Solving Common Problems with Science: Implant Abutment Screw Loosening
- Common reasons for screw loosening
- Understand how screws work
- Considerations regarding facial change and development
- Correct usage of torque limiting tools
- Practical tips and habits to avoid screw loosening
The most common prosthetic issue with single tooth implants is the loosening of the screw. In his lecture, Dr. Chandur Wadhwani looks at screws, how they work, and the science behind them. He will provide tips and habits to ensure your prosthetics are more effective.
It is well known that the screw, or screw loosening is the most common prosthetic problem with single implant restorations. What is particularly interesting is that not only does a large percentage of screws become loose, but it also occurs over time. So, when does this happen, and what are the consequences besides the known biological and mechanical issues? With the loosening of a screw, one can experience anything from mucositis to a screw fracture, from an abutment fracture to an implant fracture – all resulting in costs for the patient and the treating clinician.
Dr. Wadhwani shares seven habits that will help to understand how screws work, how to use screws optimally, identify risk factors, and how to address them.
1. Habit: Seek to understand – how do screw joints work?
The first thing to understand about screws is that as we tighten a screw, we impart a force directly onto the screw. This is a rotational force that is called torque. It is known that a vast amount of this torque is actually absorbed purely by friction, friction in different parts of the joint. The friction starts when the screw thread is entered into the internal aspect of the implant and then goes all the way through to the head and any connecting component within this device. By the time the screw is seated, most of the imparted energy has been absorbed. And with the right amount of force applied, the resulting elasticity or stretch of the screw will actually optimize the screw joint. Looking at where the ultimate tensile strength is, it is also the point where the screw has the highest risk of fracture. The goal is to achieve an axial stretch within the screw, reaching about 75%, from 66 to 75% of the ultimate tensile strength within that screw. It is known that if you under-tighten a screw, you don't stress it enough. If you over-tighten a screw and bring it to the point of limit where it will start to rupture, it will permanently deform. But this weakness within a screw appears not so much when you stretch it but when it's subjected to sideways movement. And, in dentistry, we have cyclic movements – the movement of the jaws. With this lateral flexure, take care not over tighten the screw. Because if you tighten to the point where the stretch is almost optimum and then you have sideways movement, you will diminish what's known as the preload or the stretch within the screw, and the screw will become permanently deformed. This demonstrates the importance of keeping the torque to that prescribed by the manufacturer. Problems may occur, even with the correct torque applied, when a large cantilever is present or in cases with bruxism. Lateral forces will destroy the screw. We should stay within the limits of the torque to not overstretch or understretch the screw and understand that the vast majority of the torque that we use is purely absorbed as friction. Only about 10% of the applied torque value goes to preload.
2. Habit: Put first things first – torque wrench issues
How can you achieve optimum torque, and which torque-limiting devices or machines should be used? Various devices are available, but they typically fall into two categories: mechanical and electronic. It is well known from research and industry testing those electronic devices are not always accurate or precise, while manual ones can be highly precise and accurate. Therefore, this lecture will focus on manual devices.
Manual devices are divided into two groups: beam wrench and toggle devices. A toggle or friction device consists of mechanics within the actual torque limiting device, usually, a spring and a ball bearing. Although both beam wrench and toggle devices can be precise and accurate, a spring and a ball bearing toggle device has two consequences that the beam wrench does not have. The first consequence, toggle devices are susceptible to contamination, affecting the way the spring works. This is because the toggle device is used directly in the mouth, and moisture from the mouth can get into the mechanism. If the device is not cleaned and oiled properly, the resulting contamination gets baked on during autoclaving, hardens, and ultimately affects the spring. This can cause over-torque up to 400% higher than a clean, non-contaminated device. Therefore, it is crucial to maintain these devices properly.
The other consequence of having a spring and ball bearing within the device is that they suffer from inertia, causing a time lag between the spring activating and a new rotation of the torque wrench. This inertia means the torque wrench must be used slowly to achieve the optimum torque. Experiments have shown that the optimum torque can be achieved by using this device over 4 seconds. If used more quickly, it will under-torque. There are in fact two ways to use this torque wrench to optimize the torque value. First, take it up to about 80% of its desired torque and then slow down the last 20%. Second, do it slowly over 4 seconds, which is the preferred method.
When comparing the toggle device to the beam wrench, the beam wrench has no working parts whatsoever and is, therefore, easier to maintain. Furthermore, it has no speed limitation because there is no inertia. The beam wrench can be used as fast or slow as desired, and the delivered torque will be accurate.
