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