Locked plate fixation of osteoporotic humeral shaft fractures: are two locking screws per segment enough?

Abstract

The purpose of this study was to compare the biomechanical behavior of using two versus three locking screws per bone segment in a cadaveric humerus fracture gap model.
Six matched pairs of elderly osteoporotic fresh-frozen human cadaveric humerii were used. An eight-hole locking compression plate was placed posteriorly on the humeral shaft and secured with either four or six bicortical locking screws. A 5-mm middiaphyseal gap osteotomy was created to simulate a comminuted fracture without bony contact. Specimens were tested in offset axial compression, four-point anteroposterior bending, four-point medial-lateral bending, and torsion. After the initial testing in each of these modalities, the constructs were cyclically loaded in torsion and again tested in the four loading modalities. Lastly, the fixation constructs were then tested to failure in torsion.
There were no significant differences in stiffness between the group fixed with two screws per segment and the group fixed with three screws per segment. The peak torque to failure was higher in the four-screw construct compared with the six-screw construct. The mean torque to failure was 23.5 +/- 3.7 Nm in the construct with two locking screws per segment compared with 20.4 +/- 2.8 Nm in the construct with three locking screws per segment (P = 0.030).
The addition of a third screw in the locked plate construct did not add to the mechanical stability in axial loading, bending, or torsion. In testing to failure, the addition of a third screw resulted in lower load to failure.

Full text

Page 1

Locked Plate Fixation of Osteoporotic Humeral Shaft? ?
Fractures: Are Two Locking Screws Per Segment Enough?? ?
David J. Hak, MD, MBA, Peter Althausen, MD, MBA, and Scott J. Hazelwood, PhD
Objective: The purpose of this study was to compare the
biomechanical behavior of using two versus three locking screws
per bone segment in a cadaveric humerus fracture gap model.
Methods: Six matched pairs of elderly osteoporotic fresh–frozen
human cadaveric humerii were used. An eight-hole locking
compression plate was placed posteriorly on the humeral shaft and
secured with either four or six bicortical locking screws. A 5-mm
middiaphyseal gap osteotomy was created to simulate a comminuted
fracture without bony contact. Specimens were tested in offset axial
compression, four-point anteroposterior bending, four-point medial–
lateral bending, and torsion. After the initial testing in each of these
modalities, the constructs were cyclically loaded in torsion and again
tested in the four loading modalities. Lastly, the fixation constructs
were then tested to failure in torsion.
Results: There were no significant differences in stiffness between
the group fixed with two screws per segment and the group fixed with
three screws per segment. The peak torque to failure was higher in the
four-screw construct compared with the six-screw construct. The
mean torque to failure was 23.5 6 3.7 Nm in the construct with two
locking screws per segment compared with 20.4 6 2.8 Nm in the
construct with three locking screws per segment (P = 0.030).
Conclusions: The addition of a third screw in the locked plate
construct did not add to the mechanical stability in axial loading,
bending, or torsion. In testing to failure, the addition of a third screw
resulted in lower load to failure.
Key Words: locking plate, humerus shaft fracture, biomechanics,
osteoporosis
centered
mineral density (g/cm2) was calculated using software from
the manufacturer (Hologic, Inc., Bedford, MA). An estimation
patient’s T-score was calculated by comparing the
calculated humeral shaft bone mineral density with standard
values for young adult forearms.
The proximal and distal ends of the humerii were
potted colinearly within methylmethacrylate blocks to provide
a substrate for securing the bone during testing. Mechanical
was performed in a manner previously described
in the literature.5 Each pair of humerii was initially tested
of the
testing
INTRODUCTION
Humeral shaft fractures are relatively common injuries
in the aging population.1 The majority of humeral shaft
fractures can be treated nonoperatively with high union rates
and good functional results.2 In cases in which internal fixation
is indicated, plate fixation results in a high union rate and a low
incidence of complications.3
Osteoporosis compromises screw fixation and leads to
increased failure rate after plate fixation. In fractures initially
treated nonoperatively that fail to heal, disuse leads to
increased osteopenia further compromising screw fixation.
Ring et al have demonstrated that nonunions of the upper
extremity in the elderly are extremely disabling.4 These
individuals are less able to adapt to changes and depend on an
intact upper extremity to maintain function and independence
in activities of daily living.
Locking plate fixation is being increasingly used for the
treatment of fractures in patients with osteoporosis; however,
questions remain about how many locking screws are
necessary to achieve adequate fixation. The purpose of this
biomechanical study was to examine the resistance to
displacement of paired humerii with transverse gap osteoto­
mies that have been internally stabilized with a locking plate
fixed with either two or three bicortical locking screws per
fracture segment.
MATERIALS AND METHODS
Fresh matched cadaveric humerii pairs were harvested
from six donors that included three males and three females
ranging in age from 75 to 87 years with a mean of 82 years.
Specimens were cleaned of soft tissues, covered in saline-
soaked gauze, and frozen to 220�C until preparation and
testing. The prepared specimens were kept moist with normal
saline during testing. Specimens were imaged by standard
radiographs and dual energy x-ray absorptiometry (DXA). For
DXA scanning, a standard region of interest 40 cm long
on the middiaphysis was selected and the bone

