A novel method of measuring passive quasi-stiffness in the first metatarsophalangeal joint

The first metatarsophalangeal joint (MTPJ) is the articulating joint between the first metatarsal and the proximal phalanx of the big toe. It is classified as a single synovial joint that allows motion in the sagittal and transverse planes, with sagittal plane dorsiflexion being the joint’s primary movement in gait. Approximately 65 to 75 degrees of dorsiflexion is considered necessary to enable efficient forward transfer of bodyweight during the propulsive phase of gait [1, 2]. Restricted first MTPJ range of motion can alter foot function, leading to first MTPJ pain and the development of secondary conditions such as plantar calluses and inefficient gait [1‐4]. On the opposite end of the mobility spectrum, a hypermobile or unstable first MTPJ may be associated with conditions such as hallux valgus (a forefoot deformity commonly known as “bunion”) and metatarsalgia (pain under the ball of foot) [5‐7]. Clinically, first MTPJ mobility is assessed in cases where there are foot propulsion problems (e.g., hallux limitus). Other uses of measuring MTPJ range of motion include screening and identification of people who may be at high risk of diabetic foot ulceration [8] and assessment of surgery outcome in patients treated for hallux valgus [9].

There are no standardised methods for measuring first MTPJ mobility. A common practice is to subjectively “feel” how easily the first MTPJ moves through its range of motion [10, 11]. The end-of-range quality is often described using various adjectives such as “bony block” or “normal” by a clinician. Hypermobile joints have been documented in literature as a joint with “excessive excursion with soft end-point” [12]. This subjective assessment method is descriptive and not quantifiable. First MTPJ range of motion is also sometimes visually estimated by a clinician or measured using goniometric devices. Unfortunately, neither visual estimation nor goniometric measurements are sufficiently reliable to be deemed clinically acceptable regardless of the experience of the testers [13]. Research studies have reported wide variations in the range of motion for the first MTPJ—dorsiflexion: 65 to 110 degrees; plantarflexion: 23 to 45 degrees [2, 14‐16]. One reason for the discrepancy in the range of motion is the different “zero” or starting positions for measurement of joint motion. A second factor is that the first metatarsal shaft naturally plantarflexes when the hallux dorsiflexes, shifting the reference point and hence causing large variations in the measurements. Thirdly, the amount of force applied to displace a joint to maximum range is tester-dependent and thus range of motion is not entirely objective [17]. It is possible that one tester uses slightly more force than another tester, resulting in greater range of motion.

Quasi-stiffness, which is the resistance of a joint to an external force, may be a better alternative to assess the first MTPJ mobility. Quasi-stiffness is often determined experimentally as the derivative of the torque-angle relationship [18, 19]. Although quasi-stiffness and stiffness are conceptually different by definition [18], they are mechanically equivalent when the system is behaving with passive dynamics [19]. Previous studies have reported methods in measuring the passive stiffness of other foot joints including the first ray [12, 20, 21] and the ankle [22]. There are, however, no published procedures for the measurement of the first MTPJ quasi-stiffness.

Quantifying first MTPJ quasi-stiffness can be useful in clinical assessment as well as informing orthotic prescription. With quasi-stiffness measurements, joint mobility can be assessed in an objective manner instead of relying on subjective descriptions such as “excessive excursion with soft end-point” [12]. For example, when evaluating a patient with hallux valgus, it is recommended that the first MTPJ be assessed for quantity and quality of motion [23]. Quasi-stiffness measurement of the first MTPJ can accurately reflect how stiff or hypermobile the joint is. Currently, there are methods to assess the extent of hallux valgus deformity (e.g. Manchester scale [24]) but the biomechanical risk factors associated with the development of hallux valgus is not well studied [25]. Quantifying the quasi-stiffness of the first MTPJ may provide useful information on how susceptible a joint is to developing hallux valgus, and hence contributing to screening and early intervention. In the treatment of hallux rigidus and hypermobile first MTPJ, a morton’s extension splint of varying stiffness (rigid or semi-rigid) is the acceptable orthotic modification to ‘splint’ the first MTPJ so as to restrict motion. However, the decision as to whether or not to restrict motion and by how much (choice of rigid or semi rigid) depends largely on the clinician’s experience and subjective assessment of the situation. In orthotics prescription, the clinician also makes a decision on orthotic shell stiffness (e.g. very stiff carbon fiber for more control or thin polypropylene for less stiffness and control). Such prescription is often based on trial-and-error, subjected to the clinician’s assessment. It is common that orthotics prescribed require further adjustments, which may signify that initial assessments or prescriptions may not have been accurate [26, 27]. Therefore, the underpinning motivation of this study was to develop a method for quantifying first MTPJ quasi-stiffness which may, in a long term, lead to better treatment and more accurate prescription of orthotic devices.

This study aimed to present a novel method incorporating joint range of motion and force applied to displace the first MTPJ, in order to measure passive quasi-stiffness of the first MTPJ. The intra-rater and inter-rater reliability of the proposed method were also assessed.

Participants were assessed for the first MTPJ quasi-stiffness in their dominant foot on two occasions (Sessions 1 and 2), 7 days apart. In Session 1, MTPJ quasi-stiffness was assessed by an experienced podiatrist (MH) who has a Bachelor of Podiatry (Australia) and 6.5 years of clinical experience. In Session 2, the measurements were taken by the same experienced podiatrist as well as an inexperienced tester (YC) who had no prior knowledge of podiatry and had only underwent two training sessions pertaining to measurement procedures used in this study. The measurement orders of experienced and inexperienced testers were randomised.

