Patellar tendinopathy is one of the most common injuries among runners [1]. However, there is a great difference in prevalence between different sports (between 14% and 22%) and its frequency is 2 times higher in men than in women. The highest prevalence is found in sports where there is a ballistic load with a high demand on the knee extensor apparatus, suggesting that there is a correlation between an excessive mechanical load on the tendon and the appearance of symptoms [2].

Considering the available scientific evidence [3], it is not yet clear whether structural alterations in tendon tissue may have a strong correlation with pain, but does indicate that those alterations are related to a loss of tendon function, increasing the probability of developing a symptomatic picture when the alterations are present [4].

In recent years, ultrasound has been widely used in the static and dynamic evaluation of abdominal and lumbar soft tissues [5]. In addition, the high reliability and precision that it has shown for objective measurements of oral motor function by measuring masticatory muscle thickness in healthy patients [6], Rotator Cuff tendon disease [7], or Achilles or patellar tendinopathies [8, 9] has made it possible that it has progressively replaced the traditional magnetic resonance imaging (MRI) or computed tomography (CT).

Moreover, there is literature that also strongly recommends its use not only as an evaluation tool due to its low cost, safety and portability, but also because of the possibility of obtaining quantitative measurements of tissue texture (thickness, cross-sectional areas or perimeters) and as a guide for invasive interventions such as dry needling or percutaneous electrolysis in anatomical regions of high risk due to space conflict [10, 11, 12].

The therapeutic exercise is one of the most effective conservative treatments in the management of patellar tendinopathy symptomatology [13]. Isometric contractions have shown to be an effective tool to control short-term tendon pain and increase muscle activation in the early stages of treatment [14]. In order to achieve collagen remodeling and improve long-term tendon function, a more demanding exercise involving higher mechanical stress on the tendon, such as eccentric contraction and Heavy Slow Resistance (HSR), is needed [13]. It seems that the active role of the patient triggers neurological adaptations that increase neuromuscular functionality [13]. Therefore, increasing strength and motor control are established as primary objectives to achieve results in treatments.

One of the most innovative techniques today for the treatment of tendinopathies is percutaneous electrolysis due to its powerful effect in reducing pain and increasing functionality, especially in load tendons in the lower limb [15]. There is only a modest scientific evidence demonstrating the effects of electrolysis in reducing pain and increasing function in both lower and upper extremity tendon structures, which is drawing the attention of physicians and physical therapists in both clinical and scientific arenas [16, 17, 18].

Percutaneous electrolysis is produced when the flow of a galvanic current goes through a sterile needle made of surgical steel (negative electrode). This generates a dissociation of water (H+OH) which leads to an oxidation reaction with the mineral salts of the tissue, producing mostly sodium hydroxide (NaOH) [15, 16, 17, 18, 19].

The negative electrode is attributed with the capability of liquefaction, membrane permeabilization, and an increase of metabolism and local vascularization. From a molecular point of view, increased concentrations of vascular endothelial growth factor (VEGFR) and activated peroxisome proliferator receptor (PPAR), instigators and modulators of regeneration have been found [19].

One of the possible effects that has been attributed to the technique is the change in pH of the injured area [20]. However, recent publications have emerged rejecting the existence of this mentioned action mechanism, observing that the effect is not statistically significant [21].

One of the hypotheses currently handled in publications [22] investigating evident clinical effects of electrolysis, is the clearing of algogenic and inflammatory substances that have been found in degeneration tendon areas and its periphery at initial stages of tendon degeneration. In a recent systematic review, Gomez-Chiguano et al. [23] presented results of moderate scientific evidence of electrolysis for the treatment of pain and musculoskeletal dysfunction. For this reason, the aim of our study was to determine the effects of a new treatment with percutaneous electrolysis directed to peripheral tendon, with or without the presence of degenerative zone.

The 3 patients (3 males) in this case series ranged in age from 37 to 45 (mean age 40 years $\pm$ 4.35) and reported patellar tendonopathy symptoms ranging from 5 to 8 weeks (mean 6.5 months $\pm$ 2.12). The mean NPRS score at initial examination was 6.6 ($\pm$ 0.57) and the mean VISA-P score at initial examination was 25.6 ($\pm$ 5.5). The mean thickness score at initial examination was 6.1 ($\pm$ 1.49). At 3 weeks follow-up, the patients improved by a mean NPRS of 2 points ($\pm$ 1.41; p = 0.01), VISA-P of 13.14 points ($\pm$ 9.64; p = 0.003), and to a thickness of 0.03 cm ($\pm$ 1.49; p = 0.01). At 8 weeks follow-up, the patients improved by a mean NPRS of 5.6 points ($\pm$ 1; p = 0.22), VISA-P of 41 points ($\pm$ 7.1; p = 0.001), and to a thickness of 0.14 cm ($\pm$ 1.47; p = 0.01) (Table 3).

