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Coalitions
Coalition represents abnormal fusion between two or more bones; it occurs more commonly in the bony tarsus but is also seen in the carpal bones. The condition may be congenital or acquired. Coalitions may be complete or incomplete and are bony (synostosis), cartilaginous (synchondrosis), or fibrous (syndesmosis).
Carpal coalition is often an incidental finding, although fused bones are known to be more prone to fracture. On the other hand, patients with tarsal coalition usually present in the second and third decades of life with foot pain.
Etiology
Both tarsal and carpal coalitions may be acquired as a result of trauma; this may be secondary to injury or may be surgically induced (arthrodesis). It may also result from infection (including osteomyelitis and tuberculosis) or articular disorders such as juvenile chronic arthritis and osteoarthritis.
Early research by Pfitzner, Sloman, Badgley, Harris, and others favored a theory of incorporation of accessory ossicles into adjacent tarsal bones as an explanation for congenital coalition. This theory was later disproved by the discovery that coalition was present in fetuses. After Leboucq's initial proposition in 1890 and its subsequent confirmation by Harris's anatomic studies on fetal cadavers in 1955, it is now universally accepted that congenital or developmental coalition represents a disorder of organization of primitive mesenchyme in the fifth week of gestation.
A tarsal-carpal coalition syndrome exists. This is transmitted as an autosomal dominant condition and is characterized by fusion of carpals, tarsals, and phalanges and short first metacarpals, causing brachydactyly and humeroradial fusion.
Both carpal and tarsal coalition occur in various syndromes: Ellis-van Creveld syndrome, symphalangism, arthrogryposis multiplex congenita, hand-foot-uterus syndrome, Apert syndrome, gonadal dysgenesis (Turner syndrome), Nievergelt syndrome, fetal alcohol syndrome, and Holt-Oram syndrome, as well as diastrophic dwarfism.
In some cases of tarsal coalition, a family history of identical abnormalities is obtained. Leonard reported that 39% of 98 first-degree relatives of 31 patients with peroneal spastic flat foot and partial coalition had some type of fusion themselves, although the pattern varied among the patients and their relatives. He proposed that the disorder was of autosomal dominant inheritance with genetic variability of expression.
Prevalence and Epidemiology
Coalitions have been part of the human gene pool for centuries. In 1965, Harris discovered a coalition in a Mayan archeological specimen found in Guatemala. In 1969, Heiple and Lovejoy demonstrated the presence of bilateral talocalcaneal coalitions in a pre-Columbian Indian specimen.
The earliest mention of coalition in the literature was made in 1829, when Cruveilhier produced the first known anatomic description of calcaneonavicular coalition. It was Holl, in 1880, who first theorized the relationship between tarsal coalition and peroneal spastic flatfoot.
The true prevalence of tarsal coalitions is not known; estimates range from far less than 1% of the population to 1% to 2%, although they may well be higher because patients are commonly asymptomatic. A recent dissection and CT study of cadaveric feet found that the incidence of nonosseous tarsal coalition is around 12.7%, much higher than previously thought. There is a slight male preponderance. The condition is bilateral in around 50% of cases.
The prevalence of isolated carpal coalition is quoted at 0.08% to 0.13%. It tends to occur more commonly in the black population, and there is a strong female predilection. Carpal fusions are frequently bilateral.
Clinical Presentation
Tarsal Coalition
Although some patients may be asymptomatic, patients typically present in the second or third decade of life with vague pain in the foot, which is often aggravated by minor trauma or unusual athletic activity. An earlier age at onset is rare, perhaps because, at this time, the fusion is fibrous or cartilaginous. Symptoms usually become more pronounced with progressive ossification of the coalition.
Physical examination can reveal pes planus, limited subtalar motion (worse with talocalcaneal coalition), and shortening with persistent or intermittent spasm of the peroneal muscles. The rigid foot may be held in valgus, although a varus deformity can be seen with anterior tibial spasm. A cavus foot may also be seen at presentation.
Carpal Coalition
This is mostly an asymptomatic condition. It may produce symptoms by virtue of alteration of the normal biomechanics of the wrist. This can then predispose the affected joints and surrounding soft tissues to abnormal stress, leading to discomfort or pain during repetitive strain, such as while playing musical instruments or racquet sports. Partial coalitions and cystic changes are associated with an increased incidence of pain. Fused carpal bones also lead to an increased risk of fracture. Patients with congenital lunotriquetral coalition may poorly tolerate stress loading or trauma, resulting in symptoms similar to degenerative arthritis or pseudarthrosis. Kienböck disease may also present in conjunction with carpal coalition.
Activities posing a high stress demand on the wrist movement, especially in sportsmen, result in progressive loading and thus early degenerative arthritis or pseudarthrosis, leading to earlier presentation.
Manifestations of the Disease
Tarsal Coalition
Isolated coalitions can be classified according to the bones that are affected— calcaneonavicular, talocalcaneal, talonavicular, and calcaneocuboid in decreasing order of frequency—although improved detection using CT and MRI has shown that talocalcaneal coalition is at least as common as the calcaneonavicular type.
Tarsal fusions occurring as part of a syndrome may have atypical patterns or may even involve the whole tarsus.
Calcaneonavicular Coalition
This is one of the most frequent types of coalitions and may be bilateral. The anatomy of calcaneonavicular coalition was described by Cruveilhier in 1829, nearly a century before its clinical relationship to peroneal spastic flatfoot was radiographically shown by Sloman in 1921. It may present as part of the rigid flatfoot or may be asymptomatic. Calcaneonavicular coalition is defined as abnormal coalescence of the calcaneus with the tarsal navicular bone. As with the other coalitions, it may be described as osseous (synostosis) or nonosseous (synchondrosis or syndesmosis), but it is probably best to consider these categories as points on a histopathologic continuum. The normal morphologic relationship between the calcaneus and the navicular can be described as a slender gap between the two articulated bone structures, and this occurs in a statistically normal distribution in the general population. At either end of the spectrum is found a broader gap between the two and completely continuous osseous ossification.
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