**6. Acoustic therapy**

Clinical applications for acoustic therapy include nonunions and delayed unions. Debatable applications include acceleration of fracture healing, acceleration of segmental defects healing, enhancement of bone density and quality, management of stress fracture, enhancement of bone-tendon junction healing and management of avascular necrosis of the femoral head.

LIPUS has a unique protocol of treatment, which consists of daily 20-min sessions at 30 mW/ cm2 . On the contrary, ESWT has no established protocol regarding energy levels, frequency, number of sessions and number of cycles (impulses). This heterogeneity makes it difficult for the clinician to adopt the best approach for ESWT. No studies were found on the subject for RPWT.

#### **6.1. Delayed union and nonunion**

Normally, patients with nonunion and delayed union are managed surgically for revision of a primary surgery or for biological stimulation. Those managed surgically for biological stimulation may be the best candidates for a non-invasive approach with acoustic therapy, since there is no problem with hardware and fracture reduction. Those experiencing technical problems related to the first procedure (gross bone instability, broken hardware, malalign‐ ment) should be subjected to revision surgery combined with acoustic therapy to provide also biological stimulation.

LIPUS exhibits healing rate from 67 to 92% and may challenge surgical treatment for delayed union and nonunion. Patients aged 70–79 years feature decreased healing rates (83.3 vs 86.2%), and older than 80 years feature even lower healing rates (77.8 vs 86.2%). LIPUS may also be an alternative approach to treat conservatively congenital pseudarthrosis of the tibia. Mean body mass index, open fracture, multiple prior surgical procedures, time to initiate treatment with LIPUS, type of surgical procedure, comorbidities and number of smoking years repre‐ sented no risk factor for failure with LIPUS in a cohort of 767 patients. Smaller cohorts present some conflicting data: decreased healing rate was found in late treated (more than 12 months) nonunions and smokers. Moreover, atrophic nonunions may be a risk factor for decreased healing rates. Interestingly, LIPUS combined with iliac crest autograft exhibits synergistic effect to overcome spinal pseudarthrosis created by nicotine administration, although LIPUS alone cannot [94, 107–114].

ESWT also shows healing rates that may challenge surgical treatment for nonunion and delayed union, with successful rates ranging from 63.6 to 95% using electrohydraulic or electromagnetic devices. No reports explored the effectiveness of piezoelectric devices, and RPWT. Energy density varied from 0.25 to 0.70 mJ/mm2 , 1000–10000 impulses, single or multiple sessions. Technical parameters depended on bone size and authorship. Specifically for scaphoid pseudarthrosis, energy density varied from 0.05 to 0.12 mJ/mm2 depending on patient's pain tolerance. Some studies also investigated serum level of BMP-2, NO, TGF-β1 and VEGF, which were higher in treated individuals. Again, atrophic nonunions, smoking and treatment performed at late stages (after 12 months) provided decreased healing rates [115– 124].

#### **6.2. Accelerated healing of bone defects and fractures**

The potential benefits of LIPUS and ESWT to accelerate healing of bone defects and fractures have been shown in various animal studies, but there is not sufficient clinical evidence to support their routine use.

LIPUS promoted earlier callus formation, promoted larger callus width, increased biome‐ chanical strength, reduced adverse outcomes (nonunion and delayed union), accelerated maturation of newly formed bone and healing time in distraction osteogenesis and reduced time for fracture healing. LIPUS reduced 18–36 days of healing time in conservatively treated fractures, and decreased about 30% of the healing time for surgically managed closed com‐ minuted diaphyseal tibial and femoral fractures (irrespective of implant choice). Open fractures and patients older than 60 years had pronounced benefit from LIPUS treatment. LIPUS' effectiveness increases as soon as treatment is initiated. In addition, fractures of the metatarsal, radius, scaphoid, ankle, fibula and ulna exhibited better healing rates. Smoking, diabetes, vascular insufficiency, osteoporosis, cancer, rheumatoid arthritis and obesity are risk factors for failure. A large cohort of 4190 patients showed 96% healing rate, which is greater than literature averages (93%). In that study, patients between 20 and 29 years old had greater healing rate than patients over 30 years old. Furthermore, LIPUS has no reported adverse effects [37, 56, 86, 125–132].

Only electrohydraulic devices investigated the beneficial effects of ESWT for bone defects and fracture healing. Energy density varied from 0.16 to 0.62 mJ/mm2 , 500–6000 impulses, single or multiple sessions. Increased callus formation; biomechanical properties; ALP activity; and expression of BMPs, IGF-1, eNOS, TGF-β1 and VEGF were reported. Patients subjected to ESWT exhibited better pain scores and decreased nonunion rates, but no difference of fracturerelated complications rate. Reported complications include skin petechiae, scarring to the muscle at the treatment site (only for small animals) and subcutaneous swelling. No neuronal damage has been reported even for vertebral exposure (study with small animals) [54, 80, 91, 95, 98, 133–136].

