Thumb hypoplasia is a spectrum of congenital abnormalities of the

thumb The thumb is the first digit of the hand, next to the index finger. When a person is standing in the medical anatomical position (where the palm is facing to the front), the thumb is the outermost digit. The Medical Latin English noun for thumb ...

varying from small defects to complete absence of the thumb.Riley, S.A. & Burgess, R.C. (2009). Thumb Hypoplasia. ''Journal of Hand Surgery'', vol 34A, 1564–1573 It can be isolated, when only the thumb is affected, and in 60% of the casesAshbaugh, H. & Gellman, H. (2009). Congenital Thumb Deformities and Associated Syndromes. Journal of Craniofacial Surgery, vol 20, number 4, 1039–1044 it is associated with radial dysplasia (or radial club, radius dysplasia, longitudinal radial deficiency). Radial dysplasia is the condition in which the forearm bone and the soft tissues on the thumb side are underdeveloped or absent.Manske, P.R. & Goldfarb, C.A. (2009). Congenital Failure of Formation of the Upper Limb. ''Hand Clinics'', 25, 157–170 In an embryo the upper extremities develop from week four of the gestation. During the fifth to eighth week the thumb will further develop.Hovius, S., Foucher, G. & Raimondi, P.L. (2002). ''The Pediatric Upper Limb''. London: Informa Healthcare In this period something goes wrong with the growth of the thumb but the exact cause of thumb hypoplasia is unknown. One out of every 100,000 live births shows thumb hypoplasia. In more than 50% of the cases both hands are affected, otherwise mainly the right hand is affected. About 86% of the children with hypoplastic thumb have associated abnormalities. Embryological hand development occurs simultaneously with growth and development of the cardiovascular, neurologic and hematopoietic systems. Thumb hypoplasia has been described in 30 syndromes wherein those abnormalities have been seen. A syndrome is a combination of three or more abnormalities. Examples of syndromes with an hypoplastic thumb are

Holt–Oram syndrome Holt–Oram syndrome (also called atrio-digital syndrome, atriodigital dysplasia, cardiac-limb syndrome, heart-hand syndrome type 1, HOS, ventriculo-radial syndrome) is an autosomal dominant disorder that affects bones in the arms and hands (the ...

VACTERL association The VACTERL association (also VATER association, and less accurately VACTERL syndrome) refers to a recognized group of birth defects which tend to co-occur (see below). This pattern is a recognized association, as opposed to a syndrome, because t ...

and thrombocytopenia absent radius (

TAR syndrome TAR syndrome (thrombocytopenia with absent radius) is a rare genetic disorder that is characterized by the absence of the radius bone in the forearm and a dramatically reduced platelet count. Signs and symptoms * Presents with symptoms of thro ...

Classification

In general there are five types of thumb hypoplasia, originally described by Muller in 1937 and improved by Blauth, Buck-Gramcko and Manske. - Type I: the thumb is small, normal components are present but undersized. Two muscles of the thumb, the

abductor pollicis brevis The abductor pollicis brevis is a muscle in the hand that functions as an abductor of the thumb. Structure The abductor pollicis brevis is a flat, thin muscle located just under the skin. It is a thenar muscle, and therefore contributes to the ...

and

opponens pollicis The opponens pollicis is a small, triangular muscle in the hand, which functions to oppose the thumb. It is one of the three thenar muscles. It lies deep to the abductor pollicis brevis and lateral to the flexor pollicis brevis. Structure The opp ...

, are not fully developed ,. This type requires no surgical treatment in most cases.Light, T.R. & Gaffey, J.L. (2010). Reconstruction of the Hypoplastic Thumb. Journal of Hand Surgery, vol 35A, 474–479 - Type II is characterized by a tight web space between the thumb and index finger which restricts movement, poor

thenar The thenar eminence is the mound formed at the base of the thumb on the palm of the hand by the intrinsic group of muscles of the thumb. The skin overlying this region is the area stimulated when trying to elicit a palmomental reflex. The word ...

muscles and an unstable middle joint of the thumb

metacarpophalangeal joint The metacarpophalangeal joints (MCP) are situated between the metacarpal bones and the proximal phalanges of the fingers. These joints are of the condyloid kind, formed by the reception of the rounded heads of the metacarpal bones into shallow cav ...

