Three-dimensional Morphology Of Lunate Surface In Crowe Type Ⅰ Hip Dysplasia And Implications Of Periacetabular Osteotomy Determined By Finite Element Analysis

Objective: Developmental dysplasia of the hip is an important cause of pain and arthritis in adults and adolescents.Periacetabular osteotomy through rotating the acetabular to provide ideal coverage for the femoral head has been widely used in the treatment of patients with acetabular dysplasia and achieved a good long-term effect.Not all patients have satisfactory results even the surgery was planned at a young age.Recently,variability in the extent of development of acetabular cartilage has increasingly become of interest and investigated as a major cause.Excessive coverage in any direction will cause femoroacetabular impingement.In these situations,three dimensional information of acetabular weight-bearing surface,namely lunate surface,is crucial for corrective treatment.Recently,FEA has been employed in the study of hip joint mechanical behavior and pre-clinical testing of DDH,through analyzing contact area,pressure,and Von Mises stress in the femoral and acetabular cartilages and estimatig intraoperative approximate range of rotating degrees.The purpose of the present study was to evaluate the three-dimensional morphology of acetabular lunate surface in patients with dysplastic hip using three-dimensional CT scan.The second objective was to compare the contact pressure,and Von Mises stress in the acetabular cartilages between the normal and dysplastic hip models with theoretical defects of lunate surface to verify the theoretical significance of the classification.We hope that these findings may aid precise computational biomechanical analysis,preoperative planning for PAO,achievement of satisfactory cartilaginous congruency,and judgment of postoperative prognosis in addition to postoperative treatment evaluation.Methods: Sixty seven dysplastic hips in symptomatic patients and 61 normal hips inhealthy volunteers who underwent standardized anteroposterior pelvic radiograph,the false-profile radiographs and three-dimensional CT scanning were included in this study.The size and shape of the lunate surface was measured with a fully automated imaging technique using Mimics and 3-matic software;lunate surface morphology was compared between the normal and dysplastic hips on the radial spherical coordinate system.A normal model was generated from a male without clinical or radiological abnormalities by Mimics 16.0 and MSC.Marc.Approximately,600 000 tetrahedral elements and 120 000 nodes were used for the main linear elastic and isotropic material properties of cortical bone,trabecular bone,cartilage,and acetabular labrum.The average thickness of the cartilage layer was determined to be 2 mm.The articular surface was considered as a nonlinear,frictionless.The area of acetabular cartilage,excluding the fossa,was 2070.1 mm2.Four DDH models including DDH-1 normal,DDH-2 SD,DDH-3 ASD and DDH-4 GD model,were rotated by 10°,15°and 20°each and 12 models of PAO-1 Normal 20,PAO-1 Normal 25,PAO-1 Normal 30,PAO-2 SD 20,PAO-2 SD 25,PAO-2 SD 30,PAO-3 ASD 20,PAO-3 ASD 25,PAO-3 ASD 30,PAO-4 GD 20,PAO-4 GD 25,PAO-4 GD 30 were obtained respectively.Vertically,a load of 620 N(body weight of 74 kg)was placed on the center of the superior aspect of the sacral mount.The simulated abductor force of the middle gluteal muscle was defined as 460 N.the Newton-Raphson method was applied as an iterative method,applying the load in 20 steps by an incremental loading method.Results: A general trend of inferomedial rotation of the lunate surface was observed in dysplastic hips.Dysplastic hips showed a remarkable decrease in total absolute and relative size of the lunate surface as compared to that in normal hips.The dysplastic hips were divided into four groups: superior area decrease group(SD,10.44%);anterosuperior area decrease group(ASD,40.3%);global area decrease group(GD,32.84%);global area increase group(GI,16.42%).The arc of the subgroups was decreased as compared to that in the normal acetabulum group.In the normal hip,the von Mises stress showed a uniform distribution pattern,mainly distributed in dispersed fashion on the superior region of the lunate surface,mostly concentrated on the intermediate region.The maximum value was 13.06 MPa.However,the von Mises stress in the four DDH models concentrated intensively on the edge of the acetabulum,which was higher than in the normal model.Through rotating different degree of 10°,15°,20°during PAO,the maximum value of von Mises stress on the load-bearing area of the acetabulum decreased from 24.14 MPa pre-PAO to 22.87 MPa,18.94 MPa,19.64 MPa post-PAO in DDH-1 normal model.The maximum value in DDH-2 SD reduced from 40.36 MPa pre-PAO to 35.56 MPa,33.22 MPa,and 29.45 MPa post-PAO.The maximum value in DDH-3 ASD decreased from 43.35 MPa pre-PAO to 40.34 MPa,37.99 MPa,and 30.79 MPa post-PAO.The maximum value in DDH-4 GD reduced from 46.67 MPa pre-PAO to 35.81 MPa,36.28 MPa,and 30.10 MPa post-PAO.Conclusion: We first divided the dysplastic hip lunate surface into four groups: superior area decrease group,anterosuperior area decrease group,global area decrease group and global area increase group.Three-dimensional information and fundamental morphological features of the lunate surface in dysplastic hips were significantly different from those in the normal group.The contact pressure and Von Mises stress in the acetabular cartilages differed a lot among dysplastic hip models with different defects of lunate surface.These findings may aid precise computational biomechanical analysis,preoperative planning for PAO,achievement of satisfactory cartilaginous congruency,and judgment of postoperative prognosis in addition to postoperative treatment evaluation.We hope these findings were of great clinical significance to ensure the long-term effect and to study the mechanism of the development of osteoarthritis.