Clinical Validation and Outcome Measures From Bend Ease: A Novel, Sensor-Based Digital Measurement Tool for Assessing At-Home Morning Stiffness and Spinal Range of Motion in Axial Spondyloarthritis
verfasst von:
Angela Crowley, Lori Siegel, Rebecca Grainger, Dan E. Webster, Tiancheng He, Liuqing Yang, Elina Moon, Dee-Dee Shiller, Michelle Crouthamel, Heather Jones, Phillip J. Mease, Jeffrey R. Curtis
To evaluate the accuracy, reliability, and usability of Bend Ease, a novel smartphone-based digital health technology (DHT), which objectively self-measures spinal range of motion (SRoM) and remotely assesses morning stiffness.
Methods
This phase 1 study involved healthy volunteers (HV) and patients with axial spondyloarthritis (axSpA). Participants used Bend Ease by placing a phone against their chest during a forward-flexion bend, and the application collected and processed accelerometry data to measure bend angle in both clinical and at-home settings. Bend Ease measurements were compared to the video-based method (gold standard) and functional ability questionnaires.
Results
The study included 30 HV and 30 patients with axSpA. Bend Ease accurately measured forward-flexion bend angles, demonstrating strong correlation (r = 0.74) and concordance (ρc = 0.71) with measurement by video. Impaired bending for patients with axSpA relative to HV was most pronounced upon waking (65.3° versus 88.3°, P < 0.001), with increasing bend angle improvements observed for patients with axSpA at later time points (71.0° and 75.8° at 30 min and 1 h after waking, respectively). Waking bend angle correlated with self-reported morning stiffness and functional ability scores. A minimum clinically important difference in bend angle of 14 degrees was established for patients with axSpA, providing a benchmark for improvement. Bend Ease demonstrated robust test–retest reliability, and participants reported high usability.
Conclusions
Bend Ease is an accurate, reliable, and user-friendly tool for assessing SRoM. As the first DHT to objectively evaluate morning stiffness upon waking, Bend Ease provides valuable assessments of spinal mobility when it is most impaired.
Prior Presentation: Some of the results reported in this manuscript were originally published in an abstract associated with the European Alliance of Associations for Rheumatology 2024 Congress. Available at: Crowley A et al. Bend Ease: A Novel App-Based Digital Measurement Tool for Assessing At-Home Morning Stiffness and Spinal Range of Motion in Axial Spondyloarthritis [Abstract]. Ann Rheum Dis, 2024; 83 (suppl 1): 1779. https://doi.org/10.1136/annrheumdis-2024-eular.1301.
Key Summary Points
Why carry out the study?
Axial spondyloarthritis (axSpA) is characterized by back pain and stiffness in the morning, primarily measurable by spinal range of motion (SRoM); traditional clinical assessments often capture SRoM later in the day when morning stiffness has already subsided or rely on patient-reported questionnaires.
We developed and validated Bend Ease, a novel smartphone-based digital health technology that enables patients to quickly, accurately, and objectively self-measure their SRoM.
What was learned from the study?
For the first time, serial morning SRoM measurements showed that measuring spinal mobility directly upon waking captures the greatest magnitude of impairment between patients with axSpA and healthy volunteers across all time points in this study.
Our study also established a minimum clinically important difference in waking bend angle of 14 degrees, providing a meaningful context for interpreting the efficacy of therapeutic interventions on morning stiffness for clinical care or clinical research.
This novel digital health tool represents a significant advancement over traditional questionnaire-based methods to assess morning stiffness and has potential applications across various rheumatic conditions in providing objective assessment of disease impacts and treatment efficacy.
Introduction
Axial spondyloarthritis (axSpA) is characterized by back pain and stiffness, with back stiffness measurable by spinal range of motion (SRoM)[1]. SRoM impairment typically peaks for patients with axSpA upon waking and is commonly described as morning stiffness. Stiffness can improve as the day progresses with movement and exercise [2]. Morning stiffness is part of the core outcome set for axSpA clinical trials [3] and directly impacts disease activity and physical function [3‐6]. However, morning stiffness poses significant challenges for practical and objective measurement in a clinical setting due to the relatively limited time window in which it presents after waking. In both clinical trials and routine care settings, traditional clinical assessments of axSpA, including the modified Schober test [7], are often conducted later in the day after morning stiffness has already subsided. This delay in assessment hinders the acquisition of real-time measurements of morning stiffness. Instead, commonly used approaches for assessing morning stiffness rely on patient-reported outcome instruments, such as the inclusion of two questions within the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) questionnaire evaluating the severity and duration of morning stiffness [3, 8, 9]. However, the BASDAI may be limited by reporting bias, placebo effects, or the influence of pain from concomitant diseases such as fibromyalgia [10].
