Thumb hypoplasia is a congenital hand anomaly, which ranges in severity from the presence of a small and weak thumb to complete absence of the thumb. The International Federation of Societies for Surgery of the Hand classifies thumb aplasia as a type V congenital hand abnormality that is often associated with radial longitudinal deficiency.1,2 Approximately 11.2% of all birth deformities of the hand are congenital thumb abnormalities, 4.6% of which consist of thumb hypoplasia/aplasia.3 A child with a deficient or absent thumb encounters difficulty in social and educational development because a hand without a thumb loses 40% of its usefulness. Children are able to adapt to an absent or weak thumb and learn to use a lateral pinch between the long and index fingers to compensate if surgical treatment is not performed. Autosomal dominant inheritance with variable expressivity is suspected in patients with thumb hypoplasia but not yet proven.4,5
Four weeks after fertilization, embryogenesis of the upper extremity begins with the formation of the upper-limb bud on the lateral wall of the embryo. The digits of the hand are recognizable by the sixth week. Eight weeks after fertilization, embryogenesis is complete and all limb structures are present. The majority of congenital anomalies of the upper extremity occur during this period of rapid limb development.6 Apical ectodermal ridge (AER) signaling is mediated via the fibroblast growth factor (FGF) family of proteins.7 This signaling determines proximodistal patterning and the patterning and differentiation of the individual segments of the limb. Researchers believe that abnormalities involving AER and FGF signaling molecules produce the abnormal phenotypes seen in hypoplastic thumb and other radial ray deficiencies, particularly when FGF-4 and FGF-8 are inactivated.1
Modified Blauth Classification
Thumb hypoplasia is divided into six distinct types:
||Minimal shortening and narrowing of thumb
||First web space narrowing
Metacarpophalangeal (MCP) joint instability, usually ulnar collateral ligament
Hypoplasia intrinsic thenar muscles
||A) Type II features, plus extrinsic tendon abnormalities
Hypoplastic metacarpal, stable carpometacarpal (CMC) joint
B) Type II features, plus extrinsic tendon abnormalities
Partial metacarpal aplasia, unstable CMC joint
||Pouce flottant or "floating thumb"
||Complete absence of the thumb
The most common type of thumb hypoplasia is type V, constituting 31% to 36% of cases.8
A careful upper-limb examination is necessary to determine the grade of hypoplasia and to ascertain whether other aspects of radius deficiency are present. The thumb should be inspected and examined for size, consistency, stability, and, most importantly, stability of the MCP and CMC joints. Radiographs should be reviewed to evaluate the bony architecture of the thumb.
The cardiovascular, hematopoietic, and neurologic systems develop at the same time as the hand and therefore anomalies of the thumb may indicate the presence of other systemic anomalies. In a prospective study conducted on 51 patients with 81 hypoplastic thumbs, the most commonly associated anomaly found was hypoplasia of the radius.4 Other related syndromes include Fanconis anemia, VATER (vertebral, anal, tracheo, eosophageal, and renal) syndrome, Holt-Oram syndrome, Silver syndrome, Poland syndrome, and TAR (thrombocytopenia absent radius) syndrome.
Infants with any form of thumb hypoplasia must be evaluated for any related syndromes. Life-threatening Fanconis anemia may not become clinically apparent until a child reaches 7 years of age. Children should undergo a chromosome challenge test as soon as possible; if Fanconis anemia is diagnosed, then the child can be treated with bone marrow biopsy before the onset of symptoms.
Type I hypoplasia does not require treatment because children often have no significant loss of function. Type II hypoplasia requires reconstruction to provide a stable thumb. Reconstruction includes deepening of the web space, stabilization of the MCP joint, and a Huber opponensplasty using the abductor digiti minimi. In patients with type IIIA hypoplasia, due to the abnormalities present in the extrinsic flexors and/or extensors, a tendon transfer is performed as well, usually in the form of an extensor indicis proprius to extensor pollicis longus transfer.
The surgical treatment for types IIIB, IV, and V hypoplastic thumbs is pollicization. The aim of this surgery is to aesthetically and functionally improve the hand. Surgery involves the ablation of the hypoplastic thumb if one is present and the transfer of another digit into the thumb position. The index finger is most commonly used, but one can transfer any of the other digits or even a toe if necessary.
The index finger metacarpal is cut while keeping the arteries and veins attached, and the finger is rotated 140° to 160° and replaced in the usual position of the thumb. It is necessary to shorten the index finger by removal of most of the metacarpal shaft to ensure the tip of the new thumb reaches slightly proximal to the line of the proximal interphalangeal joint of the index finger.9 This shortening is critical because it also destroys the growth plate of the metacarpal head, ensuring that no longitudinal growth occurs at the base of the new thumb. Tendons from the migrated index finger may be shortened and rerouted to provide good movement. The index finger is usually used because it can be moved into the thumb position while still maintaining all vital neurovascular components, whereas these would be lost in the transfer of anyone of the other digits. Also, the activities of the index finger are easily taken over by the long finger.
