dual plating of humeral shaft fractures: orthogonal plates biomechanically outperform side-by-side...

BASIC RESEARCH

Dual Plating of Humeral Shaft Fractures: Orthogonal PlatesBiomechanically Outperform Side-by-Side Plates

Victor Kosmopoulos PhD, Arvind D. Nana MD, MBA

Received: 26 June 2013 / Accepted: 5 November 2013

� The Association of Bone and Joint Surgeons1 2013

Abstract

Background Single large-fragment plate constructs cur-

rently are the norm for internal fixation of middiaphyseal

humerus fractures. In cases where humeral size is limited,

however, dual small-fragment locking plate constructs may

serve as an alternative. The mechanical effects of different

possible plate configurations around the humeral diaphysis

may be important, but to our knowledge, have yet to be

investigated.

Questions/purposes We used finite element analysis to

compare the simulated mechanical performance of five

different dual small-fragment locking plate construct con-

figurations for humeral middiaphyseal fracture fixation in

terms of (1) stiffness, (2) stress shielding of bone, (3) hardware

stresses, and (4) interfragmentary strain.

Methods Middiaphyseal humeral fracture fixation was

simulated using the finite element method. Three 90� and

two side-by-side seven-hole and nine-hole small-fragment

dual locking plate configurations were tested in compres-

sion, torsion, and combined loading. The configurations

chosen are based on implantation using either a posterior or

anterolateral approach.

Results All three of the 90� configurations were more

effective in restoring the intact compressive and torsional

stiffness as compared with the side-by-side configurations,

resulted in less stress shielding and stressed hardware, and

showed interfragmentary strains between 5% to 10% in

torsion and combined loading.

Conclusions The nine-hole plate anterior and seven-hole

plate lateral (90� apart) configuration provided the best

fixation. Our findings show the mechanical importance

of plate placement with relation to loading in dual-plate

fracture-fixation constructs.

Clinical Relevance The results presented provide novel

biomechanical information for the orthopaedic surgeon

considering different treatment options for middiaphyseal

humeral fractures.

Introduction

Locking plates for fracture fixation offer biologic and

mechanical advantages and therefore have increased in

popularity. Locking the screws to the plate allows the plate

to sit at a distance offset from the underlying bone surface

providing a biologic advantage for bone fracture healing by

preserving the periosteal blood supply underlying the plate

[12, 13, 26, 31]. Mechanically this provides stability without

Each author certifies that he or she, or a member of his or her

immediate family, has no funding or commercial associations

(eg, consultancies, stock ownership, equity interest, patent/licensing

arrangements, etc) that might pose a conflict of interest in connection

with the submitted article.

All ICMJE Conflict of Interest Forms for authors and Clinical

Orthopaedics and Related Research editors and board members

are on file with the publication and can be viewed on request.

Clinical Orthopaedics and Related Research neither advocates nor

endorses the use of any treatment, drug, or device. Readers are

encouraged to always seek additional information, including FDA-

approval status, of any drug or device prior to clinical use.

This work was performed at the University of North Texas Health

Science Center, Fort Worth, TX, USA.

V. Kosmopoulos (&), A. D. Nana

Bone and Joint Research Center, Department of Orthopaedic

Surgery, University of North Texas Health Science Center,

3500 Camp Bowie Boulevard (CBH 407), Fort Worth,

TX 76107, USA

e-mail: [email protected]

A. D. Nana

Department of Orthopaedic Surgery, John Peter Smith Hospital,

Fort Worth, TX, USA

123

Clin Orthop Relat Res

DOI 10.1007/s11999-013-3379-7

Clinical Orthopaedicsand Related Research®

A Publication of The Association of Bone and Joint Surgeons®

the need for the plate to match the curvature of the bone

surface and without the need to compress and maintain

friction between the plate and bone surface [4, 7, 31].

Currently, the majority of diaphyseal humeral fractures

are treated nonoperatively. In the subset treated opera-

tively, one large-fragment plate construct is considered the

norm for internal fixation. The smaller size of the humerus

in some patients, however, limits the diaphyseal shaft

length and/or diameter available for fixation, and therefore

makes using a large-fragment plate difficult. Difficulties

that arise include the number of screws that can be placed,

a resulting bulky fixation with undesirable stress shielding

[19], and having to precontour the large-fragment plate to

match the diverging anatomy of the humeral metaphysis.

