**2. From concepts to acoustic devices**

Bone modelling refers to changes in bone structure and density in response to increased loads. Bone remodelling is defined as, the almost obligatory, bone resorption that follows bone formation irrespective of mechanical loads. The first to describe that bone deposition occurs preferably on sites of compressive loads, whereas bone resorption occurs preferably on sites of tensile loads was Julius Wolff, whose observations were the foundation of Wolff's law (1892) [1].

Later, Frost realized that different ranges of intensities (magnitudes) of bone deformation elicited different biological responses. Based on that, he published the mechanostat model (1964), in which low-frequency cyclic (less than 5–10 Hz), or static, loads lower than 50–100 μstrain (desuse range) lead to bone resorption, loads from 50–100 to 1000–1500 μstrain (physiological range) do not change bone mass, loads from 1000–1500 to 3000 μstrain (overuse range) induce osteogenesis and loads greater than 3000 μstrain (pathological overuse range) may lead to fracture or stress fracture. Strain stands for relative deformation of a cell or tissue. It should be noted that the ranges of intensities proposed by Frost refer to bone tissue, not to bone cells, which normally need higher strains to elicit an osteogenic response [1–3]. For detailed information, see reference [3].

Based on Wolff's law and mechanostat model concepts, mechanical devices were developed to purposely stimulate osteogenesis. LIPUS and ESWT are applied by acoustic devices approved in many countries for clinical use in the management of bone healing disorders. On the other hand, currently, RPWT lacks evidence for its use to induce osteogenesis, but it is used to address soft tissue orthopaedic disorders.

### **2.1. LIPUS device**

Low-intensity pulsed ultrasound was developed by Duarte, and its use for accelerating fracture healing was published in 1983. Most commonly used device generates 1.5 MHz ultrasound in a pulse wave mode (duty cycle of 20%, 200 μs burst width with repetitive frequency of 1 KHz) and average intensity 30 mW/cm2 . Low-intensity ultrasound waves are produced from a piezoelectric crystal within an unfocused, circular transducer. Its effective radiating area is 3.88 cm2 , peak rarefactional pressure at specimen (nonderated) is 0.076 MPa and focal length is ∼130 mm [4–6].

#### **2.2. ESWT devices**

ESWT originally was developed for lithotripsy in order to break up and disrupt stones within genitourinary tract. Its use for osteogenesis initiated after the observation that shock waves provoked osteogenic response on the pelvis of animals during lithotripsy experiments [7].

There are three main techniques for generation of shock waves. Irrespective of the technique, production of shock waves requires the conversion of electrical energy into acoustic energy. All three devices (electrohydraulic, electromagnetic and piezoelectric) are used in orthopae‐ dics, and there is no evidence that a certain device provides better results than the other [7–10].

#### *2.2.1. Electrohydraulic device*

This is the first generation of orthopaedic shock wave devices. A high-voltage electrical discharge is applied across electrode tips—a spark gap—within a water-filled semi-ellipsoid reflector. The resultant spark heats and vaporizes the surrounding water, which, in turn, generates a gas bubble filled with water vapour that expands and produces a shock wave. The wave is reflected by the metallic surface of the semi-ellipsoid and is focused into the therapeutic zone [7, 9, 11].

Electrohydraulic shock wave devices usually are characterized by relatively large axial diameters of the focal volume and high total energy within that volume. The spark gap wears out after about 50,000 shots (impulses) and needs to be replaced [7, 11].

Technical specifications vary according to manufacturer (not all manufacturers provide complete data): energy flux density varies from 0.01 to 1.80 mJ/mm2 , focal zones vary from 0 to 95 mm (fx[-6dB]) and from 4.8 to 25 mm (fz[-6dB]), frequency varies from 0.5 to 360 Hz, and penetration depth is up to 84 mm [12–15].

#### *2.2.2. Electromagnetic device*

Based on Wolff's law and mechanostat model concepts, mechanical devices were developed to purposely stimulate osteogenesis. LIPUS and ESWT are applied by acoustic devices approved in many countries for clinical use in the management of bone healing disorders. On the other hand, currently, RPWT lacks evidence for its use to induce osteogenesis, but it is used

Low-intensity pulsed ultrasound was developed by Duarte, and its use for accelerating fracture healing was published in 1983. Most commonly used device generates 1.5 MHz ultrasound in a pulse wave mode (duty cycle of 20%, 200 μs burst width with repetitive

produced from a piezoelectric crystal within an unfocused, circular transducer. Its effective

ESWT originally was developed for lithotripsy in order to break up and disrupt stones within genitourinary tract. Its use for osteogenesis initiated after the observation that shock waves provoked osteogenic response on the pelvis of animals during lithotripsy experiments [7].

