Periacetabular Osteotomy

Periacetabular Osteotomy is a hip preserving procedure performed to correct a congenital deficiency of the acetabulum:

Acetabular Dysplasia

Acetabular dysplasia is a condition defined by inadequate development of an individual's acetabulum. The resulting acetabulum is shallow and "dish shaped" rather than "cup shaped". The upper portion (roof of the acetabulum is obliquely inclined outward rather than having the normal horizontal orientation.

Because of these abnormalities, the superior and usually anterior femoral head are incompletely covered by this dysplastic acetabulum. Individuals with acetabular dysplasia usually develop through childhood and adolescence without symptoms or knowledge of their abnormality.

Between the age of 20-30 however the patient may typically experience pain from their hip and they often seek medical evaluation and an X-ray discloses the abnormality (acetabular dysplasia). Other patients may have been treated for hip problems as an infant or child.

Acetabular dysplasia may also be associated with abnormalities in the shape of the upper femur which may contribute to the patient's hip symptoms.

Acetabular dysplasia is associated with an abnormally high stress on the outer edge (rim) of the acetabulum which leads to degeneration of the articular cartilage (arthritis). It is also possible for breakdown of the acetabular labrum (rim cartilage of the acetabulum) or a fatigue fracture of the rim of the acetabulum to occur as a result of this rim overload. Any one or a combination of these conditions can cause hip pain sufficient for the patient to seek medical evaluation and treatment.

When the diagnosis of acetabular dysplasia is made, the X-ray also usually may show signs of arthritis which is most commonly an acetabular cyst though increased bone density, a femoral head cyst, osteophytes (bone spurs), and/or cartilage thinning may also be present. If the dysplasia is left uncorrected worsening of the arthritis is predictable and may progress to a severe status within a few years and sometimes even a few months. For the patient, this means increasing hip pain, progressive loss of hip motion, and worsening functional capabilities.

Periacetabular Osteotomy (PAO) is a surgical treatment for acetabular dysplasia that preserves and enhances the patient's own hip joint rather than replacing it with an artificial part. The goal is to alleviate the patient's pain, restore function, and maximize the functional life of their dysplastic hip.

PAO is a procedure that was developed and first performed in 1984 in Bern, Switzerland by Professor Reinhold Ganz. Professor Michael Millis from the Boston Children’s Hospital, was taught by Prof Ganz the surgical technique.

Dr Solomon was privileged to train under both Prof Millis and Prof Ganz and brought the the PAO operation to Sydney in 1997 after completing his travelling Fellowship. Dr Solomon has been performing PAO’s since 1997 and has done more than 400 procedures.

The Anatomy

Two parts comprise the hip joint: a ball on the upper end of the thigh bone (femur), called the head of the femur, and a socket in the pelvis known as the acetabulum. The hip joint, like other joints, is made up of specialized structural elements that serve as precisely fitting moving parts. The head of the femur rotates freely within the smooth, concentric surface of the acetabulum. An extremely low friction tissue, hyaline cartilage, lines this joint as well as other joints in the human body. The friction between two hyaline cartilage surfaces is much less than the best man-made bearing.

A normal acetabulum "covers" the upper (superior) portion of the head of the femur as well as a partial portion of the front (anterior) and back (posterior) of the femoral head "Periacetabular" means around the acetabulum. "Osteotomy" means to cut bone. Simply put, the PAO cuts the bone around the acetabulum that joins the acetabulum to the pelvis. Once the acetabulum is detached from the rest of the pelvis by a series of carefully controlled cuts, it is rotated to a position of ideal coverage as dictated by the specific acetabulum's unique anatomy. PAO thereby re-orientates the acetabulum by changing its rotational position. The dysplastic roof that incompletely covers the femoral head is brought over the head to give the head a normal coverage and also brings the roof from an oblique to a horizontal position.

Other subtle changes typically also occur. Anterior coverage may increase. Also the shortening of the extremity and lateralization of the joint which are often a part of acetabular dysplasia can also be improved. Individual cases of dysplasia however present with their own unique deficiencies and the PAO must often be tailored to solve these unique problems. X-rays take during surgery confirm the correct position of the acetabulum and screws (typically 3) are inserted into the bone to maintain the acetabulum's new corrected position during bone healing.

A proximal femoral osteotomy (cutting and repositioning the bone of the upper femur) may be needed in some patients who undergo PAO surgery in order to correct abnormalities related to the femur. The indication for this is not always known until during the PAO operation. The femoral osteotomy is then completed during the same surgery though a second incision is necessary.

The Acetabular Labrum may have a tear due to the edge loading in the dysplastic hip. By correcting the acetabular coverage over the femoral head (The PAO surgical goal), the torn labrum is no longer in the weight bearing zone and may heal on its own. Extremely large tears are those patients with large labral cysts may need this addressed at the time of the PAO. Smaller tears are left alone and if they become a problem in the future a hip arthroscopy (key hole) can be done to address the tear.

As with any other major hip surgery, there is some risk of complications. Surgical wound infection and injury to major nerves or arteries is possible. Non-union (lack of healing) of the bone following the osteotomy is also possible.

Studies have shown PAO to be a safe operation as long as it is done by a surgeon skilled in the technique. The chance for any one of these major complications is less than one percent.