Recent literature suggests that dual small-fragment

plating constructs may be mechanically superior to one

large-fragment plate construct [32] and may have a role in

the fixation of certain fracture patterns. Furthermore, small-

fragment plates for humeral shaft fracture fixation have

shown promising clinical results [18]. Questions remain,

however, regarding how plate placement on the diaphysis

affects the mechanical performance of the dual-plate con-

struct, necessitating further research. Plates typically are

implanted using either a posterior or anterolateral

approach. The posterior approach allows for direct obser-

vation of the fracture and posterior and lateral plate

placement but requires the nerve to be dissected out

because it is in the middle of the operative field. In con-

trast, the anterolateral approach avoids direct observation

of the nerve and allows for anterior and lateral plate

placement.

We used a finite element (FE) method, a computational

simulation approach, to compare the mechanical perfor-

mance of five different dual small-fragment locking plate

constructs for humeral shaft fracture fixation under three

different loading regimens. The five dual-plate placement

configurations chosen are based on implantation using the

aforementioned surgical approaches. Using the different

constructs and the three loading regimens (eccentric com-

pression, torsion, and combined) in a FE humerus fracture

model, we aimed to determine the best plate configuration by

assessing (1) stiffness, (2) bone stress shielding, (3) hardware

stress, and (4) interfragmentary strain.

Materials and Methods

Six (one intact and five fixation) FE models were created

and analyzed using COMSOL Multiphysics1 (Version 4.3;

COMSOL, Inc, Burlington, MA, USA). The intact humeral

cortical bone diaphysis was idealized as a hollow cylinder

with a constant cross section. Dimensions were based on

reported mean values from an MRI study of 20 volunteers

with an average age of 37 years [16]. The length of the

tested portion of the diaphysis was set to 237 mm and the

diameters of the canal and outer bone cortex were 12.1 mm

and 19.3 mm, respectively [16]. Cortical bone was modeled

as an orthotropic material characterized by nine independent

technical constants (E1 = 12.0, E2 = 13.4, E3 = 20.0 GPa; G12

= 4.53, G23 = 6.23, G13 = 5.61 GPa; m12 = 0.376; m23 = 0.235,

m13 = 0.222) [2, 3] with density set to 1817 kg/m3 [22].

To study a worst-case scenario such as an unstable

comminuted fracture, a 1-cm transverse fracture gap was

created at the middiaphysis of the intact model (simulat-

ing an Orthopaedic Trauma Association 12.C.2 fracture)

[1, 5, 6, 18, 19, 23, 30, 32]. Fracture fixation was per-

formed using five different seven-hole and nine-hole small-

fragment (3.5 mm) locking-plate configurations as follows:

(1) nine-hole plate anterior and seven-hole plate lateral, 90�apart (Fig. 1A); (2) nine-hole plate lateral and seven-hole

plate anterior, 90� apart (Fig. 1B); (3) nine-hole plate

anterolateral and seven-hole plate posterolateral, 90� apart

(Fig. 1C); (4) nine-hole plate anterolateral and seven-hole

plate posterolateral, side by side 65� apart (Fig. 1D); and

(5) nine-hole plate posterolateral and seven-hole plate

posteromedial, side by side 65� apart (Fig. 1E). The plates

in all of the aforementioned models were centered to the

fracture gap and locked at a 1-mm offset distance from the

outer cortical bone cortex [1, 5, 25]. The plates are fixed

using a total of eight bicortical locking screws [18, 25, 31]

sequentially alternating between the two plates (four

screws/plate) resulting in all the models having the same

number of fixation points and equal working lengths. Such

a staggered placement of the hardware has been suggested

to reduce the risk of plate fracture [17]. Bicortical screws

were chosen to stabilize the screw-bone interface [6, 7].

The generic locking plates and screws were modeled using

stainless steel material properties (E = 193.0 GPa; m = 0.3;

q = 8000 kg/m3) [24]. Geometric properties of the seven-

hole and nine-hole small-fragment locking plates and

screws are similar to FDA-approved commercial hardware

(Table 1).

