Diacutaneous Fibrolysis (DF) is an instrumentally assisted manual therapy technique which is defined as “a specific instrumental intervention for normalizing the musculoskeletal system function after a precise diagnosis and preserving the skin’s integrity” [1]. The application of this soft tissue mobilization is assisted by specially designed, hook-shaped steel instruments and applied to different musculoskeletal structures, such as the myofascia, aponeurosis, tendons, ligaments and scar tissues [1].

Based on deep transversal massage from Cyriax, DF was developed by Kurt Ekman, a Swedish physiotherapist, who assisted James Cyriax in the 1950s [2]. As a technique eminently based on the manual skills of the therapist who applies it, it has been transmitted to the present day thanks to the master–disciple model of knowledge transmission. The disciple of Kurt Ekman, Pierre Duby, a Belgian physiotherapist and professor of anatomy at the Free University of Brussels, spread the technique internationally, particularly in Europe. Most of the clinical research on DF has been carried out in Spain, where the technique was introduced by Pierre Duby in the 1990s at the University of Zaragoza. At the University of Zaragoza, José Miguel Tricás-Moreno and María Orosia Lucha López, two Spanish physiotherapists, started the diffusion of the technique in Spain.

The hook-shaped steel instruments are designed to promote a mechanical advantage by allowing deeper and more precise tissue mobilization and a reduction in stress on the fingers of the clinician [2]. Like deep transversal massage from Cyriax, DF is applied according to the tissue healing phase and transversal to the longitudinal direction of the tissue fibers to be treated [3], seeking mechanical, circulatory and neurological effects ^[4][5][4,5].

A systematic review and meta-analysis from 2021, including six randomized controlled trials performed to evaluate the effectiveness of DF on pain, range of motion and functionality in a variety of musculoskeletal disorders, stated the efficacy of the technique for pain relief and for the improvement of functionality immediately after treatment and at follow-ups of at least three weeks [6].

Moreover, there are some systematic reviews that have included DF as a potential co-adjuvant treatment for the management of a variety of musculoskeletal disorders.

Landesa-Piñeiro and Leirós-Rodríguez performed a systematic review about physiotherapy management in lateral epicondylitis [7]. Among the 19 manuscripts analyzed, one double-blind randomized controlled trial evaluating the effectiveness of DF for the treatment of chronic lateral epicondylalgia was included [8]. The authors stated that interventions including shock waves, platelet-rich plasma, ultrasounds, friction and stretching exercises, and bandages obtained better results with less sessions in pain relief. However, the dosage of the DF intervention that appears in the review is wrong, since it describes 15 sessions of DF [7] and the original manuscript specified that the dosage in the DF group was 6 sessions [8].

Haik et al. performed a systematic review of randomized controlled trials about the effectiveness of physical therapy in subacromial pain [9]. Of the 64 studies included, one double-blind randomized controlled trial evaluating the effectiveness of DF for the treatment of subacromial impingement syndrome was included [10]. The control group of the DF study received five sessions per week of treatment that consisted of therapeutic exercises, analgesic electrotherapy and cryotherapy. The inclusion of analgesic electrotherapy and cryotherapy in the control group might have influenced the positive outcomes in pain achieved in the control group that Haik et al. attributed to supervised exercises alone [9].

Piper et al. performed a systematic review about the effectiveness of soft tissue therapy for the management of musculoskeletal disorders and injuries of the upper and lower extremities [11]. They analyzed six studies due to their low risk of bias, including the double-blind randomized controlled trial, evaluating the effectiveness of DF for the treatment of subacromial impingement syndrome [10]. Regarding DF, Piper et al. stated that “statistically significant differences favored DF, as well as sham DF over multimodal care for function, shoulder extension and external rotation” [11], but in the original manuscript, it was stated that “differences between placebo and control groups were statistically significant only in extension movement” [10], and the other differences were presented between DF and multimodal care.

Finally, Yu et al., in a review about the noninvasive management of soft tissue disorders of the shoulder [12], recommended that DF should not be offered for soft tissue disorders of the shoulder after analyzing the results of the previously mentioned double-blind randomized controlled trial [10]. Although the results were similar between the group treated with DF and the control group after three months, immediately after treatment, that is, after three weeks, including six DF sessions improved the functionality and the active extension and external rotation movements compared to the control group and the differences between the placebo and control groups were statistically significant only in extension movement.

