Background and Indication
Treatment of sialolithiasis is currently achieved by a minimally invasive gland-preserving therapy regime. The observation by van den Akker and Busemann-Sokole that salivary gland function completely recovers after stone removal later was confirmed by others.
In >80% of stones, fragmentation is necessary because of the size, impaction, and location. Of all stones in the submandibular, 80–85% and 75–80% in the parotid gland can be treated by transoral duct surgery and/or interventional sialendoscopy, including intraductal lithotripsy, which are methods of choice. However, 10–15% of stones in the submandibular gland and 20–25% in the parotid gland are not accessible with sialendoscopy but can be treated by extracorporeal shock wave lithotripsy (ESWL). The first successful fragmentation of a salivary stone in the parotid gland by ESWL was reported by Iro et al. in 1989.
Shock Waves, Devices, and Procedures in Extracorporeal Shock Wave Lithotripsy
In contrast to high-frequency ultrasound waves, which are characterized by a sinusoidal pressure course with successive compression and tensile phases, low-frequency acoustic shock waves are characterized by an extremely short time increment (10–100 ns), a short pulse duration (<1 µs), and an increase of pressure resulting in high amplitude or peak pressure (10–100 MPa) ( Fig. 24.1A ).
Low-frequency and high-energy extracorporeal shock waves are much less attenuated by the tissue compared with the high-frequency ultrasound waves and therefore penetrate to deeper layers. They are generated within a water-coupling medium, acoustically similar to human tissue, that prevents tissue-harming interactions at the body surface. If shock waves are penetrating biologic tissue, they are reacting with bodies of different impedance, e.g., stones. The shock waves induce compressive waves that spread through the stone and expansive waves that pit and cavitate stones. At the facing edge of the stone, partial reflection of the shock waves causes compressive forces. Some parts of the shock waves penetrate through the stone to its averted border and are again reflected there, causing further compressive and tensile forces. The stone fragments if these forces exceed its compressive or tensile strength. An indirect effect of shock waves are cavitation bubbles that are generated directly after the pressure/tension alternating load of the shock waves has passed the medium. The majority of the bubbles collapse after the waves pass. But bubbles which are located near to interfaces like stones collapse asymmetrically causing microjets ( Fig. 24.1B ), which contain high amounts of energy and can erode the hard interfaces of stones. Because the forces are only generated at its interfaces, central parts of a stone are fragmented in the vast majority of cases only after repeated procedures.
The extracorporeal shock waves used today are generated by electrohydraulic, piezoelectric, and electromagnetic systems ( Fig. 24.2 ). Due to the high risk of tissue maceration, electrohydraulic systems are no longer used.
In piezoelectric systems, polycrystalline piezoceramic-elements contract and extend due to high voltage impulses. Due to the spherical array, the generated waves are focused on the midpoint of the sphere, which focuses the energy exactly at one point. The shock waves then propagate into a water-coupling medium. The treatment can be performed without causing significant discomfort and with minimal anesthesia. Piezoelectric systems have relatively low effectivity requiring frequent treatment cycles. The use of the piezoelectric system was described in early publications, but is not favored currently ( Table 24.1 ).
| Author (year) | All Glands ( n ) | SMG ( n ) | PG ( n ) | Partial success (%) (complaints – Res stone ±) | Stone-free (%) (complaints ±) | Complete success (%) | Preservation of gland (%) | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| SMG | PG | All | SMG | PG | All | SMG | PG | All | SMG | PG | All | ||||
| Aidan et al. (1996) | 15 | 12 | 3 | 50 | 33 | 47 | – | – | – | 33 | 33 | 33 | 83 | 66 | 80 |
| Iro et al. (1998) | 76 | – | 76 | – | 26 | – | – | – | – | – | 50 | – | 93 | – | – |
| Külkens et al. (2001) | 42 | – | 42 | – | 27 | – | – | 67 | – | – | 67 | – | 100 | – | – |
| Zenk et al. (2004) | 191 | 191 | – | 15 | – | – | – | – | – | 35 | – | – | 95 | – | – |
In electromagnetic systems, pulsed current flows through a flat coil causing a fast changing magnetic field. At the magnetic membrane located above the coil, an anti-poled magnetic field is generated, causing electromagnetic repulsion of the membrane. The resulting electromagnetic waves are focused by a lens. In modern devices, the coil is arranged in a cylinder-like array ( Fig. 24.3 ). The waves generated are reflected by a parabolic reflector and transformed into spherical arrangement of waves, which are focused on one point ( Fig. 24.3A ). Due to the cylinder-like array, an inline-localization is possible e.g. by inserting an ultrasound transducer ( Fig. 24.3B ). The shock wave then propagates into a water-coupling medium. The energy enters the body on a relatively large surface at the level of the skin. It is focused on one small point, with a high density of electromagnetic energy in deeper regions ( Fig. 24.3A ). The favorable distribution of the energy onto the surface of the skin allows treatment without too high a level of discomfort. The more the energy can be focused on one distinct point (stone), the higher is the effect on the small area ( Figs 24.3 and 24.4 ). The maximum energy of the acoustic wave is present on the focal point. In this area, the maximal pressure and intensity can be measured. Its size is independent on the selected energy level ( Fig. 24.4A ). The size of the treatment zone depends on the selected energy level and is generally larger than the focal zone and therefore used in most centers ( Fig. 24.4B ). The total energy flux density changes in an over-proportional manner compared with the pre-set energy level ( Fig. 24.4C ). Due to these features and effects, electromagnetic systems are the most effective ( Table 24.2 ).
