The femoral head sinks too deep into the pelvis | Primary (idiopathic) or secondary to bone-softening or inflammatory disease | Diagnosed on the AP pelvis | THA restores the hip centre and grafts the medial defect
PRIMARY VERSUS SECONDARY
Critical Must-Knows
- Protrusio acetabuli means the femoral head has migrated medially, sinking past the ilioischial (Kohler) line so the head lies deep within the pelvis - the joint centre is medialised and the bone medial to the head is deficient
- Most protrusio is secondary - always hunt for an underlying cause (inflammatory arthritis, a bone-softening metabolic disease, Marfan syndrome, trauma, tumour, or infection) before labelling it primary or idiopathic
- It is diagnosed on a plain AP pelvis radiograph: the medial wall of the acetabulum lies medial to the ilioischial line, and the lateral centre-edge angle is high (greater than 40 degrees) - the opposite of the shallow, uncovered hip of dysplasia
- In the young hip with little arthritis, treatment aims to stop progression - valgus intertrochanteric osteotomy in the adult, and triradiate cartilage closure in the growing child
- Total hip arthroplasty is the definitive treatment once the hip is arthritic: the key principles are to lateralise the cup back to the true hip centre and to bone-graft the medial defect, usually with morsellised femoral head autograft
Clinical Pearls
- "The single best test is an AP pelvis radiograph - protrusio is present when the femoral head or medial acetabular wall sits medial to the ilioischial (Kohler) line
- "Protrusio is the mirror image of dysplasia: dysplasia is a shallow socket with a low centre-edge angle, protrusio is a deep socket with a high centre-edge angle
- "New bilateral protrusio in a tall young patient should prompt a search for Marfan syndrome
- "The two THA principles examiners want to hear are restore the hip centre (lateralise the cup) and graft the medial defect with morsellised autograft
Clinical Imaging
Protrusio before and after total hip arthroplasty




Critical Protrusio Acetabuli Exam Points
Definition
Protrusio acetabuli is medial migration of the femoral head into the pelvis, so that the head or medial acetabular wall lies medial to the ilioischial (Kohler) line. The hip centre is medialised and the medial acetabular bone is deficient - the opposite of the shallow, uncovered dysplastic hip.
Most Cases Are Secondary
Always look for a cause: inflammatory arthritis (rheumatoid, ankylosing spondylitis), bone-softening disease (osteomalacia, Paget, osteogenesis imperfecta), Marfan syndrome, trauma, tumour, or infection. Primary (idiopathic) protrusio is a diagnosis of exclusion.
Diagnosis and Measurement
Made on a plain AP pelvis. The medial wall lies medial to the ilioischial line, and the lateral centre-edge angle is high (greater than 40 degrees). The acetabular line crossing medial to the ilioischial line is the classic radiographic sign.
The Two THA Principles
For arthroplasty, examiners want: restore the hip centre by lateralising the cup back to the true (anatomical) position, and bone-graft the medial defect, usually with morsellised femoral head autograft. Rely on the intact peripheral rim for press-fit, not on screws into thin medial bone.
At a Glance
Protrusio Acetabuli - Quick Reference
| Question | Answer |
|---|---|
| What is it? | Medial migration of the femoral head into the pelvis, past the ilioischial (Kohler) line |
| How do I diagnose it? | AP pelvis - medial wall medial to the ilioischial line, centre-edge angle greater than 40 degrees |
| Primary or secondary? | Mostly secondary - find the cause; primary is bilateral, middle-aged women, diagnosis of exclusion |
| Commonest secondary causes | Inflammatory arthritis (rheumatoid, ankylosing spondylitis); also bone-softening disease and Marfan |
| Young hip, minimal arthritis | Valgus intertrochanteric osteotomy (adult); triradiate closure (child) |
| Arthritic hip | Total hip arthroplasty - restore the hip centre and graft the medial defect |
| The one-line mnemonic | Protrusio is dysplasia's mirror image - deep socket, high centre-edge angle |
Memory aids
PROTRUDESCauses of Secondary Protrusio
| P | Paget disease Softened, remodelling bone yields under load |
| R | Rheumatoid arthritis The commonest inflammatory cause |
| O | Osteomalacia and osteoporosis Weak periacetabular bone |
| T | Trauma Medial wall (acetabular) fracture |
| R | Radiation and tumour Bone destruction or metastasis |
| U | Untreated infection Septic destruction of the medial wall |
| D | Dysplasia syndromes and OI Osteogenesis imperfecta, fibrous dysplasia |
| E | Ehlers-Danlos and Marfan Connective tissue disorders |
| S | Spondyloarthropathy Ankylosing spondylitis |
| P | Paget disease Softened, remodelling bone yields under load | T | Trauma Medial wall (acetabular) fracture | D | Dysplasia syndromes and OI Osteogenesis imperfecta, fibrous dysplasia |
| R | Rheumatoid arthritis The commonest inflammatory cause | R | Radiation and tumour Bone destruction or metastasis | E | Ehlers-Danlos and Marfan Connective tissue disorders |
| O | Osteomalacia and osteoporosis Weak periacetabular bone | U | Untreated infection Septic destruction of the medial wall | S | Spondyloarthropathy Ankylosing spondylitis |
Hook:The femoral head PROTRUDES through the medial wall - the letters list the diseases that let it happen.