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Digital health technology (DHT) is well known to provide objective sensor measurements and has the potential to improve the quantitative assessment of morning stiffness in axSpA. By utilizing DHT, novel approaches for objective measurement of SRoM could be implemented that enhance and complement traditional self-reported methods currently used, providing a more accurate and comprehensive understanding of the impact of morning stiffness on people with axSpA. The rise of decentralized trials in which at-home assessments might be made, outside of office-based clinical research visits, also offers unique opportunities for enhanced and more frequent data capture, especially when assessments such as morning stiffness should be made at a particular time of day [11].
DHTs for several rheumatologic indications have included motion-sensing devices like accelerometers and gyroscopes to assess functional movement. Digital goniometers have been developed and validated for measurement of spinal mobility [12, 13]. Moreover, adhesive wearable sensor technology has recently demonstrated diurnal variation of mobility in patients with axSpA [14]. However, a limitation of these methods is that they often require a second person to perform the measurements on the patient or study participant, which reduces the feasibility of DHT for at-home use.
In this study, we aimed to develop a smartphone application for patients with axSpA to self-measure their SRoM using built-in-motion-sensing technology and investigate the application’s accuracy, reliability, and usability. This application, named Bend Ease, could then be used to assess SRoM in an at-home context immediately upon waking to provide an objective measure for morning stiffness.
Methods
Study Design and Participants
This was a non-drug, exploratory study to evaluate Bend Ease, a novel DHT application for assessing SRoM and morning stiffness using smartphone accelerometry. An overview of the study design is shown in Fig. 1. The study included adult participants (> 18 years old) diagnosed with axSpA by a trained, board-certified rheumatologist, and healthy volunteers (HV). Patients with axial spondylarthritis had a previous diagnosis of non-radiographic axSpA or radiographic axSpA with chronic back pain lasting at least 3 months. There were no specific requirements regarding the minimum severity of pain for study entry. The eligibility of patients with axSpA was determined by the study investigators, ensuring patients were suitable candidates based on their medical history, including clinical features, laboratory assessments, and imaging, as well as current health status. HV were required to be in good overall health and capable of standing and performing forward-bending motions safely and independently. Exclusion criteria for HV included history of significant back conditions, severe lumbar stenosis, previous lower back surgery, and any condition that increased the risk of falling while bending forward, such as postural orthostatic tachycardia syndrome, vertigo, or labyrinthitis. Participants were enrolled from the AbbVie Clinical Pharmacology Research Unit in Grayslake, IL. An estimated sample size of approximately 30 patients per group was calculated (described in Supplementary Materials).
Fig. 1
Bend Ease digital health technology and study design. A Bend Ease application usage during a forward-flexion bend task performed by a representative healthy volunteer (left) and patient with axSpA (right). Accelerometry data were collected during the bend, while video capture was used to calculate a reference angle. B Study design schematic for the phase 1 trial evaluating clinical validity of the Bend Ease application. For bend angle measurements with Bend Ease, three replicate forward flexion bends were performed at each time point, and the median of those values was used for subsequent analyses. AxSpA axial spondyloarthritis, PRO patient-reported outcome
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Bend Ease Smartphone Application Development
The study team developed Bend Ease, a newly created smartphone application for the iOS system, designed using the Swift programming language. Bend Ease collected triaxial accelerometry data at 100 Hz during forward-flexion bends while each study participant held a provided study phone flat against their chest. Forward flexion was chosen as the representative task for SRoM because of its relevance for routine functional tasks (e.g., bending over to pick up items) and its larger dynamic range compared to cervical rotation or lateral-flexion angles. As part of each forward-flexion bend angle measurement, the user taps the “start” button before their bend to initiate data collection and taps the “end” button upon completion of the bend.
