Pellegrini-Stieda Lesion

The Pellegrini-Stieda lesion is post-traumatic calcification or ossification at or near the femoral attachment of the medial collateral ligament (MCL) of the knee. On the AP radiograph it is a curvilinear or comma-shaped density lying medial to the femoral condyle, and it is one of the classic spot diagnoses in orthopaedic radiology.

What it is. Neither a true fracture nor a ligament tear, it is a form of heterotopic ossification: new bone formed within the substance of the injured MCL. "Pellegrini-Stieda disease" and "Pellegrini-Stieda syndrome" are used interchangeably with "lesion" and name the same process, not a separate disease entity.

The name. Pellegrini (1905) first described the radiographic appearance of medial knee calcification, and Stieda (1908) reported on the associated medial knee injury.

The story. It typically develops weeks after an MCL sprain or a direct blow to the medial knee. The calcification is not visible on the initial radiograph and typically appears 3 to 6 weeks after the injury, which is itself a diagnostic clue.

Where it comes up. The lesion joins anatomy (the MCL origin), pathology (heterotopic ossification), radiology (reading the AP knee) and management (conservative patience) in a single topic, and it can surface in several parts of the exam:

• Radiology viva - a spot diagnosis on the AP knee, to be told apart from avulsion fractures and other calcifications
• Sports medicine - a complication of MCL injuries in athletes, and one reason some medial knee sprains take longer than expected to settle
• Trauma - it follows knee injuries including fractures and ligament disruptions, and its delayed appearance teaches the timeline of heterotopic ossification
• Basic science - a way into the biology of heterotopic ossification: mesenchymal stem cell differentiation, BMP signalling and mineralisation
• Operative surgery - a rare indication for excision, but the principles (protect the MCL, excise the mass, rehabilitate) are examinable

The ligament. The MCL is a broad, flat ligament on the medial side of the knee. It originates from the medial femoral epicondyle and inserts onto the medial tibial metaphysis, about 5 to 7 cm distal to the joint line.

Two layers. The superficial MCL (tibial collateral ligament) is the larger, more superficial portion, running from the medial epicondyle to the proximal tibia. The deep MCL blends with the joint capsule and attaches firmly to the medial meniscus.

Where the lesion forms. At or near the femoral attachment, where the ligament is tightly bound to the medial epicondyle. The adductor magnus tendon and the medial head of gastrocnemius lie close by and may contribute to the pattern of calcification in some cases.

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The prevailing theory. The exact mechanism remains debated, but the lesion is thought to be heterotopic ossification within the damaged MCL, forming in a sequence:

1. Trauma - an MCL sprain (often a valgus injury) or a direct blow to the medial knee damages the ligament fibres at or near the femoral attachment
2. Inflammatory response - the damaged tissue releases inflammatory mediators and growth factors that stimulate mesenchymal stem cell differentiation into osteoblasts; BMP signalling is the key molecular driver
3. Mineralisation - over the following weeks calcium is deposited in the damaged tissue, forming an immature calcific deposit that progressively matures
4. Maturation - the deposit may mature into true lamellar bone with organised trabeculae, or it may remain as amorphous calcification

Familiar biology. The process parallels the heterotopic ossification seen after hip arthroplasty and spinal cord injury.

The time course. The delay between injury and a visible deposit is a hallmark of the lesion, and it is what distinguishes it from an acute avulsion fracture:

• 0 to 2 weeks - injury; the initial radiograph is normal; inflammatory phase with local swelling and pain
• 2 to 6 weeks - the calcification becomes visible on the radiograph; the patient may notice persistent medial knee pain and stiffness
• 6 weeks to 6 months - the calcification may enlarge or mature; symptoms typically plateau
• 6 to 12 months - gradual resorption in most cases; the calcification may partially or fully resolve

There is no single universally accepted classification. The most practical approach is to describe the lesion by its radiographic maturity, which correlates with clinical behaviour and management.

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History. A preceding MCL injury (valgus stress, a direct blow or a sporting injury) 3 to 6 weeks earlier, which the patient may have forgotten or considered minor. Medial knee pain comes on gradually and worsens with activity, and may be worse at night or after exercise. Stiffness makes bending difficult, especially beyond 90 degrees of flexion: pain and loss of flexion from the calcific mass are the main clinical problem.

Examination. Look for:

• Localised tenderness over the medial femoral epicondyle, the MCL origin
• Restricted knee flexion compared with the contralateral side
• Mild medial joint-line swelling or fullness
• Pain on valgus stress testing, with the MCL usually stable rather than lax (see the next section)
• A limp from pain and stiffness

Not every lesion hurts. Many are discovered incidentally on radiographs taken for other reasons and cause no symptoms. Others present late: because the calcification appears weeks after the injury, the patient returns with medial knee pain that was expected to have resolved by now.

Bilateral lesions are rare but possible, typically in patients with bilateral knee injuries or systemic conditions predisposing to heterotopic ossification.

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Interpreting the valgus stress test is the key examination skill for the underlying MCL injury that produced the lesion. Test at 30 degrees of flexion and in full extension, then grade the injury:

In the Pellegrini-Stieda knee. Valgus stress at 30 degrees typically reproduces pain, but the ligament is stable: the calcification sits within a healed MCL. Frank laxity instead signals ongoing MCL insufficiency.

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Plain radiographs. The AP knee is first-line in any patient with medial knee pain after injury, and often the only investigation needed. The lesion is a curvilinear or comma-shaped area of calcification or ossification just medial to the femoral condyle, at the expected site of the MCL femoral attachment. It lies within the MCL substance, separate from the femoral cortex (unlike an osteophyte or an avulsion fragment) and not extending into the joint (unlike a loose body), although a mature beak-type lesion may show cortical continuity.