DEEPReading the AP Pelvis for Protrusio
| D | Deep socket - head sits within the pelvis Loss of normal medial bone |
| E | Edge angle high (CE angle greater than 40 degrees) Over-coverage, the opposite of dysplasia |
| E | Examine the Kohler (ilioischial) line Medial wall lies medial to it |
| P | Plain film first, then look for the cause Bloods and further imaging as guided |
| D | Deep socket - head sits within the pelvis Loss of normal medial bone | E | Examine the Kohler (ilioischial) line Medial wall lies medial to it |
| E | Edge angle high (CE angle greater than 40 degrees) Over-coverage, the opposite of dysplasia | P | Plain film first, then look for the cause Bloods and further imaging as guided |
Hook:A protrusio hip is DEEP - deep socket, high edge angle, crosses the Kohler line on the plain film.
GRAFTTHA Principles for Protrusio
| G | Graft the medial defect Morsellised femoral head autograft |
| R | Restore the hip centre Lateralise the cup to the true position |
| A | Anchor on the peripheral rim Press-fit on intact rim bone |
| F | Fixation - avoid relying on medial screws Use rim support; a cage if the rim is deficient |
| T | Template carefully Plan offset and leg length pre-operatively |
| G | Graft the medial defect Morsellised femoral head autograft | F | Fixation - avoid relying on medial screws Use rim support; a cage if the rim is deficient |
| R | Restore the hip centre Lateralise the cup to the true position | T | Template carefully Plan offset and leg length pre-operatively |
| A | Anchor on the peripheral rim Press-fit on intact rim bone |
Hook:Think GRAFT for the protrusio THA - Graft, Restore the centre, Anchor on the rim, Fixation by rim not medial screws, Template.
Overview
Protrusio acetabuli describes a hip in which the femoral head has sunk too deep into the acetabulum, migrating medially so that the head and the floor of the socket bulge into the pelvis. The defining radiographic feature is that the medial wall of the acetabulum (or the femoral head itself) lies medial to the ilioischial line - the line that marks the normal medial border of the joint.
It is best understood as the mirror image of hip dysplasia. In dysplasia the socket is shallow and the head is uncovered (a low centre-edge angle); in protrusio the socket is too deep and the head is over-covered (a high centre-edge angle). Both deformities change where the load passes through the joint and both lead to early arthritis, but they need opposite reconstructions - lateralising and deepening for the dysplastic hip, and lateralising the cup back out of the pelvis for the protrusio hip.
For the exam, three things matter: knowing the causes (and that most cases are secondary), being able to diagnose and measure it on a plain radiograph, and understanding the principles of reconstruction, particularly the two rules of total hip arthroplasty - restore the hip centre and graft the medial defect.
Anatomy and Pathophysiology
The normal acetabulum is a deep, hemispherical socket whose medial wall is supported by the thick bone of the quadrilateral plate and stabilised by the surrounding bony columns. In a healthy hip the medial acetabular wall lies lateral to (outside) the ilioischial line, and load passing from the femoral head is spread across the dome and the columns.
How protrusio develops:
- The deformity occurs when the medial acetabular bone is too weak to resist the force transmitted by the femoral head, or when that force is abnormally high. Over time the head pushes the medial wall progressively into the pelvis.
- In bone-softening disease (osteomalacia, Paget disease, osteogenesis imperfecta) the periacetabular bone itself is weak and gradually yields.
- In inflammatory arthritis (rheumatoid, ankylosing spondylitis) synovitis erodes the medial wall and central cartilage, and the softened bone migrates medially.
