In the current study, using the DFIS, we can capture two orthogonally frames at the same time to obtain more detailed spatial information about the tridimensional heel pad. By incorporating the DFIS system and compression force plate, the in vivo viscoelastic properties of the heel pad in healthy adults were measured in the actual gait. We found that age was negatively correlated with the primary thickness of heel pad and peak strain, and was positively correlated with viscous modulus. Additionally, repetitive loading could decrease the primary thickness of heel pad and viscous modulus.
Numerous in-vivo measurement tools, such as spherical indentation system [10, 16], instrumented pendulum [15, 28], ultrasound indentation system [17, 18, 29, 30], tissue ultrasound palpation system [31], and optical coherence tomography-based air-jet indentation system [31], have been developed in the past. These quasi-static methods, however, could not replicate the mechanical condition experienced by the heel during dynamic gait cycle. De Clercq et al. [21] firstly described a novel method using cine-radiography to evaluate the in vivo compressive strain of heel pad during running in 1994. Then, this method was expanded by Gefen et al. [22] in 2001 to include simultaneous measurements of strain rate using digital radiographic fluoroscopy and contact pressure using embedded foot-ground contact pressure display, which allows for characterization of the heel pad during the full cycle of loading and unloading in normal gait. However, as the authors specially pointed out, the one-dimension lateral X-ray projection used for measuring the heel pad thickness inevitably put some limitations on the interpretation of the results, as the true nature of the heel pad deformation is three dimensional.
The average primary/ unloaded heel pad thicknesses reported in previous studies were ranged from 11.5 to 19.1 mm in healthy adults [9, 22, 32, 33]. In addition, the primary heel pad thickness was proven to be related with the gender, age and physiques of subjects [34, 35]. Maemichia et al. [34] observed the changes on heel pad thickness associated with age, and physique in 1126 healthy Japanese, demonstrating that the thickness tends to increase from ages 1-5 (male: 10.5 ± 1.6 mm; female: 9.6 ± 1.4 mm) to 30-44 years (male: 15.8 ± 2.7 mm; female: 13.9 ± 2.1 mm) and decrease from ages 30-44 to 80-96 (male: 14.2 ± 2.7 mm; female: 11.9 ± 4.0 mm) years, and the males had higher thickness than females in the corresponding age groups. What’s more, the thickness of heel pad in males is associated with the body mass and height. In our results, the median primary thicknesses at time zero and following continuous compressive loading were 15.99 and 15.72 mm respectively. It could be speculated that the inconsistency among studies may be due to the biases caused by differences on race, age, BMI, and so on, as small sample size was reported in these researches. Similar with the results in Maemichia et al. [34], we demonstrated a negative correlation between age and primary thickness both at time zero (R = -0.507) and after continuous load (R = -0.607).
The elastic modulus measured in the current study (192.55 and 197.585 kPa at time zero and following continuous loading) was similar with the result in Gefen et al .[22] (175 kPa), which was also derived from in vivo measurement as we did. However, Ledouxa et al .[5] reported a elastic modulus as high as 830 ± 30 kPa for the heel pad, basing on in-vitro compressive test, which is more than four folders of our results. The significant difference between in-vitro and in-vivo mechanical testing has been proven in previous studies, demonstrating a six-time higher stiffness and a three-time lower energy dissipation rate for in vitro testing [13,14,15]. Thus, with the aim of reinstating an actual mechanical condition during gait, the current study presented a more practicable approach to involve the whole body in measurement.
It is of particular importance to evaluate the time-dependent behaviour (i.e., viscous properties) of heel fat pad, as it has been widely recognized as the major origin of the ability of shock absorption at heel strike [36]. What’s more, the modifications on viscous properties may be even more sensitive to pathological conditions, such as diabetes, than other commonly evaluated mechanical properties (such as elasticity) [37]. Using our novel system, in consequence, clinicians could easily obtain the viscous parameter to assist the diagnoses and interventions of pathological states at heel. In the current study, we calculated two significantly different viscous modulus at rest (median: 43.9, range: 21.13 ~ 75.69 kPa۰s) and fatigue (median: 20.37, range: 10.25 ~ 70.535 kPa۰s) statuses, compared to a viscosity constant of 22 kPa۰s in Gefen et al .[22]. The viscous modulus at fatigue status in our results was closed to that of the Gefen et al. [22]. It could be speculated that the measurement condition was similar with the fatigue status following 15 min of sustaining standing or wandering that performed in the current study, as the authors of the previous study have taken their subjects to train on the platform prior to data acquisition. In our study, to test time-zero mechanical properties, the subjects were required to keep their foots on relax condition and free of loading for one hour. Thus, the viscous modulus at rest was much higher than that at fatigue status, demonstrating that the loading history of heel fat pad could obviously impact the viscous property.
This study, nevertheless, has some limitations. As a pilot study, the small sample size, inevitably, would bring in potential risk of selection bias. Then, the strain rate applied to the heel pad has been widely proven to obviously impact the mechanical properties of heel pad [5, 10, 12]. While in the stance phase of gait, it is non-possible to precisely control the strain rate as that performed in in-vitro machine-based loading. To overcome this problem, subjects were trained prior to measurement to ensure an approximate gait velocity of 1.0 m/s.