Objective: Establishment of the new method to evaluate fill of the hip stem.
Background: The fill of the hip stem is one of the important parameters to estimate the quality of planning or positioning of the cementless stem. It has been defined as a stem-canal width ratio on the A-P plain of X-ray images so far. However, it is quite a problem to get the correct AP images on basis so that positional difference may affect the measurement. According to our data, the fill was measured significantly different in 15, 30, 45, 60 degrees erroneous direction. First, we tried to figure out the fill of the hip stem three-dimensionally rather 2-dimensionally. Next, our new method was compared to conventional method.
Material and Methods: Leg CAT scans were performed on 32 hips of 20 patients (2 male, 18 female). Images of the canal of femora were reconstructed using CAD software. We made 2-types of canal model with or without lesser trochanter. The geometries of our lateral flare stems with different sizes were compared to each canal geometry in the CAD software and proper size was decided. Then images were observed from an accurate vertical direction of the coronal plain of the stem. We measured the 2-D fill on this plane and the 3-D fill of every 5 mm slice from the 5mm above to the 100mm below the head of lesser trochanter line (reference line). We also examined the stems 1-size smaller or larger than the appropriate ones.
Results: The mean age was 61.114 (range 24–82). The average of “3-D fill of Lateral flare stem was 51%/59% with/without lesser trochanter, and 2-D one was 74%/77%. The numerical and distributional results by these two methods to measure fill were alike but different. For example, in case without lesser trochanter, the 3-D fill showed the maximum value in the area just below the reference line. The maximum 2-D fill was recorded in 10mm caudal from the reference line. In general, this stem occupied much space in the distal area and around the lesser trochanter.
Future Plan: Extension of this evaluation method into various kinds of stems.
