analysis of syndesmotic screws - portfolio of natalie s....
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ROCHESTER INSTITUTE OF TECHNOLOGY
Analysis of Syndesmotic Screws
Introduction to Biomaterials
Phillip Amsler and Natalie Ferrari
5/10/2011
Introduction The general overview of this project is to determine why syndesmotic screws will break in a
patient and whether the screws should be removed. The syndesmosis screw is commonly used in the
repair of a pronation-external rotation injury to the ankle. This hardware is efficient in aiding the
stability of the tibiofibular joint to allow the ligaments surrounding the ankle to heal properly. However,
with the addition of the screw into the ankle joint, a separate surgery is needed to remove the
mechanism, weight bearing activities are postponed, and other complications can result
postoperatively. Currently, it has been brought to attention that syndesmotic screws are being
overused and could perhaps be avoided if the situation permits. According to the papers, the same
screws are implanted for syndesmotic fixation regardless of patient lifestyle, gender, or weight.
However, it is apparent that under normal operating conditions for an active person, the screws will
likely break between the fibula and tibia. To test this, we will be using bone on bone substitute and
applying force in the shear direction on unbroken syndesmotic screws. From this, we will be able to
determine the shear stress needed to break the screw and how high a person would have to potentially
jump to break them.
Background Information Based on the paper by Anna N. Miller, MD “Functional Outcomes after Syndesmotic Screw
Fixation and Removal” it is apparent that screw removal is highly beneficial to the patient3. Three
months after screw removal (referred to hardware removal), the average patient saw an increase in
flexibility, as well as a decrease in pain. These findings support the idea that the screws were limiting
the range of motion for the patient and that with physical activity, the screws will likely be broken.
However, the study does neglect weights of the patient as a factor which is something that our study
will focus on because it has been identified as a major issue leading to screw breakage and implant
failure.
Another item that this paper identifies as a factor contributing to implant failure is the length of
time the screws are left in vivo. This is not necessarily a major factor contributing to fatigue or
corrosion, but impacts the healing of the patient. As the patient becomes more active they are more
likely to engage in athletics, running, or other forms of activity that will increase the impact loading on
the screws. However, the paper claims that the longest times the screws are left in are just over 5
months and that there are not significant differences between the 3 month and 5 month patients. This
analysis seems flawed by the previous argument because it assumes that the patient will not be active in
the time leading up to the follow up surgery.
Overall the article is in favor of removing the syndesmotic screws from the ankle to increase the
range of motion. The proposed experiment would also investigate whether the hardware should be
removed based on failure mechanics. The variables to be investigated will include those which are most
notably neglected in this case study including patient weight and physical activity level.
Next, in the “Distal Tibiofibular Syndesmosis Fixation: A Cadaveric, Simulated Fracture
Stabilization Study Comparing Bioabsorbable and Metallic Single Screw Fixation” by Stephen Cox, MD,
the comparison of different materials is introduced into the problem. The study proposes the idea that
since stainless steel screws are usually removed; bio-absorbable materials such as co-polymers that will
break down and be replaced by bone tissue over time could be used as a replacement. The material
they used in the study was 82:18 poly-L-lactic acid/poly-glycolic acid (a copolymer alloy) and its
mechanical properties were compared to those of typical stainless steel screws. The study was
conducted in a cadaver’s ankle after the hardware was inserted into the bone and focused on the
fatigue failure of the screws. The test load ranged from 90 to 900 N at 1.5Hz for 1000 cycles and the co
polymer did not significantly differ from its stainless steel counterpart. The axial stiffness of the
copolymer was about 100 N/mm less than the steel at the beginning and end of the loading, but scored
higher in the angle of failure at about 50 degrees of ankle movement. It did require more torque for the
steel screw to break in the ankle however, which is an import consideration for this project.
Again the first critique of this study would have to be the time line they are using for testing. Is
it really that important that the screws don’t fail in fatigue? According to most doctors and research,
the screws don’t stay in for longer than 3 or 4 months and if they do, they are expected to break which
returns the patient to a normal range of motion anyway3. The biggest reason a screw would fail is likely
coming from athletic activity (which they shouldn’t be doing) or just being heavier than the average
person. The project will not use any cadaver ankles, however the bone substitute should be viable
enough to give an idea of how much shear stress the screws can endure. This article gives a good idea
of how to test the screws and how much force they can endure in vivo.
