Os trigonum syndrome is the most common cause of posterior ankle impingement.1 The os trigonum, which is a normal anatomic variant with a prevalence of 1.7% to 7.7%, usually remains asymptomatic.2-5 It can become symptomatic after strenuous activity or an injury such as ankle sprain.
Presentation of patients is significant for an acute ankle injury with hyper-plantar flexion or inversion.6 A second presentation includes slow-onset symptoms after an increase in amount and/or duration of activities, consistent with overuse injury.3-5 Athletes such as dancers, gymnasts, and runners present more frequently with the latter pattern of injury.
Diagnosis of os trigonum syndrome is primarily made by history and physical exam, and confirmed by radiography with or without multiplanar imaging modalities. A classic symptom is characterized by posterior ankle pain, deep to the Achilles, which is exacerbated by forced plantar flexion of the foot. This plantar flexion moment causes impingement of the os trigonum against the posterior tibial rim, compressing a thickened joint capsule or scar tissue between them, or by abnormal motion between the os trigonum and the talus. Physical examination usually demonstrates positive posterior impingement test performed by passive forced plantarflexion of the ankle.
A course of non-operative treatment can be beneficial, with a historic reported success rate of 60%.7 This non-operative regimen consists of symptomatic treatment, such as rest, ice and anti-inflammatory medications, with a decrease in aggravating activities such as forced plantar flexion (en pointe), running downhill, or overuse. A 4- to 6-week course of casting may relieve symptoms. Nonetheless, in recalcitrant cases and in active individuals who cannot afford to undergo a prolonged course of non-operative measures, surgery is warranted.
Open resection has been used in the past with good results. However, substantial recovery periods and complications such as vascular and neurologic injury, hematoma, and infection have been noted with formal open procedures.7-10 The prevalence of complications associated with open surgery has been reported to range between 10% and 24%.8-9,11 Complications could have potentially been avoided with less invasive, arthroscopic techniques that are indicated for posterior impingement syndrome.12 Endoscopic os trigonum resection through a posterior approach, as described by van Dijk,13 is now commonly used to address posterior ankle pain. Advantages include decreased morbidity, less scaring, and speedier recovery.10,14-16
- Posterolateral tenderness and pain on the forced plantar flexion test.
- Posterior impingement pain caused by os trigonum syndrome; may be associated with FHL tenosynovitis with posteromedial tenderness when the great toe is fully dorsiflexed.
- Extensive scarring due to previous injuries or surgeries of the posteromedial ankle preclude the use of this technique due to risks of neurovascular injury.
- Plain radiography: Standard weight-bearing foot radiographs, a neutral weight-bearing lateral foot and ankle radiograph (Figure 1)
- Computer tomography: Can assist in ruling out fractures of the posterior process
- Bone scan: Negative bone scan indicates os trigonum not likely source of pathology
- Magnetic resonance imaging: Study of choice to assess bone edema and soft tissue involvement (ie, the flexor hallucis longus, thickening of the posterior ligamentous structures) (Figures 2-3)
Figure 1. Plain lateral radiograph
Figures 2-3. Sagittal and axial T1 MRI images with the arrows pointed to the os trigonum
The patient is placed in a prone position with all bony prominences, including the knees, padded appropriately (Figures 4-5). A tourniquet is placed proximal to the knee. The ankle is located at the distal end of the bed, with a padded support under the distal tibia. This allows the ankle and foot to hang over the end of the table so that the ankle and hallux can be passively dorsiflexed during the procedure. A sterile bump should be available in the OR in case extra support under the ankle is required intra-operatively.
To allow slight rotation of the ipsilateral limb so that it is perpendicular to the floor, the contralateral hip can be bumped slightly higher or the bed can be tilted to the ipsilateral side with the patient appropriately secured with straps or supported at the ipsilateral hip.
Figures 4-5. Prone positioning
Standard posterolateral and posteromedial arthroscopic hindfoot portals are used. Two portals are made on either side of the Achilles tendon at approximately the level of the fibula tip (Figures 6-8).
