Abstract
Introduction
In DDH cases often have high anteversion. They also often have high hip center. THA for those cases sometimes requires subtrochanteric derotational/shortening osteotomy. To achieve good results of the surgery, accurate preoperative planning based on biomechanics of the high anteversion cases, method for accurate application of the plan, and stable fixation are very important. At ISTA 2008, we have reported that the location of the anteversion exist several centimeters below the lesser trochanter. Independently from the extent of anteversion, femoral head, grater trochanter, and lesser trochanter are aligned in the same proportion. We have also reported in 2007, in improper high anteversion cases, many cases grow osteophytes posterior side of femoral head to reduce it functionally. In 2014, we reported about development of the stem for subtrochanteric osteotomy. (ModulusR)[Fig.1] In the present study, we established systematic planning way for estimate proper derotation and shortening and apply it for the surgery.
Methods
Leg alignment during walking were well observed. According to the CT, 3D geometry of the femur, anteversion in hip joint and its compensation by the osteophyte, and knee rotation were measured. It was divided into proximal part and distal part at several centimeter below the lesser trochanter. Adequate hip local anteversion was determined by local original anteversion – compensation if IR-ER can be done. Keeping that anteversion for the proximal part, distal part was rotated as knee towards front. Thus derotation angle was decided. Using 3D CAD (Magics®) proper size of Modulus R was selected and overlapping with canal was extracted then its center of gravity was calculated. This level is decided as the height of osteotomy to obtain equal fixation to both proximal and distal part.[Fig.2] If the derotation angle is less than 15 degree, modular neck adjustment was selected first. By trial reduction and motion test, according to the instability osteotomy was performed. In the high hip center cases, original hip center was reconstructed. Shortening length was determined not to make leg elongation more than 3cm. ModulusR were used for the replacement and fixation of the osteotomy.
Results
Eight cases were operated with this procedures. Standard Modulus was used in one case. In the case rotational fixation was well obtained but proximal stress shielding happened. ModulusR was used in other seven cases. In one ModulusR case vertical clack; which was fixed by metal band; happened in proximal part by the repeated rotational adjustment. But in all ModulusR cases, weight baring could be started in 1 week and good union was observed. Every patient feels knee direction became better than before.[Fig.3,4]
Discussion
In intraoperative stability test, much better stability was obtained after derotational osteotomy was done than the adjustment only by modular neck direction. Reducing anteversion by osteotomy was supposed to have advantage. Limitation of this paper is that the adequate hip local anteversion was estimated from femoral geometry and osteophytes and knee direction during walking. Future improvement would to use 2D-3D matching while walking to determine accurate hip local anteversion.