| Author (year) | All Glands ( n ) | SMG ( n ) | PG ( n ) | Partial success (%) (complaints – Res stone ±) | Stone-free (%) (complaints ±) | Complete success (%) | Preservation of gland (%) | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| SMG | PG | All | SMG | PG | All | SMG | PG | All | SMG | PG | All | ||||
| Kater et al. (1994) | 104 | 75 | 29 | 65 | 51 | 61.5 | n.n. | 35 | 48 | 38 | – | – | 86.2 | ||
| Wehrmann et al. (1994) | 73 | n.n. | n.n. | – | – | 15 | n.n. | – | – | 52 | n.n. | ||||
| Ottaviani et al. (1996) | 80 | 56 | 24 | 25 | 42 | 30 | n.n. | 41 | 58 | 46 | n.n. | ||||
| Capaccio et al. (2004) | 322 | 234 | 88 | – | – | 77 | n.n. | 36 | 69 | 45 | 97 | – | – | ||
| Eggers et al. (2005) | 38 | 22 | 16 | 23 | 31 | 26 | – | – | 50 | 4.5 | 62.5 | 29 | n.n. | ||
| McGurk et al. (2005) | 218 | 130 | 88 | 48 | 42 | 45 | n.n. | 32 | 48 | 38 | 99 | – | – | ||
| Schmitz et al. (2008) | 167 | 126 | 59 | 60 | 27 | 55 | n.n. | 26 | 39 | 31 | 74 | – | – | ||
| Escudier et al. (2010) | 142 | 78 | 64 | 37 | 31 | 35 | n.n. | 36 | 61 | 47 | n.n. | ||||
| Guerre et al. (2011) | 1571 | 1031 | 540 | – | – | 92 | – | – | 82 | – | – | 67 | n.n. | ||
| Desmots et al. (2014) | 25 | 6 | 19 | 67 | 42 | 48 | n.n. | 17 | 42 | 36 | n.n. | ||||
Currently, with the most frequently used electromagnetic system Minilith SL-1 (Storz Medical, Tägerwilen, Switzerland), the application of shock waves is performed under simultaneous ultrasound control with an inline-localization. An integrated ultrasound is used to focus the shock wave onto the stone and continuously monitors the degree of stone fragmentation during each therapeutic session, thus avoiding side effects to the surrounding tissues. The ultrasound transducer can be moved in a 3D and 360° manner to locate and fix the stone in the head and neck region and perform targeted application of shock waves ( Fig. 24.5 ). The minimum size of the electromagnetic focus is 2.4 mm in the lateral and 25 mm in the axial dimension. The pulse frequency ranges from 0.5–4 Hz. The intensity level applied per shock wave can be set from 0.5–8 and can be modified continuously, resulting in a total energy flux density of 0.15–1.5 mJ/mm 2 ( Figs 24.4C , 24.5 ).
The purpose of ESWL is to achieve fragmentation and/or at least disintegration and mobilization of salivary stones. The mobilized stones and/or fragments may be washed out of the salivary duct system spontaneously or after sialogogue-induced salivation or may be removed by interventional sialendoscopy.
Patients can most often be treated with local anesthesia (e.g., injection of Tetracaine into various sites of the buccal mucosa and overlaying skin). In addition to this, the administration of a mild opioid (e.g., Tillidine) is helpful. Anxious patients or children can be treated under general anesthesia. An earplug is inserted in each ear canal and teeth are protected (e.g., by a gauze covered by a glove inserted into the oral cavity). The patients are positioned on a dentist’s chair and ESWL is performed under ultrasound guidance ( Fig. 24.6 ).