. This unstable thumb is best treated with reconstruction of the mentioned structures. - Type III thumbs are subclassified into two subtypes by Manske. Both involve a less developed first metacarpal and a nearly absent thenar musculature. Type III-A has a fairly stable

carpometacarpal joint The carpometacarpal (CMC) joints are five joints in the wrist that articulate the distal row of carpal bones and the proximal bases of the five metacarpal bones. The CMC joint of the thumb or the first CMC joint, also known as the trapeziometacar ...

and type III-B does not. The function of the thumb is poor. Children with type III are the most difficult patients to treat because there is not one specific treatment for the hypoplastic thumb. The limit between

pollicization {{Short description, Surgically turning an existing finger into a thumb Pollicization (or pollicisation) is a hand surgery technique in which a thumb is created from an existing finger. Typically this consists of surgically migrating the index fin ...

and reconstruction varies. Some surgeons have said that type IIIA is amenable to reconstruction and not type IIIB. Others say type IIIA is not suitable for reconstruction too. Based on the diagnosis the doctor has to decide what is needed to be done to obtain a more functional thumb, i.e. reconstruction or pollicization. In this group careful attention should be paid to anomalous tendons coming from the forearm (extrinsic muscles, like an aberrant long thumb flexor –

flexor pollicis longus The flexor pollicis longus (; FPL, Latin ''flexor'', bender; ''pollicis'', of the thumb; ''longus'', long) is a muscle in the forearm and hand that flexes the thumb. It lies in the same plane as the flexor digitorum profundus. This muscle is uniqu ...

). - Type IV is called a pouce flottant, floating thumb. This thumb has a

neurovascular bundle A neurovascular bundle is a structure that binds nerves and veins (and in some cases arteries and lymphatics) with connective tissue so that they travel in tandem through the body. Structure There are two types of neurovascular bundles: superficia ...

which connects it to the skin of the hand. There’s no evidence of thenar muscles and rarely functioning tendons. It has a few rudimentary bones. Children with type IV are difficult to reconstruct. This type is nearly always treated with an index finger pollicization to improve hand function. - Type V is no thumb at all and requires pollicization.

Cause

The cause is unknown, and likely related to genetic abnormalities. Children with Fanconi anemia can sometimes display hypoplasia of the thumb.{{citation needed, date=June 2020

Diagnosis

Three main points in diagnosing thumb hypoplasia are: width of the first web space, instability of the involved joints and function of the thumb. Thorough

physical examination In a physical examination, medical examination, or clinical examination, a medical practitioner examines a patient for any possible medical signs or symptoms of a medical condition. It generally consists of a series of questions about the patien ...

together with anatomic verification at operation reveals all the anomalies. An X-ray of the hand and thumb in two directions is always mandatory. When the pediatrician thinks the condition is associated with some kind of syndrome other tests will be done. More subtle manifestations of types I and II may not be recognized, especially when more obvious manifestations of longitudinal radial deficiency in the opposite extremity are present. Therefore, a careful examination of both hands is important.

Treatment

When it comes to treatment it is important to differentiate a thumb that needs stability, more web width and function, or a thumb that needs to be replaced by the index finger. Severe thumb hypoplasia is best treated by

of the index finger. Less severe thumb hypoplasia can be reconstructed by first web space release, ligament reconstruction and muscle or tendon transfer. It has been recommended that pollicization is performed before 12 months, but a long-term study of pollicizations performed between the age of 9 months and 16 years showed no differences in function related to age at operation. It is important to know that every reconstruction of the thumb never gives a normal thumb, because there is always a decline of function. When a child has a good index finger, wrist and fore-arm the maximum strength of the thumb will be 50% after surgery in comparison with a normal thumb. The less developed the index finger, wrist and fore-arm is, the less strength the reconstructed thumb will have after surgery.

References

Plastic surgery Congenital disorders Congenital disorders of musculoskeletal system Rare diseases Fingers