What is Hip Dysplasia?

Dysplasia is any abnormal growth of cells. It can occur on a microscopic level, or macroscopic, the latter being when the hip is not formed properly (congenitally). If it happens in the hip then it means that the socket part of the hip doesn’t cover the ball part – and there is a risk of dislocation. The left hip is more likely to be affected than the right (my left hip has most of my symptoms and looks to be the worst on xray). 7.5% of hip replacements are because of dysplasia and around one in 1000 babies are affected. The condition can be bilateral, or it can be left only and the right has the resulting effects. There are different degrees of being affected, I think it’s on a scale of one to four. The joint can be fully dislocated, a false acetabulum can form higher up on pelvis opposite the dislocated femoral head position.

Acetabular Dysplasia Explained

Acetabular dysplasia is where the acetabulum (socket) is too shallow or domed, the angle of the head can be too narrow or too wide. In other words, a poorly developed acetabulum.

There is a possible hormonal link to this condition, and it can be formed from swaddling (wrapping babies up) (I wasn’t swaddled, but there is a family story that my mother used to spend hours pushing my legs in and out trying to make my legs strong (no reason, she was just an idiot). It leads to decreased hip abduction (meaning a decreased ability to move your leg out from the side of the body, which I have). Part of diagnosing this can be the Trendelenburg sign, i.e. looking for a pelvis drop while standing on one leg. In other words, you stand on one leg and if the pelvis tilts down on the opposite side, then the abductors are weak on the leg you are standing on. An X-ray is used to diagnose it and an MRI to confirm. A different set of measurements is used to diagnose it in adults. Negative outcomes can be arthritis and AVN. The acetabulum has its own blood supply, and when there are problems, then it affects blood supply to the femoral head and leg generally, leading to AVN. A bone graft given during hip replacement can improve the blood flow slightly if new bone grows into the area but it doesn’t fully restore what was lost, and the femoral head doesn’t need a blood supply if it’s gone!

The acetabulum can be shallow, small or misshapen. Not deep means it doesn’t cover the femoral head. The angle can be wrong and the rim can be underdeveloped or too small.

Why the Socket Needs Rebuilding

In dysplasia, the socket often lacks the depth, shape, or bone stock to hold a standard cup securely. Over decades the femoral head migrates upward, eroding the true acetabulum and sometimes forming a false socket higher on the pelvis. By the time arthritis forces a replacement, the native bone may be paper-thin superiorly, sloped incorrectly, or simply missing in key areas. A standard cup pressed into that defective bed would sit proud, wobble, or dislocate easily. Rebuilding creates a new, stable foundation so the implant has something solid to grip.

How Surgeons Rebuild the Socket

Shallow or misshapen socket – rebuilt with bone grafts or metal augments. The new cup is positioned higher (called high hip centre) if the normal spot isn’t stable.

Too small or deformed – a custom or modular implant to match the shape and correct the angle.

There is more risk of leg length difference after replacement with dysplasia present. The abductors can be weak from years of misuse, and the implant may need to be adjusted (position) to improve muscle function.

All of the above is known as a technically demanding hip replacement. Generally, SuperPath approach is not suitable for a technically demanding hip replacement because bone grafts are required, cups are placed deeper or higher, implants may need to be ‘special’, and it all needs more visibility. The socket may need reshaping to correct the angles and it’s hard to do safely through SuperPath. SuperPath makes it harder to correct leg length through better visibility. Tight or distorted muscles make minimally invasive access more risky. This is all true unless the dysplasia is very mild and the surgeon very experienced.

Reconstructive Grafting Step-by-Step

This is used if the socket is too shallow or missing bone. Autograft means that bone is taken from the femoral head that is being removed – free, perfectly matched, and alive with growth factors. The head is morcellised into chips or fashioned into structural wedges, packed into defects, and shaped to restore depth and version. Metal augments or cages can be used for extra strength, and screwed or pressed into place. The new cup is anchored partly in graft and partly in natural bone. Over 6–12 months the graft incorporates, turning into living host bone that supports load forever.

