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Metatarsalphangeal Instability
- synonyms: MTP instability, MTP subluxation, MTP dislocation
- ICD-9 = 718.87(instability of ankle and foot) 838.05 (closed dislocation of MTP joint)
- Generally associated with inflammatory arthritis: rheumatoid arthritis etc.
MTP Instability Anatomy
- During normal gait , the toe-off phase results in dorsiflexion across the MTP joint. Which when associated with an inflammatory arthritis may lead to the plantar aponeurosis, plantar plate, and capsule elongating and weakening which can progress to joint subluxation and eventual dislocation.
- In advanced disease the base of the proximalphalanx may abut the neck of the metatarsal. The metatarsal head displaces plantarward, and the fat pad moves distally, leaving the metatarsal heads devoid of padding.
- Anterior drawer test: hold the metatarsal head between the index finger and thumb; grasp the base of the proximal phalanx with the other hand and apply a dorsally directed force. Dorsal displacement and pain indicate metatarsophalangeal joint instability or MTP synovitis.
- Advanced cases have MTP subluxation or dislocation with associated distal displacement of the fat pad, and plantar callosities.
MTP Instability Clinical Evaluation
- Pain and swelling in the MTP joint.
- MTP joint may be warm, erythematous and boggy.
- Anterior drawer test: hold the metatarsal head between the index finger and thumb; grasp the base of the proximal phalanx with the other hand and apply a dorsally directed force. Dorsal displacement and pain indicate metatarsophalangeal joint instability or MTP synovitis. In advanced disease MTP may be dislocated at presentation.
- Rheumatolgic evaluation and treatment of underlying systemic disorder is indicated.
MTP Instability Diagnositc Tests
- A/P and lateral xrays. Evaluate for joint subluxation / dislocation as well as joint erosions and cystic changes indicative of inflammatory arthritis.
- A/P view: subluxation is demonstrated by decreaseing clear space in the MTP joint. Dislocation is evident by the base of the proximal phalanx overlapping the metatarsal head. "Gun-barrel" sign = proximal phalanyx hyperextended to the point that is viewed end on, on the A/P xray.
MTP Instability Classification / Treatment
- Initial Treatment: extra-depth, wide toe box shoes, metatarsal pads, metatarsal arch supports. Consider a hard-soled rocker-bottom shoe.
- Rheumatolgic evaluation and treatment of underlying systemic disorder is indicated.
- Surgery, limited deformity: synovectomy to decompress the joint and remove inflammatory tissue.
- Surgery, advanced deformity: dorsal approach, resection of affected metatarsal heads with K-wire pinning for 3wks (allows fat pads to return to normal position). Usually done with a first MTP fusion. Consider concomittent correction of any associated hammer toes.
MTP Instability Follow-up Care
MTP Instability Differential Diagnosis / Associated Injury
MTP Instability omplications
MTP Instability Review References
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