Hip complaints are a common problem that can affect people of any age. While known causes such as osteoarthritis, hip dysplasia or femoroacetabular impingement (FAI) are often in the foreground, a specific anatomical variant often remains unrecognized: the retroversion of the acetabulum. This malposition can lead to significant complaints and increase the risk of degenerative changes in the hip joint. In this comprehensive article, we will examine the retroversion of the acetabulum in detail, its anatomical features, clinical consequences, diagnostic features on X-ray - in particular the cross-over sign - as well as possible treatment options discussed in detail.

Anatomy of the hip joint

The hip joint is a central ball-and-socket joint of the human body that connects the femur (thigh bone) to the pelvis. It enables a variety of movements and contributes significantly to stability and mobility.

Components of the hip joint:

• Femoral head: The spherical upper part of the femur that fits into the acetabulum.
• Acetabulum: The hip joint socket in the pelvis that accommodates the femoral head.
• Cartilage: A smooth layer covering the joint surfaces, enabling smooth movement.
• Labrum: A fibrocartilaginous ring at the edge of the acetabulum that increases joint stability.
• Joint capsule and ligaments: They surround the joint and provide additional stability.

The alignment of the acetabulum plays a crucial role in the function of the hip joint. Normally, the hip socket is slightly tilted forward, a position referred to as anteversion. This forward tilt allows for optimal freedom of movement and prevents premature collision between the femoral head and acetabulum.

What is retroversion of the hip joint socket?

Retroversion of the acetabulum is an anatomical variant in which the hip socket is tilted backwards rather than forwards (anteverted) as usual. This posterior tilt causes the anterior edge of the acetabulum to become more prominent and the posterior edge to recede.

Anatomical features of retroversion:

• Change in acetabular orientation: Instead of normal anteversion, the acetabulum shows retroversion, meaning it is tilted posteriorly.
• Prominence of the anterior acetabular rim: The anterior rim of the acetabulum protrudes further forward, resulting in reduced anterior coverage of the femoral head.
• Reduced posterior coverage: The posterior edge of the acetabulum provides less coverage for the femoral head, which can compromise the stability of the joint.

This anatomical malalignment can significantly affect the biomechanics of the hip joint and lead to various clinical problems.

Clinical consequences of acetabular retroversion

Acetabular retroversion can lead to a range of clinical symptoms and long-term consequences.

1. Femoroacetabular impingement (FAI):

One of the most common consequences of acetabular retroversion is femoroacetabular impingement, particularly the Pincer type. This results in increased contact between the anterior acetabular rim and the femoral head or neck, leading to impingement.

Symptoms of FAI:

• Groin pain: Especially with movements such as lifting the leg or rotating inward.
• Limited mobility: Difficulty with deep sitting, bending, or sporting activities.
• Snapping sounds: A audible or palpable "snapping" in the hip joint during certain movements.

In the long term, untreated FAI can lead to cartilage damage and labral lesions, increasing the risk of developing coxarthrosis.

2. Increased risk for osteoarthritis:

Due to the altered biomechanics and increased stress on certain joint areas, the risk of degenerative changes in the hip joint increases. Continuous cartilage wear can lead to the development of coxarthrosis, which is associated with chronic pain and significant movement restrictions.

3. Hip Dysplasia:

In some cases, the retroversion of the acetabulum can be associated with hip dysplasia, a malformation in which the acetabulum does not adequately cover the femoral head. This leads to joint instability and also increases the risk of arthritis.

Diagnosis of acetabular retroversion

An accurate diagnosis of acetabular retroversion is essential to initiate targeted therapeutic measures and prevent serious consequential damage such as coxarthrosis or femoroacetabular impingement (FAI) . Since the symptoms are often nonspecific and can be confused with other hip diseases, a thorough clinical examination and the use of various imaging techniques are crucial.

1. Clinical examination

An experienced orthopedist or hip specialist will first check the mobility of the hip joint . Certain clinical tests can provide indications of mechanical impingement due to the prominent anterior acetabular rim.

