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External fixation

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Review Article
External Fixation: Principles and
Applications
Abstract
Jesse E. Bible, MD, MHS
Hassan R. Mir, MD
The modularity and ease of application of modern external fixation has
expanded its potential use in the management of fractures and other
musculoskeletal conditions. In fracture care, it can be used for
provisional and definitive fixation. Short-term provisional applications
include “damage control” and periarticular fracture fixation. The
risk:benefit ratio of added stability needs to be assessed with each
fixator. Soft-tissue management is critical during pin insertion to lessen
the risk of loosening and infection. Although provisional fixation is safe
for early conversion to definitive fixation, several factors affect the
timing of definitive surgery, including the initial injury, external fixator
stability, infection, and the physiologic state of the patient.
W
From the Penn State Milton S.
Hershey Medical Center, Hershey, PA
(Dr. Bible) and Vanderbilt University
Medical Center, Nashville, TN
(Dr. Mir).
Dr. Mir or an immediate family
member serves as a paid consultant
to Smith & Nephew, and serves as
a board member, owner, officer, or
committee member of the American
Academy of Orthopaedic Surgeons,
the Foundation for Orthopedic
Trauma, and the Orthopaedic Trauma
Association. Neither Dr. Bible nor any
immediate family member has
received anything of value from or has
stock or stock options held in
a commercial company or institution
related directly or indirectly to the
subject of this article.
J Am Acad Orthop Surg 2015;23:
683-690
http://dx.doi.org/10.5435/
JAAOS-D-14-00281
Copyright 2015 by the American
Academy of Orthopaedic Surgeons.
ith Hippocrates having used
an external “shackle” device
for maintaining a tibia fracture out
to length, the concept of external
fixation is more than 2,000 years
old. Today, the principles and techniques of external fixation continue
to be an essential component in every
orthopaedic surgeon’s armamentarium. In acute fracture care, its use is
separated into two categories: provisional and definitive. Provisional
fixation is further subdivided into
“damage control” and periarticular
fracture stabilization. In both of
these short-term applications, the
surgeon must consider the impact of
the fixator on the patient and any
future needs for definitive management as well as the ability to adapt
the provisional fixator into a definitive fixator if needed.
Fixator Principles and
Biomechanics
Pins
Pins, including half-pins and transfixion pins, serve a critical role in
construct stability because of their
direct link to the bone. Although the
risk:benefit ratio of added stability
needs to be assessed with each type of
fixator, many of the options to
increase stability involve the use of
pins (Table 1). When using a fixator
for definitive fixation, the ideal
construct for stability consists of
placing one pin as close to the fracture as possible, with another pin
placed as far from the fracture as
possible within the same bone. When
using a fixator for provisional stabilization, potential areas for future
definitive fixation should be considered and avoided, if possible, to
prevent the occurrence of deep
infection arising from pin tracts
located within the zone of plate
fixation.
Pin bending strength is increased to
the fourth power of the increase in the
pin’s radius.1 This gain in stiffness is
critical because decreased pin stiffness
causes increased stress at the pin-bone
interface, leading to micromotion and
ultimate pin failure.2 Therefore, the
largest diameter pin should be used;
however, to minimize the risk of
November 2015, Vol 23, No 11
Copyright ª the American Academy of Orthopaedic Surgeons. Unauthorized reproduction of this article is prohibited.
683
External Fixation: Principles and Applications
Table 1
Methods to Manipulate an
External Fixator to Increase
Stability
Increase
Diameter of pins
Number of pins used
Pin spread
Number of planes pins are placed
Diameter of rods
Number of rods
Decrease
Pin-to-fracture distance
Bone-to-rod distance
creating a substantial stress riser that
leads to a possible fracture, the pin
should not exceed one-third diameter
of the bone.1 Similarly, the advantage
of using a smaller core diameter pin
for added pullout strength must be
weighed against its reduced bending
strength.
