Biomechanical Analysis of Pin Placement for Pediatric
Supracondylar Humerus Fractures: Does Starting Point,
Pin Size, and Number Matter?
Hilton Phillip Gottschalk, MD,* Daljeet Sagoo, DO,w Diana Glaser, PhD,*
Josh Doan, MEng,* Eric W. Edmonds, MD,* and John Schlechter, DOw
Background: Several studies have examined the biomechanical
stability of smooth wire fixation constructs used to stabilize
pediatric supracondylar humerus fractures. An analysis of
varying pin size, number, and lateral starting points has not
been performed previously.
Methods: Twenty synthetic humeri were sectioned in the mid-
olecranon fossa to simulate a supracondylar humerus fracture.
Specimens were all anatomically reduced and pinned with a
lateral-entry configuration. There were 2 main groups based on
specific lateral-entry starting point (direct lateral vs. capitellar).
Within these groups pin size (1.6 vs. 2.0mm) and number of pins
(2 vs. 3) were varied and the specimens biomechanically tested.
Each construct was tested in extension, varus, valgus, internal,
and external rotation. Data for fragment stiffness (N/mm or
Nmm/degree) were analyzed with a multivariate analysis of
variance and Bonferroni post hoc analysis (P<0.05).
Results: The capitellar starting point provided for increased
stiffness in internal and external rotation compared with a direct
lateral starting point (P<0.05). Two 2.0-mm pins were stat-
istically superior to two 1.6-mm pins in internal and external
rotation. There was no significant difference found comparing
two versus three 1.6-mm pins.
Conclusions: The best torsional resistances were found in the
capitellar starting group along with increased pin diameter. The
capitellar starting point enables the surgeon to engage sufficient
bone of the distal fragment and maximizes pin separation at the
fracture site. In our anatomically reduced fracture model, the
addition of a third pin provided no biomechanical advantage.
Clinical Relevance: Consider a capitellar starting point for the
more distally placed pin in supracondylar humerus fractures,
and if the patient’s size allows, a larger pin construct will provide
improved stiffness with regard to rotational stresses.
Key Words: supracondylar humerus fracture, pinning config-
uration, biomechanical analysis, lateral pinning
(J Pediatr Orthop 2012;32:445–451)
the most common, representing approximately 65% of
these fractures.1Historically, supracondylar humerus
fractures were treated with a variety of methods such as
closed reduction and splinting, traction, external fixation,
and closed reduction with percutaneous pinning. Wilkins
modified the Gartland classification, which is based on
the amount of fracture displacement seen on x-ray.2,3
Currently, the standard of care for most type II fractures
and all type III fractures involves closed reduction with
percutaneous pin fixation.4There continues to be debate
regarding pin configuration (lateral entry vs. cross pin fix-
ation) and number of pins for optimal stabilization.1,5–13
Because of the documented iatrogenic ulnar nerve
injury from the medial cross pin (prevalence rate up to
12%),7,11,12,14,15there has been a predilection for place-
ment of lateral divergent pins. Although recent bio-
mechanical and clinical studies have supported the
configuration of lateral smooth pin fixation5,6,8,11; there
has been wide variation in the actual starting points.
Some biomechanical studies have used a direct lateral,
extra-articular starting point, almost directly on the lat-
eral epicondyle,5,6whereas other studies have utilized a
capitellar or paraolecranon starting point (Fig. 1).8,11
This has previously been described by placing one of the
lateral pins as close to midline as possible (just lateral to
Skaggs et al11emphasized important technical
points for fixation with lateral-entry pins, specifically
maximizing the spread across the fracture site and en-
gaging sufficient bone in both proximal and distal frag-
ments. To our knowledge, no study has specifically
addressed the actual starting points for the placement of
lateral-entry pins. We retrospectively reviewed 110 cases
of supracondylar fractures at our institution, and found
that the most distal pin started within the cartilage anlage
of the capitellum in 87 patients. The remaining 23 patients
had a more direct lateral starting point (Gottschalk HP,
mong fractures around the elbow joint in the pedia-
tric population, supracondylar humerus fractures are
From the *Department of Pediatric Orthopaedic Surgery, Rady Child-
ren’s Hospital San Diego, San Diego, CA; and wChildren’s Hospital
Orange County, Orange, CA.
The authors declare no conflict of interest.
Supported by Riverside County Regional Medical Center, Department
of Orthopedic Surgery Residency Research and Education Fund and
Rady Fellowship Fund.
Reprints: John Schlechter, DO, Children’s Hospital Orange, 1310 West
Stewart Drive, Suite 508 Orange, CA 92868. E-mail: john_schlechter@
Copyrightr2012 by Lippincott Williams & Wilkins
J Pediatr Orthop?Volume 32, Number 5, July/August 2012www.pedorthopaedics.com|445
Edmonds EW. unpublished data; Fig. 2). We therefore
sought to address the question: does starting point in lat-
eral-entry pinning influence the overall construct stiffness?
