Dysplasia is any abnormal growth of cells. It can occur on a microscopic level, or macroscopically, 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 does not 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 X-ray). 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-sided only and the right has the resulting effects. There are different degrees of severity; it is graded on a scale from one to four. The joint can be fully dislocated, and a false acetabulum can form higher up on the pelvis opposite the dislocated femoral head position.

Acetabular dysplasia is where the acetabulum (socket) is too shallow or too domed; the angle of coverage 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 caused by swaddling (wrapping babies tightly). (I was not swaddled, but there is a family story that my mother spent 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 can 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 it can affect 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 does not fully restore what was lost – and the femoral head does not need a blood supply once it is replaced!

The acetabulum can be shallow, small or misshapen – not deep enough means it does not cover the femoral head adequately. The angle can be wrong and the rim can be underdeveloped or too small.

Replacement with appliances

• Shallow/misshapen is rebuilt with bone grafts or metal augments. The new cup is positioned higher (called high hip centre) if the normal spot is not 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 when dysplasia is 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, the SuperPath approach is not suitable for a technically demanding hip replacement because cups often require bone graft, 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 is hard to do safely through SuperPath. SuperPath makes it harder to correct leg length because of reduced 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

This is used if the socket is too shallow or missing bone. Autograft means bone taken from the femoral head that is being removed. Metal augments or cages can be used for extra strength. They fill the deficient area and are screwed or pressed into place. The new cup is anchored partly in graft and partly in natural bone.

Dual Mobility

This is a special implant that reduces the risk of dislocation. It can be useful in dysplasia as the muscles are weak and the cup is placed at a different angle due to bone loss.

Normal Implant

This can still be used in some cases. It depends on whether the hip was high or unstable before surgery and the soft tissue quality. The decision is made intraoperatively, once the surgeon sees how stable the joint is.

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.

Abductors are on the outer thigh and buttock, and are stretched and weakened.

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 is thin or stretched, or for any labral tears or fluid in the joint. If the joint moves very far without pain during physical examination, it can show a loose capsule (excessive range of motion).

Q&A on Hip Dysplasia and Related Topics

Q: What does decreased hip abduction mean?
A: Decreased hip abduction means you have a reduced ability to move your leg outward to the side, away from the midline of your body. The hip abduction movement is mainly controlled by the gluteus medius and gluteus minimus muscles, which stabilise the pelvis and control side-to-side leg motion. When abduction is limited, it can be caused by muscle weakness (especially in the gluteus medius or minimus), joint stiffness or arthritis in the hip, pain or contracture in the inner thigh muscles (adductors), or structural problems such as hip dysplasia or changes after injury or surgery. Functionally, decreased hip abduction can make it hard to walk normally. The pelvis may drop on the opposite side when standing on the affected leg — this is called a Trendelenburg gait.

Q: Is the acetabulum linked to blood supply somehow?
A: Yes. The acetabulum — the socket part of the hip joint — is linked to the hip’s blood supply, though it is not as dependent on it as the femoral head is. The acetabulum itself gets its blood mainly from branches of the obturator artery, the superior gluteal artery, and the inferior gluteal artery. These vessels supply the bone tissue and the labrum. The acetabular branch of the obturator artery also helps feed a small area called the acetabular notch and sometimes gives rise to the artery of the ligamentum teres, which supplies a small part of the femoral head in some people. So while the acetabulum has its own blood flow, its condition can still affect the joint’s circulation overall. If the socket is damaged, it can reduce joint space and indirectly restrict blood flow to the femoral head, increasing the risk of avascular necrosis.

Q: Would rebuilding the acetabulum with surgery restore blood supply?
A: Not directly. Rebuilding the acetabulum mainly restores the shape and stability of the joint, not its natural blood supply. Bone grafts or metal components do not reconnect the original blood vessels. Over time, new blood vessels can grow into living bone grafts, but this only restores circulation locally within the grafted area. The femoral head, if already avascular or necrotic, will not regain blood flow — that is why surgeons replace it rather than try to restore its blood supply.

Q: So the femoral head does not need a blood supply once it is replaced, right?
A: Correct — once the femoral head is replaced with an artificial implant, it no longer needs a blood supply. The natural femoral head is living bone and depends on arteries to stay alive, but the metal or ceramic implant is inert and functions mechanically. The surrounding bone still needs good blood supply for healing and fixation, but the artificial head itself does not.

Q: Does the blood supply down the rest of the leg improve after hip replacement?
A: No — a hip replacement does not directly improve blood supply to the rest of the leg. The main arteries to the leg run outside the joint capsule and are not repaired during the operation. Any perceived improvement in warmth or colour is usually from better movement and reduced inflammation, not from restored arterial flow.

Q: How does a “pelvis drop” (Trendelenburg sign) look when standing on one leg?
A: When someone has a positive Trendelenburg sign, the pelvis drops on the opposite side of the weak hip when standing on one leg. In a normal hip, the gluteus medius and minimus on the standing leg keep the pelvis level. If they are weak, the pelvis tilts downward on the side of the raised leg. For example, if you stand on your right leg and your left hip drops, the right hip abductors are weak. In walking this produces a Trendelenburg gait — a side-to-side sway.

Q: What is a joint capsule?
A: A joint capsule is a tough, flexible sleeve of tissue that surrounds a joint and holds it together. In the hip it is like a strong bag enclosing the femoral head and acetabulum. It protects and stabilises the joint, produces synovial fluid for lubrication, and carries blood vessels and nerves. It is reinforced by strong ligaments (iliofemoral, pubofemoral, ischiofemoral).

Q: Can the joint capsule be checked to see if it is thin or stretched?
A: Yes. The best non-invasive way is an MRI scan (or MRI arthrogram with contrast), which can show thickness, laxity, tears, or excess fluid. Physical examination can detect excessive range of motion or instability tests. During arthroscopy the surgeon can see and feel the capsule directly.

Q: What are instability tests, and what abnormal fluid can be in the capsule?
A: Instability tests are specific movements to see if the hip is abnormally loose, e.g. log roll test, dial test, or apprehension tests. Abnormal fluid (effusion) is excess synovial fluid inside the capsule, usually from inflammation, infection, bleeding, or cartilage/labral damage — it appears bright on MRI.