Indications/Contraindications, Advantages/Disadvantages, Limitations, Side Effects, and Complications
ESWL treatment is less commonly indicated due to recent developments with interventional sialendoscopy, sialendoscopy-assisted surgery, and combined endoscopic transcutaneous techniques. Despite these advances, 10–20% of all stones are not sialendoscopically accessible and may be treated primarily by ESWL. The success of ESWL has increased markedly through the combination with interventional sialendoscopy or transoral duct surgery, which is helpful in difficult and/or multiple sialolithiasis. If embedded in a treatment algorithm, ESWL contributes to better overall success rates.
The only absolute contraindication in electromagnetic devices is after implantation of a cardiac pacemaker. ESWL should be delayed in case of an acute sialadenitis or any other acute inflammation of the head and neck region. Relative contraindications include the presence of a complete distal duct stenosis (which should be treated prior to ESWL or simultaneously) and pregnancy.
ESWL can be performed in nearly all cases (98%), with local anesthesia. Ease of use is enhanced if ultrasound is integrated. It can be repeated as often as required. It is an expensive device, which is currently not approved by the American Food and Drug Administration. The procedure is time-consuming, requiring repeated 30-min sessions at intervals of a few weeks. Its main limitation is that the stones often cannot be sufficiently fragmented and completely cleared by the salivary flow after ESWL alone. Residual fragments may remain within the duct and may act as nidus for recurrences. But most of these disadvantages can be overcome in the majority of cases if combined treatment is performed.
In terms of safety, only minor, transient, and self-resolving side effects have been described, including pain over the treated area (15–100%), glandular swelling (3–35%), ductal bleeding (17–71%), and cutaneous petechiae (6–55%). There have only been rare reports of acute sialadenitis with or without abscess formation (2–6%), temporary hearing impairment (2–3%), temporary tinnitus (1–2%), and the loss of tooth fillings (1%). If ESWL is performed under ultrasound guidance, risk for complications or side effects are markedly reduced.
Success Rates of ESWL: Literature Review
The most important parameter, the rate of complete stone clearance, indicates the effectiveness of ESWL as a single mode of treatment. Stone clearance after electromagnetic ESWL is reported as 26–69%, and after piezoelectric ESWL as 29–81% ( Tables 24.1 , 24.2 ). The success rate is higher in stones in the parotid gland compared with stones in the submandibular gland (electromagnetic ESWL 39–69% vs 26–42%; piezoelectric ESWL 33–81% vs 29–40%).
Following the first report by Iro et al. in 1989, further clinical experiences were reported in 1992. In 1998, a prospective trial in 76 patients with parotid stones revealed complete success (stone- and complaint-free state) in 50%; partial success in 25% (complaint-free with residual fragments); and marked improvement of symptoms in 17%.
Compared with the parotid, less successful long-term results were reported after treatment of 191 patients with submandibular stones, showing complete success in 35% and partial success in 15% (complaint-free with residual fragments; Table 24.1 ).
Capaccio et al. from the Milan group reported that complete stone clearance has been achieved in 48.9% of cases, with a hilar or intraparenchymal location in the submandibular gland. In the parotid gland, 70.6% of cases with distal and 66.7% with hilar to intraparenchymal stone location were stone-free, respectively. Sialadenectomy was performed in 3.1% of patients (all with submandibular gland stones). Recurrences were observed in four patients; all of these achieved complete ultrasonographic stone clearance ( Table 24.2 ). Moreover, univariate and multivariate statistical analyses of the findings relating to 322 of the 420 patients after a median follow-up period of 58 months revealed a favorable outcome in patients with parotid gland stones in any location compared with intraductal submandibular gland stones, in stones with a diameter of <7 mm, in patients aged <46 years, and in those receiving fewer than 2000 shock waves. Ottaviani et al. reported their experience after use of ESWL in children. Seven cases were treated, all under general anesthesia. In 71.4% (5/7), complete stone clearance was achieved (67% PG, two of three cases and 75% SMG, three of four cases).
Escudier et al. reported results from the London group. ESWL was performed in 142 patients (SMG 78; PG 64). Reported success rate for all glands was 47%; for submandibular gland, 36%; and for the parotid gland, 61%. The analysis of predictors of outcome for ESWL showed that larger stones, stone location in the submandibular gland and stones with a higher radiodensity had a worse prognosis concerning stone clearance ( Table 24.2 ).