#### *6.2.1. Diabetes*

number of sessions and number of cycles (impulses). This heterogeneity makes it difficult for the clinician to adopt the best approach for ESWT. No studies were found on the subject for

Normally, patients with nonunion and delayed union are managed surgically for revision of a primary surgery or for biological stimulation. Those managed surgically for biological stimulation may be the best candidates for a non-invasive approach with acoustic therapy, since there is no problem with hardware and fracture reduction. Those experiencing technical problems related to the first procedure (gross bone instability, broken hardware, malalign‐ ment) should be subjected to revision surgery combined with acoustic therapy to provide also

LIPUS exhibits healing rate from 67 to 92% and may challenge surgical treatment for delayed union and nonunion. Patients aged 70–79 years feature decreased healing rates (83.3 vs 86.2%), and older than 80 years feature even lower healing rates (77.8 vs 86.2%). LIPUS may also be an alternative approach to treat conservatively congenital pseudarthrosis of the tibia. Mean body mass index, open fracture, multiple prior surgical procedures, time to initiate treatment with LIPUS, type of surgical procedure, comorbidities and number of smoking years repre‐ sented no risk factor for failure with LIPUS in a cohort of 767 patients. Smaller cohorts present some conflicting data: decreased healing rate was found in late treated (more than 12 months) nonunions and smokers. Moreover, atrophic nonunions may be a risk factor for decreased healing rates. Interestingly, LIPUS combined with iliac crest autograft exhibits synergistic effect to overcome spinal pseudarthrosis created by nicotine administration, although LIPUS

ESWT also shows healing rates that may challenge surgical treatment for nonunion and delayed union, with successful rates ranging from 63.6 to 95% using electrohydraulic or electromagnetic devices. No reports explored the effectiveness of piezoelectric devices, and

multiple sessions. Technical parameters depended on bone size and authorship. Specifically for scaphoid pseudarthrosis, energy density varied from 0.05 to 0.12 mJ/mm2 depending on patient's pain tolerance. Some studies also investigated serum level of BMP-2, NO, TGF-β1 and VEGF, which were higher in treated individuals. Again, atrophic nonunions, smoking and treatment performed at late stages (after 12 months) provided decreased healing rates [115–

The potential benefits of LIPUS and ESWT to accelerate healing of bone defects and fractures have been shown in various animal studies, but there is not sufficient clinical evidence to

LIPUS promoted earlier callus formation, promoted larger callus width, increased biome‐ chanical strength, reduced adverse outcomes (nonunion and delayed union), accelerated

, 1000–10000 impulses, single or

RPWT. Energy density varied from 0.25 to 0.70 mJ/mm2

**6.2. Accelerated healing of bone defects and fractures**

RPWT.

**6.1. Delayed union and nonunion**

46 Advanced Techniques in Bone Regeneration

biological stimulation.

alone cannot [94, 107–114].

support their routine use.

124].

Diabetes is a systemic disease that affects bone healing. Therefore, diabetic individuals are at risk of developing delayed unions, nonunions and pseudarthrosis. Those individuals may also exhibit impaired biomechanical strength of newly formed bone. LIPUS does not increase cellular proliferation during fracture healing in diabetic animals but increases bone healing and biomechanical properties. Additionally, LIPUS increases the expression of TGF-β1 and VEGF but not the expression of IGF-1 and PDGF-β. There are no reports on ESWT and RPWT in diabetic animals or individuals [137, 138].

#### *6.2.2. Osteoporosis*

Fracture healing slows and endochondral ossification is impaired with senescence. At the molecular aspect, fracture-induced cox-2 expression in aged rats is lower than youngsters. Thankfully, bone cells keep their mechanosensitivity; as such, acoustic stimulation accelerates fracture healing. It has been shown that LIPUS accelerates fracture healing in estrogendeficient osteoporotic bone and regains biomechanical strength so that it becomes comparable to non-osteoporotic bones also subjected to LIPUS. Furthermore, LIPUS increases the activity of ALP, and the expression of aggrecan, BMP-2/4/6, cbfa-1, cox-2, FGF-2, OPG, osteocalcin, osterix, RANKL, TGF-α1, VEGF and types I, II and X collagen. The effects of ESWT and RPWT were not investigated for fractures in osteoporotic bones [139–141].

