Hip implants are a crucial piece of medical design that helps patients regain painless movement at the hip joint. Hip implants were first introduced in 1960 and as of 2008, 80,000 hip replacements occur each year in the UK (National Joint Registry, 2008). There are two main types of hip replacement, metal-on-metal hip resurfacing, where the femoral head and the acetabulum cup are both resurfaced with metal and total hip replacement, which involves removing part of the femoral bone and replacing it with an artificial implant (Arthritis Research UK, n.d.).
Over the years, it has become abundantly clear that hip joints differ from one another. This claim is supported by the issues many patients receive when undergoing a standard hip implant, due to the fit being ‘universal’ and not custom. Having a custom implant increases the quality of life for the patient as there is less room for error in regards to the geometry of the hip. However, custom implants come at a greater expense than standard implants, due to an extensive design and testing programme prior to use.
1. Fixation Techniques
Alongside the two different types of implant are the two different types of fixation techniques. The first is cemented, where the stem and acetabulum are cemented in place. This type of fixation can be used for osteoporosis sufferers who have more porous bones (Sood, 2014) but the cement tissue has been known to ‘irritate the surrounding tissue’ (Sood, 2014). The other type of fixation is uncemented. This is where the stem is reamed or ‘specially textured to allow the bone to grow onto it… over time’ (Sood, 2014). Not using cement ‘eliminates worry about the potential breakdown of cement’ but it takes longer for the bone to grow onto the new implant (Sood, 2014). The two can also be combined to make a hybrid fixation, which ‘combines a cemented femoral component with a cementless acetabular component’ (Davey and Harris, 1989).
For this particular hip replacement case, the patient is a 55-year-old male who is suffering from osteoarthritis, which means that they suffer from joint pain as well as stiffness and so a hip replacement is needed to relieve his symptoms. From the x-rays, he appears to be suffering from valgus of the hips as the femoral shaft is below the head of the femur and so the head is distally away from the body. Based on the age of the patient and their disease, it can be assumed that they lead a relatively active life style but avoid strenuous activity. The patient will still need to remain mobile on a daily basis and would need to be able to perform simple tasks like bending down to pick things up and getting in and out of chairs and a bed. The patient would need an implant for the rest of their life, so the implant must be durable enough to avoid failing for a 20-year period.
In order to decide on the best fixation for the patient, the advantages and disadvantages need to be weighted accordingly to suit this patient. The first thing that needs to be taken into account is the patients suitability for each type of fixation. The patient is able to have both cemented and uncemented types of fixation as they don’t have a lower bone density commonly associated with osteoporosis (Sood, 2014). One of the major issues with cementing is that wear can occur and cause the implant to come loose (Sood, 2014) and can increase the chance of osteolysis occurring. Osteolysis is where there is bone loss due to there being too many particles in an area that can no longer be cleared (Oransky, 2001). This happens because the osteoclasts in the bone are activated by the cytokine TNF-alpha, and the cytoclasts are responsible for reabsorbing bone. When osteolysis occurs, there is an increased risk of aseptic loosening occurring, which can lead to a complete failure of the implant (Saleh, 2004). A positive of cemented fixation is that the implant has a lower risk of needing replacing after ten years (Dattani, 2007) and as it uses cement that dries in ten minutes, both the patient and the surgeon can be sure that the implant is securely in place without having to wait for pain and swelling to go down (Sood, 2014). One of the disadvantages of a cementless fixation is that it can ‘take up to three months for the bone material to grow into the joint component’ (Sood, 2014). However, because of this, the bond is thought to be of a better strength and offers a longer-term solution. As the patient is younger than typically anticipated and will need to continue to work and be relatively mobile, the best option for the fixation would be a cementless fixation as a cementless fixation will support a higher demand (Oransky, 2001). Alongside this, a cementless fixation reduces the risks of osteolysis occurring meaning that a second hip replacement later in life may be viable as the deterioration of the bone is minimal when compared to a cemented hip implant.
However, it is vital that when using a cementless fixation that the implant is well fitting to minimize the micromotion that occurs (Khanuja et al, 2011). The ideal range of micromotion is <20m as it is below this that predominantly bone forms around the bone, with micromotion of >150m leading to the formation of fibrous tissue (Khanuja et al, 2011). With the formation of fibrous tissue comes the increased likelihood of loosening of the implant, which can cause pain and a need for further surgeries (Büchler et al, 2014).