After treatment of 1571 patients with ESWL, Guerre and Katz, from the Paris group, published long-term results. A total of 67.2% of all stones were totally fragmented after an average of six sessions; 92% of the patients were asymptomatic, and 82% of these showed no residual fragments in ultrasound examination ( Table 24.2 ).
Altogether, the literature results reveal that larger stones tend to be more prone to impaction within the parenchyma or the ductal system and therefore are more difficult to disintegrate, mobilize, or fragment by ESWL. Currently, all parotid gland stones and submandibular gland stones with a size between 7 and 10 mm, which are not accessible with the sialendoscope and/or not adequately treatable with intraductal lithotripsy, methods of transoral duct slitting, or other combined methods, are indications for ESWL.
The site-dependent cure rate may be related to different anatomic conditions. In the submandibular gland, the duct has a bend around the posterior border of the mylohyoid muscle. The lumen of the duct system, in particular the posthilar duct is often smaller with a lumen not exceeding 1 mm. Stensen’s duct in most cases is more linear with a wider lumen. Stones therefore may be less often disintegrated and mobilized after fragmentation in submandibular glands and the fragments may be washed out spontaneously more readily in the parotid gland.
These observations were also made when intraductal lithotripsy was performed. Nevertheless, even after partial stone clearance with remaining partial intraductal fragments, the effect of ESWL frequently has successful outcomes in submandibular gland sialolithiasis due to improvement of symptoms.
Another factor that has an impact on the success rates of ESWL are differences in the composition of the stones. The parotid stone is less radio-dense and mineralized. In this context, the constitution of the saliva (i.e., the predominantly serous saliva produced by the parotid gland) and better identification of stones by ultrasound in the parotid gland may be additional factors.
ESWL applied as single mode therapy can be effective. The preservation of the gland was achieved in 74–99% of patients. Complete success (stone- and complaint-free state) was reached in 29–67% (SMG 26–41%; PG 48–69%); partial success (no or at least improved complaints with/without residual stone/fragment) in 15–92%; and a stone-free state with or without complaints in 50–92% of the treated cases/glands ( Tables 24.1 , 24.2 ).
The literature shows that complete success often cannot be achieved in a substantial number of cases and that supplemental therapy in such cases may be necessary to achieve complete success. The combination of ESWL with other treatment modalities resulted in a further increase of success rates. Katz described the removal of residual fragments after ESWL by endoscopic methods in 668 cases. After ESWL was performed alone, 63% were stone-free, but in 35% of cases, fragments were removed only after application of sialendoscopic-controlled methods.
Nahlieli et al. reported results after combined therapy in 94 patients. In 32%, ESWL alone was effective, but in 29%, an additional intraductal endoscopic assistance was needed. In 39%, an additional endoscopic-assisted extraductal approach was performed after ESWL to achieve complete stone removal. Altogether, 98% of the patients remained asymptomatic after this treatment regime.
Zenk et al. published results after analyzing over 942 patients after treatment for sialolithiasis (21.9% in parotid and 78.1% in submandibular glands). In the parotid gland, 52.4% were treated by ESWL alone (79% success rate); 25.7% by ESWL in combination with interventional sialendoscopy (89% success rate); and 21.8% by interventional sialendoscopy alone (98% success rate). In the submandibular gland, no patient was treated by ESWL alone; 2.7% by ESWL in combination with interventional sialendoscopy or transoral duct surgery (94% success rate); 4.6% were treated by interventional sialendoscopy only (93% success rate); and 94% by transoral duct surgery only (90% success rate). As reported by others, the importance of ESWL proved to have more impact on treatment of parotid gland sialolithiasis.
Koch et al. reported success rates after therapy of 38 patients with difficult and/or multiple sialolithiasis by a treatment regime dominated by ESWL and intraductal pneumatic lithotripsy. Complete success was achieved in 92%. In these cases, ESWL proved to be significant, because in 95% of these, the stones were converted from sialendoscopic inaccessible/untreatable to accessible/treatable stones.
All the results cited show ESWL has specific strengths and weaknesses. ESWL can be successful as a single treatment modality. It should be part of a comprehensive treatment concept more often. It has merit for difficult sialolithiasis. Interventional sialendoscopy, including intraductal lithotripsy is currently considered the first method of choice. However, for the 10–25% of stones that cannot be accessed using a sialendoscope or any other surgical method, ESWL is the treatment of choice and can be applied successfully, in particularly under ultrasound guidance. Consequently, if a salivary gland center intends to achieve maximum success rates, ESWL must be included within a comprehensive treatment protocol.
