Background After insertion of cementless stem, new bone formation arise around stem. This bone formation is classified as follows, proximal endosteal bone bridging (spot welds: SW), increased cortical thickness (cortical hypertrophy: CH) and distal intramedullary new bone formation (pedestal: PD) in radiographically. These osteogenesis patterns depend on implant design, material, and mode of porous coating. However, there are no reports concerning about the influence of stem intramedullary canal fill, canal shape, and bone structure of proximal femur on periprosthetic bone remodelling.
Objectives The purpose of this study was to clarify whether there are relationships between sequential changes of periprosthetic bone remodelling and radiographic variables such as intramedullary canal fill, femoral canal shape, and bone structure.
Methods Between 1993 and 1998, 57 primary cementless total hip arthroplasties (THAs) were performed with Mallory-Head titanium femoral component in 50 patients of osteoarthritis. The average age at the time of operation was 58 years old. The mean duration of follow-up was 5.2 years. Anteroposterior radiographs of the hip were made at 6 weeks, 3 months, 6 months, 1 year and every year after the operation. We evaluated the frequency and time of new bone remodelling around the stem. Canal shape was converted to numerical value by using calcar to canal isthmus ratio (CC ratio) of Dorr. The large CC ratio indicates a cylindrical shape and the small value reveals funnel shape of proximal femur. Also intamedullary canal fill was measured. Radiographic signs of stem instability were defined as progressive subsidence and radiolucent or radiodense line around the porous coating surface.
Results SW appeared in all case within a year. At the time of 3 months, proximal canal fill in 40 cases with spot welds was 77% and it was significantly larger than 72% in 17 cases without SW. CH began to appear after 6 months and it was found in 30 cases (53%) after 1 year. PD appeared in 52 cases (91%) after 1 year. The presence rate of SW, CH, and PD at final follow up period did not depend on proximal and distal canal fill. After 6 months, CC ratio of 16 cases with CH was 54% and it was significantly larger than 46% of 41 cases without CH. The appearance of SW and PD did not depend on CC ratio. Radiographic signs of stem instability were not found during follow up period in all cases.
Conclusion The time of spot welds appearance is earlier in cases with large proximal canal fill. Cortical hypertrophy appeared earlier in cases with cylindrical canal shape rather than funnel shape. However, the appearance rates of new bone formations around cementless stem are not dependent on intramedullary canal fill, shape of proximal femur, and bone structure.