Femoral Neck Fracture After Hip Joint ResurfacingHip joint resurfacing instead of a total hip joint replacement is fairly new. Resurfacing replaces the arthritic surface of the joint but removes far less bone than the traditional total hip replacement. It saves bone in the femoral neck but also puts strain on the femoral neck leading to fractures. In this study, researchers look at surgical techniques that might increase the risk of a failed implant due to femoral neck fractures.
They used 64 cadaver femora (plural for femur, the thigh bone). They examined the load and shear strain placed on the femoral neck using different positions of the implant. The implant consisted of a cap that fit over the head of the femur with a short stem down into the femoral neck.
Some implants were placed with the stem aligned in an anteverted (tilted forward) position. Some were put in a retroverted (tilted backward) position. And others were implanted in a varus (angled inward) position. Placement was 10-degrees in each direction from the midline (center) of the femoral neck axis. One group (control group) had the stem of the resurfacing component lined up with the normal (anatomic) femoral axis of the neck.
Malpositioning of the femoral implant is known to be a problem. This is the first time scientists have measured changes in the load on the femoral neck caused by different positions of the implant after resurfacing.
X-rays were taken of each femur before and after implantation of the femoral component. The X-rays helped identify the center of the femoral neck and neck-to-shaft angles. Load was placed on the femora and forces measured before implanting a joint resurfacing device.
They placed three times the body weight of a person weighing about 150 pounds. The load was intended to mimic forces on the femur while standing on one leg. Then the bones were retested using the same loads after joint resurfacing.
Shear strain and amount of load needed to cause failure (fracture) were measured. Measurements were taken from 56 different points around the femoral neck. The results showed that resurfacing with placement of the implant in good alignment protects or shields the femoral neck from fracture. This is referred to as stress shielding.
But the amount of load on the femoral neck referred to as shear strain increased even with small amounts of change in angle or tilt of the implanted component. A specific surgical technique called notching of the femoral neck is used during implantation. This also put the femoral neck at a significant increased risk of fracture.
The authors point out several items to consider. First, although the cadaver models showed a strain pattern right away, bone remodeling during healing might change these results. This study could not measure those changes.
Second, only a 10-degree change in alignment was measured. The results may not be the same for any other position of the component. Third, they did not test the component in a valgus (angled outward) position. This would be the opposite of the varus position that was tested. The results might be different for a component in the valgus position.
And finally, how and why notching decreases the bone's resistance to a fracture was not tested in this study. Further studies are needed to examine each of these additional features of joint resurfacing.
The authors suggest that different loading patterns should be tested. Force applied should be changed to represent various body weights as well. And bone quality should also be examined. Many patients do not have normal strength at the time of the joint resurfacing. This factor could also affect stress shielding of the bone beneath the implant.
Thomas Parker Vail, MD, et al. Position of Hip Resurfacing Component Affects Strain and Resistance to Fracture in the Femoral Neck. In Journal of Bone and Joint Surgery. September 2008. Vol. 90. No. 9. Pp. 1951-1960.
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