Data Sources and Searches

A search was conducted using the following databases: PubMed, EBSCOHost and Ovid according to the PRISMA statement (Page et al., 2021). The last search was carried out on the 5^th of June, 2021. Keywords including ‘patellar tendinopathy’, ‘tendinitis’, ‘tendinopathy’, ‘patellar tendon’, ‘enthesopathy’, ‘shockwave therapy’, ‘shock wave therapy’, ‘shock-wave therapy’, ‘extracorporeal shockwave’, ‘extracorporeal shockwave therapy’, ‘ESWT’, ‘focused extracorporeal shockwave’, ‘radial extracorporeal shockwave therapy’, ‘therapy’ and ‘treatment’ were used in various configurations. Reference lists of all the retrieved articles were manually checked for additional studies. The full search strategies for all databases, including any filters and limits used, are presented in Appendix A (Table A.1.).

Study Selection

The papers were checked for relevant content and were included based on the following criteria: (1) full-text article published in English; (2) randomized controlled trials (RCTs); (3) trials in which athletes with patellar tendinopathy were treated with ESWT; (4) trials in which controls underwent sham treatment or any other conservative treatment (e.g., exercise, injections, physical therapy); (5) trials containing the comprehensive description of ESWT (the following application variables were reported: the number of shocks, frequency, energy flux density or pressure, the number of sessions, frequency of treatment).

We excluded trials using ESWT to treat knee cartilage disorders. Studies in animals were not included either. Conference abstracts, proceedings, case reports, commentary articles, secondary analyses, designs of RCTs, systematic reviews, meta-analyses and narrative reviews were also excluded (Stania et al., 2019). The results of the study selection procedure are summarized in the PRISMA 2020 flow diagram (Figure 1).

Figure 1

PRISMA 2020 diagram flowchart for the study search and selection.

Data Extraction and Quality Assessment

Three authors (MS, TK, WM) independently searched for articles, screened studies, and extracted data in a blinded manner. Any disagreements between the authors were resolved through consensus, with other research team members (JM and PK) acting as arbiters. Data were extracted from each study on participant characteristics, intervention (type of shock wave therapy, description of procedures), outcomes (primary and secondary outcomes, methods and timing of assessment) and results.

The methodological quality of randomized clinical studies was rated with the Physiotherapy Evidence Database (PEDro) scale. The PEDro scale (ranging from 1 to 10 points) is a valid measure of the methodological quality of randomized clinical trials (de Morton, 2009).

Based on the PEDro score, the methodological quality of trials is rated as high (PEDro scores ≥7), medium (4 to 6), or low (PEDro scores ≤ 3) (Liao et al., 2018). The methodological quality of the included articles was assessed by three reviewers (MS, TK, WM). In the event of disagreement, consensus was sought by involving another researcher (JM).

Meta-analysis of effect estimates

Meta-analysis was undertaken where data were available from more than one study assessing the same outcome. The analysis included original papers which compared the outcomes of ESWT (as a monotherapy) with those of other conservative (non-surgical) therapies. Two outcomes were taken into consideration, i.e., pain reduction assessed with the Visual Analogue Scale and functional improvement verified using the Victorian Institute of Sports Assessment for Patella (VISA - P). Analyses were carried out for long-term scores obtained at ≥ 6 months of therapy completion (a long-term follow-up). It was not possibile to undertake the meta-analysis for the short-term scores as there was only one study which met the inclusion criteria.

Data in the meta-analysis were expressed as standardized mean differences (SMDs) and 95% confidence intervals (CIs). Heterogeneity was assessed with I² statistics: 25% was considered low, 50% moderate, and 75% high. Statistical significance was set at p < 0.05. The random effects model was applied for the pooled effect estimates.

Search Results

The search and databases yielded a total of 192 articles, of which only 7 met the inclusion criteria (Figure 1). The total pooled sample size of all of the included studies was 155 participants in the experimental group and 156 participants in the control group.

Quality Assessment

Table 1 summarizes the methodological quality of randomized clinical trials included in our review that were rated with the PEDro scale. The methodological quality of randomized clinical trials included in this review was rated as high (Lee et al., 2020; Vetrano et al., 2013; Zwerver et al., 2011) or medium (Cheng et al., 2019; Taunton et al., 2003; Wang et al., 2007).

Results of meta-analysis

The meta-analysis did not reveal significant differences between the ESWT and control groups regarding the Visual Analogue Scale long-term scores obtained at ≥ 6 months of therapy completion (SMD: -0.33; 95% CI: -4.64 to 3.98; p = 0.87) (Figure 2). However, significant heterogeneity was observed with respect to longterm pain reduction (I²= 98%, p < 0.0001).

Figure 2

Forest plot of visual analogue scale (VAS) scores – measurements at ≥ 6 months of therapy completion - ESWT and control groups.

