The Incidence of Accessory Ossicles of the Wrist

                               Dr. KS Dhillon

In the wrist, there are more than 20 known accessory ossicles (1). Most of these accessory ossicles originate from unfused secondary ossification centers. They can be considered as developmental anomalies. These ossicles are generally asymptomatic and detected incidentally as radiological findings. They can rarely become symptomatic due to impingement, trauma, and other such causes. A more common problem in clinical practice is that accessory ossicles can be misinterpreted as avulsion fractures in patients with trauma, which can lead to overtreatment or the use of cross-sectional imaging modalities such as CT which can cause further radiation exposure. Hence it is important to know the localization, frequency, and distribution of accessory ossicles.

Although there are numerous studies on the distribution and incidence of accessory ossicles of the foot and ankle, the number of studies about the accessory ossicles of the wrist is limited (2,3). Most of the studies are case reports. There are only two studies with a large patient series (4,5,6,7). Both of these studies are considerably outdated (1932, 1953). They were performed using conventional radiography (8,9). There are no recent large patient series that investigated the distribution and incidence of accessory ossicles of the wrist using digital radiography.

In the literature, the incidence of accessory ossicles of the wrist varies.  O'Rahilly (9) conducted a study in 1953. He analyzed 743 plain radiographs. He reported that the incidence of accessory ossicles of the wrist was 1.6%.  Bogart (8) conducted a study with 1452 radiographs in 1932 and found that the incidence of accessory ossicles was 0.3%. These studies were performed when analog radiography was used. They are, therefore, considerably outdated. In another study by Gursoy et al (10), the incidence of accessory ossicles of the wrist was 9.7% among 1146 radiographs. The most important reason for the significant difference between the results of these studies is the technological advancements in radiography techniques. Better image quality in digital radiography as compared with conventional radiography has enhanced the visibility of accessory ossicles on radiographs. The studies were conducted with populations of different ethnic origins and this can be another reason for the difference between the reported incidence values. The number of existing studies are, however, not sufficient to determine if there is a difference between ethnic groups in terms of the accessory ossicles of the wrist. There are numerous studies on accessory ossicles of the foot and sesamoid bones of the hand conducted with different populations, reporting different incidences (2,3,11,12,13). 

The most common accessory ossicles are os triangulare and os ulnostyloideum. Both ossicles are present close to the ulnar styloid process. A clear distinction needs to be made since ulnar styloid process fractures, various ossifications, and dystrophic/heterotopic calcifications are also localized in this area.

These ossicles may be difficult to distinguish from a fractured ulnar styloid process. Accessory bones have well-defined, regular contours, whereas the contours of a fracture are irregular. Accessory bones have a cortex. Cortical continuation cannot be seen on at least one surface of a fracture. Bogart (8) in a study of 1452 plain radiographs found only two os triangulare (0.1%). Aydinlioglu et al (14) analyzed 388 plain radiographs to investigate the incidence of os ulnostyloideum. They found the incidence to be 2.5%.

The incidence of each accessory ossicle is not known. One of the accessory ossicles that is relatively common is the os styloideum. The clinical significance of os styloideum stems from its relationship with the carpal boss. Carpal boss is a painful lump on the dorsum of the wrist. Os styloideum is defined as a separate ossicle. It may have a fibrous connection with metacarpal bones or carpus. The carpal boss is defined as a bony protuberance, fused to the base of the 2nd and 3rd metacarpal bones. 

According to Lawson (15), the incidence of os styloideum in the normal wrist is in the range of 1.3 to 3%. Greditzer et al (16) conducted a study with 16 National Hockey League players. He found os styloideum in 13 of the players (81%). This incidence is markedly higher than the results of previous studies conducted with the general population. Although the etiology of os styloideum is not entirely clear, it is believed to be due to adaptive and reactive bone formation. 

The os centrale carpi is usually associated with other congenital anomalies such as Holt–Oram syndrome, hand-foot-uterus syndrome, etc. It is rarely an isolated finding (17).  A study by Gursoy et al (10) was the first to report an incidence of os centrale carpi, which was found in seven patients, leading to an incidence of 0.6%. Os centrale carpi can be confused with scaphoid fractures. It can cause pain due to osteonecrosis (4,18,19). 

Irregular and linear lucent areas that are superposed on the scaphoid waist due to os centrale carpi may make it challenging to distinguish between this ossicle and a transverse, ununited scaphoid fracture (4). An acute scaphoid fracture usually shows irregular or noncorticated sharp margins. An os centrale carpi generally has better-defined cortical margins. It is usually dense, round, and structureless. Old fractures usually exhibit demineralization and cyst formation. It is not always possible to differentiate between these fractures and os centrale carpi, especially in patients with no history of trauma. A CT scan is useful in such cases.

Os epilunatum is another rare ossicle. It was detected only in two patients (0.2%) in a study by Gursoy et al (10). In a patient series with 1452 radiographs Bogart (8) did not find an os epilunatum. 

According to Mauler et al (6), accessory ossicles constituted an important differential diagnosis in chronic wrist pain. They presented the first case that manifested with inflammatory changes and synovitis around the os epilunatum. Another important point to consider in patients complaining about pain due to os epilunatum is concomitant scapholunate ligament tears (6). It is therefore essential to examine the adjacent anatomical structures when an accessory ossicle is identified. This is especially so in patients who planned to undergo surgical excision.

There is no significant difference between the patients with and without accessory ossicles in terms of gender and side. The mean age is higher among patients who have accessory ossicles than those who do not have the same. 

A study by Sun et al (20), investigated the distribution patterns of sesamoid bones in metatarsophalangeal (MTP) joints. The number of sesamoid bones and the number of MTP joints with sesamoids had a positive correlation with age. They found that the number of sesamoid bones and affected joints increased with age. Ossification of accessory ossicles and sesamoids usually begins before the age of 20 years and continues throughout life. These bones become visible on radiographs once they are ossified. 

In conclusion, accessory ossicles of the wrist can be confounding. They can be confused with fractures in trauma patients and are frequently ignored in patients presenting with pain. The most common accessory ossicles in the wrist are os triangulare and os ulnostyloideum, and the incidence of accessory bones increases with age.

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