**3. Results**

The sociodemographic data did not show statistically significant differences (*P* > 0.05) for the sex, age, weight and height between groups but it did show statistically significant differences (*P* < 0.05) for the body mass index (BMI) between groups (Table 1). Regarding Table 2 and Figure 3, ultrasound evaluations of the CFP and PF at the calcaneus thickness showed statistically significant differences (*P* < 0.01) with a decrease observed for the tendinopathy group with respect to the control group. For the PF midfoot and forefoot thickness, no significant differences (*P* > 0.05) were observed between groups.


**Table 1.** Sociodemographic features, pain scores and VISA-A scale of the sample.

Abbreviations: VAS, visual analogue scale. \* Mean ± standard deviation (SD) was applied. \*\* Student´s *t*-test for independent samples was performed. † Median ± interquartile range (IR) was used. ‡ Mann–Whitney *U* test was utilized. ‡‡ Fisher exact test was used.


**Table 2.** Ultrasonography measurements.

Abbreviations: PF, plantar fascia. \* Mean ± standard deviation (SD) (minimum–maximum) was applied. \*\* Student´s *t*-test for independent samples was performed. † Median ± interquartile range (IR) (minimum–maximum) was used. ‡ Mann–Whitney *U* test was utilized.

**Figure 3.** Bar graphs completed with the 95% CI to illustrate the thickness differences of calcaneus fat pad and plantar fascia at the calcaneus insertion, midfoot and forefoot between patients with chronic non-insertional Achilles tendinopathy and controls.

According to the linear regression analysis (Table 3), the prediction model for CFP thickness (*R*<sup>2</sup> = 0.382) was determined by group (presence of Achilles tendinopathy) and sex, and also the prediction model for PF thickness at calcaneus insertion (*R*<sup>2</sup> = 0.323) was determined by group and weight. The rest of the independent variables did not predict these statistically significant differences between the case and control groups.

**Table 3.** Multivariate predictive analysis for CFP and PF thicknesses for patients with Achilles tendinopathy and controls.


Abbreviations: CFP, calcaneus fat pad; PF, plantar fascia. \* Multiplay: Group (control = 0; Tendinopathy = 1); Sex (women = 0; men = 1). † *P*-value < 0.05 for a 95% confidence interval was shown. ‡ *P*-value < 0.001 for a 95% confidence interval was shown.

## **4. Discussion**

USI was considered a valid and reliable imaging method to assess soft tissue structures, architectures and sizes. Considering the strong relationship between the Achilles tendon and PF by their locations and insertions, this study may provide a new approach to the assessment and treatment of individuals with AT. This is the first study where the thickness of the PF and CPF were evaluated with USI in patients with AT, variables of interest due to ankle and foot biomechanics [13].

Foot overpronation was considered a risk factor to the predisposition of AT [31] through disturbances in ankle biomechanics producing an extra mechanical stress on the soft tissue structures, such as the PF. Those findings were related to a thickness decrease in the PF at the calcaneus, a structure that serves as a link between the PF and the Achilles tendon. In addition, Cornwall et al. [32] argued that an excessive foot pronation may increase the foot mobility and the level of stress applied through PF. Many studies have also reported an altered foot arch in patients with disturbances in PF modifies the capability of absorbing ground reaction forces [32,33]. These structural alterations were related to the Achilles tendon capacity to store and release energy during the gait [34]. Our findings showed an altered thickness in the PF at the calcaneus insertion, which could be explained by the changes in the ankle biomechanics, the extrinsic and intrinsic foot muscles observed in patients with AT.

CFP protects the rear-foot from the stress produced during the initial phase of locomotion and the heel-strike actions. CFP thickness was described as a valid and reliable method to quantify and assess the heel fat pad by USI [24]. Several authors argued that CFP has been implicated in foot and plantar disturbances, such as diabetes [35], fractures [36] and plantar heel pain [37]. To our knowledge, this is the first study to evaluate the CPF thickness in patients with AT. The results of our study showed a decrease in CPF thickness in the tendinopathy group, so it could be considered a relevant variable for the diagnosis and management of individuals with AT.

The findings of the present study did not intend to provide an explanation about the etiology of AT. In addition, several studies argued that the etiology of the tendinopathy was caused by multiple factors. The authors try to offer a novel approach to evaluate and quantify soft tissue structures that usually present disturbances in AT by USI. Thus, USI assessment of the reduction in PF in subjects with AT could be useful to carry out a follow-up for the interventions prescribed to treat AT.