No significant differences were also found between the ESWT and control groups on the pooled VISA-P scores in the long-term follow-up (SMD: 8.21; 95% CI: -39.3 to 55.73; p = 0.73; I²= 99%) (Figure 3).

Figure 3

Forest plot of Victorian Institute of Sports Assessment–Patella (VISA-P) scores – measurements at ≥ 6 months of therapy completion - ESWT and control groups.

Athletes with patellar tendinopathy – characteristics

Differences in therapeutic efficacy of ESWT could be attributed to large diversity across research materials including elite athletes during the competitive season (Lee et al., 2020; Zwerver et al., 2011), athletes in their off-season (Vetrano et al., 2013), individuals engaged in recreational sports (Wang et al., 2007) as well as patients of hospitals or sports medicine centers (Thijs et al., 2017). Each of these populations may have different etiological factors, e.g., the mechanical load on the patellar tendon in elite and non-elite athletes is much greater than that of hospital patients. Therapeutic success of ESWT is thus more difficult to reach in athletic populations as indicated by the studies of Zwerver et al. (2011) and Vetrano et al. (2013). It is also important that athletes do not engage in intensive physical activity during the therapy period. Zwerver et al. (2011) suggested that the total load on the tendon resulting from the combination of ESWT-induced collagen disorganization and the mechanical overload from sports training might be too high and the time for recovery was insufficient. Consequently, some interference with potential regenerative effects of ESWT might be expected. Therefore, some researchers recommended that their athletes would not engage in physical activity during therapy and up to six weeks following therapy completion (Vetrano et al., 2013; Vulpiani et al., 2007; Wang et al., 2007).

Researchers mainly recruited athletes with a history of over 3-month chronic patellar tendinopathy (Lee et al., 2020; Taunton et al., 2003; Wang et al., 2007; Zwerver et al., 2011). Only Thijs et al. (2017) included those who had suffered for a period of eight weeks. Timely differentiation between acute and chronic tendinopathy is essential for diagnostic and therapeutic considerations (Gabel, 1999). Zwerver et al. (2011) did not show superiority of ESWT intervention over the placebo treatment for reduction of jumper’s knee symptoms. According to the authors ESWT might fail in athletes suffering from reactive tendinopathy or early tendon dysrepair in the continuum of tendon pathology. ESWT proved beneficial in the degenerative stage of tendon disease (Zwerver et al., 2011).

Williams et al. (2017) suggested that MRI should be performed in those athletes with refractory patella tendinopathy who failed conservative treatment. MRI scans help determine the location and involvement of the fat pad. If the scans reveal intratendon changes only, ESWT might be expected to yield good outcomes. Athletes with fat pad involvement should undergo arthroscopic debridement without ESWT.

Differences in the outcomes of ESWT might also be accounted for by differences in the initial severity of jumper’s knee symptoms as assessed with the Victorian Institute of Sport Assessment–Patella questionnaire (VISA – P) (Zwerver et al., 2011).

Types of ESWT

Jumper’s knee has much more frequently been treated with focused extracorporeal shockwave therapy (Lee et al., 2020; Thijs et al., 2017; Zwerver et al., 2011) than radial ESWT (Cheng et al., 2019). Focusing of shockwaves on a small-volume tissue target requires an accurate lesion location by ultrasounds. Radial ESWT allows treatment of both well-localized and more extensive lesions (Król et al., 2009).

Van der Worp et al. (2014) compared the efficacy of radial (3 therapy sessions, 2000 pulses, 8 Hz, 2.4 bars) and focused ESWT (3 therapy sessions, 2000 pulses, 4 Hz, 0.12 mJ/mm²) in athletes with patellar tendinopathy. Both treatment approaches reduced pain complaints and improved athletes’ function. However, none of the methods proved superior.

While treating patellar tendinopathy, most researchers used three sessions of ESWT (Thijs et al., 2017; Vetrano et al., 2013; Zwerver et al., 2011), with a one-week interval in between (Cheng et al., 2019; Lee et al., 2020; Thijs et al., 2017; Zwerver et al., 2011). There were also shorter intervals of 2 days (Vetrano et al., 2013). Wang et al. (2007) only held one therapeutic session, while Cheng et al. (2019) sixteen sessions. The results of a meta-analysis revealed that duration of the intervention affected treatment results in athletes with knee tendinopathy and those with other soft tissue disorders as well (Liao et al., 2018). Long intervention periods (≥ 1 month) exhibited beneficial therapeutic effects of ESWT on reducing pain and improving functional outcomes, while shorter treatments were less effective irrespective of the type of the extracorporeal shockwave (Liao et al., 2018).

