15 Normal Variants of the Skeleton and the Soft Tissues of the Hand



10.1055/b-0034-77608

15 Normal Variants of the Skeleton and the Soft Tissues of the Hand

Schmitt, R., Schindler, G.

Nonpathologic variants of the skeleton of the hand include accessory sesamoid bones, coalescences and divisions of the carpal bones, and accessory bones. In contrast to malformations, these are not considered diseases and cause no functional disturbance. Diagnostically, these findings should always be differentiated from posttraumatic conditions. The most important variants of soft tissues of the hand are aberrant or accessory muscles. These are often asymptomatic and are discovered incidentally on MR images or during surgery.



Normal Variants of the Skeleton of the Hand



Sesamoid Bones


Sesamoids are located on the palmar side of the metacarpophalangeal and the distal interphalangeal joints. Their prevalence depends on their location, as listed in Table 15.1 .



Coalescence of the Carpals


Coalescence here refers to fusion of two carpal bones. Idiopathic coalescences, in which mostly two bones in the same carpal row are congenitally fused, are differentiated from postinflammatory or posttraumatic forms, which bridge the two carpal rows. Females and African Americans are more often affected by idiopathic coalescences.


The different possible forms of carpal fusion are listed in Fig. 15.1 . The most common are fusions of the lunate and triquetrum ( Figs. 15.215.4 ), followed by fusions of the capitate and hamate ( Fig. 15.5 ). There are four different variations of lunotriquetral coalescence, which depend of the degree of the connecting synostosis or synchondrosis ( Table 15.2 ).


The remaining coalescences are very rare ( Fig. 15.6 ). When accessory elements fuse with adjacent carpals, they form defined exostoses.












































Table 15.1 Frequency of sesamoid bones in the hand (according to Birkner)

Localization


Frequency(%)


Sesamoid radial I


95


Sesamoid ulnar I


100


Sesamoid radial II


50


Sesamoid radial III


2


Sesamoid radial IV


2


Sesamoid radial IV


<2


Sesamoid radial V


3


Sesamoid radial V


80


Sesamoid distal interphalangeal I


70


Sesamoid distal interphalangeal II


<1


Sesamoid distal interphalangeal V


<1





























Table 15.2 Variants of lunotriquetral coalescence (according to Minaar)

Type


Description of Coalescence


Peculiarity


I


Fibrous/fibrocartilaginous

 

II


Osseous incomplete


Distal notch


III


Osseous complete

 

IV


Osseous complete


Further carpal anomalies

Fig. 15.1 Diagram of carpal coalescences (according to Zimmer).
Fig. 15.2a, b Lunotriquetral coalescence Type I (according to Minaar).
Fig. 15.3 Lunotriquetral coalescence Type II (according to Minaar). T1-weighted SE sequence shows a small distal notch at the level of the coalescence
Fig. 15.4 Lunotriquetral coalescence Type III (according to Minaar). Complete synostosis.
Fig. 15.5 Capitohamate coalescence.
Fig. 15.6 Scaphotrapezial coalescence. 3D surface-shaded display of a CT dataset.
Fig. 15.7 Diagram of divided carpal bones (according to Ruckensteiner). 1 and 2 = bipartite scaphoid; 3 and 4 = bipartite lunate; 5 = triquetrum; 6 = pisiform; 7 and 8 = bipartite trapezium or trapezioid (8) with paratrapezium (7); 9, 10, 11 = tripartite trapezoid. The proximal bone (11) can also originate from an os centrale; 12 and 13 = bipartite capitate; 14 = hamate; 15 = os Vesalianum (DD: os hamulare basale)
Fig. 15.8 Bipartite scaphoid in dorsopalmar radiograph.







Table 15.3 Diagnostic criteria for a bipartite scaphoid (according to Taleisnik)



  • No history of trauma



  • Both parts of the bone have normal density; different sizes are possible



  • Sharply defined bone edges with narrow, jointlike spaces in between



  • No sign of sclerosis or cystic changes on the facing bone edges, but rounded edges



  • No periscaphoid osteoarthritis (radioscaphoid and STT joints)



Divided Carpals


Twofold or manifold divided carpal bones are very rare. In descending prevalence these involve the scaphoid, triquetrum, pisiform, trapezium, trapezoid, and capitate. Figure 15.7 lists possible divisions of the carpal bones. A typical bipartite scaphoid is shown in Fig. 15.8 .


When a divided carpal bone is discovered, a posttraumatic condition must be considered. This is especially true for the scaphoid, where fractures are often overlooked. Criteria for a truly bipartite scaphoid are summarized in Table 15.3.


A bipartite scaphoid supports the assumption that the division is congenital, but it does not constitute proof because bilateral scaphoid fractures or nonunions are also possible.

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Jul 21, 2020 | Posted by in MUSCULOSKELETAL IMAGING | Comments Off on 15 Normal Variants of the Skeleton and the Soft Tissues of the Hand
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