The fleck sign is a small bone fragment visible on a plain X-ray that signals an avulsion injury, where a ligament or tendon has pulled a chip of bone away from its attachment site. It is most classically associated with Lisfranc injuries of the midfoot, where it appears as a tiny speck of bone between the base of the first and second metatarsals, but the same term has been applied to similar avulsion fragments at the thumb, hip, and other joints. The fragment itself is usually tiny and easy to overlook, which is exactly what makes the sign so clinically important: that innocuous-looking fleck often points to a serious ligamentous disruption that needs surgery.
The Classic Fleck Sign in Lisfranc Injuries
The term “fleck sign” originated in the context of the Lisfranc joint, which is the set of articulations connecting the midfoot to the forefoot. The original description, attributed to Myerson and colleagues, identified a small avulsion fragment lying between the base of the medial cuneiform and the base of the second metatarsal. In their series, this fragment was present in roughly 90% of patients with tarsometatarsal fracture-dislocations.1International Journal of Foot and Ankle. “Fleck Sign”: Traumatic Avulsion Fracture of the Medial Cuneiform by Anterior Tibialis Tendon The fragment represents a piece of bone torn away when the Lisfranc ligament, a strong band connecting the medial cuneiform to the second metatarsal base, is ruptured under force.
Along with widening of the space between the first and second metatarsal bases, the fleck sign is considered one of the hallmark radiographic findings of a Lisfranc fracture-dislocation.2PubMed Central. Lisfranc fracture-dislocations: report of two cases But neither finding is guaranteed to appear in every case. Some Lisfranc injuries are purely ligamentous, leaving no bony fragment at all, while others involve subtle shifts that standard X-rays miss entirely.3Skeletal Radiology. Epidemiology, imaging, and treatment of Lisfranc fracture-dislocations revisited So while spotting the fleck sign is a strong positive indicator, its absence does not rule out injury.
Why Lisfranc Injuries Are So Often Missed
Lisfranc injuries have a reputation for being one of the most frequently missed diagnoses in foot trauma. The mechanism can be surprisingly mundane: a stumble off a curb, a missed step, a twist during sports. The foot swells, it hurts to walk, and an emergency department X-ray may look unremarkable to someone not trained to scrutinize the subtle alignment of the tarsometatarsal joint. The fleck sign, when present, can be the single radiographic clue that something more serious is going on beneath the swelling.
Part of the diagnostic challenge is that standard non-weight-bearing X-rays, the kind typically taken in an emergency room, can underestimate the injury. Applying body weight to the foot during imaging stresses the damaged ligaments and reveals displacement that a resting foot conceals. Research comparing the two approaches has confirmed that relying solely on non-weight-bearing views significantly increases the chance of missing Lisfranc instability.4Injury. Lisfranc injury: Refined diagnostic methodology using weightbearing and non-weightbearing radiographs When clinical suspicion is high but the initial X-ray looks clean, advanced imaging such as CT or MRI becomes essential to catch purely ligamentous injuries that leave no fleck fragment behind.5Skeletal Radiology. Epidemiology, imaging, and treatment of Lisfranc fracture-dislocations revisited
Where Exactly Does the Fleck Come From
An avulsion injury occurs when a tensile force, essentially a pull, is strong enough that the ligament or tendon yanks a piece of bone off its insertion point rather than tearing through its own fibers.6PubMed Central. Avulsion injuries: an update on radiologic findings In the midfoot, the Lisfranc ligament runs from the medial cuneiform to the base of the second metatarsal. When the forefoot is forcefully abducted or twisted, that ligament can rip a small chip from either bone. The result is a free-floating fragment between the two bones, which is the fleck that appears on imaging.
The avulsion does not always stay confined to the classic first-to-second metatarsal corridor. A study examining cases with avulsion of the Lisfranc ligament from the second metatarsal base found that the majority also had accompanying avulsion fractures along the plantar aspect of the adjacent third or fourth metatarsal bases; only a small minority had an isolated second metatarsal base fracture alone.7PubMed. Lisfranc fleck sign: characteristics and clinical outcomes following fixation using a percutaneous position Lisfranc screw This means that when you see a fleck sign, the injury is often more extensive than just one torn ligament. The entire tarsometatarsal complex may be compromised.
The Fleck Sign at the Thumb
The concept of a small avulsion fragment serving as a red flag for a larger ligamentous catastrophe has recently been extended to the thumb. When the ulnar collateral ligament (UCL) of the thumb’s metacarpophalangeal joint tears completely, the torn end can sometimes flip over and get trapped outside the adductor aponeurosis, a condition called a Stener lesion. Stener lesions do not heal on their own and require surgery. The clinical problem is distinguishing a partial tear that might heal in a cast from a complete tear with a Stener lesion that will not.
