NEW DEVELOPMENTS IN 3D PRINTING FOR KNEE REPLACEMENTS

Knee replacement surgery is one of the most successful and cost-effective surgical procedures performed in medicine to-date.^[1] Most of these procedures convey a success rate over 90%^[2], providing pain relief to those battling with their knees.

The below article outlines what knee replacement surgery is, who are the most ideal candidates, and what to expect when acquiring the surgery. Types of prosthetic knees are discussed as well, alongside surgical complications and contraindications.

What Is Knee Replacement Surgery and Who Should Get It?

Knee replacement surgery is a surgery that intends to correct long-standing knee damage through reconstructing the knee joint^[3]. It’s also known as knee arthroplasty.

Usually knee replacement is only recommended for those who have severe knee arthritis or injury, for whom non-invasive treatment options failed to manage symptoms. For these patients, symptoms often get worse, tending to impact on their ability to sleep and perform daily activities.

The procedure usually serves to replace the cartilage of the knee with a prosthetic. The cartilage is the part of the knee that is most vulnerable to becoming damaged in response to arthritis or injury. Other parts of the knee are often shielded with artificial parts^[4], depending on the requirements of the individual.

Knee Anatomy. The knee is a joint that connects the knee cap and the bones of the lower and upper parts of the leg. Wedged between these structures are two types of cartilage: articular cartilage and the meniscus. Both types cushion the bones from friction and other forces the knees encounter. The meniscus acts as the main shock absorber for the knees. These layers of cartilage hold the bones in place and are slung within the confined of ligaments that keep the knees contained.

Replacement of Cartilage. Injuries and various forms of arthritis tend to damage the cartilage, resulting in a loss of insulation in the knee. The cartilage can be damaged due to ligament inflammation, bone growths or bone fractures. Over time, enough cartilage damage results in chronic inflammation, more damage, prolonged pain and eventual difficulty with using one’s knees. Knee arthroplasty merely replaces the damaged cartilage in the knee with a prosthetic, which improves insulation, lowers pain and enhances movement.^[5]

Designing a Bio-Identical Prosthetic. In between the layers of cartilage, a cavity exists that provides enough space for the joint to perform a wide variety of rotation without placing excessive pressure on the knee. When structures in the knee become inflamed and damaged, they can sometimes reduce or increase the size of this cavity. This can result in an inability to stand, walk, sit and/or move the knees. Gauging the size of this cavity is one of the biggest hurdles that orthopedic specialists need to overcome when designing a prosthetic.

What to Expect

If you’re considering getting a knee replacement, the process consists of the following^[6]:

• History and Medical Testing

Once approached by the patient, the surgeon will take a medical history and run a few test prior to surgery to make sure they’re an ideal candidate. Part of this includes a physical examination, dental evaluation and urinary testing. Those with chronic infections, particularly urinary and dental infections, are not good candidates for knee replacement surgery. If you have a history of heart problems, you may need to consult with a cardiologist to make sure you can withstand the recovery without a problem.

• Prosthetic Design

In general, knee replacement prosthetics consist of up to three parts:

• Femoral Component. The femur is the bone of the higher leg, the lower end of which forms the top half of the knee. In a knee replacement, this is typically resurfaced with a metal plate in order to maintain its shape in the face of bone damage.
• Tibial Component. The top portion of the tibia forms the lower half of the knee. The prosthetic for the tibia is similar to that of the femur, consisting of a metal plate the covers it and gives it a better shape. This component also consists of a plastic layer that mimics the functions of the meniscus, providing a cushion for any of the impacts that the knee encounters.
• Patella Replacement. If required, the patella is either replaced or resurfaced to provide better functioning of the knee. This is usually done in a durable plastic, similarly to the prosthetic meniscus of the tibial component.

In some types of prosthetics, artificial ligament structures are also incorporated into the design. If not, the surgeon often re-arranges the ligaments in order to optimize the functioning of the knee.

Prosthetics for knee replacements are different to those for prosthetic legs, and do not operate electronically or pneumatically. These three parts are designed in such a way that mimic the recipients knee, moving in a coordinate manner that minimizes friction and protects the person’s existing bone tissue. Types are discussed in more depth below.

The damage to the knee will need to be assessed so a prosthetic can be designed by an orthopedic specialist. This is often achieved with an x-ray, CT scan or MRI of the knee. Either custom-made prosthetics or a standard set of prosthetics will be used in accordance with the person’s shape and size. Aside from custom-made ones, more than 150 designs are available on the market.

• Procedure

The surgical procedure itself can take 1-2 hours after an evaluation by an anesthetic specialist. During the procedure, the patient is typically placed under anesthetic.

Incisions are made into the knee to access the damaged areas. These usually demand removing one of the prime ligaments that hold the knee joint in place.

The damaged cartilage is then removed, as well as the surface of the bone on either side of the joint. The metal implants are then inserted to cover the bone and give it more shape.