Overall this paper is also in favor of removing the screw (that’s the assumption they have to
make in order to compare the degradable co-polymer) and also gives the general fatigue test results for
stainless steel screws. For instance the screws tested are found to have infinite life in the ankle (1
million cycles), but are known to fracture for most inactive adults in around 10 years2. The paper does
not investigate how the screws might perform in patients with increased activity level. This test, while
important, also neglects the impact loading. We feel that this is a large oversight, and would like to
investigate further into how impact loading affects the screws.
The third study investigated was “Mechanical Considerations for the Syndesmosis Screw” by
Boden, S.D, Labropoulos, P.A. The intent of the paper was to bring to light the mechanical need for the
syndesmosis screw and its ability to provide stability to the internal fixation of the fibula and medial
malleolus during the event of a pronation-external rotation fracture. A sample group of thirty
embalmed and five fresh cadaver legs were mounted to a wooden frame with 15-20 degrees of internal
rotation. The load test was performed on two groups. Group I consisted of thirteen specimens that were
subject to serially sectioning the deltoid, syndesmosis, and interosseous membrane in 1.5-centimeter
increments. Group II consisted of the other seventeen specimens that were subjected to the same
sectioning of ligaments expect the deltoid was left alone until the last step. This allowed for final
comparison between the groups. Each specimen was dissected to expose the deltoid ligament, the
anteromedial section of the joint capsule, the syndesmosis, and the distal fifteen centimeters of the
interosseous membrane. A plate was also bolted to the bottom of the foot to allow a rope and pulley
system to provide specific loading to the ankle. The pulley system was set up at the distal lateral corner
of the foot plate in order to properly pronate the foot for testing. Performing the test in this fashion
allowed directly observing and measuring the widening of the syndesmosis in response to different
loads. It also provided qualitative analysis as to whether osteotomy and rigid fixation had any affect on
the ankle when exposed to this same loading.
The resulting data from the loading model developed provided insight pertaining to how certain
situations of fractures and tearing react under uniform loads. Widening of the syndesmosis was found to
be analogous between bone that was intact and bone that was treated via osteotomy and fixation. The
baseline syndesmosis width for Group I was determined to be around 3.2 ± 0.2 millimeters for observer
number one and 3.1 ± 0.2 millimeters for observer number 2. Group II experienced very minimal
widening of 1.4 ± 0.3 millimeters when the medial malleolus and fibula were intact but experienced
similar widening to Group I after the last step when the deltoid was severed.
From this study, it was observed that while the deltoid was still intact, the syndesmosis
experienced minimal widening even though the syndesmosis and interosseous membrane were
disturbed. As a result, stability of the syndesmosis could be reinstated by rigid fixation of the fibula and
tibia, therefore avoiding trans-syndesmotic fixation. In the event that the deltoid is severed, the width of
the syndesmosis experienced an increase and was also directly proportional to any disruption to the
interosseous membrane. From this evidence, the avoidance of consistently inserting a syndesmotic
screw to provide and improve stability was demonstrated. In the event of a fibular fracture, the
necessity for a syndesmotic screw was found to be related to the height of the facture. A critical zone for
this height was 3-4.5 centimeters. Fractures occurring proximal to this zone would still need to be
supported by trans-syndesmotic stabilization, but those occurring distal to this range would not be
necessary.
This study exemplified a more in depth look at why and how fails occur. It provided more insight
on what ligaments are affected and what ligaments to not play a large role in stabilization. Also, this
study used a very interesting model to perform the testing with the jig and loading system. It offered a
good suggestion for why the screws should or shouldn’t be removed and took into consideration other
factors the previous studies did not.
The last article considered was “Operative aspects of the syndesmotic screw: Review of current
concepts” by Van Den Bekerom, M.P.J. This was a great review paper revealing the current models and
practices used in order to repair ankle injuries with the syndesmotic screws. This publication also stands
for a collection of technical characteristics of performing surgery using this mechanism and to provide
suggestions and insight for clinical practice. Although the article discusses many issues regarding the
screws, the important points that can be taken from the article related to our project are the size of the
screws, use of bioabsorbable screws, time until weight bearing, and whether the screws should be
removed before weight bearing.
According to Van Den Bekerom M.P.J., it was observed that a larger diameter screw allowed for
more resistance to a shear stress affecting the distal syndesmosis. The load was that applied was equal
to that of weight bearing. Although a larger diameter provides more resistance, a further look into the
affects of the holes left by the larger screws shows that this is not fully advantageous to the patient. The
use of bioabsorbable screws was a focus for other studies with the intent of eliminating a second
surgery and delaying recovery. There was no observable difference in the measurements performed
when comparing the metal to the bioabsorbable screws. In addition to this information, patients who
were treated with this biomaterial experienced less swelling and were able to return to regular activities
more quickly. However, biomaterials described in the paper, come with a greater price than a
manufactured screw. The largest controversy and one that our project will ideally shed light on, is when
weight bearing should be deferred until and if the screw should be removed. Weight bearing is usually
not recommended before about six weeks after surgery. Some surgeons feel that leaving the screw in
place while introducing weight bearing exercises results in no adverse effects where as others feel
weight bearing causes the screws to fracture or loosen, causing discomfort in the joint. One study
preferred that the hardware should be removed before participating in any weight bearing exercises
because leaving it in would result in irregular ankle movements causing pain and discomfort. In addition,
the screws could potentially fracture and loosen causing the joint to be unstable.
