summary Thumb hypoplasia is the congenital underdevelopment of the thumb frequently associated with partial or complete absence of the radius. Diagnosis is made clinically with hypoplasia of the thumb and thenar musculature. Radiographs help determine musculotendinous versus osseous deficiencies. Treatment may be observation, soft tissue reconstruction, pollicization, or amputation depending on the level of the deficit and presence of carpometacarpal joint instability. Epidemiology Incidence rare 1/100,000 live births Demographics male = female Anatomic location bilateral involvement in ~60% of patients right hand more common than left Etiology Pathophysiology exact cause during embryologic development has yet to be elucidated likely due to abnormalities involving the AER, progress zone, and FGF signaling molecules Associated anomalies greater than 80% of patients will have associated anomalies including VACTERL Holt-Oram thrombocytopenia-absent radius (TAR) Fanconi anemia Cornelia de Lange syndrome Okihiro syndrome Classification Treatment algorithm depends on presence of carpometacarpal joint stability Blauth Classification Type Description Treatment Type I Minor hypoplasia All musculoskeletal and neurovascular components of the digit are present, just small in size No surgical treatment required Type II All of the osseous structures are present (may be small) MCP joint ulnar collateral ligament instability Thenar hypoplasia Adduction contracture Stabilization of MCP jointRelease of first web spaceOppensplasty Type IIIA Musculotendinous and osseous deficiencies Proximal 1/3 metacarpal aplasia with CMC joint intact Absence of active motion at the MCP or IP joint Stabilization of MCP jointRelease of first web spaceOppensplasty Type IIIB Musculotendinous and osseous deficiencies. Basal (proximal 2/3) metacarpal aplasia with deficient CMC joint Absence of active motion at the MCP or IP joint. Thumb amputation & pollicization Type IV Floating thumb Attachment to the hand by the skin and digital neurovascular structures Thumb amputation & pollicization Type V Complete absence of the thumb Thumb amputation & pollicization Presentation Physical exam inspection extrinsic tendon abnormalities (frequently found in Blouth type IIIA) pollex abductus flexor pollicis longus attaches to normal insertion and the extensor tendon hypoplasia of thenar musculature variable extensor pollicis longus tendon insertion absence of skin creases indicates muscle or tendon abnormalities excessive abduction of MCP joint range of motion and instability ulnar collateral ligament laxity web-space tightness evaluation for associated anomalies is essential cardiac auscultation echocardiography kidneys ultrasound abdomen ultrasound Imaging Radiographs recommended views bilateral films of hand, wrist and forearm Studies Labs peripheral blood smear and complete blood count important to rule out Fanconi anemia Additional studies chromosomal challenge test detects Fanconi anemia before bone marrow failure Treatment Nonoperative observation indications Type I hypoplasia where augmentation of thenar musculature (thumb abduction) is not necessary Operative opposition tendon transfer (opponensplasty) indications Type I hypoplasia with insufficient thumb abduction release of first web space, opposition transfer, stabilization of MCP joint indications Type II and IIIA hypoplasia pollicization indications Type IIIB, IV, V hypoplasia Techniques Opponensplasty (opposition transfer) technique performed using either flexor digitorum superficialis abductor digiti minimi First web space deepening technique usually performed with Z-plasty Stabilization of MCP joint technique three options fusion reconstruction of UCL with FDS reconstruction of UCL with free tendon graft Pollicization technique plan skin incision to avoid skin grafts isolate index finger on its neurovascular bundles detach first dorsal and palmar interosseous muscles shorten digit by removing index finger metacarpal and epiphyseal plate stabilize index MCP joint reattach and balance musculotendinous units reconstruct long extensor tendons rebalance flexor tendons