generally use by being mounted on the knee pad. However, problems such as incorrect palpation of anatomical landmarks or misalignment of the axis of rotation of the sensor during the calibration process have shown that the instrumented knee brace causes larger standard deviations than the reference system. Also, performing the calibration of the knee brace and the sensor with passive knee flexion causes the knee movement to differ in active and weight-bearing conditions such as walking, and this brings with it measurement errors. The elastic material properties of the knee pad can also cause sensor slippage. The stretching of the material may affect the of the actual movements of the legs. Knee angle measurement in terminal flexion and extension is the measurement most affected by this problem. Thus, absolute knee angle data should be interpreted with caution, especially for sagittal knee angle terminal values. However, the deviation value may be less critical as it measures total knee motion angle or changes in total knee angle based on relative data rather than absolute data.¹⁴ All these reasons show that special attention is required for knee pads in which sensors are integrated.
Büttner et al.¹⁴ in their study verbally questioned the adaptations of the participants to the knee brace, and the participants did not report discomfort when wearing an instrumented knee brace during single measurement sessions. Büttner et al.¹⁴ used potentiometer in their study and measured sagittal knee motion during various activities during a mean monitoring time of 7.1 ± 0.8 hours, without any connectivity issues or malfunctions in data transmission.
Accelerometers are sensitive to varying acceleration and vibration modes. However, it can be problematic for mobile applications as it increases bandwidth requirements for mobile systems, and reduces sampling rate and battery life.³⁰ In measurements made with gyroscopes, it has high accuracy and sensitivity to motion in short measurement periods, while the error rate is high in long measurement periods.¹⁷
Measurement accuracy is an important topic that requires deep research when investigating the diagnostic and evaluation potential of wireless sensors. Again, the repeatability of measurement results is another important factor that determines the validity of wireless sensors. Lisiński et al.²⁵ showed high reproducibility in the measurement results obtained in both normal and maximum velocity measurements in most of the subjects. Reproducible assessments of the degree of deviation of the knee joint from the flexion and extension trajectory are also very important. All deviations from the knee joint trajectory show possible joint instability.^30,31 or a mismatch of muscle activities affecting the joint.³² and hence the effect of fatigue.³³ Precise motor control and the ability to sense and or show the position of a joint are crucial factors in determining proper knee function. Again, Lisiński et al.²⁵ evaluated this in their study and evaluated the patients’ ability to return to 60° of flexion and the speed used to reconstruct this angle, and found no significant difference between the results recorded for the left and right knee joint.
The results of most studies presented so far have been considered isolated tests (focusing on a single joint movement) and therefore did not affect the knee joint functions tested. However, in rehabilitation practices, exercises that work several joint and muscle groups are generally used instead of isolated exercises. So, it is also important to test and examine with sensors several global motor tasks that are vital to people’s daily activities, such as sitting in a chair, standing up, lunging forward, and going up and down a step.²⁵ Lisiński et al.²⁵ demonstrated successful methods for monitoring exercises performed independently in their studies.
When the oscillating motion in the sagittal plane is rapid, the acceleration measured from the triaxial accelerometer is affected by the effect of gravity and cannot reflect the true flexion angle of the leg.²² Gholami et al.¹⁸ found that the sensor signal correlated highly with sagittal angles and less with non-sagittal angles. Also, in the same study, knee joint angles were more consistent in participants compared to hip and ankle, providing higher accuracy in estimating knee angles. In addition, soft tissue artifact and sensor limitations affect the estimation accuracy of knee angle in dynamic movements such as walking.¹⁹ One study showed that angles measured during walking, especially at heel strike, have higher error rates than other kinematic parameters. The causes of error during heel strike are related to sensor technology and location and occur because the sensor is not designed to detect hyperextension of the knee joint. For fast speeds also with bigger errors have been reported significant differences in peak flexion angle during both the stance and swing phases.²² However, Gholami et al.¹⁸ achieved similar accuracy at fast and slow speeds. The flexibility of wearable sensors affects the sensor readings that occur during flexion and extension movements. If the difference in torque and sensor flexibility produced by the person is too low, the sensor torque retraction will be slow and the disparity between flexion and extension will widen. Conversely, high tension can alter gait kinematics or cause stiffness or discomfort by pulling on the skin.²²

Conclusion

As a result; knee angles are often an important outcome measure in the evaluation of biomechanical function for both clinical and research purposes. In many diseases such as stroke and osteoarthritis, abnormal knee flexion/extension patterns can be seen during the gait cycle in patients. Monitoring and tracking these kinematic changes can yield clinically relevant and relevant information and data on prognosis and treatment. This review contains important data on the points to be considered in the measurement of sagittal plane angles of the knee.
What Do We Expect In The Future?
Evaluation of participants’ passive range of motion can provide individualized guidance on the limits to which knee angle data can be expected. Studies have measured not only isolated knee motion but also knee angles with other joints during daily activities, but data on long-term performance are scarce. Future studies should provide long-term follow-up. Reducing the number of sensors in measurements should be the goal of future research to avoid data and statistical complexity. Although the developed sensor devices are quite small and light, velcro straps or an elastic belt are required for skin contact. Converting the hardware to a chip in the future will be an important step in preventing sensor slippage and belt or soft tissue artifact. Again, it is recommended to develop algorithms that prevent soft tissue artifact to improve kinematic tracking. Future work should go towards ergonomic and functional arrangements of smart knee pads that differ in design, structure and material. Modifications are needed to prevent unwanted slack in the sensors, especially during dynamic movements such as running and walking. Again, studies should be carried out to search for a generalizable calibration procedure without the need for any user for sensor calibrations. Finally, advances in technology’s wearability, usability, general appearance and feel, ease of use, time required, and clarity of training will further increase participants’ interest in technology.

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