Study Process
The study included both in-clinic and at-home measurement sessions over a total of 7 days (Fig. 1). Participants were provided with a study phone (iPhone SE 2nd generation) with Bend Ease already installed as a beta-testing application. Participants used Bend Ease while doing forward-flexion bends, and the application calculated bend angles through an algorithm developed by the study team. Participants were initially trained on how to hold the phone and perform the forward-flexion bend via a brief video tutorial built into the application (detailed in Supplementary Materials and Study Protocol Appendix) and practice tasks during a clinic visit. On average, this training session for Bend Ease lasted 15 to 20 min per study participant and also involved a walk-through of a patient brochure and a question-and-answer period. The day 1 clinic visit included video recording of each flexion bend for angle verification (considered the gold standard) by a trained research coordinator. Participants also completed the Bath Ankylosing Spondylitis Functional Index (BASFI) and BASDAI questionnaires for functional impairment assessment, along with the System Usability Scale (SUS) survey. All clinic visits occurred in the afternoon due to scheduling constraints.
For home-based assessments from days 2 to 6, participants performed forward-flexion bends at three distinct time intervals daily: upon waking (as defined by the study participant), 30 min after waking, and 1 h after waking. Measurements were taken in triplicate at each time point, with the median of those values used for subsequent analyses. Participants used a modified Patient Global Impression of Severity (PGIS) questionnaire [15] integrated within Bend Ease to evaluate their perceived difficulty of each bend task at each interval.
The day 7 clinic visit included video-captured bending tasks and completion of BASFI, BASDAI, and SUS questionnaires. A brief interview was also conducted to gain insights into the experiences of participants with Bend Ease and overall study participation.
Bend Angle Calculation with Bend Ease Application
Triaxial accelerometry data were collected during the forward flexion task and securely transferred from the application to a backend server for quality control, signal processing, and analysis. To ensure data quality, the video capture of bend angle and the corresponding accelerometry data were compared to confirm that bend activities were performed according to protocol without errant phone screen taps to prematurely end data capture or signal the end of the task. Using this review, accelerometry data measurements captured in less than 3 s or more than 30 s were associated with user error, and a time duration-based filter applying these criteria was used for at-home measurements where no video reference was available. Comparison between the “on-device,” immediately calculated bend angle, and the "backend" signal-processed result is shown in Fig. S1. Algorithms for calculation of bend angle from accelerometry data are further described in the Supplementary Materials.
Video-Based Bend Angle Method
To validate the accuracy of bend angles calculated by the Bend Ease application, video recordings were captured via a tripod-mounted smartphone camera for each clinic visit (days 1 and 7) and used as the reference angle. A computer vision pipeline using the MediaPipe [16] pose estimation model was implemented to calculate the participant’s bend angle in the video data (Supplementary Materials).
Participant-Reported Outcomes
The BASFI and BASDAI questionnaires were used to capture participant-reported functional impairment and disease activity at baseline and during in-clinic visits on days 1 and 7. BASFI is a validated questionnaire designed to assess the functional impairment of axSpA, which includes 10 items addressing daily activities, such as dressing, hygiene, and mobility [17, 18]. The overall BASFI score was correlated with forward-flexion bend angle. BASDAI is a questionnaire-based tool consisting of 6 questions that assess different aspects of disease activity in ankylosing spondylitis, including pain, stiffness, and fatigue, and is rated on a scale from 0 to 10 [9]. The averaged values for BASDAI question (Q) 5 and Q6 (related to morning stiffness severity and duration, respectively) from in-clinic visits on day 1 and day 7 were used to evaluate the correlation with functional forward bend angle.
The PGIS [15], termed in the application as the “Ease of Bend Questionnaire,” evaluated the study participants’ perception of difficulty in performing forward-bending tasks and the severity of their symptoms. The questionnaire followed a 7-item choice rubric, incorporating predefined answers described in the PGIS questionnaire. Upon asking participants to fill out the Ease of Bend Questionnaire for the first time, a free-text query was implemented that asked patients to describe in their own words a meaningful activity that involves bending forward (e.g., picking up their child or tying their shoes) to assess their level of difficulty (rating scale ranges from 1 [no difficulty] to 7 [most difficult]). The aim of this modification to the PGIS was to allow participants to understand their difficulty within the framework of a familiar and meaningful task. The questionnaire was administered across the 7-day study period, including the first clinic visit (day 1), end of study clinic visit (day 7), and each at-home bend angle measurement (day 2–6).
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System Usability Scale Survey and Semi-Structured Exit Interviews
The SUS survey was administered on clinic visit days 1 and 7 to assess the participants’ perception of the usability of the Bend Ease application. The SUS survey is a short, 10-item questionnaire (each question is asked on a Likert scale ranging from “strongly disagree” to “strongly agree”). Overall scores ranged from 0 to 100, with higher scores indicating a higher level of usability and user satisfaction [19].