Implants can be cemented in, pressed to fit precisely against the bone or a combination of both. In the case of press-fit implants, materials are used that encourage the bone to grow firmly against the implant. This lowers the risk of implant rejection and can be enhanced through the use of stem cell technology.

• Recovery

The first phase of recovery commences in the hospital. The patient remains in the recovery room for several hours till the anesthesia wears off. After gaining consciousness, the patient may remain in the hospital for 1-3 days or be discharged right away; depending on their condition.

Before the patient leaves, a doctor will make sure to manage for pain, blood clotting, and infection by prescribing the necessary medications. A physical therapist is often employed to show the patient knee exercises, which promote muscle growth and help to integrate the knee implants. The patient is advised to start off very slowly with exercise and to spend as much time recovering as possible with their leg elevated to help decrease swelling.

The second phase of recovery takes place at home. After 3-6 weeks, a patient is able to resume normal activities following surgery. The recovery time depends on whether there were surgical complications and how well the doctor’s instructions were followed.

Sitting, standing, walking and climbing stairs are advisable once at home; however they all need to be approached with caution. Avoiding falls is vital. The wound will need to be cleaned daily once properly sealed and kept wrapped in bandages. A follow-up visit will be required in a few weeks to remove stitches or staples. If advised by your surgeon or doctor, a pressure sock or stocking may be used to help circulation and prevent blood clots. The doctor will often prescribe a healthy, balanced diet in order to support the healing process.

Prior to surgery, it can be a good idea to install safety bars and rails in your home to assist you with getting up and down. Crutches or a walking cane ought to be arranged beforehand as well. Sleeping and living on the same floor is preferable, as is removing loose carpets, and arranging a shower chair or bench. Assistance with cooking, shopping and other daily activities is crucial during recovery to facilitate optimal healing.

Types of Knee Replacement Prosthetics

There are many types of knee prosthetics available on the market. The main types refer to those pre-made and those custom-made.

1. Pre-Made Prosthetics

Pre-made prosthetics have been the golden standard of knee arthroplasty since it’s first recorded appearance in the 1800’s. Unlike custom prosthetics, the orthopedic surgeon need only find the best matching prosthetic for the patient’s shape, size and constraints.^[7]

In total knee replacement surgery, one of the patient’s ligaments (posterior or anterior) are often cut away in order to insert the prosthetics. Different types of pre-made prosthetics have been made that emphasize preserving one or both ligaments in the patient. These include:

• Posterior-Stabilized Implants. These types do not have artificial ligaments and parts of the design compensate for the posterior cruciate ligaments. Usually the tibial component acts as the substitute by being raised to fit into the femoral component.
• Cruciate-Retaining Implants. Artificial ligaments are incorporated in this type of design, often used if the patient’s posterior cruciate ligament is healthy enough to remain intact post-surgery.
• Bicruciate-Retaining Implants. This design allows for both anterior and posterior ligaments to be preserved in the patient. In theory, this allows for a more natural feel to the knee replacement.

The knee itself is a socket joint, allowing it to rotate in many directions due to the way in which the parts interact inside the socket. Most knee replacement implants are not as sophisticated as a normal knee. Two types are commonly produced in regard to mobility and rotation:

• Fixed-Bearing Implants. Fixed bearing implants do not allow for knee rotation. The plastic spacer (artificial meniscus) attached to the lower tibial component is fixed, only allowing for the top component to roll off the bottom.
• Mobile-Bearing Implants. As the name suggests, this type of implant allows for increased mobility and rotation. The plastic meniscus is able to rotate above the tibial component, designed to move within the confines of a tray.

Not all knee replacements involve replacing the entire knee:

• Unicompartmental Implants refer to implants that only replace half of the knee. This includes resurfacing half of the tibial and femoral components, half of the meniscus and the full knee cap in some instances.

2. Custom-Made Prosthetics

Custom prosthetics have been on the market since 2011. Since this time, studies revealed that they have substantially improved outcomes for those getting a knee replacement. ^[8]

Generally, the process involves taking multiple CT or MRI scans of the knee at different angles and layers. Through compiling a series of 2D images, an accurate 3D image can be rendered on a computer. An orthopedic specialist then takes this model and does their best to create an accurate knee prosthetic, best tailored to suit the client’s needs. It typically takes anywhere between 6 and 8 weeks for the prosthetic to be completed.

The advantages of using custom prosthetics are that they tend to be more precise, fitting better than pre-made types. However, custom prosthetics are likely to be designed in a similar fashion to pre-made ones, with subtle variations that enhance their functionality. The only true drawbacks to custom prosthetics would be the extra time it takes to construct and potential difficulty in replacing parts if it ever wears out after long-term use.

