Abstract
Localisation and injection of the sinus tarsi can be challenging, particularly for the less experienced musculoskeletal sonographer/practitioner. The inferior extensor retinaculum arises from the sinus tarsi in the form of three roots (medial, intermediate and lateral) which are collectively termed the frondiform ligament. This is readily identified on ultrasound and can be used as a reference point to aid the safe delivery of injectate material into the sinus tarsi. This article describes the technique and its anatomical basis.
Keywords: Ultrasound, Frondiform ligament, Inferior extensor retinaculum, Sinus tarsi
Introduction
The sinus tarsi is a conical soft tissue region formed by the bordering anterosuperior calcaneus and talar neck. The base of the cone lies at the lateral aspect of the midfoot and it courses in an anterolateral to posteromedial direction. It contains fat, an arterial anastomosis, capsular and ligamentous structures (cervical and interosseous talocalcaneal ligaments) as well as the roots of the inferior extensor retinaculum [1].
Sinus tarsi syndrome is characterised by anterolateral ankle pain, and although the true aetiology is not completely understood, it is commonly a post-traumatic condition [1–3]. Targeted injection of the sinus tarsi can be of both diagnostic and therapeutic benefit and previous studies have proven that ultrasound-guided injections have superior accuracy in comparison to ‘blind’/non-guided approaches [4]. As such, understanding of the local anatomy and identification of specific sonographic landmarks improves operator confidence in the exact site of injectate deposition. Such landmarks that are readily identifiable on ultrasound are the extensor digitorum longus (EDL) tendon and inferior extensor retinaculum at the lateral margin of the ankle. The inferior extensor retinaculum forms a sling around the EDL tendon before descending and attaching into the sinus tarsi via a network of roots [5–7]. These roots, collectively known as the frondiform ligament, consist of three main components (medial, intermediate and lateral) with attachments on the anterior process of the calcaneus and undersurface of the talus [5, 8] (Fig. 1). Therefore, on ultrasound, identification of the EDL tendon can help demonstrate the inferior extensor retinaculum, which if followed distally with the probe, can then invariably identify the sinus tarsi.
Fig. 1.
Coronal orientated diagram of the inferior extensor retinaculum and frondiform ligament. T Tibia, Ta Talus, C Calcaneus, IER Inferior Extensor Retinaculum, M Medial band of frondiform ligament, I Intermediate band, L Lateral band
Ultrasound identification of the sling of the frondiform ligament
The patient is imaged in the supine position with the ankle in a neutral position. A high-frequency linear probe aligned in the transverse plane is placed over the anterior aspect of the distal tibia and talocrural joint (Fig. 2a). In this position, the long extensor tendons are readily identified in their short axis (Fig. 2b)—from medial to lateral, the tibialis anterior, the extensor hallucis longus and the common tendon of the extensor digitorum longus (EDL). Whilst still in the transverse plane, the EDL tendon is traced proximally and distally until a characteristic sling-like structure is visualised wrapping around the EDL, corresponding to the lateral and intermediate roots of the frondiform ligament (Fig. 3a and b). By following this sling inferolaterally to its base, an abrupt increase in depth in the talar cortex will be seen deep to the sling, corresponding to the lateral opening of the sinus tarsi. The calcaneal border of the sinus tarsi will also be seen as the attachment of the sling.
Fig. 2.
a Probe position transverse across the distal tibia to identify the extensor tendons. b Transverse image with doppler at the level of the talocrural joint. EDL Extensor Digitorum Longus tendon, EHL Extensor Hallucis Longus tendon, TA Tibialis Anterior tendon, asterisk dorsal pedis with intrinsic doppler flow, Ta – Talus
Fig. 3.
a Maintaining a transverse position, the probe is centred on the EDL tendon which is then followed distally to reveal the sling of the frondiform ligament. b Maintaining a transverse position inferolateral to the talocrural joint following the EDL tendon identifies the sling of the frondiform ligament formed by its lateral and intermediate bands. EDL Extensor Digitorum Longus tendon, asterisk sling of the frondiform ligament, Ta Talus, C Calcaneus
Upon visualisation of this sling, the probe can be angulated slightly along the long axis of the sling; it can be then readily traced inferolaterally following the intermediate root of the frondiform ligament in particular (Fig. 4a). The lateral opening of the funnel-shaped sinus tarsi is now identified, bordered by the neck of the talus and the anterosuperior calcaneus (Fig. 4b).