- In primary (idiopathic) protrusio there is no clear underlying disease - it is typically bilateral and more common in middle-aged women. When severe and bilateral it has historically been called an Otto pelvis.
Why it causes symptoms:
As the head migrates medially the hip centre is displaced, which shortens the effective offset and the lever arm of the abductors, leading to a weak, sometimes limping gait. The deep socket causes impingement of the femoral neck against the rim, limiting movement (especially abduction and rotation), and the abnormal load distribution drives secondary osteoarthritis. Late in the disease the patient presents with progressive pain and stiffness.
Protrusio Versus Dysplasia - the Two Opposite Hips
| Feature | Protrusio acetabuli | Developmental dysplasia |
|---|---|---|
| Socket depth | Too deep - head sunk into the pelvis | Too shallow - head uncovered |
| Lateral centre-edge angle | High (greater than 40 degrees) - over-coverage | Low (less than 20 to 25 degrees) - under-coverage |
| Medial wall versus ilioischial line | Wall lies medial to the line | Wall lies lateral to the line (normal or more lateral) |
| Typical THA correction | Lateralise the cup outward, graft the medial defect | Place cup at the true centre, may need superolateral graft |
Classification
The most useful classification for the exam is simply primary versus secondary, because it drives the search for an underlying cause. Severity can then be graded by how far the femoral head lies medial to the ilioischial line.
- Primary (idiopathic) protrusio acetabuli - no identifiable underlying disease. It is usually bilateral, more common in middle-aged women, and is a diagnosis of exclusion once secondary causes have been ruled out. The severe bilateral form is the historical Otto pelvis.
- Secondary protrusio acetabuli - the result of an identifiable disease that weakens periacetabular bone or increases central load. This is the larger group and the one examiners expect you to work through.
Clinical Presentation
Protrusio is often silent for years and only declares itself when secondary arthritis develops. The history and examination are aimed both at the hip and at the underlying cause.
Symptoms
Progressive groin pain and stiffness, often gradual. Patients notice loss of movement (especially abduction and rotation) before pain becomes severe. Ask about features of inflammatory arthritis and a family history suggesting Marfan or other connective-tissue disease.
Examination of the Hip
Reduced and painful range of movement, particularly abduction, external and internal rotation, because the deep socket causes impingement. Look for a fixed flexion deformity, leg-length difference, and an abductor (Trendelenburg) limp from the medialised hip centre.
Look for the Cause
Examine for signs of rheumatoid arthritis (hand and other joint involvement), ankylosing spondylitis (stiff spine, reduced chest expansion), and the marfanoid habitus (tall stature, long limbs, arachnodactyly, high-arched palate, lens or aortic problems).
Bilaterality
Primary protrusio and the connective-tissue and metabolic causes are typically bilateral, so always examine and image both hips even when only one is painful.
Investigations
Investigating Protrusio Acetabuli
| Investigation | What it shows | Why it matters |
|---|---|---|
| AP pelvis radiograph | Medial wall or head medial to the ilioischial line; high centre-edge angle; deep socket | The single key diagnostic test - confirms protrusio and grades severity |
| Lateral and false-profile views | Anterior and posterior wall and overall coverage | Help plan reconstruction and assess impingement |
| CT of the pelvis | Three-dimensional bone stock, wall and column integrity | Templates the cup and shows how much medial bone graft is needed |
| Bloods (inflammatory and metabolic screen) | Inflammatory markers, rheumatoid serology, calcium, phosphate, vitamin D, ALP, PTH | Identifies inflammatory or bone-softening causes |
| Targeted work-up for syndromes | Genetic and clinical assessment for Marfan or other connective tissue disease | A new bilateral protrusio in a young patient needs a cause |
Measuring it on the radiograph
The diagnosis rests on the AP pelvis:
- Ilioischial (Kohler) line - the diagnosis is made when the medial acetabular wall or the femoral head lies medial to this line. This is the most important single sign.
- Lateral centre-edge angle of Wiberg - high (greater than 40 degrees) in protrusio, reflecting over-coverage of the head (the opposite of the low angle of dysplasia).
- Acetabular crossing - quantifying how far the floor lies medial to the ilioischial line grades severity and helps planning.
Clinical Pearl
Read protrusio against dysplasia. If the femoral head is uncovered with a low centre-edge angle, think dysplasia; if the head is buried deep in a socket that crosses the ilioischial (Kohler) line with a high centre-edge angle, think protrusio. The reconstruction is the opposite in each: deepen and cover the dysplastic socket, but lateralise the protrusio cup back out of the pelvis.
