Analysis of Syndesmotic Screws

Transcription

1 ROCHESTER INSTITUTE OF TECHNOLOGY Analysis of Syndesmotic Screws Introduction to Biomaterials Phillip Amsler and Natalie Ferrari 5/10/2011

2 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 patient 3. 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

3 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 anyway 3. 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

4 million cycles), but are known to fracture for most inactive adults in around 10 years 2. 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 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

5 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 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

6 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.

7 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 fibula 2. Figure 1: The overall schematic of the test rig used. 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 Figure 2: The test rig being set up 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.

8 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. 4F For analyzing the data, we used for shear stress where F was the weight of the hangar 2 D (lbs), and D was the diameter of the screw (in). Also was used to analyze the strain in the screw D 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.

9 Shear Stress (psi) 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) Table 1: A compilation of the stresses and strains from the elastic region trial. Shear Stress Shear Strain (in/in) Figure 3: The stress-strain curve generated from the elastic region trial (see table 1)

10 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) 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) 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 Figure 4: The final failure mode of both sydesmotic screws. 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.

11 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.

12 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, 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,