The present work provides novel evidence that percutaneous electrolysis with a less invasive approach targeting the peripheral tendon in combination with therapeutic resistance exercise diminished pain, improved functionality and showed a tendency to decreases thickness in our cases with patellar tendinopathy.

Also, we found that, by means of the ultrasound evaluation it was observed that 3 of the patients presented structural alterations in the proximal insertion of the patellar tendon, in the deepest aspect at the inferior pole of the kneecap. Despite this clinical heterogeneity, there were no significant differences in the type of symptoms presented by the patients.

Our results agree with those of the clinical trial developed by Abat et al. [15], in which two groups of 30 subjects affected by patellar tendinopathy and treated with an identical eccentric exercise program were compared, but one group was also treated with electrolysis and another with electrotherapy, which showed better results in the VISA-P scale the group treated with electrolysis.

The changes in terms of decreased pain as well as increased functionality become evident long before the improvement in tissue disorganization [17, 22, 35]. These findings align with the controversial evidence widely described in the literature of whether or not the presence of structural alterations are responsible for the appearance of tendon pain [3, 30].

Therefore, the effectiveness of electrolysis in terms of pain and functionality is due not so much to the structural recovery of the tissue but to an analgesic effect due to the stimulation of the nervous tissue. We know that the periphery of the tendon is a very relevant area in terms of accumulation of algogenic substances and proliferation of both nerve endings and vascular endings [30]. It is possible that the application of galvanic current through the needle may have a modulating effect on pain and vascularization.

The type of approach chosen for treating the patellar tendon by means of electrolysis has been repeatedly used in previous studies with positive results [15, 18]. In our current case, we have sensibly modified the approach used by Abat in which intratendinous and peritendinous approaches were made. For this reason, approaches were made exclusively on the superficial interface and on the deep interface of the tendon. The absence of structural alterations in some of the tendons diagnosed with patellar tendinopathy that come to our attention does not justify, from our point of view, the intra-tendinous approach in which a certain homeostatic alteration is generated [19]. Even in patients with structural damage to the tendon, in most cases it is found in the deepest aspect of the tendon, coinciding with the deep interface of the tendon [41, 42]. There are a number of limitations to this case series. We recognize that the sample size was small (although sufficient to determine significance), in addition to differences in age and composition of men only, and the lack of a control group. The immediate therapeutic response to treatment leads us to believe that it is due to the electrolysis treatment, since the effects of exercise take at least 3 to 6 weeks to be present. The information extracted from the patients of this study comes from the clinical field, therefore it cannot be found in the same way as the information obtained from an experimental study in terms of extracting efficacy conclusions. An additional limitation to this case series is that there was no long-term follow-up of patients after their discharge from physical therapy.

The results of this retrospective case series suggest the combination of percutaneous electrolysis and therapeutic exercise may have the potential to improve pain and disability in individuals with patelar tendinopathy. Conservative management of individuals with patelar tendinopathy pain is often challenging and and the novel treatment exposed in the present work is the least invasive known in invasive physiotherapy to date, being able to prevent patients from having to undergo surgery. The clinical outcomes observed with the first description of a less invasive approach of electrolysis than those presented to date in this case series suggest that further controlled trials of this novel treatment approach are warranted.

Conceptualization—ACMF and EASR; methodology—JHV and EASR; software—ACMF; validation—all authors; formal analysis—JHV; investigation—all authors; resources—JLAP; data curation—EASR; writing—original draft preparation—JHV; writing—review and editing—EASR, CBC, RDM, DGJ and JHV; visualization—SMP; supervision—all authors; project administration—EASR; funding acquisition—EASR. All authors have read and agreed to the published version of the manuscript.

The study protocol was approved by the Ethical Committee of European University of Madrid (reference number CIPI/20/156). Informed consent was obtained from all subjects involved in the study. Written informed consent for publication must be obtained from participating patients who can be identified (including by the patients themselves). Written informed consent has been obtained from the patient(s) to publish this paper.

The authors would like to thank the 3 patients of the study for their collaboration. The authors would like to thank Samuel Fernández Carnero for his guidance.

The publication of this work has been financed by the European University of Canary Islands, C/Inocencio García 38300 La Orotava, 38300 Tenerife, Canary Islands, Spain.

The authors declare no conflict of interest.

NPRS, Numerical Pain Rating Scale; VISA-P, Lower limb functionality was measured by a Victorian Institute of Sport Assessment questionnaire; RP, rehabilitation program.