Of the various pin coatings and
designs that have been developed,
hydroxyapatite-coated pins provide a significantly improved pinbone interface and a greater
extraction torque compared with
uncoated pins.3-6 Tapered, or conical, pins were developed to obtain
purchase on both cortices and to
yield higher insertion and extraction torques as well as sound osteointegration.4 However, concern
remains regarding their potential
for loss of fixation; if the pins back
out even minimally, their fixation
significantly diminishes because of
their shape.
The weakest point of a pin is the
thread-shank junction, which forms
a large stress riser. Therefore, placing
the pin’s junction at the pin-bone
interface, at the site of the highest
stress, should be avoided. If possible,
the shank should be buried into the
proximal cortex, doubling the pin’s
stiffness.1 In addition, soft tissues
become less irritated and inflamed
684
when only the smooth shank is
exposed.
Bars
Sidebars, or rods, form the link
between bony fragments in the fixator construct. Originally, stainless
steel and aluminum alloy materials
were used. The use of carbon fiber
rods has since become more common, and compared with stainless
steel rods, they are 15% stiffer in
loading to failure.7 However, when
carbon fiber rods are used in an
entire fixator construct, it is only
85% as stiff as one created using
stainless steel rods. This finding is
attributed to the clamps being less
effective (ie, limited by clamp tightening) in connecting to carbon fiber
rods versus stainless steel rods.
Clamps and Rings
Clamps allow for multiple degrees of
freedom and adaptability in their
connection of pins and wires to bars
and rings. Simple (ie, single) clamps
connect one pin to a rod, whereas
modular (ie, universal) clamps allow
multiple pins to be connected to a rod.
In the latter, distributing the pins
symmetrically within the clamp provides the best pin fixation strength
within the clamp.8 With the use of
modular clamps, there is the possibility of uneven holding strength on
multiple pins within the clamp, thus
interfering with the rigidity of the
fixation. This problem is avoided
with the use of simple clamps.
Ring fixators are especially useful
in fractures around the joint and in
those with significant bone loss. They
allow for dynamic axial loading (ie,
weight bearing) and joint motion
during treatment. However, their
application and utility require a substantial learning curve. Although best
popularized by their limb-lengthening
ability using multiplanar fixators,
more straightforward ring constructs
can be applied, such as the hybrid
frame. This frame combines the advantages of ring fixators in the periarticular region and the simplicity of
planar half-pins in the diaphyseal
region.
Construct Design
Fixator configurations are subdivided
according to whether they are unilateral/bilateral or uniplanar/multiplanar. Although bilateral frames (ie,
placed on both sides of the bone) are
stiffer, they can be cumbersome to
apply and hold a higher potential for
pin infection compared with unilateral frames. Similarly, uniplanar
frames are less obstructive for softtissue access but are four to seven
times weaker when stressed in the
plane orthogonal to the pins.1 In
addition to increasing the number of
planes to increase construct stability,
other methods can be employed
(Table 1). Although anatomic safe
zones may be a limiting factor, the
pins and bars should be aligned with
the bending axis of the bone. Likewise, when a ring fixator is used for
oblique fractures, placing angled pins
parallel to the fracture line to create
a structural parallelogram is more
effective at reducing shear than is the
use of transverse pins.9,10
MRI Compatibility
Surgeons should be familiar with the
MRI compatibility of the external fixation systems used in their hospitals.
Although most modern external fixation systems are MRI-compatible,
some older systems are not. Paperwork from the device manufacturer
should be readily available online to
confirm compatibility and to minimize
delays in the acquisition of MRIs for
other injuries, such as spinal injuries.
Application Technique
Significant debate exists regarding
techniques for pin insertion. Soft-tissue
Journal of the American Academy of Orthopaedic Surgeons
Copyright ª the American Academy of Orthopaedic Surgeons. Unauthorized reproduction of this article is prohibited.
Jesse E. Bible, MD, MHS, and Hassan R. Mir, MD
management is critical and should
entail choosing an anatomic site
without a large soft-tissue sleeve,
making an adequate skin incision,
spreading tissues to bone, using
cannulation during drill/pin insertion
with the use of protective sleeves, and
stabilizing soft tissues around the
pin to prevent motion. It is thought
that excessive motion of muscle and
skin around bone results in local
inflammation, thus leading to pintract infections.11 The most common method of reducing motion is
the use of a gentle compressive dressing around the pin to create a bolster between the skin and the clamp,
such as a roll of gauze wrapped
around the pin.