Page 2

nondestructively on an Instron (Norwood, MA) mechanical
testing machine. Baseline load versus deformation curves were
obtained for the intact humerii using three loading modalities;
offset axial compression, four-point bending (in both
posterior–anterior and lateral–medial directions), and torsion.
Stiffness of the construct was determined from the linear
portion of the load deformation curve.
A test fixture with the upper loading supports spaced
5 cm apart and the lower loading supports spaced 15 cm apart
was used for four-point bending. Loads with a maximum
force of 200 N were applied to the humerii in a lateral–medial
and posterior–anterior direction. Offset axial loading was
performed by securing the distal potted block in a custom
fixture and loading the proximal humerus 4 cm medial to its
central axis. A maximum force of 100 N was applied to
simulate an inferiorly directed vertical force on the humeral
head surface. Torsion was tested by applying a maximum load
of 4.5 Nm in external rotation along the central longitudinal
axis of the humerus. During torsional testing, the specimens
were held rigidly without an applied axial load.
The humerii were then cycled in torsion between 0 and
4.5 Nm at 1 Hz for 1000 cycles to simulate upper extremity use
during the early postoperative period and then retested in the
three loading modalities to obtain load versus deformation
curves.
Right and left paired humerii were randomly instru­
mented with an eight-hole narrow large-fragment locking plate
(Synthes, Paoli, PA). The plate was positioned on the posterior
aspect of the humeral shaft and centered at the middiaphyseal
level. A locking drill sleeve was threaded into the plate and
a 4.3-mm diameter drill bit used to drill a hole at right angles to
the long axis of the plate. Five-millimeter diameter locking
screws were inserted and tightened to a maximum torque of
3.4 Nm. Screws longer than would be chosen clinically were
used to ensure that the far cortex was fully engaged by the
threaded portion of the screw. Paired humerii were instru­
mented with either two bicortical screws (placed in holes 1, 3,
6, and 8), or three bicortical screws (placed in holes 1, 2, 3, 6,
7, and 8) per fracture segment. A 5-mm transverse gap
osteotomy was then created with a hand saw at the midpoint of
the plate to simulate a comminuted fracture without cortical
bone contact (Fig. 1).
The specimens were then tested nondestructively in
the manner described previously. Finally, each humerus was
loaded until failure in torsion to determine the maximum
torque to failure. The site and pattern of fixation failure were
recorded.
The resistance to displacement was determined by the
slope of the load versus deformation curves for the two
constructs under the three loading sequences before and
after cycling. Statistical analyses were performed on both the
stiffness values and on normalized stiffness values expressed
as a percentage of the intact stiffness, which were obtained
before instrumentation to minimize any differences between
right- versus left-sided variations. An analysis of variance was
performed to determine differences between the measured
stiffness values and normalized stiffness values for each
plate at each loading modality before and after cyclic loading
(P , 0.05 significant). In addition, statistical analyses were
performed using paired t tests to determine the significance of
any differences between the two plates in stiffness before
cyclic loading, stiffness after cyclic loading, and maximum
torque to failure (P , 0.05 significant).
RESULTS
Humerii ranged in length from 28 to 34 cm with a mean
of 32 cm. Pretesting radiographs demonstrated no evidence of
fracture or pathologic lesion in any specimen. DXA scanning
was consistent with osteoporotic bone in all specimens with
estimated T-scores ranging from 22.3 to 24.9 with a mean of
23.7. Two-tailed t test demonstrated no inherent right–left
differences in the intact samples before plating.
There were no significant differences between the
precycle stiffness or normalized stiffness and those after
1000 cycles of physiological loading (Table 1, Figs. 2–5).
Specimens were least stiff relative to the intact bone in
anteroposterior bending. Bending in this direction corresponds
to the plate’s smallest moment of inertia (thinnest cross-
section). Constructs were stiffest in medial–lateral bending.
Bending in this direction corresponds to the plate’s largest
moment of inertia (thickest portion of the plate). In addition,
constructs tested in medial–lateral bending were significantly
stiffer than their corresponding intact values (Table 1).
TABLE 1. Summary of Mean Stiffness Values (6 Standard
Deviations) in Different Modes of Mechanical Testing
Two Screws
Loading Mode
per Segment
Three Screws
per Segment
Axial loading (N/mm) Intact stiffness: 602 6 290
Before cycling
After cycling
Torsional loading (N-mm/degree) Intact stiffness: 682 6 131
Before cycling
After cycling
Four-point anteroposterior bending (N/mm) Intact stiffness: 489 6 202
Before cycling 101 6 39
After cycling 104 6 43
Four-point medial–lateral bending (N/mm) Intact stiffness: 424 6 145
444 6 210
509 6 196
454 6 128
578 6 296
498 6 121
492 6 74
553 6 95
606 6 181
132 6 41
111 6 17
Before cycling
After cycling
521 6 67
555 6 50
571 6 90
593 6 58
FIGURE 1.
specimen
a
segment (screws placed in holes 1, 3, 6, and 8). In the
contralateral humerus, a third bicortical screw was placed
in each segment (screws placed in holes 1, 2, 3, 6, 7, and 8).
fixed with two bicortical locking screws per
Five-millimeter transverse osteotomy gap in

