Protrusio Acetabuli

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.

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.

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.

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.

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.

Quantitative criteria (Armbuster): the commonly quoted measurement thresholds define protrusio when the acetabular line lies medial to the ilioischial (Kohler) line by at least 3 mm in men, 6 mm in women, and 3 mm in children. Combined with a centre-edge angle greater than 40 degrees, these figures turn the qualitative "crosses the line" sign into a measurable diagnosis.

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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.

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.

• 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.

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.

• 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.