Objective Assessment of Orofacial Muscle Strength: Validation of an Alternative Low-Cost Measurement Device

Abstract

Objective: Accurate measurement of orofacial muscle strength is essential for phenotyping patients with obstructive sleep apnea (OSA), and particularly those with hypotonic phenotypes. High costs associated with gold standard devices such as the Iowa Oral Performance Instrument (IOPI^®) limit their clinical use. This study aims to validate the Sandway^® manometer, a low-cost alternative to the IOPI^®, by comparing its performance against the IOPI^® and the Tongue Digital Spoon (TDS) in both laboratory and clinical settings. Methods: We conducted a two-phase study. In the laboratory phase, pressure readings from the IOPI^® and Sandway^® devices were compared using standardized force application methods. In the clinical phase, 60 patients with moderate-to-severe untreated OSA underwent orofacial strength evaluation using IOPI^®, Sandway^®, and TDS devices. Intraclass Correlation Coefficient (ICC), Bland–Altman analysis, and Pearson correlation were used to assess reliability and agreement. Results: The Sandway^® showed excellent reliability for anterior tongue and lip strength measures (ICC = 0.978). Bland–Altman plots revealed minimal bias and narrow limits of agreement compared to the IOPI^®, indicating strong agreement for both tongue and lip measurements. A high correlation was also observed between Sandway^® and TDS results (r = 0.863, p < 0.001), supporting secondary validation. Conclusions: The Sandway^® manometer demonstrates high reliability and strong agreement with gold standard instruments, representing a valid, accessible alternative for objective assessment of orofacial muscle strength in clinical practice.

1. Introduction

Obstructive sleep apnea (OSA) is a highly prevalent condition, affecting an estimated 4–30% of the global population [1], and it is characterized by recurrent episodes of upper airway obstruction during sleep, leading to severe cardiovascular, metabolic, and neurological consequences [2,3,4]. Different phenotypes have been described to better understand the pathophysiology of OSA [5], one of which is the hypotonic phenotype, characterized by the inability of the upper airway muscles to maintain their patency during sleep.

The clinical evaluation of patients with OSA includes a comprehensive orofacial myofunctional assessment as an essential component for achieving more precise phenotyping. This functional evaluation goes beyond anatomical observations, allowing clinicians to characterize muscular ineffectiveness that may contribute to upper airway collapsibility. Muscle weakness identified in clinical practice is often associated with reduced muscle tone and may correlate with the hypotonic endotype described in the pathophysiology of OSA [5]. One of the key parameters in this context is the strength of orofacial muscles, particularly the tongue and lips, which play a critical role in maintaining airway patency during sleep. These parameters can be objectively assessed using validated tools such as the Iowa Oral Performance Instrument (IOPI^®) [6], considered the gold standard for isometric tongue and lip pressure measurements, or the Tongue Digital Spoon (TDS) [7], a more recent and cost-effective alternative that has shown its validity.

However, the relatively high cost of commercially available devices for assessing orofacial and myofunctional strength—such as the IOPI^®—represents a substantial barrier to their routine use in clinical practice. This economic limitation can negatively affect both diagnostic accuracy and the ability to monitor therapeutic progress, ultimately impacting patient outcomes. In fact, one recent survey concluded that costs of orofacial/myofunctional devices are a significant barrier for clinicians, patients, and caregivers treating OSA, and this barrier is greater for people and places with lower economic resources, resulting in the need to rely on alternative, home-made products as a cost-reduction strategy for the patient/family [8].

Therefore, there is a need to identify and validate lower cost alternatives to objectively measure orofacial strength to improve widespread implementation of objective measurement for enhanced and equitable patient care.

