Holding power impaired in rheumatoid femoral heads. Cadaveric study of fracture fixation devices

A measurement was made of the holding strength and the energy needed to extract a NoLok hip screw, a von Bahr screw, and a Hansson hookpin from cadaveric femoral heads. The specimens were obtained from female subjects aged 65 years or more, with 36 specimens each from rheumatoid (RA) and nonrheumatoid (non-RA) donors. Retraction of the implants was made by a continuous uniaxial pullout at 10 mm/min. For each type of device, the holding strength in rheumatoid femoral heads was less than in non-RA specimens. In rheumatoid specimens the maximum holding strength for the NoLok screw (1,622 N) was higher than that of the other two devices, whereas the von Bahr screw (1,177 N) had a higher maximum holding strength than the Hansson hook-pin (603 N). In non-RA, there was no difference in maximum holding power between the NoLok screw (2,549 N) and the von Bahr screw (2,282 N); however, both had a higher holding strength than the Hansson hook-pin (851 N). A rapid fall off was experienced in the force required to continue extraction of both types of screws, whereas for the Hansson hook-pin the strength decreased slowly. For each type of device, the energy needed for extraction of the implant was less in the RA group femoral heads, while there were no differences in total extraction energy between devices.