Biomechanical effects of fascial hydrorelease: a cadaveric study

Abstract

Background

We aimed to investigate the influence of hydrorelease (HR) on the gliding resistance force between the aponeurotic fascia and epimysial fascia of tibialis anterior and between two epimysial fasciae of tibialis posterior and flexor digitorum longus using a biomechanical testing system.

Methods

In this cadaveric comparative study, 12 paired legs amputated above the knee joint from six fresh-frozen specimens were divided into two groups. The distal insertions of the target tendons of the tibialis anterior and posterior were detached and sutured to a force gauge for tension measurement during tendon pull. These tendons were representatives of the layer between the aponeurotic and epimysial fasciae of the tibialis anterior and between the epimysial fasciae of the tibialis posterior and flexor digitorum longus. For the baseline, the position where the tension of the target tendon was approximately 15 N was determined to eliminate creep. In the HR group, the baseline test position was replicated, and force was measured. The intervention was an HR injection between the specified fascial layers. The main outcome was the gliding resistance force between the aponeurotic and epimysial fasciae and between two epimysial fasciae.

Results

The resistance force between the aponeurotic and epimysial fasciae in the HR group was 6.4% lower than that in the control group (P = 0.02). The resistance force between two epimysial fasciae in the HR group was 4.3% lower than that in the control group (P < 0.01).

Conclusions

The gliding resistance force significantly decreased after HR in the layer between the aponeurotic and epimysial fasciae and between two epimysial fasciae in this cadaveric study.

Background

Hydrorelease (HR), or hydrodissection, is the ultrasound (US)-guided injection of a solution into the fascia without anesthesia [1, 2]. HR is indicated for nerve disorders and myofascial pain syndrome (MPS) [1, 2]. Physical findings of MPS include tenderness and localized hyperirritable nodules within palpable taut bands of skeletal muscle fibers and fascia [3, 4]; these are the targets of HR. Recently, US has been utilized to confirm MPS findings, such as thickening or stacking of the fascia or densification [5]. Furthermore, the solution can be injected into the inter-fascia layers, inside the aponeurotic fascia (APF), or intramuscularly using US [3, 6–7].

Several reports discuss the clinical outcomes related to significant pain reduction after HR for MPS and fascia-related disorders [6,7,8,9]. A study revealed a significant improvement in the shoulder range of motion (ROM) of flexion, extension, abduction, external rotation, and internal rotation after HR of the coracohumeral ligament in patients with a global limitation of shoulder ROM [9]. However, the mechanisms of pain relief or ROM improvement remain to be thoroughly investigated. Speculated mechanisms underlying HR in patients with MPS include improved hydration and gliding between fascial layers, as well as the release of entrapment of small nerve fibers of both A- and C-fibers [10]. However, the biomechanical effects of HR on intra-APF and between the APF and epimysium (EPI) have not been investigated. Only one cadaveric study has revealed significant reduction in mean peak gliding resistance after HR around the median nerve at the carpal tunnel [11].

Therefore, this study aimed to investigate the influence of HR on the gliding resistance force between the APF and EPI and between two EPIs using fresh-frozen cadavers and a biomechanical testing system. We hypothesized that the resistance force would decrease after HR.

Methods

Specimen Preparation

The Ethics Committee of our institution approved the study protocol for obtaining, utilizing, and disposing of fresh frozen human cadaveric legs (No. 1-2-68) in accordance with the Japan Surgical Society and Japanese Association of Anatomists Guidelines for Cadaver Dissection in Education and Research of Clinical Medicine. Twelve paired legs from six fresh-frozen specimens (mean age at death, 82.5 years; range, 72–90 years) with no apparent scars from previous surgery or leg trauma were obtained through donations to the university anatomy program. All individuals provided informed consent before death. The specimens were thawed at room temperature for at least 24 h before testing. Each specimen was positioned supine and moistened to preserve tissue integrity during the test.

