Abstract
Background
Excessive laxity of the first ray has been implicated in various foot disorders. Accurate assessment is critical to define the most appropriate treatment. However, assessment currently depends largely on clinical examination and lacks objectivity. We evaluated the performance of an automated electromechanical tool developed to measure the relative dorsal mobility of the medial column compared to the lateral rays, and assessed the intra- and inter-examiner reliability of this device.
Methods
Two examiners assessed 30 feet of 17 volunteers. Data were collected 15 times on each foot. The intraclass correlation coefficient (ICC), Bland-Altman (B&A) graphical analysis, and the standard error of measurement (SEM) were calculated.
Results
Inter-rater and intra-rater ICC values (>0.9) were excellent. The Bland-Altman analysis revealed a mean bias of 0.07 mm between examiners.
Conclusion
This novel electromechanical device has demonstrated reliability in measuring first ray laxity, supporting its use in future studies aimed at quantifying first ray hyperlaxity.
Level of evidence
Level II, Prospective Reliability Study.
1
Introduction
First ray hyperlaxity refers to an increased flexibility in the joints of the entire medial column ,,,, . This leads to a biomechanical imbalance between the first ray and the lesser metatarsals, given the critical role of the first ray in overall foot function during the gait cycle , that can become disabling and require surgical solutions . Hallux valgus deformity, metatarsalgia, metatarsal stress fractures, flatfoot deformity and Lisfranc arthritis have been described as a result of first ray hypermobility , . The diagnosis of first ray hyperlaxity is crucial for determining the most appropriate treatment strategy for each patient , . Given the dynamic nature of this condition, mobility should be assessed dynamically by means of a reliable and reproducible scientific approach. Nevertheless, clinical assessment of first ray mobility is still largely based on empirical methods. This may lead to over- or under-treatment, as first ray hyperlaxity is thought to be crucial not only in the pathogenesis of hallux valgus, but also in increasing the risk of recurrence . Scientific work on strategies to limit recurrence after hallux valgus treatment has increased exponentially over the last decade. The Lapidus procedure, including its modified version, is widely regarded as one of the most effective surgical strategies for addressing first ray laxity, reducing recurrence rates, and managing recurrent hallux valgus ,,,, . It is therefore a highly contentious and clinically relevant topic. Considerable resources have been devoted to this area, resulting not only in a significant increase in scientific publications but also in the development of devices aimed at facilitating the Lapidus procedure. Despite these efforts, however, there remains no validated method to scientifically define and quantify first ray hypermobility.
In 1928 Morton introduced a manual examination method to assess first ray stability applying a dorsal force beneath the head of the first metatarsal with one hand while stabilizing the lesser metatarsals with the other. Other manual assessment methods were subsequently published ,, , but were later deemed unreliable due to high variability even among experienced clinicians ,, .
Over the years, various different manual maneuvers or handheld rulers have been presented ,,, , trying to improve the quantification component of manual examination. However, the fundamental lack of accuracy and objectivity regarding the force applied during the measurement remains unaddressed.
Klaue et al., Glasoe et al. and Morgan et al. presented different devices to quantify first ray hyperlaxity ,,,,, . These tools apply a manual or, respectively, an automated fixed force under the first metatarsal head to obtain a dorsal displacement of the first ray in the sagittal plane. The measurement is done keeping the lesser metatarsals immobilized. One of the most common criticisms of the various methods of measurement proposed to date has been the approximation resulting from measuring in the sagittal plane a deformation that is in fact three-dimensional ,, , and involves the cuneiform bones as well . However, the sagittal plane remains the most crucial for maintaining the balance of the foot’s tripod structure, as originally described by Cotton in 1936 . The primary force-weight vectors are transmitted through this plane, directly influencing load distribution and postural balance in the physiological mechanics of gait. During the stance phase, ground reaction forces directly apply an upward load to the forefoot . Ideally, these forces should be evenly distributed between the first metatarsal (M1) and the lesser metatarsals (M2-M5). However, when the medial column is unstable, M1 fails to bear its share of the load, leading to compensatory weight transfer onto the lateral rays. Unlike previous solutions , , a new automated device, named LaxiPed, provides an objective measurement of first ray mobility, while applying the same amount of force to the medial column and the lateral rays. It then measures the relative dorsal mobility between these two segments of the forefoot. This measurement is called the First Ray Relative Mobility (FRRM).
