Biomechanical Investigation of Plate Working Length on Fatigue Characteristics of Locking Plate Constructs in Human Cadaveric Distal Metaphyseal Femoral Fracture Models

1. Biomechanical Investigation of Plate Working Length on Fatigue Characteristics of Locking Plate Constructs in
Human Cadaveric Distal Metaphyseal Femoral Fracture Models
1
Ricci, W; 2
Tornetta, P; +3
Zheng, Y; 3
Soileau, R; 3
Cartner, J.; 3
Hartsell, Z., 3
Ewing, M.
1
Washington University School of Medicine, St. Louis, MO, 2
Boston University School of Medicine, Boston, MA
3
Smith & Nephew, Inc., Memphis, TN
Yanming.zheng@smith-nephew.com
INTRODUCTION:
Successful application of plate and screw fracture fixation requires a
stable construct with proper stiffness for the entire fracture healing
period [1-2]. It is known that plate working length, i.e. the distance
between the first screws on either side of the fracture site, is one of the
most important surgeon-controlled factors determining the flexibility of
the construct especially when a bridging technique is used. It is less
clear how working length is related to the fatigue characteristics of a
given construct. Furthermore, it is of interest to understand how the
plate working length will influence construct stiffness and fatigue
stability with osteoporotic bone stock as compared with a healthy bone
condition.
The purpose of this study was to evaluate the fatigue performance and
construct stiffness of locked plate constructs for different plate working
lengths, using a cyclical physiologically relevant loading condition, in a
human cadaveric distal metaphyseal femoral fracture model. As the first
step, the study was performed with non-osteoporotic human cadaveric
femora. Specifically, differences in construct stiffness at various cycle
counts, fatigue endurance, screw loosening, and failure mode were
evaluated for two different plate working lengths.
METHODS:
Seven (7) matched pairs of fresh-frozen non-osteoporotic (T-score ≥ -
1.5 based on bone mineral density screening evaluation) human
cadaveric femora were used for this study. An oblique distal
metaphyseal defect that was 1 cm on the medial side and spanning 3
screw holes on the lateral side was created and bridged, with either 1 or
5 screw holes unfilled (i.e. “short” or “long” working lengths), using a
distal femur locking plate system (Smith & Nephew, Memphis, TN)
(Figure 1). The left and right femurs of each pair were randomly
assigned to either a short or long working length group. Each plate was
provisionally secured to the bone so that no gap existed between the
plate and bone upon instrumentation. Screws were then inserted and
tightened to approximately 4 N-m of torque. Distal fixation in all
specimens was with five 5.7 mm bicortical cannulated locking screws.
The remaining fixation was with six (6) or four (4) 4.5 mm locking self-
tapping cortex screws for the short and long working lengths,
respectively. After instrumentation, the femora were resected below the
lesser trochanter 36 cm above the knee joint line and were potted in a
pair of custom-made loading fixtures through which a physiological
combined loading configuration per ISO 7206-4 (2002(E)) was applied.
All specimens were subjected to a staircase axial cyclical load (starting
at 445 N with an increased load increment of 111 N every 25,000
cycles) at 1 Hz until failure. During the entire period of testing, fracture
gap closure was monitored through a pair of contact sensing plates.
Failure was defined as fracture gap closure or a loss of maintenance in
load. Construct stiffness was evaluated initially and after each
subsequent 12,500 cycle interval. After fatigue failure, screw removal
torque was measured using a torquemeter. Statistical analyses were
conducted using paired Student’s t-tests.
RESULTS:
Failure Modes: Testing of all of the fourteen constructs stopped due
to closure of fracture gap on the medial side. The modes of failure for
short working length constructs were most commonly plate fracture (6
of 7) with only one construct failure due to failure of a screw. The
modes of failure for long working length constructs were more mixed,
indicating stress concentration was less focused on the plate. Plate
fracture occurred in four constructs but the plate/screw/bone interface
failed in the other three. The short working length constructs survived
higher loads levels (1,001 N (1), 890 N (4), and 779 N (2)) when
compared to the long working length constructs (890 N (1), 779 (4), and
668 N (2)). This corresponded to a significantly higher fatigue life for
the short working length constructs (103,734 ± 17,623 cycles) compared
to the long working length constructs (86,090 ± 12,792 cycles),
(p=0.04).
Stiffness: Short working length constructs demonstrated higher
average stiffness at baseline and at all cycle counts compared to the long
working length constructs (p< 0.013). Based on the first 62,500 cycles
when none of the constructs from either group had failed, stiffness of the
short working length constructs decreased at a higher average rate (2.8
times, p=0.009) than the long working length constructs (Figure 2 dotted
line).
Screw Loosening: For the short working length constructs, the two
screws nearest to the fracture gap in the proximal fragment loosened the
most. For the long working length constructs, the two screws nearest to
the fracture gap on the distal fragment loosened the most. The most
proximal screw for both the short and long working length constructs
loosened the least.
0.00
200.00
400.00
600.00
800.00
1000.00
1200.00
1400.00
0.0 12.5 25.0 37.5 50.0 62.5 75.0 87.5 100.0 112.5 125.0 137.5
Cycles (1000X)
Stiffness(lbf/in)
Short Working Length
Long Working Length
DISCUSSION:
Results from this study show that the short and long working length
constructs fail in different modes. The short working length constructs
tend to fail by plate fracture whereas long working length constructs, in
lieu of their more diverse stress concentration, have a less predictable
mode of failure that is likely sensitive to other construct parameters such
as bone quality, screw purchase, etc. The higher stiffness seen with the
short working length constructs is intuitive. These short working length
constructs had a faster rate of stiffness decline, but surprisingly, had an
overall longer fatigue life than the long working length constructs.
Analysis of screw loosening suggests that the screws nearest the fracture
gap were stressed the most: those in the proximal fragment for the short
working length; and those in the distal fragment for the long working
length constructs. These data provide insight for the surgeon to
maximize construct fatigue properties when treating comminuted
fractures in non-osteoporotic bone.
REFERENCES:
1. Sommer C, et al, J Orthop Trauma. 2004;18:571-7.
2.Smith W, et al, J Bone Joint Surg Am. 2007;89:2298-2307
Figure 1: Photographs
showing: a) bone fracture
pattern in anterior view, b)
screw pattern for short
working length where the two
screws nearest the defect were
set in unicortical medial
fixation due to the wedge-
shape fracture pattern (pre-test
construct), and c) screw pattern
for long working length (tested
construct).
Figure 2: Stiffness of constructs during testing. The two
dotted lines show stiffness change trends with time.
a b c
Poster No. 1754 • 56th Annual Meeting of the Orthopaedic Research Society