Page 3

Axial Stiffness 4 Point AP Stiffness
post-cyclepre-cycle
0+---<..:..:.:...:..:.:.
60.,------------------,
10
50 +-----+--------------1
c::
Q)
E
'(3
Q)
Co
VI
_
u
40 +------+---------+----1
30 +-----1-
ltl-
.:
'0
20
post-cyclepre-cycle
20
o +-----'~~
60
80
100 +---+--+-------l
120 +---+--+------+---+-----1
180 -,-----------------,
160 +------------+-----1
<f:.
40
~
140 +-----=------=;=~__+--__i
E
'(3
Q)
Co
VI
t)
ltl-
....o
c::
Torsional Stiffness
4 Point ML Stiffness
post-cyclepre-cycle
o +-----''-'--'-''-'-'-'
50
100
250 .,------------------.,
200 +----------+---+------1
l::
Q)
E
'(3
Q)
Co
ltl­
....o
<f:.
150 +---+--=±=-----=±::=
VI-
l::
U
post-cyclepre-cycle
100
80
l::
Gl
E
'0
Gl
Co
III
ti
ltl
:5
....
0
60
40
~0
20
0
FIGURE 2. Normalized axial load testing showed no significant
difference in stiffness between humerii instrumented with two
locking screws per fragment versus three locking screws per
segment.
There were no significant differences in stiffness or
normalized stiffness between the group fixed with two
screws per segment and the group fixed with three screws per
segment for any loading modalities before or after cyclic
testing. Similar values were seen in all modes of testing
(Figs. 2–5).
The peak torque to failure was higher in the four-screw
construct compared with the six-screw construct. The mean
peak load was 23.5 6 3.7 Nm in the four-screw constructs and
was actually less, 20.4 6 2.8 Nm, in the six-screw constructs.
A paired t test revealed a P value of 0.030.
All final failures in the four-screw constructs con­
sisted of spiral fractures through the distal screw hole.
Spiral fractures also occurred in the six-screw constructs
with four occurring through the distal screw hole and two
FIGURE 4. Normalized four-point anteroposterior bending
showed no significant difference in stiffness between humerii
instrumented with two locking screws per fragment versus
three locking screws per segment.
through the middle screw hole. There were no cases of
screw pullout.
DISCUSSION
When using locked plate fixation, the number of screws
and the number of cortices needed per segment continues to
be a topic of debate. In this study, we found no mechanical
advantage for using more than two locking screws per bone
segment in an osteoporotic humerus fracture gap model. In
this gap osteotomy model, the mechanical properties of the
construct appeared to be most influenced by the mechanical
properties of the plate between the two inner locking screws.
The distance between the two inner screws was not altered
between the two groups. The torque to failure was actually less
FIGURE 3. Normalized torsional load testing showed no
significant difference in stiffness between humerii instru­
mented with two locking screws per fragment versus three
locking screws per segment.
FIGURE 5. Normalized four- point medial–lateral bending
showed no significant difference in stiffness between humerii
instrumented with two locking screws per fragment versus
three locking screws per segment.

