# Applied Mechatronic: A Sensor-Based Modification of an External Fixator According to [PERSON]
## 1 Introduction  
A bone fracture can be partial or complete and is usually caused by a fall from a height, accident, osteoporosis, and overuse [1]. In [2], the authors describe four phases of secondary bone fracture healing: hematoma formation (1-2 weeks), granulation tissue formation (2-4 weeks), bone callus formation (4-8 weeks), and bone remodeling (8-12+ weeks). Callus is the new bone tissue formed at the fracture site. It is elastic and hardens over time, reaching the stiffness of healthy bones by the end of healing. The callus deflection is greatest at the beginning and the lowest at the end of healing, when it is equivalent to the deflection of a healthy bone. The bone healing process requires a short distance between fracture fragments with very small freedom of movement until new bone tissue develops. Until the twentiethcentury, immobilization was the primary technique to hold bone fragments in place. Bone fixation implants emerged with the development of medicine. There are two types of severe bone fracture fixation. Internal fixation involves inserting the entire fixation implant under the skin, while external fixation uses pins inserted into the bone, passed through the soft tissues and skin, and attached to a frame outside the body [3]. Discomfort and possible pin tract infection are considered disadvantages of external fixation [4]. An external fixator, according to [PERSON], is a minimally invasive 3D fixation system typically used to treat open fractures with a higher risk of infection, complex fractures, and fractures with severe trauma of the surrounding nerves and blood vessels [5]. It consists of bars, adjustable clamps, and clamp-carriers designed to be highly adaptable to different fracture fragments position. Long bones fractures (femur, tibia, etc.) are the most common types of injuries that, in addition to expected discomfort, can lead to reduced or lost limb function [6; 7; 8] and, also, to complications such as deep infection, vascular or nerve damage [9], delayed union, malunion, and nonunion [10; 11; 12; 13]. Delayed union occurs when bone does not heal at the expected rate, malunion is followed by bone healing but not in the proper position, whereas nonunion occurs when broken fragments fail to be connected by the new bone [10]. The authors of [11] conducted a study on the rate and predictions of patient readmission for surgical treatment due to bone healing complications (8% of patients were readmitted to the hospital). In [12], the authors found that vulnerability of the soft tissues surrounding the fracture increases the susceptibility to non-union and infection. The authors of [13] discuss a morbidity associated with a long bone nonunion. In addition, several studies [14; 15; 16; 17] discuss emergency fracture care in cattle orthopedics. The authors of [14; 15; 17] discuss external skeletal fixation (ESF) as a rigid immobilization, while the author of [16] discusses long bone fractures complications. However, the external fixation on the cattle is outside of the scope of this study. Integration of a sensor technology into the external fixator is a strategy that provides a more accurate decision on bone healing. A modified external fixator according to [PERSON] is an electrically active system in which force is detected by a sensor integrated into the existing structure [18]. At the beginning of the treatment, the sensor readings correspond to half of the patient's weight, implying that the broken leg was not loaded. When the callus hardens, the sensor deflection changes due to the axial load. As the force in the leg increases the force detected by the sensor drops to near-zero levels. In the experiments, three different axial loads delivered by the Shimadzu universal electromechanical testing machine (UETM) (50 N, 250 N, and 500 N) are compared with axial forces detected by the sensor, with three springs of different rigidity (k = 1.5 N/mm, k = 3.8 N/mm, and k = 7.5 N/mm) used to simulate different stages of callus hardening [2].