Skip to main content
NIHPA Author Manuscripts logoLink to NIHPA Author Manuscripts
. Author manuscript; available in PMC: 2020 Jul 1.
Published in final edited form as: Transl Sports Med. 2019 Mar 1;2(4):164–172. doi: 10.1002/tsm2.78

Immediate effect of photobiomodulation therapy on Achilles tendon morphology and mechanical properties: an exploratory study

Patrick Corrigan a, Daniel H Cortes b, Karin Grävare Silbernagel a
PMCID: PMC6860369  NIHMSID: NIHMS1014931  PMID: 31742249

Abstract

Objectives:

Evaluate the immediate (within 4 hours) effects of laser-induced photobiomodulation (PBM) therapy on Achilles tendon morphology and mechanical properties in healthy and pathologic tendons.

Materials and Methods:

Twenty people with healthy Achilles tendons and twelve people with Achilles tendinopathy participated. One Achilles tendon received PBM treatment following an established protocol and the contralateral side received a placebo treatment. Achilles tendon morphology and mechanical properties were evaluated bilaterally with ultrasound imaging and continuous shear wave elastography immediately before treatment, immediately after treatment, then 2- and 4-hours after treatment.

Results:

There were no immediate effects of PBM on tendon morphology or mechanical properties when comparing the PBM-treated side and placebo-treated side within each cohort. Additionally, the effects of PBM did not differ between healthy and pathologic Achilles tendons.

Conclusion:

When treated with a laser-induced PBM treatment, healthy and pathologic Achilles tendons do not have immediate (within 4 hours) changes in tendon morphology or mechanical properties. These findings suggest that PBM therapy can be administered before other clinical treatments or high-load activities.

Keywords: Tendinopathy, Tendinitis, Laser Therapy, Material Properties, Elastography, Ultrasound

Introduction

The Achilles tendon is a collagenous tissue that is designed to efficiently store and release elastic energy during locomotor activities. The efficiency of this energy transfer relies on many factors, including the integrity of the Achilles tendon.16 Achilles tendinopathy is an overuse injury that reduces the integrity of the tendon.7,8 This manifests as fusiform thickening of the tendon and reduced mechanical properties.913 Pain during loading activities and functional impairments accompany these structural changes and lead patients to seek treatment.

Exercise therapy is currently the standard of care for treating Achilles tendinopathy.14 Throughout a long-term exercise program, the mechanical properties (e.g. strength and stiffness) of the Achilles tendon increase.15,16 Interestingly though, there appears to be immediate (within 30 minutes) and short-term (within 7 days) reductions in mechanical properties after a single bout of exercise.17,18 Taken together, these findings illustrate that the Achilles tendon is a metabolically active tissue and undergoes complex remodeling in response to exercise. There are, however, other treatments (e.g. injection therapies, shock-wave therapy, laser therapy) that are commonly used as adjuncts to exercise that may influence the remodeling process. In order to improve outcomes and tailor treatments for patients with Achilles tendinopathy, the mechanisms that explain the effectiveness of each adjunct treatment need elucidated.

Laser therapy has been investigated for the treatment of tendinopathies, with mixed outcomes in human-subjects research.19 For Achilles tendinopathy specifically, Tumilty et al20 found that after 12 weeks of treatment patients who receive exercise plus laser therapy have better outcomes than patients who receive only exercise therapy. Furthermore, Stergioulas et al21 noted that patients who are treated with only exercise therapy for 12 weeks have similar outcomes to patients who are treated with exercise plus laser therapy for 4 weeks. These positive outcomes may be attributed to the physiologic effects of laser therapy, which include reducing inflammation,22,23 and cell apoptosis,24 inhibiting activity of matrix metalloproteinases (MMPs),25 and increasing collagen synthesis26,27 and angiogenesis.28 However, changes in patient-reported symptoms (e.g. pain) have been the primary focus for evaluating the efficacy of laser therapy rather than also considering tissue-level changes.19,21,2932 This is problematic since full symptomatic recovery does not ensure full structural or functional recovery of the musculotendinous unit.33,34 In order to optimize clinical outcomes for patients with Achilles tendinopathy, there is a need to further evaluate the physiological effects of laser therapy on human tendon.

Laser therapy, similar to exercise therapy, may induce temporary changes to the tendon’s morphology and mechanical properties. This would affect the efficiency of the muscle-tendon unit and potentially increase the tendon’s susceptibility to further tissue damage. Additionally, laser therapy may affect the tendon’s response to exercise therapy if administered beforehand. Therefore, the purpose of this exploratory study is to evaluate the immediate (within 4 hours) effects of a laser-induced photobiomodulation (PBM) treatment on Achilles tendon morphology and mechanical properties in healthy and degenerative tendons. We hypothesize that changes in tendon morphology and mechanical properties will be greater in a laser-treated side compared to a placebo-treated side in people with healthy Achilles tendons and people with Achilles tendinopathy. Additionally, we hypothesize that changes in tendon morphology and mechanical properties would be greater in tendinopathic Achilles tendons compared to healthy Achilles tendons when treated with PBM.

