Carpal coalitions are relatively common anatomical variants present in approximately 0.1% of the population and are characterized by the union osseous (synostosis) and non- osseous of two or more carpal bones.
Carpal coalition may represent a congenital or an acquired anomaly seen in healthy individuals,
may present independently or in association with syndromic or metabolic disorders. Intercarpal fusion is genetically transmissible (non-sex linked,
Mandelian dominant pattern of inheritance).
Among the syndromes most related to our clinical cases is symphalangism which consists of the ankylosis of the interphalangeal joints of the fingers or toes,
less common of the metacarpophalangeal joints,
mainly affecting the fifth finger (55%),
the fourth finger (5%) and the third finger (1%),
clinically may show rigidity or present as an incidental finding on radiographs.
It can be associated with many conditions,
including Apert syndrome,
Carpal / Tarsal Coalition or Cushing's Symphalangism (proximal interphalangeal)
Carpal coalition may result from multiple inflammatory arthropathies including rheumatoid arthritis,
juvenile arthritis,
psoriatic arthritis,
and Reiter’s syndrome among others.
Metaplastic conversion of mesodermal derivatives such as fibrous,
cartilaginous,
and ligamentous tissue to bone may also lead to intercarpal fusion,
union of the pisiform with either the triquetrum or hamate is accomplished via this mechanism.
Trauma-associated fusion may reflect primary injury to the carpus,
or alternatively,
surgical arthrodesis performed to maintain stability and partial mobility of the joint.
Isolated fusions typically involve two bones within the same row,
while syndromic associated fusions are quite often multiple in nature.
Forms of carpal coalition: osseous coalition (synostosis) and non - osseous coalition.
Osseous coalition: carpals are united as a single osseous block.
Non - osseous coalition: carpals are united either by cartilage (synchondrosis),
fibrous tissue (syndesmosis) or some combination of the two.
Symptoms may manifest secondary to biomechanical stress at the site of fusion or fractures,
but mostly patients are asymptomatic.
Carpals located in the ulnar region,
occupying the same row,
are most frequently involved.
Coalition between the lunate and triquetrum is the most common variant,
the highest prevalence of this anomaly is seen in individuals of West African descent,
with reports as high as 9.5 % in some groups.
The American population,
the prevalence ranges between 0.08 % and 0.13 %,
with an estimated frequency of 0.1 % among white Americans and 1.6 % among Americans of African origin.
Second in frequency is coalition between the capitate and hamate,
which demonstrates similar variations in prevalence.
The reported prevalence in East Africans (0.14 %) and Caucasians (0.25–0.29 %) is known to be lower than in those of West African descent (0.4–0.8 %).
Additionally,
pisiform–hamate coalition is believed to occur most often in African populations,
with frequencies ranging from 0.11 % to 0.76 %.
Other,
less common,
forms of carpal coalition have been reported; however,
their prevalence has not been the subject of extensive research.
The prevalence of this condition has yet to be studied specifically in Hispanic and Afro-Caribbean populations.
LT coalition comprised nearly 90 % of the identified cases in our sample.
Non-osseous coalitions are less likely to be identified by radiography compared to skeletal analysis or physical dissection.
There is no standardized classification system that encompasses all potential variants of intercarpal fusion.
In 1952,
De Villiers Minnaar proposed a four-type classfication scheme for lunate - triquetrum coalition based on radiographic assessment; it neglects the association between intercarpal fusion and the presence of co-existing anomalies throughout the skeleton and Minnaar’s system only addresses variations in osseous coalition (Minnaar Type II and III),
compiling the considerably more extensive variation associated with non-osseous coalition into a single discrete category (Minnaar Type I)
Burnett classified coalition anomalies as falling into one of two discrete categories: osseous versus non-osseous.
Additionally,
it avoids the inconsistent and narrow scope of associated anomalies described by Minnaar’s scheme.
Carpal coalition is largely an asymptomatic condition discovered incidentally on radiographs taken for unrelated reasons.
Neither osseous nor non-osseous coalition appears to have a discernible effect on wrist function.
However,
symptoms may manifest because the biomechanical alterations that result from structural fusion at the wrist.
Loss of movement between the fused bones and a compensatory increase in motion at surrounding joints,
theoretically predisposes individuals to recurrent sprains and pain under conditions of excessive physical stress.
Asymptomatic cases of coalition rarely require anything more than conservative management.
However,
in cases where the severity and duration of pain warrants surgical intervention,
limited wrist arthrodesis,
has demonstrated good reliability.