Management
Management depends on the age of the patient, the underlying cause, and how much arthritis has developed. The aim early in the disease is to halt progression; once the hip is arthritic the aim is reconstruction.
- Treat the underlying cause - medical control of inflammatory arthritis and correction of metabolic bone disease (vitamin D, treatment of osteomalacia or Paget disease) can slow progression and are essential before considering surgery.
- Skeletally immature child - if recognised before the triradiate cartilage closes, closure or arrest of the triradiate cartilage (sometimes combined with a femoral osteotomy) can stop further medial migration.
- Young adult, minimal arthritis - a valgus intertrochanteric proximal femoral osteotomy redirects load and can relieve symptoms and slow progression while the patient's own joint is preserved.
Surgical Technique
The technically demanding part of the operation is the acetabular reconstruction. The femoral side is usually managed as in routine arthroplasty, but careful templating of offset and leg length is essential because protrusio distorts both.
- Use a familiar approach (posterior, direct lateral, or direct anterior) - the choice matters less than safe delivery of an often incarcerated femoral head from a deep socket.
- Because the head sits deep within the pelvis, dislocation can be difficult and can fracture fragile bone; an in-situ femoral neck cut to deliver the head in pieces is a safe option when dislocation is risky.
- Preserve the resected femoral head - it is the source of morsellised autograft for the medial defect.
Complications
Key Complications
| Complication | Why it happens | How to limit it |
|---|---|---|
| Failure to restore the hip centre | Cup seated in the deep medial defect | Lateralise the cup and graft the medial defect to rebuild the true centre |
| Aseptic loosening | Poor initial fixation in deficient or grafted medial bone | Press-fit on the intact rim; cage or ring if the rim is inadequate |
| Medial wall perforation or fracture | Reaming or impacting into thin, soft medial bone | Gentle technique, recognise fragile bone, protect the medial wall |
| Graft non-union or resorption | Failure of the medial bone graft to incorporate | Use morsellised autograft on a viable, well-prepared bed |
| Heterotopic ossification | Soft-tissue trauma during a demanding reconstruction | Recognised common complication - usually managed conservatively |
| Leg-length and offset error | Complex deformity not planned for | Careful pre-operative templating of both hips |
The deep socket can trap the femoral head
In severe protrusio the femoral head can be incarcerated within the cavernous acetabulum, making dislocation at surgery difficult and risking fracture of fragile bone. Plan the approach, consider an in-situ neck cut to deliver the head safely, and handle soft, osteoporotic medial bone with care.
Postoperative Care and Rehabilitation
- Weight-bearing - many surgeons protect weight-bearing for a period after a medially grafted reconstruction to allow the bone graft to incorporate before full load is applied; the exact regimen depends on the quality of fixation and the surgeon's judgement.
- Thromboprophylaxis and early mobilisation - standard arthroplasty venous thromboembolism prophylaxis and supervised physiotherapy to restore movement and abductor strength, which is often weak because of the previously medialised hip centre.
- Manage the underlying disease - continue medical control of inflammatory arthritis and treatment of metabolic bone disease, and review steroid and disease-modifying therapy with the rheumatology team to support bone healing and reduce infection risk.
- Follow-up imaging - serial radiographs to confirm the graft has incorporated, the cup remains well-fixed, and the restored hip centre is maintained, with attention to any sign of medial migration or loosening.
Outcomes and Prognosis
Published series of total hip arthroplasty for protrusio report good and durable results when the principles are followed - particularly the use of an uncemented cup with bone grafting of the medial defect and restoration of the hip centre.
- A systematic review of 18 studies (783 hips, mean follow-up about 8.9 years) found good outcomes with uncemented cups plus bone graft, identified restoration of the anatomical hip centre as paramount for durability, and noted that routine screw augmentation is not recommended.
- The commonest complications across series were aseptic loosening (which required revision) and heterotopic ossification (usually managed conservatively).
- A cohort using press-fitted cups reported that stable, well-fixed cups did well even when the centre was slightly medialised, with similar hip scores whether the centre was fully restored or left medialised - underlining that solid initial fixation on the rim is the key driver of a good result.
- Overall the prognosis after well-executed reconstruction is comparable to routine arthroplasty, while the natural history without treatment is progressive medial migration, stiffness and secondary osteoarthritis.