The legs were amputated 20 cm above the knee joint line. The proximal end of the femoral bone was exposed for 15 cm, fixed using acrylic resin (Ostron II; GC), and poured into a cylindrical mold. The femoral cylinder was secured with an aluminum clamp, connected to the manipulator (Technology Service), and fixed to the clamp (Fig. 1a). The knee was fixed at 30° flexion using wires, and the ankle was similarly fixed in a neutral position. The distal insertions of the target tendons of the tibialis anterior and posterior were detached and sutured to a force gauge to measure the tension in real time; these tendons were selected as representatives of the layer between the APF and EPI and between two EPIs, respectively (Fig. 1b).

Fig. 1

(a) Experimental setup with a clamp, manipulator, and force gauge. Downward arrow: target point for (1) the layer between the APF and EPI for the tibialis anterior. Upward arrow: target point for (2) the layer between two EPIs of the tibialis posterior and flexor digitorum longus. (b) Schemas of the layer (1) between the APF and EPI and (2) between two EPIs. Smaller arrow: target point for (1) the layer between the APF and EPI of the tibialis anterior. Larger arrow: target point for (2) the layer between two EPIs of the tibialis posterior and flexor digitorum longus. APF: aponeurotic fascia; EPI: epimysium; HR: hydrorelease; SF: superficial fascia

US-guided injection

A 10–2 MHz linear transducer (Aixplorer v12 and SL10-2; Supersonic Imagine, Aix-en-Provence, France) was used in this study, and a US-trained orthopedic specialist conducted the examination. Injections were performed under a short-axis view using a 25 G needle and the in-plane technique (Fig. 2). The target points were between the APF and EPI of the tibialis anterior and between EPIs of the tibialis posterior and flexor digitorum longus (Fig. 1b). Four injections were administered for each target (Fig. 1a). The first injection, HR1, was administered in the middle third of the leg; this was followed by injections HR2, HR3, and HR4, each performed 2.5 cm distal to the previous one. Considering previous clinical publications about HR [7, 9], all injections comprised 2.5 mL of 0.9% saline with blue ink.

Fig. 2

Short-axis ultrasound images of ultrasound-guided injection. (a) Anterolateral aspect of the middle third of the lower leg. (b) Medial aspect of the middle third of the lower leg

Testing protocol

One side of the paired leg was assigned to the HR group and the other to the control group. First, for the HR group, as a baseline test, the position where the tension of the anterior or posterior tibialis equaled approximately 15 N (named “force 0 h” [Table 1]) was determined for each leg. During the creep removal process, the baseline test position was adjusted to maintain a force of approximately 15 N through 10 repetitions of pulling from the relaxation to the baseline test position. HR1 was exclusively performed in the HR group. In Test 1, the position of the baseline test was reproduced, and “force 1 h” was acquired. This process was repeated four times, and the values “force 1 h–4 h” were documented (Table 1). Second, for the control group, all procedures were performed except for the HR intervention, resulting in the acquisition of “force 0c–4c.” For instance, the reduction in gliding resistance due to HR1 was calculated as force 1c divided by force 0c.

Table 1 Protocol and statistical analysis

Confirmation by dissection

Within 1 min of the biomechanical tests, dissection was promptly commenced and completed in approximately 10 min to confirm where the ink was. A large U-shaped skin incision, 5 cm from the injected point, was created. The superficial fascia, APF, and EPI were carefully exposed to confirm the presence of saline in the intended layer as directed by the operator (Fig. 3a). Regarding the lateral area, the ink was in loose connective tissue between the APF and EPI of the tibialis anterior. Additionally, some property changes may have occurred in the loose connective tissue of the fascia between the APF and EPI (Fig. 3b). We named this status “lubricated fascia.”

Similarly, regarding the medial area, the ink was in loose connective tissue between the two EPIs of the tibialis posterior and flexor digitorum longus.

