The Blauth classification of thumb hypoplasia is useful in determining treatment options. Types IV (floating thumb) (Fig. 39-1) and V (absent thumb) (Fig. 39-2) are straightforward to identify clinically. However, it can be challenging to differentiate type IIIA (stable carpometacarpal [CMC] joint) from type IIIB (unstable CMC joint), and the trapezium and trapezoid are ossified only at 5 to 6 years of age, making radiographs less useful in decision making.

Serial examination of a child is required to differentiate between types IIIA and IIIB. A newborn uses digital grasp, and the infant begins to use the thumb in grasp at about 1 year of age. If the child uses the thumb in manipulating objects, this suggests that a type IIIA deficiency is present. If the child prefers to grasp objects between the index and long finger web space, this web appears wider, and the index finger is pronated toward the thumb. This suggests type IIIB hypoplasia.

The index finger may be stiff and hypoplastic to varying degrees in children with type IV and type V hypoplasia, which will affect the result of pollicization.

The child should be examined for signs of other systematic abnormalities, such as VACTERL (i.e., vertebral abnormalities, anal atresia, cardiac abnormalities, tracheoesophageal fistula and/or esophageal atresia, renal agenesis and dysplasia, and limb defects), Fanconi anemia, and Holt-Oltram syndrome, before considering surgical reconstruction.

Radiographs of the hands, wrists, and forearms are useful in determining the degree of metacarpal and phalangeal hypoplasia of the thumb and index finger. They can also identify other associated upper extremity anomalies, such as radial deficiency.

The number of carpal bones seen on the radiograph gives a rough estimate of the age of the child (Fig. 39-3A). The order of ossification of carpal bones is detailed in Figure 39-3B. About one center appears per year from the age of 1 year to 7 years.

The presence of a tapered metacarpal without a base confirms type IIIB hypoplasia. Type IIIA hypoplasia is associated with full length of the metacarpal (Fig. 39-4).