Screw design alters the effects of stress relaxation on pullout

Stress relaxation during pullout of a pedicle screw decreases the peak load and stiffness of the bone-screw interface. However, it is unknown whether this can be generalized to all types of screw designs. This study aimed to show whether screw design altered the effects of stress relaxation on the mechanical performance of the pedicle screw during pullout. Twelve calf vertebrae were obtained: six vertebrae were instrumented with 7.5x40 mm conical pedicle screws and the other six with 5.0x40 mm cylindrical pedicle screws. The screws with two different designs were pulled out using either a standard pullout or a stress relaxation pullout protocol. Both bone-screw interfaces had lower stiffness in the stress relaxation pullout model than in the standard pullout model, but it was significant in only the cylindrical design group (P<0.05). However, the stress relaxation and standard pullout models did not yield any difference in peak loads in either screw type. Although stress relaxation at the bone-screw interface can alter the mechanical performance of the screw, this may be eliminated by modifying the screw design. A better understanding of viscoelastic properties of the bone-screw interface may help improve implant design and thus, clinical outcomes.     

Vertical transmission of Kaposi's sarcoma

AIDS related Kaposi's sarcoma is commonly seen in homosexual men, only occasionally in men and women with heterosexually acquired HIV, and extremely rarely in children. The case of an HIV infected mother and her vertically infected child who both developed visceral Kaposi's sarcoma is reported. It is proposed that the putative Kaposi's sarcoma agent may also be transmitted vertically.