The knee is the largest joint in the human body, consisting of the femur, tibia, and patella, functioning as a hinge that allows the flexion and extension of the leg. Muscles and ligaments hold the joint together, providing stability and enabling movement. The articular surfaces of the three bones are covered by cartilage, which cushions the contact between joint surfaces, reducing friction during movement. The knee comprises three compartments: the inner compartment, the outer compartment, and the femoro-patellar joint.
Certain diseases lead to the irreversible destruction of joint surfaces, and only a prosthesis can permanently resolve the symptoms.
In the initial stage, osteoarthritis may improve with weight loss and targeted rehabilitation focusing on muscle strengthening and stretching. Cartilage supplements, pain management with ice and anti-inflammatories, and joint injections can also be used. As the disease progresses, sports become impossible, and daily activities such as walking or climbing stairs are limited. Pain may also be present at rest or during the night. In advanced stages, conservative treatments are often ineffective, and prosthetic surgery is necessary to resolve symptoms, removing or significantly alleviating pain and restoring joint movement.
Personally, I recommend surgery for patients with severe osteoarthritis, evident from X-rays showing bone-on-bone contact, and experiencing any of the following symptoms:
- Persistent pain despite pain relief therapies
- Constant pain even at rest
- Difficulty standing for extended periods
- Inability to perform daily activities like walking or climbing stairs
- Pain while walking
- No benefit or limited benefit from physiotherapy and conservative treatments
There are various types of knee prostheses: total surface replacement, partial or mono-compartmental prosthesis, and revision prosthesis. The surgical procedure removes damaged cartilage and bone, replacing it with a prosthetic implant. Prostheses are made of various materials: titanium, polyethylene, chrome, cobalt, and cement often used to secure them firmly to the bone. The choice of implant type varies from person to person, depending on the type of arthritis, the patient’s age and activity level, and the quality of the bone. Typically, a small amount of bone is removed from the tibia, femur, and patella during surgery to eliminate damaged structures and create good-quality bone surfaces for prosthetic placement. Most ligaments and tendons are left in place, allowing natural knee movement.
Total Knee Arthroplasty
In total knee arthroplasty, the diseased bone and cartilage are removed and replaced with prosthetic components, usually composed of metal and polyethylene. This combination of materials has shown excellent results with minimal long-term wear despite use.
The prosthesis consists of four components:
- Femoral shield: Made of metal, usually titanium, coated with alloys that reduce friction and wear during movement (commonly chrome-cobalt). Once positioned, it wraps around the articular part of the femur.
- Tibial component: Made of titanium, usually equipped with a small stem that enters a few centimeters into the tibia bone to increase stability.
- Insert: Placed on the tibial component, made of polyethylene, a highly resistant plastic material that generates minimal friction during prosthetic movement.
- Polyethylene button: Replacing the patellar component, applied to the patella when this bone is damaged by arthritis or in cases where the joint is affected by a rheumatologic disease.
Each knee prosthesis procedure is customized based on the patient’s needs, with significant variations from case to case. General steps of the procedure include:
- Administering anesthesia and, according to the patient’s preference, sedation for comfort during the procedure.
- Surgical incision in front of the knee to access the joint.
- Removal of damaged cartilage and bone to prepare the bone ends for the implantation of prosthetic components.
- Ensuring that the prosthetic components are correctly positioned on the bone.
- Testing the new joint.
- Closing the incision.
Partial or Uni-compartmental Prosthesis
In some patients, arthritis develops in only one area of the knee (inner compartment, outer compartment, or femoro-patellar joint). In these cases, partial prosthesis may be the best solution. Instead of replacing the entire knee, patients with partial damage can undergo a partial prosthesis procedure, which involves less surgical stress, less blood loss, less pain, and a faster functional recovery. Additionally, the healthy joint portion is preserved. There are partial prostheses for the inner compartment, the outer compartment, and the patella. Partial prostheses have smaller femoral and tibial components but are made of the same materials; the surgical technique is similar but involves a smaller incision.
Some patients develop selective patellar arthritis, experiencing symptoms predominantly during flexion movements such as rising from a chair or climbing stairs. A smaller prosthesis has been developed to address this condition and is called the femoro-patellar prosthesis.
Materials in Knee Prosthesis
In recent years, prosthesis designs and the number of available sizes have significantly increased, allowing true customization of knee prostheses. The most commonly used metal is titanium, coated with a chrome-cobalt alloy that has shown excellent long-term resistance. Chrome-cobalt contains nickel, to which some patients may be allergic. It is unclear whether nickel skin allergies can lead to joint complications; however, nickel-free prostheses are available for allergic patients. Cross-linked ultra-high molecular weight polyethylene is highly wear-resistant. Typically, knee prostheses are fixed to the bone with cement. This ensures immediate and solid anchoring, with a very long lifespan. In certain patients, such as young individuals with good bone quality, non-cemented prostheses made of a material that integrates with the bone can be used.