What Is a Carpal Coalition?
Carpal coalitions, defined by the abnormal union of two or more carpal bones, are common anatomical variations affecting 0.1 percent of the population. There are two main carpal coalition types: non-osseous and osseous (synostosis). The carpals are joined as a single osseous block in the osseous coalition; in the non-osseous coalition, however, the affected carpals are joined by fibrous tissue (syndesmosis), cartilage (synchondrosis), or a combination of the two. It may be preferable to refer to these separate entities as osseous and non-osseous coalitions, as they more precisely characterize the nature of the union, even if they are sometimes called complete and imperfect coalitions, respectively.
The coalition of the carpus is typically observed in otherwise healthy individuals and may be a congenital or acquired defect. The former most frequently manifests as a standalone event; it can also happen associated with recognized metabolic problems or as part of a syndrome. Two bones in the same row are usually involved in isolated fusions, but numerous bones are frequently involved in syndromic-associated fusions. Though this ailment has been observed in almost every possible combination, carpals in the ulnar area, on the same row, are the most commonly implicated. Intercarpal fusion is also genetically transmissible, indicating a dominant Mendelian inheritance pattern unrelated to sex.
What Are the Causes of the Carpal Coalition?
Carpal fusion is a misnomer because an impairment of normal segmentation of the carpal mesenchyme causes the anomaly. The carpal coalition is less problematic because its definition simply denotes a link between two bones. The carpal coalition is either an acquired or congenital aberration; the former is caused by a failure to differentiate during the embryonic stage. The term "fusion" should not be used to characterize this aberration because it refers to the inadequate delineation of cartilaginous precursors rather than the joining of two previously different structures. On the other hand, a number of inflammatory arthropathies, such as psoriatic arthritis, juvenile arthritis, rheumatoid arthritis, and Reiter's syndrome, can cause carpal coalition.
Intercarpal fusion may also result from the metaplastic conversion of mesodermal derivatives such as fibrous, cartilaginous, and ligamentous tissue to bone. This mechanism is responsible for connecting the pisiform with the triquetrum or hamate. Trauma-associated fusion may indicate primary carpus damage, or it may indicate a surgical arthrodesis necessary to preserve joint stability and partial motion. After a thorough examination of pertinent medical data, none of the patients had ever experienced trauma, arthritis, or prior wrist surgery.
What Is the Classification for the Carpal Coalition?
Currently, no established classification system includes all probable intercarpal fusion types. De Villiers Minnaar presented a four-type classification method based on radiographic examination for the LT coalition in 1952. This approach has since become the most extensively used classification mechanism for the carpal coalition. However, its application to all kinds of carpal synostosis is questionable, as it ignores the correlation between co-existing abnormalities throughout the skeleton and intercarpal fusion. Additionally, Minnaar's method covers only osseous coalition variations (Minnaar Type II and III), combining the significantly broader diversity related to non-osseous coalitions into a single discrete group (Minnaar Type I).
Minaar Type [2] categorized the range of lunotriquetral coalitions into four categories:
- type I: proximal fibrous or cartilaginous coalition.
- type II: incomplete bony fusion with distal notch.
- type III: complete bony fusion.
- type IV: complete bony fusion with other carpal anomalies.
What Are the Radiographic Appearances of the Carpal Coalition?
The radiological appearance of the scapholunate interosseous joint space expanding is frequently observed, even when the scapholunate interosseous ligament remains intact. The second most common coalition is between the bones capitate and hamate. A few other uncommon fusions, including pisiform-hamate, trapezium-trapezoid, and capitate-trapezoid, have also been reported.
Although they rarely cause symptoms, the pain has been associated with partial fusion and cystic abnormalities in the neighboring bones. In conjunction with other malformations, coalitions within the bones of different carpal rows, or coalitions across carpal bones, massive carpal fusion, and the distal extremities of the ulna or radius can be seen in conditions such as arthrogryposis, symphalangism, Holt-Oram syndrome, Turner syndrome, or acrocephalosyndactyly syndromes.
What Are the Clinical Significances?
Carpal coalition is a mostly asymptomatic syndrome that was unintentionally found on radiographs taken for unrelated purposes. There isn't a noticeable impact on wrist function from either the osseous or non-osseous coalition. Nonetheless, the biomechanical changes brought about by structural fusion at the wrist may cause symptoms.
In extreme physical stress, a person may be more susceptible to recurrent sprains and pain due to the loss of motion between the fused bones and the corresponding increase at adjacent joints. This might apply especially to non-osseous coalitions, where insufficient intra-articular cartilage production could result in morphological and clinical conditions that mimic degenerative arthritis. Moreover, patients with osseous coalition appear to be more susceptible to fractures due to the preservation of cavities, grooves, and notches at the fusion site.
How Is the Carpal Coalition Managed?
Conservative therapy is usually sufficient in cases of coalition that are asymptomatic or very slightly symptomatic. On the other hand, limited wrist arthrodesis, accomplished by compressing the affected bones, has shown good resiliency with tolerable effects on wrist motion following surgery in situations where the intensity and duration of pain justify surgical intervention. A complete recovery requires postoperative immobilization, which may take two to five months to achieve until radiographic evidence of fusion. In addition, comprehensive rehabilitation following surgery is necessary to guarantee the best possible recovery of wrist range of motion and functional grip strength.
Conclusion:
All carpal synostosis patients manifested asymptomatically, with the most significant percentage of instances reported in Afro-Caribbean (primarily Haitian) populations. The lunate-triquetral coalition was the most commonly isolated variety in the research population. Even though a complete understanding of carpal kinematics is still lacking, more research using advanced dynamic imaging on patients with carpal coalition may help clarify our understanding of carpal biomechanics and functional wrist anatomy.
