Subtalar Joint Pathology

BRADLEY J. DUNLAP

RICHARD D. FERKEL

Pathology of the subtalar joint continues to become more widely recognized as the knowledge of normal anatomy and arthroscopic identification of abnormal anatomy grows. Indications for subtalar arthroscopy are numerous and range from treatment of arthritis to arthroscopic bullet removal and are described in detail in Chapter 14.¹ In this chapter, common and unusual pathologic processes of the subtalar joint, their arthroscopic treatment, and results are discussed. In the time period between 1990 and January 1, 2014, we have performed 691 subtalar arthroscopies for various pathologic conditions which are discussed in this chapter.

PAINFUL OS TRIGONUM

Introduction

Posterior ankle pain and subtalar (hindfoot) pain are less common than anterior hindfoot pain. One must have a high index of suspicion to make the correct diagnosis. There are many causes of posterior hindfoot pain, and these are listed in Table 15-1.² An injury to the trigonal process or os trigonum is among the most common sources of posterior hindfoot pain.

Anatomy

The anatomy of the posterior ankle is less well understood than in other areas of the foot and ankle, and there is more disagreement regarding the nomenclature of the structures. The posterior process of the talus is composed of lateral and medial tubercles, which are separated by the fibro-osseous tunnel for the flexor hallucis longus (FHL) (Fig. 15-1A-C). The lateral tubercle tends to be larger and more prominent. The os trigonum has been identified in a 12-week embryo and 2-month-old fetus.³, ⁴ It is an inconstant accessory bone of the talus that arises from a separate ossification center posterior to the lateral tubercle. This ossification center appears radiographically in puberty and typically mineralizes at the age of 8 to 10 years in girls and 11 to 13 years in boys, or 1 year after its appearance.⁵ When the ossicle is noted to be separate from the lateral tubercle, either by failure to unite in puberty in 8% of girls and 14% of boys or by subsequent fracture, it becomes an os trigonum. There is confusion regarding the terminology of the os trigonum in the literature. Rosenmuller first described the os trigonum in 1804, and Bardeleben coined the name “os trigonum.”⁶ The terms “nonunion,” “ununited,” and “free” have all been used when a radiographic lucency separates the os trigonum from the lateral tubercle of the talus.⁷, ⁸, ⁹ Confusion also arises when a large, intact lateral tubercle is referred to as a “fused os trigonum.”⁷ Sarrafian refers to this more appropriately as a trigonal process,¹⁰ and others have referred to it as the posterior process of the talus or the Stieda process after the German anatomist.¹¹ There is wide disparity in the literature regarding the incidence of the os trigonum. For clarity, we will use the terms “os trigonum” and “trigonal process” to denote the radiographically separate and intact descriptions, respectively (Fig. 15-2). The os trigonum is reported to have an incidence of 2.7% to 7.7%.¹⁰. Burman
and Lapidus studied 1,000 feet x-rays and noted that the os trigonum was present in 50% of the feet.⁷ Some authors have reported that the os trigonum is seen twice as often unilaterally than bilaterally.⁷, ¹², ¹³, ¹⁴, ¹⁵, ¹⁶, ¹⁷ In contrast, Sarrafian reported that an os trigonum was more common bilaterally than unilaterally.¹⁰

Table 15-1. Etiologic Classification of Posterior Ankle Impingement Syndrome

Pathology	Example
Trigonal process	Fracture (acute or chronic) Synchondrosis injury True compression
FHL dysfunction	Tenosynovitis
Tibiotalar pathology	Posterior capsuloligamentous injury Osteochondritis Fracture
Subtalar pathology	Osteochondritis Arthritis
Other	Calcified inflammatory tissue Posterior distal tibial osteophytes Prominent calcaneus posterior process osteophytes
Combined	FHL tenosynovitis and synchondrosis injury
Posterior ankle ligament sprain	Tear TTFL or PITFL
FHL, flexor hallucis longus; TTFL, transverse tibiofibular ligament; PITFL, posterior inferior tibiofibular ligament. Modified from Maquirriain J. Posterior ankle impingement syndrome. J Am Acad Orthop Surg 2005;13:365-371.

FIGURE 15-1. Subtalar anatomy of a right ankle. (A) Lateral drawing of the subtalar joint. Note the positions of the anterior, lateral and posterior processes of the talus. (B) Axial drawing showing the medial and lateral tubercles with position of the FHL and os trigonum.

There is a wide variation in the size of the os trigonum. A large os trigonum needs to be differentiated from a bipartite talus or talus partitus.¹⁸ In addition, an os trigonum can be bipartite or multipartite (Fig. 15-3A, B).

