Periosteal Reaction Patterns

What it is. Periosteum is a bilaminar membrane, and its inner cambium layer contains osteoprogenitor cells. Any process that lifts, irritates or inflames the periosteum triggers osteoid deposition.

Why the first film can be normal. Newly laid osteoid is radiolucent, so periosteal reaction is invisible on plain film for approximately 10-21 days after the insult. That is why the film in an early stress fracture or early osteomyelitis may be normal.

Is it really there? Look for a line of mineralised density lying parallel to the cortex and separated from it by a lucent gap, best appreciated tangentially at the bone margin. Confirm it on the orthogonal view: a genuine reaction persists, while an overlying structure moves. Then describe it: continuous (unbroken along its length) or interrupted, thin or thick, and whether it has an intact outer margin.

The only classification that matters is by aggressiveness, and each pattern carries a rate:

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Naming the pattern. Use the descriptive term, then say what it implies:

• Solid — a homogeneous, thick band fused to cortex, from slow processes lasting more than 2 months
• Single lamella or thin shell — one thin line, the earliest visible reaction, and indeterminate at that moment
• Lamellated (onion-skin) — multiple concentric layers, implying repeated cycles of lifting and ossification
• Spiculated perpendicular (hair-on-end) — fine parallel spicules at 90 degrees to cortex, along Sharpey fibres
• Spiculated divergent (sunburst) — spicules radiating from a focal point
• Codman triangle — an angled cuff of ossified periosteum at the margin of the lesion, the central portion having been destroyed
• Undulating or wavy — thick, irregular and mature, seen in venous stasis, chronic infection and hypertrophic osteoarthropathy

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Saying it out loud. Pattern first, then extent and associated findings, then what it means and what you would do: "There is an interrupted, lamellated periosteal reaction along the mid-diaphysis of the femur over approximately 8 cm, with an associated permeative lytic lesion and a soft tissue mass. The pattern indicates an aggressive process; my differential is Ewing sarcoma, osteosarcoma and osteomyelitis, and I would want an MRI of the whole bone and a chest CT."

Mimics. Several things look like periosteal reaction and are not:

• Nutrient foramen or vascular groove — a lucent line, not a dense one, oblique and unicortical
• Cortical striations of hyperparathyroidism — intracortical tunnelling, not a separate outer line
• Overlapping soft tissue folds, skin folds, plaster or dressing edges — cross the bone margin and continue into soft tissue
• Myositis ossificans — mineralisation in the soft tissue, with a lucent cleft from an intact cortex and a mature peripheral rim (zoning)
• Parosteal osteosarcoma — a dense juxtacortical mass with a cleft, not a layered reaction
• Ossifying haematoma or avulsive cortical irregularity at the distal posteromedial femur in adolescents — a benign, self-limiting cortical desmoid
• Periosteal chondroma — cortical saucerisation with a buttress of mature periosteal bone at each end

Plain films first. Orthogonal films of the whole bone come first, always, and always include both joints. They confirm the reaction is real, define matrix and zone of transition, and may make the diagnosis outright: stress fracture, venous stasis, physiological infantile periostitis. They change management by determining whether anything further is needed at all.

MRI of the whole affected bone, with contrast, is mandatory for every interrupted pattern. It defines marrow extent, skip lesions, soft tissue mass, neurovascular involvement and joint invasion, all of which determine the resection plan, and it distinguishes a rim-enhancing subperiosteal abscess from a solid enhancing tumour mass.

Thin-slice CT of the lesion is for the questions of matrix, cortical integrity, sequestrum or the nidus of an osteoid osteoma. It is superior to MRI for mineralisation, and it does not replace MRI for staging.

Chest radiograph, then CT chest, has two indications: staging any suspected primary bone sarcoma, and finding the lung primary in bilateral symmetrical hypertrophic osteoarthropathy. It changes management by upstaging, or by redirecting the whole workup away from the limb.

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Bloods. FBC, CRP, ESR, ALP, LDH, calcium and a myeloma screen separate infection from tumour imperfectly but usefully. Raised ALP and LDH carry prognostic weight in osteosarcoma and Ewing sarcoma, and a myeloma screen is mandatory in any adult over 50 with permeative bone lesions.

Biopsy, at the treating sarcoma centre only. Image-guided core biopsy is taken along the future resection incision, and never before MRI staging is complete. A biopsy tract placed by a non-specialist can convert a limb-salvage case into an amputation.

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When no further imaging is needed. These films need nothing further:

• Physiological periostitis of infancy in a well 3-month-old with a normal examination
• Unilateral undulating tibial periostitis in a patient with obvious venous ulceration and lipodermatosclerosis
• A solid fusiform reaction at a known fracture site, healing as expected

Related pages. The aggressive interrupted patterns, and what makes them: Osteosarcoma for the sunburst and Codman triangle, Ewing Sarcoma for onion-skin — noting neither pattern belongs to either tumour exclusively — plus Langerhans Cell Histiocytosis and Chondrosarcoma, whose endosteal scalloping matters more than its periostitis. The surface tumours read off the same film: Parosteal Osteosarcoma and Periosteal Osteosarcoma. The great mimic in both directions: Myositis Ossificans — peripheral maturation, the reverse zoning to a sarcoma. Infection, which reproduces every aggressive pattern: Osteomyelitis Pathophysiology, Septic Arthritis Pathophysiology and Septic Arthritis of the Paediatric Hip. The benign continuous patterns: Stress Fractures Overview, Stress Fractures of the Femoral Neck and Osteoid Osteoma. Symmetrical and multifocal periostitis: Hypertrophic Osteoarthropathy — image the chest — plus Caffey Disease, Scurvy and Non-Accidental Injury, where reactions of differing ages are the discriminator.