Materials and Methods

Study Design

Two cohorts of participants were enrolled in this study. The first cohort included participants with healthy Achilles tendons and the second cohort included participants with Achilles tendinopathy. After determining each participant’s eligibility, demographics, anthropometrics, and a subjective history of current and previous lower body injuries were gathered. Achilles tendon morphology and mechanical properties were acquired bilaterally at baseline. PBM treatment was then administered to one Achilles tendon following an established protocol20 and a placebo treatment was administered to the other Achilles tendon. Tendon morphology and mechanical properties were measured bilaterally immediately after the PBM treatment, 2 hours after the PBM treatment, and 4 hours after the PBM treatment. To reduce the effects of loading between measurements, participants were asked to refrain from performing activities thought to influence mechanical properties (e.g. running, jumping, and stretching). A single, experienced evaluator performed all experimental procedures. Prior to data collection, each participant provided written consent after being thoroughly introduced to the study procedures and potential risks. This study was approved by the Institutional Review Board at the University of Delaware.

Participants

Twenty participants with healthy Achilles tendons were enrolled and their descriptive information are reported in Table 1. Healthy participants were at least 18 years old with no history of Achilles tendon injury or pain. To ensure the healthy participants had no underlying Achilles tendon pathology or tissue degeneration (i.e. asymptomatic Achilles tendon degeneration), they were screened with ultrasound imaging, a clinical evaluation performed by a licensed physical therapist, and the Victorian Institute of Sports Assessment - Achilles (VISA-A) questionnaire.35 If there were clinical signs of Achilles tendon pathology (e.g. pain to palpation, palpable tendon thickening) or tissue degeneration (e.g. fusiform thickening or hypoechoic regions visualized with ultrasound) during the screening procedures, the potential participant was excluded.

Table 1.

Demographics for healthy and tendinopathy cohorts.

Healthy Cohort (n=20) Tendinopathy Cohort (n=12)
Age (years) 29 (4) 59 (8)
Height (cm) 177 (8) 173 (11)
Weight (kg) 82 (18) 78 (12)
Sex (Male: Female) 11: 9 8: 4
Duration of Symptoms (months) N/A 29 (23) Range: 11–98
Pain to palpation (NPRS) Placebo N/A 1.4 (1.8) Range: 0–8
Laser N/A 3.7 (2.6) Range: 0–5
VISA-A Score Placebo 100 (0.2) Range: 99–100 85 (11) Range: 63–100
Laser 100 (0.2) Range: 99–100 79 (11) Range: 63–100

VISA-A– Victorian Institute of Sports Assessment- Achilles Questionnaire

NPRS– Numeric Pain Rating Scale

N/A– Not Applicable

Twelve participants with midportion Achilles tendinopathy were enrolled and their descriptive information are reported in Table 1. Participants were included in this cohort if they were at least 18 years old, had a clinical diagnosis of midportion Achilles tendinopathy, and had ultrasound-confirmed tendinosis.36 Achilles tendinosis was defined as a minimum of 2 mm of midportion tendon thickening as previously reported in the literature.10,37,38 Briefly, tendon thickening is measured by taking the difference in tendon thickness at the thickest part of the tendon and the tendon’s thickness 2 cm proximal to the osteotendinous junction.38 Participants were excluded from the tendinopathic cohort if they presented with pathology in addition to midportion Achilles tendinopathy (e.g. insertional Achilles tendinopathy, retrocalcaneal bursitis, history of Achilles tendon rupture). These diagnoses were ruled out by screening with ultrasound imaging and a clinical evaluation. Exclusion criteria for both cohorts included currently taking heat or light sensitive medications, an open wound in the treatment area, a diagnosis of peripheral neuropathy, or previously received laser therapy to either Achilles tendon.

Laser Treatment

Laser-induced PBM treatment was administered to one Achilles tendon and a placebo treatment to the other Achilles tendon. The order of treatment was randomized. The placebo treatment was identical to the actual treatment, except the finger-trigger that activates the laser was not depressed. Participants were blinded to treatment. Blinding was maintained during the treatment by positioning participants in prone and having them wear noise-cancelling headphones with white noise playing. These procedures ensured that the participant could not see when the finger-trigger was depressed or hear the beeping that occurs when the laser is active.

For the healthy cohort, the side that received treatment (i.e. right or left) was determined with a computer-generated randomization scheme. This resulted in 11 dominant and 9 non-dominant limbs being treated. Limb dominance was defined as the participants reported preferred kicking leg. For the tendinopathic cohort, the side that received treatment was the side with greater local tendon thickening, which is also typically the side of worse symptoms. This resulted in 4 dominant and 8 non-dominant limbs being treated in the tendinopathic cohort.

PBM treatment was delivered following a previously reported protocol.20 This protocol was selected since it has been shown to improve clinical outcomes for patients with Achilles tendinopathy.20 Briefly, a LightForce EX unit (LiteCure LLC, Newark, DE, USA) was used for treatment. The tendon was irradiated with an in-contact, constant scanning motion (2.5–5 cm/second per manufacturer) from the osteotendinous junction of the Achilles tendon to 10 cm proximal. Ultrasound imaging was used to locate the osteotendinous junction and a tape measure was used to measure 10 cm proximal. Marks were made on the skin to visualize the treatment area. The medial, lateral, and posterior aspects of the Achilles tendon were each treated for 30 seconds (i.e. total treatment time of 1 minute and 30 seconds).

