History of the PAO: The Original Patients and Their 30-Year Outcomes

Periacetabular Osteotomy (PAO) surgery is a hip preservation surgery used for treating hip dysplasia in teenager and adult patients.  In hip dysplasia the socket of the hip joint is often shallow, and does not fully cover the ball/head of the femur.  This can make the hip unstable and cause abnormal forces in the joint, which can cause pain and early arthritis. 

The First PAOs

The first PAO was first performed in April 1984 by Dr. Reinhold Ganz in Bern, Switzerland.  From 1984 through 1987, 63 patients had PAO surgery (total of 75 hips).  This first group (“cohort,” in research terms) has been followed since, which gives insight into the long-term outcomes of PAO surgery.   Information about the patients and surgery was initially reported in a research article in 1988.  Since then, 10-, 20-, and 30-year updates have been reported for this group.

Patients were 13 to 56-years-old when they had PAO surgery (average age 29).  Before the PAO, previous surgeries attempting to correct hip dysplasia had been performed in 23 hips.  There was “significant arthritis” in 18 hips at the time of surgery (Ganz et al, 1988).

Fun fact time! Here are some other interesting details about the initial group of patients to have PAO surgery:

  • One of the original patients was 56-years-old, and had mild arthritis on x-rays when she had her PAO.  11 years after her PAO she did not have a progression of arthritis, and still had a good clinical score with regards to pain, range of motion, and walking function (Siebenrock et al, 1999).
  • For patients who had both right and left PAOs, the time between surgeries ranged from 2 weeks to 1 year in this early group (Ganz et al, 1988).
  • The first patient to undergo PAO surgery was 13 years old, and she also had surgery on her femur (valgus intertrochanic osteotomy) at the same time.  She had another hip surgery 3 years later because she was still having some hip instability.  23 years after her original PAO surgery, she still had a good clinical result (Steppacher et al, 2008).

10-Year Outcomes

At the 10-year follow up researchers were able to get information about 71 of the original 75 hips operated on.  The researchers reported outcomes such as whether or not the patient ended up needing a total hip replacement (THR) since their PAO.  Researchers also reported on overall joint symptoms and function by using measures of pain, range of motion, and walking function.  Around 10 years post-PAO, the following outcomes were reported:

  • 12 patients had a THR (19%).
  • One patient had a hip fusion.
  • The other 58/71 PAO hips (82%) were “preserved” (meaning no THR or hip fusion).
  • 73% of the group were considered to still have good or excellent clinical results.  This means that they had little pain, good range of motion, and good walking function.

The researchers also looked at the data to determine if there were factors related to the patient or surgery that predicted worse outcomes.  Patients who had moderate-to-severe arthritis, who were older, and who had labral tears at the time of surgery tended to have poorer outcomes.  The researchers also found that how the socket was repositioned during surgery also affected outcomes.  When the slope of the socket (the “acetabular index”) was higher or lower than the normal 0-10-degree range, and when there wasn’t enough coverage in the front of the hip after the PAO, outcomes were sometimes not as good (Siebenrock et al, 1999). 

20-Year Outcomes

At the 20-year follow up, the researchers were able to get information about 68 of the original 75 hips.  By this time, 14 more of the original PAO hips had been converted to a THR.  The other 41/68 hips (60%) were preserved.  The researchers noted that there was a large difference in outcomes for patients who had little-to-no arthritis before their PAO versus those who had moderate-to-severe arthritis.  75% of hips that had little-to-no arthritis (Tonnis scores 0-1) were preserved at 20 years compared to only 13% of hips that had moderate-to-severe arthritis (Tonnis scores 2-3). 

The study found 6 factors that predicted a poorer outcome.  Patient-related factors included older age at the time of their PAO; higher pain and worse range of motion and walking function before surgery; positive anterior impingement test before surgery; limping before surgery; and hip joint arthritis before their PAO.  The researchers also found that patients who had bone over-coverage after surgery (“increased extrusion index”) were also more likely to have poorer outcomes (Steppacher et al, 2008).

