This is a very rare situation, in which the extra digit is on the ring, middle or index finger. Of these fingers, the index finger is most often affected, whereas the ring finger is rarely affected.

This type of polydactyly can be associated with syndactyly, cleft hand and several syndromes.

Polysyndactyly presents various degrees of syndactyly affecting fingers three and four.

This is characterized by hand and arm abnormalities. The following are specific characteristics:

- Malformed or absent (aplasia) thumb

- A thumb that looks more like a finger

- Partial or complete absence of a radius

- Shortening and radial deviation of the forearms

- Triphalangeal thumb

- Duplication of the thumb (preaxial polydactyly)

People with the combination of Duane anomaly and radial ray malformations may have a variety of other signs and symptoms. These features include:

- Unusually shaped ears

- Hearing loss

- Heart and kidney defects

- A distinctive facial appearance

- An inward- and downward-turning foot (a clubfoot)

- Fused vertebrae.

The triphalangeal thumb has a different appearance than normal thumbs. The appearance can differ widely; the thumb can be a longer thumb, it can be deviated in the radio-ulnar plane (clinodactyly), thumb strength can be diminished. In the case of a five fingered-hand it has a finger-like appearance, with the position in the plane of the four fingers, thenar muscle deficiency, and additional length. There is often a combination with radial polydactyly.

Classification is performed by using x-ray imaging to see the bone structures.

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 and opponens pollicis, are not fully developed

. This type requires no surgical treatment in most cases.

- Type II is characterized by a tight web space between the thumb and index finger which restricts movement, poor thenar muscles and an unstable middle joint of the thumb metacarpophalangeal joint. 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 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 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).

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

Generally, triphalangeal thumbs are non-opposable. In contrast to most people with opposable thumbs, a person suffering from TPT cannot easily place his or her thumb opposite the other four digits of the same hand. The opposable thumb's ability to effortlessly utilize fingers in a "pinch" formation is critical in precision gripping. For the thumb to adequately grip, certain thumb criteria must be met (e.g. suitable position and length, stable joints and good thenar muscle strength). Because triphalangeal thumbs cannot easily oppose and do not possess many of the optimal qualities found in most opposable thumbs, they tend to cause the hand to be less effective in use and, therefore, prove to be more problematic in daily life.

Thumb hypoplasia is a spectrum of congenital abnormalities of the thumb varying from small defects to absolute retardation of the thumb. It can be isolated, when only the thumb is affected, and in 60% of the cases 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.

In an embryo the upper extremities develop from week four of the gestation. During the fifth to eighth week the thumb will further develop. 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, VACTERL association and thrombocytopenia absent radius (TAR syndrome).

The Wassel classification is used to categorise radial polydactyly, based upon the most proximal level of skeletal duplication.

Characteristics are:

- A fibrous band instead of the fibula

- Short deformed leg

- Absence of the lateral part of the ankle joint (due to absence of the distal end of the fibula), and what is left is unstable; the foot has an equinovalgus deformity

- Possible absence of part of the foot requiring surgical intervention to bring the foot into normal function, or amputation.

- Possible absence of one or two toes on the foot

- Possible conjoined toes or metatarsals

Partial or total absence of fibula is among the most frequent limb anomalies. It is the most common long bone deficiency and is the most common skeletal deformity in the leg. It most often is unilateral (present only on one side). It may also present as bilateral (affecting both legs). Paraxial fibular hemimelia is the most common manifestation in which only the postaxial portion of the limb is affected. It is commonly seen as a complete terminal deficiency, where the lateral rays of the foot are also affected. Hemimelia can also be intercalary in which case the foot remains unaffected. Although the missing bone is easily identified, this condition is not simply a missing bone. Males are affected twice as often as females in most series.

Schmitt Gillenwater Kelly syndrome is a rare autosomal dominant congenital disorder consisting of radial hypoplasia, triphalangeal thumbs, hypospadias, and maxillary diastema.

Hemimelia comprises

- Fibular hemimelia, Congenital longitudinal deficiency of the fibula or Fibular longitudinal meromelia

- Tibial hemimelia, Congenital longitudenal deficiency of the tibia, Congenital aplasia and dysplasia of the tibia with intact fibula, Congenital longitudinal deficiency of the tibia or Tibial longitudinal meromelia

- Radial Hemimelia, Congenital longitudinal deficiency of the radius, Radial clubhand, Radial longitudinal meromelia or Radial ray agenesis

- Ulnar hemimelia, Congenital longitudinal deficiency of the ulna, Ulnar clubhand or Ulnar longitudinal meromelia

People who are affected by Liebenberg Syndrome suffer from three main symptoms:

1. Dysplasia (improper formation) of the bony components of the elbow

2. Abnormal shape of carpal bones

3. Brachydactyly, a symptom where the fingers and toes are shorter than normal.

Fibular hemimelia or longitudinal fibular deficiency is "the congenital absence of the fibula and it is the most common congenital absence of long bone of the extremities." It is the shortening of the fibula at birth, or the complete lack thereof. In humans, the disorder can be noted by ultrasound in utero to prepare for amputation after birth or complex bone lengthening surgery. The amputation usually takes place at six months with removal of portions of the legs to prepare them for prosthetic use. The other treatments which include repeated corrective osteotomies and leg-lengthening surgery (Ilizarov apparatus) are costly and associated with residual deformity.