The PBM treatment was administered with a 3 cm2 ball applicator. A blend of 810:980 nm laser light in a 20:80 power ratio was used. The total treatment area was 67.5 cm2, which was calculated from the length of the treatment area (10 cm), beam diameter (1.95 cm), and the area of the laser end caps (3.0 cm2 for each aspect). The power output was 10 W pulsed at a frequency of 100 Hz, resulting in an average power output of 5 W. This led to an average power density of 74.07 mW/cm2. Applying the treatment for 1 minute and 30 seconds resulted in a total dose of 450 Joules (150 Joules per aspect) at 6.66 J/cm2.

Tendon Morphology

Achilles tendon morphology was evaluated with B-mode ultrasound imaging using a LOGIQ e ultrasound system (GE Healthcare, Chicago, IL, USA) with a wide-band linear array probe (5.0–13.0 MHz). Participants were positioned prone with their feet hanging naturally over the edge of a treatment table. Images were taken at the same location along the length of the Achilles tendon at each time point by marking and recording the distance from the osteotendinous junction of the Achilles tendon to the center of the treatment area. For the healthy cohort, images were acquired immediately distal to the myotendinous junction of the soleus. For the tendinopathic cohort, the location of greatest tendon thickness was used on the PBM-treated side and an anatomically matched location was used on the placebo-treated side.

Three long- and short-axis images of each Achilles tendon were taken at each time point to measure Achilles tendon thickness and cross-sectional area. Ultrasound images were exported from the ultrasound scanner to an external computer where measurements were made with OsiriX imaging software (Pixmeo SARL, Bernex, Switzerland). Measurements were taken in a random order by a single assessor who was blinded to treatment. Intra-rater reliability for measuring Achilles tendon thickness and cross-sectional area using ultrasound imaging has been found to be excellent.39

Tendon Mechanical Properties

Achilles tendon mechanical properties were quantified with continuous shear wave elastography (cSWE) bilaterally at the same location of morphologic measurements. cSWE has been shown to be a valid and reliable method for quantifying Achilles tendon mechanical properties and is described in detail elsewhere.40 Briefly, an external actuator (Mini-Shaker Type 4810, Bruel & Kjaer, Norcross, GA, USA) generates a series of continuous shear waves that propagate along the length of the Achilles tendon at six known frequencies (322, 358, 402, 460, 536, 643Hz). While these waves propagate, a SonixMDP Q+ ultrasound system (Ultrasonix, Vancouver, Canada) with a L14–5/38 probe and a 128-channel data acquisition device collect raw radiofrequency data (frame rate=6438 frames/s). These data are used to track tendon displacement and estimate wave speed. Static shear modulus and viscosity of the tendon are then calculated on a pixel-wise basis using viscoelastic modeling. Static shear modulus and viscosity is then averaged for the selected region of interest (i.e. the Achilles tendon), which is identified on a B-mode ultrasound image. Lastly, a dynamic shear modulus at a frequency of 400 Hz was calculated using static shear modulus, viscosity and the Voigt model. Dynamic shear modulus is calculated since it is more comparable to elastography techniques that estimate tendon mechanical properties by measuring the group velocity of a broadband shear wave (e.g. Supersonic Shear Imaging).

During cSWE procedures, participants were secured to a platform in 10° of ankle dorsiflexion. The dorsiflexed angle ensured that the tendon was within the linear elastic region (i.e. constant elastic modulus). The feet were secured to minimize muscle activity and movement. Three trials on each tendon were performed at each time point and the average was used for analysis.

Statistical Analysis

Descriptive data are reported as means and standard deviations (SD). The variables of interest were tendon thickness, cross-sectional area, static shear modulus, viscosity, and dynamic shear modulus. The immediate (within 4 hours) effects between laser- and placebo-treated sides were compared with 2 (laser-treated side vs. placebo-treated side) x 4 (time) repeated measures ANOVA for each variable of interest. This analysis was performed separately for each cohort (i.e. tendinopathic and healthy). The immediate effects between healthy and tendinopathic tendons treated with laser were compared with a 2 (laser-treated side tendinopathic cohort vs. laser-treated side healthy cohort) x 4 (time) mixed model ANOVA for each variable of interest. For all analyses, Greenhouse-Geisser corrections were applied when assumptions of ANOVA were violated. In addition to raw values, changes in tendon morphology and mechanical properties were calculated as the difference between baseline and each follow-up time point. Changes were compared against the minimal detectible change (MDC95%) for each variable of interest. MDC95% for each variable are as follows: Static shear modulus= 5.1 kPa; Viscosity= 3.0 Pa*s; Dynamic shear modulus= 11.0 kPa; Tendon Thickness= 0.089 mm; Cross-sectional area= 0.009 cm2.