This article provided a lot of great information with regards to what we would like to observe in
our own study but it did caution readers about extrapolating data. The studies used cadavers in order to
retrieve their data which does not replace a living, human leg. Changes in bone composition and
ligament degeneration are factors that are hard to replicate and draw conclusions from. For our
purposes, this provides great insight into what surgeons prefer and why.
Methods
In order to analyze the failure mechanics of the syndesmotic screws, a pair of syndesmotic
fixation screws were obtained from the research lab of Mark L. Prasarn, MD at the University of
Rochester. (We are very
grateful for this donation
since these screws would
have cost more than $100
per screw otherwise) To
analyze the syndesmotic
screws, a pair of cancellous
bone blocks were obtained
so that they could
represent the tibia and
fibula. They were
arranged in a fashion
similar to figure 1. The 2
inch screw is the typical
length used for ankle
fixation, and the .153
inches was an
approximation of the
distance between the tibia and fibula2.
In order to keep the spacing between the
bone specimens constant, a pair of Allan Wrenches
were oiled and placed between the two bone
specimens. The oil was used so that there is a
minimum frictional effect between the two screws
(see assumptions: all loads are supported by screw in
shear), and the Allan wrenches are used to make sure
that there was no bending moment in the screw from
uneven spacing (see above assumption).
With this basic rig constructed, it was attached to a
table using a C-clamp to keep the rig static under
loading. A ruler was taped next to the rig, and a
tripod was used to record deflections in the screw.
Finally a mass hangar was used to load and unload
the weights from test rig. The general purpose of this rig was to replicate the shear forces experienced
between the fibula and tibia bones when applying weight on one’s foot.
Figure 2: The test rig being set up
Figure 1: The overall schematic of the test rig used.
In order to create a realistic model for this experiment we needed to make several assumptions.
First and foremost was the friction between the bone segments needed to be neglected. This allows us
to assume that all loading is applied in shear to the screw which will ultimately break the screw. In
addition, we must assume that there is no bending moment in the screw. This is likely not 100% true,
but the spacing between the two was controlled by using Allan wrenches, so this is an assumption we
can make.
For analyzing the data, we used 2
4
D
F
for shear stress where F was the weight of the hangar
(lbs), and D was the diameter of the screw (in). Also D
was used to analyze the strain in the screw
where δ was the deflection (in), and D was the diameter of the screw (in).
Figure 3: An example of the deflection and ultimate failure from loading.
Results The results of this experiment came in two trials. First, the experiment was conducted in the
mechanics of materials lab with a total weight of 57.4 pounds. We were hopeful that this weight would
be enough to cause a failure, however this only put the screw under elastic deformation, which is
evident from the stress strain curve. Below are the results for the elastic region of a syndesmotic screw.
Trial 1 (elastic region) Weight (lbs) Disp (mm) Disp (in) Shear Stress (psi) Strain (in/in)
0.0 0 0.000 0 0.000
5.4 0.5 0.020 720 0.201
10.4 1 0.039 1383 0.402
15.4 1.5 0.059 2046 0.603
20.4 2 0.079 2709 0.803
25.4 2.4 0.094 3372 0.964
30.4 2.9 0.114 4035 1.165
35.4 3.2 0.126 4697 1.286
40.4 3.3 0.130 5360 1.326
45.4 3.6 0.142 6023 1.446
50.4 4 0.157 6686 1.607
55.4 4.2 0.165 7349 1.687
57.4 4.4 0.173 7614 1.768
0
1000
2000
3000
4000
5000
6000
7000
8000
0.0 0.5 1.0 1.5 2.0
She
ar S
tre
ss (
psi
)
Shear Strain (in/in)
Shear Stress
Table 1: A compilation of the stresses and strains from the elastic region trial.
Figure 3: The stress-strain curve generated from the elastic region trial (see table 1)
In the second trial we used weights from the student life center in order to increase the total
weight. It should be noted that gym weights have a high uncertainty in their values (~15%) so there will
be a higher amount of error in these stress calculations.
