Rectangular femoral stems can successfully accommodate the medullary canal in patients with severe hip dysplasia operated on with total hip arthroplasty and a shortening osteotomy: A morphometric study

The current study aimed to objectively evaluate the fit of a rectangular, tapered stem to the severely dysplastic hips on the basis of the proximal femoral anatomy and the dimensional properties of the stem. It was hypothesized that the stem size planned with accordance to the diaphyseal canal width alone can accommodate the distal femur successfully with no sizing mismatch. Forty-six patients (53 hips) suffering from secondary osteoarthritis due to hip dysplasia scheduled for total hip arthroplasty (THA) with a subtrochanteric transverse shortening osteotomy were included. All hips were Crowe type 4. All patients underwent preoperative computed tomography imaging. Height of femoral head center (HCH) was determined. Medullary canal diameter measurements at different levels were made. These were made at a level (1) 35% of HCH above the lesser trochanter (DT +35), (2) 70% of HCH below the lesser trochanter (DT -70), and (3) at the level of isthmus (Di). Medullary canal flare indices were calculated from the individual parameter ratios. Similar measurements were carried out for the different sizes of the femoral stem. The mean DT +35 was 41.9 ± 6.4 mm, the mean DT-70 was 17.3 ± 2.2 mm, and the mean Di was 12.8 ± 1.9 mm. In all femurs, the width of the isthmus was wider than that of the corresponding femoral stem isthmus. The femoral stem sizes established with respect to the diaphyseal width of the femur were all compatible with the isthmus width of the femur. Tapered and rectangular stems can accommodate the proximal femur above and below the osteotomy level in Crowe type IV hip dyplasia patients, thereby constituting a viable option as the femoral stem in this patient group with technically demanding difficulties.