Neill M. Pounder

Low intensity pulsed ultrasound for fracture healing: a review of the clinical evidence and the associated biological mechanism of action.

Low intensity pulsed ultrasound is used in the clinical treatment of fractures and other osseous defects. Level I clinical studies demonstrate the ability of a specific ultrasound signal (1.5 MHz ultrasound pulsed at 1 kHz, 20% duty cycle, 30 mW/cm(2) intensity (SATA)) to accelerate the healing time in fresh tibia, radius and scaphoid fractures by up to 40%. Additionally, the same ultrasound signal has been shown to be effective at resolving all types of nonunions of all ages, following a wide range of fracture types and primary fracture management techniques. Recently, significant efforts have resulted in a more comprehensive understanding of the biological mechanism of action that produces the documented clinical outcomes. Low intensity pulsed ultrasound has been demonstrated to accelerate in vivo all stages of the fracture repair process (inflammation, soft callus formation, hard callus formation). In particular, accelerated mineralisation has been demonstrated in vitro with increases in osteocalcin, alkaline phosphatase, VEGF and MMP-13 expression. Integrins, a family of mechanoreceptors present on a wide range of cells involved in the fracture healing process, have been shown to be activated by the ultrasound signal. Downstream of the integrin activation, focal adhesions occur on the surface of cells with the activation of multiple signalling pathways, including the ERK, NF-kappabeta, and PI3 kinase pathways. These pathways have been directly linked to the production of COX-2 and prostaglandin, which are key to the processes of mineralisation and endochondral ossification in fracture healing.

Osteocytes exposed to far field of therapeutic ultrasound promotes osteogenic cellular activities in pre-osteoblasts through soluble factors

Low intensity pulsed ultrasound (LIPUS) was reported to accelerate the rate of fracture healing. When LIPUS is applied to fractures transcutaneously, bone tissues at different depths are exposed to different ultrasound fields. Measurement of LIPUS shows pressure variations in near field (nearby transducer); uniform profile was found beyond it (far field). Moreover, we have reported that the therapeutic effect of LIPUS is dependent on the axial distance of ultrasound beam in rat fracture model. However, the mechanisms of how different axial distances of LIPUS influence the mechanotransduction of bone cells are not understood. To understand the cellular mechanisms underlying far field LIPUS on enhanced fracture healing in rat model, the present study investigated the effect of ultrasound axial distances on (1) osteocyte, the mechanosensor, and (2) mechanotransduction between osteocyte and pre-osteoblast (bone-forming cell) through paracrine signaling. We hypothesized that far field LIPUS could enhance the osteogenic activities of osteoblasts via paracrine factors secreted from osteocytes. The objective of this study was to investigate the effect of axial distances of LIPUS on osteocytes and osteocyte-osteoblast mechanotransduction. In this study, LIPUS (plane; 2.2 cm in diameter, 1.5MHz sine wave, ISATA=30 mW/cm(2)) was applied to osteocytes (mechanosensor) at three axial distances: 0mm (near field), 60mm (mid-near field) and 130 mm (far field). The conditioned medium of osteocytes (OCM) collected from these three groups were used to culture pre-osteoblasts (effector cell). In this study, (1) the direct effect of ultrasound fields on the mechanosensitivity of osteocytes; and (2) the osteogenic effect of different OCM treatments on pre-osteoblasts were assessed. The immunostaining results indicated the ultrasound beam at far field resulted in more β-catenin nuclear translocation in osteocytes than all other groups. This indicated that osteocytes could detect the acoustic differences of LIPUS at various axial distances. Furthermore, we found that the soluble factors secreted by far field LIPUS exposed osteocytes could further promote pre-osteoblasts cell migration, maturation (transition of cell proliferation into osteogenic differentiation), and matrix calcification. In summary, our results of this present study indicated that axial distance beyond near field could transmit ultrasound energy to osteocyte more efficiently. The LIPUS exposed osteocytes conveyed mechanical signals to pre-osteoblasts and regulated their osteogenic cellular activities via paracrine factors secretion. The soluble factors secreted by far field exposed osteocytes led to promotion in migration and maturation in pre-osteoblasts. This finding demonstrated the positive effects of far field LIPUS on stimulating osteocytes and promoting mechanotransduction between osteocytes and osteoblasts.