Researchers have described a “displaced fleck sign” on thumb X-rays: a small bone fragment that has been pulled away from the proximal phalanx or metacarpal and sits displaced from its expected location. In a series of over 200 patients with thumb UCL injuries, about 7.5% showed this displaced fleck. Every single one of those patients had gross instability on clinical exam and underwent surgery. All had Grade III ruptures, and roughly 94% had a confirmed Stener lesion at operation. The sign had a positive predictive value of 100% for a complete rupture.8The Journal of Hand Surgery. The Displaced Fleck Sign: Description of a Radiographic Finding Consistent with Grade III Thumb Ulnar Collateral Ligament Tears with Stener Lesions A similar preliminary analysis of 200 patients found nearly identical numbers, with 93% of those with a positive fleck sign having a Stener lesion confirmed surgically.9American Association for Hand Surgery. The Fleck Sign: Description of a Radiographic Finding Consistent with Grade 3 Thumb UCL Tears with Stener Lesions
What makes this useful is timing. A plain X-ray of the thumb is fast and inexpensive. If the displaced fleck sign is present, the surgeon can proceed directly to operative planning with high confidence, potentially avoiding the delay and cost of an MRI. Since the sign is rare (appearing in fewer than 1 in 10 UCL injuries), its absence does not rule out a Stener lesion, but its presence is an extremely reliable indicator that surgery is needed.
Avulsion Flecks Around the Knee
The knee has its own family of small avulsion fractures that serve as radiographic sentinels for serious ligament damage. The most well-known is the Segond fracture, a small chip pulled from the lateral margin of the proximal tibia. Despite its modest size, a Segond fracture is considered pathognomonic for ligamentous damage in the knee, most often an anterior cruciate ligament (ACL) tear, though the medial collateral ligament can also be involved.10PubMed Central. The Segond Fracture as a Vital Diagnostic Tool for Anterior Cruciate Ligament Injury in Acute Settings: A Case Report The presence of a Segond fracture on an X-ray taken after an acute knee injury should immediately prompt further investigation for ACL disruption.11Orthopedic Clinics of North America. Diagnosis of Chronic Injury to the Anterior Cruciate Ligament
On the opposite side of the knee, the “arcuate sign” describes a small avulsion fracture at the tip (styloid process) of the fibular head. This fragment indicates damage to the posterolateral corner of the knee, a complex of ligaments and tendons that stabilizes the outer back part of the joint. MRI studies of patients with this avulsion have shown that the bony fragments are attached to the fibular collateral ligament, the biceps femoris tendon, or both, and that tears of the posterolateral capsule accompany the avulsion in about two-thirds of cases.12PubMed. MR evaluation of the “arcuate” sign of posterolateral knee instability Even when no visible bony fragment exists on X-ray, MRI may reveal bone marrow swelling at the medial aspect of the fibular head as a subtler version of the same injury pattern.13PubMed. Arcuate sign of posterolateral knee injuries: anatomic, radiographic, and MR imaging data related to patterns of injury
These knee avulsions are not typically labeled “fleck signs” in the way the midfoot and thumb variants are, but they operate on the same principle: a small piece of bone, easily dismissed as trivial, that betrays a much larger ligamentous injury lurking beneath the surface.
The Fleck Sign in Pediatric Hip Dislocations
The fleck sign has also been described in a less common context: pediatric traumatic hip dislocation. Hip dislocations in children are rare, and the immature cartilage around a child’s acetabulum (the hip socket) behaves differently from adult bone under stress. In a case report of a nine-year-old boy who sustained a posterior hip dislocation during rugby, CT imaging revealed a small bony fragment near the posterior acetabular rim, termed the “acetabular fleck sign.” Follow-up MRI showed that the fragment was continuous with an osteochondral piece attached to the labrum. Open surgery confirmed a posterior labral osteochondral avulsion, which was repaired with anchors.14PubMed Central. Traumatic hip dislocation in a child with an acetabular fleck sign indicating posterior osteochondral avulsion: a case report
The authors suggested that recognizing this acetabular fleck sign early could help clinicians identify osteochondral damage that might otherwise be missed on plain films, prompting timely surgical repair and reducing the risk of recurrent instability. It is a niche application of the concept, but it illustrates how the same radiographic logic, a small fragment signaling a more serious structural problem, can surface wherever ligaments and cartilage meet bone.
Distinguishing a Fleck From a Normal Accessory Bone
One practical challenge in interpreting fleck signs is that the foot is full of naturally occurring small bones, called accessory ossicles, that can look remarkably similar to an avulsion fragment on X-ray. Accessory ossicles are developmental variants present from birth; they have smooth, rounded cortical margins and sit in predictable locations. An avulsion fragment, by contrast, tends to have irregular edges, may be displaced from its expected position, and usually appears in a clinical setting where there was a specific injury.
Radiologists use a few contextual clues to tell the two apart. An accessory ossicle typically has a well-defined cortex all the way around, with no surrounding soft-tissue swelling. An acute avulsion fragment often has a jagged margin that looks like it would fit back into a donor site on an adjacent bone, and there is usually localized swelling or bruising. History matters enormously: a patient who twisted their foot and now cannot bear weight, with a small bone chip near the Lisfranc joint, is a very different clinical picture from an incidental finding on an X-ray taken for another reason. When there is any ambiguity, CT can resolve whether the fragment is corticated (smooth, old, and developmental) or freshly fractured.
