Abstract
Introduction
The advance of surgical technique and implant design have led to improvement in total hip arthroplasty (THA), and short stem THA is now gaining number as a treatment option for younger patients to preserve bone in the proximal femur for a future revision. The SMF stem is shorter stem, and requires a slightly higher neck resection and implanted in slight varus to contact at lateral cortex. Developmental dysplasia of the hip (DDH) is the most common cause to hip osteoarthritis (OA) in Japan, and the morphology of the dysplastic femur is narrow canal and increased anteversion. Thus, the purpose of this study is to evaluate the SMF stem design can fit for Japanese patients, using CT based 3-D template planning.
Methods
We evaluated 30 patients who required THA in our institution. Inclusion criteria are hip OA, but cases with post-trauma, post-osteotomy, and any other hip disease in childhood are excluded. Patients were selected with their femoral anteversion, based on the dispersion of anteversion in dysplastic hip population, which was reported by Noble and collegues in 2003. Preoperative planning with ZedHip software (Lexi, Japan) was performed by established protocol. The center of socket was placed at 15mm proximal from teardrop and medialized to primary acetabulum, with 40° of radiographic inclination and 20° of anteversion. Neck resection of femur was 20mm proximal from the top of smaller trochanter, and stem was placed with lateral fit at distal and medial fit at calcar with appropriate size. Stem offset was selected by leg extension and balanced shenton line. Finally, ROM simulation was performed and the socket anteversion was arranged to achieve the optimal ROM. And then, SMF stem alignment and appropriate fitting was evaluated in Japanese OA cases.
Results
Within 30 cases, 28 were female and 2 were male. Mean age was 66.0±9.6, and 8 (26.7%) were primary OA and 22 (73.3%) were secondary OA, from dysplastic hip. SMF stem was placed at average 2.7° varus and 7.2° flexion against femoral axis [Fig. 1]. The average degree of femoral anteversion was 34.0°±10.9 at pre-implantation and 22.7°±9.9 after implantation, which means average 11.4° retroverted [Fig. 2]. Stem offset and neck length variation was acceptable in 28/30 cases (93.3%). Finally, the mean flexion was 130°±7.1 and abduction was 56.0°±5.5, and optimal ROM was achieved 19/30 cases (63.3%). We cannot find any correlation between femoral anteversion pre-implantion and ROM post-implantation, however positive correlation was found between flexion angle and offset length after implantation (r2=0.48).
Conclusion
The stem design of SMF is unique to preserve bone stock, but impossible to control anteversion after implantation, so it should be evaluated if it could fit to dysplastic hip. In this 3-D simulation study, SMF stem can be appropriately fitted in most of Japanese cases. Neck length and offset is important to achieve optimal ROM. Large diameter head (≥32mm) and high-offset stem would be suggested, and it may spread the indication for most of the cases regardless of the anteversion of femur.
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