Fig. 3

Dissection after injection between the APF and EPI of the tibialis anterior. (a) Before separation from the leg. (b, c) After separation from the leg. The loose connective tissue of the fascia between the APF and EPI, termed “lubricated fascia,” is found. APF: aponeurotic fascia; EPI: epimysium

Statistical analysis

First, a two-factor repeated analysis of variance was performed. Subsequently, as a post hoc test, an unpaired t-test was conducted to compare the differences between two groups of equivalent tests; for instance, comparisons were performed between “force 1 h” divided by “force 0 h” and “force 1c” divided by “force 0c” (Table 1). Additionally, a paired t-test was conducted to compare the changes from the previous test within each group, such as “force 1 h” divided by “force 0 h” versus “force 2 h” divided by “force 0 h” (Table 1). All statistical analyses were performed using IBM SPSS Statistics for Windows, version 28.0 (IBM Corp., Armonk, NY, USA). Statistical significance was set at P < 0.05.

Results

Results for the tibialis anterior

Figure 4a and Table 2a show the test results for the tibialis anterior. A two-way repeated measures ANOVA was conducted to examine the change of the test (F1, F2, F3, F4) and Condition (Control, Hydrorelease) on force. About TA, the mean force for the control condition at test1, test2, test3, and test4 were 99.2% (SD = 0.7), 98.5% (SD = 0.9), 98.0% (SD = 0.8), and 97.4 (SD = 1.3), respectively. Also, the mean force for the HR condition at test1, test2, test3, and test4 were 96.1% (SD = 1.9), 93.4% (SD = 3.3), 91.9% (SD = 4.3), and 91.0 (SD = 4.6), respectively. The analysis revealed a significant main effect of Condition, F(1, 88) = 21.82, p = 0.005, ηp² = 0.81, a significant main effect of test, F(3, 88) = 20.46, p = 0.001, ηp² = 0.80, and a significant interaction between Condition and test, F(3, 88) = 7.21, p < 0.001, ηp² = 0.59. Holm’s-adjusted post hoc comparisons indicated that forces of HR group at test1, test2, test3, and test4 were significantly lower than at forces of Control group (p = 0.009, 0.012, 0.016, and 0.017). After the four HRs, the force decreased by 6.4% compared to the control group.

Results for the tibialis posterior

Figure 4b and Table 2b show the results of the tests for the tibialis posterior. A two-way repeated measures ANOVA was conducted to examine the change of the test (F1, F2, F3, F4) and Condition (Control, Hydrorelease) on force. About TP, the mean force for the control condition at test1, test2, test3, and test4 were 99.9% (SD = 0.6), 99.9% (SD = 0.8), 99.8% (SD = 1.2), and 99.1 (SD = 0.7), respectively. Also, the mean force for the HR condition at test1, test2, test3, and test4 were 97.9% (SD = 1.2), 96.7% (SD = 1.8), 95.7% (SD = 2.1), and 94.8 (SD = 2.4), respectively. The analysis revealed a significant main effect of Condition, F(1, 88) = 30.57, p = 0.003, ηp² = 0.86, a significant main effect of test, F(3, 88) = 18.94, p < 0.001, ηp² = 0.879, and a significant interaction between Condition and test, F(3, 88) = 5.18, p = 0.001, ηp² = 0.51. Holm’s-adjusted post hoc comparisons indicated that forces of HR group at test1, test2, test3, and test4 were significantly lower than at forces of Control group (p = 0.014, 0.008, 0.005, and 0.008). These findings suggest that the HR were effective in reducing forces. After the four HRs, the force decreased by 4.3% compared to the control group.

Fig. 4

Results of the tests for (a) tibialis anterior and (b) tibialis posterior. Each result is presented as a percentage of the baseline test. F: force. Vertical arrows: paired t-test; horizontal arrows: unpaired t-test; HR: hydrorelease; F: force

A post hoc power analysis was performed to determine the study’s power. The statistical power was 0.998-1.000, which was sufficient.

Table 2a Result of tibialis anterior of mean and 95%CI

Table 2b Result of tibialis posterior of means and 95%CI

Discussion

This study revealed that the gliding resistance force between the selected fasciae significantly decreased after HR. The force decreased by 6.4% after HR between the APF and EPI and by 4.3% after HR between the two EPIs.