Recently, one laboratory-based study found that generic, low-cost, pressure testing manometers exhibit a near perfect level of agreement in pressure readings when compared with current clinical gold standard devices [9]. These findings suggest that such devices may serve as reliable alternatives for objective muscle strength assessment. However, it is important to note that the study was not completed using human patients, limiting its external validity and clinical applicability. In addition, the generic manometer evaluated is not currently manufactured or distributed in Europe, which poses a practical limitation for immediate clinical adoption in that context. Despite these constraints, the study highlights the potential utility of generic, low-cost, pressure testing manometers, representing a promising method to facilitate objective measurement of orofacial strength while addressing known barriers of care at reasonable cost.

Given the above, the aim of this study was to assess the performance of the Sandway^®, a cost-economic alternative device, by assessing its reliability and accuracy through comparison with the IOPI^® and the TDS in both laboratory-based and patient-testing contexts.

2. Materials and Methods

2.1. Laboratory Measurements

The first part of this study was conducted in the laboratory, comparing measurements taken by the following two devices: (A) the IOPI^® Pro Deluxe Kit (IOPI Medical LLC, Woodinville, WA, USA, EEUU), which presents a measurement range of 0–100 kPa, an accuracy of 2 kPa, and a liquid crystal display (LCD) screen, and (B) the Sandway^® model SW-512C (Sandway Technology Guangdong Co, Ltd., Humen Town, Dongguan, Guangdong, China) with a range of 0–103 kPa, accuracy 2 kPa, and an LCD screen. Both instruments present their measurement results in kPa units. We used the original IOPI^® connector and bulb to reduce any measurement bias.

Our first series of measurements were taken by applying pressure in the bulb midline with a Digital Calliper Parkside^® (Walter Werkzeuge Salzburg Co., Anif, Austria), which presents a measurement range of 0–150 mm, accuracy of 0.01 mm, and an LCD screen. We then took a new series of measurements, applying a weight plate (different weights) over the bulb as the only site of contact (Figure 1). Measurements obtained are presented in

Table 1. In-lab measurements.

IOPI vs. Sandway Measurements
Model: Digital Calliper
		Measurement (kPa)
Sample	Pressure Applied in mm	IOPI	Sandway
1	9	5	5.33
2	8	9	9.12
3	7	14	14.15
4	6	21	21.09
5	5	29	29.63
6	4	39	39.85
7	3	52	52.58
8	2	66	66.95
Model: Weight Plate
		Measurement (kPa)
Sample	Plate	IOPI	Sandway
1	Screw	8	8.05
2	1kg	25	25.6
3	2kg	41	41.53
4	3kg	58	58.24
5	4kg	64	64.63

.

2.2. Observational Study

In the second part of this research, we designed an observational study including 60 patients with OSA, with the following criteria. Inclusion comprised participants aged 18 to 75 years old, with a moderate-to-severe OSA diagnosis and no other treatment for OSA. Exclusion comprised participants with neurological or cognitive impairment, alcoholism, craniofacial malformations, neoplastic comorbidity, or having been treated for OSA.

We obtained the approval of the Malaga Ethical Committee (Protocol Registration Code AWGAP-2024-2, approval date 27 March 2025) and registered it in the UK’s Clinical Study Registry (ISRCTN49446863. Registration date 6 May 2025).

We conducted a clinical evaluation of participants following the previously published Tongue+ Protocol [10], Orofacial Myofunctional Evaluation Protocol with Scores (OMES) [11], and orofacial strength evaluation using the IOPI^®, the Sandway^®, and the TDS as described in its validation study [7].

To assess orofacial strength with both the IOPI^® and the Sandway^®, three consecutive measurements were obtained for the anterior tongue and the lips, with 30 s of rest between each trial. For the tongue assessment, participants were instructed to press their tongue as firmly as possible against the bulb, which was positioned with its proximal edge on the incisive papilla, and extending posteriorly along the hard palate, a maneuver that reflects anterior lingual pressure primarily related to the genioglossus function. For the buccinator assessment, the balloon was placed between the gingival mucosa and the inner cheek, and participants were asked to compress it, with the resulting pressure value considered an indicator of buccinator strength.