The Natural History of an untreated shallow hip is to develop osteoarthritis in the patients 30’s or 40’s

The periacetabular osteotomy is designed to delay the onset and progression of hip arthritis. There is still a possibility that despite the surgery hip arthritis will continue to progress and very rarely may develop sooner if a complication arises.

The Operation and Aftercare

A cell saver is used in the operation to reduce the degree of blood loss.

A Spinal Anaesthetic as well as a General Anaesthetic is used during the surgery. A urinary catheter is placed in your bladder for 3 to 4 days. An incision is made just above the groin and is about 12-15cm in length. The surgery takes approximately 90-120 minutes. Patients spend about one hour following surgery in the recovery room where nurses closely monitor them. It is common to feel numb on the side of the leg for 6-12 months due to a small skin nerve always getting cut or stretched as it crosses the wound. It is common for the vaginal or scrotal skin to swell for about 1 week post operatively. The swelling will always resolve.

Post-surgical priorities are pain management, preventing infection, and the prevention of deep vein thrombosis (blood clots in large veins), and pulmonary emboli (blood clots traveling through veins to the lungs). Mobilisation usually occurs within 24hrs following surgery.

Patients begin physotherapy as soon as possible to improve hip motion and muscle function and to learn to use appropriate assistive devices such as crutches or a walker. During the first eight weeks following surgery, the operated hip should bear no more than a limited weight of 10Kg’s. Placing full weight on the operated side prior healing may lead to screw breakage and for the osteotomy to lose its position. Too vigorous exercise such as resistive exercise against weights can also cause failure. If failure occurs, re-operation may be necessary and the chance of developing arthritis is greatly increased.

The usual hospital stay is 5 days and depends on how rapidly pain subsides and progress in physical therapy. At discharge pain medication is prescribed as well as aspirin to prevent blood clots. Some degree of pain after discharge is natural which may increase or decrease on different days but the general trend should be toward decreasing pain. Some patients may sense an occasional "click" or "pop" in or around the hip. Numbness and a tingling sensation is common around the incision area. It is usual to feel numb on the outer aspect of the thigh due to bruising of a small skin nerve. This usually resolves but if not it does not cause any functional problems in the future.

After discharge if you experience severe or consistent pain or have redness, swelling and/or wound drainage please contact the hospital.

Follow-up outpatient visits are necessary to monitor progress by X-ray and physical examination. The first follow-up visit is usually scheduled about 6 weeks after surgery and the second at 12 weeks.

At 8 weeks after the surgery the patient is allowed to be full weight bearing and work toward discontinuing use of the crutches. Muscle strengthening exercises often with the help of a physical therapist are also started. Progress in walking depends on return of muscle strength. The majority of patients are walking without support by 3 months after the surgery Subsequent follow-up visits are at 6 months, 1 year and 2 years after surgery and then at 5 year intervals. Screws are usually removed after 6 to 8 months as a small day only procedure with minimal discomfort.

Osteotomy should not be thought of as an inferior second choice to total hip replacement that the young patient with early arthritis must undergo because he or she is too young for total hip replacement. The results after PAO, which preserves the patient's own hip, justify its use and the long term results can be better than what the patient could have obtained from a hip replacement. The patient's own hip is a living tissue with self-maintenance capabilities, whereas deterioration with time is inevitable for an artificial part.


Appropriately selected patients have an 85% chance of a successful operation and pain free hip 20 years after the surgery. The Ganz PAO procedure has only been around for just over 30 years and in the early days we did not know who was the best suited candidate for the surgery. The long term studies include patients who would be poor candidates for a PAO and better off with a Hip Replacement. As a result of the studies we now know who is best suited for the surgery which in turn leads to better outcomes.

Results of periacetabular ostetomies show the majority of patients with poor results did not have an immediate poor result but were benefited by the PAO for a variable period (up to 12 years) before requiring further surgery.

Pre-operative early arthritis, age older than 30, a stiff hip and severe pre-operative pain are predictors of a poor outcome and a PAO is usually contraindicated.

For patients who develop a poor result some time after their PAO surgery the cause is typically advancing hip arthritis. These patients are almost always best treated by total hip replacement surgery. For these patients the previous PAO has typically enhanced the acetabular bone with the increased femoral head coverage. Enhancement of a dysplastic acetabulum contributes to the success of a later total hip replacement by making the stability of the prosthetic acetabulum more reliable.


The Final Word

Dr Michael Solomon has over 20 years experience in Periacetabular Osteotomy surgery, a complex operation to effectively treat acetabular dysplasia. He was trained by Prof Millis and Prof Ganz and has used his skills to treat over 400 patients from Sydney and other parts of Australia to improve the outcomes of their dysplastic hips.

Dr Solomon takes a cautious approach when recommending PAO surgery and carefully weighs up the benefits and predicted outcomes in each individual before making a decision as to whether surgery would benefit the patient or not.

Patients need to expect around a 6 month recovery period and need to remember that the dysplastic hip can never be made into a normal hip however a well performed operation with the correct indications is likely to last 25 yrs or more in the majority of patients and relieve them of pain and discomfort.

Updated Jan 2019