What is the diagnosis and treatment for a patient with a painful bony prominence on the lateral foot, negative X-ray for fracture, and a history of repetitive ankle motion, possibly due to sports or activity?

Painful Lateral Foot Bony Prominence with Negative X-ray

This clinical presentation most likely represents a stress fracture (particularly of the fifth metatarsal base or lateral process of the talus), peroneal tendon pathology with associated os peroneum syndrome, or an occult fracture—and MRI without contrast is the definitive next imaging study to establish the diagnosis. 1

Most Likely Diagnoses Based on Clinical Context

Stress Fracture (Most Common with Repetitive Activity)

• Stress fractures account for up to 20% of all sports medicine injuries and frequently occur before radiographic changes are visible. 1
• The lateral foot is particularly susceptible in athletes involved in running, jumping, and repetitive ankle motion. 1
• Pain on lateral calcaneal compression (calcaneal squeeze test) is highly suspicious for calcaneal stress fracture. 2
• Symptoms typically worsen progressively following increased activity or change to harder walking surfaces. 2

Lateral Process Fracture of the Talus ("Snowboarder's Fracture")

• This injury causes lateral and posterior ankle pain with an antalgic gait, particularly following inversion trauma. 2
• Radiographs have only 78% sensitivity for talar fractures compared to CT, making occult fractures common. 1
• The lateral process can be difficult to visualize on standard radiographs and may be erroneously diagnosed as a lateral ankle sprain. 1

Painful Os Peroneum Syndrome (POPS)

• This represents a spectrum of conditions causing plantar lateral foot pain, including os peroneum fracture, peroneus longus tendon tears, or hypertrophic peroneal tubercle causing tenosynovitis. 3, 4
• The os peroneum is a sesamoid bone within the peroneus longus tendon near the cuboid, and its involvement is frequently misdiagnosed. 5
• Acute fracture or diastasis of a multipartite os peroneum can result in discontinuity of the peroneus longus tendon. 3
• A hypertrophic peroneal tubercle on the lateral calcaneus can entrap the peroneus longus tendon during excursion, causing chronic friction and tenosynovitis. 3, 4

Peroneal Tendon Tears

• Most acute peroneus brevis tears are longitudinal, occur adjacent to the tip of the fibula, and present with lateral ankle pain. 6
• Peroneus longus tears more commonly occur at the cuboid tunnel level and may initially be managed nonoperatively unless associated with stenosing tendonitis. 6

Key Physical Examination Findings to Differentiate

Localize Point Tenderness Precisely

• Palpate the lateral malleolus, base of the fifth metatarsal, lateral process of the talus, lateral calcaneal wall, and along the peroneal tendon course. 2
• Tenderness at the lateral calcaneal wall slightly posterior to the subtalar joint facet suggests calcaneal stress fracture. 2
• Tenderness along the cuboid tunnel (plantar lateral midfoot) suggests os peroneum pathology or peroneus longus tendon involvement. 3

Perform Specific Provocative Tests

• Calcaneal compression test (medial-to-lateral squeeze): Pain suggests stress fracture. 2
• Single stance heel rise and varus inversion stress test: Localizes tenderness along the distal peroneus longus tendon at the cuboid tunnel in POPS. 3
• Resisted plantarflexion of the first ray: Reproduces pain along the peroneus longus tendon course in POPS. 3

Timing of Examination Matters

• Always reexamine 3-5 days post-injury, as excessive swelling and pain within the first 48 hours can limit the initial examination. 2

Definitive Imaging Strategy

MRI Without Contrast (First-Line Advanced Imaging)

• MRI is the reference standard for occult fractures with bone marrow edema patterns, particularly in inversion injuries and persistent lateral ankle pain. 1
• Stress injuries of bone, including those in "weekend warriors," are best depicted by MRI. 1
• MRI has 93-96% sensitivity and 100% specificity for occult fractures and is superior for evaluating soft-tissue injuries, ligamentous pathology, and tendon tears. 7
• MRI can identify peroneal tendon tears, tenosynovitis, and os peroneum fractures that are invisible on radiographs. 8, 5

CT Without Contrast (Alternative if MRI Unavailable)

• CT is useful for radiographically occult fractures and demonstrates only 87% and 78% sensitivity for calcaneal and talar fractures respectively compared to radiography, but superior to plain films. 1
• CT excels at identifying cortical and subcortical involvement, particularly for lateral process fractures of the talus and subtle calcaneal fractures. 1
• If radiographs are negative but pain persists >1 week, MRI without contrast or CT without contrast are equivalent alternatives according to ACR guidelines. 2

Technetium Bone Scan (Historical Alternative)

• Bone scanning has 92.9% sensitivity for stress injuries and shows abnormalities days to weeks earlier than radiographs. 1
• However, bone scan cannot differentiate stress reactions from actual stress fractures and has been largely supplanted by MRI. 1
• Consider bone scanning if MRI is contraindicated or unavailable. 2

Treatment Algorithm Based on Diagnosis

If Stress Fracture Confirmed

• Protect and immobilize the foot immediately. 2
• Rest and activity modification with non-weight-bearing or protected weight-bearing depending on fracture location and severity. 2
• Fifth metatarsal base fractures (Jones fractures) and lateral process fractures often require orthopedic referral due to high nonunion rates.

If Peroneal Tendon Pathology or POPS Confirmed

• Most acute peroneus brevis tears require surgical treatment because they represent a mechanical abnormality. 6
• Peroneus longus tears at the cuboid tunnel may initially be managed nonoperatively with immobilization, NSAIDs, and activity modification. 6
• If associated with stenosing tendonitis or hypertrophic peroneal tubercle, surgical debridement and tenodesis may be required. 6, 4

Conservative Management for Stress Reactions (Not Fractures)

• PRICE protocol (protection, rest, ice, compression, elevation). 9
• NSAIDs for pain control (naproxen 250-500 mg twice daily or ibuprofen). 9
• Activity modification with gradual return to activity once pain-free. 2

Common Pitfalls to Avoid

Don't Assume "Just a Sprain"

• 15% of ligamentous injuries on MRI demonstrate no fracture on radiography, and conversely, occult fractures can present with minimal swelling mimicking sprains. 1
• Lateral process fractures are frequently misdiagnosed as lateral ankle sprains. 1, 2

Don't Overlook Stress Fractures

• Symptoms often occur before the fracture is radiographically visible, requiring advanced imaging (MRI or bone scan) for early diagnosis. 2
• Patients often report increased walking activity or change to harder surfaces before symptom onset. 2

Don't Delay Imaging if Pain Persists

• If pain persists >1 week despite negative radiographs, advanced imaging is indicated. 2
• Waiting for repeat radiographs to show callus formation (typically 2-3 weeks) delays diagnosis and appropriate treatment. 1

Don't Miss Os Peroneum Pathology

• Os peroneum involvement in peroneus longus tendon disorders is frequently misdiagnosed by radiologists. 5
• Look for changes in os peroneum morphology or position on radiographs, which may indicate proximal or distal tendon tears. 5

When to Refer

Immediate Orthopedic/Podiatric Referral

• Displaced fractures or fractures at high-risk locations (Jones fracture, lateral process of talus). 2
• Complete peroneal tendon ruptures or tears requiring surgical repair. 6
• No improvement after 6-8 weeks of appropriate conservative treatment. 2