Objective: To explore the changes of bone and risk of micro-fracture in femoral head after removal of cannulated screws following femoral neck fracture healing under the impact force of daily stress.
Methods: A total of 42 specimens of normal hip joint were collected from 21 adult fresh cadaveric pelvic specimens. Wiberg central-edge (CE) angle, bone mineral density, diameter of femoral head, neck-shaft angle, and anteversion angle of femoral neck were measured. Then, the 3 cannulated screws were implanted according to the AO recommended method and removed to simulate the complete anatomical union of femoral neck fracture. The morphology of screw canal in the femoral head was observed by CT. Finally, the specimens were immobilized vertically within the impact device in an upside-down manner, and the femoral heads were impacted vertically. Every specimen was impacted at 200, 600, and 1 980 N for 20 times with the impacting device. After impact, every specimen was scanned by CT to observe the morphology changes of screw canal in the femoral head. Micro-fractures in the femoral head could be confirmed when there was change in the morphology of screw canal, and statistical software was used to analyze the risk factors associated with micro-fractures.
Results: After impact at 200 and 600 N, CT showed that the morphology of screw canal of all specimens did not change significantly compared with the original. After impact at 1 980 N, there were protrusion and narrowing in the screw canal of the 22 femoral head specimens (11 pelvic specimens), showing obvious changes compared with the original screw canal, indicating that there were micro-fractures in the femoral head. The incidence of micro-fracture was 52.38% (11/21). logistic regression results showed that there was correlation between micro-fracture and bone mineral density ( P= 0.039), but no correlation was found with CE angle, diameter of femoral head, neck-shaft angle, and anteversion angle ( P>0.05).
Conclusion: The micro-fractures in the femoral head may occur when the femoral head is impacted by daily activities after removal of cannulated screws for femoral neck fractures, and such micro-fractures are associated with decreased bone density of the femoral head.
目的: 探讨股骨颈骨折内固定物取出后，在日常活动撞击力作用下股骨头内部骨质变化情况以及微骨折发生风险。.
方法: 取 21 具成人新鲜尸体骨盆标本，共获得 42 个骨质及软组织完整的髋关节标本。首先测量标本股骨颈处骨密度、CE 角、股骨头直径、颈干角及前倾角。然后参照 AO 推荐方法植入 3 枚空心钉后取出，模拟股骨颈骨折完全解剖愈合空心钉取出后状态，CT 扫描观察股骨头内钉道形态。最后，模拟直立情况下股骨头受到撞击情况，分别用 200、600、1 980 N 力量撞击标本 20 次。CT 扫描观察撞击后股骨头内钉道形态变化情况，统计分析微骨折发生危险因素。.
结果: CT 扫描观察撞击力量为 200、600 N 时，股骨头钉道形态无明显变化，提示股骨头内无微骨折发生；撞击力量达 1 980 N 时，11 具标本 22 个股骨头的钉道形态与撞击前相比有明显变化，钉道内有凸起及钉道狭窄，提示股骨头内发生微骨折，微骨折发生率为 52.38%（11/21）。多因素分析显示，骨密度是股骨颈骨折内固定物取出后发生股骨头内部微骨折的危险因素（ P=0.039），CE 角、股骨头直径、颈干角、前倾角与股骨头内部微骨折发生无相关性（ P>0.05）。.
结论: 股骨颈骨折内固定物取出后，股骨头受到日常活动撞击力可能出现微骨折，并且微骨折的发生与股骨头骨密度下降相关。.
Keywords: Femoral neck fracture; cannulated screw; internal fixation; micro-fracture.