Acetabular Reconstruction in Complex Hip Replacement

Acetabular reconstruction in hip replacement rebuilds a hip socket (the acetabulum) that fracture, dysplasia or a failed implant has eroded. Dr. Rakesh Mahajan plans it over four to five days: a 3D CT reconstruction maps every bone defect, the implant is chosen from jumbo cups, metal augments, cages and 3D-printed custom implants, and a posterior approach exposes the acetabulum while protecting the sciatic nerve.

That deliberate pace is the core of Dr. Rakesh Mahajan's method for complex hip replacement. As Vice Chairman and Head of Orthopaedics at BLK-Max Super Speciality Hospital, New Delhi, he treats the complex hip as a problem in geometry: the implant can only be as good as the surgeon's understanding of what bone is missing and where. This article follows his sequence from examination to the final X-ray check. The complete guide to complex total hip arthroplasty explains what makes a hip complex in the first place, and the article on complex and revision hip replacement covers the individual conditions and his cases.

Step 1: find the cause of failure

Planning starts with a thorough investigation of why the hip has failed. The physical examination then concentrates on two questions that change the implant choice.

Abductor function

The abductors, chiefly the gluteus medius and gluteus minimus, hold the pelvis level when the patient stands on one leg and keep the hip in its socket. They must be present and working. Where they are weak or absent, Dr. Mahajan plans for a dual mobility (tripolar) hip, a design in which motion takes place at two bearing surfaces instead of one, which greatly improves stability at the cost of a somewhat smaller range of movement. Why that choice is worth making is explained in the revision article.

How much leg length hip replacement can correct

In Dr. Mahajan's practice, hip replacement implants can restore up to 2 cm of leg length, and no more. Leg length is the second question in the examination, and he always obtains long-leg films alongside a clinical measurement of limb length, because an unequal leg can be the very thing troubling the patient and loading the spine.

His rule on correction is firm. A surgeon who promises the patient that a shortening will be corrected must be careful not to go beyond 2 cm. Any remaining discrepancy is managed with a shoe raise.

Step 2: image the bone defect with X-ray and 3D CT

X-rays: AP pelvis and Judet views

The standard films are an anteroposterior (AP) view of the pelvis and Judet views, the two oblique projections (obturator oblique and iliac oblique) that show the anterior and posterior walls and columns of the acetabulum. Dr. Mahajan uses them to judge whether the acetabular roof and walls are intact.

3D CT reconstruction

If there is any doubt, he moves to a CT scan with 3D reconstruction. This shows how much bone is missing on the femoral side and in the acetabulum, and therefore what has to be rebuilt:

• a deficient posterior wall or posterior column must be reconstructed;
• a missing anterior wall or anterior column is prepared for in the same way;
• a deficient dome, the roof of the socket, needs its own solution.

Posterior wall deficiency is the most common and most troublesome pattern in his experience.

Classification as a guide, not a rulebook

Formal classification systems grade acetabular bone loss from minimal to severe; Dr. Mahajan mentioned the AO classification and regards such systems as a simple guide that is hardly used in practice. The logic he follows is simpler: minimal bone loss can be managed with a standard hemispherical cup and screws; a wall deficit, usually posterior, needs metal augments; and progressively larger defects need progressively more substantial reconstruction.

Step 3: acetabular reconstruction implants

Every implant the reconstruction might need must be in the operating room before the hip is opened. The practical lesson of complex hip surgery, in Dr. Mahajan's words, is that the surgeon must never open the hip, discover that a particular component is needed and find it unavailable. He tells patients that the team will take its time. Planning takes four or five days and several discussions. The team works on a physical model of the patient's pelvis, marking the defect and the reconstruction on it with reference to the 3D films. AI-based planning now also helps the team visualise how the implant will sit before surgery.

The reconstruction options

The range of options he described includes:

• Jumbo cups. Oversized acetabular cups, up to about 65 mm, used when the posterior column, posterior wall and anterior column remain stable. The cup gains its stability from the anterior and posterior walls.
• High hip centre cups. A cup placed above the anatomical centre of rotation, where bone stock is better.
• Impaction or structural bone grafting. Packing morselised graft into a contained defect, or using a solid block of graft to rebuild a missing segment.
• Metal augments. Porous metal blocks that fill wall defects and sit with the cup.
• Titanium shells and cages. For the most severe defects, where the anterior and posterior walls of the acetabulum have both gone, he uses the Burch-Schneider cage, an anti-protrusio device with flanges that are screwed to the surrounding pelvic bone so that the cup can sit inside it.
• 3D-printed custom implants. Made to the patient's own anatomy from the 3D CT, these are needed especially when childhood disease has left the cup or stem undeveloped. Sometimes the femoral canal is narrower than 6 mm and no standard stem will fit. Several manufacturers now make these implants to order. In one 3D-printed model he showed, the anterior wall of the acetabulum was entirely missing.