Results

For the healthy cohort, descriptive data obtained at baseline and each post-treatment time point are presented in Table 2. There were no significant treatment by time interactions for measures of tendon morphology or mechanical properties (p=0.273–0.922; η2partial=0.008–0.066) for the healthy cohort. On the placebo-treated side, there were no changes in tendon morphology or mechanical properties greater than the MDC95% (Figure 15). On the laser-treated side, there was an increase in tendon thickness and a decrease in cross-sectional area detected 2-hours after treatment and a decrease in tendon thickness 4-hours after treatment that exceeded the MDC95%. There were however no changes in mechanical properties greater than the MDC95% on the laser-treated side.

Table 2.

Achilles tendon morphology and mechanical properties at baseline and after photobiomodulation therapy in participants with healthy Achilles tendons.

Baseline Immediate 2 hours 4 hours p-value Interaction Effect (η2partial)
Thickness (mm) Placebo 4.645 (0.551) 4.646 (0.607) 4.628 (0.563) 4.627 (0.601) 0.273 0.066
Laser 4.731 (0.661) 4.704 (0.673) 4.842 (0.610) 4.625 (0.586)
Tendon CSA (cm2) Placebo 0.553 (0.128) 0.556 (0.129) 0.549 (0.115) 0.552 (0.112) 0.922 0.008
Laser 0.562 (0.132) 0.558 (0.129) 0.552 (0.123) 0.556 (0.123)
Static Shear Modulus (kPa) Placebo 95.0 (16.0) 94.6 (15.8) 94.8 (15.4) 94.0 (13.0) 0.819 0.013
Laser 90.9 (14.7) 90.4 (14.9) 94.1 (10.7) 93.0 (12.5)
Viscosity (Pa*s) Placebo 57.4 (13.4) 55.5 (12.4) 58.4 (13.3) 58.5 (11.5) 0.886 0.011
Laser 57.1 (10.7) 57.8 (13.1) 58.5 (10.1) 57.3 (8.9)
Dynamic Shear Modulus (kPa) Placebo 226.8 (49.4) 218.0 (46.2) 230.7 (49.0) 230.6 (42.2) 0.812 0.017
Laser 224.4 (39.0) 227.6 (49.2) 229.2 (37.4) 225.0 (32.2)

CSA– Cross-sectional area

Figure 1.

Figure 1.

Change in Achilles tendon thickness in the four-hour period following photobiomodulation therapy.

Figure 5.

Figure 5.

Change in Achilles tendon dynamic shear modulus in the four-hour period following photobiomodulation therapy.

For the tendinopathic cohort, descriptive data obtained at baseline and each post-treatment time point are presented in Table 3. There were no significant treatment by time interactions for measures of tendon morphology or mechanical properties (p=0.316–0.752; η2partial=0.035–0.100) for the tendinopathic cohort. On the placebo-treated side, there was a decrease in tendon thickness 2-hours and 4-hours after treatment (Figure 1) and a decrease in cross-sectional area immediately after treatment and 4-hours after treatment (Figure 2). There were no changes in mechanical properties greater than the MDC95% on the placebo side (Figure 35). On the laser-treated side, there was a decrease in cross-sectional area at all time points after treatment that exceeded the MDC95% (Figure 2) and a decrease in static shear modulus 4-hours after treatment that exceeded the MDC95% (Figure 3). There were no changes in tendon thickness, viscosity, or dynamic shear modulus greater than the MDC95% on the laser-treated side (Figure 1, 4, and 5).

Table 3.

Achilles tendon morphology and mechanical properties at baseline and after photobiomodulation therapy in participants with Achilles tendinopathy.

Baseline Immediate 2 hours 4 hours p-value Interaction Effect (η2partial)
Thickness (mm) Placebo 6.531 (1.301) 6.487 (1.206) 6.366 (1.210) 6.390 (1.514) 0.455 0.070
Laser 8.585 (2.364) 8.548 (2.391) 8.657 (2.634) 8.611 (2.504)
Tendon CSA (cm2) Placebo 0.757 (0.234) 0.745 (0.224) 0.752 (0.219) 0.740 (0.206) 0.316 0.100
Laser 1.102 (0.445) 1.048 (0.418) 1.062 (0.440) 1.049 (0.431)
Static Shear Modulus (kPa) Placebo 95.5 (19.7) 99.9 (16.8) 97.6 (12.6) 95.5 (19.4) 0.381 0.088
Laser 103.2 (21.5) 98.6 (16.5) 102.6 (18.4) 95.4 (19.7)
Viscosity (Pa*s) Placebo 54.7 (10.7) 54.9 (12.1) 54.3 (14.3) 54.9 (13.7) 0.729 0.038
Laser 53.6 (10.6) 52.6 (10.3) 51.3 (12.0) 56.5 (10.9)
Dynamic Shear Modulus (kPa) Placebo 216.7 (38.1) 217.5 (42.8) 215.7 (49.8) 217.5 (50.9) 0.752 0.035
Laser 212.9 (41.1) 209.0 (37.9) 204.8 (44.2) 221.7 (42.1)

CSA– Cross-sectional area

Figure 2.

Figure 2.

Change in Achilles tendon cross-sectional area in the four-hour period following photobiomodulation therapy.

Figure 3.

Figure 3.

Change in Achilles tendon static shear modulus in the four-hour period following photobiomodulation therapy.

Figure 4.