This study seeks to evaluate the inter- and intra-examiner reliability of LaxiPed, an innovative automated tool developed to quantitatively assess the dorsal mobility of the first ray in relation to the lesser rays.
2
Materials & methods
2.1
Subject enrollment
This study included 17 participants (N = 30 feet), 11 men and six women, recruited from the clinic staff pool. The mean age was 39 years. All participants were adults. The inclusion criteria required participants to have no neurological or systemic conditions that could impact the foot and ankle. Exclusion criteria for each foot included a history of recent trauma, previous foot or ankle surgery, or presence of pain. Data collection took place in March 2024. The participation was voluntary, and each participant gave informed consent before the test session. Ethical approval for the study was granted by the local ethics committee (Req-2021–01473).
2.2
Description of LaxiPed
LaxiPed is an automated forefoot arthrometer designed to quantitatively assess forefoot instability. The device measures the relative displacement, in millimeters, between M1 and M2 to M5 as a function of the applied force, ranging from 0 to 100 Newtons. A previous version of the device was presented in the past . This new version, shown in Fig. 1 , features multiple improvements. The force is applied in an automated manner, the patient is seated, and the foot is in a Resting Calcaneal Stance Position (RCSP). These enhancements minimize operator influence on the measurements . Before testing, the patient places their foot on an adjustable platform designed to accommodate different foot sizes. The knee should be flexed at 90 degrees, with the ankle in neutral position. The patient’s leg is secured to the device with an elastic strap, which improves measurement repeatability by further stabilizing the leg, and the device’s measurement module automatically aligns beneath the heads of the M1 and M2-M5 metatarsals. Two optical sensors precisely adjust the two bearings supporting M1 and M2-M5 within the transverse plane. The module then applies an equal force to both bearings while simultaneously measuring their relative displacement. The device is connected to a screen displaying the real-time progression of the measurement. The relative dorsal mobility of the first ray is quantified as the First Ray Relative Mobility (FRRM) value. The FRRM is calculated as the difference between the relative mobility of the first ray at equilibrium, obtained under maximal applied force (50 N), and its position at rest.
The Laxiped device. The dimensions of the device are approximately those of a shoebox. It is connected to a computer platform with two screens that display in real-time the progression of the curve and the data related to the measurements.
The measurements obtained follow a characteristic S-shaped curve, as shown in Fig. 2 , which can be divided into three parts. In the first part of the curve, called the Resting Zone, an initial plateau represents an increasing force that only tensions the soft tissues without causing displacement. The initial position corresponds to Relative Mobility at Force 1 (RMF1). The initial motion varies based on the relative position of the first ray to the lesser rays in the sagittal plane, influenced by the foot structure (higher or lower, whether cavus, rectus, or planus) . This is followed by the Elastic Zone, where increasing force results in increasing displacement. The slope of this section of the curve correlates with the laxity of the first ray. Eventually, the Equilibrium Zone is reached, marked by a second plateau where additional force no longer increases the displacement of the first ray. This corresponds to Relative Mobility at Force 2 (RMF2) and occurs between 60 N and 100 N of applied force. The maximum force delivered, 100 N, is the result of two equal and parallel vectors of 50 N applied symmetrically to the forefoot. This amount of force fully translates the first ray into dorsiflexion, following Glasoe’s recommendations , ( Fig. 3 ). The First Ray Relative dorsal Mobility (FRRM) is then calculated at these two specific points during the process using the equation: FRRM = RMF2 − RMF1. The result represents the maximum range of motion.
The graph illustrates a sigmoid curve divided into three distinct zones. On the left, the segment of the curve where increasing force primarily tensions the soft tissues without causing significant deformation. The central portion corresponds to the deformation phase. The slope of the tangent in this area is an expression of foot laxity. The plateau on the right represents the equilibrium zone, where further increases in applied force no longer result in any additional relative displacement between the two bearings.
Left: The foot in a resting position, corresponding to the initial segment of the sigmoid curve. Right: The device’s two bearings support the lesser metatarsal heads and the first metatarsal head, with a maximal force of 100 N evenly distributed and applied, corresponding to the right part of the curve.
