Knee joint replacement procedures have rapidly become one of the most frequently performed, as well as life-enhancing, surgeries. Many people who suffer the ravages of advanced degenerative arthritis can receive much-needed pain relief, together with improved mobility, which ultimately leads to a better quality of life. Knee joint replacements can be constructed of various materials and have evolved into very patient- and condition-specific configurations, so that components as well as surgical techniques can be tailor-made to specific patient needs. Two of the important variables involving knee joint replacement include the types of materials used in components and how they are implanted into the body.
Cemented Knee Replacement
The cemented knee joint replacement is one of two major forms of component implantation. Implant construction is specific to the use, or lack of use, of bone cement. In the case of cemented components, the surfaces of the implants that will ultimately interface with the cut surfaces of the bones in the knee are essentially smooth, but not polished.
The bone cement, which acts more as a grout than an adhesive, attaches to the surface of the implant as well as the small honeycomb openings of the bone, primarily because the components are inserted and placed into position under pressure, which helps disperse the cement and fill the small openings in the bone.
Un-Cemented / Press-Fit Knee Replacement
This type of knee joint replacement consists of implants with metallic surfaces; those that interface with bone are coated with a rough, porous surface that allows for and stimulates actual bone growth into the small openings between the metallic particles. This process is also known as box-fixation or bio-ingrowth implants; bio meaning biologic referring to bone, and ingrowth referring to the actual adherence of bone to the metallic implant backing. Implant components are often coated with special materials, such as a substance called hydroxyapatite, which can serve as a platform for bone growth, as well as a stimulus for it.
Partial Knee Replacement
Partial knee replacements are typically performed for advanced arthritis that is well-localized to one femoral condyle, or knob of the thigh bone, while the remainder of the knee joint displays healthy cartilage. Because knee joints are divided into compartments, normally three, the term uni-compartmental or uni-condylar also refers to partial, one-sided knee replacements.
In uni-compartmental knee replacement surgery, the femur, or thigh bone, and the tibia, or shin bone, are resurfaced, just like in the conventional total knee replacement. A plastic spacer is also inserted between the metallic implants. This surgical procedure often requires less invasive techniques and can result in less pain, a shorter hospital stay and quicker recovery.
Rotating Hinge Knee Joint
The rotating hinge knee joint is typically not used for primary knee replacement surgery. This type of knee replacement is also known as a constrained prosthesis, which means that very little if any natural knee motion or mechanics is allowed.
Rotating hinge knee replacements have been essentially reserved for patients who are undergoing revision knee surgery for reasons such as previous infection, tumor formation or severe implant loosening with bone destruction. These situations are typically observed in the elderly and more sedentary populations.
By design, the rotating hinge knee, or RHK, does allow for some minimal natural rotation of the tibia, or shin bone, during bending and straightening. The femoral and tibial components are physically attached to each other with a metallic peg, much the same as a door hinge, which allows only for bending and straightening.
Patello-Femoral Replacement
Patello-femoral joint replacement refers to a procedure that involves the resurfacing of the articular surface, or underside, of the patella, or knee cap, and the groove in which the patella glides during bending and straightening of the knee, called the trochlea. Patello-femoral replacements are typically performed when damage to joint cartilage surfaces is localized and limited to only these two areas, and the remainder of the knee is deemed healthy.
The implant utilized for the patellar under-surface is made of the same high density plastic as the spacers used in knee replacements, and the trochlea implant, which resurfaces the groove or notch at end of the femur, is made of the same highly polished metallic compounds as the femoral, or thigh bone, prosthesis.