Figures 6-7. Posterior and lateral views of the portals related to the anatomical landmarks
Figure 8. Axial diagram demonstrates the directions of the posterolateral and posteromedial portals on both sides of the Achilles tendon towards the Os trigonum
In most cases, this procedure is performed in the outpatient setting under general anesthesia (preferable) or regional anesthesia with sedation. After the patient is positioned in the manner described above, and the anatomic landmarks and portals are marked, the tourniquet is inflated. Irrigation using normal saline is set on a low pump pressure of 40 mmHg. A 4.0 mm, 30° arthroscope is used, as is a standard full radius resector, with larger full radius resector on hand for more involved bony sculpting. A bipolar electrocautery probe can assist in the release of fibrotic tissue, but is not mandatory. Curettes, elevators, arthroscopy punches and graspers, hemostats, and small osteotomes should be available.
The posterolateral portal is made first (Figure 9). A vertical stab incision is made through skin, hugging the lateral border of the Achilles. The subcutaneous tissue is split by a curved mosquito clamp, which is then inserted deeper and aimed at the second ray
The postero medial portal is made in the same way, with the blade just medial to the Achilles tendon and angled anteriorly to midline. Once skin is incised, a curved hemostat is again used, with the curve aimed anterior to the Achilles tendon and hugging its undersurface, spreading tissues anterior to it. This hemostat is advanced until it interacts with the hemostat inserted in the posterolateral portal. The instruments are mobilized against each other to provide room. The hemostats are spread during removal to dilate the fascial openings.
At this point, a full-radius shaver with the opening positioned laterally is inserted into the posteromedial portal. It is aimed towards the posterolateral aspect of the subtalar joint, medial to the peroneal tendons and just distal to the tip of the fibula (Figure 10). The surgeon should be able to palpate the instrument posterolateral under the peroneal tendons, postero-distal to the fibula.
Figures 9-10. The posterolateral portal is made (left). A full-radius shaver is inserted into the posteromedial portal (right).
The trochar, followed by the scope with the eyes pointed laterally, is then inserted into the posterolateral portal and aimed towards the second ray, until it touches the shaver (Figure 11). Fluid is then turned on and the shaver is backed off until its opening can be safely visualized by the scope.
Figure 11. Trochar inserted into the posterolateral portal
Loose connective tissue behind the ankle joint is debrided under direct visualization until a working space is created (Figure 12). The shaver is then turned off to visualize its initial tract towards the subtalar joint, created when it was first inserted before being withdrawn to the level visualized by the scope (Figure 13).
The scope is then inserted deeper into this potential tract and the shaver is turned on again, debriding gently through fibers of ligament of Ranvier and the fascia of the deep posterior compartment until the subtalar joint and the os trigonum overlying it are visualized (Figure 14). The shaver is placed just proximal to the os trigonum and is pushed deeper towards the posterior aspect of the ankle joint, then swept medially across the level of the ankle joint with the opening aimed disto-laterally (Figure 15, indicated by gold arrow), debriding some of the posterior capsule of the ankle joint.
The great toe is plantar and dorsiflexed to identify location of the FHL tendon, which should have at least 1.5-2 cm of excursion distinguishing it from other accessory muscles. The scope is moved medially, as mentioned above, until the tendon of FHL is visualized and protected by always pointing the teeth of the shaver away from it. This is the most medial extent of our debridement as the FHL (Figure 16) protects us from the more medial posterior tibial neurovascular bundle.
At this point, a better picture is obtained of posterior subtalar and ankle joint with proper medial (FHL) and lateral constraints (peroneals). The ankle is hyper-plantar flexed to visualize the impingement. After impingement is noted, an elevator is used as a probe to push on the os trigonum and identify the motion (Figure 17, yellow arrow) that exists between it and the posterior talus. Once this is identified, it is the plane for the debridement of tethering soft tissue.