Thermal damage to bone is thought
to play a potential role in pin loosening. The severity of damage is
related to the maximum temperature
and the amount of time that bone is
exposed to increased temperatures
during pin insertion.12 Irreversible
changes, including osteocyte death
and alkaline phosphatase inactivation, are seen at 122°F (50°C).12-14
Methods to decrease temperatures
during pin insertion include predrilling, irrigation during drilling, and
power insertion of the pin.
A significantly longer duration of
temperatures $131°F (55°C) was
seen when a hand technique was used
for pin insertion versus a power
technique, both at 300 and 700
rpm.12 However, all techniques easily
reached temperatures $194°F (90°
C). Although this study was based on
older pins without advance flutes that
are commonly in use today, it suggests that time in contact with bone,
which causes frictional heat, plays
a significant role and should be limited by using a power technique.
More importantly, the study revealed
that predrilling before manual pin
insertion lowered temperatures by
more than half compared with pin
insertion without predrilling.12
The dogma of predrilling has been
lessened by recent pin designs that
incorporate a modified drill point
with flutes and cut lead for tapping.
Additionally, the new modified
thread pitch theoretically allows for
the advancement speed of the pin to
be controlled, thus avoiding stripping
the near cortex when the cutting tip
hits the far cortex. However, Seitz
et al15 reported a 22% reduction in
bone purchase of self-drilling pins
compared with predrilled pins. The
authors also observed a visible
“wobble factor” when predrilled
pins were inserted by hand. This
wobble leads to conical deformation
and subsequent instability of the
near cortex, causing increased stress
in the far cortex.11
Fixator Configurations
Most external fixators placed in the
United States today are for provisional
stabilization. As discussed, these
frames should be simple constructs
and should be applied with consideration of their impact on the patient’s
physiology and care, any future
definitive fixation, and implant cost.
Additionally, during the insertion of
any external fixation device within
the pediatric population, care must be
taken to avoid the epiphysis and open
physis.
Humerus
Placement of an external fixator on
the humerus can be especially useful
after revascularization, in patients
with burns or grossly contaminated
open fractures, and in obese patients
when splinting is unlikely to hold
the fracture in acceptable alignment. Pins (5 mm) are placed anterolaterally in the proximal humerus,
taking care to avoid damage to
the axillary and radial nerves, and
posterolaterally (4 to 5 mm) in the
distal humerus, avoiding the olecranon
fossa (Figure 1).
Elbow
“Floating elbows” can be stabilized
with posterolateral distal humerus pins
(4 to 5 mm) and subcutaneous proximal ulna pins (4 mm). A hinged fixator
can be used for definitive treatment of
certain periarticular fractures or ligament instability. The details involved in
its application and management are
beyond the scope of this article but can
be referenced at Chen and Julka16 and
Wegmann et al.17
Forearm
Given its subcutaneous placement,
the ulna is best used for forearm stabilization. Pins measuring 4 mm and
3 mm are subcutaneously placed
proximally and distally, respectively.
Proximal radius pin placement
should be avoided because of the
variable location of the posterior interosseous nerve. Similarly, the
superficial radial nerve is at risk during distal radius pin insertion.
Distal Radius
For distal radius fractures, restoration
of alignment can be achieved with
spanning and nonspanning external
fixation of the wrist. The proximal
pins (3 to 4 mm) should be placed
posterior to the radial artery, with the
superficial radial nerve protected.
Distal pins (3 mm) are inserted into the
base of the second metacarpal using
a small incision to identify the terminal
branches of the superficial radial nerve
and to sharply elevate off the first dorsal interosseous muscle. When tightening the construct, overdistraction of
the wrist joint should be avoided
because this can cause difficulty with
finger flexion as well as potentially play
a role in the development of complex
regional pain syndrome.18
Pelvis
An anteriorly placed fixator can be
used to close an anterior-posterior
compression injury, open a lateral
November 2015, Vol 23, No 11
Copyright ª the American Academy of Orthopaedic Surgeons. Unauthorized reproduction of this article is prohibited.