Where the causes come from. Tumour (osteosarcoma, Ewing sarcoma, metastasis, leukaemia, lymphoma), haemorrhage and haematological disease (subperiosteal haematoma, scurvy, haemophilia, thalassaemia), infection (acute and chronic osteomyelitis, subperiosteal abscess, syphilis) and hypertrophic pulmonary osteoarthropathy, with physiological, pressure and traumatic causes alongside.

The infant. Symmetrical, smooth, single-layer periosteal new bone along the femora, tibiae and humeri between roughly 1 and 4 months of age is normal. Labelling it non-accidental injury without metaphyseal corner fractures or rib fractures is a serious error.

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Work through the film in this order.

1. Is the reaction continuous or interrupted? Trace the outer margin of the new bone along its whole length before looking at anything else, and ask only whether it is unbroken. Continuous with an intact outer margin means the periosteum kept pace: benign, healed or slow. Interrupted, lamellated, spiculated or a Codman triangle means the lesion outran it: aggressive. This single question sorts the differential into two halves before you consider age or site.

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2. Is it solitary or multifocal, unilateral or symmetrical? Look at the opposite limb and at any other available films, and ask specifically whether lesions are of the same age or different ages.

Symmetrical bilateral periostitis of the distal long bones in an adult means hypertrophic osteoarthropathy, so image the chest, not the leg; symmetrical diaphyseal periostitis in a 2-month-old is physiological. Multiple lesions of different ages in an infant mean non-accidental injury, and multiple lesions in an adult over 50 mean metastasis or myeloma.

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3. Where in the bone is it, and how old is the patient? Name the segment (epiphysis, metaphysis, diaphysis) and pair it with the decade; do not skip the age. Epiphyseal lesions rarely produce periosteal reaction at all.

• Metaphysis, aged 10-20, aggressive reaction: osteosarcoma
• Diaphysis, aged 5-20, onion-skin: Ewing sarcoma or osteomyelitis
• Metaphysis, under 5, with fever: acute osteomyelitis
• Diaphysis, over 40, with cortical destruction: metastasis or lymphoma

4. Is there matrix, and what kind? Look inside the lesion and inside any soft tissue mass for mineralisation, and classify it as osteoid, chondroid or absent, with CT if the plain film is equivocal. Fluffy, cloud-like amorphous mineralisation extending into the soft tissue mass is osteoid, and means osteosarcoma. Rings-and-arcs or popcorn calcification is chondroid, and means chondrosarcoma, which classically causes endosteal scalloping greater than two-thirds of cortical thickness rather than florid periostitis. No matrix with permeative lysis favours Ewing sarcoma, lymphoma or infection.

5. Is there a soft tissue mass, and is it out of proportion? Compare the size of the soft tissue component with the amount of bone destruction, and look for a rim-enhancing collection tracking beneath the periosteum. A large mass with relatively modest bone destruction is characteristic of Ewing sarcoma and lymphoma of bone. A rim-enhancing subperiosteal collection with adjacent muscle oedema is a subperiosteal abscess, and no mass at all with a fusiform solid reaction favours stress fracture or osteoid osteoma.

The mass, not the spicules. A spiculated reaction with no soft tissue mass is far less worrying than the same reaction with a large mineralised mass. The mass drives the diagnosis and the biopsy.

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6. What are the systemic and biochemical features? Take the temperature, send CRP, white cell count and ESR, examine for clubbing, and ask directly about night pain, weight loss, smoking and diet. Fever with a raised CRP and white cell count points to infection, but Ewing sarcoma produces fever and a raised ESR and is the classic mimic, so this finding narrows nothing on its own. Clubbing means hypertrophic osteoarthropathy, bleeding gums and dietary restriction in a toddler mean scurvy, and night pain abolished by aspirin means osteoid osteoma.

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7. What did it look like before, and what does it look like now? Serial films are the most underrated discriminator. Find the old films; if there are none, take a new one at a defined interval rather than guessing, and 2 to 3 weeks is enough to answer the question. An aggressive pattern that consolidates over 3-6 weeks on antibiotics was infection, a pattern that progresses despite treatment is tumour, and any reaction that changes appreciably over 2 weeks is by definition an active, rapid process.

Healing changes the pattern. An aggressive lamellated reaction that is treated, with antibiotics, chemotherapy or rest, consolidates into a solid shell. A solid reaction on a single film may be an aggressive lesion caught late, so always ask for the old films.