Some fibers of the posterior talofibular ligament (PTFL) and the retinaculum overlying the FHL are attached to the os trigonum. These should be debrided, but the remainder of the attachment to the talus should be preserved (Figure 18). An elevator or curette can then be used to probe the os trigonum and its fibrous junction to the posterior talus. This junction can be loosened and the os trigonum mobilized free with a combination of the freer, curette, resector, and/or electrocautery. It is useful to leave the os trigonum attached by very minimal tissue to the posterior talus. The os trigonum is then grasped with a hemostat or grasper and rolled free (Figure 19). The os trigonum is extracted out of the ankle, preferably en bloc, through either portal (Figure 20). The portal may need to be extended to allow for removal of a larger fragment.
Figures 18-19. The FHL tunnel is debrided while the shaver blade is turned away from the FHL tendon (arrow).
At this point, the posterior aspect of the talus is sculpted with the shaver or bur to remove edges of the excision site, eliminate offending bone (Figure 21) that may be contributing to further posterior impingement, and allow for a smooth gliding surface for the FHL (Figures 22-24). It is not uncommon for the calcaneal articular surface of the subtalar joint to be exposed posteriorly, as the os trigonum can have an underlying articular surface that articulates with the dorsum of the calcaneus.
Final inspection (Figure 25) and dynamic arthroscopic visualization of hyper-tibiotalar plantar flexion (Figure 26) and FHL gliding with great toe motion is completed to confirm that there is no longer any bony posterior impingement or FHL mal-gliding/stenosis/low lying muscle belly (Figures 22-24). When there is an associated FHL stenosing tenosynovitis, the retinaculum overlying the tendon can be released with arthroscopic scissors.
Pearls and Pitfalls
- Positioning is critical for the success of the procedure. This is achieved by having a bump under the distal anterior tibia and having the foot slightly overhang the edge of the operative table.17 The foot should be in vertical orientation, achieved by bumping up the contralateral pelvis or tilting of the bed.
- A thigh tourniquet should be used. Ankle and calf tourniquets can tether the posterior leg musculature.17
- General anesthesia is preferable to regional anesthesia as it is important to limit the patient's motion. Converting convert to general anesthesia in the middle of the case, with the patient in the prone position, is not easy.17
- The FHL tendon is the limit to medial dissection: Motion of the great toe helps identify its location in the soft tissues. The FHL should be distinguished from other accessory muscles by having an excursion of at least 1.5-2 cm.18
- Portals may need to be extended prior to pulling out large os trigonum fragments as these fragments may be trapped in the soft tissues.17
- Patients are place in a post-operative boot, with weight-bearing as tolerated.
- The boot remains on until the follow-up appointment in 2 weeks for wound assessment and suture removal.
- Patients are encouraged to come out of the boot on day 3 for range-of-motion (ROM) exercises of the ankle and great toe three times per day; they are also encouraged to sleep without the boot.
- At 2 weeks, patients are weaned from the boot and can start working with a physical therapist on progressive ankle strengthening, ROM, gait training, and proprioception with a focus on sport-specific activities and return to sports.
- The literature indicates that the results of endoscopic treatment of posterior ankle impingement and open surgery are at least comparable.15
- “Hindfoot endoscopy appears to cause less morbidity than open ankle surgery and facilitates quick recovery.”15
- Van Dijk15 reviewed 55 consecutive patients with posterior ankle impingement, which was treated with endoscopic removal of bone fragments and/or scar tissue. In the osseous impingement, non-traumatic group, Van Dijk found:
- Median time to return to work: 2 weeks
- Median time to return to sports activities: 8 weeks
- Change from baseline in Tegner score: 1 point
- Median change in AOFAS hindfoot score: 12 points.
- Van Dijk15 also found that patients treated for posterior ankle impingement caused by overuse, specifically the osseous impingement group, had better results than those treated following trauma.
- Amendola et al 16 reported good functional and clinical results, high rate of return to work and sports, and no significant complications in a retrospective review of 23 patients treated with hindfoot endoscopy, 11 of which were done for excision of os trigonum.
- Minimal complications are encountered with endoscopic os trigonum resection as compared to open surgery, with decreased morbidity, less scaring, and speedier recovery.10,14-16,18
- Potentially, without adequate learning curve in hindfoot arthroscopy, the FHL tendon could be injured, as could the posterior tibial neurovascular structures.
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