30-Year Outcomes

This is the first study to follow patients for 30 years after their PAOs.  In 2017, the researchers were able to report outcomes 27-32 years after the original PAO surgeries for 72 hips.  Here are some of the most-recent outcomes reported in this study: 

  • 42/72 hips (56%) hips had undergone THR, 1 fusion, 7 (9%) had a Merle D’Aubinge score of less than 15 and 4 others (5%) had worse arthritis. 
  • 29% of the original PAO hips were preserved and did not have pain or worsened arthritis on x-rays.

The researchers continued to study factors that seemed to predict worse outcomes.  There were 10 factors found to be associated with failure:

  • Eight occurred preoperatively: age greater than 40 at the time of surgery, Merle D’Aubinge and Postel score less than 15 (this measure looks at pain, range of motion, and walking function, with lower scores suggesting poorer status), Harris Hip score less than 70 (this measure looks at pain, function, and hip range of motion, with lower scores suggesting poorer status), positive anterior impingement test, positive posterior impingement test, hip internal rotation range of motion of less than 20 degrees, and moderate-to-severe arthritis (Tonnis osteoarthritis grade > 1).
  • Two factors associated with failure occurred postoperatively:  anterior over-coverage (too much bony coverage in the front of the hip) and acetabular retroversion (hip socket positioned to face relatively more towards the back of the body).

Based on the 30-year outcomes for the original group of patients having PAO surgery, the authors predicted that patients with hip dysplasia with the following characteristics will have the best outcomes from PAO surgery at 30 years:

  • Tonnis osteoarthritis score of 0-1 (minimal-to-no arthritis) at the time of surgery.
  • < 40-years old at the time of PAO surgery.
  • Little-to-no pain before PAO surgery, no positive impingement test before surgery, and no limp when walking before PAO surgery.

 (Learch et al, 2017). 

Final Notes

The information from these four studies has helped advanced PAO surgery so that surgeons can better-predict which patient will have a good outcome, and has also helped advance surgical techniques.  It is important to note that these studies represent the first few years of PAO surgery.  Patient selection and PAO surgical technique have changed over the years as more surgeons have become trained in this surgery, and many more patients have been studied.  The information learned from these studies is extremely valuable to understand long-term outcomes for patients after PAO surgery.  However, it is very possible that the outcomes for patients who have had PAO surgery in the 90s, 2000s, 2010s, and beyond may look different.  This may be especially true as surgeons now have a better sense of which patients are good candidates for PAOs and surgical techniques and technology are constantly improving.  Other hospital institutions and surgeons have started to report long-term outcomes for patients after PAO surgery.  It will be interesting to continue to follow previous, current, and future patients to learn more about outcomes after surgery over time.  This information will continue to help patients and surgeons have better expectations about long-term outcomes for patients undergoing PAO surgery.

References and Links to Studies

1. Ganz R, Klaue K, Vinh TS, Mast JW. A new periacetabular osteotomy for the treatment of hip dysplasias. Technique and preliminary results. Clin Orthop Relat Res. 1988;232:26–36.


2. Lerch TD, Steppacher SD, Liechti EF, Tannast M, Siebenrock KA. One-third of Hips After Periacetabular Osteotomy Survive 30 Years With Good Clinical Results, No Progression of Arthritis, or Conversion to THA. Clin Orthop Relat Res. 2017;475(4):1154–1168. doi:10.1007/s11999-016-5169-5


3. Siebenrock KA, Schöll E, Lottenbach M, Ganz R. Bernese periacetabular osteotomy. Clin Orthop Relat Res. 1999;363:9–20. doi: 10.1097/00003086-199906000-00003


4. Steppacher SD, Tannast M, Ganz R, Siebenrock KA. Mean 20-year followup of Bernese periacetabular osteotomy. Clin Orthop Relat Res. 2008;466:1633–1644. doi: 10.1007/s11999-008-0242-3.


Contributors: Cynthia Porcari and Nancy Muir