A thirteen-year-old boy fell on the right forearm and sustained 25°
dorsally displaced radial and ulnar diaphyseal fractures. On the same day,
closed reduction and flexible intramedullary nailing of both bones was
performed with 2.5-mm titanium nails (Bäramed Instrumente, Schwenningen,
Germany) with the patient under general anesthesia. In accordance with our
protocol, the nails were not bent before insertion, no cast was applied, and
the patient was discharged on the third postoperative day.
One month later, the patient returned with marked forearm bowing after
another fall on the same arm. Radiographs demonstrated a 21° angulated
refracture of both bones (Fig.
1). The malalignment was easily reduced with the patient under
general anesthesia with the nails in situ. The angulation was slightly
overcorrected to achieve an anatomical axis of both the radius and the ulna.
In total, one additional cycle of deformation was applied to the nails. The
limb was placed in a cast for one week, and then immobilization was
discontinued at the patient's request. Five months later, the fractures had
healed without complications and the nails were removed
(Fig. 2).
To evaluate the mechanical strength of previously bent flexible
intramedullary nails, we subjected ten titanium and eighteen stainless steel
3.0-mm nails (Bäramed Instrumente) to controlled bending by firmly
clamping one end of the nail to a stable surface and subjecting the other end
to an increasing lateral bending force
(Fig. 3). We evaluated changes
in tensile strength, proof stress, and plastic strain with use of two simple
representative parameters: spring constant (D = bending force [N]/deflection
[°]) and point of first plastic deformation.
As an analogy to our clinical case, a lever arm of 16 cm was used. For
every increment of 6 N, the degree of elastic deformation was recorded up to
the point of plastic, irreversible, bowing. At that time, the nail was
manually bent to a deformity of 21° and then was reduced back to the
original straight position in order to emulate the intervention performed in
our patient. The experiment was subsequently repeated with the previously bent
nail. Finally, the nail was subjected to five cycles of reversed bending to
21° to evaluate if repetitive deformation causes implant fracture in
vitro.
Average forces to achieve plastic deformation and modified spring constants
were compared between the native and previously bent nails with use of the
Wilcoxon and the t test for dependent samples, respectively.
Macroscopic examination of the deformed area of the bent implants after one
cycle of reversed bending to 21° demonstrated no evidence of metal
fracture or fatigue, such as orange peeling (Lueder's lines), kinking, or
necking. Furthermore, the state of the polish appeared to be unaffected and
without surface irregularities, which would indicate damage to the passivating
oxide layer that protects contemporary orthopaedic biomaterials from
accelerated corrosion. Even five cycles of reversed bending of the steel and
titanium nails did not cause implant fracture.
The average force required for permanent deformation of the previously bent
nails (an indicator of proof stress) was reduced significantly by 37% for both
the titanium nails and the stainless steel nails (from 21 ± 3.2 to 13.2
± 2.5 N for the titanium nails [p < 0.01] and from 25 ± 2.3
to 15.7 ± 4.7 N for the stainless steel nails [p < 0.001]) after one
cycle of reversed bending to 21° (Fig.
4). The average modified spring constant (an indicator of
resistance to bending or stiffness) decreased by 15.1% (from 0.814 ±
0.054 to 0.691 ± 0.071 N/°) for titanium nails (p < 0.001) and
by 12.2% (from 0.991 ± 0.218 to 0.870 ± 0.156 N/°) for
stainless steel nails (p < 0.001) (Fig.
5). Overall, the steel nails were stiffer and stronger than the
titanium nails were.
Proof stress in metals is defined as the stress that leads to a 0.2%
permanent plastic deformation. The term tensile strength signifies the stress
at implant failure, whereas the term elongation refers to the relative change
in length at this point. Intramedullary nails are produced from cold drawn
wire, rendering high proof strength as well as elongation, generally referred
to by material scientists as toughness and ductility, respectively. Metals of
this type usually show a reduction of cross section when strained repeatedly
into the plastic range, ultimately leading to instability by collapse.
However, this was not evident even after five cycles of reversed bending in
the present study.
On the basis of this background, the conditions for a successful
realignment of bones by reversing the accidental plastic bending of the
implant must meet the following conditions. First, one cycle of reversing the
plastic deformation to the original geometry must not produce microfractures,
necking, or any other deterioration of the polished surface of the implant as
such changes not only would cause failure but also would promote corrosion.
Second, loss of strength after one cycle must not lead to inherent
instability, which would promote fracture nonunion.
Even in our trauma center, where more than 100 flexible intramedullary
nailing procedures are performed annually, a refracture with the nails in
place is only rarely encountered. Therefore, no formal guidelines for the
management of this situation are available. In isolated cases, operative
replacement of the nails has been
advocated2-4.
In one report, an ulnar nonunion occurred in association with closed reduction
of the refracture with the nails in situ, requiring decortication and plate
osteosynthesis5.
However, no conclusive evidence was given with regard to the cause of the
nonunion.
Closed reduction with the intramedullary nails in place is easily
performed, requires minimal time, and avoids another invasive procedure. The
disadvantages, as shown in the present study, are compromised proof stress as
demonstrated by the lower forces needed for plastic deformation of the
previously bent nails and the small, but significant, difference in spring
constants. Therefore, the patient who is treated with closed reduction of a
bent intramedullary nail should be instructed to avoid any excessive forces to
the forearm until fracture union has been documented radiographically. On the
basis of the results of the present study, the decrease in mechanical
stability is sufficient to call for extra precautions, such as casting for
three to four weeks. Many clinicians recommend routine additional bracing in
any case, even after the initial
operation6.
It is our practice not to apply a cast after the primary flexible
intramedullary nailing of a forearm fracture. The advantage of this approach
includes a rapid return to daily activities, an ability to participate fully
in school activities if the dominant extremity is affected, and avoidance of
the discomfort and inconvenience of cast immobilization. Postoperative pain is
minimal and is easily controlled with nonsteroidal analgesic medication. The
patient is instructed to avoid excessive loading of the involved limb until
adequate callus formation is observed on radiographs made at approximately
four weeks and is advised to refrain from sports for six to eight weeks. The
nails usually are removed by means of an outpatient procedure after four to
six months. On the average, we perform about ten forearm fracture-nailing
procedures per year. The fracture in our patient was the first forearm
refracture with the flexible intramedullary nails in place at our institution
in four years.
An important question is whether routine prebending of flexible
intramedullary nails for the stabilization of femoral, humeral, or tibial
fractures compromises their stability. This maneuver is standard practice to
provide the desired three-point fixation in such cases. However, because the
nail is only subjected to the first half of the bending cycle that was
simulated in our study, the loss of mechanical stability should be less. On
the other hand, repetitive bending of the nail before insertion probably
should be avoided.
For standardization and methodological purposes, 3.0-mm nails were used for
the experiment instead of the 2.5-mm nails that had been used in our patient.
While different diameters may have different absolute spring constants and
yield strength, the relative change in these parameters after bending should
remain the same.
To our knowledge, we are the first to describe the successful closed
reduction of an angulated secondary forearm fracture with flexible
intramedullary nails in situ. However, our discussions with colleagues have
suggested that many pediatric orthopaedic surgeons have had similar
experiences. As long as precautions are taken to avoid excessive strain on the
reduced nails, the maneuver is safe, effective, and minimally invasive. If
maximum postoperative stability is desired, as in the case of active athletes
who wish to return to their sport as soon as possible, replacement of the bent
nails may be a better option.