The most important issue in the assessment of fracture healing is to acquire information on the restoration of mechanical integrity of the bone. To measure bending stiffness at the healing fracture site, we focused on the use of echo tracking (ET) that was a technique measuring minute displacement of bone surface by detecting a wave pattern in a radiofrequency echo signal with an accuracy of 2.6 μ. The purpose of this study was to assure that the ET system could quantitatively assess the progress, retardation or arrest of healing by detecting bending stiffness at the fracture site.

With the ET system, eight tibial fractures in 7 patients with an average age of 37 years (range: 24–69) were measured. Two tibiae in 2 patients were treated conservatively with a cast, and 6 tibiae in 5 patients were treated with internal fixation (intramedullary nailing: 4, plating: 1, screw 1). Patients assumed supine position, and the affected lower leg was held horizontally with the antero-medial aspect faced upwards. The fibula head and the lateral malleolus were supported and held tight by a Vacufix ®. A 7.5 Hz ultrasound probe was placed on each antero-medial aspect of the proximal and distal fragments along its long axis. Each probe was equipped with a multi-ET system with 5 tracking points with each span of 10 mm. A load of 25 N was applied at a rate of 5 N/second using a force gauge parallel to the direction of the probe and these probes detected the bending angle between the proximal and distal fragments. An ET angle was defined as the sum of the inclinations of both fragments. In the patients treated with a cast, the contralateral side was also measured and served as a control. Fracture healing was assessed time sequentially with an interval of 2 or 3 weeks during the treatment.

None of the patients complained of pain, or no other complication related to this measurement occurred. In the patient (patient:M) treated with a cast, the ET angle exponentially decreased as time elapsed (y = 1.4035e-0.1053x, R = 0.9754) and the radiographic appearance showed normal healing. Including this case, in all patients with radiographic normal healing, the ET angle exponentially decreased. However, in patients with retarded healing (patient:N), the decrease of the angle was extremely slow(y = 0.2769e-0.0096x, R = 0.815). In patients with non union (patient:T), the angle stayed at the same level.

With this method, noninvasive assessment of bending stiffness at the healing site was achieved. Bending angle measured by ET diminished over time exponentially in patients with normal healing. On the contrary, in patients with healing arrest, no significant decrease of the bending angle was recognized. It was demonstrated that the echo tracking method could be applicable clinically to evaluate fracture healing as a versatile, quantitative and noninvasive technique.