Figure 4.

Change in Achilles tendon viscosity in the four-hour period following photobiomodulation therapy.

When comparing the immediate effects between healthy and tendinopathic tendons treated with laser, there were no significant group by time interaction effects for mechanical properties (Dynamic shear modulus: p=0.586; η2partial=0.021, Static shear modulus: p=0.461; η2partial=0.028, Viscosity: p=0.514; η2partial=0.025). Additionally, there were no significant group by time interaction effects for measures of tendon morphology (tendon thickness: p=0.673; η2partial=0.016, cross-sectional area: p=0.062; η2partial=0.084).

Discussion

This exploratory study aimed to evaluate the immediate (within 4 hours) effects of a laser-induced PBM treatment on Achilles tendon morphology and mechanical properties. The main finding was that for both healthy and tendinopathy cohorts there were no differences between the laser-treated and placebo-treated sides over time. Additionally, the effects of PBM treatment did not differ between healthy and tendinopathic Achilles tendons. However, since there was a reduction in tendon cross-sectional area that exceeded the MDC95% following PBM that was not seen with the placebo treatment, the effect of PBM on tendon cross-sectional area warrants further investigation.

Research on laser therapy is currently limited to basic research that use animal models and clinical research that does not necessarily investigate the physiologic mechanisms responsible for positive outcomes. Since increased collagen synthesis26,27 and decreased cell apoptosis24 are proposed physiologic effects of laser therapy, measuring changes in tendon mechanical properties following laser treatment may be critical to improve dosage guidelines and elucidate mechanisms responsible for clinical outcomes. The current study used cSWE to evaluate the immediate (within 4 hours) effects of a single laser treatment on human Achilles tendon mechanical properties. This was the first step to evaluate the physiologic effects of laser therapy on tendon mechanical properties. Future research is needed to understand how tendon mechanical properties respond throughout a typical laser therapy program since positive clinical outcomes are commonly reported with longer follow-up periods (e.g. 12 weeks) and programs that involve multiple treatments (e.g. 2 treatments per week for 4 weeks).20,31,32

Tendon morphology is frequently evaluated with ultrasound and magnetic resonance imaging in clinical research and relates to symptom severity and physical activity levels in patients with Achilles tendinopathy.38 Clinical trials have reported positive changes in tendon morphology (i.e. reduction in tendon size) and pain when treated with exercise therapy.41,42 Additionally, Tumilty et al.20 found reductions in tendon thickness following a treatment program that incorporated both exercise and laser therapy. Findings from these studies 20,41,42 suggest that treatment of Achilles tendinopathy should focus on normalizing tendon structure, yet a previous systematic review concluded that improvements in symptoms and function cannot be explained by changes in tendon structure.34 Similar to Tumilty et al., the current study found that laser treatment led to reductions in tendon cross-sectional area that exceeded measurement error in participants with Achilles tendinopathy. This indicates that tendon cross-sectional area may be a variable of interest for monitoring treatment response in patients with Achilles tendinopathy.

In addition to changes in tendon morphology, laser therapy has previously been shown to reduce short- and long-term pain following treatment.20,21,29 Reductions in pain may be due to the anti-inflammatory effects of laser therapy or remodeling of the tendon’s structure. Evidence also suggests that laser therapy may increase pain-pressure threshold.22 Collectively, it appears advantageous to administer laser treatment early in a treatment session to reduce pain so that more loading exercises can be performed. To investigate this notion further, we performed a secondary analysis evaluating the immediate effects of PBM on pain. We hypothesized that pain to palpation would decrease on the laser-treated side in patients who presented with palpatory pain (n=10). At each of the four time points a single examiner palpated the midportion of the Achilles tendon using a pinch-grip technique moving up the tendon from distal to proximal. Participants were prone and asked to report the greatest amount of pain experienced during palpation. Pain was reported using the Numeric Pain Rating Scale (NPRS), which is an eleven-point scale ranging from 0 (no pain) to 10 (worst imaginable pain).43 This technique has been previously reported in Achilles tendinopathy literature and has good intra-rater reliability.44 The immediate effects of PBM on pain to palpation was analyzed with a one-way repeated measures ANOVA. Changes in pain to palpation were also calculated and compared against a minimal clinically important difference (MCID) of two points on the NPRS.45 We found no immediate effects of PBM on pain to palpation (Baseline: 4.4(2.1), Immediate: 3.9(2.2), 2-hours: 3.3(1.9), 4-hours: 3.4(1.8); p=0.07; η2partial=0.22). Additionally, reductions in pain were smaller than the MCID. These results suggest that a single PBM treatment does not lead to clinically meaningful reductions in pain to palpation in patients with Achilles tendinopathy. This contradicts Bjordal et al22 who showed an immediate increase in pain-pressure threshold following a low-level laser treatment. However, comparing our findings is difficult since there were major differences in study designs (i.e. subjects who participated in Bjordal et al22 performed pain inducing activity before treatment), laser treatment parameters, and methods for assessing pain. Further research is needed to investigate the analgesic effects of laser therapy.