Hemifacial hypertrophy (also termed facial hemihypertrophy, facial hemihyperplasia, or Friedreich's disease) abbreviated as (HFH) is rare congenital disease characterized by unilateral enlargement of the head and teeth. It is classified as true HFH (THFH) with unilateral enlargement of the viscerocranium, and partial HFH (PHFH) in which not all structures are enlarged. Hemifacial hypertrophy can cause a wide spectrum of defects or may involve only muscle or bone. it is usually treated surgically. It is believed to be a minor form of hemihypertrophy.

The most common and defining features of BGS are craniosynostosis and radial ray deficiency. The observations of these features allow for a diagnosis of BGS to be made, as these symptoms characterize the syndrome. Craniosynostosis involves the pre-mature fusion of bones in the skull. The coronal craniosynostosis that is commonly seen in patients with BGS results in the fusion of the skull along the coronal suture. Because of the changes in how the bones of the skull are connected together, people with BGS will have an abnormally shaped head, known as brachycephaly. Features commonly seen in those with coronal craniosynostosis are bulging eyes, shallow eye pockets, and a prominent forehead. Radial ray deficiency is another clinical characteristic of those with BGS, and results in the under-development (hypoplasia) or the absence (aplasia) of the bones in the arms and the hands. These bones include the radius, the carpal bones associated with the radius and the thumb. Oligodactyly can also result from radial ray deficiency, meaning that someone with BGS may have fewer than five fingers. Radial ray deficiency that is associated with syndromes (such as BGS) occurs bi-laterally, affecting both arms.

Some of the other clinical characteristics sometimes associated with this disorder are growth retardation and poikiloderma. Although the presentation of BGS may differ between individuals, these characteristics are often observed. People with BGS may have stunted growth, short stature and misshapen kneecaps. Poikiloderma may also be present in people with this syndrome, meaning that their skin may have regions of hyperpigmentation and hypopigmentation, or regions where the skin is missing (atrophy).

Radial aplasia is a congenital defect which affects the formation of the radius bone in the arm. The radius is the lateral bone which connects to the wrist via articulation with the carpal bones. A child born with this condition has either a short or absent radius bone in one or both of his or her arm(s). Radial aplasia also results in the thumb being either partly formed or completely absent from the hand. Radial aplasia is connected with the condition VACTERL association. The cause for radial aplasia in unknown, but it widely believed to occur within the first ten weeks of gestation.

Presenting at birth, features of the disorder include moderately severe IUGR, microcephaly, craniosynostosis, moderately severe post uterine growth retardation, deafness, deep set eyes, cryptorchidism, truncal obesity and acanthosis nigricans, small teeth, prognathism, dislocated radial heads without generalized skeletal dysplasia, however, tall vertebrae, moderate mental retardation, hypothyroidism, insulin resistance, hypoparathyroidism.

Renal (kidney) defects are seen in approximately 50 percent of patients with VACTERL association. In addition, up to 35 percent of patients with VACTERL association have a single umbilical artery (there are usually two arteries and one vein) which is often associated with additional kidney or urologic problems. Renal abnormalities in VACTERL association can be severe, with incomplete formation of one or both kidneys or urologic abnormalities such as obstruction of outflow of urine from the kidneys or severe reflux (backflow) of urine into the kidneys from the bladder. These problems can cause kidney failure early in life and may require kidney transplant. Many of these problems can be corrected surgically before any damage can occur.

The following features are observed with VACTERL association:

- V - Vertebral anomalies

- A - Anorectal malformations

- C - Cardiovascular anomalies

- T - Tracheoesophageal fistula

- E - Esophageal atresia

- R - Renal (Kidney) and/or radial anomalies

- L - Limb defects

Although it was not conclusive whether VACTERL should be defined by at least two or three component defects, it is typically defined by the presence of at least three of the above congenital malformations.

Ectromelia is a congenital condition where long bones are missing or underdeveloped.

Examples include:

- Amelia

- Hemimelia

- Phocomelia

- Sirenomelia

RAPADILINO syndrome is an autosomal recessive disorder characterized by:

- RA: radial ray defect

- PA: patellar aplasia, arched or cleft palate

- DI: diarrhea, dislocated joints

- LI: little size (short stature), limb malformation

- NO: nose slender and normal intelligence.

It is more prevalent in Finland than elsewhere in the world.

It has been associated with the gene RECQL4. This is also associated with Rothmund-Thomson syndrome and Baller-Gerold syndrome.

Liebenberg Syndrome is a rare autosomal genetic disease that involves a deletion mutation upstream of the PITX1 gene, which is one that's responsible for the body's organization, specifically in forming lower limbs. In animal studies, when this deletion was introduced to developing birds, their wing buds were noted to take on limb-like structures.

The condition was first described by Dr. F. Liebenberg in 1973 while he followed multiple generations of a South African family, but it has since been noticed in other family lineages across the world.

Encephalocraniocutaneous lipomatosis (ECCL), otherwise known as Haberland syndrome, is a rare condition primarily affecting the brain, eyes, and skin of the head and face. It is characterized by unilateral subcutaneous and intracranial lipomas, alopecia, unilateral porencephalic cysts, epibulbar choristoma and other ophtalmic abnormalities.

It was named after Haberland and Perou who first described it.

Dysmelia can refer to

- missing (aplasia) limbs: amelia, oligodactyly, congenital amputation e.g. Tibial or Radial aplasia

- malformation of limbs: shortening (micromelia, rhizomelia or mesomelia), ectrodactyly, phocomelia, meromelia, syndactyly, brachydactyly, club foot

- too many limbs: polymelia, polydactyly, polysyndactyly

- others: Tetraamelia, hemimelia, Symbrachydactyly