# Applied Mechatronic: A Sensor-Based Modification of an External Fixator According to [PERSON]
## 2 Related Work  
External fixator systems are characterized by their simplicity and versatility of use, ability to reduce soft-tissue damage, stability at the bone screw interface, and rigidity [19]. The decision to remove an external fixator is typically based on the interpretation of clinical results and the experience of an orthopedist. In [20] the authors discuss the lack of standardized methods for assessing bone union that result in significant disagreements among orthopedists in both clinical practice and research. Currently, X-rays and magnetic resonance imaging are the primary but unsafe diagnostic tool used to assess the callus formation and the mechanical stability of the bone. Other clinical evaluation parameters assessed by the orthopedist include the pain, soft tissue healing, (in)stability of the patient, and weight bearing [21]. In addition, mechanical assessment tools can be used to decide about the stiffness of the callus from the early stages of its formation until complete union. Direct methods (i.e., assessing the fracture stiffness by measuring the deflection at the fracture site) have the major disadvantage of removing the external fixator for each measurement. Indirect approaches (i.e., measuring strain via external fixation) have been more extensively applied. However, only a few studies have examined the displacement of external fixator components as a mean of monitoring the fracture healing.  
In [22] the authors present design of a dynamic external fixator system used to treat open tibial fractures. It consists of an inner rod with axial motion in the outer tube for dynamization to improve bone healing. Clamps for rods and tube provide six degrees of freedom. An open clamp is designed to limit tube movement. Distal nuts are used for bone gap closure function. This design is developed based on the benefits of dynamization (a gradually decreasing construct-stability of a fixator as a therapy used for the final phase of fracture healing), increased union rate and the simplicity of mono-lateral frame. It is simple to use, safe, and provide a stable structure in which rigidity and alignment can be adjusted as needed during the treatment. In [23], the authors describe three-step dynamization of the hexapod circular external fixator that is primary based on the experience of an orthopedist, with no set criteria. The first step is to loosen two struts arbitrarily, then remove the loosened struts, and finally remove the remaining struts one at a time. The authors of [24] propose a non-invasive method based on changes in stress transmission, the location of stress-sensitive points, and the displacement in an external fixator system during healing process of tibial fractures. Their research focuses on developing finite element models for fractures treated with unilateral external fixation. The intersections of the lowest screw and the bone cortex, as well as the second screw and the connecting rod, can be used as sensitive points for monitoring the healing. As a next research step, the authors propose investigating the feasibility of placing non-invasive, compact sensors on external fixators. This can assist orthopedists to accurately assess fracture healing, allowing for more informed treatment decisions, including removal of the external fixator.  
The orthopedist can remove the vertical rod of the fixator to check the bone healing process. If the pain persists during the orthopedist's manual pressure at both sides of the fixator, the vertical rod should be reinserted into the device and treatment should be continued. To facilitate the assessment of the callus hardening without requiring numerous X-ray images, we have modified an external fixator according to [PERSON] [25] that detects micromovements between bone fragments and have provided data on callus elasticity based on sensor readings.