PAO (periacetabular osteotomy) is a hip preservation surgery that repositions the hip socket to better cover the ball of the femur. It’s the standard surgical treatment for hip dysplasia in young adults, designed to delay or prevent the need for a total hip replacement. The procedure involves making controlled cuts in the pelvic bone around the hip socket, freeing it so the surgeon can rotate it into a more protective position, then securing it with screws.
Why PAO Surgery Is Performed
In a normal hip, the socket (acetabulum) wraps around the top of the thighbone like a deep cup, distributing weight evenly across a broad area of cartilage. In hip dysplasia, the socket is too shallow. This means a smaller area of cartilage bears all the force of walking, running, and standing. Over time, that concentrated pressure wears the cartilage down and leads to early arthritis, often by a person’s 30s or 40s.
PAO corrects the underlying geometry. By rotating the socket to cover more of the femoral head, the surgery spreads load across a larger surface area, protecting the cartilage and buying the joint decades of function. Most patients are between their late teens and early 40s.
Who Qualifies for PAO
Surgeons use specific measurements on hip X-rays to determine whether someone needs the procedure. The most important is the center-edge angle, which measures how far the socket extends over the femoral head. A normal angle is about 25 to 40 degrees. Patients typically qualify for PAO when this angle falls below 26 degrees, meaning the socket isn’t covering enough of the ball. Two other criteria are commonly used: femoral head coverage below 75%, and a pelvic angle measurement (the ilioischial angle) above roughly 86 degrees.
Not everyone with dysplasia is a good candidate. The surgery works best when the joint cartilage is still in good condition. Patients with moderate to advanced arthritis (Tönnis grade 2 or higher on X-ray) are generally not offered PAO because the cartilage damage is already too far along for repositioning to help. Other contraindications include a BMI over 35, poor hip congruence on imaging, limited range of motion (less than 90 degrees of flexion or less than 20 degrees of abduction), and poor alignment between the ball and socket.
What Happens During the Procedure
PAO is a complex operation that involves making a series of precise cuts in the pelvic bone to free the socket fragment while keeping the blood supply intact. The surgeon works through an incision at the front of the hip and makes cuts in several locations: the pubic bone in front, the bone below the socket (the ischium), the area directly above the socket, and the column of bone behind it. A fifth connecting cut links the back two together. Each cut is carefully angled to avoid penetrating the joint surface itself.
Once the socket fragment is fully freed, the surgeon rotates it into the corrected position. The fragment is typically brought forward and outward to increase coverage of the femoral head, particularly in the front and to the side. Fine adjustments to rotation control how much the socket faces forward or to the side. The surgeon confirms the new position using real-time X-ray imaging (fluoroscopy), then locks the fragment in place with three fully threaded cortical screws inserted from the top of the pelvis down into the bone above the socket.
The bone eventually heals around these cuts just as a fracture would, incorporating the socket into its new position permanently.
Recovery and Return to Activity
Recovery from PAO is a months-long process. For the first six weeks, you’ll use crutches and limit yourself to 50% weight bearing on the surgical leg. This protects the healing bone cuts while they begin to fuse. During this phase, physical therapy focuses on gentle range-of-motion exercises and preventing muscle wasting.
After the initial six weeks, weight bearing gradually increases. Most rehabilitation protocols follow a phased approach that gets progressively more demanding. Low-level impact activities like jogging and straight-line agility drills are typically introduced around weeks 20 to 26 (roughly five to six months after surgery). Full return to sports involving jumping, cutting, and pivoting comes at six months or later, depending on individual progress and the demands of the activity.
About 37% of patients eventually request a second smaller surgery to remove the screws because of soft tissue irritation. The screws sit near the iliac crest (the top ridge of your pelvis), and in leaner patients especially, they can be felt under the skin or cause discomfort with certain movements. Screw removal is a straightforward procedure compared to the original surgery.
Nerve Injury and Other Risks
The most common complication is injury to the lateral femoral cutaneous nerve, a sensory nerve that runs near the surgical approach and provides feeling to the outer thigh. In one prospective study that carefully tested patients at multiple time points, 74% had some degree of nerve involvement at ten days after surgery. This sounds alarming, but the number drops steadily: 60% at three months, 57% at six months, and 48% at one year. Many of these cases involve mild numbness or tingling on the outer thigh rather than pain. The nerve injury did not affect hip function scores, though it was associated with a negative impact on mental health measures at the one-year mark.
Other potential complications include the bone cuts failing to heal (non-union), fracture through the cuts before healing is complete, blood vessel injury, infection, and under- or over-correction of the socket position. These are uncommon in experienced surgical centers, and PAO outcomes are strongly influenced by surgeon volume. This is not a procedure where choosing a surgeon who does it regularly is optional.
Long-Term Outcomes
The landmark long-term study of PAO, following the original patients treated by the surgeon who developed the technique, found that 60% of hips were still preserved at 20 years. That means six out of ten patients had not gone on to need a hip replacement two decades after surgery. For a population that would otherwise face arthritis and joint replacement in their 30s or 40s, pushing that timeline into the 50s or 60s is a meaningful gain, since hip replacements performed at older ages tend to last the rest of a patient’s life without needing revision.
Outcomes are better for patients who go into surgery with minimal cartilage damage and good joint congruence. This is why surgeons emphasize catching dysplasia early and operating before arthritis sets in. Patients with Tönnis grade 0 (no arthritis) or grade 1 (minimal changes) at the time of surgery have the best long-term results.