2. Potential Mechanisms of Action of Diacutaneous Fibrolysis

DF, such as Cyriax’s massage, is supposed to accelerate and ameliorate the healing process in connective tissues, through mechanisms such as hyperemia with consequent increased blood flow to the tissue and elimination of the augmented cross-linking of collagen, as well as a decrease in the preponderance of disorganized type III collagen, and mechanoreceptor stimulation [13]. During the remodeling process of the connective tissues after injury, the proliferation of fibroblasts may lead to matrix accumulation by increasing the production of collagen type I and III, unbalanced with the degradation [14]. Moreover, fibroblast dysfunction has been related with hypoxia [15]. In a typical connective tissue disorder such as Dupuytren’s disease [16], fibroblasts proliferate and differentiate into myofibroblasts with an augmented contractile force. This force is beneficial for physiological tissue remodeling but it can become excessive, leading to connective tissue contracture with a subsequent deficit for function [17]. Tendinopathy, which is understood as maladaptation to mechanical loading, has been related to a degenerative process more than to an inflammatory one [13]. Tendon degeneration has been characterized by collagen fiber disorganization, disruption and angiogenesis [18]. In joint capsules of long-term immobilized joints suffering from hypomobility, increased disorganization of fibroblasts, high collagen density and irregular collagen fibers, as well as the disappearance of adipocytes in the synovial membrane, have been found [19]. Thus, due to these underlying mechanisms related to failed remodeling in the connective tissue, it might be supposed that a reduction in cross-linking and the disorganization of collagen and better blood flow will favor adequate tissue healing. There are no studies in DF supporting the previous mechanisms nor the underlying mechanism behind them. Nevertheless, some studies developed in animal models have analyzed the tissue effects of other instrumentally assisted soft tissue mobilization approaches. They shed some light on the issue. Kami et al. developed a study to highlight the effects of mechanical massage manipulation in the recovering of a blunt muscle injury in the gastrocnemius muscle of the rat. The blunt muscle injury was induced by a hit with 1.57 joules [20]. Massage was performed for 10 min per day for 25 days [14]. Collagenous fiber increased in the non-treated group with respect to the massage group. Muscle fibrosis is characterized by the accumulation of extracellular matrix, due to an imbalance between the synthesis and degradation of matrix components. An excessive number of fibroblasts and myofibroblasts can lead to extracellular matrix augmentation. In the massage group, the apoptosis of myofibroblasts was increased and showed a more orderly arrangement of sarcomeres within one myofibril [14]. Kassolik et al. studied the effects of the spiral friction technique, applied for 6 min per day, for 60 days, in the middle part of the tail of male rats, on the dense connective tissue of the tendon [21]. The diameter of collagen fibrils was smaller in the massage group with respect to the non-treated group. Tendon tissue in young rats is characterized by a smaller fibril diameter [21]. Andrzejewski et al. studied the effects of spiral friction movements on the collagen fibers of the rear long flexor muscle of the digits tendons from rats subjected to running training for 5 min per day, for 70 consecutive days [22]. In the same way as Kassolok et al., they found a higher percentage of the smallest fibers in the tendons [22]. Davidson et al. studied the effects of instrumentally assisted soft tissue mobilization in the Achilles tendons of rats with collagenase-induced tendinopathy [23]. The tendons received friction in a longitudinal manner from distal to proximal and from proximal to distal with an aluminum instrument for 3 min, four times, on post-collagenase injection days 21, 25, 29, and 33 [23]. In the tendons that received the soft tissue mobilization technique, more fibroblasts, stimulated extracellular matrix production and fibronectin (an extracellular matrix adhesion protein) were found [23]. Gehlsen et al., using the same methodology but with different pressure levels, discovered the largest number of fibroblasts after six sessions with the heaviest pressure [24]. Again using instrumentally assisted soft tissue mobilization in the collagenase-induced injured Achilles tendons of rabbits, Imai et al. found better dynamic viscoelasticity and a larger cross-sectional area in the treated tendons [18]. Collagen was better aligned in the treated tendons and a lower proportion of type III collagen was found, suggesting a better remodeling phase [18]. In 2009, Loghmani and Warden studied the effects of instrumentally assisted cross-fiber massage on the healing process of knee medial collateral ligament injuries [25]. The cross-fiber massage (transversal to the longitudinal direction of the ligament) was started 1 week post-injury and three sessions of 1 min per week were performed. Changes were evaluated at 4 and 12 weeks post-injury. At 4 weeks, the post-injury-treated ligaments had better orientation and formation of collagen fibers, with minimal differences at 12 weeks post-injury [25]. In 2013, the same authors, with a similar methodology, found enhanced tissue perfusion due to an increase in the proportion of arteriole-sized blood vessels in the tibial third of the ligament [26]. Subtle mechanical stretching consisting of 20 to 30% strain for 10 min, twice a day, for seven days, in mouse subcutaneous connective tissue of the back was related to a lesser presence of type-1 procollagen (in vivo) [27]. Some studies have been performed in healthy human subjects to study the effects of DF on the neural underlying mechanisms of muscular relaxation empirically observed with the technique. Lévénez et al. found a reduction in passive tension, with augmentation of the maximal passive range of motion in ankle dorsiflexion and a reduction in the deep tendon reflex from the triceps surae, but not in the Hoffmann’s reflex, immediately after 10 min of DF on the gastrocnemius, the soleus and the Achilles tendon [28]. Veszely et al., with a similar methodology, found similar results but also the maintenance of the changes at the 30 min follow-up [29]. It has been stated that if the experiment was performed with cutaneous anesthesia of the treated area, the behavior of the Hoffmann’s and deep tendon reflexes did not change, thus it seems that cutaneous and aponeurotic afferents did not participate in the neural answer. Moreover, the difference in behavior of reflex responses might be due more to a change in compliance of the myofascia-tendinous structure transmitting tendon knock than to a change in neuromuscular spindle sensitivity [30]. López-de-Celis et al. evaluated the immediate effects and the effects at the 30 min follow-up of a single session of DF on the neuromuscular properties of gastrocnemius in healthy subjects. A single 10 min session of DF was applied to the same regions as in the previous studies and was implemented only to one of the lower extremities. The other extremity acted as a control. After treatment, treated gastrocnemius were less rigid, showed a more relaxed state, and its contraction velocity was diminished. No differences between limbs were observed at follow-up [31]. Leite et al. [32], conducted research to study the changes in neuromuscular properties of lateral gastrocnemius immediately after a single 10 min session of DF in one group or sham DF in the other. The DF group generated a higher force and higher degree of muscle activation, measured with electromyography after DF [32].