Three different loading conditions were applied to test

the fixation constructs [8, 11]: (1) eccentric compression;

(2) torsion; and (3) combined eccentric compression and

torsion (Fig. 2). The eccentric load was inferiorly directed

and applied in 20-N increments to a maximum of 100 N a

distance 40 mm posteromedial to the central longitudinal

humeral axis at a rotation of 23.3� from the frontal plane

[10]. This off-center eccentric loading produces combined

bending and compressive loads on the humeral diaphysis.

Torsion was applied in 1.0-Nm increments from 0.5 Nm to

a maximum of 4.5 Nm along the central longitudinal axis

of the humerus. Combined loading included simultaneous

application of the eccentric compressive and torsional

loads (eg, first load step = 20 N compression and 0.5 Nm

Kosmopoulos and Nana Clinical Orthopaedics and Related Research1

123

torsion; last load step = 100 N compression and 4.5 Nm

torsion).

All outcomes, other than stiffness, are reported at the

maximum loads simulated. The extrinsic compressive stiff-

ness for each simulation was calculated as the slope of the

applied compressive load to deformation curve. An analo-

gous rotational stiffness was calculated as the slope of the

applied torsional load to the resulting rotational deformation.

Stiffness of each of the fixation constructs was compared

with the intact humerus stiffness for each respective load-

ing regimen with positive percentile differences denoting

increases and negative percentile differences denoting

decreases in stiffness. Von Mises stress distributions were

computed for the bone, screws, and plates. Average bone

stress analysis results for each of the fixation constructs were

compared with the intact humerus model. In this manner,

negative percentile changes denote decreased bone tissue

stresses as compared with the intact humerus (indicative of

bone stress shielding after fixation). To characterize the load-

sharing performance of the construct for the screws, two

additional measures were calculated. The first measure,

denoted as the maximum-minimum range, is calculated as

the difference in mean stress between the highest and lowest

stressed screws. The second, the screw-to-screw fluctuation,

is calculated as the difference in mean stress between adja-

cent screws. Lower values in these measures represent better

load-sharing characteristics among the screws. Finally, the

interfragmentary strain was characterized by dividing the

interfragmentary motion by the fracture gap size. The

interfragmentary motion for each construct was calculated

Fig. 1A–E The (A) nine-hole plate anterior and seven-hole plate

lateral, 90� apart (Model A); (B) nine-hole plate lateral and seven-

hole plate anterior, 90� apart (Model B); (C) nine-hole plate

anterolateral and seven-hole plate posterolateral, 90� apart (Model

C); (D) nine-hole plate anterolateral and seven-hole plate posterolat-

eral, side by side 65� apart (Model D); and (E) nine-hole plate

posterolateral and seven-hole plate posteromedial, side by side 65�apart (Model E) fixation construct configurations are shown. The

humeral diaphysis is light gray and the plates and screws are dark

gray. Lateral and anterior directions are oriented to the left and

bottom, respectively.

Table 1. Geometric properties of the small-fragment plates and

screws used in Models A to E

Description Length

(mm)

Width

(mm)

Hole

spacing

(mm)

Number

of holes

3.5-mm small-

fragment

9-hole plate

140 11 14.5 9

3.5-mm small-

fragment

7-hole plate

111 11 14.5 7

Core

length

(mm)

Core

diameter

(mm)

Head

height

(mm)

Head

diameter

(mm)

Locking screw 22 2.7 3.2 6.8

Fig. 2A–C The nine-hole plate anterior and seven-hole plate lateral,

90� configuration (Model A) is used to illustrate the simulated (A)

eccentric compression, (B) torsion, and (C) combined loading

conditions. The force (F) was applied in 20-N increments to a

maximum of 100 N. Torsion was applied in 1-Nm increments from 0.5

Nm to a maximum of 4.5 Nm. The superior (Sup) and inferior (Inf)

cross-sectional bone surfaces used to calculate the interfragmentary

strain also are shown.

Dual Plate Configurations

123

by adding the maximum three-dimensional displacement of

the cross-sectional bone surface superior and cross-sectional

bone surface inferior to the fracture (Fig. 2).

Results

All three of the 90� configurations (Models A through C)

were nearly equally effective in restoring the intact com-

pressive stiffness showing less than a 2% difference in

compression and combined loading, respectively (Fig. 3A).