685
External Fixation: Principles and Applications
anterolateral femur and the anteromedial tibia (Figure 3, A). Proximal tibia pins should be placed at
least 14 mm distal to the articular
surface to avoid joint penetration. A
single long bar or smaller crossing
bars from each segment can be used
to span the knee joint, ensuring that
no radiopaque clamps overlie the
joint line. The fixator should be
locked into a slight amount of flexion, measuring approximately 5° to
15°. A posterior splint may be used
to provide additional stability.
Figure 1
Tibia
The plane of insertion for fixator pins into the distal humerus.
compression injury, or translate a vertical injury (Figure 2). A small incision
and dissection 2 cm posterior to the
anterior-superior iliac spine is carried
down to bone to avoid injury to the
lateral femoral cutaneous nerve. Pins
are directed posteromedially between
the inner and outer tables of the gluteal
pillar. If needed, a second pin on each
side can be placed more posterior on
the crest and angled more horizontally. It may be advantageous to use
a pin with a blunt tip to avoid penetration of the inner and outer tables.
The placement of subcristal pins into
the iliac crest in an anteroposterior
fashion is a newer technique that does
not require the use of fluoroscopy.19
Supra-acetabular pins have become
more commonly used in the acute
setting as well as in situations that
require long-term anterior external
fixation because they allow for more
powerful control of the hemipelvis.20,21 These pins require the
use of fluoroscopic imaging for safe
placement. Additional anterior
constructs have also been described
that combine iliac wing and supraacetabular pins to provide multiplanar stability.22
686
Posterior pelvic external fixation
has been described with the use of
pelvic C-clamps applied to the posterior ilium above the greater sciatic
notch; however, this procedure has
been associated with complications
related to errant pin placement if
done without fluoroscopy. An alternative construct with the use of the
pelvic C-clamp is to place the pins
into the greater trochanters so that
the C-clamp acts in the same fashion
as a pelvic binder or sheet, and the
pins can be placed relatively safely.23
Femur
Femoral shaft fractures are stabilized
using pins (5 mm) placed anterolaterally or directly lateral, both
proximally and distally. The distal pins
must be placed with consideration to
avoid the suprapatellar pouch. Lateral
pin placement does not interfere with
future prone positioning during the
treatment of other concurrent injuries.
Knee
Knee dislocations, distal femur fractures, and tibial plateau fractures are
all stabilized using pins (5 mm) in the
The subcutaneous anteromedial surface of the tibia is used to place pins
perpendicular to either the anteromedial or posterior faces of the
tibial cortex. Distal pins should be
placed using blunt dissection to avoid
injury to the anterior tibial vessels
and the deep peroneal nerve.
Periarticular Ankle
The configuration most commonly
used for significantly swollen ankle
and pilon fractures involves a delta
frame with anteromedially placed
tibial shaft pins and a transcalcaneal
pin (Figure 3, B). The insertion of
a transfixation pin, compared with
a half-pin, into the calcaneus aids in
fracture reduction and stabilization.
It is inserted distal and posterior to
the neurovascular bundle. The safest
medial calcaneus placement is posterior to the halfway point from the
posteroinferior calcaneus to the
inferior medial malleolus and posterior to the one-third mark from the
posteroinferior calcaneus to the
navicular tuberosity24 (Figure 4).
Blunt dissection should be carried
down to bone to avoid injury to the
lateral plantar and medial calcaneal
nerves. To provide for additional
stability and to prevent equinus,
a posterior splint may be applied.
Alternatively, additional pins may be
placed medially into the talar neck,
Journal of the American Academy of Orthopaedic Surgeons
Copyright ª the American Academy of Orthopaedic Surgeons. Unauthorized reproduction of this article is prohibited.
