Bone stimulators or bone growth stimulators are devices which are used for enhancing the union of bone. Nonunion [failure of the fracture to unite] or delayed union [fracture unites slower than expected] are common and significant problems in the bone union. Internal or external fixation, bone grafting, and more radically and rarely amputation are the recommended invasive options.
The reported incidence of nonunion in various bones is
- Tibia – 35%
- Femur – 19%
- Humerus- 7.5%
- Forearm bones – 15.5%
- Clavicle – 2%
Bone healing may be manipulated by external and internal stimuli. External stimuli are biomechanical and internal are biological. A variety of biological, mechanical, and physical interventions have been developed to enhance fracture healing. Physical methods to stimulate bone healing include electrical bone stimulators, low-intensity pulsed ultrasound, and extracorporeal shock waves.
These modalities are less invasive to patients, cause less morbidity and less costly.
But how effective these bone stimulators are?
Types of Bone Stimulators
Bone stimulators can be external bone growth stimulators which are superficial or percutaneous and implantable bone growth stimulators which need to be implanted. Depending on, how they produce the action, bone simulators may be of different types, as described below.
Electric or Electromagnetic
Electrical and electromagnetic fields work similar to mechanical stress applications. They create a strain gradient which leads to fluid driving through the canaliculi from high to low pressure. This exposes the osteocyte membranes to flow-related shear stress and electrical potentials causing their activation.
A variety of instruments have been developed to be delivered to electrical and electromagnetic fields to fracture sites, each being categorized into one of three types:
- Direct-current stimulators
- Inductive coupling stimulators or pulsed electromagnetic fields
- Capacitive coupling stimulators
Direct-current bone stimulators need either implanted or percutaneously applied insulated electrodes. Other two are not invasive
Implantable bone stimulators have the advantage of providing constant stimulation of bone directly at the fracture site but have more problems like infection rates and painful implant.
Inductive coupling devices [ pulsed electromagnetic fields] can be placed under casting material or used through the material. An external battery is provided. These create low-level electromagnetic signals, which after reaching a fracture site, are converted to electric currents and thought to mimic the body’s normal physiologic processes. The heavy weight of these devices may cause poor compliance.
Capacitive coupling devices are small, lightweight devices which use an external power source for frequencies of 20–200 kHz and fracture site electric fields of 1–100 mV/cm. Patients must change batteries daily and irritation of skin is a common problem.
Low-intensity pulsed ultrasound [LIPUS]
Low-intensity pulsed ultrasound produces a mechanical signal which transmits through soft tissue and bone, leading to micromotion at the fracture site causing an increased expression of cyclooxygenase- 240, which leads to increased prostaglandin E-2 at the fracture site, and increased mineralization.
Micromotion also causes fluid flow in the tissues and extracellular matrix leading to increased cell permeability and nutrient levels at the fracture site. A small rise in temperature also occurs and results in increased local blood flow. There is increased new vessel formation or angiogenesis may be prompted by increased cytokines and interleukin 8. Increase in the incorporation of calcium ions in cultures of cartilage and bone cells stimulate the expression of numerous genes. There is an increase in the formation of soft callus and early onset of endochondral ossification.
LIPUS is said to be promising for healing fresh fractures and treatment of delayed union or nonunions.
Extracorporeal Shock Waves
Extracorporeal shock waves method is recently being studied and not used as a standard treatment for bone fractures.
Current Evidence for Bone Stimulators
One of the major drawbacks of bone stimulators is the lack of a sufficient number of studies. Therefore the proper recommendations cannot be made.
For the treatment of delayed unions and nonunions, all other bone stimulators have a Grade C recommendation which means that further validation is required.
Moreover, there insufficient evidence to recommend one stimulator over another.
The conclusion is that we do not have enough data on bone stimulators to say yes or no its use in bone fracture healing. There remains a need to conduct, large, and definitive trials.
How and When to Use Bone Stimulators
The best person to decide, in the wake of weak evidence and no clear-cut recommendations, is your treating doctor. Your fracture pattern, your age, treatment method, insurance coverage etc are few of the factors that may determine prescription of the bone stimulators.