As a result of geometric equivalency in torsion, Models A

through C behaved equally as did Models D and E. Models

A through C were more effective however, in restoring the

intact torsional stiffness (Fig. 3B). Model E, with the plates

placed side by side posteriorly, was the only construct to

exceed the intact compressive stiffness.

All of the fixation models resulted in some degree of

stress shielding by redistributing the load and consequently

reducing the average stress on the humerus (Fig. 4). The

side-by-side configuration from Model D showed the least

stress shielding in compression, whereas Models A through

C, with the 90� configuration, were better in torsion and

combined loading.

The fixation plates in Models A through C were, on

average, less stressed in torsion and combined loading than

the plates from the side-by-side constructs (Models D and

E). Model E however, with the posteriorly placed plates,

was significantly more effective in reducing plate stresses

in compression (Fig. 5). The highest stress concentrations

were located near and around the unused screw holes for

each plate and at the neck of the screws just below the

plates for all the constructs studied (Fig. 6). Generally, as

evidenced by the lower mean screw stresses, Models A

through C were better at reducing screw stress in torsion

and combined loading than the side-by-side plate con-

structs (Models D and E) (Table 2). Using the screw-to-

screw fluctuation and maximum-minimum range measures

however, Model E showed the best load-sharing charac-

teristics between screws in combined loading.

The 90� configurations tested in Models B and C

resulted in the highest (9.4%) and lowest (7.4%) inter-

fragmentary strains in combined loading, respectively

(Fig. 7). In torsion, the 90� constructs (Models A through

C) resulted in lower interfragmentary strains than the side-

by-side constructs (Models D and E). Models D and E

however, showed the highest (5.0%) and lowest (4.4%)

strain at the fracture site in compression, respectively.

Fig. 3A–B The relative stiffness of each of the tested constructs as a

percentage of (A) the intact compressive (317 N/mm) and com-

bined compressive (293 N/mm) stiffness, and (B) the intact torsional

(5 Nm/degree) and combined torsional (5 Nm/degree) stiffness are

shown. Positive percentile differences denote increases and negative

percentile differences denote decreases as compared with the intact

stiffness.

Fig. 4 The percentages of change in mean von Mises bone stress as

compared with the intact humeral stress in compression (3.4 MPa),

torsion (5.4 MPa), and combined loading (6.7 MPa) are shown. The

results are shown for each of the five construct configurations tested

(Models A through E) in maximum compression (100 N), torsion

(4.5 Nm), and combined (100 N, 4.5 Nm) loading. In torsion, as a

result of geometric equivalency, results are identical among Models A

to C and between Models D and E.


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Discussion

The smaller size of the humerus in some patients may limit

application of the more commonly used large-fragment

plate constructs. In such cases, large-fragment plates may

limit the number of screws that can be inserted (ie, less

holes/unit length), and lead to increased stress shielding

from the greater mismatch in load transfer between bone

and plate [19, 32]. Dual small-fragment plate constructs

may offer a promising alternative. Dual 3.5-mm locking

plates offer advantages over one large fragment plate

including: (1) the 3.5 mm plate is more easily contoured;

(2) the 3.5 mm plate width easily accommodates small

bone diameters; and (3) dual 3.5 mm plates require a

smaller incision and working length compared with one

large fragment plate. The biomechanical benefits related to

different placement configurations of the two plates,

however, remain unanswered. Using FE modeling, we

compared the performance of five different small-fragment

dual-plate configurations for fixation of middiaphyseal

humeral fractures by evaluating (1) stiffness, (2) bone

stress shielding, (3) hardware stresses, and (4) interfrag-

mentary strain.

Limitations

The results presented are based on the specific plates and

screws modeled and may not be representative for other

plates and screws. Nevertheless, the tested plates and

screws were sized to be similar to what currently is com-

mercially available. Additionally, the use of FE modeling

offers the advantage of studying bone stress shielding as an

Fig. 5 The mean von Mises plate stress comparisons among each of

the five construct configurations tested (Models A through E) in

maximum compression (100 N), torsion (4.5 Nm), and combined

(100 N, 4.5 Nm) loading are shown. In torsion, as a result of

geometric equivalency, the results are identical among Models A

through C and between Models D and E.