2. Standard vs Custom implant
The other important decision that needs to be made about the patient’s implant is whether they should have a standard or a custom implant. The standard implant is offered to everyone, regardless of their specific needs such as femur length, hip socket size, disease causing the need for a hip replacement or the varus or valgus of the hip joint. A custom implant on the other hand is specifically designed for the patient and takes all of these things into account. It can be designed by use of CAD CAM methods to maximise the ‘fit and fill’ of the stem in the bone (Mirza et al, 2010).
The standard implant has been designed so that it could fit a large number of people, based on the average features of previous patients. This can suffice in many instances as typical patients will follow the same basic patterns that the standard implant has been based on. However, as design and analysis techniques are advancing, it has become easier in recent years to design and make custom hip implants that are a perfect fit for the patient in question without having to use an invasive method. This works better for those that have abnormalities of the femur or hip joint, such as a shorter or longer than normal bone length.
One of the largest differences between standard and custom implants is the cost, something which oftentimes convinces a surgeon to use a standard implant, even when a custom implant would be superior. Some surgeons have even decided to stop using custom implants, despite their high satisfactory rate, due to them being deemed cost ineffective and providing minimal improvement over the standard hip implant (Reize & Wülker, 2007). The reason that custom implants are more expensive is because an accurate model of the patient’s femur and hip joint needs to be generated and then an implant has to be designed to fit the patient exactly. This modelled implant would then have to undergo finite element analysis to ensure that the stresses the bone and implant are subjected too are suitable for the patient.
Despite the initial surgery and implant being more expensive, as the surgeon would have to alter their usual technique due to the custom implant being ‘abnormal’, a patient with a custom implant would be at a decreased risk of dislocation and therefore would be less likely to need a second surgery. One of the reasons is because the neck length of the implant would be customised to fit the patient and so the muscles of the patient would be of sufficient tension to keep the implant within the socket (Raaymakers et al, 2014). As well, the stem length can be chosen to suit the specific patient’s needs, ensuring that there isn’t excess stress shielding that can’t be prevented in a standard stem. Alongside this, the custom implant would typically last longer because it is designed to be as ‘biomechanically and physiologically close to the patient’s normal hip’ as is possible (Sandiford, 2011), again reducing the need for revision surgery.
3. Stress shielding
Both custom and standard implants would undergo some stress shielding, as the implant has a higher stiffness than the bone and so it takes some of the stress off of the bone. This effect causes a decrease in bone density due to the bone not having to take as much load (Millis, 2013). The stress shielding that could occur is related to the stiffness of the material used in the femoral stem of the implant. The lower the young’s modulus of the material, the less the bone is broken down by the osteoclasts (Huiskes, 1992) and so the bone undergoes less resorption. Often, titanium is used to reduce the effects of stress shielding as it has a lower young’s modulus than other commonly used metals, such as Co-Cr alloys (Niinomi and Nakai, 2011). This means that the bone wouldn’t undergo as much bone resorption due to the fact that the bone density would not have decreased as much as it would have if a stiffer material had been used. Whilst keeping the young’s modulus of the implant as low as possible to match that of the bone, the strength of the implant must also be taken into account, which is why the young’s moduli still differ greatly between the bone and the stem (Niinomi and Nakai, 2011). Another way to decrease the stress shielding is by using a shorter femoral stem. Although longer stems decrease the induced stresses in the femur, there is an increased ‘deformation in the distal region compared to the proximal region’ which in turn increases the stress shielding (Amirouche et al, n.d.). A short stem has less stress shielding due it improving the proximal stress distribution and this has been tested both experimentally and through finite element analysis (Goshulak, 2014). The stress shielding must be low so that the hip implant does not undergo migration, which is the ‘permanent change of position of a component’ inside the bone after it has undergone a loading event (Hua, 2018).
4. Joint Stability
Alongside stress shielding, it is important that joint stability is maintained or improved with a total hip replacement. The implant stem would need to be adapted so that it matches the specific bone geometry of the intended patient (Yang et al, 2014). The bone geometry includes the length of the femur, the size of the natural femoral head, the angle of the hip joint and the femoral offset of the hip. A custom implant can provide a personalised proximal medial curve in the hip implant, which will subsequently reduce the stress as it fits the bone properly. When using a standard implant, the geometry of the femur and hip joint is not taken into account and so the stem may not be the right fit or of the right angle for the patient.