2.3
Assessment protocol
Inter-rater and intra-rater reliability were assessed by two different investigators: an orthopedic surgeon and a resident specializing in foot and ankle surgery, both of whom were randomly assigned as Investigator 1 and 2. Neither investigator had prior experience with the tool. For each foot, three measurement trials were performed. Inter-rater reliability was evaluated using Trials 1 and 2, conducted by Investigators 1 and 2, respectively. Intra-rater reliability (test-retest) was assessed using Trials 1 and 3, both performed by Investigator 1. A five-minute interval was maintained between each trial, and the foot was removed from the device after every set of measurements to reset baseline conditions. During each trial, three initial loading cycles were performed solely to precondition the soft tissues and establish consistent tension. These cycles were excluded from the analysis. Five subsequent measurement cycles were then recorded and used to calculate the variables of interest. Each trial thus comprised eight loading cycles in total, with only the final five used for data analysis. This sequence was repeated for all three trials.
For each measurement set, mean values were computed for the RMF1, RMF2, and FRRM variables. The measurements were carried out over three days.
2.4
Statistics
We used the formula provided by Giraudeau and Mary to determine the sample size, setting a confidence interval (CI) width (w = 0.2), a type I error rate (alpha = 0.05), and five repeated measurements (m = 5). This calculation yielded a required sample size of 27 feet. Descriptive statistics were used to compute the mean, standard deviation (SD), and standard error of measurement (SEM). The intraclass coefficient (ICC) was used to assess inter-rater and intra-rater reliabilities with a single-rater, absolute-consistency and two-way random-effects model, assuming that the examiners represented the broader examiner pool. ICC values were calculated with 95 % confidence intervals and categorized as poor (<0.5), moderate (0.5–0.75), good (0.75–0.9) or excellent (>0.9). Analysis of statistics was performed using SPSS Statistics (version 26, IBM Corp, Armonk, NY). A Bland-Altman analysis was conducted to assess the systematic error between investigators 1 and 2.
3
Results
Of the 19 participants initially recruited for this study, 2 were excluded due to not meeting the inclusion criteria (bilateral previous surgery, pain) and measurements from 2 feet were excluded due to missing data. The remaining participants had a mean age of 39 years (range: 27–57 years; SD: 8.4). In the intra-rater analysis, the mean First Ray Relative dorsal Mobility (FRRM) was 6.64 mm, while in the inter-rater analysis yielded a mean value of 6.70 mm. Both intra- and inter-rater reliability for FRRM demonstrated excellent intraclass correlation coefficient ICC values (≥0.97), indicating high measurement consistency. For the variables RMF1 and RMF2, the intra-rater analysis showed mean values of 4.09 mm and 10.73 mm, respectively, whereas the inter-rater analysis produced mean values of 4.06 mm and 10.76 mm. ICC values ranged from 0.96 to 0.97 for test-retest reliability and from 0.92 to 0.94 for inter-rater reliability. These findings are summarized in Table 1 . The Bland-Altman (B&A) plot for FRRM inter-rater measurements, presented in Fig. 4 , shows a mean bias of 0.074 mm, with limits of agreement spanning from −2.20 mm to 2.05 mm.
Table 1
Results are described as mean values of the measured variables. ICC: intraclass correlation coefficient; SEM: standard error of measurement. The FRRM values reached 0.97 for both the inter-rater and intra-rater measurements. This parameter allows to mitigate the effect of the foot position.
| Intra-rater (test-retest) | Inter-rater | |||||
|---|---|---|---|---|---|---|
| Variables | Mean±SD (mm) | ICC [95 % CI] | SEM | Mean±SD (mm) | ICC [95 % CI] | SEM |
| RMF1 | 4.09 ± 3.67 | 0.97 [0.93, 0.99] | 0.225 | 4.06 ± 3.87 | 0.94 [0.88, 0.97] | 0.327 |
| RMF2 | 10.73 ± 3.58 | 0.96 [0.91, 0.98] | 0.260 | 10.76 ± 3.64 | 0.92 [0.83, 0.96] | 0.368 |
| FRRM | 6.64 ± 3.05 | 0.97 [0.94, 0.99] | 0.191 | 6.70 ± 2.94 | 0.97 [0.93, 0.98] | 0.198 |
Bland & Altman chart illustrating the First Ray Relative Mobility (FRRM) results for inter-rater reliability graphical analysis.
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