To determine whether the patient presents with a normal or hypotonic muscle endotype, we used published and validated reference values that are commonly applied in clinical practice [6,7].

In daily clinical practice, three measurements are typically obtained, with the highest value taken as the reference. However, given the design of our study and in order to minimize measurement bias, we chose to average the three measurements, thus reducing variability. This strategy not only mitigates the potential influence of fatigue or muscular exhaustion resulting from consecutive trials in the same patient, but also facilitates the statistical analysis, which required the comparison of one measure against one measure.

For the TDS we took three measures of the anterior tongue and calculated the mean as the definitive value.

For each participant, all measurements were performed by the same investigator during a single clinical consultation, following the order IOPI^®—Sandway^®—TDS, to ensure standardized conditions for statistical analysis.

2.3. Statistical Analysis

Reliability of the Sandway^® was calculated by using the intraclass correlation coefficient (ICC), which quantifies the degree of consistency and agreement between repeated measurements performed under similar conditions, and which is expressed as a value between 0 and 1, with values closer to 1 indicating a stronger reliability. We applied the following reliability interpretations: <0.5, Poor, 0.5–0.75, Moderate, 0.75–0.9, Good, ≥0.9, and Excellent [12,13,14].

Bland–Altman analysis was performed to assess the agreement between IOPI^® and Sandway^® allowing for the visualization and quantification of agreement between two different measurement techniques by plotting the differences against the mean of the paired measurements. The mean bias and the 95% limits of agreement (mean difference ± 1.96 × standard deviation of the differences) were calculated.

The Pearson coefficient was calculated to assess correlation between the Sandway^® and TDS measurements. It was not feasible to perform a Bland–Altman plot to evaluate the agreement between the Sandway^® and the TDS devices, as they operate based on different physical principles and express their measurements in distinct units (kPa and g/cm³, respectively). This methodological limitation precluded a direct assessment of their interchangeability.

3. Results

A total of 60 adult patients were enrolled in the present study. The demographic and clinical characteristics of the sample are summarized in

Table 2. Population characteristics.

Age
Mean	46.05
SD	16.67
Sex
Male	35 (58.33%)
Female	25 (41.67%)
BMI	n (%)
Normal	15 (25%)
Overweight	17 (28.34%)
Obesity Type 1	18 (30%)
Obesity Type 2	8 (13.33%)
Obesity Type 3	2 (3.33%)
AHI	n (%)
Moderate	35 (58.33%)
Severe	25 (41.67%)

, including variables such as age, sex distribution, body mass index (BMI), and Apnea Hipopnea Index (AHI). The study population consisted of both normotonic and hypotonic individuals, with a predominant male subgroup and moderate AHI. The complete set of individual measurement values—obtained from both the reference IOPI^®, the alternative Sandway^®, and the Tongue Digital Spoon—is provided in the Supplementary Materials, ensuring transparency and reproducibility of the analysis. All participants completed the measurement protocol without adverse events or protocol deviations, and data collection was completed as planned for each subject.

3.1. Sandway^® Reliability

The analysis of test–retest reliability for the Sandway^® device yielded an Intraclass Correlation Coefficient (ICC 3,1) of 0.978, with a 95% confidence interval ranging from 0.967 to 0.986, confirming the stability and reproducibility of the instrument under standardized conditions (

Table 3. Intraclass correlation for Sandway^®.

Sandway® Intraclass Correlation 3,1
Type	Point Estimate	Lower 95% CI	Upper 95% CI
ICC 3,1	0.978	0.967	0.986

). According to commonly accepted interpretation thresholds, this level of agreement reflects excellent reliability.

