Os Trigonum Syndrome

By
Robert Rinaldi, DPM
Posted February 24th, 2010

The Os trigonum is the most commonly occurring accessory bone in the foot, occurring in an estimated eight to ten percent of the population. This interesting bone sits at the most posterior aspect of the talus, at the border of the talus and the calcaneous. The Os trigonum is truly a congenital development, but does not begin to become evident until adolescence, when it is visible in an x-ray. Too often the Os trigonum is misdiagnosed as a fracture of the posterior process of the talus, also called stieda’s process.
POSTERIOR TALAR IMPINGEMENT
A crushing pressure on the Os trigonum (OT) can cause persistent pain behind the ankle. This is often called Os trigonum syndrome or posterior talar impingment syndrome. The mechanism of this injury occurs when the athlete drives the foot into extreme plantar flexion. As an example, it commonly occurrs in football and soccer during a hard kick. Ballet dancers on point may experience this injury, as well as skaters performing a jump maneuver.
In all instances the resulting pain will cause a limitation of motion. The pain is deep and unmistakably in the back of the ankle, and for this reason it is often confused with an Achilles injury. Fractured posterior process of the talus or stress and tearing of the fibrous band holding the Os trigonum in place causes the pain. It limits activity and signals a mechanical problem in the foot. In many athletes, including runners, this injury is the result of an ankle sprain, but becomes overshadowed with a diagnosis not made until the resolution of lateral ankle pain has occurred.
Lateral ankle instability can also exacerbate OT syndrome as the talus can be slipping in micro movements out of its position under the tibia because of lateral ankle ligament laxity. This forward migration of the talus will cause the OT to impinge between the talus and the calcaneous. As a result, the runner often suffers from overuse of the flexor hallicus longus muscle (FHL), which courses just medial to the Os trigonum. This may result in tendonitis of the FHL.
This area of the foot is thought to be composed of simple anatomy, but in reality it is very complex. An impinged Os trigonum or tearing of the associated fibrous bands will cause chronic pain. A fracture of the posterior process of the talus will cause similar constant pain with motion. FHL tendonitis can also present very similar symptoms. X-ray alone cannot establish the diagnosis; an MRI will be needed along with a precise clinical exam.
You should think of the Os trigonum as a serious problem if pain is primarily on the lateral side and just behind the ankle, also if pain can be reproduced with extreme plantar flexion of the ankle. The runner may find that pain is associated with the toe-off phase of gait. A simple and confirming test that can easily be performed in the office involves the use of a local anesthesia injection into the area of the OT. This injection should immediately relieve pain if OT syndrome is present and the cause of pain.
APPROPRIATE TREATMENT
Os trigomum syndrome treatment has been debated on a professional level for years. As an example, surgical resection of the OT in the dancer, with symptoms in the back of the ankle, is often considered necessary. However, I disagree, as a conservative approach to this problem can be effective, complete, and timely. Often the athlete can return to a full range of training with no limitations in a very short period of time. A conservative treatment plan that utilizes all the available options is best for the athlete. Rest from motion will cause a decompression of pressures and reduction of inflammation. A Cam walker is convenient to use and offers complete immobilization of ankle activity. Crutch walking is not necessary with use of the cam walker. Injection therapy utilizing ultra sound guidance is accurate, safe and very affective. This is a situation where cortisone is safe and should be included as a primary treatment. The cortisone will reduce inflammation surrounding the ossicle. This should be done with ultra sound guidance to avoid a deposition of steroid into the passing FHL tendon. Lateral ankle instability can exacerbate symptoms and must be ruled out as a primary cause of recurring impingement syndrome.
SUMMARY
Pain in the back of the ankle can be a complex and difficult condition to accurately diagnose. Clinical exams including diagnostic anesthesia injection, biomechanical gait studies, X-ray, and MRI should all be used to gather the information necessary to establish an accurate diagnosis. Conversely, Os trigonum syndrome often responds to trigger point injection therapy with cortisone.
Robert Rinaldi is a board-certified podiatrist and podiatric surgeon at the Gifford Medical Center in Randolph, VT. He is a fellow and a founding member of the American Academy of Podiatric Sports Medicine, and a podiatric consultant to the Dartmouth College track and cross-country teams. He is a former nationally ranked long-distance runner, having competed in 25 world-class marathons. You can reach him at Gifford Sports Medicine and Surgery Clinics in Randolph, VT, or at the Sharon Health Clinic in Sharon, VT, 802-728-2490 or 802-763-8000 or at rrinaldi@giffordmed.org.