The lateral view is always obtained alongside the AP as part of the standard knee series, but it is less helpful. The calcification is superimposed on the femur, though a large deposit may be projected anteriorly.

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MRI is not routinely required but is useful when:

• the diagnosis is uncertain, to confirm the calcification lies within the MCL and exclude other pathology
• there is concern about associated ligament or meniscal injury
• surgical excision is being considered, to plan the approach and assess MCL integrity

The lesion returns low signal on all sequences (T1, T2 and PD) at the MCL femoral attachment, consistent with calcification. There may be surrounding soft-tissue oedema in the acute phase.

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CT is occasionally used for pre-operative planning. It defines the exact size, shape and location of a mature ossified lesion and its relationship to the femoral cortex, which helps plan the approach and the extent of excision.

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Differential diagnosis. The most common mistake is confusing the lesion with a medial femoral condyle avulsion fracture or a medial meniscus calcification. The alternatives, and what separates them:

• Avulsion fracture of the medial epicondyle - a detached piece of cortical bone rather than calcification within the MCL; if a density is seen on a film taken immediately after trauma, think avulsion instead
• Chondrocalcinosis (CPPD) - calcification within the meniscus or articular cartilage, inside the joint rather than at the MCL
• Loose body in the medial gutter - a mobile calcified body within the joint, not fixed at the MCL attachment
• Femoral cortex irregularity - a cortical ridge or groove may mimic the lesion; look for continuity with the cortex
• Calcific tendinitis of the medial gastrocnemius - can cause medial knee pain, but the calcification is more posterior and distal
• Infection or osteomyelitis - periosteal reaction at the medial condyle, with systemic signs and a different clinical picture
• Other soft-tissue calcification - from renal failure, tumoral calcinosis or previous surgery

Conservative management is the mainstay. The vast majority of lesions resolve with non-operative treatment, most settling over 3 to 12 months, and patience is key before considering surgery. Treatment follows a progressive approach.

Acute phase (0-6 weeks). Rest from provocative activities, with protected weight-bearing on crutches if needed, to avoid stressing the healing MCL; a hinged knee brace protects it from valgus stress. Simple analgesia or NSAIDs control pain and inflammation, and cryotherapy (ice) reduces local swelling. Avoid passive stretching or forced flexion.

Subacute phase (6 weeks to 3 months). Gentle active-assisted range-of-motion exercises regain flexion without aggravating the calcification. Progress weight-bearing as tolerated, strengthen the quadriceps and hamstrings, and continue NSAIDs if needed. The calcification may now be visible on the radiograph, and is expected at this stage.

Remodelling phase (3-12 months). Progressive strengthening and stretching, then a gradual return to sport and full activity. Serial radiographs may show a gradual reduction in the size of the calcification, and most patients achieve full or near-full recovery by 6 to 12 months. Document range of motion and pain scores throughout, because that record is what justifies the timing of any intervention.

When to operate. The indications for excision are:

• persistent, functionally limiting pain despite at least 6 to 12 months of conservative treatment
• significant restriction of knee flexion (a mechanical block) from a large, mature lesion
• a beak-type (Type III) lesion causing impingement or a mechanical block

Surgical technique. Excision of the calcified or ossified mass, with careful preservation of the MCL:

1. Medial incision over the area of calcification
2. Identify and protect the MCL fibres
3. Carefully excise the calcific or ossific mass from within the ligament substance
4. Preserve or repair the MCL if its integrity is compromised
5. Immobilise in a hinged brace after surgery, followed by gradual rehabilitation

Outcomes. Reported results of excision are generally good, but the evidence base is limited to small case series and case reports. The key principle is to exhaust conservative treatment first.

Percutaneous treatment. Ultrasound-guided percutaneous treatment of the deposit has been reported in individual cases (see the Evidence Base).

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Prognosis. The overall prognosis is very good. Most lesions resolve or become asymptomatic within 6 to 12 months of conservative management.

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Grading. The reference grading for heterotopic ossification, around the hip, is the Brooker classification:

• I - islands of bone in soft tissue
• II - spurs leaving over 1 cm between opposing surfaces
• III - spurs leaving under 1 cm
• IV - apparent ankylosis

Risk factors include male sex, prior heterotopic ossification, ankylosing spondylitis or DISH, and extensive surgical dissection.

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• No specific guidelines exist for the Pellegrini-Stieda lesion from any major orthopaedic society (AAOS, BOA, EFORT). Management is based on expert opinion, case series, and general principles of heterotopic ossification treatment.
• Conservative management is the universal standard across all regions: rest, protected mobilisation, physiotherapy, and NSAIDs. The time course and expected recovery are the same regardless of geography.
• Surgical excision is reported in case series from multiple countries (Japan, India, Europe, North America) with broadly similar indications: persistent symptoms after at least 6 to 12 months of failed conservative treatment, with a mechanical block from a mature lesion.
• Prevention: some surgeons recommend a short course of NSAIDs after MCL injury to reduce the risk of heterotopic ossification, extrapolating from the hip arthroplasty literature, but there is no high-quality evidence specific to the Pellegrini-Stieda lesion.
• Global epidemiology: the lesion is reported worldwide with no clear geographic or ethnic predilection. It is more common in athletes and active individuals who sustain MCL injuries, and may be under-diagnosed in regions where knee injuries are managed non-operatively without follow-up radiography.
• Minimally invasive techniques such as ultrasound-guided percutaneous lavage and extracorporeal shockwave therapy have been reported in individual cases, but remain experimental without Level 1 or 2 evidence.