With regard to radial ESWT, the number of shocks per session was 2000 (Cheng et al., 2019; van der Worp et al., 2014); the pulse frequency was 8 Hz (van der Worp et al., 2014) or 9–12 Hz (Cheng et al., 2019). The number of focused ESWT pulses was 1500 to 2500 and the pulse frequency was 4 Hz (Lee et al., 2020; Wang et al., 2007; Zwerver et al., 2011). For neuromuscular dysfunction, the effect of shockwave therapy tended to be dose-dependent and caused symptom improvement over time (Wang et al., 2007). The intensity of radial shockwaves applied for patellar tendinopathy was 1.5–3 bars (Cheng et al., 2019; van der Worp et al., 2014), whereas in focused shockwave therapy, the energy flux density was 0.08–0.58 mJ/mm²(Lee et al., 2020; Taunton et al., 2003; Wang et al., 2007; Zwerver et al., 2011).

Vetrano et al. (2013) recommended that chronic tendinopathies should be treated with focused ESWT at an energy flux density of 0.08 to 0.17 mJ/mm². A meta-analysis of randomized controlled trials also showed that athletes with knee tendinopathies and other soft tissue disorders treated with low-energy (below 0.2 mJ/mm²) focused shockwaves exhibited higher rates of treatment success and noticeable functional improvement compared to those who received high-energy treatments. The inverse results were observed for radial shockwave therapy (Liao et al., 2018).

Depending on jumper’s knee manifestations, focused extracorporeal shockwaves were typically applied to the most painful spot within the patellar tendon (Thijs et al., 2017; Wang et al., 2007; Zwerver et al., 2010, 2011), a maximum of two specific points of tenderness (Taunton et al., 2003) or on the bone-tissue junction of the patellar or tibial insertions of the patellar tendon (Peers et al., 2003; Vetrano et al., 2013). Radial shockwaves were directed onto the distal pole of the patella and patellar tendon. The area of maximal tenderness was treated circumferentially, starting at the most painful site (Furia et al., 2013).

Klonschinski et al. (2011) concluded that application of local anesthesia inhibited the effect of low-energy extracorporeal shockwave therapy on nociceptors and therefore, should not be used during extracorporeal shockwave therapies. Otherwise, low-energy extracorporeal shockwave therapy is usually quite well tolerated (Vetrano et al., 2013; Wang et al., 2007; Zwerver et al., 2011). Only rarely have adverse effects been observed during or after ESWT interventions. Some athletes may complain of transient post-therapy skin reddening, but no bruising (Vetrano et al., 2013), moderate pain that occurs during the treatment, but resolves following treatment completion (Peers et al., 2003) or transient numbness and hypoesthesia (Wang et al., 2007).

ESWT can be used as monotherapy (Taunton et al., 2003; Wang et al., 2007; Zwerver et al., 2011) or can be combined with other treatment modalities, e.g., eccentric exercises (Lee et al., 2020; Thijs et al., 2017; van der Worp et al., 2014). However, a combination therapy had no additional effect compared to the sham-shockwave/eccentric group (Thijs et al., 2017). Those authors also warned against using very intensive eccentric exercises in combination with ESWT (Thijs et al., 2017).

Evaluations of ESWT efficacy

The efficacy of ESWT for patellar tendinopathy was mainly evaluated using subjective measures (Thijs et al., 2017; Vulpiani et al., 2007; Wang et al., 2007; Zwerver et al., 2011) such as the Visual Analog Scale for pain (VAS Pain) (Cheng et al., 2019; Furia et al., 2013; Lee et al., 2020; Peers et al., 2003) and the Victorian Institute of Sport Assessment–Patella (VISA – P) questionnaire (Lee et al., 2020; Wang et al., 2007; Zwerver et al., 2010, 2011). The VISA score is a reliable index of the severity of patellar tendinopathy and shows excellent short-term test-retest, and inter-tester reliability as well as good short-term stability (Visentini et al., 1998). It should be noted though that the Likert score (Thijs et al., 2017), the Blazine Scale and its modification (Vetrano et al., 2013; Vulpiani et al., 2007) and the Roles and Maudsley score (Furia et al., 2013; Peers et al., 2003) were also used.

Some other authors relied on objective measurements, including ultrasound examination, to evaluate the vascularization in the area of the patellar tendon (Taunton et al., 2003; Wang et al., 2007), dimension and thickness of the patellar tendon, the presence of edema or swelling within the tendon (Wang et al., 2007). Cheng et al. (2019) explored the effects of extracorporeal shockwave therapy in athletes with patellar tendinopathy using isokinetic strength tests. Tendon mechanical properties were also assessed with dynamometry (Lee et al., 2020).

Literature analysis clearly indicates a need for randomized clinical trials with objective measurements of ESWT outcomes in athletes with patellar tendinopathy.