Upon completion of the study, bend angle data were presented to the user to allow them to comment on the utility of the returned results during semi-structured exit interviews. The interview questions were designed to assess the participants’ views on the frequency and duration of the surveys, application rating scales, suggestions for additional features, and views on the usefulness of sharing results with physicians in clinical care (detailed in Supplementary Materials). All interviews were manually transcribed and subsequently examined by a team member to identify patterns or themes. Consistent themes were also identified using a large language model (ChatGPT version 3.5).
Statistical Analysis
In bend angle analyses, the median value of the three replicate bends at each time point over at-home days was used for comparison with changes in digital bend measurements from wakeup to the two subsequent time points of wake + 30 min and wake + 1 h. Data from both HV and axSpA cohorts were used for accuracy of flexion measured by Bend Ease and test–retest reliability of Bend Ease. Day 1 clinic visit data were used to calculate the agreement between the bend angles measured by the Bend Ease application and those obtained from video recordings using Pearson’s correlation and Lin’s concordance correlation coefficients. Data from at-home measurements on days 2 to 6 were used for the test–retest reliability of Bend Ease angle measurements, using the wakeup timepoint, by calculating the intraclass correlation coefficient (ICC). This ICC was derived from a linear mixed-effects model incorporating random effects for participants and fixed effects for different days. Among patients with axSpA, Pearson’s correlation was used to evaluate the association between Bend Ease angles (calculated as the mean scores from the 5-day wakeup measurements) and both the BASFI total score and mean scores of BASDAI Q5 and Q6 (from the day 1 and day 7 clinic visit). The proportion of patients with axSpA experiencing less than 1 h of morning stiffness, as measured by BASDAI Q6, was also determined.
To investigate differences in Bend Ease angle measurements across cohorts, both at home and in clinic, linear mixed-effects models were used. For in-clinic data, the participant identification (ID) number was included as a random intercept. To analyze at-home measurements, the dependent variable in the linear mixed-effect model was bend angle, while fixed effects included the study group (HV versus axSpA patients), the wake period (consisting of wakeup, wakeup + 30 min, and wakeup + 1 h), and their interaction; participant ID was also incorporated as a random intercept. The R package “lmerTest” (version 3.1–2) was used to estimate the least-squares means. Pairwise contrasts between least-squares means were conducted using the R package “emmeans" (version 1.10.0) with Bonferroni’s correction. P values for all linear mixed-effect models were calculated using Satterthwaite’s degrees of freedom method.
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Based on the receiver-operator characteristic curves between the bend angle change and PGIS value change, a minimum clinically important difference (MCID) was derived by finding the maximal bend angle value using Youden’s index [20, 21]. The proportion of all participants experiencing MCID bend angle changes from wakeup were then analyzed.
Ethical Compliance
Studies were conducted per the International Conference on Harmonisation guidelines, applicable regulations, and the Declaration of Helsinki. Study-related documents were reviewed and approved by the Advarra Institutional Review Board (OHRP/FDA-registered IRB#00000971). All participants provided written, informed consent.
Results
Study Participants
A total of 30 HV and 30 patients with a prior diagnosis of axSpA (non-radiographic axSpA or ankylosing spondylitis) were enrolled in the study (Fig. 1). Baseline demographics were generally comparable between HV and axSpA populations (Table 1), with a mean participant age of ~ 48 years. Among patients with axSpA, 27% were male, the mean time since diagnosis was 7 years, and 47% had a positive HLA-B27 result. The mean overall BASFI score for these patients at baseline was 4.3, and the average BASDAI score for Q5 and Q6 was 5.9, indicative of severe morning stiffness.