Important clinical tests:

• Flexion-internal rotation-adduction test (FADIR test):
• The patient lies on their back, the leg is brought into flexion, internal rotation, and adduction.
• Pain in the groin suggests femoroacetabular impingement (Pincer type), which is favored by retroversion.
• Loss of internal rotation with above-average good external rotation of the hip: due to the retroversion of the acetabulum, there is a collision of the femoral neck with the anterior acetabular rim during internal rotation, whereas there is usually no bony limitation during external rotation, resulting in very good mobility.
• Drehmann sign:
• When bending the hip, the leg deviates uncontrolled outward.
• This may indicate labral injury or arthrosis development .
• Pain provocation tests:
• Targeted pressure on the groin or buttocks can provide indications of overload due to the incorrect acetabular orientation.

2. X-ray examination – The cross-over sign as a key factor

The most important diagnostic tool for detecting acetabular retroversion is the conventional X-ray of the pelvis in anterior-posterior (AP) view. Here, the so-called Cross-Over Sign (COS) is of central importance.

The Cross-Over Sign (COS) – A clear indication of retroversion

• Normally, the anterior and posterior acetabular rim run parallel, without crossing.
• In retroversion, the anterior acetabular rim crosses the posterior rim and appears further inside than the latter.
• This indicates that the acetabulum is tilted backwards and there is inadequate coverage of the femoral head.

Further X-ray signs for retroversion:

• Posterior wall sign:
• Normally, the posterior wall of the acetabulum lies medial to the femoral head.
• In retroversion, this shifts further laterally , which confirms the lack of posterior coverage.
• Ischial spine sign:
• The ischial spine (spina ischiadica) is not visible on a normal X-ray.
• If it is clearly visible, this indicates an abnormal acetabular orientation.

3. Advanced imaging: CT and MRI

Since the X-ray image only allows a two-dimensional representation , a computed tomography (CT) with 3D reconstruction may be required for a precise analysis of the acetabular position.

CT scan to determine the retroversion angle

• A CT-based angle measurement enables accurate calculation of the acetabular orientation.
• Normal values:
• Anteversion: approx. 15°–20°
• Retroversion: < 0°

MRI to assess accompanying damage

• Labrum lesions: Due to the incorrect positioning of the acetabulum, the labrum can become pinched or damaged.
• Cartilage damage: Excessive stress on certain joint areas can lead to premature osteoarthritis (coxarthrosis) .
• Joint effusion: Fluid accumulation in the joint may indicate an inflammatory reaction.

Treatment options for acetabular retroversion

Depending on the severity of the symptoms and existing accompanying damage, there are conservative and surgical treatment approaches.

1. Conservative therapy – When can it help?

In early stages, a physiotherapeutic treatment and targeted adjustment of the load can help reduce complaints.

Important measures:

• Strengthening of the hip musculature:
• Stabilization of the hip joint through targeted training of the abductors, flexors, and gluteal muscles.
• Reducing impinging movements:
• Avoiding deep flexion and internal rotation to avoid further stressing the labrum.
• Weight reduction:
• Reducing pressure on the hip joint can alleviate discomfort.
• Pain management:
• Anti-inflammatory medications (NSAIDs) can reduce pain.
• Hyaluronic acid or PRP injections to improve joint lubrication.

2. Surgical therapy – When is surgery necessary?

If conservative measures are insufficient and damage to the labrum or cartilage already exists, surgical correction of the acetabular position may be necessary.

Arthroscopic therapy (minimally invasive method):

• In cases of mild retroversion , an arthroscopy with labrum refixation and bone smoothing can be performed.

Periacetabular osteotomy (PAO):

• If the malposition is severe , a PAO according to Ganz can help by surgically repositioning the acetabulum.

Total hip replacement (THR):

• If the retroversion has already caused severe osteoarthritis of the hip , replacement with an artificial hip joint is the only option.

Conclusion: Why early diagnosis is crucial in the treatment of this hip condition

The retroversion of the acetabulum is a frequently overlooked cause of chronic hip pain and can lead to early osteoarthritis (coxarthrosis) in the long term. Especially the femoroacetabular impingement (FAI) is a common accompanying feature of this anatomical malalignment.

Important points summarized:

✔ Early diagnosis is essential! – The cross-over sign on X-ray is a crucial indicator.
✔ Take symptoms seriously! – Persistent groin pain should not be ignored.
✔ Conservative measures can help in early stages.
✔ Surgical interventions like periacetabular osteotomy (PAO) can support joint preservation.
✔ In advanced osteoarthritis a hip replacement (hip TEP) may be necessary.

If you suffer from unexplained hip pain , a visit to a specialized hip orthopedist can be crucial to avoid long-term damage. Let yourself be advised by an endoprosthetics center with hip specialists !