# Reliability and Validity of the Orthotic Deformation Test
## 4 Discussion  
An ODT is performed when a clinician intends to evaluate the degree of stiffness that a polypropylene custom foot orthosis retains after use. This simple test does not require specific equipment; it can accordingly be performed when a patient is being clinically evaluated. Our findings showed that this test is valid for interpreting the degree of deformation suffered by a polypropylene custom foot orthosis. However, our data suggest that clinical experience is necessary to properly interpret an ODT. The medial longitudinal arch is responsible for absorbing and distributing the weight-bearing loading that the foot supports during walking and running. In the non-weight-bearing position, there are no GRFs on the foot. However, during supporting activities GRFs exert pressure beneath both the rearfoot and the forefoot. Both the rearfoot and the forefoot tend to approximate closer together over the foot dorsum. The degree of dorsiflexion movement or arch collapse depends on the stiffness and flexibility-related characteristics of the joints located between the forefoot and the rearfoot; the joints compounding the medial column of the foot play particularly important roles [35]. Higher levels of medial arch collapse translate into both higher levels of tensile stress in plantar structures such as the plantar aponeurosis, plantar ligaments, and extrinsic foot tendons and higher levels of dorsal compression stress of medial column joints [36]. Furthermore, the windlass mechanism begins when GRFs acting over the hallux generate dorsiflexion movement between the first metatarsal and hallux (i.e., the metatarso-phalangeal joint) during the propulsive phase of the gait cycle [37]. Rotation of the hallux over the first metatarsal generates plantar aponeurosis tension due to its insertion in the proximal phalanx of the hallux. That process establishes the effective windlass mechanism [38]. Excessive arch foot collapse results in ineffective dorsiflexion of the first metatarso-phalangeal joint and delayed onset of the windlass mechanism [39]. That condition has been associated with painful osseous and soft tissue pathologies such as plantar fascitis [38; 40; 41]. It has been demonstrated that restarting the windlass mechanism via  
Figure 6: Differences between positive and negative tests for the researcher with a high level of experience. Panel (**A**) (**top**): The compression device applied 150 N of vertical force. Panel (**B**) (**middle**): middle side. The medial longitudinal arch was deformed vertically 3 mm. Panel (**C**) (**bottom**): The medial longitudinal arch was deformed by 10% of the maximum height of the medial longitudinal arch. plantar approaches ameliorates the pain of patients with persistent plantar fasciiitis [42]. Therefore, appropriate stiffness of custom foot orthoses can help prevent excessive foot collapse and the delayed-onset windlass mechanism, thereby contributing to reducing the pain associated with plantar fascitis. The results of this study suggest that the ODT is useful for evaluating the effectiveness of the stiffness of custom polypropylene foot orthoses. Findings revealed that the application of a vertical force of to 150 N over the custom foot orthosis can deform an orthosis with positive tests by more than 5 mm compared with orthoses associated with a negative test, which exhibited reduced arch collapse (Figure 6A). This force is similar to what a clinician can exert with his thumb [32] when performing the ODT. Although, the medial longitudinal arch supports 179 N during walking and up to 358 N during running [43], a vertical force applied by a thumb appears sufficient to evaluate the degree of orthotic deformation. The graphs of displacement versus force (Figure 5) revealed that the greatest disparity in displacement between orthoses, characterised by positive and negative tests, occurred during the initial 50 N; the remainder of the curve exhibited a constant difference in deformation.