This is a problem as an ill-fitting implant can cause a disparity in the length of the legs, which can cause pains in the back from the patient trying to self-rectify, as well as pain in the hips, from too much force being exuded. The neck of the femoral stem should be positioned in such a way that anteversion (anterior rotation) and retroversion (proximal rotation) do not occur excessively as retroversion in particular can cause dislocation (Raaymakers et al, 2014). One of the main goals of the surgery is to reduce pain and improve the quality of life of the patient, therefore it is paramount that considerations regarding joint stability are made during the design of the implant.
5. Femoral Offset
It is important to take into account the femoral offset of the hip, which is the ‘distance from the centre of rotation of the femoral head to a line bisecting the long axis of the femur’ (Lecerf et al, 2009). The femoral offset must be determined prior to surgery taking place and in order for the patient to not undergo impingement at the hip joint, the femoral offset must be increased. This is because if the femoral offset is decreased, then not only is there a risk of impingement occurring but there would be an increase in the wear of the joint due to the fact that more force would be needed to move the leg and so the joint would have an increase in reaction forces on it (The Bone School, n.d.). If the femoral offset isn’t restored then the abductor muscle can become weak and this can lead to limping, which in turn could cause back and further hip pain (Mirza, 2010). The femoral offset can be chosen when a custom implant is made but cannot be altered when a standard implant is used.
6. Femoral Head
The size of the femoral head is another consideration that needs to be taken into account. Up until the past few years, the femoral head size that was used most often used has a 22mm diameter (Ganapathi, n.d.) This is a lot smaller than the natural head size of the femur, which is often between 40 and 50mm in diameter (Milner and Boldsen, 2012). In more recent times, surgeons have started to increases the size of the femoral head that they use on a regular basis, to around 28mm, in order to combat the issue of dislocation that often occurred with the smaller in diameter femoral heads (Ganapathi, n.d.). It is important that the issue of dislocation is dealt with, as every time a patient has a dislocation of the hip implant, they have to go back into surgery to get it fixed. Specific diameters of femoral heads are more suited to patients of a given age and activity level, thus the diameter selected by surgeons is often heavily influenced by these traits. For more flexible patients, which often coincides with younger female patients, a larger in diameter femoral head would be a better choice in order to decrease the ‘risk of osseous impingement at the extremes of motion’ (Malik et al, 2007). The most suitable head size for the patient is the 36mm diameter femoral head as it has been shown that increasing the diameter to this much can reduce the risk of dislocation significantly and this positive outweighs the risk of wear that comes with a larger head size (Zjilstra, 2017).
Having a larger head size also creates a larger head-neck ratio which means that there can be a larger range of motion before the joint undergoes impingement (Karadsheh, n.d.). Alongside this, the larger head-neck ratio leads to an increase in the generated safe-zone and allows surgeons to have a larger margin of error when it comes to the surgery itself, something that will benefit the patient should the surgeon not be able to fully avoid an error (Li, 2010). Having a custom implant would be better when designing a large head-neck ratio, as it could take into account the other features that the bone has that could affect the head-neck ratio, whilst also taking into account the size and shape of the stem. As this would be a primary hip replacement, a surgeon is likely going to want to give the patient a shorter stem, as longer stems are normally saved for hip replacement revisions (Mirza, 2010).
Alongside the size of the femoral head, the material that it’s made out of should also be taken into account, with reference to the acetabulum cup. The material that has been known to cause the least amount of wear and so can allow for a greater head size is ceramic. Another advantage of ceramics when compared to metals is that the wear caused by movement doesn’t allow for microscopic metal particles to get into the blood stream, as there isn’t any metal involved (Hip Replacement and Recovery, n.d.). However, ceramic femoral heads are costlier than their metal counterparts so the price of this must be taken into account (Hip Replacement and Recovery, n.d.).
7. Conclusion
In conclusion, the implant that would be the most effective in treating this patient is the custom implant with a 36mm in diameter femoral head and with a cementless fixation. The combination of the custom implant and the cementless fixation together should allow for a perfect fit of the stem within the bone. Whilst the standard implant would have been cheaper, the patient is relatively young and custom implants have been known to last longer than the standard ones and so using a custom implant should reduce the risk of needing additional surgeries to either replace the all or part of the implant, which will reduce the long-term costs. The customised geometry of the implant within the bone and the size of the femoral head will help to ensure that no dislocation occurs, again meaning there is less chance of another surgery being needed. The cementless fixation will provide a strong bond with the bone resulting in a lower risk of aseptic loosening compared to if a cemented fixation, again reducing the risk of further surgeries occuring. The combination of the customised stem, 36mm femoral head and cementless fixation should allow the patient to regain mobility that he may have lost and be able to continue about his daily life comfortably for many more years to come, without the worry of further surgeries and unnecessary pain.
