Abstract
[Introduction]
Short tapered wedge-shaped cementless (TW) stems have been widely used for several years. The concept of fixation of TW stem is wedge-fit fixation in the proximal metaphysis. Developmental dysplasia of the hip (DDH) has anatomical abnormality, such as excessive femoral anteversion, short femoral neck length, narrow femoral cavity, or proximal-distal mismatching of the femoral canal. Therefore, Mismatching between stem and bone might be occurred in DDH. We evaluated intramedullary matching of short TW stem for DDH by three dimensional (3D) digital template in order to clarify whether mismatching between stem and bone is seen in DDH implanted short TW stem.
[Materials and Methods]
One hundred hips (92 patients) with DDH were performed preoperative simulation for total hip arthroplasty by 3D digital template system (ZedHip: Lexi, Tokyo, Japan). The average age was 63.5 years old. There were 12 males and 80 females. The average bone mass index was 21.5 kg/m2. Femoral canal shape was normal in 71, champagne-flute in 16 and stovepipe in 13 hips. Bone quality was classified into type A in 23, type B in 74 and type C in 3 hips. Preoperative computed tomography data were used for 3D digital template and reconstructed to 3D femoral model. Short TW stem (Taperloc Complete Microplasty: Biomet, Warsaw, IN) model constructed from computer-assisted design was matched to the reconstructed femoral model. Short TW stem model was in principle implanted according to the femoral neck anteversion with neutral alignment (varus and valgus < 2 degrees, flexion and extension < 2 degrees) at the coronal and sagittal plane of the femur. Stem size was determined in order to obtain the largest intramedullary matching at the coronal plane. Area of stem fitting with the cortical bone was investigated at 10 mm intervals above and below of mid minor trochanter. Intramedullary matching pattern was classified into proximal mediolateral metaphyseal fit, proximal flare fit and diaphyseal fit at multiple reconstructed planes of the 3D femoral model according to stem fitting area.
[Results]
Ninety-three percent of stem could be implanted with neutral alignment at the coronal plane, and 86 percent at the sagittal plane. The average stem anteversion was 31.4 degrees. Over 70 percent of stem could be fit with the medial and lateral cortical bone at 10 mm above and below of mid minor trochanter. Intramedullary matching pattern was proximal mediolateral metaphyseal fit in 49%, proximal flare fit in 44% and diaphyseal fit in 7%.
[Discussion and Conclusion]
Early migration or failure of osteointegration of TW stem was recently published. Diaphyseal fixation or mismatching between stem and bone is considered as risk factors of early failure of TW stem. In this study, proximal metaphyseal fit, such as mediolateral metaphyseal fit or flare fit, could be achieved in 93% of DDH patients. Mismatching between stem and bone, such as diaphyseal fit, was observed only in 7% of DDH. Short TW stem is good choice for DDH in order to avoid of diaphyseal fixation followed by early migration of stem and decreased osteointegration.
