Definition
The foot can contain a number of ossicles or small bones. There are 2 types, sesamoid bones and accessory bones. Sesamoids are bones that develop inside a tendon, where the tendon passes over a bony prominence (e.g. 1st MTP Jt). The foot may also contain ossicles that are not associated with a tendon, but result from developmental variations. The process of ossification progresses from a primary ossification centre, until the bone is completely ossified. Irregularly shaped bones such as the tarsal bones may develop a secondary centre and in some individuals complete ossification does not occur. The secondary centre remains separate from the rest of the bone, forming an accessory ossicle.
Common Accessory Bones
Os trigonum – the separated posteriolateral tubercle of the talus.
- 2.5% to 14% of normal feet
- Can be round, oval or triangular and of variable size.
Os tibiale externum (Accessory navicular) – located posteriomedial aspect tuberosity of navicular, within insertion fibres of Tibialis posterior
- Can vary in size, shape and position
- 2-12% of population may have this ossicle.
- Generally does not ossify until 9 years of age and approximately½
- that occur may fuse to the navicular.
- Insertion of Tibialis posterior tendon into accessory bone displaces the tendon, allowing foot to deviate into valgus / flatfoot position
Os Peroneum – Located within the Peroneus Brevis tendon
Can vary considerably in size and even number
Os Vesalianum – located proximal to well formed tip of the tuberosity of the 5th metatarsal.
- Quite rare in occurrence
- Size and shape vary considerably.
- Commonly partite
Os supra naviculare ( os talonaviculare dorsale) – located dorsal aspect talo navicular joint
Os intermetatarseum – Located between the medial cuneiform and the 1st and 2nd metatarsal bones
Os calcaneus secondarius – located dorsally at the anterior process of the calcaneus , at the junction of the calcaneus , cuboid, head of the talus and navicular
Os subfibulare – located distal to the tip of the fibular malleolus. Occurs approximately 10% population
Os subtibiale – located distal to the tip of the tibial malleolus.
- Occur approximately 10% population
- Less common accessory bones
- Os cuneo -1-metatarsale-1-plantare
- Os sustentaculi
- Os infranaviculare
- Os supranaviculare
- Os supracalcaneum
- Os intercuneiforme
Differential Diagnosis
Fracture – There is a need to differentiate from avulsion fracture fragments in the event of trauma.
- Os Vesalianum, differentiate from a fracture of the base of the 5th metatarsal base or non-united or fragmented apophysis.
- Os trigonum, differentiate from avulsion fracture of the talus (Sheperd’s Fracture).
Os subfibulare, differentiate from an avulsion fracture of the lateral malleolus. Os subtibiale, differentiate from an avulsion fracture of the medial malleolus.
Nonunion fracture
- Avascular Necrosis – this can occur when the accessory ossicle lies within a tendon e.g. Accessory Navicular, Os Peroneum.
- Osteoarthritis
- Posterior Talar Impingement (Os trigonum)
Investigations
- X-Ray. These may not be helpful if the accessory bone is not ossified.
- Bone scan, increase uptake on bone scan can indicate that a lesion is symptomatic, although non specific.
- CT scan, identifies unusual accessory bones and the anatomical positioning can help explain any symptoms.
- MR Scan.
Management
Can depend on which accessory bone has become symptomatic
- Orthotic Devices
- Cast immobilisation
- Cortisone Injections
- Surgical intervention
References