In this study we used subject-specific finite element analysis to investigate the mechanical effects of rotational acetabular osteotomy (RAO) on the hip joint and analysed the correlation between various radiological measurements and mechanical stress in the hip joint.

We evaluated 13 hips in 12 patients (two men and ten women, mean age at surgery 32.0 years; 19 to 46) with developmental dysplasia of the hip (DDH) who were treated by RAO.

Subject-specific finite element models were constructed from CT data. The centre–edge (CE) angle, acetabular head index (AHI), acetabular angle and acetabular roof angle (ARA) were measured on anteroposterior pelvic radiographs taken before and after RAO. The relationship between equivalent stress in the hip joint and radiological measurements was analysed.

The equivalent stress in the acetabulum decreased from 4.1 MPa (2.7 to 6.5) pre-operatively to 2.8 MPa (1.8 to 3.6) post-operatively (p < 0.01). There was a moderate correlation between equivalent stress in the acetabulum and the radiological measurements: CE angle (R = –0.645, p < 0.01); AHI (R = –0.603, p < 0.01); acetabular angle (R = 0.484, p = 0.02); and ARA (R = 0.572, p < 0.01).

The equivalent stress in the acetabulum of patients with DDH decreased after RAO. Correction of the CE angle, AHI and ARA was considered to be important in reducing the mechanical stress in the hip joint.

Cite this article: Bone Joint J 2015;97-B:492–7.

Aims: The concept of balancing ßexion and extension gap during total knee arthroplasty (TKA) is reported to be crucial. However, difþculties in 1)deþning the ideal distraction force to create a gap, and 2)equalizing ßexion and extension gap are often encountered during TKA for rheumatoid arthritis (RA). This study was performed to analyze these difþculties biomechanically in vivo. Methods: 35 knee at randomly selected TKA for RA were studied as follows. After soft tissue balancing, distraction force for both gaps was applied by ligament balancer. Force was gradually increased with recording the length of the gap created by consecutive force, in order to obtain load-elongation curve for each case. Results: Load-elongation curve showed various patterns, indicating soft tissues including ligamentous structure has been altered its biomechanical property due to the variety of rheumatoid pathology. If ideal force for extension gap was determined at the point when low stiffness changes to high stiffness in the curve, it would be about 120–200N. This force differed reasonably in each case. However, measured ßexion gap curve hardly reached this force in more than 60% of the35 knee, presumably due to functional loss of posterior structures. Interestingly, this phenomenon was often unpredictable at examination before the operation. Conclusions: Ideal tension for þlling the gap with implant seems to be different in each case from load-elongation curve analysis. From this study, we raise question to the concept of equal ßexion and extension gap. This concept, although sounds attractive, is often difþcult to obtain in rheumatoid knee. This observation may suggest which type of TKA (þxed or mobile) is safer for replacing the rheumatoid knee.