Page 4

when three screws were used compared with two. The three
closely spaced screws may create a linear stress riser leading to
reduced resistance to fracture.
Authors have provided various recommendations for the
minimum number of locking screws and the minimum number
of cortices required per segment. Based on the clinical obser­
vation of radial and ulnar shaft radiolucencies at the bone–
screw interface with the PC-Fix (Point Contact Fixator, Synthes,
Paoli, PA) implant, which uses unicortical screw fixation,
Hertel et al recommended obtaining at least three cortices per
segment.6 In good quality bone, Gautier and Sommer have
recommended using a minimum two screws per segment with
at least three cortices for simple fractures and at least four
cortices for comminuted fractures. In other cases such as
osteoporotic bone, they have recommended a minimum of three
screws per segment.7 In osteoporotic bone, Wagner has
recommended the use of at least three locking screws in each
main fragment with at least one of them being bicortical.8
Because rotational forces predominate in the upper extremity,
Stoffel et al recommended the use of three or four locking screws
in each main humerus fragment to improve torsional rigidity.9
Screw location and plate length may be as important as
screw number, because it influences the construct stiffness and
amount of motion seen at the fracture. Stoffel et al examined
different screw configurations using a 12-hole 4.5-mm
titanium locking compression plate in composite synthetic
bone cylinders. They found that axial stiffness and torsional
rigidity were mainly influenced by the distance between the
fracture site and the closest screw. Moving the screw one hole
farther from the fracture the construct became almost twice as
flexible in compression and torsion.9 Similar observations
have been noted with nonlocking plate fixation. In a study
using composite foam blocks, To ¨rnkvist et al showed that the
bending strength of screw–plate fixation can be increased by
using a longer plate with screws spaced further apart.10 In
a cadaveric ulnar osteotomy model, Sanders et al concluded
that the length of the plate is more important than the number
of screws and stated that once the working length (defined as
the distance between the fracture and the nearest screw) is
minimized and the plate length maximized, only two screws
need to be inserted on each side of the fracture.11 Similarly, in
our study using the same length plate, we found no advantage
to the placement of a third locking screw in between two
equally spaced screws.
Internal fixation of long bone fractures in the elderly
patient is challenging secondary to the problems of osteo­
porotic bone.4 In osteoporotic bone, poor screw purchase
results in sequential loosening of nonlocked screws and sub­
sequent levering of the plate away from the bone. Loss of
nonlocked screw purchase in osteoporotic bone is an impor­
tant factor leading to failure of internal fixation of humeral
shaft fracture fixation. The development of locking plates has
provided an alternative to standard compression plates. Lock­
ing plates can provide fracture fixation without the undesirable
effects on periosteal vascularization and mechanical draw­
backs that are encountered with standard compression plates.12
In a retrospective clinical study comparing the use of
locked plates and standard plates in the treatment of humeral
nonunions and delayed unions, the authors suggested that
locking plates may be a more reliable implant. There was one
hardware failure in the 14 patients treated with standard plates
but no failures in the 19 patients treated with a locked plate
construct.13 Ring et al have reported on 24 patients with
osteoporosis with humeral diaphyseal nonunions or delayed
unions treated with a locking compression plate.14 Twenty-two
of the patients united with the initial surgical treatment,
whereas the remaining two that initially received demineral­
ized bone healed after the addition of iliac crest bone graft.
The authors noted no loss of fixation or implant breakage.
They used a combination of standard screws to bring the plate
to the bone, and locking screws, but did not specifically state
the number of locking screws used per segment. Regardless of
the number of locking screws used, it is important that locking
screws are placed accurately, because deviation in the inse­
rtion angle greater than 5� can significantly decrease the
fixation stability.15
In standard plates, the biomechanical effect of different
fixation constructs in torsion differs from that seen in bending.
To ¨rnkvist et al showed that in torsion, the fixation strength was
dependent on the number of screws securing the plate.10 In
comparison, in the locking plate construct, we did not see
an improvement in the fixation stability with the addition of
a third screw per fracture segment. The locking plate
eliminates screw pullout as a means of failure in torsion as
demonstrated by our study.
One potential advantage of the locked plate construct is
prevention of screw loosening through repeated loading.
Although we did cycle the implants to simulate sequential
load, we limited this to 1000 cycles to maintain the integrity of
the cadaveric bone. No difference was observed between the
precycle and postcycle testing results. This lack of effect has
been noted by other authors during humeral biomechanical
testing. 16 It is possible that a difference in pre- and postcycling
stiffness in any modality may have been detected with greater
numbers.
There are several limitations of the present study. Our
model examined placement of only one length plate with
specific screw configuration. The distance between the two
inner most screws remained unchanged between the two
groups. This screw position, which represents the plate work­
ing length, plays a critical role on the mechanical properties
of the construct. Although DXA scanning was performed on
these specimens obtained from an elderly population, this
method does not necessarily correlate with standard hip, wrist,
and spine DXA values. Microfractures can occur during non­
destructive testing, especially in osteoporotic bone, and this
may have influenced subsequent modes of mechanical testing.
We did not perform any radiographic evaluation after mech­
anical testing, which may have identified such microfractures.
Finally, a significant limitation of this study was the small
sample size, resulting in statistical analysis with relatively
weak power.
Biomechanics of the fixation device is only one of many
factors to be considered in the operative treatment of humeral
shaft fractures. Although numerous factors affect operative
treatment decisions, it is worthwhile for the orthopaedic
surgeon to have comparative biomechanical information of
different plating constructs. Because the amount of motion at