All medical devices have to undergo constant safety checks, even after they have started to be in use. It is important that manufacturers and users keep up to date with any new legislation that is brought in to ensure that their devices still follow it. To ensure that the device is working as it should be, there should be regular maintenance checks scheduled and regular training of staff and patients to make sure that they can use the device efficiently. At the end of the device’s lifecycle, it must be disposed of safely and in the correct way to ensure that it does not pose a risk when it is no longer in use. The purpose of this report is to discuss the possible safety issues of the Jaco assistive robot manufactured by Kinova Robotics, as well as the legislation and maintenance it would need to undergo during its use as well as the best way to decontaminate and dispose of it.
The Jaco assistive robot is a three-fingered robotic arm that is attached to a wheelchair for use by people who do not have full mobility of their upper extremities. This device can be battery powered or can be connected directly to a power supply, depending on the situation of the user. Before being released onto the market, it is important the device underwent a process Failure Mode Effect Analysis, which is used to detect any potential failures during the manufacturing process (Kamm, 2005). This will eliminate any major safety problems and will highlight any safety concerns that need watching. On top of this, it will affirm that the legislation is being followed, which is crucial for the device to be deemed safe. One of the main safety issues with this device would be its chance of over-heating, which could cause harm to the user or to other around them. This is because the arm is an electronic device and so heat gets conducted via the movement of electrons (Schelling et al, 2005). The risk of overheating can be reduced by having an internal system that alerts the user by signalling that it is getting too hot. As well, ensuring that the arm is covered in insulating material so that it is not hot to touch would protect the user from immediate harm. As well, it is important that the user use only the provided charging equipment and battery pack, otherwise they could run the risk of damaging the arm and this may contribute to overheating. Alongside this, the device runs the risk of causing an electric shock. For this reason, it is important that the device does not get submerged into water and that it is kept away from water sources when being charged to minimise this risk. This should be noted in the instruction manual for the device. Throughout its life, it is important that the device undergoes planned preventative maintenance, to ensure that the device is running as it should and to prevent failure of the device (Basri et al, 2017).
Since the Jaco robot is classed as medical device, it must follow all the same legislation as any other medical device on the market. Across the EU, all medical devices must follow the legislation of the Medical Device Directives, which in the UK is enforced by the Medicines and Healthcare Products Regulatory Agency (MHRA) (Conformance, 2017). The most vital legislation is the Medical Device Regulations 2002. This states that a medical device must have a CE marking for it to be allowed to be used as a medical device (Legislation.gov.uk, 2002). Alongside this, it states that it is the manufacturers responsibility to ensure that the device has the correct documentation and that they register with the competent authority (ILO, 2009), which in the UK is the MHRA. The competent body is responsible for designating whether an organisation follows the requirements to be a notified body (GOV.uk, 2017b). As well as following the Medical Device Regulations, the device must also follow the General Product Safety Regulations 2005 as the device is intended to be used by a consumer (GOV.uk, 2017a). Both of these regulations fall under the Consumer Protection Act 1987 (GOV.uk, 2017a) and they are in place to make sure that the devices are safe to use and will not cause harm to any member of the public.
Should the robot not meet the required legislations, the correct bodies must be notified. If the device is not CE marked, then the device will have to undergo a clinical investigation in order to obtain the marking and this must be told to the MHRA at least 60 days before the clinical investigation is set to begin (GOV.uk, 2014). As well as this, a notified body must be contacted in order to gain the CE marking, as they are the ones who issue it (GOV.uk, 2017b). The notified body will then run all the necessary tests to make sure that the device is suitable to be used and that it follows all of the legislation that has been set out. It is important that a notified body is contacted as they are responsible for making sure that a device is safe for public use. However, it is the manufacturers’ job to make sure that the correct bodies have been notified. Depending on the class of the device, the device will have to conform to different requirements (Conformance, 2017). This device is classed as a class I device, as it is non-invasive and is classed as an assistive technology (Stefanov, 2018). When looking to obtain CE marking, it is important to follow the correct process. It is paramount that the directives and standards that are applicable to your design are identified so that the correct documentation can be obtained (GOV.uk, 2012). The product must then be tested to check its conformity to its relevant classification by a notified body (GOV.uk, 2012).