There are three main techniques for generation of shock waves. Irrespective of the technique, production of shock waves requires the conversion of electrical energy into acoustic energy. All three devices (electrohydraulic, electromagnetic and piezoelectric) are used in orthopae‐ dics, and there is no evidence that a certain device provides better results than the other [7–10].

This is the first generation of orthopaedic shock wave devices. A high-voltage electrical discharge is applied across electrode tips—a spark gap—within a water-filled semi-ellipsoid reflector. The resultant spark heats and vaporizes the surrounding water, which, in turn, generates a gas bubble filled with water vapour that expands and produces a shock wave. The wave is reflected by the metallic surface of the semi-ellipsoid and is focused into the therapeutic

Electrohydraulic shock wave devices usually are characterized by relatively large axial diameters of the focal volume and high total energy within that volume. The spark gap wears

Technical specifications vary according to manufacturer (not all manufacturers provide

to 95 mm (fx[-6dB]) and from 4.8 to 25 mm (fz[-6dB]), frequency varies from 0.5 to 360 Hz, and

out after about 50,000 shots (impulses) and needs to be replaced [7, 11].

complete data): energy flux density varies from 0.01 to 1.80 mJ/mm2

penetration depth is up to 84 mm [12–15].

, peak rarefactional pressure at specimen (nonderated) is 0.076 MPa

. Low-intensity ultrasound waves are

, focal zones vary from 0

to address soft tissue orthopaedic disorders.

30 Advanced Techniques in Bone Regeneration

frequency of 1 KHz) and average intensity 30 mW/cm2

**2.1. LIPUS device**

radiating area is 3.88 cm2

*2.2.1. Electrohydraulic device*

zone [7, 9, 11].

**2.2. ESWT devices**

and focal length is ∼130 mm [4–6].

Within this device, there is an electromagnetic coil and a metal membrane besides the coil both embedded in a water medium. A high current pulse is released through the coil, generating a strong magnetic field, which repels the membrane rapidly away from the coil, therefore pushing the surrounding water to produce a shock wave. The shock wave is focused with an acoustic lens to the therapeutic zone. The lens can be used for several hundred thousand impulses with no need to replace the elements [7, 9, 11].

A variation of the electromagnetic device uses a repelling membrane formed as a cylinder and the sound waves are reflected by a surrounding parabolic reflector [11].

Technical specifications vary according to the manufacturer (not all manufacturers provide complete data): energy density flux varies from 0.01 to 0.55 mJ/mm2 , frequency varies from 1 to 8 Hz shock waves, penetration depth is up to 80 mm and focal zone varies from 0 to 65 mm [16, 17].

#### *2.2.3. Piezoelectric device*

Within this device, a few hundred to some thousand piezoelectric crystals—usually more than a thousand—are arranged in a spherical surface filled with water. A high pulse discharge is applied to the crystals, which immediately contract and expand (piezoelectric effect) generat‐ ing a shock wave in the surrounding fluid. The emitted energy of each crystal is fairly weak, but reaches higher energy at the focus where all shock waves gather together. The focal zone is relatively small and cigar shaped. Because of the spherical shape of the device's surface, this device has an extremely precise focus and a high energy density within a well-confined focal volume. In addition, the large aperture of the source allows for almost pain-free treatment because of the low-pressure at the skin entry zone [7, 9, 11].

Technical specifications vary according to the manufacturer (not all manufacturers provide complete data): energy flux density ranges from 0.03 to 0.4 mJ/mm2 , frequency ranges from 1 to 8 Hz, pressure ranges from 11.5 to 82.2 MPa, focal size ranges from 1.2 to 4.8 mm (fx[-6dB] = fy[-6dB]) and from 1.2 to 14.1 mm (fz[-6dB]) and penetration depth ranges from 5 to 40 mm [18–20].

#### *2.2.4. RPWT*

Radial pressure waves are produced pneumatically (ballistically). A projectile is accelerated with compressed air, or an electromagnetic field, within a guiding tube (cylindrical piston) and strikes a metal applicator placed on the patient's skin. The projectile produces stress waves in the applicator that transforms their kinetic energy into a radially expanding pressure—or pulse—wave towards tissue [21, 22].

Technical specifications vary according to model and manufacturer (not all manufacturers provide complete data): energy density flux is up to 0.55 mJ/mm2 , frequency ranges from 1 to 22 Hz, pressure ranges from 1.0 to 5.0 bar and penetration depth is up to 60 mm [23–26].