#### *6.2.3. Bone-implant osseointegration*

Osseointegration of implants is an important step for recovery of biomechanical strength of bone. Facilitation of this biological process may decrease recovery time and the risk of hardware failure. LIPUS accelerates osseointegration of titanium screws in tibias and femurs, porous hydroxyapatite ceramic and miniscrew implants. Histologically, LIPUS-induced osseointegration provides denser trabecular microstructure at implant-bone interface and thicker newly formed bone. Those findings suggest acoustic therapy may be used as adjunctive therapy to increase hardware lifetime (e.g. for arthroplasties) and decrease recovery time. No reports were found on the subject for ESWT and RPWT [142–144].

#### *6.2.4. Bone graft substitutes*

Bone graft substitutes provide an osteoconductive scaffold for filling large osseous defects, and they are an alternative for autologous bone graft, which adds morbidity to the patient. Acoustic therapy provides osteoinductive stimulation for bone. Therefore, combination of acoustic therapy and bone graft substitutes may be a finer alternative to treat fractures associated with large defects. A report showed LIPUS increased bone formation in ulna defect filled with β-tricalcium phosphate (bone graft substitute). In addition, LIPUS did not alter resorption rate of the bone graft substitute. The influence of ESWT and RPWT on large osseous defects filled with bone graft substitutes needs to be explored [86].

#### *6.2.5. Bone-tendon junction*

Healing at bone-tendon junction is crucial for tendon repairs (e.g. quadriceps tendon repair, rotator cuff repair, calcaneal tendon repair) and ligament reconstruction (e.g. anterior cruciate ligament of the knee reconstruction and medial patellofemoral ligament reconstruction) to ensure early recovery and improved biomechanical strength. Acoustic therapy may be used as adjunctive therapy in those situations since LIPUS and ESWT were found to enhance healing of bone-tendon junction. Histologically, those acoustic therapies promoted better remodelling of the newly formed trabecular bone, increased bone mineral density and improved tendonto-bone collagen fibre reconnection [145–147].

#### *6.2.6. Stress fractures*

Stress fractures are pathological overuse injuries common in athletes and military recruits. Those injuries result from repetitive loading beyond the regenerative capacity of bone, and represent failure of the adaptive mechanisms of bone to mechanical loads. Results regarding this subject are variable.

LIPUS at 30 mW/cm2 used to treat incomplete stress injury of the posteromedial tibia, fibula, or second to fourth metatarsals was ineffective to accelerate recovery during a 4-week treatment. On the other hand, LIPUS at 100 mW/cm2 accelerated stress fracture healing of ulnae even in the presence of non-steroidal anti-inflammatory drugs, which normally delay fracture healing. In addition, athletes with delayed or nonunions of stress fractures of tibia or fifth metatarsus experienced bone healing within 6–14 weeks of exposure to electromagnetic ESWT [148–150].

#### **6.3. Intact bone**

*6.2.3. Bone-implant osseointegration*

48 Advanced Techniques in Bone Regeneration

*6.2.4. Bone graft substitutes*

*6.2.5. Bone-tendon junction*

*6.2.6. Stress fractures*

this subject are variable.

to-bone collagen fibre reconnection [145–147].

treatment. On the other hand, LIPUS at 100 mW/cm2

Osseointegration of implants is an important step for recovery of biomechanical strength of bone. Facilitation of this biological process may decrease recovery time and the risk of hardware failure. LIPUS accelerates osseointegration of titanium screws in tibias and femurs, porous hydroxyapatite ceramic and miniscrew implants. Histologically, LIPUS-induced osseointegration provides denser trabecular microstructure at implant-bone interface and thicker newly formed bone. Those findings suggest acoustic therapy may be used as adjunctive therapy to increase hardware lifetime (e.g. for arthroplasties) and decrease recovery time. No

Bone graft substitutes provide an osteoconductive scaffold for filling large osseous defects, and they are an alternative for autologous bone graft, which adds morbidity to the patient. Acoustic therapy provides osteoinductive stimulation for bone. Therefore, combination of acoustic therapy and bone graft substitutes may be a finer alternative to treat fractures associated with large defects. A report showed LIPUS increased bone formation in ulna defect filled with β-tricalcium phosphate (bone graft substitute). In addition, LIPUS did not alter resorption rate of the bone graft substitute. The influence of ESWT and RPWT on large osseous

Healing at bone-tendon junction is crucial for tendon repairs (e.g. quadriceps tendon repair, rotator cuff repair, calcaneal tendon repair) and ligament reconstruction (e.g. anterior cruciate ligament of the knee reconstruction and medial patellofemoral ligament reconstruction) to ensure early recovery and improved biomechanical strength. Acoustic therapy may be used as adjunctive therapy in those situations since LIPUS and ESWT were found to enhance healing of bone-tendon junction. Histologically, those acoustic therapies promoted better remodelling of the newly formed trabecular bone, increased bone mineral density and improved tendon-