So, we know speed and maintenance are factors. We also know that the automotive industry uses torque wrenches very similar to those we use, but ours are on a micro-scale. So, are we both equally accurate and precise in using these devices? The person who changes your tires is mandated to calibrate and check their torque wrench daily or weekly. However, when we surveyed 400 dentists, we discovered that only 6% knew what we were talking about or ever calibrated their torque wrenches. This finding underscores the need to understand how screws work and how to optimize torque because inaccurate tools can lead to imprecise results. Such inaccuracies could translate into screws loosening sooner than expected or more issues with screws breaking down. As per ISO 6789, which governs us and our industry, we should calibrate our torque wrenches at least once a year, except when exposed to extreme temperatures or damage. An autoclave is considered an extreme temperature environment. Thus, checking your torque wrench frequently is crucial, especially if you have the toggle type. The toggle type is an excellent device, but it requires regular maintenance. You need to use it for at least four seconds to achieve accuracy and precision, and check it regularly.
3. Habit: Begin with the end in mind – occlusal changes
Why do I mention that screw loosening can occur over a period of years? This is an intriguing phenomenon that I have encountered in one of my cases, a 70-year-old male patient. When I took photographs of the restoration in 2012, the occlusion appeared to be perfect with regular incisal edges. However, when the patient returned for a regular check five years later, there was a noticeable difference in the incisal height.
So we investigate the clinical situation. We know that human faces continue to grow and develop throughout their lives, which can impact the position of an implant in relation to the natural dentition over time. We also know there is an age when people experience a growth spurt that can further affect the position of the implant. Furthermore, facial growth can be predicted based on the pattern of the face. In a study of a group of adolescents and mature adults by Bernard, he measures the change in the position of implants relative to natural teeth over a period of four years on average -- he finds the change is similar in both groups. The resulting change in occlusion can alter the mechanics of the implant crown, increase the lateral forces on the screw, which is the weakness of the screw, and lead to screw loosening.
The facial form can somewhat predict these changes to occlusion over time. If we have a brachiocephalic face type, characterized by a short wide face, it tends to grow sideways. Over time, the implants may look like they were placed more palatally because the face and dentition have grown. If we compare it to the dolichocephalic face type or long face, growth is towards the anterior. The impact is more profound in these patients because the lateral incisors start to look much shorter. Be aware of the facial form with your patient to be accurate, follow the occlusion, and adjust it accordingly to prevent screw loosening.
Another feature of facial growth and development is mesial drift. The teeth move anteriorly over time due to wear and again due to dental alveolar growth. This means we get open contacts, which, when recorded, occur in about 50% of cases. Once again, this results in a difference in occlusion, which will again impact the way the screw works. So we can anticipate some screws that will loosen even if we do everything optimally. Be aware that the occlusion will change. So how can you help that occlusion and help slow down that change? Well, we can do what orthodontists do. We can provide a retainer to help limit tooth movement and help protect our restoration and the screw within.
4. Habit: Think win/win – Preventing bad outcomes
It is also important to inform the patient and explain what might occur. Tell them about normal facial growth and that monitoring is needed to prevent these complications. The best habit is to inform your patients so they know what to anticipate. I give my patients a form at the very beginning of their restoration explaining what will change and monitoring, not only from a restorative point of view with the occlusal changes but also from a disease perspective.
5. Habit: Sharpen WHAT you see – parallax effect
When we surveyed over 400 dentists, many dentists didn't understand where to place the beam according to the marker. The marker beam's width represents a certain torque, about four Newton centimeters in the example of the Nobel Biocare mechanical torque limiting device. This translates to an error of up to four Newton centimeters if the beam is placed incorrectly. So, where should the beam be placed? When reading the Newton centimeters, the beam marker should be in the center of the beam. Furthermore, beam markers all suffer from parallax; their position appears different when viewed from different angles. It can only be read correctly by looking at it from exactly 90 degrees. Otherwise, the parallax effect means that you don't necessarily see the right marking on the beam length, and depending on the angle of view, you'll either under-torque or over-torque. We have to understand the tools that we are using.
6. Habit: Synergy – Component precision
Synergy means that your componentry must fit. If your components fit and interlock well, the screw has a minimum amount of work to do. The friction effect will change the preload value if they do not fit. So you must use components that fit and are compatible with each other. As we know from the literature, the vast majority of third-party components are at risk for failure in fit. To ensure compatible components, it is recommended to use only components delivered by the original manufacturer, not by a third party. Using the right components and applying the correct force optimizes seating. If you can use the original componentry, outcomes are improved, with less screw loosening effects, such as fracture, and higher predictability.
7. Habit: Be Proactive – Optimum tightening technique
Different opinions and methods may be presented when you ask how I should tighten the screw on this implant componentry. Realistically, if we are scientists, there should only be one way that we tighten a screw depending on the material we use. The different surface affects the screws because most of the energy goes and is absorbed as friction during tightening. Surface effects on the screw can reduce this friction. One of them is you could use gold. Some screws have a gold layer of 23 karat gold on them. And what they do is when you tighten them up, it reduces the friction, increasing the preload. Surfaces and surface coatings contribute to the preload and stability of the screw joint system.
These are my seven tips to contribute to our understanding of how the screw works and how to optimize your approach to screw tightening to limit the number one problem with single-tooth implant restorations.
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