Vibration Testing Procedures for Bone Stiffness Assessment in Fractures Treated with External Fixation LORENZA MATTEI , 1 ANTONIA LONGO, 1 FRANCESCA DI PUCCIO, 1 ENRICO CIULLI, 1 and STEFANO MARCHETTI 2 1 Department of Civil and Industrial Engineering, University of Pisa, Largo Lucio Lazzarino 2, 56126 Pisa, Italy; and 2 Department of Translational Research and of New Surgical and Medical Technologies, University of Pisa, Pisa, Italy (Received 10 August 2016; accepted 26 November 2016) Associate Editor Mona Kamal Marei oversaw the review of this article. Abstract—A bone healing assessment is crucial for the successful treatment of fractures, particularly in terms of the timing of support devices. However, in clinical practice, this assessment is only made qualitatively through bone manipulation and X-rays, and hence cannot be repeated as often as might be required. The present study reconsiders the quantitative method of frequency response analysis for healing assessments, and specifically for fractures treated with an external fixator. The novelty consists in the fact that bone excitation and response are achieved through fixator pins, thus overcoming the problem of transmission through soft-tissues and their damping effect. The main objective was to develop and validate a test procedure in order to characterize the treated bone. More than 80 tests were performed on a tibia phantom alone, a phantom with pins, and a phantom with a complete fixator. Different excitation techniques and input–output combinations were compared. The results demonstrated the effectiveness of a procedure based on impact tests using a micro-hammer. Pins and fixator were demonstrated to influence the frequency response of the phantom by increasing the number of resonant frequencies. This procedure will be applied in future studies to monitor healing both in in vitro and in vivo conditions. Keywords—Fracture healing, Bone stiffness, Experimental modal analysis, External fixator, Mechanical vibrations. INTRODUCTION Fracture healing is a physiological process through which the bone restores its original physical and mechanical properties. Healing occurs through several stages from inflammation to consolidation and remodeling, during which bone stiffness and strength continuously increase. 10,12 A successful treatment of fractures (e.g. support planning and rehabilitation) should be timed according to the healing process. Nonetheless, the assessment of bone healing is still an open issue, as it is a complex and patient-specific phenomenon. In clinical practice, the healing assess- ment is based on plane radiographs and manual examination of fracture mobility, which have two main drawbacks: firstly, the patient may be exposed to radiation many times during his/her treatment and, secondly, it depends on a subjective evaluation. External fixators are widely used to improve the stabilization and temporization of long bone fractures. They combine different types of elements, such as pins and rods, and are also made from different materials 9 (Fig. 1). Successful treatment with external fixators requires the correct timing for their removal and therefore a reliable healing assessment. In fact, an early fixator removal can lead to bone refracture in up to 11% of cases, 5 while a delayed removal results in unnecessary prolonged and costly treatments. Cur- rently, fixator removal follows an x-ray evaluation. It is clear that quantitative techniques are necessary to monitor healing and very important for improving the clinical outcomes of fractures treatments. 1,6,13,19 A few quantitative methods, mainly based on the evalu- ation of bone stiffness, have been proposed in the lit- erature, as reviewed in Refs. 1,6,13,19. However, these methods have not been widely accepted or used by clinicians. They include vibrational analysis, which was proposed in the 1990s: impact 3,7,14,18 and spectral 15 tests were performed on fractured tibias, both in in vivo 3,7,15,18 and ex vivo 14,15 conditions. In all these studies, the frequency response functions (FRFs) of the tibia/lower limb were evaluated, thus providing the Address correspondence to Lorenza Mattei, Department of Civil and Industrial Engineering, University of Pisa, Largo Lucio Laz- zarino 2, 56126 Pisa, Italy. Electronic mail: l.mattei@ing.unipi.it Annals of Biomedical Engineering (Ó 2016) DOI: 10.1007/s10439-016-1769-1 Ó 2016 Biomedical Engineering Society