A 71 year old woman was referred to the orthopaedic outpatient clinic with chronic right ankle pain. She was also concerned regarding the appearance of the foot and was having difficulty with normal footwear. Of note, 18 months previously, she had sustained an injury to the right ankle and recalled an audible crack at the time of the event. At the time, 24 following the injury, she had attended the accident and emergency department, and a diagnosis of ankle sprain had been made based on normal appearance on radiographs. There was no relevant past medical history on systematic questioning.

Clinical examination revealed a flat, everted foot with an obvious valgus heel position. Weakness of inversion and an inability to raise the affected heel off the ground was evident. From behind, the forefoot appeared grossly abducted, and pain was elicited on both ankle and subtalar joint movements.

Magnetic resonance imaging of the right ankle revealed the presence of a midsubstance tear affecting the tibialis posterior tendon with surrounding oedema (fig 1). A triple arthrodesis was performed with good results.

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Figure 1

 Magnetic resonance image demonstrating a midsubstance tear affecting the tibialis posterior tendon with surrounding oedema (white arrow).

DISCUSSION

The tibialis posterior functions primarily as a plantar flexor and invertor of the foot.¹ During gait, the mechanical demand on the tendon is high, particularly just after heel strike as the hindfoot moves from a position of loaded eversion into increasing inversion.² It is vulnerable to injury owing to a localised hypovascularity in a critical zone just posterior to the medial malleolus, which is the most common site of tendon rupture.³

Despite being a well documented clinical entity, rupture of the tibialis posterior tendon can be misdiagnosed. Mann and Thompson⁴ reported a series of 17 patients with posterior tibial tendon rupture. They revealed an average time to treatment of 43 months and in all but two of the patients, incorrect diagnosis had been made on initial assessment.

In almost all patients who sustain a rupture of the tibialis posterior tendon, there is a history of ankle trauma.^4–6 Typically the mechanism of injury is forced eversion. Younger patients, usually athletes, tend to sustain a traumatic avulsion of the tendon at its insertion into the navicular, while middle aged or elderly individuals often suffer a mid-substance inflammatory or degenerative type tear,² as in our case.

Initial symptoms are usually related to the tendon pathology, including pain along the course of the tendon, weakness, and loss of the medial longitudinal arch, resulting in the appearance of a flatfoot deformity.⁷ On examination, weakness of inversion is evident. Toe raises are difficult, and there is often delay in or lack of heel varus with toe raising on the affected side when viewed from behind.⁷ The “too many toes” sign,⁸ which is the appearance of more of the lesser digits on the affected side when viewed from behind, may be apparent and is the result of forefoot abduction. A defect may be palpable along the course of the tendon. With time, a progressive planovalgus foot develops, and symptoms may be related to the fixed position of the foot and the development of degenerative joint disease.⁷

Plain radiographs are of limited value, as rupture of the tibialis posterior tendon is essentially a clinical diagnosis, although they may be of value to exclude underlying bony pathology. The tibialis posterior tendon is best imaged either by ultrasound⁹ or magnetic resonance imaging.^10,11

Non-operative treatment is limited to those patients with a partial or longitudinal tear, or where surgery is contraindicated because of significant systemic disease or advanced age. Options include shoe modifications, orthotic support, and bracing.¹² Surgical treatment may involve soft tissue procedures such as primary repair or tendon transfer, flat foot reconstruction, or arthrodesis, or a combination of techniques depending on the stage of presentation, the presence of collapse of the medial longitudinal arch, or the development of degenerative joint disease.⁷

In conclusion, rupture of the tibialis posterior tendon should be considered in the differential diagnosis when assessing ankle trauma particularly in the presence of normal radiographs. In those patients where there is a high index of suspicion of tibialis posterior tendon rupture, re-evaluation at an early stage when pain and swelling have subsided may help to confirm the diagnosis. Early specialist referral is also essential.