3.2. Level of Agreement Between Sandway^® and IOPI^®

The Bland–Altman analysis was conducted to assess the level of agreement between the Sandway^® and the IOPI^® devices in the measurement of tongue strength. The mean difference (bias) between the two instruments was −0.034 kPa, with a 95% confidence interval ranging from −0.253 to 0.186, indicating minimal systematic bias. The limits of agreement, calculated as the mean difference ± 1.96 standard deviations, extended from −1.698 kPa (95% CI: −2.078 to −1.318) to 1.631 kPa (95% CI: 1.251 to 2.011) (

Table 4. Bland–Altman analysis and plot (Sandway^®/IOPI^®)—tongue.

Sandway®—IOPI® Tongue
Bias and Limits	Point Value	Lower 95% CI	Upper 95% CI
Mean difference + 1.96 SD	1.631	1.251	2.011
Mean difference	−0.034	−0.253	0.186
Mean difference − 1.96 SD	−1.698	−2.078	−1.318

and Figure 2). Importantly, no trend toward proportional bias was observed in the Bland–Altman plot, suggesting that the level of agreement remained consistent across the spectrum of measured values. Overall, these findings support a strong level of agreement between the Sandway^® and the IOPI^® for tongue pressure measurement.

A Bland–Altman analysis was also performed to evaluate the agreement between the Sandway^® and the IOPI^® devices in the assessment of lip strength. The mean difference (bias) between the two measurements was −0.072 kPa, with a 95% confidence interval ranging from −0.242 to 0.098, indicating a negligible systematic difference between the devices. The limits of agreement (mean difference ± 1.96 SD) ranged from −1.656 kPa (95% CI: −1.067 to −1.067) to 1.217 kPa (95% CI: 0.923 to 1.512). As with the tongue strength analysis, the distribution of differences appeared symmetrical, and no evidence of proportional bias was detected in the plot (

Table 5. Bland–Altman analysis and plot (Sandway^®/IOPI^®)—lips.

Sandway®—IOPI® Lips
Bias and Limits	Point Value	Lower 95% CI	Upper 95% CI
Mean difference + 1.96 SD	1.217	0.923	1.512
Mean difference	−0.072	−0.242	0.098
Mean difference − 1.96 SD	–	−1.656	−1.067

and Figure 3). These results suggest a strong level of agreement between the two devices in measuring lip strength.

3.3. Correlation Sandway^®—Tongue Digital Spoon

A Pearson correlation analysis was performed, resulting in a Pearson’s r value of 0.863 and a 95% confidence interval ranging from 0.780 to 0.916, indicating a high degree of association between the two measurement tools. Prior to the analysis, the assumption of normality was tested using the Shapiro–Wilk test, which yielded a value of 0.962, supporting the validity of the correlation approach (

Table 6. Correlation Sandway^® and Tongue Digital Spoon analysis.

Pearson’s Correlation Analysis - Sandway® and Tongue Digital Spoon
	Pearson’s r	p Value	Lower 95% CI	Upper 95% CI
Sandway® - TDS	0.863	<0.001	0.780	0.916
Shapiro - Wilk Test for Multivariate Normality		0.962	p = 0.024

and Figure 4). The results revealed a strong and statistically significant positive correlation between both devices.

4. Discussion

The Sandway^® manometer represents a cost-effective alternative that demonstrates excellent reliability and accuracy compared to the gold standard IOPI^® device.

Abnormal oro-pharyngeal development and neuromuscular dysfunction contribute to reduced muscle tone, impaired oral functions, and airway collapsibility during sleep in OSA [15,16,17,18,19,20]. Therefore, OSA patients require a thorough myofunctional assessment, which includes the objective measurement of orofacial muscle strength.