Despite the vast degree of research including bio-
mechanical analysis on pin configuration, there is scant
information comparing the size of the smooth pins used
(1.6 vs. 2.0mm smooth pin).7,9Traditionally, 1.6-mm
smooth pins (0.062 inch) have been utilized for percuta-
neous pinning; however, recent literature has described the
use of 2.0-mm smooth pins (5/64 inch).7,10Another goal of
this study was to address the question: does the size of the
smooth pin directly affect the construct stiffness?
Several studies have tauted the use of a third pin to aid
in fracture stabilization.5,8,11Skaggs et al11recommended
FIGURE 1. Starting points for lateral-entry pin fixation. A, The most distal pin starts within the cartilage anlage of the capitellum,
or paraolecranon. B, The most distal pin starts extra-articular in a more direct lateral starting point.
FIGURE 2. A, Antero-posterior (AP) radiograph demonstrating a capitellar starting point of the distal pin. B, AP radiograph
demonstrating a direct lateral starting point.
Gottschalk et al J Pediatr Orthop ? Volume 32, Number 5, July/August 2012
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using a third lateral pin if there was concern about fracture
stability. Bloom et al5demonstrated that adding an addi-
tional third smooth pin would increase the construct stiff-
ness in a malreduced fracture model. In the current study,
an additional third smooth pin was added to a well-reduced,
2-pin construct to test the effect of pin number on construct
stiffness. With these data, we would also be able to identify
whether a 2 larger pin construct was equivalent or stiffer
than a 3 smaller pin construct. The purpose of this study
was to utilize a biomechanical model of a well-reduced distal
humerus fracture to compare construct stiffness for pin-
entry location, pin size, and number of pins.
Biomechanical testing was performed on 20 syn-
thetic humeri (Sawbones Model #1028, Pacific Research
Laboratories, Vashon Island, WA). There were 2 main
groups based on lateral-entry starting point: direct lateral
versus capitellar (Fig. 3). Within each group, there were 3
different fixation types: two 1.6-mm pins, three 1.6-mm
pins, and two 2.0-mm pins. To ensure consistency, all
holes were predrilled using a 1.5-mm drill bit using a
custom pin guide (Fig. 4). The trajectory of the lateral
column pin was constant for all constructs. It started
extra-articular and roughly paralleled the lateral meta-
physeal flare of the humerus.6The most distal pin was
placed so that it crossed the fracture site at the medial
edge of the coronoid fossa.6In the capitellar group, the
pin engaged the capitellum and simulated a “para-
olecranon” starting point,16as would be done in a real
patient. Once all pin trajectories were created, a cut was
made through the olecranon fossa to simulate a trans-
verse supracondylar humerus fracture. A custom-built jig
and band saw was used to make the cuts, approximately
33mm proximal to the distal most aspect of the trochlear
point. The humeri were then anatomically reduced and
fixated with 2 smooth pins; followed by a third pin after
the initial testing, and then with the 2 larger pins.
After fracture stabilization, the humeral shafts
were embedded in a 2-part epoxy resin (Bondo-Marhyde,
FIGURE 3. Schematic representation of various pin configurations used in this study.
FIGURE 4. To ensure consistency, all holes were predrilled
using a 1.5-mm drill bit and custom pin guide.
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Atlanta, GA) and secured with custom fixation rigs to a
biaxial servohydraulic MTS858 test frame (MTS Co.,
Eden Prairie, MN). The stability of the distal fragment
was then evaluated by conducting mechanical testing in
varus, valgus, extension, internal rotation, and external
rotation. For extension, varus, and valgus, constructs
were tested by applying a translational force through the
distal fragment at 0.5mm/s to a maximum of 4mm of
displacement. For internal and external rotation, con-
structs were tested at 0.5degrees/s to an end point of 10
degrees, respectively. Data for displacement (mm), force
(N), rotation (degrees), and torque (N/mm) were sampled
at 10Hz during every test. Data for stiffness (N/mm or
Nm/degree) was calculated from the loading phase of the
last 2 cycles of each test and compared.
All data were analyzed using MATLAB custom
biomechanical application. From the processed data, the
following parameters were extracted and served as de-
pendent variables: stiffness in flexion/extension, internal/
external rotation, and varus/valgus. A multivariate anal-
ysis of variance was used to evaluate the influence of pin
diameter on these parameters. Statistical significance was
determined using a P value of 0.05.
The construct stiffness data are presented in Table 1.
The capitellar starting point had a stiffer construct
compared with the direct lateral construct in internal and
external rotation (P=0.0001 for both; Fig. 5). There was
no statistical difference in varus, valgus, or extension.