Page 5

the fracture site influences the biologic reaction of bone, the
biomechanical stability of a fracture fixation implant plays an
important role in fracture healing.
CONCLUSIONS
In summary, we could not show a mechanical benefit
from the addition of a third bicortical locking screw per
fracture fragment in the manner tested. It is important to know
that locking screws are substantially more expensive than
standard screws. Although the cost of fixation failure and need
for reoperation vastly exceed the total costs for an internal
fixation implant, the indiscriminate use of unnecessary locking
screws increases overall treatment costs. Clinically, whether
two bicortical locking screws per fracture fragment provide
sufficient fixation in comminuted osteoporotic humerus shaft
fractures will need to be examined in future clinical studies.
REFERENCES
1. Baron JA, Karagas M, Barrett J, et al. Basic epidemiology of fractures of
the upper end and lower limb among Americans over 65 years of age.
Epidemiology. 1996;7:612–618.
2. Sarmiento? ?A, Kinman PB, Galvin EG, et al. Functional bracing of
fractures of the shaft of the humerus. J Bone Joint Surg Am. 1977;59:
596–601.
3. Gregory PR, Sanders RW. Compression plating versus intramedullary
fixation of humeral shaft fractures. J Am Acad Orthop Surg. 1997;5:
215–223.
4. Ring, D,? Perey, BH, Jupiter, JB. ?The functional outcome of ununited
fractures of the humeral diaphysis in older patients. J Bone Joint Surg Am.
1999;81:177–190.
5. Jazrawi LM, Bai B, Simon JA, et al. A biomechanical comparison of
Schuli nuts or cement augmented screws for plating of humeral fractures.
Clin Orthop Relat Res. 2000;377:235–240.
6. Hertel? ?R, Eijer H, Meisser A, et al. Biomechanical and biological
considerations relating to the clinical use of the Point Contact-Fixator—
evaluation of the device handling test in the treatment of diaphyseal
fractures of the radius and/or ulna. Injury. 2001;32(Suppl 2):SB10–SB14.
7. Gautier E, Sommer C. Guidelines for the clinical application of the LCP.
Injury. 2003;34(Suppl 2):B63–B76.
8. Wagner M. General principles for the clinical use of the LCP. Injury Int J
Care Injured. 2003;34:S-B31–S-B42.
9. Stoffel K, Dieter U, Stachowiak G, et al. Biomechanical testing of the
LCP—how can stability in locked internal fixators be controlled? Injury
Int J Care Injured. 2003;34:S-B11–S-B19.
10. To ¨rnkvist H, Hearn TC, Schatzker J. The strength of plate fixation in
relation to the number and spacing of bone screws. J Orthop Trauma.
1996;10:204–208.
11. Sanders R, Haidukewych GJ, Milne T, et al. Minimal versus maximal
plate fixation techniques of the ulna: the biomechanical effect of number
of screws and plate length. J Orthop Trauma. 2002;16:166–171.
12. Frigg,? ? R. Locking compression plate (LCP). An osteosynthesis
plate based on the dynamic compression plate and the point contact
fixator (PC-Fix). Injury. 2001;32:63–66.
13. Wenzl ME, Porte T, Fuchs S, et al. Delayed and non-union of the humeral
diaphysis—compression plate or internal fixator? Injury Int J Care
Injured. 2004;35:55–60.
14. Ring D, Kloen P, Kadzielski J, et al. Locking compression plates for
osteoporotic nonunions of the diaphyseal humerus. Clin Orthop Relat
Res. 2004;425:50–54.
15. Ka ¨a ¨b MJ, Frenk A, Schmeling A, et al. Locked internal fixator: sensitivity
of screw/plate stability to correct insertion angle of the screw. J Orthop
Trauma. 2004;18:483–487.
16. Zimmerman MC, Waite AM, Deehan M, et al. A biomechanical analysis
of four humeral fracture fixation systems. J Orthop Trauma. 1994;8:
233–239.