Table 1
Baseline characteristics of healthy volunteers and patients with axial spondyloarthritis from the bend ease study
Characteristic
Heathy volunteers N = 30
Patients with axSpA N = 30
Female, n (%)
18 (60.0)
22 (73.3)
Age in years, mean (SD)
46.6 (14.4)
48.5 (14.1)
Years since diagnosis, mean (SD)
N/A
6.9 (11.6)
Race,an (%)
American Indian or Alaska Native
1 (3.3)
1 (3.3)
Asian
3 (10.0)
2 (6.7)
Black or African American
5 (16.7)
0
White
23 (76.7)
28 (93.3)
Body mass index (kg/m2), mean (SD)
26.0 (4.2)
29.6 (7.3)
Overall BASFI score, mean (SD)
N/A
4.27 (2.24)
BASDAI sum scoreb, mean (SD)
N/A
5.52 (2.14)
Average BASDAI score for questions 5 and 6, mean (SD)
N/A
5.93 (2.38)
HLA-B27 status,cn (%)
Positive
N/A
14 (53.8)
Negative
N/A
12 (46.2)
Not available
N/A
4 (13.3)
X-ray and/or MRI documentation of active inflammation or structural lesion, n (%)
N/A
26 (86.7)
axSpA axial spondyloarthritis, BASDAI Bath Ankylosing Spondylitis Disease Activity Index, BASFI Bath Ankylosing Spondylitis Functional Index, HLA-B27 human leukocyte antigen B27, N/A not available, SD standard deviation
aIndividuals can be included as part of one or more racial groups
bCalculated by the sum of questions 1 to 4 plus the mean of questions 5 and 6, with the resulting total then divided by 5
cData were not available for 4 patients in the axSpA cohort
Bend Ease Adherence, Accuracy, and Reliability
Adherence to the protocol for all at-home measurements was 94.0% (Fig. 2A), with each Bend Ease measurement taking approximately 9 s on average (29 s for triplicate bend tasks). All patients were able to provide all 3 of the Bend Ease assessments at each of the time points requested. Bend Ease angle measurements demonstrated strong correlation (r = 0.74, Fig. 2B) and concordance (ρc = 0.71, Fig. 2C), spanning a range from 55° to 110°, when compared to the video-based method in the clinic.
Fig. 2
At-home measurement adherence and accuracy of Bend Ease application. A Binary heatmap of at-home adherence to measurements per the protocol in all study participants (n = 60). Blue represents valid measurements; white represents missing or invalid data. Patients are listed in the order in which they were enrolled in the study. Measurements were taken in triplicate at each time point, with the median of those values used for subsequent analyses. Non-compliant data collection for patients 4 and 5 was due to user-based technical difficulties with the application that were later corrected. Of note, non-compliant data still contributed to overall patient adherence results. B Bend angle measurement correlation between video-based reference and Bend Ease angle measurements captured in a clinical setting. C Bland–Altman measurement comparison between video-based reference and Bend Ease angle measurements. CCC concordance correlation coefficient, r Pearson correlation coefficient
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When using Bend Ease to measure forward bends in a clinical setting (always during the afternoon), a significantly greater mean bend angle was observed in HV compared to patients with axSpA (86.6° versus 76.7°, P = 0.002; Fig. 3A). Bend angles measured in the clinic with Bend Ease were inversely proportional to BASFI scores (r = – 0.47, P = 0.008), with higher levels of self-reported functional impairment associated with a smaller bend angle (Fig. 3B). Notably, the association with BASFI was stronger for bend angle measurements conducted at home after waking (r = – 0.59, P < 0.001; Fig. 3C). Across all at-home bend angle measurements, Bend Ease demonstrated robust test–retest reliability (ICC = 0.84, Fig. 3D).
Fig. 3
Bend Ease reliability and association with functional indices from patient-reported outcomes. A Differences in Bend Ease forward-flexion bend angle measured between healthy volunteers and patients with axSpA in a clinical setting. For Bend Ease measurements, 3 replicate forward flexion bends were performed at each time point, and the median of those values was used to calculate the bend angle for a given time point. B Correlation between BASFI and Bend Ease angle measured in a clinical setting during the afternoon among the axSpA cohort. C Correlation between BASFI and Bend Ease angle measured upon waking in an at-home setting among patients with axSpA. D Representative 2-day comparison of waking bend angle for all study participants. Test–retest reliability ICC is derived from all at-home waking measurements. axSpA axial spondyloarthritis, BASFI Bath Ankylosing Spondylitis Functional Index, ICC intraclass correlation coefficient, r Pearson correlation coefficient
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At-Home Bend Angle Time Course and Bend Angle MCID
The mean bend angle difference between HV and axSpA cohorts was largest at waking (HV, 88.3°; patients with axSpA, 65.3°; P < 0.001) (Fig. 4A). Bend angle measurements showed a statistically significant increase at each time point after waking for the axSpA cohort; however, the observed changes in bend angle were consistently lower in the axSpA cohort compared to the HV cohort at all time points. Specifically, at 30 min after waking, the bend angle was 71.0° for HV versus 91.9° for patients with axSpA (P < 0.001). Similarly, at 1 h after waking, the bend angle was 75.8° for HV versus 91.5° for patients with axSpA (P < 0.001). Additionally, the marginal means controlled for wake periods were 90.6° and 70.7°, respectively, for HV and patient cohorts, resulting in a between-group difference of 19.9° (P < 0.001). The waking bend angle correlated with BASDAI Q5 self-reported severity of morning stiffness (r = – 0.39, P = 0.03; Fig. 4B). However, BASDAI Q6 self-reported duration of morning stiffness was not significantly associated with waking bend angle or the observed change in bend angle after 1 h (r = – 0.23; P = 0.22).