Interpretations of the results of the resistance force

There are few related studies which is helpful to explanate the mechanism of HR or HD. Evers et al. reported that gliding resistance decreased after HD of the median nerve [11]. Similar to our study, HD was performed to loose connective tissue. They speculated the mechanical mobilization can be a possible mechanism of HD. This is the most related study. To our knowledge, there are no further publications which can explanate the mechanisms of HD or HR. So, the theoretical discussion or speculation can mainly be performed today.

When a tendon is pulled for a certain distance, the resistance force can be divided into the force pulling it and the gliding resistance between the tendon and its surrounding structures. Because HR was performed outside the EPI, the force change of the muscle fibers inside the EPI during the test was assumed to be approximately equal between the HR and control groups. Therefore, the force change may be mainly attributable to the change in the force of the gliding resistance between the APF and EPI or between two EPIs. Considering this, the reason the force decreased after HR should be discussed, focusing on the loose connective tissue between the APF and EPI or between two EPIs. Although we could not investigate this thoroughly, some property changes may have occurred in the loose connective tissue between the APF and EPI (Fig. 3). We speculated that these property changes probably occurred due to the solution and may be an important reason for the decrease in the force after HR. Additionally, another important reason is the physical separation between the APF and EPI or between two EPIs following needle insertion and injection of a certain volume of saline solution. However, the reasons for this decrease in force are speculative. Therefore, future studies are necessary to investigate the reason for the decrease in the resistance force caused by HR.

Although we have only investigated the change in force, we speculate that the decrease in the force in this study (believed to be due to the change in the gliding between the two fasciae) is one of the main reasons why HR relieves pain in MPS and similar disorders. Therefore, future studies should investigate why force decrease is related to pain relief by HR.

Clinical implications

Thickening or stacking of the fascia on US [9], densification [5], and the associated laydown of extracellular content can be observed in MPS [12,13,14]. The myofascial trigger point is a key factor in the pathophysiology of MPS [4]. Therefore, thickened fascia on US imaging and observations of tenderness should be regarded as important findings for considering HR as a treatment option [3]. In this cadaveric study, the HR areas were normal without thickened fascia or obvious MPS. This study investigated the decrease in the force in human cadavers without specific fascial alterations. Because the resistance force is expected to be higher in patients with MPS or similar pathologies, future experiments on HR using animal models with such pathologies are necessary.

Study limitations

The three main limitations of this study are as follows: First, the specimens were fresh frozen cadavers; some reactions that occur in living patients do not occur in cadavers. Thus, the behavior of the solution after injection would also be different [15]. Second, in patients with clinical MPS, some conditions, including the resistance force, would be different from those in the normal fresh-frozen cadavers used in this study. Third, the amputation of a leg and the injection of a substance can result in alterations in pressure, which may influence the spread of the substance and the effect of HR. Consequently, the findings of this study may diverge from those observed in living patients. Fourth, while it is common for cadaveric research, the use of six cadavers as the sample is not enough numbers for these experiments. Thus, the result of this experiment have to be regarded as preliminary research. Further study with more specimen will be required in the future. Fifth, since there were no previous studies that examined the same topic as this study, it was not possible to estimate the effect size before the experiment, and thus a preliminary sample size calculation could not be performed. Sample size was calculated based on the result of first and second specimens regarding the force gauge measurement of HR and control group. Because the effect size from the result was calculated over 2.1, the effect size was set at 2.0. It was determined that 6 specimens for each group were required to obtain 80% power to detect a significant difference in force measurement between the HR and control group with an alpha level of 0.05. Additionally, the statistical power of a post hoc power analysis was 0.998-1.000. Although there may be sample bias or other factors, there may be a sufficient statistical power.

Despite these limitations, this study is the first to quantitatively evaluate the resistance force on the fascia after HR using a biomechanical testing system.

Conclusions

In this preliminary cadaveric study, the gliding resistance force significantly decreased after HR in the layer between the APF and EPI or between two EPIs. Further studies on living animals and humans are necessary.