Partial Knee Replacement, also known as Unicondylar Knee Replacement or Unicompartmental Knee Replacement is a surgical procedure performed to alleviate pain and mobility issues in patients who have limited damage to knee joints. The damage could be due to arthritis, rheumatoid arthritis or trauma. Before going into details of partial knee replacements, we must understand the anatomy of a knee joint.
Knee Joint Anatomy
Knee is the largest joint in our body, where bones of the upper and lower leg meet. It is one of the most complex and extensively used joints which moves like a hinge and facilitates movements like sitting, squatting, walking, and jumping. The knee joint comprises of the following bones:
- Lower end of thigh bone (femur).
- Upper ends of shin bone (tibia) and the smaller shin bone next to tibia (fibula).
- Kneecap, a bone at the front of the knee (patella).
Ends of these bones are covered with articular cartilage, which acts as a shock absorber and allows gliding of bones against each other during movements.
Between the thigh bone and the shin bone, there are two pads of connective tissue (meniscus) that help in reducing friction between the bones. They are:
- Lateral meniscus
- Medial meniscus
Joint capsule (synovial joint capsule) is a dense fibrous connective tissue that is attached to these bones via specialized attachment zones. It forms a sleeve around the joint space, holding bones together and providing active and passive stability to the joint.
Joint capsule is made of bundles of collagen fibers and is lined with synovium. It forms a seal that secretes synovial fluid. Synovial fluid circulates around the joint bones and helps provide lubrication and nutrients to the joint.
The joint capsule is made of two layers:
- Outer membrane made of dense connective tissue
- Inner membrane called the synovium, which secretes synovial fluid.
The outer membrane is attached to the ends of the bones and is supported by ligaments and tendons. Nerves and blood vessels pass through the joint capsule.1
Bursae are synovial fluid-filled sacs found between the moving structures in a joint, they reduce wear and tear. A knee joint has four bursae:
- Supra-patella bursa
- Pre-patella bursa
- Infra-patella bursa
- Semimembranosus 2
Ligaments of the knee hold the bones together and provide stability to a knee joint. Four main ligaments of the knee are:
- Collateral ligaments: Medial and lateral collateral ligament present on either side of the knee, control sideways motion of the joint.
- Cruciate ligaments: Anterior and posterior cruciate ligaments are found inside the knee joint and cross each other to form an ‘X’. They control the joint’s motion forward and backward.
Tendons connect muscles to bones.
- Quadriceps tendon connects muscles in front of the thigh (quadriceps muscles) to the kneecap.3
- Patellar tendon is actually a ligament that connects the bottom of the kneecap to the top of the shin bone.
Quadriceps muscles, quadriceps tendon, and patellar tendon helps in straightening the knee.4
Muscles supporting the knee are:
- Hamstrings: helps is bending the knees.
- Quadriceps: helps in straightening the knees.
Partial Damage to Knee
In cases of advanced osteoarthritis where non-invasive treatment methods do not relieve symptoms, knee replacement surgeries are advised.
The knee is divided into three major compartments:
- Medial compartment
- Lateral compartment
- Patellofemoral compartment
In a partial knee replacement (or unicompartmental knee replacement) only the affected compartment of the knee is replaced. Whereas in a total knee replacement all three compartments of the knee are replaced.
It is mostly indicated in osteoarthritis and is increasingly considered for patients below 55 years of age.5
Osteoarthritis happens due to breakdown of cartilage, bony changes of the joint, deterioration of tendons and ligaments, and various degrees of inflammation of the synovium.6
When only one of the compartments of the knee joint is affected, partial knee replacement is recommended. It is a more conservative surgical approach as compared to a total knee replacement since only the affected compartment of the knee is replaced. However, it is usually not advised in obese patients.6 7
Campbell in 1940, reported his preliminary results on the interposition of Vitallium plates in the medial compartment of arthritic knees. In early 1950s McKeever stated that osteoarthritis could affect only a single compartment in a knee joint; and that, in such cases, the replacement of the entire joint may not be required.
Partial knee replacements were designed to preserve most of the bone and anatomy, unlike total knee replacements. The use of smaller implants resulted in a shorter recovery period.
In 1957, McKeever introduced Vitallium tibial plateau prosthesis.
MacIntosh in 1958, came up with a tibial plateau made of acrylic; which was subsequently followed up by another variety, made of Vitallium, in 1964.