Jesse E. Bible, MD, MHS, and Hassan R. Mir, MD
cuneiforms, or first metatarsal base,
or laterally into the cuboid or fifth
metatarsal base. For cuneiform pin
placement, the pin should enter the
dorsal half of the medial cuneiform
to avoid the structures plantar to the
midfoot arch.
Figure 2
Calcaneus
A medially placed spanning external
fixator can facilitate restoration of
anatomic height and length of the calcaneus while awaiting definitive treatment. Half-pins (5 mm) are placed
medial to lateral in the distal tibia,
medial cuneiform, and calcaneal
tuberosity. After bar placement,
a laminar spreader and/or compressordistractor device is used to strategically reestablish length as well as
correct the varus and translation
deformity.25
Conversion to Definitive
Treatment
Several factors that must be considered in the timing of conversion from
external fixation to definitive treatment include (1) condition of the soft
tissues, (2) the initial injury, (3) the
need for further surgical débridement, (4) fasciotomy wounds, (5) the
condition of external fixator pins, (6)
external fixator stability, (7) bone or
soft-tissue loss, (8) vascular injury,
(9) infection, and (10) the physiologic state of the patient. With so
many variables affecting timing,
there is a paucity of data regarding
the safety and optimal timing of
delayed conversion.
For femoral shaft fractures, early
definitive stabilization is thought to
reduce the risks of decubitus ulcers,
pneumonia, and venous thromboembolic disease. Nowotarski et al26 retrospectively reviewed 59 femoral shaft
fractures (19 open) that were initially
stabilized using an external fixator
and later converted to intramedullary
nailing (IMN) at an average of 7 days
Possible pelvic external fixation options include supra-acetabular pin (A and B),
orthogonal pin construct (both iliac crest and supra-acetabular pins) (C), or
subcristal pin (D).
(range, 1 to 49 days). Overall, observational studies have reported similar
satisfactory results of early (4 to 7
days) conversion to IMN.27,28
Blachut el al29 reported on 41 open
tibia shaft fractures initially managed
with external fixation and conversion
to IMN with an average of 17 days
(range, 6 to 52 days) for fixator
placement. Two infections, two
nonunions, and one delayed union
were seen. Bhandari et al30 combined
data from prior studies in which patients underwent provisional external
fixation followed by IMN of the
femur or tibia. Based on these studies,
the authors found an average plausible infection rate of 3.6% and 9%
for acute conversion of femur and
tibia fractures, respectively. They also
reported average plausible union
rates of 98% and 90%, respectively.
It was determined that a length of
fixation of #28 days reduced the risk
of infection by 83%. This study has
many limitations: it combined several
level IV retrospective studies, and
many patients undergoing later conversion likely had other confounding
medical conditions, thus increasing
the infection rate.
Although conversion to IMN is
most frequently performed as a single
procedure, a staged conversion, or
“pin holiday,” before definitive fixation is sometimes warranted. The
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Copyright ª the American Academy of Orthopaedic Surgeons. Unauthorized reproduction of this article is prohibited.
687
External Fixation: Principles and Applications
Figure 3
External Fixation for
Definitive Management
A, Anterolateral femur and anteromedial tibia half-pins can be used to span the knee
in a tibial plateau fracture. Note that the tibia pins are placed distal to the position of the
future incision and plate placement. B, An anteromedial tibia half-pin and a calcaneal
transfixation pin can be used to create a delta frame for tibial pilon fractures.
Figure 4
Besides the use of formal open
reduction and internal fixation or
IMN, other fixation options include
limited internal fixation supplemented with external fixation and
solely definitive external fixation.
Early limited internal fixation with
external fixation can be a successful
strategy in the acute management of
difficult periarticular fractures; this
approach greatly aids in later definitive open reduction and internal fixation (Figure 5). This is especially
true in pilon fractures with a long
oblique fracture extension into the
diaphysis that can involve significant
reduction difficulties at the time of
definitive fixation (ie, 7 to 21 days
from injury).31
If external fixation is used solely for
definitive management, callus formation (ie, secondary bone healing)
around the fracture should be expected because of the relative stability
imparted by the external fixator.
