The mechanical, physiological and therapeutic effects of eccentric exercise combined with extracorporeal shockwave therapy in athletes with patellar tendinopathy

Abstract

Patellar tendinopathy is one the most common injuries in jumping athletes. Changes in tendons' mechanical and physiological properties are the two proposed forms of pathogenesis. Whether tendon strain and vascularity are related to pain and dysfunction in subjects with patellar tendinopathy has not been established. Also, despite the efficacy of eccentric exercise when applied alone and combined with extracorporeal shockwave therapy being reported, the underlying treatment mechanisms of pain and dysfunction are not clear. This project aimed to explore the mechanical, physiological and therapeutic effects of eccentric exercise when applied as a single treatment and as an adjunct to extracorporeal shockwave therapy. In order to achieve this aim, there were two pilot studies and one main randomized-controlled trial. The objectives of pilot study 1 was to assess the test-retest reliability in assessing strain using ultrasonography and dynamometry and to compare possible differences in these tendon mechanical properties between jumping athletes with patellar tendinopathy and healthy controls. In pilot study 2, a semi-quantitative measurement of vascularity by Power Doppler was correlated with subjective grading and assessed for its test-retest reliability. In the main study, using these measurement tools, possible relationships between tendon strain, tendon vascularity, pain and dysfunction were assessed in 34 athletes at pre-, immediately, and 6 weeks post-intervention of a 12-week eccentric exercise with and without extracorporeal shockwave therapy in the initial 6 weeks. The treatment efficacy was compared between when exercise was prescribed as a single and combined intervention with extracorporeal shockwave therapy at immediately, 6 weeks and 1 year post-intervention. At 1 year post-intervention, pre-intervention intrinsic and extrinsic factors influencing treatment successfulness was assessed. Results from Study 1: Twenty-one male basketball, volleyball and handball players with patellar tendinopathy for more than 3 months were compared with 13 healthy controls who were matched by age and activity level. In vivo mechanical properties of their patellar tendon was examined by ultrasonography and dynamometry. Good intra-rater reliability was observed for tendon strain with ICC = 0.85. Tendon strain was significantly lower by around 30% (P < 0.05) in subjects with patellar tendinopathy compared with healthy controls. No significant difference in tendon resting length and maximum tendon force (all P > 0.05) was observed between the two groups. In study 2: Forty-three male athletes with chronic patellar tendinopathy underwent Power Doppler ultrasonography on their pathologic tendon. The vascularity was graded by an experienced radiographer and semi-quantified by a customized software program. Vascular index (VI) was calculated as the ratio of the number of colour pixels to the total number of pixels within a standardized selected area of patellar tendon. The VI calculated from the Power Doppler images illustrated good correlation with a subjective grading scale ({478} = 0.94; P = 0.000). Twenty-four (55.8%) subjects presented a low vascularized tendon of grade 0 to 1 and 19 subjects (44.2%) presented a high vascularized tendon of grade 2 to 3. The intra-rater reliability showed good repeatability with an ICC of 0.83. In the main study: Thirty-four male jumping athletes with patellar tendinopathy for more than 3 months participated in the randomized controlled trial. Subjects were randomized into exercise as a single or combined intervention with weekly extracorporeal shockwave therapy in the initial 6 weeks. Tendon vascularization and strain of the patellar tendon were examined together with the intensities of self-perceived pain (maximal pain in the past 7 days and pain during the single-legged declined-squat test) using the Visual Analogue Scale (VAS). The Victorian Institute of Sport Assessment (VISA-p) questionnaire was used to reflect functional disability. Before intervention, a significant negative correlation between tendon strain and maximal self-perceived pain over 7 days (r = -0.49, P = 0.005), and pain during the single-legged declined-squat test (r = -0.37, P = 0.04) were detected. Tendon VI was positively correlated to maximal self-perceived pain over 7 days (r = 0.40, P = 0.03). These results elucidated the relationships between tendons' mechanical and physiological properties with pain in jumbing athletes with patellar tendinopathy. After 12 weeks of eccentric exercise as a single or combined intervention with extracorporeal shockwave therapy, 30 subjects completed the treatment programme. A significant increase in tendon strain, reduced intensity of pain, as well as improvement in dysfunction was evidenced at immediately and 6 weeks post-intervention (all P < 0.05) with no significant group effect (all P > 0.05). The direction of intervention-induced changes in tendon VI was significantly affected by whether the tendons were low-or high-vascularized (P = 0.04). In the low-vascularized tendons, a trend of significant increase in tendon VI was observed at immediately post-intervention (P = 0.031) with a significant increase at 6 weeks post-intervention (P = 0.021) than pre-intervention. Combining the exercise programme with ESWT induced a greater increase in VI immediately post-intervention (P = 0.008). In the high-vascularized tendons, all but one subject had reduced tendon VI at 6 weeks post-intervention. Accounting for the subjects with reduced VI in this high-vascularized group, a greater reduction in tendon VI was associated with a greater reduction in the intensity of pain in the past 7 days (r = -0.95, P = 0.01). Taken together, the relationships between tendons' mechanical and physiological properties and pain were further confirmed. Also, the treatment mechanism underlying exercise and extracorporeal shockwave therapy might relate to modulations of tendons' mechanical and physiological properties. At 1 year post-intervention, the overall improvement in the functional scores was about 50% in both groups (P > 0.05). There was no significant group difference in success rate (54.5% & 69.2%, P = 0.68). In subjects with unilateral patellar tendinopathy, only subjects with pre-intervention vascular index below the group mean could be successfully treated. Five main conclusions could be drawn from the findings of this project: 1) the measurement methods on tendon strain using dynamometry and sonography and tendon vascularity using Power Doppler are reliable to quantify the mechanical and physiological properties of the patellar tendon. 2) Riding on these methods, we observed that tendon strain is reduced in pathologic patellar tendons and about 44% of the tendinopathic tendons were moderately-highly vascularized. 3) Tendons' mechanical and physiological properties are related to tendon pain in jumping athletes with patellar tendinopathy. 4) Bi-directional modulation of tendon vascularity related to the pre-intervention vascular state was observed after interventions. Extracorporeal shockwave therapy could enhance further vascularization in low-vascularized tendons during the treatment period. 5) 12 weeks of eccentric exercise when performed as a single or combined intervention with extracorporeal shockwave therapy have significant and similar effects on reducing pain and dysfunction in in-season jumping athletes with patellar tendinopathy. Pre-intervention vascularity might be a predictor of treatment successfulness. To conclude, tendon strain and vascularity are related to pain and dysfunction in subjects with patellar tendinopathy. The underlying treatment mechanisms for pain and dysfunction of eccentric exercise when applied alone and combined with extracorporeal shockwave therapy are partially related to the intervention-induced modulation of tendon strain and vascularity.