Fracture Healing Enhancement With Low Intensity Pulsed Ultrasound at a Critical Application Angle

Low-intensity pulsed ultrasound (LIPUS) was shown to have dose-dependent enhancement effect on the osteogenic activity of human periosteal cells that played an important role in fracture healing. It was hypothesized that the stimulatory effects of LIPUS on the periosteal cells could be optimized by adjusting the ultrasound delivered at its critical angle to the surface of bone. This increased the transmission of ultrasound waves on periosteum. By using a rat femoral fracture model, the stimulatory effects of LIPUS transmitted at 0°, 22°, 35° and 48°, and the sham-treatment control were investigated. Treatment efficacy was assessed using radiography, micro-computed tomography (micro-CT), histomorphometry and torsional test. The results showed that callus mineralization and bridging, biomechanical properties were significantly enhanced in the 35° group over the control and 0° groups after week 8. LIPUS transmitted at 35°, which could be the critical application angle, showed the best enhancement effects among all the other groups. LIPUS transmitted at a critical application angle may have greater enhancement effects in fracture healing.

Investigation of rat bone fracture healing using pulsed 1.5 MHz, 30 mW/cm2 burst ultrasound – Axial distance dependency

This study investigated the effect of LIPUS on fracture healing when fractures were exposed to ultrasound at three axial distances: z=0 mm, 60 mm, and 130 mm. We applied LIPUS to rat fracture at these three axial distances mimicking the exposure condition of human fractures at different depths under the soft tissue. Measurement of LIPUS shows pressure variations in near field (nearby transducer); uniform profile was found beyond it (far field). We asked whether different positions of the fracture within the ultrasound field cause inconsistent biological effect during the healing process. Closed femoral fractured Sprague-Dawley rats were randomized into control, near-field (0mm), mid-near field (60 mm) or far-field (130 mm) groups. Daily LIPUS treatment (plane, but apodized source, see details in the text; 2.2 cm in diameter; 1.5 MHz sine waves repeating at 1 kHz PRF; spatial average temporal average intensity, ISATA=30 mW/cm(2)) was given to fracture site at the three axial distances. Weekly radiographs and endpoint microCT, histomorphometry, and mechanical tests were performed. The results showed that the 130 mm group had the highest tissue mineral density; and significantly higher mechanical properties than control at week 4. The 60 mm and 0 mm groups had significantly higher (i.e. p<0.05) woven bone percentage than control group in radiological, microCT and histomorphometry measurements. In general, LIPUS at far field augmented callus mineralization and mechanical properties; while near field and mid-near field enhanced woven bone formation. Our results indicated the therapeutic effect of LIPUS is dependent on the axial distance of the ultrasound beam. Therefore, the depth of fracture under the soft tissue affects the biological effect of LIPUS. Clinicians have to be aware of the fracture depth when LIPUS is applied transcutaneously.

Design evolution enhances patient compliance for low-intensity pulsed ultrasound device usage

Poor patient compliance or nonadherence with prescribed treatments can have a significant unfavorable impact on medical costs and clinical outcomes. In the current study, voice-of-the-customer research was conducted to aid in the development of a next-generation low-intensity pulsed ultrasound (LIPUS) bone healing product. An opportunity to improve patient compliance reporting was identified, resulting in the incorporation into the next-generation device of a visual calendar that provides direct feedback to the patient, indicating days for which they successfully completed treatment. Further investigation was done on whether inclusion of the visual calendar improved patient adherence to the prescribed therapy (20 minutes of daily treatment) over a 6-month period. Thus, 12,984 data files were analyzed from patients prescribed either the earlier- or the next-generation LIPUS device. Over the 6-month period, overall patient compliance was 83.8% with the next-generation LIPUS device, compared with 74.2% for the previous version (p<0.0001). Incorporation of the calendar feature resulted in compliance never decreasing below 76% over the analysis period, whereas compliance with the earlier-generation product fell to 51%. A literature review on the LIPUS device shows a correlation between clinical effectiveness and compliance rates more than 70%. Incorporation of stakeholder feedback throughout the design and innovation process of a next-generation LIPUS device resulted in a measurable improvement in patient adherence, which may help to optimize clinical outcomes.