# Experimental Analysis of Plantar Fascia Mechanical Properties in Subjects with Foot Pathologies
## 1 Introduction  
The Plantar Fascia (PF) is a connective tissue composed of type I collagen fibers arranged in a longitudinal direction, elastic fibers and an extracellular matrix highly rich in hyaluron [1]. This thin fibrous band connects the plantar heel to the metatarsophalangeal joints [2]. PF is thought to be attached to the Achilles tendon via the superficial aligned trabeculae of the calcaneus, even if the continuity of collagen fibers between the Achilles tendon and the PF through the calcaneus is still debated [3]. PF has a fundamental biomechanical role in supporting the medial longitudinal arch of the foot [4; 5; 6] and is capable of storing strain energy and converting it into propulsive force, behaving as a quasi-elastic tissue [7], but it is in general characterized by relevant viscoelastic behavior [8]. Moreover, it has been recently demonstrated that fascia play an important role in proprioception, compartment syndromes and fibromyalgiagia [2, 5]. The most common pathological condition that the PF can undergo is plantar fascitis [9], which typically affects workers and athletes whose activities have a high impact on the foot, causing inferior heel pain [10, 11]. Another pathology affecting the PF is plantar fibromatosisis, also called Ledderhose disease, which is a benign nodular formation usually in the medial part of the PF causing pain during gait [12, 13]. Such disease is commonly treated by a partial or total apneurootomy, although other options have recently been described [14, 15]. In addition to diseases directly involving the PF, there are various systemic pathologies, primarily diabetes, that affect foot biomechanics in different ways. Diabetic patients are subject to foot problems for multifactorial reasons [16]: Peripheral neuropathy, peripheral vascular disease, delayed bonehealing and altered immune function resulting in a diabetic foot with chronic ulcers and neuropathic arthropathy that overall subverts the normal biomechanics of the foot [17,18]. Recent studies have focused on the importance of understanding the biomechanics of the diabetic foot to define the appropriate treatment and prevent complications for each of its different pathological aspects [19,20]. This investigation must involve also the PF, as one of the elements of the foot region assuming distinctive structural behavior. For example, a selective PF release has been suggested to redistribute the pressure on the sole for healing forefoot chronic ulcers in diabetic patients [21]. Few works in the literature have been carried out on the experimental testing of the mechanical properties of PF [3,8,22]; moreover, the possible effects of structural impairments on the mechanical behavior of fascia are not well understood. Experimental mechanical studies on PF of patients affected by Ledderhose disease are almost absent, even though clinical evidence show that the healthy fascial tissue is progressively replaced by abnormal thick collagen fibers, leading to sclerosis of the entire PF [23], associated with an overall foot stiffening effect. This is generally complemented by an inflammatory reaction of the tissues around the nodules, causing pain and swelling that can lead to impairments in foot biomechanics [24]. Some studies have evaluated the PF properties in the diabetic foot. It is known that diabetes causes an increased thickness of Achilles tendon and PF, more evident in the presence of neuropathy, and it is supposed that this could contribute to an increase in the tensile force maintained throughout the whole gait cycle. The thickness of the PF and the Achilles tendon was measured by means of ultrasound tests highlighting a thickening of both these structures in patients affected by diabetes, in particular in those who were affected also by neuropathy [21]. The thickening of the PF was associated by the authors with a more rigid foot, leading to alterations of the gait cycle and of the plantar pressure distribution. [PERSON] et al.