Fig. 6A–B The von Mises hardware stress distributions at the

maximum combined load simulated (4.5 Nm torsion, 100 N

compression) for the best-performing configuration with the (A)

nine-hole plate anterior and seven-hole plate lateral, 90� apart (Model

A); and worst-performing configuration with the (B) nine-hole plate

anterolateral and seven-hole plate posterolateral, side by side 65�apart (Model D). Increasing element stress magnitudes are illustrated

from red to blue in the color bar legend and scaled to a maximum of

300 MPa to allow for direct visual comparison.

Table 2. Mean screw stress, fluctuation, maximum-minimum range,

and locations for the highest stressed screws*

Loading Model

A B C D E

Compression

Mean (MPa) 9.5 9.6 9.6 11.6 6.7

Screw-to-screw (MPa) 1.5 1.6 1.6 2.5 2.5

Maximum-minimum (MPa) 5.4 5.5 5.4 8.8 4.4

Screw locations with highest stress 3, 6 4, 5 4, 5 4, 5 2, 7

Torsion�

Mean (MPa) 37.4 37.4 37.4 42.7 42.7




Combined

Mean (MPa) 38.9 38.9 38.9 44.7 43.8




* Details in Materials and Methods; �because of geometric equiva-

lency in torsion, the measures for Models A through C and for Models

D and E were equal.


123

outcome measure. Similar to an experimental cadaveric

study, the computational simulations presented do not

evaluate the in vivo bone remodeling response expected

after internal fixation. Clinically, the mechanics of fixation

are expected to change with stress shielding (eg, bone

resorption) or bone fracture healing, especially with union

of the fracture gap. Fatigue, like with remodeling, is

another time-related response that was not simulated in this

study. Although fatigue microdamage and crack propaga-

tion would provide additional valuable information, it also

would add to the complexity of this numerical study and

outcome variables considered [14]. Screw fixation, for

example, was fixed in the models presented. Clinically,

after cyclic loading, the behavior at the screw-bone inter-

face may weaken [9]. Even with these limitations,

however, the comparisons as presented provide worthwhile

and novel information for the orthopaedic surgeon con-

sidering different treatment options. The stress results from

the study, for example, detail the changes in load transfer

for the bone and each part of the construct otherwise

difficult to measure clinically and experimentally.

Construct stiffness is of high importance. It governs the

performance of the fixation system as indicated, for exam-

ple using the other interrelated outcomes measures reported

(bone stress shielding, hardware stress, and interfragmentary

strain). Excessive stiffness of the fixation construct reduces

stress and strain on the bone and may lead to bone resorption

[13, 28] and in time screw loosening and construct failure

[4, 7, 15, 25]. However, excessive reductions in stiffness may

lead to increasing screw and plate stress and early fatigue

failure of the construct [7, 25, 26]. Stiffness resulting from

the compressive loading simulations of the humerus may be

especially important during crutch weightbearing [18, 19].

Moreover, torsional loading also is of interest in the analysis

of humeral fracture fixation constructs because it has been

reported as a predominant loading mode and possible cause

for nonunion of humeral fractures [7, 8, 29, 30]. In com-

pression and torsion, and consequently combined loading,

the 90� configurations (Models A through C) were found to

outperform the side-by-side constructs in stiffness recovery

as compared with the intact humerus. In our simulations, the

stiffness was relatively insensitive to exchanging the seven-

hole and nine-hole plates, as is done between Models A and

B, or rotating the 90� configuration, as is done between

Models A and C. In contrast, the side-by-side posterior

placement of the plates (Model E), closer to the site of

compressive load application, reduced bending loads on the

plates and increased the construct stiffness beyond that of the

intact humerus.

Ideally, when loaded, the fixation construct design

should be balanced by reducing bone stress shielding yet

maintaining adequate fixation. Model E, with the highest

compressive stiffness, also had the highest levels of aver-

age stress shielding for all three of the loading conditions

tested. Relative to each other however, all the model

configurations tested resulted in less than a 5% difference

in stress shielding. Although in compression and combined

loading the tested configurations resulted in an approxi-

mately 40% to 45% reduction in bone tissue stress, this was

not the case in torsion. The configurations were less

effective in shielding bone shear stress. As recommended

in the literature [1, 5, 25], all of the locking plates modeled

were offset 1 mm from the cortex avoiding undue stress

shielding and contact below the plate. This advantage with

locked plates has been suggested to prevent local bone

necrosis [20].