Stress fractures are pathological overuse injuries common in athletes and military recruits. Those injuries result from repetitive loading beyond the regenerative capacity of bone, and represent failure of the adaptive mechanisms of bone to mechanical loads. Results regarding

LIPUS at 30 mW/cm2 used to treat incomplete stress injury of the posteromedial tibia, fibula, or second to fourth metatarsals was ineffective to accelerate recovery during a 4-week

even in the presence of non-steroidal anti-inflammatory drugs, which normally delay fracture healing. In addition, athletes with delayed or nonunions of stress fractures of tibia or fifth

accelerated stress fracture healing of ulnae

reports were found on the subject for ESWT and RPWT [142–144].

defects filled with bone graft substitutes needs to be explored [86].

Despite fractures, bone is subject to other diseases that alter its biomechanical strength, such as osteoporosis; or produce disabling pain, such as avascular necrosis of the femoral head. Acoustic therapy may be used for prevention and treatment of some bone disorders.

#### *6.3.1. Healthy bone*

It is not known how healthy and intact bone reacts to acoustic loading. Most studies focus on pathological conditions, such as fractures and osteoporosis. The understanding of the normal response of bone to acoustic loads within the physiological range and overuse range is required to ameliorate the comprehension of tissue behaviour in pathological situations, and to prevent some disorders; for instance, stress fractures and osteoporosis.

Intact and healthy bones subjected to LIPUS experience increased density of trabecular spongiosa, and increased activity of FAK, ERK-1/2 and IRS-1. Electrohydraulic ESWT (from 0.15 to 0.47 mJ/mm2 , 500–6000 impulses, single session), in turn, promotes angiogenesis, increased cellular population and bone formation, increased activity of ERK-1/2 and Akt, and increased TGF-β1 production, but no difference on biomechanical tests was found following ESWT exposure [54, 79, 90, 95, 151–153].

#### *6.3.2. Osteoporosis*

Studies demonstrated that LIPUS does not increase bone mineral density of osteoporotic bones and does not prevent osteoporosis as measured by dual energy X-ray absorptiometry. However, in those studies the exposure to LIPUS occurred within a short time (from 4 to 12 weeks), and the population of some investigations was heterogeneous. Additionally, histo‐ logical and molecular analysis of osteoporotic bones subjected to LIPUS showed increased bone formation, normal density of trabecular spongiosa, decreased disruption of trabecular spongiosa and greater expression of cbfa-1 (although lower than controls) [37, 153–157]. Therefore, the authors believe LIPUS possesses beneficial effects for treating osteoporosis.

Electromagnetic ESWT exhibited more pronounced effects on osteoporotic intact bones than LIPUS since ESWT showed increased bone mineral density and decreased bone loss [158].

#### *6.3.3. Immature bone*

Concern exists about possible negative effects of ESWT on ephiphyseal plaque in skeletally immature individuals; therefore, ESWT is not formally indicated for children. Contrarily, LIPUS is not contraindicated for skeletally immature individuals. Two studies addressed the effects of ESWT on epiphyseal plaques of animals. It was found that electrohydraulic or electromagnetic ESWT, from 0.38 to 0.60 mJ/mm2 , 1500–3000 impulses, single or multiple sessions, did not harm epiphyseal plaque cells and did not impair growth. Furthermore, histological analysis revealed increased number of chondrocytes in the proliferative zone and increased thickness of the epiphyseal plaque, suggesting a possible role for growth stimulation. No studies were found for LIPUS that could suggest a possible role for growth stimulation in skeletally immature individuals [96, 97].

#### *6.3.4. Avascular necrosis of the femoral head*

Patients who develop avascular necrosis of the femoral head experience groin pain and disability, and further may necessitate joint replacement. A novel possible approach for initial stages of that condition, when bone collapse and osteoarthritis have not established yet, is acoustic therapy. Experimental studies with avascular necrosis of the femoral head models showed that LIPUS and electrohydraulic ESWT increase neovascularization, osteogenesis, osteogenic differentiation of bone marrow cells, decreased size of fat cells—which substitute dead bone—and biomechanical strength of bone. Increased expression of proliferative factors, such as BMP-2, FGF, IGF-1, NO and VEGF, was also found. Furthermore, a clinical and an experimental research revealed that electrohydraulic ESWT may be more effective than core decompression and non-vascularized fibular grafting in patients with early-stage disease; reverts osteonecrosis by one stage; decelerates, or stops, disease' progression; and decreases pain and functional disability [10, 38, 149, 159, 160].