There are several limitations to this study. One limitation is how power density was calculated for the laser treatment. The World Association of Laser Therapy recommends a power density of less than 100mW/cm2 for irradiating the Achilles tendon,46 but it is unclear if this should be calculated over the spot size of the laser or over the treatment area. A power density of 74.07 mW/cm2, which was calculated over the entire treatment area, was used for the PBM treatment in the current study. This calculation however neglects to consider that the laser treatment targeted the same tissue from three different directions (i.e. medial, lateral and posterior) and therefore could be considered to have a power density of 222mW/cm2. Regardless of how power density was calculated, the PBM treatment parameters were based on Tumilty et al.20, who demonstrated positive clinical outcomes in patients with Achilles tendinopathy. A second limitation is heterogeneity within the tendinopathic cohort. Participants with and without pain to palpation were included in the tendinopathic cohort and there was large variability in the patient’s duration of symptoms. This may partially explain why the short-term effects of PBM treatment did not differ from the effects of a placebo treatment. However, the relatively high VISA-A scores indicate a conservative sample was used (i.e. greater changes might be expected in a sample with more severe pathology). A third limitation is that the tendinopathic cohort had a relatively small sample size and an a priori power analysis was not completed. A post-hoc power analysis was performed using the determined interaction effect for tendon cross-sectional area in the tendinopathic cohort (η2partial=0.1) with α=0.05 and 80% power. This showed that a sample size of 36 subjects would be needed to detect differences between PBM and placebo treatments in the tendinopathic cohort. This study was however the first study to explore the immediate effects of laser therapy with innovative techniques and therefore an a priori power analysis was not feasible. Lastly, the healthy and tendinopathic cohorts were not matched for possible covariates (e.g. age, sex). Therefore, the comparison between healthy and tendinopathic tendon treated with PBM treatment should be interpreted with caution.

Conclusion

Laser therapy does not have immediate (within 4 hours) effects on Achilles tendon morphology or mechanical properties when administered with the presented laser parameters. This was found in both healthy and degenerative Achilles tendons. Findings suggest that tendon cross-sectional area may be a variable of interest for monitoring treatment response in future research and that laser therapy can be administered at any time during a clinical treatment session without influencing other treatments.

Perspectives

This is the first study to investigate the effects of a laser-induced photobiomodulation therapy on human Achilles tendon structural properties. Our findings suggest that a single treatment does not have immediate effects on morphology or mechanical properties in healthy and pathologic tendons. In order to optimize outcomes for patients with Achilles tendinopathy, further research is needed to understand the mechanisms that explain the efficacy of laser therapy.

Figure 6.

Figure 6.

Legend for Figures 15.

Acknowledgements

Research reported in this publication was supported by the National Institute of Arthritis and Musculoskeletal and Skin Diseases and the National Institute of General Medicine Sciences of the National Institutes of Health under Award Numbers R01-AR072034-01A1, R21-AR067390 and P30-GM103333. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. This research was also supported by the Delaware Bioscience Center for Advanced Technology.

Footnotes

Conflict of Interest Statement

The authors have no conflicts of interests to disclose. The laser equipment used in this study was provided by LiteCure LLC as part of a grant received from the Delaware Bioscience Center for Advanced Technology.

