ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

3.1 Lag-screw
Table 1. Material Properties.
Figure 2 shows a schematic representation of the lag-screw. We proposed a new lagscrew
design, which included four concave cutouts from a main screw with sub screw
that fit inside one of the concave sections. This designed is named the Rectangle Double
Lag-Screw System. This design reduced the cross-sectional area of the entire lag-screw
when compared with a traditional double-screw system, and the inhibition of femoral
head rotation could also be maintained. Moreover, insertion of the lag-screw into the
relatively small femoral head of a person of Japanese heritage was easily done. Each
lag-screw was composed of three parts with material properties from two PEEK types.
The PEEK of the lowest elastic modulus is placed in the middle portion as a buffer
material so it allows for the elastic deformation of the long axis direction. For the FEM
analysis the separate parts were connected with a bonded condition.
Fig 2. Schematic drawing of Rectangle Double Lag-screw System 2-A shows the
Rectangle Double Lag-screw System. 2-B shows cross section of the Rectangle
Double Lag-screw System. 2-C shows an exploded view of Rectangle Double
Lag-screw System.
3.2 Stem
Figure 3 shows a schematic representation of the stem. The stem is composed of four
parts, the high section, the lag-screw insertion section, the bending section and the
lower section. The connecting surfaces of each part were designed in a cone-shape. The
PEEK with the highest elastic modulus was used in the lag-screw insertion section. The
PEEK with the medium elastic modulus was used immediately proximal and distal to
the lag-screw insertion section and the PEEK with the lowest elastic modulus was used
in the lower section. This allows for elastic deformation in the direction of the long axis
and at the distal end of the stem. For the FEM analysis the separate parts were
connected with a bonded condition.