Sinus tarsi implant insertion for mobile flatfoot

In people with mobile flatfoot, the foot arch is effaced only on weight bearing. Manipulation or standing on tiptoe can restore it to normal appearance.

• Most children go through a self-resolving phase of mobile flatfoot during growth.

• In some children, it can be permanent as a result of neuromuscular disorders, skeletal dysplasias or ligamentous laxity.

• In adults, mobile flatfoot is common and may be associated with posterior tibial tendon insufficiency.

The condition is usually asymptomatic, particularly in children, but some people may have foot pain.

Orthotics and physiotherapy are normally used to treat children and young adults. Depending on the underlying cause, treatments may include corticosteroid injections (in adults), surgical decompression, tendon augmentation, and osteotomy or lengthening of the calcaneum.

A number of different devices can be used for this procedure.

The procedure (also known as subtalar arthroereisis) can be performed with the patient under general or local anaesthesia. Exact technique and instrumentation vary. The sinus tarsi (between the calcaneum and the talus) is accessed by a lateral incision. A trial implant may be used, with intraoperative imaging and simulated weight bearing, to direct appropriate placement and degree of correction before a sized implant is inserted. Adjunctive bone or soft tissue procedures may also be carried out.

Compression dressing or plaster cast (particularly with adjunctive procedures) and modified footwear and/or orthotics may be used postoperatively.

The implant may need to be removed, particularly in children; exact timing for this varies.

In a case series of 54 patients (68 feet), 24%, 42%, 27% and 6% of patients (or their parents) respectively reported '100%', '75%', '50%' and 'either 25% or no' resolution of symptoms (mean follow-up 2 years).

In a case series of 37 patients (65 feet), 59% (22 out of 37) reported pain before and 5% (2 out of 37) after the procedure (mean follow-up 26.5 months). In a case series of 23 patients (28 feet), the mean pain score decreased from 3.2 preoperatively to 1.6 postoperatively on a scale from 4 (severe pain) to 1 (no pain; p<0.0001; mean follow-up 44 months).

The Specialist Advisers listed key efficacy outcomes as quality of life, pain relief, X-ray angles, gait analysis, normal foot shape and footwear, clinical scoring scales and long-term correction.

In 7 case series, less than 1% (2 out of 234), 5% (4 out of 80), 5% (3 out of 65), 7% (3 out of 41), 36% (8 out of 22) and 39% (11 out of 28) of feet and 10% of patients (exact number not stated) required implant removal (follow-up from 3 months to 10 years).

In the case series of 23 patients, 1 patient had a fractured talus 6 years after implantation.

Further studies reported avascular necrosis in 1 foot 10 years after bilateral surgery; bilateral intraosseous talus cysts and osteophytes in 1 patient after 2.5 years; talus bony sclerosis in 1 patient at 4 years; and talus spur formation in 1 foot at 3 months.

The case series of 54 patients reported implant extrusion in 9% of feet after 1 year. A case series of 49 patients reported fragments in the sinus tarsi (unclear whether bone or implant) in 1 foot (follow-up not stated).

The Specialist Advisers considered theoretical or anecdotal adverse events to include sural nerve injury and complete loss of subtalar movement.