A Review of Bone Growth Stimulation for Fracture Treatment

Steve B. Behrens; Matthew E. Deren; Keith O. Monchik

Disclosures

Curr Orthop Pract. 2013;24(1):84-91. 

In This Article

Current Bone Growth Stimulator Technology

Direct current (DC) in the 1960s through 1970s was the predecessor to modern day bone growth stimulator technology. DC stimulators consist of surgically implanted wire leads of various lengths placed directly at the fracture or fusion site.[21] Most commonly used during initial spinal fusion procedures,27 these stimulators also are implanted during fixation and bone grafting of nonunions.[28] A subcutaneous lithium battery powers the unit with 5–100 mA DC for 6 months, and is later removed during a second procedure.[21] Bone growth prevents the embedded wire leads from removal. DC stimulators provide constant uniform current at the target site during the entire battery life, eliminating concerns about patient compliance.[21] The disadvantages of DC stimulators are battery life of approximately 6-8 months, difficulty placing hardware, shortcircuits from leads touching other lead wires (or any metal), risk of infection, and a second procedure for hardware removal.[29]

Modern stimulators can be classified into two groups: electromagnetic or ultrasound (Table 1). Electromagnetic stimulators are further divided into inductive or capacitive coupling devices.[28] Inductive coupling, otherwise known as pulsed electromagnetic fields (PEMF), was popularized in the 1970s, with over 250 published clinical research studies and 200 basic science studies supporting its efficacy.[1] The first PEMF device became available in 1979, and used an externally applied coil sized for fracture location. The unit may be used through or placed under casting material, with the patient wearing an external battery for up to 10 hours of daily use.[30] By creating an electrical signal in bone after energizing the coil, the device enhances the treatment of nonunions, using the bioelectrical principles of bone healing.[31] PEMFs create low-level electromagnetic signals, which after reaching a fracture site, are converted to electric currents.[32] It is thought that the PEMF mimics the body's normal physiologic processes.[31] The PEMF signal is a complex waveform that is often biphasic and quasirectangular, fluctuating in amplitude and frequency.[33] Patient noncompliance can be caused by the heavy weight of these units.

Capacitive coupling (CC), popularized in the 1980s by Brighton and Pollack,[34] uses an external power source for frequencies of 20–200 kHz and fracture site electric fields of 1–100 mV/cm.[28,35] The external battery pack is connected to two wires and electrodes applied on the skin at the fracture site.[30] When using the unit for 24 hours, patients must change batteries daily. The units, though small and lightweight, may cause irritation of the skin from the electrodes.[31] Lorich et al.36 published an article on the biochemical pathways mediating the response of bone cells to CC, stating that this electrostimulation regulates gated ion channels to increase the flux of calcium within the cells.[36]

The last electrical stimulator technology is a combined magnetic field (CMF) that became popular in the 1990s. The CMF technology combines a static DC electric field and a sinusoidal waveform[1] produced by external coils worn for 30 minutes daily. The ease and brevity of daily use may improve patient compliance for these devices.[31] Affecting cell signaling, likely through intracellular stores of calcium, these stimulators increase calmodulin levels and result in bone cell proliferation in vitro.[37]

Low-intensity pulsed ultrasound (LIPUS) is a unique, noninvasive, and low-risk treatment option. LIPUS produces a mechanical signal which transmits through soft tissue and bone, producing micromotion at the fracture site detected by integrin cellular receptors.[12,38,39] Signaling through integrins results in increased expression of cyclooxygenase- 240, which leads to increased prostaglandin E-2 at the fracture site, and increased mineralization.[41] Micromotion also results from acoustic cavitation and shearing from LIPUS waves, causing fluid flow in the tissues and extracellular matrix[42] resulting in increased cell permeability, local blood pressure, and nutrient levels at the fracture site.[43] 44 LIPUS results in small increases in local temperature less than 11C, which may affect enzymes such as matrix metalloprotease-1,[45] and increase local blood flow to dissipate heat.[46] The increased angiogenesis may be prompted by increased cytokines, such as vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF), and interleukin-8 (IL-8).[47] Increased levels of intracellular Ca2+ are critical to the cellular signaling stimulated by LIPUS.[48,49]

Much literature has misconceived electrical current, not current density, as the essential factor in the efficacy of electrical stimulation.[50–52] The current density should not exceed 625 mA/in2 to avoid bone degradation.[53] Each method of energy has benefits and potential drawbacks based on the component specification, such as size, weight, portability, patient compliance, and cost.

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