Former studies evaluating PAO correction mostly evaluated the lateral cover of the femoral head as indicator for operation success. Hartig-Andreasen describes a CE angle less than 30° or higher than 40° as a factor for an increased conversion to total hip arthroplasty after PAO . The group around Albers showed results predicting a poor clinical outcome if the postoperative CE angle was below 22° . Steppacher et al. showed similar results using the femoral head extrusion index . Only the group around Beaule did not detect an influence of the CE angle on the postoperative outcome using the WOMAC score as indicator . In our study the ASA at 12 o’clock resembles the measurement position of the CE angle but is not comparable in absolute numbers. We did not detect a correlation between PROMs as well as in osteoarthritis progression in regard to this angle. A reason may be a good postoperative lateral coverage of the femoral head in the majority of all our cases without cases with severe under or over coverage.
The study’s participants showed an increased postoperative acetabular version (male 36.6 ± 13.2°, female 30.1 ± 8.0°) in comparison to healthy patients (male 18 ± 4,5°, female 21 ± 5°). Up to now only two studies analyzed the acetabular orientation after PAO and isolated a decreased acetabular version to be a negative factor for good outcome and progression of osteoarthritis [4, 5]. Since the analysis of the acetabular version can be highly biased by pelvic tilt and other inherent limitations of conventional radiographs we decided to use MRI as a more precise diagnostic tool [6,7,8]. This makes comparison with other studies more difficult. The reason that we could not detect acetabular retroversion to be a significant factor maybe that we did not have any cases of severe retroverted acetabula with the lowest having an anteversion of 14,5°.
Instead we detected that cases with progression of osteoarthritis had a general larger acetabular anteversion. Patient outcome itself was not influenced by acetabular anteversion.
Further we detected a correlation between reduced anterocranial coverage and progression of osteoarthritis. Since no reference values for the ASA 10 and 11 o’clock exist, it is only possible to compare already established parameters like the anterior ASA at 15 o’clock (AASA) as well as the posterior ASA at 9 o’clock (PASA). In comparison to healthy patients (male: AASA 64 ± 6°, PASA 102 ± 8°; female: AASA 63 ± 6°, PASA 105 ± 8°) both anterior and posterior femoral head cover were still decreased postoperatively . Fuji et al. showed in deformity analysis of not treated dysplasia cases using CT a reduced AASA (male: 42,1 ± 6°, female: 41,3 ± 7,7°) and PASA (male: 84,4 ± 6,2°, female: 91,3 ± 6,8°) and in comparison to a control collective (AASA: 60,7 ± 9°, PASA: 104,5 ± 9,3°) a posterior and anterior undercoverage . Our study population, especially male patients, showed in comparison a reduced AASA as well as a slightly increased PASA in regard to these dysplastic patients. The horizontal ASA showed a global undercoverage for dysplastic hips (male: 117.9 ± 24°, female: 134.4 ± 11.4°) in regard to a healthy collective especially for male patients (male: 167 ± 11°, female: 169 ± 10°) . Since global coverage had no significant influence on patient outcome and progression of osteoarthritis, an uneven anterior/posterior balance of cover i.e. reduced anterocranial cover has possibly more influence on the long-term outcome. The decreased ASAA, especially in comparison to other dysplasia patients’ collectives, as well as the increased acetabular anteversion, suspects an increased version of the acetabular fragment after surgery with a less prominent anterior acetabular rim and cover. Ibrahim et al. analyzed patients with treated cam type deformities and showed that an increased anterior cover is a negative predictor for functional outcome . Since there is a wide coexistence of dysplasia and femoroacetabular impingement , which maybe even increases after acetabular correction, a slightly more anteverted placement may be a reasonable orientation of the acetabular fragment. At the same time an exaggerated anteversion can lead to a possible hip instability and progression of osteoarthritis.
Different research groups already established, that a heightened femoral torsion or a retrotorsion may induce osteoarthritis [25, 26]. Research articles evaluating femoral torsion describe different average mean values from 10.4–24.1° for healthy patients [27,28,29]. Different studies specify that patients with dysplastic hips have an increased antetorsion in comparison to healthy patients [13, 30]. Akiyama et al. describe for dysplastic hip not only an increased anteversion but also a more diverging value for the femoral torsion depending on anterior and posterior coverage of the acetabulum in comparison to healthy patients . Overall, we did not detect a correlation between femoral torsion and PROMs or progression of osteoarthritis after PAO.
The combined acetabular and femoral version, first described by McKibbin 1970, is in our opinion a parameter with increasing relevance . The developed McKibbin index divides the measurements in a group with regular combined version between 20 and 60° as well as below 20° and above 60° with increased incidences of pain. Especially a reduced combined version below 20° is associated with osteoarthritis . Kohno et al. show in a retrospective assessment of 100 dysplastic hips that patients with increased combined anteversion have an early development of pain . Since no patients in our study had a combined anteversion below 20° we could not evaluate these casuistic. In regard to other studies we did not find a difference between patients with a combined version between 20 and 60° and above 60° in regard to pain, PROMs and progress of osteoarthritis.
Cam Impingement and the corresponding increased alpha angle have a significant influence on the outcome after PAO. Beaule et al. describe a correlation between a preoperative increased alpha angle and worse postoperative WOMAC during the follow-up . Albers et al. show in a retrospective follow-up, that patients with normal head/neck ratio had a better outcome as well as less progression of osteoarthritis over 11 years . Since all patients in our study with a preoperative significant heightened alpha angle received an intraoperative femoroplasty, no severe cases of cam impingement occurred postoperatively. Although the group with progression of osteoarthritis showed a significant increased mean alpha angle at 14 o’clock, the alpha angle remained below 50°. Overall, patients with an increased cranial and posterocranial alpha angle (10, 11 and 12 o’clock) had a postoperative decreased GTO. Since the posterocranial head neck junction is the insertion zone for the femoral head vessels a possible correction is difficult. These results reflect that patients with larger head asphericity may have a reduced outcome after PAO. Goronzy et al- showed prospectively in a 5-year-follow-up-study after PAO an equal outcome for patients without cam deformity and surgical correction of the cam deformity .
Limitations to our study were the lack of standardized preoperative MRIs for better understanding of preoperative acetabular orientation. Most patients received preoperatively an external MRI which was not comparable with the follow-up MRI with sufficient quality. Still comparison of global acetabular cover (combined ASA) and acetabular orientation amplified by femoral torsion enables us to consequently make a sound conclusion for postoperative placement even without standardized preoperative MRI imaging. Although we had a good follow-up rate of 85.9%, only 66 (66.7%) hip MRIs could be obtained, leaving 33 without 3 dimensional imaging. Control collectives for 3-dimensional measurements in dysplastic hips originate from Asian countries maybe describing a different pelvic morphology and making comparisons difficult. Also, the time period of the follow-up rate of 5 years and the number of MRIs might not be sufficient to asses enough progression of osteoarthritis or decline in PROMs to filter out certain poor placements of the acetabulum.