Surgeons disagree over the best time to perform the surgery, but most surgeries are performed between 6 months and 1 year of age. Some surgeons propose that it is better to operate later because of the difficulties that arise when working with small anatomical structures, whereas others believe that operating earlier is better so that a child can develop proper fine coordination of hand movements.9
Some parents refuse this surgery because they do not want to accept the risk of transferring the index finger. Others refuse pollicization for cultural or cosmetic reasons. Patients who refuse pollicization can choose to have a free microvascular second toe/metatarsal bone/toe joint transfer. A study performed at the Division of Plastic and Reconstructive Surgery at Niigata University Hospital found that, while pollicization of the index finger led to a thumb with a more normal appearance, transfer of the free vascularized metatarsophalangeal joint led to greater hand and grip strength and is therefore a viable alternative to pollicization.10 Typically, two teams of surgeons are involved with the simultaneous dissection of the hand and foot.11
Four important parameters must be considered when judging the success of the pollicization procedure including mobility at the MP and interphalageal joints, opposition that allows pinch and grip, sensibility, and quality of the skin.12 Although achieving a completely normal hand is an unrealistic goal, pollicization greatly improves the cosmetic appearance and function of the hand. The original status of the transposed digit and the surrounding musculature is the key in determining the results of pollicization.6,13-15 A mobile index finger transferred to the thumb position provides stability for grasp and mobility for pinch, whereas a stiff index finger provides a stable thumb for grasp but will not participate in pinch.16 The best result is a thumb with 30% of normal strength, good opposition, and good sensation.
In a long-term retrospective study, Manske and colleagues17 reported that total active motion of the pollicized digit averaged 98°. Average grip strength was 21%, and average pinch strength was: opposition 26%; apposition 26%; and tripod 23%. The time required to perform activities averaged 22% longer than the standard for a normal hand.1,17 Difficulties after pollicization entail the handling or manipulation of small objects, and this difficulty increases when grip strength is needed as well (eg, buttons or small snaps).18 Pollicization is the treatment of choice for children with type IIIB, IV, or V thumb hypoplasia/aplasia; however, second-toe metatarsal bone transfer is an acceptable alternative when parents are resistant to pollicization.
A 2-year-old girl with a history of L index pollicization for Pouce flottant presents for treatment of right thumb hypoplasia.
- Tay SC, Moran SL, Shin AY, Cooney WP 3rd. The hypoplastic thumb. J Am Acad Orthop Surg. 2006; 14:354-366.
- Swanson AB, Swanson GD, Tada K. A classification for congenital limb malformation. J Hand Surg Am. 1983; 8:693-702.
- Lister G. Pollex abductus in hypoplastic and duplication of the thumb. J Hand Surg Am. 1991; 16:626-633.
- Abdel-Ghani H, Amro S. Characteristics of patients with hypoplastic thumb: A prospective study of 51 patients with the results of surgical treatment. J Pediatr Orthop B. 2004; 13:127-138.
- Pashayan H, Fraser FC, McIntyre JM, Dunbar JS. Bilateral aplasia of the tibia, polydactyly and absent thumb in father and daughter. J Bone Joint Surg Br. 1971; 53:495-499.
- Kozin SH. Upper-extremity congenital anomalies. J Bone Joint Surg Am. 2003; 85:1564-1576.
- Sun X, Mariani FV, Martin GR. Functions of FGF signaling from the apical ectodermal ridge in limb development. Nature. 2002; 418:501-508.
- Kleinman WB. Management of thumb hypoplasia. Hand Clin. 1990; 6:617-641.
- Flatt A. The Care of Congenital Hand Anomalies. St. Louis, MO: Quality Medical Publishing; 1994:96-120.
- Shibata M, Yoshizu T, Seki T, Goto M, Saito H, Tajima T. Reconstruction of a congenital hypoplastic thumb with use of a free vascularized metatarsophalangeal joint. J Bone Joint Surg Am. 1998; 80:1469-1476.
- Tu YK, Yeh WL, Sananpanich K, et al. Microsurgical second toe-metatarsal bone transfer for reconstructing congenital radial deficiency with hypoplastic thumb. J Reconstr Microsurg. 2004; 20:215-225.
- Egloff DV, Verdan C. Pollicization of the index finger for reconstruction of the congenitally hypoplastic or absent thumb. J Hand Surg Am. 1983; 8:839-848.
- McCarroll HR. Congenital anomalies: A 25-year overview. J Hand Surg Am. 2000; 25:1007-1037.
- Kozin SH, Weiss AA, Webber JB, Betz RR, Clancy M, Steel H. Index finger pollicization for congenital aplasia or hypoplasia of the thumb. J Hand Surg Am. 1992; 17:880-884.
- Clark DI, Chell J, Davis TR. Pollicisation of the index finger: A 27-year follow-up study. J Bone Joint Surg Br. 1998; 80:631-635.
- Green DP, Hotchkiss RN, Pederson WC, Wolfe SW, eds. Thumb hypoplasia. In: Green's Operative Hand Surgery. 5th ed. New York, NY: Churchill Livingstone; 2005:1445-1455.
- Manske PR, Rotman MB, Dailey LA. Long-term functional results after pollicization for the congenitally deficient thumb. J Hand Surg Am. 1992; 17:1064-1072.
- Staines KG. Functional outcome for children with thumb aplasia undergoing pollicization. Plastic Reconstr Surg. 2005; 116:1314-1323.