Evidence Base and Key Studies
Total Hip Arthroplasty in Protrusio Acetabuli (Systematic Review)
- Systematic review of 18 studies (8 prospective, 10 retrospective) covering 783 hips, mean age 60 years, followed for a mean of about 8.9 years
- Eighty percent of patients were female and most had inflammatory arthritis as the underlying cause
- Good outcomes were achieved with uncemented cups combined with bone graft, with restoration of the anatomical hip centre highlighted as paramount for durability
- Screw augmentation for fixation was not recommended unless absolutely necessary; the commonest complications were aseptic loosening (needing revision) and heterotopic ossification (managed conservatively)
Is Restoration of the Hip Centre Mandatory in THA for Protrusio?
- Retrospective review of 26 cementless THAs in 22 patients with protrusio, mean follow-up 5.1 years
- Press-fit of the cup was prioritised over hip-centre restoration; the centre was restored in 17 cups and left medialised in 9
- All 26 cups remained stable at final follow-up, including the medialised press-fitted cups
- Modified Harris Hip Scores were similar between the restored and medialised groups (about 83.6 versus 83.8)
Exam Viva Scenarios
Use these scenarios to practise clinical reasoning and management decisions
Define and Diagnose Protrusio (~3 min)
"You are shown an AP pelvis radiograph of a 55-year-old woman with bilateral deep-seated hips. The examiner asks what protrusio acetabuli is and how you would confirm and investigate it."
Definition: Protrusio acetabuli is medial migration of the femoral head into the pelvis, so that the femoral head or the medial acetabular wall lies medial to the ilioischial (Kohler) line. The hip centre is medialised and the medial bone is deficient - it is the mirror image of dysplasia.
Radiographic confirmation: On the AP pelvis the medial wall lies medial to the ilioischial line, and the lateral centre-edge angle is high, greater than 40 degrees, reflecting over-coverage. I would grade severity by how far the head lies medial to the line and image both hips because it is often bilateral.
Finding the cause: Most protrusio is secondary, so I would look for inflammatory arthritis, a bone-softening disease, or a connective-tissue disorder. I would take a focused history and examination, then send inflammatory markers and rheumatoid serology, and a metabolic bone screen (calcium, phosphate, vitamin D, alkaline phosphatase, PTH). In a tall young patient I would consider Marfan syndrome. Primary protrusio is a diagnosis of exclusion.
Reconstructing the Protrusio Hip (~4 min)
"A 62-year-old woman with long-standing rheumatoid arthritis has a painful, stiff right hip. The radiograph shows protrusio with secondary osteoarthritis. She wants surgery. How would you plan and perform the reconstruction?"
Decision: She has an arthritic, painful protrusio hip, so total hip arthroplasty is the definitive treatment. I would optimise her medically first - control of rheumatoid disease, review of disease-modifying and steroid therapy, and assessment of bone quality.
Planning: I would obtain an AP pelvis, lateral views, and usually a CT to assess the medial defect and bone stock, and template carefully because protrusio alters both leg length and offset.
The two principles: First, restore the hip centre - I would lateralise the cup back to the true anatomical centre rather than seating it in the deep medial defect. Second, graft the medial defect with morsellised femoral head autograft taken from her own resected head, creating a supporting bed for the cup. I would rely on press-fit fixation on the intact peripheral rim and avoid depending on screws into thin medial bone. If the rim could not support the cup, I would use a reinforcement ring or cage spanning from solid bone above to below the acetabulum.
Caution: Rheumatoid and steroid-treated bone is fragile, so I would handle the medial wall gently to avoid perforation and watch for an incarcerated head when dislocating the hip. I would warn her about heterotopic ossification and the small risk of further medial migration if fixation is suboptimal.
The Young Hip with Protrusio (~3 min)
"A 24-year-old man is found to have bilateral protrusio with only mild joint space change. He is tall with long fingers. How would you approach him?"
First, the cause: A tall young patient with long fingers and bilateral protrusio should be assessed for Marfan syndrome and other connective-tissue disorders - I would look for the marfanoid habitus, arachnodactyly, lens and aortic problems, and involve genetics and cardiology as appropriate.
Joint-preserving treatment: Because he is young with minimal arthritis, the aim is to halt progression and preserve his own joint rather than to replace it. In an adult with a still-preserved joint, a valgus intertrochanteric proximal femoral osteotomy can redirect load and relieve symptoms. Had he been skeletally immature, closure or arrest of the triradiate cartilage could have stopped further migration.
Long term: I would monitor him clinically and radiologically, treat any underlying disease, and reserve total hip arthroplasty for when secondary arthritis becomes symptomatic.