Size of Pin
The 2.0-mm pins had a stiffer construct than the 1.6-
mm pins in internal and external rotation in the direct
lateral group (P=0.0001; Fig. 6). There was no statistical
difference noted in varus, valgus, or extension. However,
in the capitellar starting point group, the 2.0-mm pins
were stiffer than the 1.6-mm pins in varus, internal, and
external rotation (P=0.031, 0.0001, 0.0001, respectively).
There was no statistical difference noted in valgus or
Number of Pins
With respect to the number of pins (two 1.6-mm pins
vs. three 1.6-mm pins), no differences were noted in any of
the modes of stress in either the direct lateral or capitellar
starting point groups. When comparing two 2.0-mm pins
versus three 1.6-mm pins, the 2.0-mm pin construct was
stiffer in internal and external rotation utilizing either of the
2 starting point groups (P=0.0001; Fig. 7). There was no
statistical difference noted in varus, valgus, or extension.
The gold standard for displaced supracondylar hu-
merus fractures is in close reduction with percutaneous
pinning. The goals of treatment are to provide anatomic
reduction, stability, and prevent postoperative deformity
including cubitus varus.4Controversy remains around the
exact pin configuration. Zionts et al17demonstrated that
TABLE 1. Stiffness Data for Construct and Direction of Mechanical Loading
Values are mean±SD.
FIGURE 5. Torsional stiffness (N mm/degree) between starting points (direct lateral vs. capitellar) internal rotation (A) and
external rotation (B) (P<0.0001).
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the most stable configuration to torque resistance is a
crossed pin technique. This was noted to be 25% more
rigid than the 3 lateral pins but was not statistically sig-
nificant. However, the crossed pins were 37% stronger
than the 2 lateral parallel pins and 80% stronger than the
2 lateral pins crossing at the fracture site (P<0.05 for
both comparisons). Although cross pins seem to provide
additional torsional stability, the incidence of an ulnar
nerve injury ranges from 1% to 12%.8,11,18–20This has led
most surgeons to use only lateral-entry starting points.
Recent literature has supported the use of lateral-entry
pins both clinically and biomechanically.4–6,8,10–12,21
However, little has been documented with regard to exact
starting points on the lateral side.
Effect of Starting Point on Fracture Stability
Several biomechanical studies have illustrations
depicting a direct lateral starting point, staying extra-ar-
ticular with all pins.5,6Other studies and texts have shown
a more paraolecranon or capitellar starting point for their
most distal pins.8,16,21Our study showed an increase in
construct stiffness with a more capitellar starting point
with regard to internal and external rotation. By starting
within the capitellar anlage, several advantages are pro-
vided: (1) ability to engage sufficient bone of the distal
fragment, (2) maximize separation of the pins at the
fracture site, and (3) allow sufficient room for the place-
ment of a third lateral pin, if warranted. Anecdotally, we
looked at 110 cases of displaced type III supracondylar
humerus fractures over the last 2 years, to see whether
there was a pattern in lateral entry starting points. In 79%
of the cases, the most distal pin was placed within the
capitellar anlage/capitellum. At our institution, this pin is
supplemented with a lateral column pin (usually starting
extra-articular on the lateral side) and for type III frac-
tures, a third pin is routinely added. Because the most
distal pin communicates with the joint, all pins are re-
moved at 3 weeks, and the child is allowed to start range
of motion at that time.
Effect of Number of Pins on Fracture Stability
Proper pinning technique and understanding of the
fracture pattern is required to avoid failures. Sankar
et al22reported 7 cases of failed fixation using lateral-
entry pins; 4 due to only 1 pin in the distal fragment, 2
with inadequate pin separation at the fracture site, and 1
with no bicortical purchase. They did not report whether
there was an anatomic reduction versus a slightly malre-
duced fracture. Bloom et al5performed a biomechanical
study on fracture pinning in slightly malreduced fractures.
He concluded that anatomically reduced fractures can be
treated with 2 pins; however, an additional third pin is
prudent for slightly internally rotated fractures to im-
prove stability and prevent additional loss of reduction.
The current study supports this conclusion; we observed no
difference in construct stiffness with regard to the number of
pins placed. We were able to achieve an anatomic reduction
in each sawbone model secondary to predrilling of the pin
tracts before cutting the bone.
FIGURE 6. Torsional stiffness (N mm/degree) comparing 1.6-mm pin construct versus 2.0-mm pin construct in direct lateral
starting group. A, Internal rotation. B, External rotation (P<0.0001).
FIGURE 7. Torsional stiffness (N mm/degree) between a 2?2.0mm pin construct versus a 3?1.6-mm pin construct in direct
lateral starting group. A, Internal rotation. B, External rotation (P<0.0001).
J Pediatr Orthop ?Volume 32, Number 5, July/August 2012Does Starting Point, Pin Size, and Number Matter?
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