3D Printed Prosthetics

Unlike conventional custom-made prosthetics, 3D printed prosthetics are nearly 100% accurate; fully customizable to the patient’s anatomy. With 3D printed knee replacements, there is no room for ill-fitting, making them the best currently available option for knee replacements.

Like other custom prosthetics, an accurate 3D image is rendered on a computer from a compilation of 2D images of the knee. This image is then printed in three dimensions to form an exceedingly accurate replica of the patient’s knee joint. This type of prosthetic typically eliminates complications derived from an ill-fitted replacement.

This ground-breaking surgery is the future of all implant surgeries. Already multiple fields and branches of medicine are working on 3D printing, but knee replacements are way ahead of the game, already on offer to patients. The success rates are the highest for this type of knee replacement surgery. Compared to traditional joint replacement surgery, patients report quicker recovery times and better functioning of the prosthetic.

Future 3D printing techniques are expected to make use of cellular components in order to replicate fully functioning knee parts; such as cartilage, bone, and muscle. Stem cell therapy can be used to avoid prosthetic rejection, thus enhancing the efficacy of these future techniques.

The process is still being perfected; however it is being performed on patients at top medical establishments. If you need new joints, it’s worthwhile to discuss this option with your orthopedic doctor as the results are likely to be of long-lasting benefit.

Complications and Prevention

The below points described complications that are common during and after knee replacement as well as how they are best prevented:

• Mismatched Prosthetics. In traditional knee implants, the process of matching the prosthetic and recipient limbs is often difficult. If the prosthetic limb is mismatched, it can cause more pain than the original knee condition did in the first place. In some cases, this can result in the need for additional surgery and a better prosthetic implant. 3D printing minimizes the risk of having this complication.
• Surgical Inaccuracy. Part of the surgery demands re-sectioning tissues in the knee. This helps to insert the prosthetic and integrate it properly for optimal use. Surgeons don’t always get it right, resulting in misalignment of the surrounding tissues, which often gives rise to poor results, damage to surrounding tissues and demands additional surgery to correct. Robot-assisted surgeries have been implemented in top hospitals to improve upon accuracy with great results.^[9]
• Prosthetic Rejection. The recipient’s body may reject the implant as a foreign body, resulting in increased inflammation and pain. In some cases, the body can begin to demolish the foreign body, working to dissolve it over time. This can greatly reduce the lifespan of the prosthetic limb.
• Osteolysis. In some patients, the inflammation arising from the surgery causes the bone tissue to break down. This results in a loosening of the prosthetic, increased pain and a worse prognosis.
• Post-Surgical Infections. The physiological stress of surgery often gives rise to infections after the event. Prosthetic joint infections are another frequent complication of replacement surgeries. Infections can prolong recovery time and may also interfere with the healing process, promoting sub-optimal results. Antibiotics are usually prescribed in order to prevent this complication.
• Difficult Recovery and Rehabilitation. If any complications were experienced during the surgery, the recovery time can be extended and made more difficult. It may also be more of a challenge to get used to walking on one’s prosthetic limb and rehabilitation may require additional physical therapy sessions, adding to the total cost of receiving a knee replacement.
• Blood Clotting. The risk of blood clots increases after any surgical procedure. Blood thinners are often prescribed after surgery to avoid this complication. Increased pain in the area, redness, swelling, shortness of breath and chest pain are all signs of developing blood clots. If these symptoms are experienced during recovery, it’s important to notify your doctor and make an appointment.

Complications are less common when this surgery is conducted in an establishment that performs them frequently.

Contraindications

Individuals with severe sepsis, systemic infection, and late-stage or untreatable peripheral arterial disease should never receive a knee replacement.^[10]

Arthritic patients for whom the surgery is indicated may not be able to get the procedure if they are already taking high doses of narcotic drugs for pain relief. High doses of narcotics will greatly reduce the effectiveness of anesthetic. The doctor may arrange a prescription schedule for the patient to cut down and stop before surgery can commence, if possible.

Those with the following may be at an increased risk for complications:

• Obesity
• Extremely brittle bones or insufficient bone to manage reconstruction
• Poor ability to build muscle^[11]
• Terminal illnesses
• Blood clotting disorders and related cardiovascular diseases
• Inflammatory skin conditions
• Neuropathic pain

In severe cases, surgery is not advised for any of the above conditions. Additionally, those who cannot comply during and after surgery should not opt for the surgery.

The elderly who are at a high risk for dementia may wish to avoid knee replacement surgery as it greatly increases the risk for delirium, depression, and anxiety post-surgery.

Conclusion

For the vast majority of patients in extreme pain as a result of arthritis or injury, knee arthroplasty greatly alleviates pain and improves the quality of life. Complications are minimal by comparison to many other surgeries and the prognosis is generally good when the patient complies to the recovery regimen. Surgical techniques and prosthetics continue to improve the efficacy of knee replacements, with 3D printing expected to replace all currently available prosthetics on the market.

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