Lateral Approach Anatomy

Arthroscopic evaluation of the subtalar joint has traditionally been performed using a lateral approach. Three portals are recommended for visualization and instrumentation of the subtalar joint using the lateral approach (Fig. 2-6). The anatomic landmarks for lateral portal placement include the lateral malleolus, the anterior process of the calcaneus, the sinus tarsi, and the Achilles tendon. The lateral malleolus and anterior process of the calcaneus are easy to palpate. The sinus tarsi is a palpable depression between the distal tip of the fibula and the anterior process of the calcaneus, although it can be filled with large amounts of adipose tissue. Inversion and eversion of the foot may be helpful in palpating the sinus tarsi.

FIGURE 2-6 A, Three standard portals are used to approach the subtalar joint: anterior, middle, and posterior. B, Anatomic landmarks are used for placement of the lateral portals: the distal fibula, the anterior process of the calcaneus, and the sinus tarsi between the fibula and calcaneus. The patient is in the lateral decubitus position.

The point of entry for the anterior portal is usually about 2 cm anterior and 1 cm distal to the tip of the distal fibula, directing the instrument slightly upward and about 40 degrees posteriorly. The location of the portal in cadaveric dissection studies is an average of 28 mm (range, 23 to 35 mm) anterior to the tip of the fibula (Fig. 2-7).⁷ Structures at risk when placing the portal include the dorsal intermediate cutaneous branch of the superficial peroneal nerve, the dorsal lateral cutaneous branch of the sural nerve, the peroneus tertius tendon, and the small branch of the lesser saphenous vein. The dorsal intermediate cutaneous branch of the superficial peroneal nerve is located an average of 17 mm (range, 0 to 28 mm) anterior to the portal. The dorsal lateral cutaneous branch of the sural nerve (indentified in 8 of 15 specimens) was located an average of 8 mm (range, 2 to 12 mm) inferior to the anterior portal.⁷ The peroneus tertius tendon was located an average distance of 21 mm (range, 8 to 33 mm) anterior to the portal. A small branch of the lesser saphenous vein consistently coursed along the anterolateral aspect of the foot in the vicinity of the anterior portal. It is located an average of 2 mm (range, 0 to 5 mm) from the anterior portal and was lacerated 20% of cases in one report of portal safety.⁷ With use of the anterior portal, care must be taken to avoid injury to the dorsal intermediate cutaneous branch of the superficial peroneal nerve as it divides on the dorsum of the foot. A small branch of the lesser saphenous vein is also at risk, although damage to the structure is unlikely to cause significant problems.

FIGURE 2-7 Some structures are at risk with placement of the anterior and posterior portals. The posterior portal is just in front of the sural nerve, and the anterior portal is near the branches of the superficial peroneal nerve.

Meticulous attention to proper portal creation technique minimizes the risk of injury to superficial nerve and vascular structures. The nick and spread technique is recommended; only the skin is incised with a no. 10 scalpel blade, the subcutaneous tissues are bluntly dissected using a mosquito forceps, and the capsule is penetrated with a blunt trocar.

The middle portal is located approximately 1 cm anterior to the tip of the fibula, directly over the sinus tarsi. It places no significant anatomic structures at risk.

The posterior portal is located an average of 25 mm (range, 20 to 28 mm) posterior and 6 mm (range, 0 to 10 mm) proximal to the tip of the fibula.⁷ Compared with other subtalar portals, the posterior portal used for the lateral approach is more likely to cause nerve or vessel damage. Structures subject to injury with posterior portal placement include the sural nerve, lesser a saphenous vein, peroneal tendons, and the Achilles tendon. Great care must be taken during posterior portal placement to avoid injury, especially to the sural nerve and lesser saphenous vein.

The sural nerve and lesser saphenous vein run parallel to each other along the posterolateral aspect of the ankle, with the nerve coursing posterior to the vein at the level of the ankle joint. In 7 of 10 cases reported by Frey,⁷ the posterior portal was located posterior to the sural nerve, and in two cases, it was found to be anterior to the nerve. The average distance from the sural nerve to the posterior portal was 4 mm (range, 8 mm posterior to 6 mm anterior). In one case, the sural nerve was transected during portal placement, and in another, a small laceration was made in the lesser saphenous vein. The peroneal tendon sheath was located an average of 11 mm (range, 6 to 16 mm) anterior to the portal, and the Achilles tendon was an average of 15 mm posterior (range, 10 to 20 mm) to the portal. Neither tendon was damaged in the series, but the tendons’ proximity should be considered.

Accessory portals for posterior subtalar arthroscopy have been described in the literature. The accessory anterolateral and posterolateral portals are used as needed for viewing and instrumentation. The accessory anterolateral portal is usually slightly anterior and superior to the standard anterior portal. The accessory posterolateral portal is made behind the peroneal tendons, lateral to the standard posterior portal.^12–16

Posterior Approach Anatomy

Posterior subtalar arthroscopy can be performed from a purely posterior approach using posterolateral and posteromedial portals (Fig. 2-8).^17,18 This is a two-portal endoscopic approach to the hindfoot, which becomes arthroscopic after it enters the subtalar joint. The posterior approach can be accomplished with the patient in the prone position. The posterior approach has been reported to provide better access to the medial and anterolateral aspects of the posterior subtalar joint,^17,18 but the posterolateral and posteromedial portals used to approach the subtalar joint are the same as those used to approach the posterior aspect of the ankle and the hindfoot.

