ACETABULAR PLACEMENT ANGLES DETERMINED BY PELVIC LOCAL LANDMARKS

Summary Study showed a simple acetabular placement plane formed by pelvic landmarks. The plane was adjusted by changing one of the landmarks to a fixed value for best representing the native acetabular orientation based on CT generated 3D pelvi Introduction Correct acetabular cup placement is a critical step to prevent dislocation in the total hip arthroplasty. There are many mechanical alignment devices available but they are usually only referencing to the body long axis and the table therefore are lack of accuracy. Recently more accurate guide was achieved by image or imageless hip navigation system. But they add more cost, steps and time. The purpose of this study was to find a simple acetabular cup placement plane by selcting bonny land marks. The plane was adjusted with a fixed value by comparing it to native acetabular orientation in CT constructed 3D pelvi. Methods 274 anonymous CT pelvic scans from skeletally mature, normal Caucasian population with age range of 20–93 years old (mean age=64). The population included 164 males and 110 females (mean age for male=63 and female=65, P=0.40). CT data was converted to virtual bones using custom CT analytical software (SOMA™ V4.0). The acetabular anteversion angle was measured against coronal plane as AA defined by Murray. The inclination angle was measured from transverse plane. The native acetabular rim plane was constructed by three rim points of Ilium, ischium and pubis. The pelvic plane was based on and modified from previously reported alignment pelvic land marks. Anterior Inferior Iliac Spine (AIIS) was added to two local landmarks of Anterior Superior Iliac Spine (ASIS) and a point direct Lateral to Greater Sciatic Notch (LGSN). AIIS, ASIS and LGSN formed the local placement plane. The distance from three LGSN9s to greater sciatic notch were 70mm, 75mm and 80mm. Three planes from three LGSN points were analyzed for anteversion and inclination angles. Results were compared with the same angles from native acetabulum. Student T test was performed with confidence level at P=0.05. Results The mean anteversion angle/standard deviation for native acetabulum:25.7°/6.4° (male=24.6°/5.7°; female=27.3°/7.0°); plane LGSN+70mm: 21.9° /6.3°, (male= 20.3° /5.7°; female= 24.3° /6.4°); LGSN+75mm: 24.9°/6.3° (male= 23.3°/5.8°, female= 27.3°/6.2°); LGSN+80mm: 27.7°/6.1° (male= 26.1°/5.6°, female= 30.1°/6.0°). The mean inclination angle/standard deviation for native acetabulum were 51.5°/4.4° (male= 51.5°/4.2°, female= 51.5°/4.4°), plane LGSN+70mm: 51.8°/7.9° (male=53.3°/7.9°, female= 49.5°/7.4°), LGSN+75mm: 50.7°/7.8° (male= 52.3°/7.8°, female= 48.3°/7.3°); LGSN+80mm: 49.6°/7.8° (male= 51.2°/7.7°, female= 47.2°/7.2°). Student T test showed both anteversion and inclination angles of plane LGSN+75mm were not significantlydifferent from that of native acetabulum (P=0.12 and 0.11). (Table 1) The anteversion angle and inclination angle distribution are shown in Figure 1 and 2. Discussion/Conclusion Unlike previously reported landmark methods, landmarks in this study were verified in large bone data base with exactly same measurements. The direct lateral point from GSN can be projected intraoperatively by calibrated hand along with other fixed landmarks to form an imaginative acetabular cup placement plane. This method also can be used for the imageless computer navigation as well as the mechanical alignment device.