3D printing individualized heel cup for improving the self-reported pain of plantar fasciitis

Plantar heel pain (PHP), also known as plantar fasciitis, jogger’s heel, tennis heel, and policeman’s heel [1], is a painful syndrome that occurs around the calcaneus. Although the term itself might be vague, the chronic pain that originates at the plantar fascia following weight bearing is the unfortunate reality of many. The pain, which could lead to a reduction in sporting and everyday activities, has been described as burning, aching, and occasionally lancinating [2]. It is the most common foot musculoskeletal disorder encountered by health professionals, that has a negative impact on health-related quality of life in those who experienced it [3, 4]. This condition occurs in approximately 2 million Americans per year, while the prevalence rates in UK and Australia are 4.6 and 17.4%, respectively [5‐7]. Besides athletic and non-athletic populations, it has also observed in the sedentary population [8‐10]. It isn’t gender-specific, and while it generally affects the mid-aged; it has also been observed in children and elderly [9, 11]. Despite the high prevalence of PHP, its real etiology is poorly understood. Previous studies have suggested that it is probably multifactorial, and related with plantar fasciitis, heel spur, calcaneal apophysitis, bursitis, posterior calcaneal exostosis, local bruises, Achilles tendonitis, and trapped nerve [12, 13]. The repetitive micro-trauma associated with persistent load-bearing caused by excessive stress at the calcaneal attachment of the plantar fascia has shown to be a major risk factor for heel pain [14].

The initial treatments for PHP include padding, strapping of the foot, orthotic insoles, oral anti-inflammatories, and a local corticosteroid injection [15‐19]. If the therapy program is effective, patients usually have a positive response within 6 weeks of initiation treatment. The second line treatment includes orthotic devices, night splints, repeat corticosteroid or botulinum toxin injections, a course of physical therapy, and cast immobilization during activity [15, 18, 20‐25]. For most patients, clinical response after this line of treatment usually occurs within 2–3 months [26‐28]. If the symptoms are still present within 1 year after the first and/or second line treatment, the patients should face the final line of treatment, which usually includes plantar fasciotomy, heel spur resection, nerve release, and other kinds of surgeries [12, 29]. However, simple removal of the heel spur does not seem to promote the successful outcome in the surgical treatment in most cases, i.e. the combination of plantar fasciotomy and nerve release appear to be necessary [30, 31]. In sum, the three lines of treatment are gradually complementing, where the last two lines of treatment, and especially the surgical treatment, are avoidable if the first line of treatment is successful. The decision to include analgesia or anti-inflammatory medication in line 1 and line 2 treatment is considered only where its use has been shown to detrimental or to have an off-label effect [32]. The key factor for successful rehabilitation in the early period is altering the kinetic chain of the lower limb [33, 34].

Heel cup, or foot orthosis, is the most common rehabilitation method used for the initiation treatment. The traditional fabrication process of orthosis includes plastics injection molding and plaster molding. The fabricated products are usually non-individualized, heavy, plump and uncomfortable to wear [35], and compared with individualized heel cups, the pre-fabricated products are not very effective in some cases [36]. In order to overcome these shortcomings of traditional orthosis, additive manufacturing (AM) has been introduced to the field. The new technology, also known as rapid prototyping (RP) or 3D printing, has unique advantages in medical application. It allows the fabrication of complex 3D physical objects from large sizes to microstructures in a wide range of materials, like plastics, metals, nylons, and biocompatible ones [37‐42]. Furthermore, it is possible to acquire the digital models of the human body by combining the 3D scanning technology and rebuilding or repairing surface anatomy by 3D printing [43]. Based on this premise, we designed and tested the therapeutic effect of individualized heel cup fabricated using 3D printing. The digital models of heel cup were designed to closely fit the surface of human feet and were tested on 16 participants with plantar heel pain who were enrolled in the study. Additionally, the functionary mechanism of 3D printing heel cup was explored by finite element (FE) simulation.

The experiment was divided into three parts: heel cup design and fabrication, clinical evaluation and FE model simulation. All methods in this study were carried out in accordance with relevant guidelines and regulations. All experimental protocols in this study were approved by the committee of Drum Tower Hospital affiliated to Medical School of Nanjing University.