Traditionally, this evaluation has been carried out using the Iowa Oral Performance Instrument (IOPI^®), a device widely recognized for its clinical utility and whose reference values have been well-established in both healthy and pathological populations [21,22]. However, despite its proven validity and reliability, the high economic cost of the IOPI^® limits its accessibility in routine clinical practice, especially in resource-limited settings or in multidisciplinary teams where multiple assessments are required. The retail price available online for the IOPI^® Trainer System is EUR 1771.20 (excluding taxes) as available on the official distributor website https://logopedicum.com/producto/monitor-iopi-trainer-system/ (accessed on 23 September 2025). In contrast, the Sandway^® manometer is available on Amazon.es, https://www.amazon.es/Manómetro-diferencial-digital-medició-103-42KPA/dp/B08Y56SKSQ (accessed on 23 September 2025) for EUR 55.81 plus shipping. The prices were updated as of 23 September 2025.

As a result, the integration of orofacial strength testing into standard diagnostic workflows remains restricted, potentially delaying accurate phenotyping and targeted intervention in hypotonic patients. Therefore, there is a growing demand for the development and validation of cost-effective alternatives that allow for widespread implementation in clinical and research contexts. Such devices would broaden access to functional assessment, monitoring of therapeutic progress, and personalization of myofunctional therapy protocols.

In 2024, Curtis et al. [9] published a pivotal study validating a low-cost digital manometer designed to assess both orofacial and respiratory muscle strength. This research laid the foundation for the present study by demonstrating the feasibility of employing affordable tools for functional assessment in research settings. Conducted in a controlled laboratory environment, their study compared the performance of the low-cost manometer against two widely accepted clinical reference devices: the IOPI^® for evaluating orofacial muscle strength and the MicroRPM^® for respiratory pressure measurements. Their results revealed a substantial level of agreement between the low-cost device and the established standards and the IOPI (ρc = 0.988) and the MicroRPM (ρc = 0.990). These findings underscored the potential of such alternative devices to deliver accurate and reliable measurements at a fraction of the cost, thereby supporting their incorporation into routine functional assessments.

In our study, we developed a hybrid measurement system combining components of the IOPI^® and the Sandway^® devices, with the aim of minimizing potential measurement bias. Specifically, we utilized the original IOPI^® connector and bulb—elements with demonstrated consistency and standardization in clinical assessment—while integrating them with the Sandway^® pressure measurement unit. By modifying only the core sensor while maintaining the physical interface for the patient, we ensured that any observed differences in performance could be attributed primarily to the measurement system itself, rather than to variability in bulb mechanics or patient interaction. This methodological approach enhanced the internal validity of our validation process, allowing for a more accurate comparison between instruments that share similar mechanical inputs but differ in sensing technology.

Following promising results obtained during the initial in-laboratory testing phase—where the device demonstrated consistent accuracy—we proceeded to design an observational study aimed at evaluating its performance in a clinical setting. The goal was to assess the device’s reliability under conditions that mirror routine clinical use, thereby determining its practical applicability. The findings of our clinical study are consistent with those previously reported by Curtis et al. [9], as well as with our own laboratory data, and further support the robustness of the device by demonstrating excellent test–retest reliability. In this context, we employed the Intraclass Correlation Coefficient (ICC), a statistical measure used to evaluate the reliability of a measurement instrument [12,13,14]. The high ICC values observed in our study underscore the device’s reproducibility and its potential for consistent use across repeated clinical assessments, which is crucial for both diagnosis and longitudinal monitoring of orofacial muscle function.

Once measurement consistency has been established, the next critical step in validating any new instrument is to assess its level of agreement with the current gold standard. The Bland–Altman analysis [23] revealed that the differences between the Sandway^® and the reference IOPI^® were symmetrically distributed around the bias line, with the majority of points falling within the 95% limits of agreement. Importantly, no significant trend toward proportional bias was observed, indicating that discrepancies between devices did not increase or decrease systematically with the magnitude of the measurement. These findings provide strong evidence of excellent agreement between both devices and suggest that, within the range of clinical values tested, they may be used interchangeably in the assessment of orofacial muscle strength.

