J. Biomedical Science and Engineering, 2010, 3, 35-38
doi:10.4236/jbise.2010.31005 Published Online January 2010 (http://www.SciRP.org/journal/jbise/
JBiSE
).
Published Online January 2010 in SciRes. http://www.scirp.org/journal/jbise
How cross screw length influences the stiffness of
intramedullary nail systems
S. V. Karuppiah1, A. J. Johnstone1, D. E. T. Shepherd2
1Orthopaedic Unit, Aberdeen Royal Infirmary, Aberdeen, UK;
2School of Mechanical Engineering, University of Birmingham, Edgbaston, Birmingham, UK.
Received 25 October 2009; revised 20 November 2009; accepted 25 November 2009.
ABSTRACT
Fractures of long bones are commonly treated with
intramedullary (IM) nails and they have been shown
to have a very high success rate. Recently we have
concerns with the use of the newer IM nailing sys-
tems, that uses longer cross screws, which have been
developed with variation in implant designs. We be-
lieve that the newer implants provide less fracture
stability, due to decreased stiffness of the IM nailing
system. The aim of this study was to biomechanically
determine the influence of the length of cross screw
on the stiffness of the IM nailing system, using a
composite model. Our test results confirmed our sus-
picion that the newer IM nailing system using longer
cross screw-length is less stiff than traditional nailing
systems using shorter cross screw length, during axial
loading.
Keywords: Intramedullary Nail; Mechanical Testing;
Screw; Stiffness
1. INTRODUCTION
Orthopaedic implants have been popular in the man-
agement of fractures for the last six decades. There are
various types of implants available for fixation of bone
fractures, intramedullary (IM) nails being one of them.
These are popular for the fixation of diaphyseal (shaft)
fractures of the femur, tibia and humerus with high suc-
cess rates. It is one of the accepted and most widely used
methods of treating, transverse and short oblique, axially
stable fractures of the femoral diaphysis [1,2,3,4]. It pro-
vides an excellent and prompt return of function with a
low rate of complication and non-unions [5].
With the introduction of inter locking screws (cross
screws) it has been also possible to provide rotational
stability and precise reconstruction of the anatomy in
communited fractures using the IM nailing system. The
IM nailing system provide stiffness and stability of the
bone segments, all factors which are considered to be
essential preconditions for its increased success of bone
union, while maintaining the function of the joints and
soft tissues [6,7]. From time to time further changes in
the design of the implant has been made, including the
placement of the cross screws, to extend the use of
femoral IM nails in all types of femoral fractures.
There is recent concern in our clinical practice re-
garding the use of newer femoral IM nailing system,
using longer cross screw-length, designed to accommo-
date in extreme distal femoral metaphyseal fractures as
compared to the traditional system which were designed
to be used in diaphyseal fractures.
The anatomy of the human distal femoral bone is
wider in diameter compared to the narrow diaphyseal
bone. In order to accommodate the distal wider femoral
bone fragment, the newer intramedullary nailing system
uses longer cross screw-length that could be threaded
from one cortex to the other.
The concept of stability is crucial in fracture surgery.
Stability determines the amount of strain (i.e. relative
change in fracture gap) at the fracture site, and strain
determines the type of healing. Excessive strain could
result in delayed or non-union of the fractures [8]. We
believe that the longer cross screw-length, used in the
newer IM nailing system, provides less stability to the
fracture fixation due to decreased stiffness of the IM
nailing system. The aim of this study was to determine
the influence of the cross screw length on the stiffness of
fracture fixation when using IM nailing systems.
2. MATERIALS AND METHODS
2.1. Materials
The intramedullary nails were simulated using stainless
steel tubes with a length of 150 mm, outside diameter of
12 mm and inside diameter of 10 mm. These dimensions
were similar to intramedullary nails used surgically. The
distal hole in the intramedullary nail was 10 mm from the
distal end. The distal bone end was simulated using
stainless steel cylinders. In the study three different cylin-
ders were used to represent different parts of the femur,
with the dimensions comparable to clinical measure

36 S. V. Karuppiah et al. / J. Biomedical Science and Engineering 3 (2010) 35-38
SciRes Copyright © 2010 JBiSE
Figure 1. Test set-up.
ments using x-rays:
 outside diameter 50 mm, wall thickness 5 mm,
height 50 mm (proximal diaphyses bone with a nar-
row canal and a thick cortical wall);
 outside diameter 75 mm, wall thickness 3 mm,
height 50 mm (metaphyseal junction);
 outside diameter 100 mm, wall thickness 3 mm,
height 50 mm (distal condylar bone with a wide
canal and a thin cortical wall).
