34 Thumb Hypoplasia
Thumb hypoplasia can cause significant loss of function because it is the most important digit of the hand. Most patients require surgical intervention which is based on the type of hypoplasia. Treatment ranges from first webspace widening to index finger pollicization.
The thumb is the prime ray of the hand. Its size, position, mobility, and relationship to other digits is critical for function. In the first few months of life, the thumb is adducted and flexed within the palm. Large object grasp starts around 6 months of age, and small object pinch starts around 9 months of age. By this time, the first ray has gained independence from the palm and by a year of age, it has become the most important ray of the hand. In the young toddler, the thumb is used creatively and independently to explore and manipulate the environment.
The thumb is considered hypoplastic if any portions of the ray including bony or soft-tissue components are underdeveloped or absent. Children with thumb hypoplasia can suffer from other associated conditions that affect the heart, blood, skeleton, and other systems. The widely accepted classification system for thumb hypoplasia was initially proposed by Blauth and later modified by Manske. This system is concise and practical as it guides surgical treatment (Fig. 34‑1).
With the exception of mild hypoplasia, most patients with thumb hypoplasia require surgical intervention. Treatment ranges from simple procedures such as first webspace widening to more complex procedures such as index finger pollicization. Over the past 30 years, there have been refinements in surgical techniques that improve both appearance and function. Recent advances in outcome instruments hold promise for better functional delineation of the impact of surgical interventions for these conditions.
Correct diagnosis starts with a thorough physical examination and plain radiographs. Although it is difficult to accurately assess ligament stability and active function in a newborn, an accurate assessment of deficient anatomy is possible. Accordingly, a preliminary diagnosis can be given early on and then modified as the child grows. Once the child starts to use the hand, a correct diagnosis is much easier to establish. Physical examination and radiographs should guide classification of the thumb using the Blauth system (Fig. 34‑1).
34.2.1 Associated Conditions
It is critical for the hand surgeon to be aware of conditions that are associated with thumb hypoplasia. Oftentimes—especially in those with blood conditions—these children manifest radial deficiency as their only physical difference. Accordingly, the hand surgeon can be instrumental in diagnosing associated conditions. These conditions include, but are not limited to, Fanconi anemia, Holt–Oram syndrome, and VACTERL (vertebral anomalies, anal atresia, cardiac abnormalities, tracheoesophageal fistula, renal agenesis, and limb defects) complex.
Type I: Mild Hypoplasia
In this mildest type of hypoplasia, the thumb is thinner and shorter than normal. The joints show normal range of motion, and flexion and extension creases are easily visible. There may be slightly smaller thenar intrinsic muscles, mainly the abductor pollicis and opponens pollicis; however, all of the intrinsic muscles are present. The joints are stable. The first webspace deficiency may be slight to moderate.
Type II: Moderate Hypoplasia
In this type of hypoplasia (Fig. 34‑2), the bones of the ray are present but small. The radial carpal bones (trapezium and scaphoid) may be small and cause slight radial deviation of the hand. The first webspace is narrow and the thumb adducted. The metacarpophalangeal (MP) joint collateral ligaments are lax and the median-innervated thenar muscles are hypoplastic or absent. There may be extrinsic flexor and/or extensor deficiencies and these usually manifest by the lack of flexion or extension creases.
Type III: Severe Hypoplasia
The degree of thumb skeletal shortening and joint laxity is much more severe in these hands. These thumbs have been divided into subtypes A and B, based on the length of the metacarpal and stability of the carpometacarpal (CMC) joint. In type A (Fig. 34‑3), the CMC joint is stable, the metacarpal is narrow, and the child uses the thumb. In type B (Fig. 34‑4), the CMC joint is unstable, the metacarpal is incomplete, and the child does not use the thumb. Type IIIA thumbs have severely hypoplastic thenar muscles and usually lack an extrinsic flexor. The collateral ligaments at the MP joint are lax and prevent sustained large object grasp—children usually use the head of the metacarpal for grasp. The extrinsic muscles insert abnormally and conspire to act primarily as radial deviators and not as primary flexors or extensors. This is known as “pollex abductus.” Many anatomical variations of this anomaly exist, but the functional result is identical: when the extrinsic muscles activate, the thumb radially deviates. Type IIIB thumbs share many of the same characteristics as type IIIA thumbs with greater degree of hypoplasia. In addition, the CMC joint is unstable in these thumbs. Most children with these thumbs will not use the thumb routinely; instead, they automatically use the index and middle fingers.
Type IV: Floating Thumb
The floating thumb (pouce flottant, French; pendeldaumen, German) is only attached to the hand by a soft-tissue pedicle that contains a neurovascular bundle (Fig. 34‑5). There is no metacarpal. The thumb has a rudimentary nail and one or two small phalanges. It is functionless.
Type V: Aplasia
In this type of hypoplasia, the thumb is completely absent (Fig. 34‑6). The radial side of the hand is hypoplastic. There is often an associated deficiency of the radius; and if so, there may be concomitant hypoplasia or stiffness of the index finger. If the index finger is normal, many patients will autopronate the index finger and scissor-grasp with the middle finger. The pulp of the index finger widens and the digit pronates, sitting in a more abducted position and thus widening the intermetacarpal space.
Other Types: Five-Fingered Hand
In this type, the most radial digit looks and functions like a finger (Fig. 34‑7). It usually lies in the same plane as the ulnar four digits and is nonopposable. Severe deficiency of the first webspace is present. The metacarpal has a distal growth center and the intrinsic muscles are those that typically motor a finger, not a thumb.