Fig. 4
Time course of at-home bend angle measurements using bend ease and correlation with BASDAI. A Time course of all at-home median bend angle measurements using Bend Ease upon waking, 30 min after waking, and 1 h after waking. Three replicate forward-flexion bends are performed at each time point, and the median of those values was used to calculate the bend angle for a given time point. B Correlation between BASDAI Q5 assessing the severity of morning stiffness and the waking bend angle. axSpA ankylosing spondyloarthritis, BASDAI Bath Ankylosing Spondylitis Disease Activity Index, Q5 question 5, r Pearson correlation coefficient
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For the modified PGIS questionnaire, HV reported minimal difficulty bending during all morning time points (mean score upon waking: 1.6; wake + 30 min: 1.4; wake + 1 h, 1.4; Fig. 5A). By contrast, patients with axSpA reported markedly greater difficulty in bending upon waking (mean score: 4.5) and a lessening of bend difficulty at each subsequent time point measured (wake + 30 min: 3.9; wake + 1 h, 3.4), although not to the levels reported by HV even an hour after waking. Our MCID for bend difficulty (i.e., a change of 2 severity categories on the PGIS scale) aligned closely with the observed inflection point of bend angle differences (Fig. 5B and C). Compared to the waking bend angle, 13% and 34% of patients with axSpA experienced bend angle change MCID at 30 min and 1 h after waking, respectively, whereas only a small proportion of HV met MCID criteria (1% and 5% at the respective time points), supporting the suitability of this MCID threshold to distinguish meaningful change (Fig. 5D). When anchoring to this change, an MCID for waking bend angle of 14° was derived that demonstrated 69% sensitivity and 62% specificity for detecting a two-category change in bend difficulty for patients with axSpA. Other MCID thresholds for bend difficulty, including a one- and three-category change in PGIS, were also explored but found to result in high proportions of false positives (e.g., HV showing bend angle MCID) or appeared too stringent (Figure S2). As orthogonal evidence for a two-category MCID threshold, 63% of patients with axSpA were found to experience less than 1 h of morning stiffness as measured by BASDAI Q6 (Figure S3).
Fig. 5
Establishment of minimally important difference in bend angle. A Cross-comparison of PGIS changes relative to baseline severity score and wake angle changes relative to baseline waking angle. For bend angle measurements with Bend Ease, 3 replicate forward-flexion bends are performed at each time point, and the median of those values was used to calculate the bend angle for a given time point. B The derived MCID for bend angle of 14 degrees is shown. C Receiver-operator curve demonstrating the sensitivity and specificity performance of the 14-degree MCID to capture a two-category or more change in PGIS for bend difficulty. D Proportion of participants in each group who experienced bend angle change MCID at 30 min and 1 h after waking compared to the waking bend angle. axSpA axial spondyloarthritis, MCID minimal clinically important difference, PGIS Patient Global Impression of Severity
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Bend Ease Usability and Participant Feedback
Study participants reported high usability (median: 95.3; maximum possible: 100.0) and learnability (median: 100.0; maximum possible: 100.0) scores for Bend Ease based on ratings from the SUS survey (n = 60/60; Table S1). The end of study interviews took approximately 10 min per participant. As a common theme of these interviews, patients expressed positive feedback regarding the application’s user-friendly interface and ease of use. They also appreciated the utility of having objective, functional data in providing a more accurate representation of their symptoms to their doctors, as opposed to relying solely on subjective reporting. Additionally, patients valued the ability to continuously capture and track their symptoms over time, which was reported to provide a sense of agency in managing their health between visits to their healthcare provider (Table S2). Consistent themes were identified by both manual assessment and ChatGPT.
