A 38-year-old man rolled his ankle during a football match and turned up at hospital with the standard picture: swelling and tenderness over the outer ligaments and the top of the foot, painful restriction of ankle movement, and a limp.
The first two X-ray views showed nothing. It was the oblique view that produced a small, sharply outlined piece of bone sitting next to the front of his heel bone, in exactly the spot where a chunk gets torn off in a certain kind of ankle injury.
It was not a fracture. CT confirmed an os calcaneus secundarius, an accessory bone roughly 15 millimeters across, well corticated on every side and nestled between the anterior process of the calcaneus, the cuboid, and the talonavicular joint. He had been carrying it since childhood.
An Inversion Sprain and a Suspicious Oval on the Oblique View
The distinction matters because the fracture it imitates, an avulsion of the anterior calcaneal process, is treated very differently from a bruised accessory bone.
On CT, the giveaways are geometry and surface. An accessory ossicle is smooth and continuously corticated all the way around because it has been a separate bone since it formed. A fresh fracture fragment has an interrupted, irregular edge where it separated from its parent bone, with a matching defect on the other side.
The ossicle itself is a leftover of skeletal development, forming from a secondary ossification center that never fused with the calcaneus. Roughly two dozen accessory ossicles have been cataloged in the foot and ankle, most of them asymptomatic and most of them discovered by accident. Radiologists have long warned that they are easily mistaken for fracture fragments or loose bodies.
Nobody Agrees on How Many People Carry It
Prevalence estimates vary enormously depending on how you look for it. The case report cites a range of 0.6% to 7% across anatomical and radiologic series. A classic examination of 1,367 dry calcanei found the trait 47 times, or 3.4%. A recent systematic review with meta-analysis concluded the epidemiology remains poorly established, with published figures spanning 0.1% to 15.2%. A large radiographic study of Turkish patients found it in just 0.42%.
That spread reflects a real methodological problem: what counts as an os calcaneus secundarius, and whether you can see it depends heavily on which imaging you use and which projection you take. In this patient, the standard frontal and lateral views were unremarkable. Only the three-quarter oblique showed it.
Why Radiologists Miss This in Both Directions
The most striking data on the problem come from a German level I trauma center. Researchers re-examined 611 CT scans of the entire foot taken after acute trauma or for postoperative checks. They found 14 accessory ossicles at the anterior calcaneal process, 12 of them typical examples of this one, plus 56 genuine pathologies, including 44 fractures and 12 pseudarthroses.
Then came the uncomfortable part. Seven of the 12 ossicles and 25 of the 56 pathologies were not mentioned in the original CT reports at all. That is 58% of the normal variants and 46% of the actual injuries going unremarked in a region that reports keep skipping.
The team identified the features that separate the two. The accessory bone sits anteromedially in a concave notch on the calcaneal facet and shows continuous corticalization. A fracture shows cortical interruption and irregularity. As the ossicle grows larger, degenerative changes can appear within it.
Misreading runs both ways, and both errors cost something. Calling a normal variant a fracture means unnecessary immobilization, repeat imaging, and sometimes surgery. Calling a fracture a normal variant means a real injury goes untreated.
When the Spare Part Actually Starts Hurting
Most people with this ossicle never know. But an inversion sprain can injure the fibrous or cartilaginous junction between the accessory bone and the calcaneus, and then it becomes the source of pain rather than an incidental finding.
One published case describes a 51-year-old man with three decades of load-dependent ankle pain who was finally diagnosed with a traumatized calcaneus secundarius after an acute sprain. Surgical excision resolved his symptoms completely, and he was still asymptomatic more than a year later.
Depending on size and alignment, the ossicle can alter mechanics around the subtalar joint. Those authors advise considering it in patients with persistent sinus tarsi syndrome, painfully limited subtalar motion, or repeated ankle sprains. It can also be mistaken for a calcaneonavicular coalition. When conservative treatment fails, surgical removal is an option, and endoscopic resection has been reported.
For the ordinary reader who has just sprained an ankle, the useful point is modest. If an X-ray shows a small bone fragment near the heel after a twisting injury, that finding alone does not settle whether anything broke. Comparison views of the other foot help, since accessory ossicles are often bilateral, and CT can usually resolve it. Ankle pain that persists for weeks after a sprain is worth taking back to a clinician rather than writing it off as slow healing.
Key Questions Answered
What is an os calcaneus secundarius?
An accessory bone near the front of the heel bone, formed from a secondary ossification center that never fused. It is a normal developmental variant, present from childhood, not the result of an injury.
How common is it?
Estimates range widely. The case report cites 0.6% to 7%, a dry-bone study of 1,367 calcanei found 3.4%, one large radiographic series found 0.42%, and a recent meta-analysis noted published figures spanning 0.1% to 15.2%.
Why is it mistaken for a fracture?
It sits in the same location as an avulsion fracture of the anterior calcaneal process, and on plain X-ray after an ankle sprain, a small bone fragment there looks like a broken piece.
How do doctors tell the difference?
CT is the most reliable tool. The accessory bone is smoothly and completely corticated and sits in a concave notch; a fracture shows interrupted, irregular edges with a matching defect on the parent bone.
Can it actually cause pain?
Yes. An inversion sprain can injure the junction between the ossicle and the calcaneus, producing localized pain and swelling. Conservative treatment often helps, and excision is an option when it does not.
What should someone do after an ankle sprain that keeps hurting?
Return to a clinician. Persistent pain weeks after a sprain warrants reassessment, which may include comparison views of the uninjured foot or a CT scan.