The results of the present study demonstrated that the individualized heel cup could alter the dynamics of the heel, and could significantly improve the self-reported pain caused by the plantar heel pain. Almost all of the participants reported a definite alleviation of heel pain, while FE simulation revealed the causation of this phenomenon. The biomechanical abnormalities and mechanical overload could lead to the excessive tensile strain, responsible for the plantar fasciitics [47]. What’s more, in order to resist arch elongation, the tensile loads of the plantar fascia increase during the stance and the heel rising stage [48]. The plantar fascia is very susceptible to injury under long and repeating overload, which may shorten its effective length and initiate the development of plantar fasciitics. According to the FE simulation, the warping of heel cups obviously exerts the opposite force on the plantar soft tissue and calcaneus bone, which in turn might directly cause the heel pain. Furthermore, the plantar pressure analysis exhibited that the load on mid foot during walking and jogging also decreased, which meant the direct load from plantar fascia had also been reduced. Compared with patients who did not wear a cup, the variety of longitudinal foot arch angle was smaller after wearing the heel cup. The work of fascia to resist collapse of the arch during gait become lighter. Through this way, it could release the tensile stress of the fascia and reduce the bending angle of foot arch.

Besides negatively affecting the fascia through increased tension, the biomechanical factors can also disrupt energy dissipation in the heel, especially in the heel pad, which is an organized elastic adipose tissue very important for shock absorbtion [49]. Heel pad is compressed during standing, walking or running. The repetitive, cumulative fatigued micro-trauma can disrupt the balance between fatigue damage and routine repair [50]. The irreversible damage leads to an increased inflammatory, edema, and heel pad thickness. Due to these alterations, the stiffness as well as the capacity to attenuate shock, decrease. As shown in Fig. 1, the patient’s foot was 3D scanned without external pressure, while the shape of the plantar heel soft tissue was physiologic. The individualized form of heel cup was designed based on the surface morphology of patient’s plantar heel so that it can fit the skin closely. The nylon heel cup can absorb energy and buffer the shock, due to good quality material that supports elasticity and stiffness. In such manner, the heel cup can provide a good supporting role for the heel pad. The FE simulation also supported this assumption. The function of heel pad was transformed from bearing load to transfer stress, while the stress applied on the heel pad notably decreased. As shown in Fig. 5, the 3D printing heel cup bore the load; the stresses applied on the inner/outer sides were 3.6793 ± 0.819 and 4.3176 ± 1.7533 MPa, respectively.

Unlike other pre-fabricated commodities, the individualized heel cup needs to be customized. The common design for commodities with a flat bottom and a low side wall could neither enwrap nor support the heel effectively. The soft tissue becomes deformed as the heel comes into contact with these insoles. The 3D printing heel cups could completely fit the patients’ individual physiological curve, thus minimizing the deformation degree of the soft tissue. In order to achieve therapeutic effect, the tensile modulus should be fitting. To verify this hypothesis, we set up a simplified model and analyzed it by FE simulation. According to the stress nephogram shown in Fig. 6, the stress applied on the soft tissue reduced if the heel cup was worn. The deformation degree was inversely proportional to the strength of the heel cup within certain range. On the other hand, the cup could absorb energy and be crushed simultaneously if the tensile modulus were too low, or if structure design was irrational. As shown in Fig. 6c, although the compression ratio and stress of soft tissue were smaller, the cup gets completely crushed with a low tensile modulus. As the strength of the cup increased (Fig. 6e), the compression ratio increased from 22 to 35% (3.3 mm vs. 5.2 mm), but had no influence on the stress from soft tissue. In our previous study, silica gel, nylon, nylon glass fiber, and photosensitive resin were tested by volunteers. The silica gel was too soft to protect the heel, while the photosensitive resin was too hard. The suitable strength, comfort, and curative effect made nylon and nylon glass fiber were the most appropriate materials for fabricating 3D printing heel cups. Nylon was finally selected as a raw material due to affordable cost, as well as short fabricating period that approximately takes about 5 h. Briefly speaking, 15 min for 3D scanning, 20 min for post-processing and designing, and 3–4 h for 3D printing.

However, there are still some limitations in this study. Due to the limited instruments, the stress nephogram of plantar pressure could not clearly show the variation in stress, but only calculate the overall load. Because of the use of FE simulation, the imaging examination methods were not employed to achieve the real time images of plantar heel under different conditions. What’s more, the number of participants was not large enough to provide strong clinical evidence for the obtained results. Thus, the main objective of our future study is to enlarge the sample size, including dividing participants in accordance with ages, symptom levels, and exercise status, and to evaluate the symptoms improvement more systematically. Moreover, the study to discuss the application potential of individualized heel cups in the athletes, especially for the runners and walkers, for the purpose of prevention and rehabilitation sports injury is in progress.

In this study, we fabricated individualized heel cup using 3D printing technology. To verify its effectiveness, an in vivo experiment was conducted using a gait analysis system to measure the plantar pressure, while the clinical therapeutic effect was evaluated in some participants. Additionally, FE simulation was introduced to explore its mechanism. In conclusion, the individualized heel cup made by the combination of 3D scanning and 3D printing resulted to be an effective method for the treatment of plantar heel pain.