First MTPJ mobility was assessed with the participants in a non-weight-bearing position. Participants were asked to lie on the examination table in supine position, with a soft block placed just proximal to the Achilles area of the dominant leg. The foot was held at the ankle in its neutral positon by the tester’s non-working hand. The full range of motion of the first MTPJ was initially identified by the tester by moving the hallux into maximal dorsiflexion and back to the neutral starting positions. A line representing the moment arm was marked from the tuberosity of the first metatarsal head to just beneath the tuberosity of first distal phalange in the medial aspect of the foot (Fig. 1a). The moment arm was measured in millimetres (mm). To minimise parallax error, a 2 MP (1600 × 1200 pixels) resolution camera (Logitech Web Pro 9000, Logitech, CA, USA; Carl Zeiss Lens Tessar 2.0/3.7) was set up perpendicular to the side of the foot. Hence it can be assumed that the dorsiflexion movement of the first MTPJ occurs in a plane perpendicular to the optical axis of the camera. Measurement of force applied to move the first toe into dorsiflexion was done using the Finger TPS tactile pressure sensing system (Pressure Profile System Inc., Los Angeles, CA, US). With Finger TPS pressure sensor fitted on the thumb of the tester’s working hand (Fig. 1b), a perpendicular force was applied in the direction of dorsiflexion, under the “x” marking of the distal end of the first proximal phalanx, producing rotation about the first MTPJ (Fig. 2). The camera recorded the angular displacement of the first MTPJ in synchronization with the Finger TPS system at 25 Hz. Dartfish video analysis software (Dartfish Ltd., Fribourg, Switzerland) was later used to determine the joint angular displacement (joint angle). In order to check the accuracy of the angle measurements, a goniometer displaying a series of angles (5°, 15°, 25°, 35°, 45°, 55°, order randomised) was videotaped. These angles were manually digitised twice in Dartfish by the inexperienced tester (YC) who was blinded from the goniometric readings. The root mean squared (RMS) difference was 2 degrees across all angles when compared to actual goniometric angles.

The process of measuring first MTPJ range of motion was as follows: a) The first MTPJ was moved into maximal dorsiflexion passively by the tester; this data point was excluded in the analysis because joint quasi-stiffness close to the end range cannot be assumed linear [22]; b) The force applied to the first toe was slowly released, allowing the toe to move from maximum dorsiflexion back towards the starting neutral position; c) The tester paused briefly for about 2 seconds at three intermediate positions to allow static measurement of torque and angular displacement [(T₁,θ₁), (T₂,θ₂), (T₃,θ₃)] before returning the big toe to its neutral position; and d). This process was repeated three times providing a total of nine data points [(T₁,θ₁), … (T₉,θ₉)] to plot a torque-angular displacement graph (Fig. 3).

The quasi-stiffness (k) of the first MTPJ joint can be calculated based on the following equation:where T = Torque = Force (in N) x Moment Arm (in mm), and θ = angular displacement (in degrees). By plotting a torque-angular displacement graph, the MTPJ quasi-stiffness was determined as the slope of the plot (Fig. 3).

Statistical analysis was performed using IBM SPSS Statistics 21 (Armonk, NY) and Microsoft Excel. The first MTPJ quasi-stiffness was the only dependent variable. Descriptive statistics were used to summarise the group data in means and standard deviations. Repeated measurements which were performed twice by the experienced tester were used to assess between-day intra-rater reliability using the Bland and Altman plot and intra-class correlation coefficient [ICC (3,1)]. The ICCs were interpreted as slight (<.20), fair (.21–.40), moderate (.41–.60), substantial (.61–.80), and almost perfect (> .80) [28]. The standard error of measurement (SEM) was also calculated to provide an estimate of the amount of error associated with the measurement. Similarly, inter-rater reliability between the experienced and inexperienced testers was assessed using Bland Altman plots, ICC and SEM. Due to technical error in video recording, we were unable to determine the angular displacement data of three participants assessed by the inexperienced tester. Hence, inter-rater reliability analyses were performed using ten participants’ data.

First MTPJ dorsiflexion angles (θ₁, …,θ₉) used to plot the torque-angular displacement graphs ranged between 3.2 to 47 degrees (median = 24 degrees). Table 1 illustrates the descriptive and reliability statistics results. Overall, there was a large variation in the measured MTPJ quasi-stiffness values, ranging from 0.66 to 53.4 Nmm/degrees. The between-day intra-rater reliability for the experienced tester was moderate (ICC = .568). Although the mean difference between Session 1 and Session 2 was small as a group (0.68 Nmm/degrees), the measurement error was rather large with a SEM of 7.7 Nmm/degrees. Visual inspection of the Bland Altman plot showed random scatter of the points between the limits of agreement, indicating homoscedastic data as well as an absence of systematic bias (Fig. 4a).

When comparing the MTPJ quasi-stiffness measurements between the experienced and inexperienced testers, the inter-rater reliability was poor (Table 1). The poor agreement between the two raters was reflected by the ICC (-.447), large mean difference (7.36 Nmm/ degrees) and large measurement error (SEM = 11.3 Nmm/degrees). Although ICC is commonly interpreted between 0 and 1, ICC as a correlation can be negative when the variance within subjects is greater than that for the raters [29]. Visual inspection for the Bland Altman plot showed random scatter of the points between the limits of agreement, indicating homoscedastic data as well as an absence of systematic bias (Fig. 4b).

This study reported a novel method to quantify first MTPJ quasi-stiffness in a clinical setting using pressure sensors and video analysis. The between-day intra-rater reliability was moderate for the experienced tester but the inter-rater reliability was poor between the experienced and inexperienced testers.

This study provided preliminary results that first MTPJ passive quasi-stiffness can be quantified from torque and angular displacement measurements using simple equipment in clinical settings. The tester’s experience affects the consistency in joint quasi-stiffness measurements. Future studies can aim at refining the measurement protocol, and to develop instrumentation that can accurately quantify foot joint quasi-stiffness.