What Happens After the Fleck Is Found
Spotting the fleck sign is the diagnostic starting point, not the end of the workup. In the midfoot, identifying the fragment prompts advanced imaging to map the full extent of ligamentous damage and guide the decision between conservative treatment and surgery. Many Lisfranc injuries with demonstrated instability ultimately require operative fixation, because the midfoot is a load-bearing architecture that does not tolerate even small amounts of malalignment. If the bones shift and heal in the wrong position, chronic pain, arthritis, and loss of the foot’s arch can follow.
At the thumb, a positive displaced fleck sign essentially fast-tracks the patient toward surgical repair, given the near-certainty of a Stener lesion.15The Journal of Hand Surgery. The Displaced Fleck Sign: Description of a Radiographic Finding Consistent with Grade III Thumb Ulnar Collateral Ligament Tears with Stener Lesions For knee avulsions like the Segond fracture, the fragment itself is rarely the main concern. The focus shifts immediately to the ACL and other supporting structures, because the long-term stability of the knee depends on addressing those soft-tissue injuries, often arthroscopically.
In all these locations, the overarching clinical message is the same: the fleck is the messenger, not the disease. The bone chip itself is often tiny and would heal without trouble on its own. What matters is the ligament or tendon that pulled it off, and whether that structure’s disruption has destabilized the joint. A missed fleck sign does not just mean a missed fracture fragment; it means a missed opportunity to diagnose and treat a joint that may become chronically unstable if left alone.
When the Fleck Sign Is Absent but the Injury Is Present
The most dangerous scenario is a purely ligamentous injury that produces no bony fragment at all. In the midfoot, a Lisfranc ligament can tear cleanly through its substance without avulsing bone, leaving the X-ray with nothing obvious to see. The same applies at the knee: an ACL can rupture without creating a Segond fragment, and a posterolateral corner injury can occur without producing the arcuate sign. The fleck sign is a highly specific indicator, meaning that when you see it you can be confident the injury is real, but it is not sensitive enough to catch every case.
This specificity-sensitivity gap is a recurring theme in musculoskeletal imaging. Clinicians are taught to rely on the patient’s story and physical exam findings as much as the X-ray. Midfoot tenderness at the Lisfranc joint, inability to bear weight, plantar ecchymosis (bruising on the sole of the foot), and pain with passive abduction of the forefoot are all clinical signs that should trigger further investigation even when the X-ray looks normal. Weight-bearing views, CT, and MRI each add diagnostic power, and current guidance emphasizes using these liberally when clinical suspicion is high.16Injury. Lisfranc injury: Refined diagnostic methodology using weightbearing and non-weightbearing radiographs
Peroneal Tendon Avulsions and the Outer Ankle
Another region where small avulsion fragments carry outsized clinical importance is the outer ankle. The peroneal tendons run behind the fibula (the outer ankle bone) and are held in place by a band of tissue called the superior peroneal retinaculum. In an ankle injury, forceful dorsiflexion or inversion can tear the retinaculum away from the fibula, sometimes taking a thin flake of bone with it. This “rim fracture” or fleck at the distal fibula is the radiographic clue to peroneal tendon subluxation or dislocation.
Peroneal tendon instability is easy to miss in the acute setting because the symptoms overlap heavily with a common lateral ankle sprain: swelling behind the outer ankle bone, pain with movement, difficulty bearing weight. When treated conservatively, failure rates as high as 50% have been reported, meaning many of these patients end up needing surgery anyway after months of persistent instability and pain. Chronic peroneal tendon instability almost invariably requires surgical reconstruction, though the outcomes from repair of the retinacular and tendinous structures are generally good to excellent.17PubMed. Peroneal tendon dislocation The small avulsion flake at the fibular rim, when spotted early, can steer the patient toward appropriate surgical referral before the problem becomes chronic.
How Mechanism of Injury Shapes What to Look For
Because avulsion fragments can appear in so many joints, the mechanism of injury is what directs the clinician’s eye to the right spot on the X-ray. A twisting fall with the foot planted and the forefoot forced outward points toward the Lisfranc joint. A skier’s thumb injury, where the pole forces the thumb into extreme abduction, points toward the UCL. A valgus blow to the knee with rotational force raises concern for the ACL and posterolateral corner. A forceful ankle inversion or dorsiflexion brings peroneal tendon avulsion into the differential. And a high-energy posterior hip dislocation in a child, although rare, should prompt a careful look at the acetabular rim.
In each case, the radiologist or emergency physician is essentially asking the same question: did a ligament pull hard enough to rip bone? The answer often hides in a fragment so small that it could easily be mistaken for an artifact or dismissed as insignificant. Learning to recognize the fleck sign in its various anatomic homes is one of those skills that, once acquired, changes how a clinician reads every post-trauma film. A two-millimeter chip of bone stops being noise and starts being a diagnosis.