Trial 2 (Plastic Failure)
Weight (lbs) Disp (mm) Disp (in) Shear Stress (psi) Strain (in/in)
0.0 0 0 0 0.000
45.4 4.5 0.177 6023 1.808
65.4 4.8 0.189 8675 1.928
85.4 5 0.197 11326 2.009 Table 2: The results from the first plastic deformation study and fracture.
Trial 3 (Plastic Failure)
Weight (lbs) Disp (mm) Disp (in) Shear Stress (psi) Strain (in/in)
0.0 0 0 0 0.000
25.4 1.4 0.055 3372 0.562
50.4 2.8 0.110 6686 1.125
75.4 4.2 0.165 10000 1.687
80.4 4.7 0.185 10663 1.888
85.4 4.7 0.185 11326 1.888 Table 3: The results from the second plastic deformation study and fracture.
The results of the plastic
deformation experiments were not
what we were expecting. From the x-
rays we were expecting there to be
some sort of brittle failure in under
shear stress, however the screws used
in this experiment merely bent causing
the cancellous bone specimens to fracture before the screws did. According to this model, it is more
likely for the ankle bone to break before the screws do, which is not only not desirable, it is also
unrealistic. The screws can clearly break in a patient without there being any adverse effects on the
patient’s ankle.
Figure 4: The final failure mode of both sydesmotic screws.
Discussion From the test rig, it was concluded that the screws will fail under normal loading. Although the
failure mode was different from in vivo studies because the screws plastically deformed and did not
completely fracture, they did not withstand the stressed applied to them. During the experiment, it was
hypothesized that the amount of weight provided in an on campus lab would be sufficient. Soon after
beginning testing, it was realized more would be needed. Some limitations of this test would be to
change how the weights were loaded to the rig. For this test, weight loading was very tedious and noisy.
Also, because the weight hanger was removed each time weight was added it was assumed that the
screw could be experiencing elastic deformation each time it was loaded and unloaded. This would
result in weakening the screw after each iteration. It was also decided that impact testing would be a
better approach to replicate what occurs when the screws break in vivo. Lastly, the limited amount of
screws available for the study restricted the amount of times the test could be performed.
Recommendations for future work of this kind would be to find a more accurate and efficient way to
load the weight and to have a larger sample size.
After performing this test, it was established that the screws definitely need to be removed after
properly healing has taken place. Agreeing with the current studies out there, if using the current
material the screws is not safe to leave in vivo once the bone has healed. The screws will undoubtedly
loosen and or break after the patient begins applying pressure to the ankle. As for changing the current
material, this test did not investigate the performance of bioabsorbable screws. However, it would be a
major cause for concern as to whether the screws would be able to stabilize a weight bearing joint while
absorbing into the body. The current material performs as it should and until a material can outperform
the syndesmotic screws, it is believed that they will prevail.
Conclusions Overall, the results from the experiment were very exciting yet surprising. It was interesting to
observe that the screws could only support about 85 lbs before beginning to fracture. From this, the
screws would most definitely need to be removed once the bone has healed properly. The patient
would have to abstain from weight bearing exercises and schedule a second surgery. Also, with the
current material only causing an inconvenience and not a problem with it’s function, a biodegradable
screw is not a necessity. More testing would need to be performed in order to conclude whether a
biodegradable screw can withstand the forces applied in such a circumstance.
References
1. Boden, S.D, Labropoulos, P.A., McCorwin, P., Lestini, W.F., Hurwitz, S.R. (1989). “Mechanical
consideration for the syndesmosis screw. A cadaver study.” J Bone Joint Surg Am, 71,
1548-1555.
2. “Distal Tibiofibular Syndesmosis Fixation: A Cadaveric, Simulated Fracture Stabilization Study
Comparing Bioabsorbable and Metallic Single Screw Fixation” Stephen Cox, MD, Debi P.
Mukherjee, ScD, Alan L. Ogden, BS, Raymond H. Mayuex, BS, Kalia K. Sadasivan, MD,
James A. Albright, MD, and William S. Pietrzak, PhD
3. “Functional Outcomes After Syndesmotic Screw Fixation and Removal”, Anna N. Miller, MD,
*Omesh Paul, MD, *Sreevathsa Boraiah, MD,† Robert J. Parker, BS, *David L. Helfet,
MD,*and Dean G. Lorich, MD*
4. Van Den Bekerom, M.P.J., Hogervorst, M., Bolhuis, H.W., Niek van Dijk, C. (2008). “Operative
aspects of the syndesmotic screw: Review of current concepts.” Int. J. Care
Injured,39,491-498.