However, if supplemented with internal fixation, a component of absolute
stability can be added, allowing for
primary bone healing.
Pin-site Care and Infections
Illustration demonstrating the nerves most at risk during medial calcaneal pin
placement. Note the small window for safe placement in the posteroinferior portion
of the calcaneus, although the medial calcaneal nerve (MCN) is still at risk of injury.
LPN = lateral plantar nerve, MPLPN = most posterior lateral plantar nerve, MPN =
medial plantar nerve, PTA = posterior tibial artery, PTN = posterior tibial nerve
most notable circumstance is concern
for pin-site infection. A common
practice involves removing the fixator, débriding the pin sites, placing
688
the extremity in a splint or traction,
and administering antibiotics before
returning to the operating room for
definitive IMN.
Pin-site infections have a reported
incidence ranging from zero to
66.7%.32-37 Mahan et al38 reported
on 214 pins examined at time of pin
removal; 74.8% had bacteria present
on them, including 37.5% with virulent Staphylococcus aureus and
9.4% with Escherichia coli. The
authors also found a significant
correlation between loose pins and
infection, supporting the belief that
soft-tissue motion is an important
factor leading to infection.
Methods of pin-site care vary considerably. Lethaby et al39 looked at all
studies since 1950 regarding pin-site
care. The authors found insufficient
Journal of the American Academy of Orthopaedic Surgeons
Copyright ª the American Academy of Orthopaedic Surgeons. Unauthorized reproduction of this article is prohibited.
Jesse E. Bible, MD, MHS, and Hassan R. Mir, MD
Figure 5
A pilon fracture definitively treated with limited internal fixation and external fixation in a patient with uncontrolled diabetes.
Preoperative AP (A) and lateral (B) radiographs, AP (C) and lateral (D) radiographs following external fixator placement
during initial hospitalization, and postoperative AP (E) and lateral (F) radiographs after definitive treatment with limited
internal fixation and external fixation.
evidence that any particular strategy
of pin-site care minimizes infection
rates.
Given the high incidence of pin-site
complications, any problematic pin
site should raise suspicion of a pinsite infection and be dealt with
accordingly. Checketts et al40 devised
a classification system for pin-site
infection that aids in the formulation
of treatment options (Table 2). It
should be stressed that if a pin is
found to be loose, it should be
removed and replaced depending on
the effect on overall construct
stability.
Summary
External fixation has a vital role in
both provisional and definitive fracture fixation. In provisional stabilization, the surgeon must consider the
impact of the fixator on the patient’s
care and definitive management, as
well as the ability to adapt the provisional fixator into a definitive fixator if needed. The basic principles
and techniques of external fixation
should be applied to maximize
the fixator’s potential to promote
Table 2
Pin-tract Infection Classification and Treatment40
Grade
1
2
3
4
5
6
Appearance
Treatment
Slight erythema, little discharge
Erythema, discharge, and pain in
soft tissue
Grade 2 but no improvement
with antibiotics
Soft-tissue infection involving
several pins
Grade 4 and radiographic evidence
of bone involvement
Infection after fixator removal
(clinical and radiographic)
fracture healing and
potential complications.
minimize
References
Evidence-based Medicine: Levels of
evidence are described in the table of
contents. In this article, references
33, 35, and 39 are level I studies.
References 32, 34, and 36 are level II
studies. References 18, 19, 26-30,
and 38 are level III studies. References 1-17, 20-24, 31, 37, and 40 are
level V expert opinion.
Improved pin care
Topical and/or oral antibiotics
Remove pin and change antibiotic
regimen
Remove any loose pins
Remove entire fixator construct
and curettage pin tract
Débridement, irrigation, and
systemic antibiotics
References printed in bold type are
those published within the past 5
years.
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Copyright ª the American Academy of Orthopaedic Surgeons. Unauthorized reproduction of this article is prohibited.
689
External Fixation: Principles and Applications
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Journal of the American Academy of Orthopaedic Surgeons
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