External adjuncts to enhance fracture healing: What is the role of ultrasound?

Summary

Current methods of fracture care use various adjuncts aimed at decreasing time to fracture union and improving fracture union rates. Among the most commonly used modalities, low-intensity pulsed ultrasound is emerging as a safe, cost-effective and reliable treatment for both fresh fractures and fracture nonunions. Both in vivo and in vitro basic science studies have helped to elucidate potential mechanisms of ultrasound action and a number of prospective, randomised, double-blind, placebo-controlled trials exist demonstrating the clinical efficacy of low-intensity pulsed ultrasound. This article will review the evidence for the use of low-intensity pulsed ultrasound in fracture care.

Introduction

While advances in the operative and non-operative care of fractures continue to improve patient outcomes, under the best of circumstances, recovery times are often on the order of months. This can have profound personal and economic consequences for patients and their families, underscoring the tremendous cost to the health care system. When fractures fail to heal, costs become even greater. Five to 10% of the 5.6 million fractures that occur annually in the US are complicated by delayed healing or nonunion.³ As an example, the tibia is the most commonly fractured long bone and accounts for 35–65% of all nonunions. This has a substantial economic impact when one considers re-operations, secondary surgical procedures, and prolonged physical therapy in treating these fractures.¹⁶ The need to decrease healthcare spending has led to interest in modalities which can enhance and hasten healing of fractures, diminish the incidence of nonunions, and in the event of delayed or nonunion treat them effectively.

Many modalities have been used in an attempt to accelerate fracture healing and prevent delayed and/or nonunions. These include autogenous and allogenic bone grafting, alteration of the mechanical stiffness of the fixation devices, electromagnetic fields, high-frequency low-magnitude mechanical stimuli, and ultrasound. Disadvantages of bone grafting and implantation of electrical stimulators are that these procedures must be performed in the operating room and often require a hospital admission. The result is added morbidity to the patient and added cost to the health care system.

Ultrasound has many medical applications, including diagnostic, operative, and therapeutic usages.¹⁵ Extracorporeal shock wave therapy (orthotripsy) has been used in the treatment of nonunions, with the premise that these high-energy waves cause microfracture of the trabeculae and through this tissue damage, encourage the reparative process to resume, leading to fracture union.35, 39, 43 In fact, prospective, non-randomised studies report more than 700 patients with documented healing success rates of 62–83%.²⁸ The appeal of this treatment option is that it performed externally. However, the practicality of shock wave therapy is questionable because it is painful and its use requires anaesthesia and often a hospital admission following treatment.35, 39, 43

As a result of continuing investigation into superior methods of external stimulation of fracture healing, low-intensity pulsed ultrasound has emerged as a safe and effective modality to enhance fracture healing. As with shock wave treatment, low-intensity pulsed ultrasound is applied externally, however, it is painless and can be applied by the patient on a daily basis from the patient's home. In October 1994, the United States Food and Drug Administration approved the use of ultrasound in fresh fractures and subsequently approved its use for established nonunions in February 2000.³⁶ A number of prospective, randomised, double-blind, placebo-controlled trials have demonstrated the efficacy of ultrasound in accelerating fracture healing. This is likely due to the influence of ultrasound at each key stage of fracture healing, including inflammation, repair, and remodeling. Moreover, ultrasound has been shown to affect angiogenesis, chondrogenesis, and osteogenesis.³⁶ The purpose of this review is to describe how ultrasound enhances fracture healing at a cellular and molecular level, describe in vitro and animal studies, and then to review relevant clinical fracture and nonunion studies.