References

  • 1.Lichtwark GA, Wilson AM. Optimal muscle fascicle length and tendon stiffness for maximising gastrocnemius efficiency during human walking and running. J Theor Biol 2008;252(4):662–673. [DOI] [PubMed] [Google Scholar]
  • 2.Lichtwark GA, Wilson AM. Is Achilles tendon compliance optimised for maximum muscle efficiency during locomotion? J Biomech 2007;40(8):1768–1775. [DOI] [PubMed] [Google Scholar]
  • 3.Lichtwark GA, Barclay CJ. The influence of tendon compliance on muscle power output and efficiency during cyclic contractions. J Exp Biol 2010;213(5):707–714. [DOI] [PubMed] [Google Scholar]
  • 4.Lichtwark GA, Wilson AM. In vivo mechanical properties of the human Achilles tendon during one-legged hopping. J Exp Biol 2005;208(Pt 24):4715–4725. [DOI] [PubMed] [Google Scholar]
  • 5.Arampatzis A Influence of the muscle-tendon unit’s mechanical and morphological properties on running economy. J Exp Biol 2006;209(17):3345–3357. [DOI] [PubMed] [Google Scholar]
  • 6.Albracht K, Arampatzis A. Exercise-induced changes in triceps surae tendon stiffness and muscle strength affect running economy in humans. Eur J Appl Physiol 2013;113(6):1605–1615. [DOI] [PubMed] [Google Scholar]
  • 7.Kannus P, Józsa L. Histopathological changes preceding spontaneous rupture of a tendon. A controlled study of 891 patients. J Bone Joint Surg Am 1991;73(10):1507–1525. [PubMed] [Google Scholar]
  • 8.Järvinen M, Józsa L, Kannus P, Järvinen TL, Kvist M, Leadbetter W. Histopathological findings in chronic tendon disorders. Scand J Med Sci Sports 1997;7(2):86–95. [DOI] [PubMed] [Google Scholar]
  • 9.van Dijk CN, van Sterkenburg MN, Wiegerinck JI, Karlsson J, Maffulli N. Terminology for Achilles tendon related disorders. Knee Surg Sport Traumatol Arthrosc 2011;19(5):835–841. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 10.Chang Y-J, Kulig K. The neuromechanical adaptations to Achilles tendinosis. J Physiol 2015;15:3373–3387. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 11.Arya S, Kulig K. Tendinopathy alters mechanical and material properties of the Achilles tendon. J Appl Physiol 2010;108(3):670–675. [DOI] [PubMed] [Google Scholar]
  • 12.Child S, Bryant AL, Clark R a, Crossley KM. Mechanical properties of the achilles tendon aponeurosis are altered in athletes with achilles tendinopathy. Am J Sports Med 2010;38(9):1885–1893. [DOI] [PubMed] [Google Scholar]
  • 13.Obst SJ, Heales LJ, Schrader BL, et al. Are the Mechanical or Material Properties of the Achilles and Patellar Tendons Altered in Tendinopathy? A Systematic Review with Meta-analysis. Sport Med 2018;(0123456789):1–20. [DOI] [PubMed] [Google Scholar]
  • 14.Martin RL, Chimenti R, Cuddeford T, et al. Achilles Pain, Stiffness, and Muscle Power Deficits: Midportion Achilles Tendinopathy Revision 2018. J Orthop Sport Phys Ther 2018;48(5):A1–A38. [DOI] [PubMed] [Google Scholar]
  • 15.Bohm S, Mersmann F, Arampatzis A. Human tendon adaptation in response to mechanical loading: a systematic review and meta-analysis of exercise intervention studies on healthy adults. Sport Med - Open 2015;1(1):7. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 16.Wiesinger H-P, Kösters A, Müller E, Seynnes OR. Effects of Increased Loading on In Vivo Tendon Properties: A Systematic Review. Med Sci Sports Exerc 2015;47(9):1885–1895. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 17.Obst SJ, Barrett RS, Newsham-West R. Immediate effect of exercise on Achilles tendon properties: Systematic review. Med Sci Sports Exerc 2013;45(8):1534–1544. [DOI] [PubMed] [Google Scholar]
  • 18.Tardioli A, Malliaras P, Maffulli N. Immediate and short-term effects of exercise on tendon structure: biochemical, biomechanical and imaging responses. Br Med Bull 2012;103(1):169–202. [DOI] [PubMed] [Google Scholar]
  • 19.Tumilty S, Munn J, et al. Low Level Laser Treatment of Tendinopathy: a systematic review with meta-analysis. Photomed Laser Surg 2010;28(1):3–16. [DOI] [PubMed] [Google Scholar]
  • 20.Tumilty S, Mani R, Baxter GD. Photobiomodulation and eccentric exercise for Achilles tendinopathy: a randomized controlled trial. Lasers Med Sci 2016;31(1):127–135. [DOI] [PubMed] [Google Scholar]
  • 21.Stergioulas A, Stergioula M, Aarskog R, Lopes-Martins R a B, Bjordal JM. Effects of low-level laser therapy and eccentric exercises in the treatment of recreational athletes with chronic achilles tendinopathy. Am J Sports Med 2008;36(5):881–887. [DOI] [PubMed] [Google Scholar]
  • 22.Bjordal JM, Lopes-Martins R a B, Iversen VV. A randomised, placebo controlled trial of low level laser therapy for activated Achilles tendinitis with microdialysis measurement of peritendinous prostaglandin E2 concentrations. Br J Sports Med 2006;40(1):76–80. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 23.Marcos RL, Leal-Junior ECP, Arnold G, et al. Low-level laser therapy in collagenase-induced Achilles tendinitis in rats: Analyses of biochemical and biomechanical aspects. J Orthop Res 2012;30(12):1945–1951. [DOI] [PubMed] [Google Scholar]
  • 24.Carnevalli CMM, Soares CP, Zângaro RA, Pinheiro ALB, Silva NS. Laser Light Prevents Apoptosis on Cho K-1 Cell Line. J Clin Laser Med Surg 2003;21(4):193–196. [DOI] [PubMed] [Google Scholar]