MCQ Practice Points
Clinical Pearl
Q: A 55-year-old woman has bilateral hips where the medial acetabular wall lies medial to the ilioischial line, with a centre-edge angle of 45 degrees. What is the diagnosis? A: Protrusio acetabuli - medialisation past the ilioischial (Kohler) line with a high centre-edge angle (greater than 40 degrees) is the defining picture. A low centre-edge angle with an uncovered head would instead be dysplasia.
Clinical Pearl
Q: A 24-year-old tall man with arachnodactyly has bilateral protrusio and minimal arthritis. What is the best next step? A: Assess for Marfan syndrome and, for the hip, choose joint-preserving surgery (valgus intertrochanteric osteotomy) rather than arthroplasty, because he is young with a preserved joint.
Clinical Pearl
Q: At total hip arthroplasty for protrusio, what are the two key reconstruction principles? A: Restore the hip centre by lateralising the cup to the true position, and bone-graft the medial defect with morsellised femoral head autograft, fixing on the intact peripheral rim. Routine screw augmentation is not recommended.
The defining radiographic sign
Protrusio is present when the medial acetabular wall or femoral head lies medial to the ilioischial (Kohler) line on the AP pelvis. This single fact is the most commonly tested point.
Centre-edge angle direction
The lateral centre-edge angle is high (greater than 40 degrees) in protrusio (over-coverage) and low in dysplasia (under-coverage). Examiners love this opposite pairing.
Most cases are secondary
The favourite distractor is calling it primary. Rheumatoid arthritis is the classic secondary cause; primary (idiopathic) protrusio is a diagnosis of exclusion in middle-aged women and is the basis of the Otto pelvis.
The THA answer
The correct reconstruction is to lateralise the cup to the true hip centre and bone-graft the medial defect with morsellised autograft, fixing on the intact rim. Routine screw augmentation is the wrong answer.
Young versus old
For the young hip with minimal arthritis the answer is a valgus intertrochanteric osteotomy (or triradiate closure in a child), not arthroplasty.
Marfan trigger
A tall young patient with bilateral protrusio is a classic prompt to choose Marfan syndrome as the underlying cause.
Guidelines, Registries & Global Practice
- No single international guideline governs protrusio acetabuli specifically; practice rests on the age-based treatment ladder described in classic reviews - triradiate cartilage closure in the child, valgus intertrochanteric osteotomy in the young adult with a preserved joint, and total hip arthroplasty once the hip is arthritic.
- The consensus on reconstruction across published series is consistent worldwide: use an uncemented cup with bone grafting of the medial defect, aim to restore the hip centre, rely on the intact peripheral rim for fixation, and avoid routine screw augmentation - the principles confirmed by the 2024 systematic review.
- Registry data (NJR, AJRR, AOANJRR and the Scandinavian registries) do not track protrusio as a separate diagnosis, but their broad message applies: well-fixed uncemented cups on supportive bone do well, and reconstructions that fail to achieve solid initial fixation are at higher risk of aseptic loosening and revision.
- Global practice variation mainly reflects the underlying disease mix and resource setting - inflammatory arthritis dominates where rheumatology care identifies these hips, while access to CT planning, modern uncemented cups and reinforcement cages varies between settings rather than any disagreement on the surgical principles.
PROTRUSIO ACETABULI
Clinical summary
Definition
- •Medial migration of the femoral head into the pelvis
- •Head or medial wall lies medial to the ilioischial (Kohler) line
- •Lateral centre-edge angle high (greater than 40 degrees) - over-coverage
- •The mirror image of dysplasia
Causes
- •Most are secondary - always find the cause
- •Inflammatory: rheumatoid arthritis, ankylosing spondylitis
- •Bone-softening: osteomalacia, Paget, osteogenesis imperfecta
- •Connective tissue: Marfan and Ehlers-Danlos; also trauma, tumour, infection
- •Primary (idiopathic) - bilateral, middle-aged women, diagnosis of exclusion (Otto pelvis)
Management Ladder
- •Treat the underlying cause first
- •Child: closure or arrest of the triradiate cartilage
- •Young adult, minimal arthritis: valgus intertrochanteric osteotomy
- •Arthritic hip: total hip arthroplasty
THA Principles
- •Restore the hip centre - lateralise the cup to the true position
- •Graft the medial defect with morsellised femoral head autograft
- •Press-fit on the intact rim; cage or ring if the rim is deficient
- •Avoid relying on medial screws; template offset and leg length