The Land of Os: Accessory Ossicles of the Foot Susan Cross, Anshul Rastogi, Rosy Jalan; Dept of Radiology, Barts Health NHS Trust, London, UK Contact: susan.cross@bartshealth.nhs.uk Pictorial review Abstract number EE34
Purpose of exhibit Foot pain is often attributed to the presence of accessory ossicles causing impingement or inflammation However, these are normal variants and additional imaging findings should be sought before attributing these alone as a cause for the symptoms We will provide a review of the clinically important accessory ossicles of the foot We present a series of cases with multimodality imaging, which make the distinction between normal ossicles, fractures and inflammatory processes of these ossicles.
Os Trigonum Found posterior to the lateral tubercle of the talus (arrow) The cartilaginous synchondrosis between the ossicle and the talus usually fuses forming the trigonal (Steida) process A separate os persists in 7-14% of the population, often bilaterally It is difficult to distinguish this from an old ununited fracture of the lateral tubercle (Shepherd s fracture)
Caused by disruption of the cartilage synchondrosis between the os trigonum and the lateral talar tubercle as a result of repetitive microtrauma or forced plantar flexion of the foot Other aetiologies include: Trigonal process fracture Flexor Hallucis Longus Tenosynovitis Os Trigonum Syndrome Posterior tibial talar impingement by bone block Ref: Karasick D et al. AJR :166. 1996 Intraarticular loose bodies
Clinical history: 25 yr old football player with 5 month history of posterior ankle pain and intermittent swelling. Figure 1 (a-d): Sagittal and Coronal T1W and STIR images demonstrate abnormal degenerative subchondral cyst formation related to the synchondrosis between the os trigonum and talus (blue arrows). Note the florid Flexor Hallucis Longus tenosynovitis (red arrows). (a) (b) (c) (d)
Clinical History: 23 yr old ballet dancer with history of previous excision of os trigonum, with recurrence of posterior ankle pain Figure 2: Sagittal T1W and STIR images of hindfoot. Appearances are similar to os trigonum syndrome, however this is a fracture of the lateral tubercle of the talar process (blue arrow). Note it is more medially sited than the true os trigonum
Os Naviculare Located posterior to the posteromedial aspect of the navicular Posterior tibial tendon often has a broad attachment into the ossicle 3 types according to the Geist classification Type 2 is most commonly associated with medial foot pain
Geist classification TYPE 1: Small (3mm) ossicle within the distal tibialis posterior tendon (os tibialis externum) Usually asymptomatic TYPE 2: Large, up to 12mm, in 10% of the population Connected to the adjacent navicular tubercle by a fibrous or cartilaginous synchondrosis Majority of the posterior tibial tendon inserts on this ossicle TYPE 3: The medial tubercle of the navicular is very large and known as the cornuate process This can result in altered stresses and premature tendon degeneration
MRI is the most specific imaging modality to assess the synchondrosis, related bone marrow oedema and posterior tibial morphology Figure 3: Coronal T2W fat saturated image of the mid and hindfoot. Note the abnormal fluid signal within the synchondrosis of the Type 2 Os with the navicular implying disruption. (blue arrow) There is also subchondral bone marrow oedema in keeping with microtrabecular bony injury. The posterior tibial tendon should be carefully evaluated for tendinopathy or frank tears. Treatment can be surgical (Kidner procedure) to remove the os naviculare and reattach the posterior tibial tendon to the navicular
Located just proximal to the base of the 5th metatarsal, within the substance of the peroneus longus tendon at the cuboid tunnel Painful os peroneum syndrome has a spectrum of conditions including: acute os peroneum fracture or diastasis of a multipartite os with resultant peroneus longus injury Chronic os peroneum fracture with resultant stenosing peroneus longus tenosynovitis Os peroneum Prominent peroneal tubercle on the calcaneum with ensuing impingement on the peroneus longus and os. Ref: Brigidio M et al. Radiology 237; 2005
Clinical details: 40 yr old female with lateral foot pain Axial T2 fat saturated, sagittal T1W and STIR images of the foot: Demonstrate os peroneum (white arrow) which has abnormal bone marrow oedema related to the synchondrosis (red arrows). The peroneus longus is intact but is tendinopathic (blue arrow)
Hallux sesamoids The medial and lateral sesamoids are located within the flexor hallucis brevis tendons at level of the 1st Metatarsal head and are incorporated into the 1st MTP articular capsule They provide mechanical advantage during flexion of the great toe, reduce friction and elevate the 1st metatarsal to partially distribute weight bearing forces to the lateral aspect of the forefoot The medial sesamoid is more likely to be involved with traumatic abnormalities The lateral sesamoid is more commonly affected by ischaemic changes with osteonecrosis
Clinical history: 40 yr old male with chronic pain in medial aspect forefoot Coronal T2W fat saturated image of the forefoot: Demonstrates abnormal fluid signal related to the medial sesamoid in keeping with seasmoiditis Corresponding Ultrasound image over theplantar aspect of the forefoot demonstrates hypoechoic oedema superficial to the medial sesamoid
Clinical details: forefoot pain, worse on walking Axial T2 Fat saturated image of forefoot: Demonstrates lateral hallux sesamoiditis (white arrow). Note also abnormal fluid signal between 1st MT head and medial sesamoid (green arrow) and Flexor Hallucis Brevis tenosynovitis
Deltoid ossicles Located within the deltoid ligament ; Usually a result of previous trauma Axial T1W and Coronal T2W fat saturated images of the ankle: Demonstrate abnormal well corticated bony fragments within the thickened inflamed deltoid ligament (blue arrows)
Conclusion Foot pain is often attributed to the presence of accessory ossicles causing impingment or inflammation. However, these are normal variants and additional imaging findings should be sought before attributing these alone as a cause for the symptoms. Making the distinction is clinically important in influencing subsequent management.
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