Discussion
Bend Ease is the first validated digital health technology developed for patients with axSpA to self-measure their SRoM with a smartphone. This unique capability allows patients to conveniently assess their SRoM at home, directly after waking up when SRoM is most impaired due to axSpA-related morning stiffness. By contrast, traditional clinical assessment methods often miss this crucial window as they are conducted after morning stiffness has subsided. Our results indicate that Bend Ease had a high degree of accuracy compared to well-established video-based methods conducted in the clinic, high protocol adherence rates when used at home, and consistent test–retest reliability. Bend Ease measurements also showed a significant inverse correlation with self-reported functional impairment (BASFI) and morning stiffness severity (BASDAI Q5), highlighting its ability to capture clinically relevant aspects of axSpA. Of note, however, the correlation with morning stiffness duration (BASDAI Q6) was not significant, perhaps due to a large proportion of patients (37%) experiencing morning stiffness for more than 1 h, which exceeded the time range evaluated during this study. In comparing patients diagnosed with axSpA to healthy individuals, the axSpA cohort consistently had a smaller range of forward flexion at all time points, with the most pronounced difference occurring at waking. During the first hour after waking, patients with axSpA showed a gradual improvement in their bend angle, which corresponded to their self-reported difficulty of bending as recorded by the PGIS integrated within Bend Ease.
In addition to its technical capabilities, Bend Ease demonstrated strong usability and learnability, as evidenced by the positive results obtained from the SUS survey. Moreover, semi-structured interviews with participants at the end of the study revealed their appreciation for being able to objectively track symptoms over time and communicate this information to their healthcare providers, thereby helping to bridge the gap between subjective patient-reported symptoms and clinical evaluation.
The ubiquity of smartphones in the population [22], the familiarity with smartphone interaction patterns, and the mature sensor technology used in the application means that Bend Ease can be made widely accessible for remote patient monitoring. Moreover, the use of motion data to calculate bending, as opposed to video capture, preserves patient privacy as there is no need to transmit video-based images that could potentially compromise their identities. The relative speed of measurement (~ 9 s for each of the three assessments) and positive feedback from study participants also indicate that patient burden of measurement using Bend Ease in either a clinical trial or a routine care setting would be minimal.
Our study also establishes an MCID for the novel biometric of waking bend angle (determined to be 14°), providing a meaningful context for assessing the efficacy of interventions in clinical care or clinical research. When using a two-category change in severity categories on the PGIS, approximately one-third of the axSpA cohort experienced a bend angle MCID from waking to 1 h after waking. While this threshold for bend difficulty was conservative relative to the suggested meaningfulness threshold reported for PGIS of ≥ 1 [15], it closely aligned with the inflection point of bend angle difference observed in this study. The suitability of this threshold was further supported by its improved signal-to-noise ratio compared to using a one- or three-category change. By providing an objective measure of physical function, our 14-degree MCID bridges the gap between subjective assessments and traditional clinical measures. Moreover, the establishment of a bend angle MCID aligns with treat-to-target strategies used in axSpA management [23]. This threshold could guide earlier treatment adjustments, even before commonly used 3-month follow-up periods, providing clinicians with a supplementary parameter alongside traditional markers such as inflammatory scores or imaging findings.
There are several limitations to the Bend Ease application and the study design. While forward flexion was selected due to its significant relevance in daily functional tasks (e.g., bending over to pick up items) and its larger dynamic range, other clinically meaningful bending tasks such as side bend or rotational movements were not measured. In general, the study was not powered to differentiate between confounding diseases, including fibromyalgia, or distinguish between non-radiographic and radiographic axSpA. Bend Ease does not account for morning stiffness in peripheral joints. Additionally, this study only captured bend angle measurements during the first hour after waking, whereas SRoM might continue to improve in the approximately one-third of patients who experienced morning stiffness lasting ≥ 1 h. Future studies should consider monitoring morning stiffness beyond the current 1-h period to accommodate patients with prolonged stiffness durations. Moreover, studies conducted in overnight clinical settings to assess SRoM during morning hours would be valuable to further validate our findings. There were also some limitations in the instructions provided on how to use Bend Ease, which did not address questions such as whether participants could place the phone in their front pocket while bending. More specific instructions or confirmation regarding where on the anterior of the chest to hold the phone (e.g., fourth thoracic vertebra) could also have been provided. While Bend Ease has the potential to address SRoM measurement in other inflammatory back pain diseases such as axial involvement in psoriatic arthritis, these patient populations were not included in this validation study.
Despite these limitations, the ability to perform recurrent self-measurements using Bend Ease, potentially on a daily basis, allows for a granular understanding of the time course regarding the effectiveness of interventions on morning stiffness. In contrast, patient-reported outcomes such as BASDAI rely on patient recall over a period of 7 days. By obtaining measurements within a single day, particularly over the morning hours when stiffness is most prominent, it may be possible to establish a "functional improvement curve" for SRoM, which could differ across inflammatory conditions relative to conditions characterized by impairment due to structural damage or diseases like fibromyalgia.