The implant was reported to provide an overall pain relief in most patients, on 6 years follow-up. However, it was noted that the lack of fixation of the device could lead to migration and unsatisfactory results. Hence McKeever added a keel to this prosthesis, for attachment.
Those days, the success rate of partial knee replacements was poor. Over the past decades, partial knee replacements have evolved significantly. Recent advances in technology have led to improved implant designs and surgical techniques. Besides, better prognosis of these surgeries is also attributed to appropriate patient selection.
Owing to the fact that partial knee replacements are minimally invasive surgeries, the popularity of such surgeries and demand of these implants has increased manifold.
Types of implant used
The choice of implant depends on which compartment of the knee is damaged and whether the anterior cruciate ligament (ACL) is intact.
- Unicondylar fixed bearing – is most commonly used implant for partial knee replacements.
- Unicondylar mobile bearing – is the other type of implant used for these procedures. The placement of these implants is more technically demanding.
Computer-assisted techniques have also been developed for knee replacement surgeries, in which a 3D image of the joint permits more precision in the removal of the damaged tissues and implant positioning.7
Popular partial knee replacement implants used today are
- Unispacer (Smith & Nephew, Inc., Memphis, TN)
- ConforMIS iFORMA8
Partial knee replacements are minimally invasive procedures. It is an advanced technique which involves minimal damage to muscles, ligaments, and tendons of the joint. This surgery has a faster recovery period and less postoperative pain as compared to total knee replacements.
A partial knee replacement surgery requires an incision in front of the knee. Damaged cartilage is then removed from the knee compartment involved.
Subsequently, the ends of femur and tibia are covered with metal implants which are then fixed to the respective bones. An acrylic (plastic) component of the implant is inserted between the implants to facilitate gliding movements.
The implant fixation can be either done by a cement or cementless. Sutures or stitches are placed to seal the incision.
During surgery, while assessing the joint, the surgeon may decide to perform a total knee replacement instead, if he deems necessary. Patients have to rely on surgeons for clinical judgment. Usually, surgeons discuss this probability with patients before surgery.
This entire procedure is usually carried out while the patient is under general anesthesia or regional anesthesia (spinal or epidural) and takes approximately 1 to 2 hours.
After surgery patients are rested in a recovery room to be closely monitored by nurses while recovering from anesthesia.
The surgery involves 2 days of hospitalization and a total recovery period of 3 to 4 weeks. However, it may be possible to apply weight on the joint with support, immediately and carry out routine activities like walking or visiting the lavatory.9 10 11
Partial Knee Replacement Cost
In India, a partial knee replacement surgery may cost you approximately Rs 2.6 to 3.2 lakhs INR. However, the prices vary depending on the type of medical facilities chosen and the surgeon’s fee. Hence this price range is not absolute but is meant as a guide for patients before they plan their surgery.
Similar to most surgeries, partial knee replacements come with additional costs, like pre-operative tests and medications; post-operative rehabilitation and medications; follow-up visits; the cost of physiotherapy, and miscellaneous costs like crutches or canes.
Some government and charitable hospitals in India may provide the same surgery at a subsidized rate.
Besides, implants manufactured in India, are less expensive than imported implants, but most patients are not aware of this option. Many surgeons do not prefer the use of Indian manufactured implants because they have been used for not more than 9 years and their long-term success rates haven’t been established yet.
If you already have a medical insurance, then the good news is that most of the medical insurance companies in India now cover knee replacements. However, such claims are only accepted provided the policy has commenced 2 to 3 years prior to surgery; especially in cases with degenerative conditions like arthritis and osteoporosis.
Patients need to talk to their surgeon and medical insurance company, well in advance. It may be prudent to find out, how much of the cost will be reimbursed. Although the coverage may not amount to the total cost, it would definitely make the surgery affordable.
Recovery after Partial Knee Replacement
Recovery after partial knee replacements takes around 3 to 4 weeks, after which it is possible to walk without assistance. This procedure is characterized by less amount of pain and swelling as compared to total knee replacements.
Medications like opioids, NSAIDs, and local anesthetics are used for pain management.
Although these medicines help in making recovery more comfortable and faster, they need to be discontinued as and when your doctor suggests.
It is important to enroll into a physical-therapy program along with home exercises for optimal outcome of a partial knee replacement surgery. Exercising and maintaining mobility will help prevent scarring of the replaced joint and improve muscle strength around the joint.
