This retrospective cohort study, which investigated preoperative factors associated with BMD loss in zone 7 after THA, found that preoperative CFI and lumbar BMD on the lateral side were significantly associated with periprosthetic BMD loss. These findings emphasize the importance of monitoring patients for severe BMD loss after THA, especially patients with lower lumbar BMD and a stovepipe-shaped proximal femur.
BMD loss around the implant has been frequently observed after THA [13, 14]. The most important cause of BMD loss is stress shielding, which is mainly influenced by stem design. A comparison of patients undergoing THA with Zweymüller stem and fit-and-fill stem implants found that both mechanical stress and zone 7 BMD loss around the implant were lower in the Zweymüller stem group than in the fit-and-fill stem group [4].
In addition to stem design, BMD loss may be associated with patient-specific or operative factors. For example, excessive stem anteversion mismatched with anatomical canal anteversion has been found to result in stem point contact with the cortical bone in the distal portion, affecting proximal periprosthetic zone 7 BMD loss after THA [15, 16]. An investigation of postoperative zone 7 BMD in groups of patients with normal preoperative lumbar BMD and patients with osteopenia and osteoporosis found that BMD loss was significantly higher in the osteopenia and osteoporosis groups than in the normal group [17], indicating that periprosthetic BMD loss is associated with bone quality as well as stress shielding [14].
Because zone 7 BMD loss occurs within 1 year postoperatively [17], it is important to take steps to prevent this BMD loss. Many drugs have been used to prevent BMD loss around the implant after THA. For example, bisphosphonate was reported to prevent BMD loss around the implant [10, 11, 18–23]. Moreover, bisphosphonate treatment was associated with a lower risk of aseptic revision in patients undergoing primary THA for osteoarthritis [21]. However, long-term continuous bisphosphonate treatment has been associated with atypical periprosthetic fractures [22, 23]. Periprosthetic fracture rates following primary THA were reported to be 1.1% in a large US cohort [22] and 0.64% over 10 years in the Swedish hip registry [23], with bisphosphonate use associated with a higher risk of periprosthetic fractures in younger patients with normal bone quantity [23]. Another effective agent that can prevent BMD loss around the implant is teriparatide. A randomized controlled trial found that teriparatide and alendronate were equally effective for preventing zone 7 BMD loss [11]. Moreover, switching from teriparatide to alendronate has been shown to be effective [12]. Although several drugs can prevent BMD loss around the implant after THA, care is necessary regarding the side effects and economic burden of these agents.
The shape of the femoral medullary cavity has been reported to be related to BMD loss around the implant. Postoperative zone 7 BMD was found to be significantly lower in stovepipe-shaped than in champagne-flute-shaped cavities when using taper-wedge-type stems [24]. The stovepipe-shaped type of medullary cavity has a small CFI, making these results similar to those of the present study. By contrast, a comparison of postoperative changes in BMD of any zone between three types of medullary cavities, stovepipe, normal, and champagne-flute shaped, using Zweymüller-type stems found no statistically significant differences in relative changes between the three groups [25]. Our study found that patients with low CFI, such as those with stovepipe-shaped medullary cavities, showed a greater reduction in postoperative zone 7 BMD than patients with high CFI, such as those with champagne-flute-shaped cavities.
Limitation
This study had several limitations. First, this study only investigated patients with Zweymüller-type stems. Studies of other stem types, such as taper-wedge stems, may yield different results. Second, the total number of evaluated subjects was small, as many subjects had to be excluded.