The hole in the cylinder was 25 mm from the top end
of the cylinder.
2.2. Methods
An Instron 1822 materials testing machine (Instron Ltd.,
High Wycombe, UK) was used for mechanically testing
the intramedullary nails. A customised stainless steel
clamp was designed to connect the proximal portion of
the intramedullary nail to the cross-head of the testing
machine. The stainless steel cylinder, which represented
the femur, was attached to the base of the testing ma-
chine. The distal end of the intramedullary nail was se-
cured to the cylinders with a single stainless steel rod of
diameter 5 mm; this represented the cross screw. The test
set-up is shown in (Figure 1).
The intramedullary nail was subjected to an axial
compressive load, by lowering the cross-head of the
testing machine; the load was applied at a rate of 0.05
N/s. During the testing load and displacement was re-

S. V. Karuppiah et al. / J. Biomedical Science and Engineering 3 (2010) 35-38
SciRes Copyright © 2010 JBiSE
37
0
0.5
1
1.5
2
2.5
0.0 0.10.2 0.30.4 0.5 0.60.7 0.80.9 1.0
Displacement (mm)
Load (kN)
C
A
B
Figure 2. Graphs of load against displacement for a cylinder of outside diameter: A) 50 mm. Curve
fit y=7.1293x4 - 11.956x3+7.7119x2 + 1.0647x+0.0035, R2 = 0.9995; B) 75 mm. Curve fit
y=0.1341x4-0.4407x3+ 0.3588x2+0.5726x-0.0012, R2 = 0.9999; C) 100 mm Curve fit y =
0.2709x4-0.7248x3 + 0.6727x2+0.0352x + 0.0091, R2 = 0.9998.
corded. Testing continued until a maximum displace-
ment of 1 mm or the maximum force of 2 kN was
reached. A displacement of 1 mm was chosen; as the rod
and the cylinder underwent mechanical deformation be-
yond 1mm. A load of 2 kN represents about three times
body weight for a 70 kg individual [9]. Each test con-
figuration was tested three times.
Graphs of load against displacement were plotted
(Figure 2 shows a typical example) and a forth-order
polynomial fitted to the results. This polynomial fit en-
ables us to use the same fit for all data. The stiffness was
then determined from the gradient of the graph at a dis-
placement of 0.5 mm.
3. RESULTS
The mean stiffness of the intramedullary nail system
decreases with increasing diameter of the cylinder,
which represents the femoral bone. The mean (± stan-
dard deviation) stiffness of the system was 3298 ± 144
N/mm, 657 ± 10 N/mm and 297 ± 16 N/mm at diameters
of 50 mm, 75 mm and 100 mm, respectively.
4. DISCUSSION
The principle aim of fracture fixation is to provide sta-
bility of the bone fragments and restoration of normal
anatomy. Intramedullary nails act as a scaffold and are
devised to hold together the proximal and distal ends of
the long bone for a conducive environment for fracture
healing. The placement of the cross screw and the design
of the implant can influence the stability of fracture fixa-
tion and hence bone union [10,11].
The proximal (diaphyseal) femoral canal is narrow in
which the intramedullary nail has a ‘snug fit’ against the
bone. Hence, the inter bone-nail distance is minimal or
none. In the current study it has been shown that with
this set-up it has a mean stiffness of 3298 N/mm. With
increasing bone diameter, the stiffness of the system
decreases, leading to an increased deflection for a given
load. At the distal condylar part of the femur, there is a
wide canal and a thin cortex. The distance of the cross
screw from the bone is increased in this case.
In the clinical situation, when the patient bears weight
the greater the diameter of the canal, the greater the de-
flection will be. As shown in this experiment, there is a
proportionate decrease in axial stiffness of the intrame-
dullary nail with increasing cross screw-length. Longer
cross screw-length, used in the newer intramedullary
nailing system to treat distal femoral fractures, will be
far less stiff during weight bearing than the traditional
IM nailing system, which uses shorter cross screw-
lengths. The newer IM nails hence provide less stability
of fracture fixation and may potentially cause delayed
union or non-union when used to treat femoral fractures.
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