Alongside gaining a CE marking for the device, the manufacturer must be sure to have an established risk management process. The ISO 14971 is used so that manufacturers can ‘identify the hazards associated with medical devices’ in order evaluate said risks and be able to control them (ISO, 2010). It is vital that a device undergoes risk analysis so that the device can be considered safe (Palanichamy, n.d.). The basic structure of risk management is to identity all of the possible hazards and determine the severity of them (Palanichamy, n.d.). Should the hazard be of an acceptable risk level, then the next hazard can be dealt with. Should the risk be deemed unacceptable, then mitigation must be applied. Mitigation is the process of doing something about the situation, with the ideal scenario eliminating the failure cause and if need be, reducing the impacts of said failures (Stefanov, 2018).
When the device is given to the user, it is important that the correct documentation is given too. This would include the user guide, a maintenance log and any important numbers for them to contact should something go wrong. In order to make sure that the patient is comfortable and confident when using the device, it would be a good idea to have an expert on hand to show them how it works and provide any help should there be any difficulties using the device. This could be done in a controlled setting, such as a hospital, before the patient receives the device so that any mishaps that occur won’t hurt anybody else or damage anything. As well, this would be the best place to make sure that the controller for the device is connected to the wheelchair in a way that is well suited to the user and to check that everything works in the way it should. When the device is in the home, it should be kept away from heat sources so it does not overheat, even when not in use, as this could damage the device (Kinova Robotics, 2017). This is especially important for finding a suitable charging point for the device and it must be ensured that there is a suitable power outlet for the arm to get charger at. As well, when starting the device, there should be ample room to ensure that the arm does not hit anything.
The Jaco assistive robot is intended to be used by a single user attached to their own personal wheelchair. However, this does not mean that it cannot be used by other people, either by attaching the robotic arm and controller to their own wheelchair or through the use of a communal wheelchair. One of the advantages of having the Jaco arm as a personal device is that the user will get constant use of it and so can improve their abilities very quickly. As well, there would be less of a need to clean it constantly as it would not be necessary to decontaminate between users to prevent infections (NHS Salisbury, n.d.). A positive to the Jaco robot being used in a care home is that it provides many people with the chance to do tasks for themselves that they otherwise cannot do. It could also be used within a special school to aid the learning of both a class and the individual. As well, the device is expensive and so it would not be cost efficient to have a personal one for every single patient. However, it is recommended that the device is not cleaned more than 3 times a day and so that would limit the number of users per day, as it would need to be decontaminated after each use. Despite this, the Jaco robot is more suited to an individual user who does not live inside a care home and wants to be able to be more independent due to the logistics.
There are varying degrees of maintenance required for the Jaco assistive robot. The first is the day to day cleaning which would be undertaken by the carer or patient. Another type of maintenance would be the planned preventive maintenance of the device. This is where maintenance is scheduled in to ensure that catastrophic failures do not happen (Medstrom, n.d.). This could be carried out via the hospital issuing the device or through a third party. The preferred maintenance method is through the use of a clinical department at the patient’s hospital as there would already be a rapport between the two parties which would make the patient feel as though they can trust the servicer more.
In order for a patient to understand and use the device successfully, they will need to have support from staff who have had training themselves. Staff training is important as it allows them to fully understand the equipment and how to use it as well as to be able to recognise any faults the device may have (MHRA, 2015). The best way for the staff to be trained is through a training day run by an expert on the Jaco assistive robot so that the staff can feel confident in answering any questions that the patient may have for them.
If the device is used by a single person, the decontamination process would not need to be that in depth. The device is considered low risk and so the recommended decontamination procedure is to clean it (MHRA, 2015). The arm is not intended to be sterile (Kinova Robotics, 2017) and it is suggested that it is cleaned no more than 3 times in one day. This would be sufficient to decontaminate the device as it isn’t intended to be used by those with infectious diseases. When the device gets to the end of its lifecycyle, it will need to be decommissioned, which involves making the device ‘safe and unusable, while minimising damage to the environment’ (MHRA, 2015). Should the device have any reusable parts, then these must be placed in bags and labelled accordingly (HSE, 2004). Any remaining parts should then be disposed of in the correct manner. Some parts of the arm can be recycled whilst others must be sent back to the manufacturer in order for them to dispose of all remaining parts properly.
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