Landmarks for placing the cup

Two anatomical landmarks guide the position of the acetabular component:

• The transverse acetabular ligament, which spans the bottom of the socket at the six o'clock position. Dr. Mahajan uses it as the reference that keeps the inclination of the cup correct.
• The teardrop, a radiographic landmark at the base of the acetabulum. Dr. Mahajan checks it on the X-ray to confirm proper placement of the shell, and correct placement helps maintain the horizontal offset of the hip.

Step 4: protect the nerves and vessels

Screws and retractors around the acetabulum pass close to structures that cannot be damaged without lasting harm. Dr. Mahajan describes the area as a dead end that the surgeon is entering, and lists the minimum anatomy every surgeon must respect:

1. The sciatic nerve at the greater sciatic notch and the inferior gluteal region. The nerve must be identified, and no Hohmann retractor or other instrument should be placed in the notch, or the nerve is bound to be damaged.
2. The superior gluteal vessels. They supply the abductor muscles; damage them and the abductors lose their blood supply, undermining the stability of the new hip.
3. The obturator canal. Nothing should be placed there, because the obturator nerve, artery and vein run through it.

Step 5: prepare the femur without perforating it

On the femoral side, the hardest step is reaming the medullary canal. In complex cases the bone is usually osteoporotic after multiple operations, and the anatomy is distorted, so the reamer can easily puncture the cortex and the tip of the stem ends up outside the bone.

Dr. Mahajan borrows the technique of femoral nailing. A guide wire is passed down the canal, and the canal is reamed sequentially over it under C-arm (live X-ray) guidance. It takes considerable time and patience. The surgeon must also work slowly and meticulously, watching the blood loss and controlling bleeding vessels as they appear.

Step 6: restore the offsets

The hip has two offsets, and the reconstruction must balance them:

• Vertical offset governs the length of the limb.
• Horizontal offset, the distance from the centre of the femoral head to the line of the femoral shaft, governs stability. Restoring it tensions the abductors and keeps the hip from dislocating.

The right ratio between the two is what produces a hip that is both the correct length and stable.

Cemented or uncemented hip: which lasts longer

Dr. Mahajan prefers uncemented cups, which registry data favour in younger patients; cages and shells come in both cemented and uncemented versions. In older patients, around 70 and above, he uses cemented cups, which in his experience give another 15 years or so. Cemented hips also cost less than uncemented ones; in one young patient with a failed femoral neck fracture fixation, he chose a cemented hip because the patient's budget was limited.

Best surgical approach for complex hip replacement

For hips that need acetabular reconstruction, Dr. Mahajan prefers a posterior approach; for straightforward primary hips, anterior and lateral approaches work well.

Posterior approach for acetabular reconstruction

Where the acetabulum has to be rebuilt, he prefers the posterior approach, either the Moore (Southern) approach or the Kocher-Langenbeck approach. In these posterior approaches the sciatic nerve is identified first and the acetabulum is widely visible, which makes reconstruction easier. The piriformis tendon is his guide: he locates it first, follows it to its attachment, and then deals with the rest of the short external rotators.

He is equally firm on exposure. Complex hips are not the place for small, minimally invasive incisions; he advises extensive incisions that show the whole field, and locating the sciatic nerve before anything else.

Anterior and lateral approaches for primary hips

For a straightforward primary hip replacement, other approaches work well: the anterior (Smith-Petersen) approach, the anterolateral approach and the Hardinge (direct lateral) approach. They are good for primary hips, but where the acetabulum needs reconstruction, Dr. Mahajan's choice remains the Kocher-Langenbeck or Southern approach with the sciatic nerve defined.

What a successful acetabular reconstruction looks like

On the post-operative X-ray, Dr. Mahajan checks the position of the shell against the teardrop, whether the construct is stable, and whether the offsets have been restored. In one case he presented, an acetabulum with no anterior or posterior wall, a so-called wandering acetabulum, was rebuilt with a Burch-Schneider cage that gave stability to the acetabular shell. In another, where an acetabular fracture had opened up but the anterior wall was intact, he first fixed the posterior wall with a plate and screws and placed the hip replacement only once the socket was stable. The full set of cases is in the article on complex and revision hip replacement. Indicative prices for complex and revision procedures are on the cost page for other hip and knee replacement procedures.