Introduction

Range of motion (ROM) simulation of the hip is useful to understand the maximum impingement free ROM in total hip arthroplasty (THA). In spite of a complex multi-directional movement of the hip in daily life, most of the previous reports have evaluated the ROM only in specific directions such as flexion-extension, abduction-adduction, and internal - external rotation at 0° or 90° of hip flexion. Therefore, we developed ROM simulation software (THA analyzer) to measure impingement free ROM in any positions of the hip. Recent designs of the hip implants give a wider ROM by increasing the head diameter and then, bone to bone impingement can be a ROM limit factor particularly in a combination of deep flexion, adduction and internal rotation of the hip. Therefore, the purpose of this study were to observe an individual variation in the pattern of the bone impingement ROM in normal hip bone models using this software, to classify the bone impingement ROM mapping types and to clarify the factors affecting the bone impingement type.

In total hip arthroplasty (THA), acetabular cup orientation is critical for avoiding edge-loading and implant-implant impingement, which may lead to serious complications such as dislocation, mechanical loosening, accelerated wear, or implant breakage. Many studies recommended to place the acetabular cup radiographically at an inclination of <50° to avoid edge-loading. Simultaneously, larger prosthetic ROMs than the patients’ ROM during daily activities are needed to minimize impingement related complications. Several three-dimensional computer simulation studies have been done for optimal cup orientation to avoid prosthetic impingement within possible hip ROMs in the late 1990s. However, the reference angles in the directions of flexion, extension, external rotation and internal rotation at 90 ° flexion as possible hip ROMs have not been consistent in previous simulation studies. Thus, different reference angles of hip ROMs resulted in different optimal cup orientation. Therefore, to give accurate information about the reference hip ROM, we measured passive hip ROMs intraoperatively using a navigation system in 91 patients.

Pelvic and femoral coordinate systems referred a functional pelvic plane in the supine position and a retrocondylar plane, respectively. The neutral position of the hip ROM was defined as the position in which corresponding axes of the pelvic and femoral coordinate systems were parallel. Maximum flexion, extension, external rotation and abduction were 120°, 36 °, 43 ° and 55 °, respectively. Moreover, we investigated the hip ROM during five traditional Japanese hip positions which required large hip flexion and internal rotation angles in five healthy female volunteers by a 3D image matching technique using an open-configuration MRI. Maximum flexion was 122 ° and maximum internal rotation was 40 ° at more than 90 ° of flexion position. Therefore, we recommended using 120 ° for flexion, 40 ° for extension, 40 ° for external rotation and 40 ° for internal rotation at 90 ° flexion as the reference ROM when calculating an optimum cup orientation.

We calculated radiographic cup anteversion, when radiographic cup inclination was 40 °, without prosthesis impingement in the reference hip ROMs using computer aided design models of prosthesis, which included a cementless CentPillar stem with a head 32mm in diameter and cementless Trident cup with a flat liner. The results showed the optimal cup target zone existed when the stem anteversion was between 20 ° and 45 °. The size of the target zone was widest when the stem anteversion was 30 °, and then it was plus or minus 5 ° of inclination and anteversion from the center of the zone. To eliminate outliers of cup orientation form the target zone, a computer assisted system such as navigation is recommended.

Although there are several reports of excellent long-term survival after cemented total hip arthroplasty (THA), cemented acetabular components are prone to become loose when compared with femoral components. On the other hand, the survival of cementless acetabular components has been reported to be equal or better than cemented ones and the use of cementless acetabular components is increasing. However, most of the reports on survival after THA are for patients with primary hip osteoarthritis (OA) and there is no report of 20-year survival of cementless THA for patients with hip dysplasia. It is supposed to be more difficult to fix cementless acetabular components for OA secondary to hip dysplasia than primary OA. The purposes of this study were to review retrospectively the 20-year survival of cemented and cementless THA for hip dysplasia and to compare the effect of fixation methods on the long-term survival for patients with hip dysplasia. We retrospectively reviewed all patients with OA secondary to hip dysplasia treated with a cemented Bioceram hip system between 1981 and 1987, and a cementless cancellous metal Lübeck hip system between 1987 and 1991. We excluded patients aged more than 60 years, males, and Crowe 4 hips. The studied subjects were 70 hips of cemented THA (Group-C) and 57 hips of cementless THA (Group-UC). Both hip implants had a 28-mm alumina head on polyethylene articulation. The mean age at operation was 50.5 years (range, 36–60 years) in Group-C and 50.0 years (range, 29–60 years) in Group-UC. The mean BMI was 23.2 kg/m² in Group-C (range, 17.3–29.3 kg/m²) and 22.9 kg/m² in Group-UC (range, 18.8–28.0 kg/m²). There were no significant differences in age and BMI between the two groups. The average follow-up period was 18.0 years in Group-C and 18.4 years in Group-UC. In Group-C, revision was performed in 33 hips due to aseptic cup loosening (30 hips), stem loosening (one hip), and loosening of both components (two hips). In Group-UC, revision was performed in 10 hips due to stem fracture secondary to distal fixation (4 hips), cup loosening (three hips), polyethylene breakage (two hips), and extensive osteolysis around the stem (one hip). The survival at 20 years regarding any revision as the endpoint was 51% in Group-C and 84% in Group-UC. This difference was significant using Log-rank test (P=0.006). The cup survival at 20 years was 54% in Group-C and 92% in Group-UC. This difference was also significant (P = 0.0003). The stem survival at 20 years was 95% in Group-C and 92% in Group-UC. This difference was not significant (P = 0.4826). Cementless THA showed a higher survival rate at 20 years for hip dysplasia than cemented THA because of the excellent survival of the acetabular component without cement. We conclude that cementless THA with the cancellous metal Lübeck hip system led to better longevity at 20 years than cemented THA with the Bioceram for patients with OA secondary to hip dysplasia.