Functional Improvement and Ultrasound Tissue Characterisation of the Achilles Tendon Following Sodium Hyaluronate Injection for Plantaris Friction Syndrome

Introduction Midportion Achilles tendinopathy is a common source of disability, with typical features including pain, swelling and dysfunction [Roche]. The causes are multifactorial [Roche]. First line treatment is typically non operative and includes an eccentric loading program [Alfredson-1998], high volume injection [Chan], platelet rich plasma [de Vos JAMA 2010], sclerosing agent injections [Alfredson-2005] , shock wave therapy [Lynen][Saxena] and for resistant cases, surgery [Alfredson-2011].

Plantaris friction syndrome (PFS) One possible cause of midportion Achilles tendinopathy is plantaris friction syndrome (PFS). In this subset of patients, the pain is located to the postero-medial aspect of the Achilles tendon. The plantaris tendon is a small vestigial tendon originating from the lateral femoral condyle, crossing obliquely to the medial side of the Achilles tendon and inserting into the medial side of the calcaneum. The anatomy at the insertion is highly variable. Over 80% of plantaris tendons insert separately to the Achilles tendon [Daseler]. Biomechanical evidence suggests that the plantaris tendon is stiffer and stronger than the Achilles tendon [Lintz]. Under load therefore, differential movement between the tendons may occur. Subsequently the plantaris tendon can become tendinopathic and thickened. Adhesions may develop between it and the adjacent Achilles tendon [Bedi], initiating medial midportion Achilles tendinopathy. Sensitization of the nerve endings in the fat layer between the Achilles and plantaris has also been demonstrated. The optimal treatment of PFS is currently unclear. Treatments include physiotherapy, injection and surgical excision [Bedi][Pearce][Weinberg][Calder-2014][Alfredson-2011].

Sodium Hyaluronate One potential treatment is injection with sodium hyaluronate which is a viscosupplement solution. It has primarily been used for joint lubrication in arthropathy, but can also be used for tendinopathy [Lynen]. Ostenil Tendon is a sodium hyaluronate solution that potentially acts in several ways to reduce pain. One mode is to act as a lubricant between tendons or between the tendon and its sheath [Akasaka 2005][Akasaka 2006][Nishida][Kumar][St Onge]. It may also promote tendon healing [Chen][Yagishta]. According to unpublished work, Ostenil Tendon significantly reduced pain at 6 weeks for peroneal and also midportion Achilles tendinopathy [Company literature]. It may also act as a medium to allow free passage of nutrients to the tendon [Hagberg]. Sodium hyaluronate injection placed in the interval between the plantaris tendon and Achilles tendon could potentially reduce friction and subsequently pain. Although the effect of sodium hyaluronate injections have been reported in the hip and knee, no previous studies have reported its efficacy for PFS. The primary aim of the current study is to assess the impact of sodium hyaluronate injections in patients with PFS.

Ultrasound tissue characterization Methods of assessing the Achilles tendon for tendinopathy continue to be debated. Common methods include ultrasound and magnetic resonance imaging [Khan][Astrom]. A novel assessment tool is ultrasound tissue characterization (UTC) which uses ultrasound to quantify tendon structure [van Schie][Docking-2015][Docking-2016][Rosengarten][Bedi].

Structural integrity of the tendon matrix is categorized in four echo types (1-green, 2-blue, 3-red, 4-black). Echo types 1 and 2 reflect aligned fibrillar structure, whereas types 3 and 4 indicate disorganised tissue [Bedi]. The entire length of the tendon can be analyzed with a minimum detectable difference in structure to be 1% [Docking-2015]. Bedi et al showed significant improvement in UTC following excision of plantaris for midportion Achilles tendinopathy [Bedi]. However this study was limited because UTC images were not performed at a uniform time pre- and post- surgery [Bedi]. Masci et al found improvement in UTC echo-structure at 6 month follow-up post plantaris excision combined with improvement in VISA-A scores [Masci]. It is debated as to whether fibrillar integrity and matrix organisation correlate with symptoms. For example, a study in elite Australian football players showed that UTC changed following pre-season training, but all players remained asymptomatic at the start and end of the study [Rosengarten]. Ultrasound studies have also questioned the relationship of tendon structure in tendinopathy and patient function [Khan][McAuliffe]. In a study of gymnasts, clinical assessment has only moderate correlation with ultrasound and MR appearances [Emerson].