Ultrasound characteristics of the mid-portion of the Achilles tendon in runners: a systematic review protocol

Achilles tendinopathy is a common musculoskeletal disorder characterised by pain and swelling in and around the Achilles tendon (AT) accompanied by impaired physical function [1]. AT injuries are the most common overuse syndrome of the lower limb, with a prevalence of up to 18% in recreational and competitive runners [2]. The condition has a cumulative lifetime incidence of approximately 24% in athletes. Running-related injuries have a prevalence between 11 and 85% or 2.5 to 59 per 1000 h of running [3]. The incidence of mid-portion Achilles tendinopathy in the general adult populations (21–60 years) has been reported with an incidence of 2.35 per 1000 and strongly associated with sporting activities [4]. The mid-portion of the AT, located 2 to 6 cm proximal to the calcaneal insertion, is the most commonly injured region, representing 55 to 65% of cases of Achilles tendinopathy [5]. It is likely this is due to the hypovascular nature of the tendon in this region which limits its ability to heal and regenerate after injury [6]. This is in contrast to insertional Achilles tendinopathy which is considered a different entity, with a different injury mechanism [7].

The AT is subject to considerable tensile and elastic loads during running which predominate in the mid-portion predisposing runners to Achilles tendinopathy [8]. The AT plays a crucial role in the transfer of calf muscle forces and production of coordinated movement. Alteration of the internal structure of the tendon by the degenerative process, which accompanies ageing, weakens the tissue when load exceeds its functional capacity, predisposing to further injury [9].

Musculoskeletal ultrasound imaging is a highly sensitive, reliable and non-invasive tool, and has been widely used to assess AT structure [10‐12]. Ultrasonography may provide a clinical tool for the early detection of tendon abnormalities in the pre-symptomatic state, which may enable the identification of runners who have an increased risk of developing mid-portion Achilles tendinopathy. Currently, there is no standardised criteria that is used to stage tendinopathy according to intratendinous changes. Diagnostic ultrasound is regarded as the gold standard for imaging tendon-based injuries [13]. Several tendon characteristics can be assessed using ultrasound and include the presence of hypoechoic thickening, the loss of normal fibrillar pattern, irregularity of the tendon margins [14, 15], vascularisation [16], AT strain or displacement [17], and AT thickness and cross-sectional area (CSA) [18].

Sonographic tendon abnormalities are not always associated with patient-reported pain, with evidence suggesting that tendon degeneration starts long before the onset of symptoms [19, 20]. Up to 90% of tendons with ultrasound evidence of tendinopathic changes are asymptomatic [21, 22], and similar pathological features may be found in asymptomatic and symptomatic tendons [23]. Therefore, abnormal ultrasound findings may be present in the absence of clinical symptoms. Despite the important functional role of the AT in weight-bearing activity, particularly running, the clinical significance and frequency of abnormal sonographic characteristics in runners is unclear.