Abstract
Introduction
In THA of DDH cases, sometimes shortening and/or derotational subtrochanteric osteotomy is required, for cases with high hip center and/or high anteversion. Initial fixation is one of the most important problems after subtrochanteric osteotomy. To prevent rotational displacement V-osteotomy or step osteotomy is often used. Even though until the osteotomy part unites, additional fixation is required. When a stem with distal load transfer was used body weight can be transferred to healthy part, so early full weight bearing should be expected. However, the muscles around hip joint can pull up proximal part, so it would be possible osteotomy part to be split. When a stem with proximal load transfer, body weight would be loaded on non united osteotomy part. So full weight bearing could sometimes be postponed. A stem that has both proximal and distal load transfer, and has facility for prevent rotation, those situations would possibly be simply solved.
Lima (Italy) has a proximal load transfer conical stem with fins “Modulus” and a distal load transfer conical stem “Revision.” Combining these two stems, a stem as explained prior was designed. In the present study, applicability of the stem for cases such as subtrochanteric osteotomy was assessed.
Materials and Methods
The stem geometry data were provided by the maker. Forty CAT scan DICOM data, 12 male, 28 female, 58+/−13.8 Y.O. were transferred to STL geometry data using Mimics®. Then using Magics® fit studies were done. For each femur, most suitable size of conventional “Modulus” and double conical stem “Modulus R” were selected to have tight fit making greater trochanter top height as head center height, then the area where stem core attaches to the inside surface of the canal were determined.
Results
Three cases were excluded, as they are very small and could not find small enough stems. With the rest, 37 cases “Modulus R” could obtain the same proximal contact area and in average 26.9% more area in distal area. (Fig.1)
Discussion
Double conical stem could have been expected to useful for the cases subtrochanteric osteotomies. In the actual case with 42 mm higher hip center, final elongation was determined as 25 mm. Then 17mm shortening subtrochanteric osteotomy was designed. (Fig. 2) Very tight fixation including rotational stability was obtained during the surgery. The patient could stand up since the next day. Full weight bearing was obtained in one week. Good bone formation was obtained in 2M. (Fig.3)