In future investigations, it would be beneficial to validate the newly established MCID by examining the impact of pharmacologic or non-pharmacologic interventions on bend angles. This could help identify potential placebo effects and longitudinal change in SRoM over longer periods in relation to specific treatments, as well as further validate the long-term reliability and sustainability of Bend Ease. Additionally, including larger patient registries that represent the broader spectrum of patients with axSpA observed in clinical practice, should be considered in future research. Exploring additional assessments, such as the Bath Ankylosing Spondylitis Metrology Index (BASMI) [18], could offer a more comprehensive analysis of spinal mobility and strengthen validation efforts. Furthermore, the functionality of Bend Ease could be expanded to include assessments of lateral bending and rotation, alongside forward flexion, which are clinically relevant in axSpA. Integrating the ASDAS in future studies could also provide a more complete picture of disease activity.
Conclusions
Our results demonstrated strong correlation and concordance between Bend Ease and video-based methods, indicating the application’s accuracy in measuring SRoM. Bend Ease successfully assessed morning stiffness and was used by participants to track changes in their bend angles over a 1-h period after waking, which would not be feasible in a clinical or research setting. Comparisons with established self-reporting instruments showed that Bend Ease effectively captures functional impairment, aligning with measures such as BASFI and BASDAI. The high adherence rates and usability scores further support the practicality and user-friendly nature of Bend Ease for at-home measurements. Lastly, semi-structured exit interviews revealed positive feedback regarding the usability of Bend Ease and its ability to accurately capture and record symptoms. Overall, Bend Ease represents a novel and promising fit-for-purpose tool for monitoring and managing axSpA and related conditions in clinical practice and as part of evaluation in research settings.
Acknowledgements
AbbVie and the authors thank the patients, study sites, and investigators who participated in these clinical trials.
Medical Writing/Editorial Assistance
AbbVie and the authors thank the participants, study sites, and investigators who participated in these clinical trials. No honoraria or payments were made for authorship. Medical writing support was provided by Matthew Eckwahl, PhD, of AbbVie. Editorial support was provided by Michael Austin of AbbVie.
Declarations
Conflict of Interest
Financial arrangements of the authors with companies whose products may be related to the present manuscript are listed, as declared by the authors. Angela Crowley: Speaker fees from AbbVie, GSK, Horizon, and Janssen; consulting fees from AstraZeneca, Novartis, and UCB; and research funding from AbbVie, Lilly, Labcorp, SetPoint Medical, and Sun Pharma. Lori Siegel: Speaker and consulting fees from AbbVie, Amgen, Boehringer Ingelheim, GSK, Novartis, UCB, and Sanofi-Genzyme. Rebecca Grainger: Speaker fees from AbbVie, Pfizer, Cornerstone, and Janssen; consultant for AbbVie and Novartis. Philip J. Mease: Speaker fees from AbbVie, Amgen, Lilly, Janssen, Novartis, Pfizer, and UCB; consultant for AbbVie, Acelyrin, Amgen, BMS, CorEvitas, Cullinan, Lilly, Inmagene, Janssen, MoonLake, Novartis, Pfizer, and UCB; grant/research support from AbbVie, Acelyrin, Amgen, BMS, Lilly Janssen, Novartis, Pfizer, and UCB. Jeffrey R. Curtis: Consulting fees and research support from AbbVie, Amgen, Bristol Myers Squibb, Janssen, CorEvitas, Lilly, Novartis, Sanofi, Pfizer, and UCB. Dan E. Webster, Tiancheng He, Liuqing Yang, Elina Moon, Dee-Dee Shiller, Michelle Crouthamel, and Heather Jones: Employees of AbbVie and may hold stock or options.
Ethical Approval
Studies were conducted per the International Conference on Harmonisation guidelines, applicable regulations, and the Declaration of Helsinki. Study-related documents were reviewed and approved by the Advarra Institutional Review Board (OHRP/FDA-registered IRB#00000971). All participants provided written, informed consent.
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Clinical Validation and Outcome Measures From Bend Ease: A Novel, Sensor-Based Digital Measurement Tool for Assessing At-Home Morning Stiffness and Spinal Range of Motion in Axial Spondyloarthritis
verfasst von
Angela Crowley Lori Siegel Rebecca Grainger Dan E. Webster Tiancheng He Liuqing Yang Elina Moon Dee-Dee Shiller Michelle Crouthamel Heather Jones Phillip J. Mease Jeffrey R. Curtis
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