On the other hand, a Pearson correlation analysis was conducted to explore the linear relationship between the Sandway^® and the TDS, revealing an excellent and statistically significant correlation. While correlation does not imply equivalence or perfect agreement, the high Pearson coefficient provides evidence that both devices track muscular performance in a consistent manner. This strong association supports the convergent validity of the Sandway^®, suggesting that it is capable of capturing relevant variations in orofacial muscle strength. Furthermore, the Sandway^® successfully discriminated between normotonic and hypotonic subjects, reinforcing its clinical utility in the context of orofacial myofunctional evaluation in patients with sleep-disordered breathing.

A comprehensive myofunctional evaluation is essential for accurately identifying orofacial dysfunctions. This type of assessment goes beyond anatomical observations, emphasizing the functional performance of the muscles and soft tissues involved in upper airway patency [22,24,25,26]. Functional deficits in these areas may contribute to airway collapsibility. Therefore, a detailed functional profile not only aids in diagnosis but also provides a baseline that enables clinicians to personalize therapeutic interventions.

One such approach is myofunctional therapy (MFT), that employs targeted exercises to retrain and optimize the function of oropharyngeal structures [14]. MFT aims not only to enhance the strength and coordination of orofacial muscles, but also to restore functional patterns such as appropriate nasal breathing, correct tongue posture, and effective swallowing mechanics [27,28,29,30]. These elements are essential for maintaining upper airway patency, particularly during sleep, where muscle tone naturally decreases. By improving muscle responsiveness, endurance, proprioception, and neuromuscular control, MFT fosters more efficient and sustained orofacial function. Consequently, this contributes to the stabilization of the upper airway, and reduces the propensity for collapse in patients with OSA [31,32].

As an extended application of our findings, it is important to highlight that the IOPI^® has been extensively employed in both the assessment and rehabilitation of patients with dysphagia [33]. Reduced tongue strength is a frequent characteristic in individuals with oropharyngeal dysphagia and has been closely linked to impaired swallowing efficiency, delayed oral transit time, and an elevated risk of aspiration. A substantial body of evidence supports the use of tongue-strengthening exercises guided by IOPI^® feedback, showing significant improvements in swallowing function—particularly in populations with neurogenic dysphagia, such as individuals recovering from stroke or those diagnosed with Parkinson’s disease [34,35]. Moreover, the IOPI^® has demonstrated high utility in tracking patient progress over time, enabling clinicians to personalize treatment plans and monitor therapeutic outcomes [36].

As part of the limitations of our study, we were unable to obtain detailed information regarding the Sandway^®, as well as the specific software used by the IOPI^®, to perform a thorough technical analysis. Nevertheless, for practical purposes, the available information on measurement capacity, units, and related specifications was considered sufficient for the aims of our study. Another limitation of the study is the potential fatigue experienced by patients during successive measurements. To mitigate this bias, we applied the same order of device testing for all participants. Nevertheless, based on the results obtained, we interpret that no significant muscle fatigue occurred in this series of patients. We also acknowledge as a limitation of this study that we averaged the measurements to reduce variability, which differs from the standard practice in both clinical and research settings where the maximum value is typically used. Our approach may hinder direct comparison with results from other studies. Nevertheless, the aim of our study was to compare measurements across devices rather than between observers. In addition, we did not assess the outcomes of any intervention; therefore, we considered comparison with other results unnecessary.

Although our intention was to minimize measurement bias by using the original IOPI^® connector and disposable bulbs, we acknowledge that a logical next step would be the design of a cost-effective, compatible connector and bulb specifically for the Sandway^® or any similar device. This would allow for greater autonomy and broader implementation, without the need to rely exclusively on proprietary consumables. However, the development of such an alternative would necessitate a new validation process, including comparative studies against the gold standard, to ensure that accuracy and reliability are not compromised.

5. Conclusions

Myofunctional assessment is essential in patients with OSA to achieve precise endotyping and to plan the therapeutic strategy. The economical Sandway^® manometer demonstrates excellent reliability and accuracy, thereby enhancing accessibility and supporting its potential for widespread implementation in clinical practice.