Hip pain is a common problem that can affect people of all ages. While well-known causes such as osteoarthritis, hip dysplasia, or femoroacetabular impingement (FAI) are often the primary focus, one specific anatomical variant frequently goes undetected: acetabular retroversion. This malposition can lead to significant pain and increase the risk of degenerative changes in the hip joint. In this comprehensive article, we will examine acetabular retroversion in detail, discussing its anatomical features, clinical consequences, diagnostic characteristics on radiographs—especially the crossover sign—and potential treatment options.

Anatomy of the hip joint

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

Components of the hip joint:

• Femoral head: The spherical upper part of the thigh bone that fits into the hip socket.
• Acetabulum: The hip socket in the pelvis that holds the femoral head.
• Cartilage: A smooth layer that covers the joint surfaces and allows for 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 orientation of the acetabulum plays a crucial role in the function of the hip joint. Normally, the acetabulum is slightly tilted forward, a position known as anteversion. This forward tilt allows for optimal range of motion and prevents premature contact between the femoral head and acetabulum.

What is a retroversion of the acetabulum?

Acetabular retroversion is an anatomical variation in which the hip socket is not tilted forward (anteverted) as usual, but backward. This backward tilt causes the anterior rim of the acetabulum to become more prominent and the posterior rim to recede.

Anatomical features of retroversion:

• Change in pan orientation: Instead of the normal anteversion, the pan shows a retroversion, which means that it is tilted backwards.
• 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 rim of the acetabulum provides less coverage for the femoral head, which can affect the stability of the joint.

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

Clinical consequences of acetabular retroversion

Retroversion of the hip socket 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 involves increased contact between the anterior acetabular rim and the femoral head or neck, leading to impingement.

Symptoms of FAI:

• Pain in the groin: Especially during movements such as lifting the leg or turning inwards.
• Limited mobility: Difficulty sitting low, bending over, or participating in sports activities.
• Snapping noises: An audible or perceptible "snapping" in the hip joint during certain movements.

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

2. Increased risk of coxarthrosis:

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

3. Hip dysplasia:

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

Diagnosis of acetabular retroversion

Accurate diagnosis of acetabular retroversion is essential for initiating targeted therapeutic measures and preventing osteoarthritis of the hip or femoroacetabular impingement (FAI) clinical examination and the use of various imaging techniques crucial.

1. Clinical examination

An experienced orthopedic surgeon or hip specialist will first check the mobility of the hip joint . Certain clinical tests can provide indications of mechanical impingement caused by the prominent anterior rim of the acetabulum.

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 .
• In cases of groin pain, this suggests femoroacetabular impingement (pincer type) , which is favored by retroversion.
• Absence of internal rotation with above-average external rotation of the hip: due to the retroversion of the acetabulum of the hip joint, in internal rotation the femoral neck impinges on the anterior rim of the acetabulum, whereas in external rotation there is usually no bony limitation at all and therefore very good mobility.
• Drehmann sign:
• When bending the hip, the leg uncontrollably swings outwards.
• This could be an indication of a labral tear or the development of osteoarthritis .
• Pain provocation tests:
• Targeted pressure on the groin or buttocks can indicate overload due to incorrect acetabular cup position.

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

The most important diagnostic tool for detecting acetabular retroversion is the conventional anterior-posterior (AP) pelvic X-ray . The so-called cross-over sign (COS) of central importance in this view.

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

• Normally, the anterior and posterior edges of the acetabulum run parallel without crossing each other.
• In a retro version, the front edge of the pan crosses the rear edge and therefore appears further inwards than the latter.
• This shows that the acetabulum is tilted backwards and that there is an incorrect covering of the femoral head.

Other X-ray signs of retroversion:

• Posterior wall sign:
• posterior wall of the acetabulum lies medial to the femoral head.
• In a retroversion, this shifts further laterally , which proves the lack of rear roofing.
• Ischial spine sign:
• The ischial tuberosity (spina ischiadica) is not visible in a normal X-ray image.
• If it is clearly visible, this suggests an abnormal pan orientation .

3. Advanced imaging: CT and MRI

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

CT scan to determine the retroversion angle

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

MRI for assessing concomitant damage

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

Treatment options for acetabular retroversion

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

1. Conservative therapy – When can it help?

In early stages, physiotherapy treatment and targeted adjustment of the load help to reduce discomfort.

Important measures:

• Strengthening the hip muscles:
• Stabilization of the hip joint through targeted training of the abductors, flexors and gluteal muscles .
• Reduction of impinging movements:
• Avoid deep flexion and internal rotation to prevent further stress on the labrum.
• Weight loss:
• Reducing pressure on the hip joint can alleviate discomfort.
• Pain management:
• Anti-inflammatory drugs (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, arthroscopy with labral refixation and bone smoothing can be performed.

Periacetabular osteotomy (PAO):

• If the malposition is severe PAO according to Ganz help by surgically repositioning the hip socket.

Total hip replacement (THR):

• If the retroversion has already severe coxarthrosis , replacement with an artificial hip joint the only option.

Conclusion: Why early diagnosis is crucial for this hip dysplasia

Retroversion of the acetabulum is a frequently overlooked cause of chronic hip pain and can lead to premature osteoarthritis (coxarthrosis) . Femoroacetabular impingement (FAI) is a particularly common consequence of this anatomical malposition.

Key points summarized:

✔ Early diagnosis is essential! – The crossover sign on the X-ray is a crucial indicator.
✔ Take symptoms seriously! – Persistent groin pain should not be ignored.
✔ Conservative measures can help in the early stages.
✔ Surgical procedures such as periacetabular osteotomy (PAO) can help preserve the joint.
✔ In advanced osteoarthritis, a total hip replacement (THR) be necessary.

If you are suffering from unexplained hip pain , a visit to a specialized hip orthopedist be crucial to preventing long-term damage. Seek advice from an endoprosthetics center with hip specialists !