Page 222 - Advances in Biomechanics and Tissue Regeneration

218 11. ANALYSIS OF THE BIOMECHANICAL BEHAVIOR OF INTRAMEDULLARY NAILING

bones, the classic and traditional anterograde nails introduced in a retrograde manner have shown better results in
terms of stability of this distal fracture of the femur compared to those specifically designed as retrograde nails for
this type of fracture [20].
Despite the new designs, anterograde nails continue to be used with good clinical results in these complex fractures,
although some nails have increased the number of distal screws and the possibility of placing them on different planes
of space to increase the stability of the fracture [21]. With respect to its diaphyseal location, Montanini et al. [22] on an
FE model of the diaphyseal fracture treated with intramedullary nails found that immediately after the fracture, the
loading should not exceed the critical tensions of breakage of the implant into the hole in the proximal screw. These
stresses in the nail disappear once the fracture is consolidated, which has been clinically confirmed [23–25].
Comparing only the biomechanical behavior of the intramedullary nails and osteosynthesis plates in fractures at the
diaphyseal location using FE models, plates can allow greater stability to the focus of the fracture, while nails suffer
increased deformities in the monopodal support. The increase in the diameter of the nail is critical because an increase
of 2mm decreases the deformation of the nails by 40% [26] and therefore increases the chances of failure. This work
confirms the results published by Heiney et al. [27] in the case of unstable distal fractures of the femur, where com-
paring nails with plates showed that there was a greater chance of failure of the implant in nails than in plates.
A debated and unsolved point is nail locking. On some models, the proximal screw is only one oblique from the
greater to the lesser trochanter, while in other models there are two proximal transverse screws. Placement depends on
the type of fracture, the type of screw, its diameter, stresses caused by screws and brittle points in the nail holes of
unplaced screws, its proximity to the fracture focus, placement plans, etc.; these points remain clinically and biome-
chanically unclarified. In terms of the diameter of the screws, there must be a compromise between a maximum value,
which does not exceed 50% of the diameter of the nail to ensure its resistance [28], and a minimum value, which ensures
their resistance to the loads and stresses to which they are subjected. The smaller is the diameter of the nail, smaller
should be the diameter of the screws. This is why some models of unreamed nails have increased their diameter to
allow larger diameter screws.
The proximity of the distal screws to the fracture focus is an important point. The closer the screws are to fracture
focus increases the stresses and forces supported by screws, while if they are further away from the focus of fracture,
rotational stability improves and their chances of failure are lower [29]. They must always be positioned perpendicular
to the axis of the nail with at least two screws, except in transverse fractures without comminution in which a unique
screw may be sufficient.
Wahnert et al. [30] compared the stability achieved with different types of distal screws (screws and coiled sheets)
and with a nail plate using an artificial model of osteoporotic femoral distal fracture subjected to rotational and axial
loads. The conclusion is that in these fractures and against rotational stresses, distal locking of the nail with four screws
with different angles is greater regarding the stability granted to the fracture than other types of locking systems with
two screws lateral to medial or using a coiled sheet, and is similar to the stability granted with the nail plate. Locking
with four distal screws obtained the best biomechanical results in terms of joint stability against rotational and axial
stresses, and results on distal bolts have been confirmed in clinical studies [21]. Chen et al. [19], in a biomechanical study
using FEs and artificial bones, explored the stresses in the screws and the rigidity of osteosynthesis in a retrograde nail
in the treatment of distal femoral fractures. They came to the conclusion that distal screws are more important with
respect to the stability of the fracture than proximal screws. A screw placed next to a fracture increases the rigidity
of the mounting in oblique fractures, although this increase is not transcendent in transverse fractures and an unplaced
screw determines an increase in stresses in the whole nail by 70%, facilitating their failure by breakage of the implant.
Nail material has also been studied. P erez et al. [31] examined the biomechanical behavior of nails of stainless steel
and titanium using an FE model in femoral fractures in children. The model is not applicable to fractures in adults or to
the behavior of the intramedullary nails, but the conclusion is that titanium behaves best, since stainless steel creates
stress-shielding areas in the bone that increase the risk of refracture once implants are removed. However, Kaiser et al.
[32] obtained different conclusions in terms of nail material. In this case, they compared intramedullary nails of steel or
titanium in the stabilization of diaphyseal femoral fractures using artificial bones, and concluded that steel allows
greater stiffness to the mounting and that titanium must only be used in cases of allergy to metals or in cases where
future scans by magnetic resonance imaging are needed. The behavior of different materials for intramedullary nails in
the treatment of diaphyseal fractures of the distal femur in adult has not yet been studied.
In conclusion, primitive stainless steel anterograde intramedullary nails with a static combination of screws are
nowadays the reference treatment for femoral fractures from zones 2 to 5 of Wiss; however, there has been no clear
demonstration of the superiority of other nails and techniques specifically designed for the treatment of certain frac-
tures. Broad discussions on the material to be used (stainless steel or titanium), on the route of entry of the nails (anter-
ograde or retrograde), reamed or unreamed, and the placement and types of distal screws depending on the type of
fracture to treat are still ongoing.

I. BIOMECHANICS