The Role of Screws in Socket Reconstruction

Screws are the unsung heroes of a dysplastic rebuild. Native bone may be eggshell-thin or sclerotic; a press-fit cup alone would crack it or migrate. Surgeons drill pilot holes and drive 2–6 titanium screws through the cup’s dome holes into the ilium, ischium, or pubis – whichever solid landmarks remain. These act like tent pegs, locking the cup instantly while the graft incorporates. Screws also neutralise shear forces from the abductors that try to lever the cup out. In severe cases a reinforcement cage with multiple flanges is screwed to the pelvis first, then the cup is cemented inside the cage for double insurance. Screw placement is planned on preoperative CT; intraoperative navigation or fluoroscopy confirms they stay inside bone and miss neurovascular bundles.

Dual-Mobility Cups in Dysplasia

This is a special implant that reduces the risk of dislocation. It can be good in dysplasia as the muscles are weak and the cup is placed at a different angle due to bone loss. The large polyethylene liner articulates with both a small femoral head and the metal shell, giving an extra range of stable motion before the neck impinges. Even if the cup sits 10–15° outside the safe zone, dual mobility forgives malposition better than fixed-bearing designs.

When a Normal Implant Still Works

This can still be used in some cases. It depends on if the hip was high or unstable before surgery, the soft tissue quality. The decision is made intraoperatively, once the surgeon sees how stable the joint is. A trial reduction with a standard cup may prove rock-solid if the dysplasia was borderline and the capsule robust.

Understanding “High Hip Centre”

High hip means that the femoral head sits higher than normal in relation to the pelvis i.e. the acetabulum is shallow or underdeveloped – so the head rises up over time, in other words, the ball is partially out of the socket. Placing the new cup at this high centre avoids grafting superior defects but lengthens the leg and slackens abductors further. Surgeons compensate by distalising the femur or using lateralised liners, yet a permanent limp can result if the offset is wrong.

Abductors and Soft-Tissue Quality

Abductors are on the outer thigh and buttock, and are stretched and weakened. Years of Trendelenburg gait leave the gluteus medius and minimus atrophied and fatty. Weak abductors increase dislocation risk and cause a lurching walk post-op until rehab rebuilds them.

Soft tissue quality means the strength of the muscles, tendons and ligaments and joint capsule, especially the abductors, because a weak hip can be unstable after surgery.

Capsule support means the joint capsule is thin or stretched, meaning there is less support. (The joint capsule is like a bag of tissue that supports the bone in place around the hip, it holds the ball and socket and also lubricates and carries blood). MRI can check its condition, to see if it’s thin or stretched, or for any labral tears or fluid in the joint. If the joint moves very far without pain during physical exam, it can show a loose capsule (an excessive range of motion).

Pre-Op Tests That Guide Reconstruction

Beyond standard X-rays, a 3D CT quantifies bone defects, maps screw trajectories, and allows virtual templating of augments. Judet views (oblique X-rays) reveal anterior and posterior column stock. EOS imaging measures global spinopelvic alignment – hyperlordosis or flat back change the functional safe zone for the cup. MRI assesses abductor muscle belly fat fraction; greater than 50% replacement predicts poor recovery. Abduction stress views under anaesthesia quantify laxity.

What to Ask Your Surgeon

• Will you need graft, augments, or both?
• How many screws and where?
• Can we avoid high hip centre?
• Is dual mobility indicated?
• Will you repair or reconstruct the capsule?
• What leg-length plan do you have?
• How will you tension the abductors?

Recovery Differences

Expect restricted weight-bearing for 6–12 weeks while graft incorporates. Abductor strengthening starts day one but progresses slowly to avoid pull-off. Walking aids are mandatory longer than routine THR. Dislocation precautions remain strict for 3–6 months. Serial X-rays monitor graft union and cup migration. Most patients notice the leg-length change immediately; a shoe lift may be needed permanently if offset cannot be restored.