FIGURE 2-8 Posterior subtalar arthroscopy can be performed using posterolateral and posteromedial portals.

The patient is placed in the prone position. The posterolateral portal usually is made at or slightly above the level of the tip of the lateral malleolus. The posterolateral portal is lateral to the Achilles tendon. The posteromedial portal is made just medial to the Achilles tendon. In the horizontal plane, the posteromedial portal is located at approximately the same level as the posterolateral portal. The important anatomic landmarks are the Achilles tendon and the lateral malleolus.

The medial aspect of the posterior subtalar joint is tighter than on the lateral side, possibly increasing the risk of iatrogenic cartilage damage. The tibial nerve, the posterior tibial artery, and the medial calcaneal nerve are in proximity and can be at risk when the posteromedial portal is used. Several investigators have studied the relative safety of the posterior portals for hindfoot endoscopy in anatomic specimens.^17–20 Mekhail and colleagues determined the average distance between the point of entry of the posteromedial arthroscope and the posterior tibial neurovascular bundle to be 1.0 cm. The closest distance was 8 mm.¹⁹ Sitler evaluated the safety of posterior ankle arthroscopy in 13 cadaver specimens.¹⁸ Posteromedial and posterolateral portals were established in each specimen, and after performing arthroscopy to determine how much of the talar dome could be visualized, plastic cannulas were placed into the portal locations. Magnetic resonance imaging (MRI) was used to document the distance of the cannulas to neurovascular structures, and these measurements were correlated with the results of anatomic dissections. The average distance between the posteromedial cannula and the tibial nerve was 6.4 mm (range, 0 to 16.2 mm). The distance between the posterior tibial artery and the cannula was on average 9.6 mm (range, 2.4 to 20.1 mm). The average distance between the cannula and the medial calcaneal nerve was 17.1 mm (range, 19 to 31 mm). The location of the posterolateral portal in relation to the tip of the lateral malleolus is an important measurement to determine the proximity of the portal to relevant anatomic structures. It appears that the posterolateral and posteromedial portals used for subtalar arthroscopy are relatively safe, are reproducible, and can be used for the treatment of intra-articular and extra-articular hindfoot pathology.

Os Trigonum Gross Anatomy

Because the os trigonum can be approached through the subtalar joint, it is included in the description of relevant gross anatomy (Fig. 2-9).^21–23 Terminology regarding the anatomy of the posterior aspect of the talus and the subtalar joint can be confusing. The posterior surface of the talus (“process posterior”) consists of the posterior medial and the posterior lateral processes, also known as tubercles. Between these tubercles glides the flexor hallucis longus tendon in its small sulcus, which is directed obliquely, downward, and inward, angling anteriorly (Fig. 2-10).

FIGURE 2-9 A, The posterior lateral process, or tubercle, of the talus is called Stieda’s process. Occasionally, an accessory ossicle communicates with the posterolateral tubercle, and it is called the os trigonum. B, A lateral radiograph of the foot shows the os trigonum.

FIGURE 2-10 Between the tubercles of the posterior talus, the flexor hallucis longus (FHL) tendon glides in a small sulcus. The FHL is directed obliquely, downward, and inward, angling anteriorly.

The size of the posterolateral process of the talus, also called Stieda’s process, can vary markedly. It courses in continuity inferiorly with the posterolateral aspect of the articular surface of the talus, making it a partially intra-articular structure, and it is therefore possible to examine it with the arthroscope. Its superior surface, however, is nonarticular and provides an insertion site for the posterior talofibular ligament and a portion of the fibuloastragalocalcaneal ligament of Rouvière and Canela Lazaro. Occasionally, an accessory ossicle communicates with the posterolateral tubercle, and this is called the os trigonum. It also has multiple surfaces: anterior, inferior, and posterior. The anterior surface articulates with the posterolateral tubercle and the inferior surface with the os calcis. The posterior surface is nonarticular. When the os trigonum fuses, it is called a trigonal process and becomes essentially a large, lateral, articulating tubercle. It is often difficult to discern a fracture of Stieda’s process from an os trigonum based on radiographs alone. Computed tomography scans and views of the opposite foot are helpful.

The medial tubercle of the talus is an extension of the medial talar articular surface. It provides an attachment site for a portion of the deep and superficial deltoid ligament, the medial talocalcaneal ligament, and part of the fibrous tunnel overlying the flexor hallucis longus tendon.

None of these terms should be confused with the lateral process of the talus, which is a distinctly different structure located on the midlateral aspect of the talus. It provides an insertion site for the lateral talocalcaneal ligament. The lateral process of the talus is of arthroscopic importance because it is a palpable landmark and can be identified intra-articularly on the medial side of the lateral gutter of the subtalar joint. The lateral talocalcaneal ligament coming off the lateral process makes up a thick portion of the lateral capsule of the subtalar joint (Fig. 2-11).

FIGURE 2-11 A, The lateral talocalcaneal ligament comes off the lateral process of the talus and makes up a thick portion of the lateral capsule of the subtalar joint. B, The capsule inserts on the lateral aspect of the calcaneus and makes up the floor of the lateral gutter of the subtalar joint. C, The lateral capsule of the subtalar joint can be seen, along with the location of the lateral talocalcaneal ligament fibers (arrow).