An accessory bone or supernumerary bone is a bone that is not normally present in the body, but can be found as a variant in a significant number of people. It poses a risk of being misdiagnosis as on radiography.[T.E. Keats, M.W. Anderson, Atlas of normal roentgen variants that may simulate disease. 7th edition, Mosby Inc. 2001, ]
Wrist and hand
[[File:Sesamoid bones of the fingers.jpg|thumb|220px|Prevalence and locations of sesamoid bones of the hand.
- Location and structure:
- Prevalences: ]]
|
| 1.5% R, 2.4% L |
| 1.3% R, 2.1% L |
| 0.5% R, 2.1% L |
| 1.2% R, 1.2% L |
| 1% R, 0.9% L |
| 1% R, 0.9% L |
| 0.3% R, 0.9% L |
| 0.8% R, 0.3% L |
| 0.5% R |
| 0.3% L |
| 0.3% R, 0.3% L |
| 0.3% L |
| 0.3% R |
| 0.3% L |
| 0.3% L |
Os ulnostyloideum
The
os ulnostyloideum is an ulnar styloid process that is not fused to the rest of the
ulna bone.
[R. O'Rahilly. A survey of carpal and tarsal anomalies. J Bone Joint Surg Am. 1953; 35: 626–642] On
, an
os ulnostyloideum is sometimes mistaken for an avulsion fracture of the styloid process. However, the distinction between these is extremely difficult.
[T.E. Keats, M.W. Anderson. Atlas of normal roentgen variants that may simulate disease. 7th edition, Mosby Inc. 2001 ] It is alleged that the os ulnostyloideum has a close relationship with or is synonymous with the os triquetrum secundarium.
[A. Köhler. Röntgenology. The borderlands of the normal and early pathological in the skiagram. Translated by Arthur Turnbull. Ed. 2. Baillière, Tindall & Cox, Londen, 1935]
Os centrale
The
os carpi centrale (also briefly
os centrale) is, where present, located on the dorsal side of the wrist between the
scaphoid bone, the
trapezoid bone and
capitate bone, radially to the deep fossa of the capitate. The bone is present in almost every human embryo of 17–49 mm length, but then usually fuses with the ulnar side of the scaphoid. Sometimes it fuses with the capitate or the trapezoid. The literature also refers to an
os centrale at the palm of the carpus, but this existence is questioned.
In most primates, including and , the os centrale is an independent bone that is attached to the scaphoid by strong ligaments. Conversely, in African apes and humans, the os centrale normally fuses to the scaphoid early in development.[A.H. Schultz. Characters common to higher primates and characters specific for man. Quart Rev Biol. 1936; 11: 259–283; 425–455] A good number of scholars have construed the scaphoid-centrale fusion as a functional adaptation to knuckle-walking,
Ankle
Accessory bones at the ankle mainly include:
-
Os subtibiale, with a prevalence of approximately 1%.
-
Os subfibulare, with a prevalence of approximately 0.2%.
Os trigonum (further described below) may also be seen on an ankle X-ray.
Foot
|
|
| Always present |
| 0.4% |
| 0.2% |
| 0.1% |
| 4.3% |
| 2–13% |
|
| 7–25% |
| Up to 26% |
| 2–21% |
| 1–13% |
| 1.0–3.5% |
| 0.6–7% |
| 0.2–2.4% |
| 0.1–1% |
| 0.5% |
Accessory navicular
An accessory navicular bone, also called
os tibiale externum, occasionally develops in front of the
ankle towards the inside of the foot. This bone may be present in approximately 2–21% of the general population and is usually asymptomatic.
When it is symptomatic, surgery may be necessary.
The Geist classification divides the accessory navicular bones into three types.
-
Type 1: An os tibiale externum is a 2–3 mm sesamoid bone in the distal posterior tibialis tendon. Usually asymptomatic.
-
Type 2: Triangular or heart-shaped ossicle measuring up to 12 mm, which represents a secondary ossification center connected to the navicular tuberosity by a 1–2 mm layer of fibrocartilage or hyaline cartilage. Portions of the posterior tibialis tendon sometimes insert onto the accessory ossicle, which can cause dysfunction, and therefore, symptoms.
-
Type 3: A cornuate navicular bone represents an enlarged navicular tuberosity, which may represent a fused Type 2 accessory bone. Occasionally symptomatic due to bunion formation.
Os trigonum
The
os trigonum or
accessory talus represents a failure of fusion of the lateral tubercle of the posterior process of the
talus bone. Is estimated to be present in 7–25% of adults.
It can be mistaken for an avulsion fracture of lateral tubercle of talus (Shepherd fracture) or a fracture of the
Stieda process. In most cases, Os Trigonum will go unnoticed, but with some ankle injuries it can get trapped between the heel and ankle bones which irritates the surrounding structures, leading to Os Trigonum Syndrome.
Less common accessory bones
[[File:Tarsalia accessoria.png|thumb|600px|left|Image also including the locations of rare accessory bones of the foot:
1=Os cuneometatarsale I plantare, 2=os uncinatum, 3=os sesamoideum tibialis posterior, 4=os sesamoideum peroneum, 5=os cuboideum secundarium, 6=os trochleare calcanei, 7=os in sinus tarsi, 8=os sustentaculum tali, 9=os talocalcaneale posterius, 10=os aponeurosis plantaris, 11=os subcalcaneum, 12=os sesamoideum tibialis anterior, 13=os cuneometatarsale I tibiale, 14=os intermetatarsale I, 15=os cuneometatarsale II dorsale, 16=os paracuneiforme, 17=os cuneonaviculare, 18=os intercuneiforme, 19=os intermetatarsale IV, 20=/os talonaviculare, 21=os vesalianum pedis, 22=os tibiale externum, 23=os talotibiale dorsale, 24=os supratalare, 25=os calcaneus secundarius, 26=os subtibiale, 27=os subfibulare, 28=os retinaculi, 29=os calcaneus accessorius, 30=os trigonum, 31=os supracalcaneum, 32=os tendinis calcanei]]
Other locations
Neck
-
Nodules in the posterior margin of the nuchal ligament form bone tissue in approximately 11% of males and 3–5% in females after the third decade of life, and may then be regarded to be .
Shoulder
-
An os acromiale forms when any of its four ossification centers fail to fuse. These four ossification centers are called (from tip to base) pre-acromion, meso-acromion, meta-acromion, and basi-acromion. In most cases, the first three fuse at 15–18 years, whereas the base part fuses to the scapular spine at 12 years. Such failure to fuse occurs in between 1% and 15% of cases.
Vertebral column
-
An Oppenheimer ossicle is found in approximately 4% (range 1–7%) of individuals.
Knee
-
The fabella is present in 10% to 30% of individuals.
See also