  • 25.Marcos RL, Arnold G, Magnenet V, Rahouadj R, Magdalou J, Lopes-Martins RÁB. Biomechanical and biochemical protective effect of low-level laser therapy for achilles tendinitis. J Mech Behav Biomed Mater 2014;29:272–285. [DOI] [PubMed] [Google Scholar]
  • 26.Reddy GK, Stehno-Bittel L, Enwemeka CS. Laser photostimulation of collagen production in healing rabbit Achilles tendons. Lasers Surg Med 1998;22:281–287. [DOI] [PubMed] [Google Scholar]
  • 27.Wood VT, Pinfildi CE, Neves MAI, Parizoto NA, Hochman B, Ferreira LM. Collagen changes and realignment induced by low-level laser therapy and low-intensity ultrasound in the calcaneal tendon. Lasers Surg Med 2010;42(6):559–565. [DOI] [PubMed] [Google Scholar]
  • 28.Salate ACB, Barbosa G, Gaspar P, et al. Effect of In-Ga-Al-P diode laser irradiation on angiogenesis in partial ruptures of Achilles tendon in rats. Photomed Laser Surg 2005;23(5):470–475. [DOI] [PubMed] [Google Scholar]
  • 29.Takenori A, Ikuhiro M, Shogo U, et al. Immediate pain relief effect of low level laser therapy for sports injuries: Randomized, double-blind placebo clinical trial. J Sci Med Sport 2016;19(12):980–983. [DOI] [PubMed] [Google Scholar]
  • 30.Bjordal JM, Couppe C, Ljunggren AE. Low Level Laser Therapy for Tendinopathy. Evidence of A Dose–Response Pattern. Phys Ther Rev 2001;6(2):91–99. [Google Scholar]
  • 31.Tumilty S, McDonough S, Hurley D a., Baxter GD. Clinical effectiveness of low-level laser therapy as an adjunct to eccentric exercise for the treatment of Achilles’ tendinopathy: A randomized controlled trial. Arch Phys Med Rehabil 2012;93(5):733–739. [DOI] [PubMed] [Google Scholar]
  • 32.Stergioulas A, Stergioula M, Aarskog R, Lopes-Martins RA, Bjordal JM. Effects of Low-Level Laser Therapy and Eccentric Exercises in the Treatment of Recreational Athletes with Chronic Achilles Tendinopathy. Am J Sports Med 2008;36(5):881–887. [DOI] [PubMed] [Google Scholar]
  • 33.Silbernagel KG, Thomeé R, Eriksson BI, Karlsson J. Full symptomatic recovery does not ensure full recovery of muscle-tendon function in patients with Achilles tendinopathy. Br J Sports Med 2007;41(4):276–280. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 34.Drew BT, Smith TO, Littlewood C, Sturrock B. Do structural changes (eg, collagen/matrix) explain the response to therapeutic exercises in tendinopathy: a systematic review. Br J Sports Med 2014;48(12):966–972. [DOI] [PubMed] [Google Scholar]
  • 35.Robinson JM, Cook JL, Purdam C, et al. The VISA-A questionnaire: a valid and reliable index of the clinical severity of Achilles tendinopathy. Br J Sports Med 2001;35(5):335–341. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 36.Maffulli N, Kenward MG, Testa V, Capasso G, Regine R, King JB. Clinical diagnosis of Achilles tendinopathy with tendinosis. Clin J Sport Med 2003;13(1):11–15. [DOI] [PubMed] [Google Scholar]
  • 37.Pförringer W, Pfister A, Kuntz G. The Treatment of Achilles Paratendinitis: results of a double-blind, placebo-controlled study with a deproteinized hemodialysate. Clin J Sport Med 1994;4(2):92–99. [Google Scholar]
  • 38.Corrigan P, Cortes DH, Pontiggia L, Silbernagel KG. The degree of tendinosis is related to symptom severity and physical activity levels in patients with midportion Achilles tendinopathy. Int J Sports Phys Ther 2018;13(2):196–207. [PMC free article] [PubMed] [Google Scholar]
  • 39.McAuliffe S, McCreesh K, Purtill H, O’Sullivan K. A systematic review of the reliability of diagnostic ultrasound imaging in measuring tendon size: Is the error clinically acceptable? Phys Ther Sport 2017;26:52–63. [DOI] [PubMed] [Google Scholar]
  • 40.Cortes DH, Suydam SM, Silbernagel KG, Buchanan TS, Elliott DM. Continuous Shear Wave Elastography: A New Method to Measure Viscoelastic Properties of Tendons in Vivo. Ultrasound Med Biol 2015;41(6):1518–1529. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 41.Ohberg L, Lorentzon R, Alfredson H. Eccentric training in patients with chronic Achilles tendinosis: normalised tendon structure and decreased thickness at follow up. Br J Sports Med 2004;38(1):8–11. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 42.Shalabi A, Kristoffersen-Wilberg M, Svensson L, Aspelin P, Movin T. Eccentric Training of the Gastrocnemius-Soleus Complex in Chronic Achilles Tendinopathy Results in Decreased Tendon Volume and Intratendinous Signal as Evaluated by MRI. Am J Sports Med 2004;32(5):1286–1296. [DOI] [PubMed] [Google Scholar]
  • 43.Williamson A, Hoggart B. Pain:a review of three commonly used rating scales. J Clin Nurs 2005;14(7):798–804. [DOI] [PubMed] [Google Scholar]
  • 44.Silbernagel KG, Thomeé R, Thomeé P, Karlsson J. Eccentric overload training for patients with chronic Achilles tendon pain--a randomised controlled study with reliability testing of the evaluation methods. Scand J Med Sci Sports 2001;11(4):197–206. [DOI] [PubMed] [Google Scholar]
  • 45.Farrar JT, Young JP, LaMoreaux L, Werth JL, Poole RM. Clinical importance of changes in chronic pain intensity measured on an 11-point numerical pain rating scale. Pain 2001;94(2):149–158. [DOI] [PubMed] [Google Scholar]
  • 46.WALT. Dosage Recommendations and Scientific Guidelines World Association of Laser Therapy. 2010. http://waltza.co.za/documentation-links/recommendations/dosage-recommendations/. Accessed 22 August 2018.

RESOURCES