Fig. 4.
a A minor adjustment, angulating along the long axis of the sling formed by the lateral and intermediate bands of the frondiform ligament identifies the lateral opening of the sinus tarsi. b Short axis across the sinus tarsi. EDL Extensor Digitorum Longus tendon, single asterisk lateral band of the frondiform ligament, double asterisk intermediate band of the frondiform ligament, Ta Talus, C Calcaneus, dashed circle proposed preferred target area for delivery of injectate
Injection technique
Once the lateral opening of the sinus tarsi has been located the probe is adjusted so that the maximum depth of the fat-laden sinus tarsi is appreciated. Using an out of plane technique a needle is passed from anterior to posterior into the depths of the sinus tarsi. The authors’ favour to place the needle obliquely towards the calcaneus, deep to the attachment of the intermediate band (Fig. 4). This is a slight variant from the delivery of injectate into the mid-portion of the sinus tarsi as described in previous cadaveric studies of ultrasound-guided injections such as that by Wisniewski et al. [4], but still constitutes accurate injection into the sinus tarsi.
Identification of the needle tip can be challenging but with a careful manipulation of the probe this can be achieved (Fig. 5). Often the needle will make contact with the edge of the calcaneus, and it is at this point the injection can take place. As with all injection procedures, it is prudent to aspirate prior to delivery of the injectate material to ensure the needle tip is not within a blood vessel (the sinus tarsi has a rich network of blood vessels).
Fig. 5.
Short axis image across the sinus tarsi during out-of-plane injection with needle encircled. Dashed line injectate containing a small hyperechoic gas bubble superiorly, asterisk intermediate band of the frondiform ligament, Ta Talus, C Calcaneus
Discussion
Appreciation of the anatomy of the sinus tarsi is paramount for any interventional sonographer given that injection into it is used for diagnostic as well as therapeutic purposes in the context of sinus tarsi syndrome. Use of the readily identifiable landmarks outlined in this report maximise confidence that injectate has been delivered accurately.
Sinus tarsi syndrome itself is clinically relevant to a wide variety of musculoskeletal sonographers and practitioners as it presents across a wide scope of patients, ranging from professional athletes to the overweight and those of more advanced age. This is partly a result of the condition’s aetiology. It is predominantly of a traumatic origin with subsequent talocrural instability but is also reported in advanced posterior tibialis tendon insufficiency [9]. It can result from either a single traumatic event or repetitive ankle sprains; it has been postulated that ligamentous injury results in synovitis and fibrotic infiltration into the sinus tarsi [2, 10]. Typical mechanisms include inversion injuries but in jumping athletes, injury to the posterior subtalar joint can be sustained via a ‘whiplash’ type mechanism secondary to anterior translation of the talus over the calcaneus [11].
Within the calcaneal side of the sinus tarsi, a distended synovial recess of the subtalar joint can often be visualised and subsequently injected in cases with combined sinus tarsi and subtalar joint pathology. Indeed, guided injection via the sinus tarsi is also a recognised approach for diagnostic and therapeutic injection of the posterior subtalar joint [12]. Many centres inject the sinus tarsi under ultrasound guidance as a surrogate for injections of the posterior subtalar joint. This may be indicated in cases where standard sonographically guided injections of the posterior subtalar joint are complicated by challenging post-traumatic anatomy [13] or because ultrasound-guided techniques are deemed less costly in terms of time and complexity as compared to fluoroscopic or CT guided procedures.
The frondiform ligament is a readily demonstrable landmark on ultrasound that can be used to identify the sinus tarsi with confidence. Visualisation of it can be aided by distension of the gruberi bursa; a bursa that extends from the sinus tarsi along the frondiform ligament (between the talus and EDL) [14]. The degree of bursal distension and subsequent extension along the frondiform ligament can be so significant that it can mimic tenosynovitis of the EDL tendon [5].
In our institution, it is the general consensus that the intermediate band, in particular, is consistently identifiable whereas delineation of the medial band can vary between patients. Thus, identification of the intermediate band is helpful when performing effective therapeutic and/or diagnostic guided injections within the sinus tarsi, as it provides a landmark deep to which the injectate can be deposited.
Authors’ contributions
All authors contributed to conception/design, manuscript drafting, final approval for publication and agreement for accountability for all aspects of the work.
Compliance with ethical standards
Conflicts of interest/Competing interests
None.
Consent to participate
Consent was gained from the participant for use of ultrasound images.
Consent for publication
Consent was gained from the participant for use of ultrasound images.
Footnotes
Publisher's Note
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