The os trigonum has three surfaces. The anterior surface of the os trigonum connects with the lateral tubercle by
fibrous, fibrocartilaginous, or cartilaginous tissue. The inferior surface is in continuity with the posterior calcaneal articular surface or the posterior subtalar joint. The large posterior surface of the os trigonum and/or trigonal process is nonarticular and serves as an attachment site for ligaments. These ligaments include the posterior talofibular ligament (PTFL), the talar component of the fibuloastragalocalcaneal ligament of Rouvière and Canela Lazaro and the Y-shaped bifurcate ligament, of which one limb attaches to the lateral tubercle and the other to the medial tubercle.¹⁰, ¹⁹
The FHL passes through the bifurcate ligament as the tendon courses between the two tubercles (Fig. 15-1C).

FIGURE 15-1. (Continued) (C) Anatomy of the posterior aspect of the subtalar joint; note the course of the FHL tendon. (Illustration by Susan Brust.)

FIGURE 15-2. Posterior talus morphology. (A) Os trigonum. (B) Stieda or trigonal process. (Illustration by Susan Brust.)

FIGURE 15-3. Bipartite os trigonum. (A) Coronal CT reconstruction demonstrating the bipartite os trigonum. (B) Sagittal CT reconstruction showing the position of the bipartite os trigonum.

CLASSIFICATION

Watson and Dobas have described a staging system that includes four anatomical variants of the posterolateral talus: type 1, normal posterolateral talar process; type 2, elongated posterolateral talar process or Stieda process; type 3, accessory bone or os trigonum; and type 4, os trigonum fused with posterolateral talar process by syndesmosis or synchondrosis.²⁰

Clinical Presentation and Treatment

The most common presentation of a symptomatic os trigonum is pain. The pain is typically located posterolaterally and may be noted during activities requiring extreme plantar flexion such as jumping or kicking a ball or during relevé in ballet. This has been referred to as posterior ankle block, posterior ankle impingement, and the os trigonum syndrome.⁸, ²¹, ²², ²³ Not surprisingly, the majority of clinical reports published to date have involved dancers or other athletes.⁴, ⁸, ²¹, ²⁴, ²⁵, ²⁶ Physical examination demonstrates pain in the posterior ankle, which is worse with forced plantar flexion. There may also be pain over the FHL and swelling posteriorly. A variety of radiographic studies can be used to help make the diagnosis. Plain radiographs should include three views of the ankle to look for the os trigonum or fracture of the trigonal process. In addition, a lateral view of the ankle “en pointe” or in full plantar flexion can show the posterior bony impingement (Figs. 15-4A, B and 15-5A). CT scan can be helpful to look for multiple fragments and hidden posterior pathology (Fig. 15-5B). MRI is very helpful and can demonstrate fluid with edema in the posterior talar region (Fig. 15-6A, B). MRI also has the advantage of showing injuries to the articular surfaces, ligaments, and tendons. Bone scans are typically positive but are not specific and not used as often with the advent of high-quality MRI (Fig. 15-7). Local anesthetic injection can also help confirm the diagnosis and can be combined with a corticosteroid for treatment purposes.

Conservative treatments, including activity modification, nonsteroidal anti-inflammatory agents, ice, injections and immobilization, are recommended initial treatments. A cast may be used if an acute fracture is noted. When these conservative measures fail, surgical excision often provides relief. While open surgical excision has been reported to yield good results,⁴, ⁸, ²¹, ²⁴, ²⁵, ²⁶ arthroscopic excision has equally good results with the advantage of smaller incisions, minimal surgical morbidity, and shorter

recovery time.²⁷, ²⁸, ²⁹, ³⁰ In addition, arthroscopy allows complete visualization of the subtalar joint to detect and treat any associated abnormalities. A specific contraindication to the arthroscopic technique would be concurrent “hallux saltans” or stenosing tenosynovitis of the FHL, which would require open FHL debridement through a medial approach. The os trigonum would also be removed via this approach. It is always critical to differentiate FHL tendinitis from posterior impingement (Table 15-2) (see Chap. 10). Another contraindication would be simultaneous posterior tarsal tunnel nerve entrapment, which would also require a medial incision.

FIGURE 15-4. Plantar flexion of the ankle. (A) Lateral x-ray shows abutment of the os trigonum against the distal tibia and posterior calcaneus in a ballet dancer. (B) Illustration showing posterior ankle impingement. (Figure B from Stoller DW. Stoller’s atlas of orthopaedics and sports medicine. Philadelphia, PA: Lippincott Williams & Wilkins, 2007, with permission.)

FIGURE 15-5. Painful os trigonum. (A) Preoperative lateral x-ray demonstrating a large os trigonum. (B) Bilateral axial CT scan showing os trigonum on the right but absent on the left.