Physiotherapy and exercises are usually recommended for 4 to 6 months post-surgery. They help in strengthening muscles, increasing the range of motion, and preventing scarring of the replaced joint.
Periodic follow-ups are essential post-surgery, as advised by your surgeon.
Having an artificial joint is an important aspect of your medical history, and this information should be always shared, before you undergo any medical treatment in the future, including dental treatments.12
Indications for Partial Knee Replacement
Partial knee replacements are usually recommended in:
- Unicompartmental disease.
- Patients older than 60 years of age, who aspire to perform routine activities comfortably, post-surgery.
- Patients who are not obese (less than 82 kgs in weight).
- Patients having intact joint ligaments (specifically ACL).
Recently, partial knee replacements have been recommended for younger patients with moderate arthrosis. This procedure allows preservation of more bone making revision surgery easier.
Younger patients eventually need a revision surgery, as knee implants have a lifespan of 15 to 20 years. Partial knee replacements have thus proven to be advantageous in younger patients, as well.
The outcome of a partial knee replacement depends more on the type of arthritis and amount of damage to the joint, rather than the patient’s age.13
Contraindications of Partial knee Replacement
Partial knee replacements are not recommended in cases of:
- Inflammatory arthritis.
- Rheumatoid arthritis.
- Significant knee stiffness.
- Anterior cruciate ligament (ACL) deficiency.
- Younger patients with high activity.
- Heavy laborers.
- Overweight patients.14
Comparison of Partial Knee Replacement with Total Knee Replacement
A study in 2015, conducted a comprehensive search of studies comparing partial knee replacements and total knee replacements, performed in the same patient. It was found that a joint with partial knee replacement provided statistically greater range of motion.
Furthermore, no differences were observed between partial knee replacements and total knee replacements with respect to joint functions, post-surgical complications, and survivor rates of the implants.15
Most of the failures of partial knee replacements have been found to be due to loosening of the implant and increased wear and tear due to delayed revision surgery. Another most common cause of failure is arthroscopic degeneration in opposite compartment of the joint.
In partial knee replacements, long-term failure rate is said to be 1.8 times higher than total knee replacements. Also, the cumulative revision rate has changed in total knee replacements, with advancement in technology, whereas the same is not true for partial knee replacements.16
Let us discuss some points of comparison between partial knee replacements and total knee replacements.
Advantages of Partial Knee Replacement
When compared to total knee replacements, partial knee replacements have certain advantages:
- Quicker recovery.
- Less blood loss.
- Less cost.
- Minimally invasive involves preservation of most of the joint anatomy which is not damaged. Hence, more normal knee movements can be achieved.
- Less post-operative pain.
- Higher initial success rate.
- Better aesthetics post-surgery.
- Longer lasting.
- Easy to convert to a total knee replacement.
- Easier revision surgery.
Disadvantages of Partial Knee Replacement
Just like any other surgery, partial knee replacements come with the possibility of complications, such as blood clots, continued pain, infection, injury to nerves and vessels, and risks associated with anesthesia. However certain complications are specific to these surgeries, like:
- Stress fractures, involving the shin bone (tibia).
- Tibial component collapse
The risk of complications increase due to faulty implant or if the patient has a chronic illness.
Implant failure can happen due to patient’s active lifestyle, wear and tear, faulty implant design or placement, and loosening of the implant.
The lifespan of an implant is 15 to 20 years, hence younger patients who undergo partial knee replacements have to go through a revision surgery at a later stage.
A second surgery may also be needed to correct a faulty implant; in the event of implant failure; dislocation or loosening.
A revision surgery is more complicated and requires special implant tools. Apart from being more expensive, revision surgeries require a longer recovery period. Unfortunately, revision surgeries may give unsatisfactory results as compared to primary partial knee replacements.
The associated risks of partial knee replacements include:
- Higher rate of revision surgery.
- Complicated revision surgery.
- Worse function after revision surgery.
It has been established that proper patient selection is essential for the success of partial knee replacements. Based on patient’s joint condition, general health, and lifestyle post- surgery, a surgeon decides whether to choose a partial knee replacement or otherwise.
It is important for you to speak to your doctor and get a clearer picture about the procedure, its suitability in your joint condition, implant selection, and not to mention the costs. If you help your doctor in providing you the optimal treatment, you can benefit in receiving satisfactory results and a pain-free life ahead.
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