The stability of the fixation system is influenced by

hardware factors including the number of screws, type of

screws (ie, bicortical, unicortical), working length, plate

offset from the bone cortex, and placement of the hardware

[1, 5–7, 18, 25, 31]. Other than plate placement, these

aforementioned hardware variables were controlled in this

study based on recommendations from the literature. Four

screws per fragment were used based on findings that

additional screws did not show a significant increase in

torsional stiffness [25]. In gauging performance, a goal of

the fixation system should be to reduce and more evenly

distribute the applied stress among the hardware compo-

nents of the construct [27]. This in turn will help limit

stress risers and extend the fatigue life and strength of the

system. O’Toole et al. [18], comparing single-plate locking

and nonlocking 3.5-mm small-fragment constructs for

humeral shaft fixation, reported both constructs withstood

strenuous fatigue and axially failed above anticipated

physiologic loads. Results from all of the dual-plate

Fig. 7 The results for percent interfragmentary strain for the five

construct configurations tested (Models A through E) are shown. The

results are reported at the maximum compressive (100 N), torsional

(4.5 Nm), and combined (100 N, 4.5 Nm) loads. In torsion, as a result

of geometric equivalency, the results are identical among Models A

through C and between Models D and E.


123

locking constructs compared in our study show the

highest stress concentrations occurring at the neck of the

screws just below the screw head and plate. These mod-

eling results confirm clinical findings suggesting this may

be the most likely location for hardware failure [27, 29].

This is especially true for the highest stressed screws

which varied in location based on plate configuration and

type of loading. In compression, Models A and E gener-

ally outperformed the other constructs by more evenly

distributing the applied load among the two plates, with

the larger nine-hole plates sustaining a slightly greater

portion of the stress. As evidenced by the lower mean

screw stress, screw-to-screw fluctuations, and maximum-

minimum ranges, these models also outperformed the

other constructs with respect to load sharing between

screws. However, the highest stressed screws in Model A

were located adjacent to the fracture on the nine-hole

plate. In contrast, the highest stressed screws in Model E

were at the far ends of the seven-hole plate. This is

indicative of the greater bending loads on Model A and

the greater axial loads on Model E. Thus, understanding

how the plate will be loaded based on its placement is

important to reduce hardware stress and improve the load-

sharing characteristics of the construct. This may help

prolong and/or prevent screw failure.

Fixation with locked plate constructs aims to minimize

motion while tolerating an increased fracture gap [4]. Some

motion at the fracture site is favored to promote secondary

bone healing [4, 21, 27]. Secondary bone healing has been

reported to occur when interfragmentary strain is kept

between 2% to 10% [4]. Although biologic responses were

not addressed in our study, all of the tested configurations

did satisfy this interfragmentary strain criteria for all the

loading regimens simulated. Strain at the fracture site was

highest (exceeding 7%) for all the models when combined

loading was applied.

Based on the simulations performed and relative

comparison between outcomes from this study, the 90�configuration with a nine-hole plate placed anteriorly and

a seven-hole plate placed laterally (Model A) was found

to mechanically outperform the side-by-side constructs

and slightly outperform the other 90� configurations

studied. As loading of the humerus in vivo is likely a

combination of compression and torsion, this configura-

tion was one of the most effective in restoring the intact

stiffness and reducing bone stress shielding and hardware

stress while meeting the suggested interfragmentary strain

criteria. Future studies are needed comparing large-frag-

ment plates, nonlocking small-fragment plates, and nail

fixation to the proposed dual small-fragment constructs to

help establish optimal fixation for middiaphyseal humeral

fractures. Although further clinical studies are needed

to confirm our main findings, the mechanical findings

presented using dual 3.5-mm small-fragment locking

plate configurations are promising.

Acknowledgments We thank the University of North Texas Health

Science Center for providing the computational resources needed to

complete this study.

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dual plating of humeral shaft fractures: orthogonal plates biomechanically outperform side-by-side...

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