This 70-year-old football coach presents for a review of his left foot. He tells you he has always had ‘flat feet’ but that his left foot has changed in appearance over the past few years. It is not painful, he is able to play some social football with his friends and can walk freely with his wife. On examination he has marked pes planus which is far more pronounced on the left side. He has callus over the medial arch of this left foot. He has no localised tenderness. His foot is supple and appears to correct. His tibialis posterior strength is excellent. His heel does not completely move into varus with plantarflexion.
There is a loss of the medial longitudinal arch on the left side. There is a pronounced sag at the naviculocuneiform joint. There are similar, but less pronounced, findings on the lateral view of the right foot. The AP image of the left foot shows abduction of the left forefoot (relative to the right).
The case was discussed at length with the patient and his wife. It was explained that it was difficult to be certain about the natural history of his condition. It was explained that it was entirely possible that he may develop symptoms over time and that there might be a need for surgery. Given his age it was felt that the most predictable procedure would be a midfoot arthrodesis (rather than a procedure to reconstruct the midfoot) and that this could be performed at any time. At this stage however it was not felt that surgery would offer him any advantage over his current situation (as he was entirely asymptomatic). He was advised to continue to manage this with observation, shoe selection and a medially posted orthotic.
Meary’s angle is often measured to assess for pes planus on a weight-bearing lateral x-ray. This is an angle formed between the long axis of the talus and first metatarsal. This line is used as a measurement of collapse of the longitudinal arch. Collapse may occur at the talonavicular joint, naviculo-cuneiform, or cuneiform-metatarsal joints. In the normal foot, the midline axis of the talus is in line with the midline axis of the first metatarsal. An angle that is greater than 4° convex downward is considered pes planus with an angle of between 15-30° considered moderate , and greater than 30° severe. An angle greater than 4 degrees convex upward is considered a pes cavus.
The most common cause of this an acquired midfoot deformity is dysfunction of the tibialis posterior tendon. Middle aged women are most commonly affected while a history of pes planus and increasing age are also risk factors. Without a normally functioning tibialis posterior tendon, the ligaments and joint capsules become weak and the deformity develops. The first sign of this condition is usually an insidious onset of medial foot pain. The pain often predates the loss of the medial longitudinal arch (which is generally something that occurs as the condition proceeds). When diagnosed early tibialis posterior dysfunction can be effectively treated by supportive footwear and an orthotic to support the medial arch. Surgical treatment in the early stages involves a hind-foot osteotomy and tendon transfer. In more advances cases, or in older patients, arthrodesis of the hindfoot and/or ankle is generally needed.
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