FIGURE 15-6. Painful os trigonum. (A) Sagittal STIR image demonstrating an edematous os trigonum with a cyst both in the os trigonum and across the synchondrosis. (B) Axial T2 image demonstrates multiple cysts in the posterior talus.

FIGURE 15-7. Three-phase bone scan demonstrating marked increased activity of the os trigonum, representing inflammation and probably loosening.

Table 15-2. Flexor Hallucis Longus Tendinitis versus Posterior Impingement of the Ankle

FHL Tendinitis	Posterior Impingement
Posteromedial location	Posterolateral location
Tenderness over the FHL tendon	Tenderness behind the fibula
Pain or triggering with motion of the hallux	Pain with plantar flexion of the ankle
Thomasen sign	Plantar flexion sign
Condition mistaken for posterior tibial tendinitis	Condition mistaken for peroneal tendinitis
From Hamilton WG, Bauman PA. Foot and ankle injuries in dancers. In: Coughlin MJ, Mann RA, Saltzman CL, eds. Surgery of the foot and ankle, 8th ed. Philadelphia, PA: Mosby, 2007:1603-1632.

FIGURE 15-8. Arthroscopic setup for excision of os trigonum in a right ankle. Note the arthroscope is in the central portal, instruments are through the posterolateral portal, and inflow is through the anterolateral portal.

FIGURE 15-9. Excision of os trigonum, right ankle. (A1 and A2) The os trigonum has a fibrous attachment that can be clearly seen with subtalar arthroscopy and palpated with a probe to determine its looseness.

Author’s Preferred Technique: Os Trigonum Removal

The patient is operated upon in the supine position, as described in Chapters 4 and 7. The os trigonum is best visualized from the central portal, using the posterolateral portal for instrumentation and the anterolateral portal for inflow (Fig. 15-8). Alternatively, a 70^° arthroscope can be used from the anterolateral portal to look around the corner to see the os trigonum, and instrumentation can be done posterolaterally. The os trigonum (Fig. 15-9A1, A2) is evaluated with a probe and also dynamically by moving the ankle and subtalar joints. An unstable os trigonum reveals significant motion at its fibrous attachment to the talus and irregularity and sometimes chondromalacia and fibrosis at its insertion. Extreme caution is needed when excising the os trigonum to avoid injuring the FHL and neurovascular bundle, which are just medial to it. A banana knife is inserted, and the fibrous attachments posterolateral to the talus are released as the os is peeled from the soft tissue, capsule, and PTFL

(Fig. 15-9B1, B2). A reverse-angle curette is then used to free the os trigonum from the posterior talus by cutting the fibrous attachments (Fig. 15-9C1, C2). A shaver, Freer, or basket scissors can also be used to release the surrounding ligaments and help “shell out” the os trigonum from the capsuloligamentous structures. A large os trigonum can be removed piecemeal or can be reduced in size using a burr prior to removal (Fig. 15-9D). Once the fragment is loose, it is removed with a grasper or an extra large pituitary rongeur (“Jaws of Life”) (Fig. 15-9E1, E2). After removal, the FHL tendon is clearly seen (Fig. 15-9F, G), and it is checked for tears, tenosynovitis, or bony impingement and debrided accordingly via the central and posterolateral portals. Visualization through the posterolateral portal also gives an excellent picture of the subtalar and ankle joints with the FHL tendon medially (Fig. 15-9H). The joint is then inspected for any loose bodies and irrigated. The wounds are closed with nonabsorbable suture. A postoperative lateral x-ray is done to verify complete excision of the os trigonum (Fig. 15-9I).

FIGURE 15-9. (Continued) (B1 and B2) A banana knife is used to release the soft tissue attachments, including the PTFL attachment to the os trigonum, using extreme care to avoid injuring the FHL and neurovascular bundle medially. (C1 and C2) A reverse-angle curette is used to free the os trigonum from the synchondrosis and posterior talus. (D) Arthroscopic view utilizing a burr to reduce the size of a large os trigonum fragment.

FIGURE 15-9. (Continued) (E1 and E2) Once the os trigonum is loose, it is removed with a grasper or a pituitary rongeur. (F) After the os trigonum is removed from the subtalar joint through the posterolateral portal, the FHL is well seen posteromedially. (G) Inspection of the FHL via tendoscopy after the os trigonum has been removed. (H) After os trigonum removal, the subtalar and ankle (above) joints can be seen with the FHL positioned medially.

FIGURE 15-9. (Continued) (I) Postoperative lateral x-ray of patient in Figure 15-5A. Note the total excision of the os trigonum. (Illustrations by Susan Brust.)

Postoperative Care

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