The knee is one of the largest and most complex joints in the body. It acts as a hinge, allowing the lower leg to flex and straighten. However, years of use can take a toll on the knee joint. Damaged or diseased cartilage and bone can cause severe pain and limit mobility. Some common causes of knee joint failure include:

Osteoarthritis: The gradual breakdown and loss of cartilage in the knee is the most common reason for joint failure. Cartilage cushions the bones and allows for smooth movement, but osteoarthritis causes it to thin and wear away over time.

Rheumatoid Arthritis: An autoimmune disorder where the body's immune system attacks the joint lining, causing pain and stiffness. It can progress to substantial cartilage and bone damage requiring replacement.

Past Injuries: Severe trauma like fractures or torn ligaments may accelerate arthritis in the long term. Even after reconstruction, the knee may degrade to the point where replacement surgery provides the best option.

Uncontrolled Obesity: Extra weight puts additional stress on weight-bearing joints like the knees, increasing the chances of damage. Losing weight can help manage arthritis symptoms but may not reverse joint degeneration in advanced cases.

The Surgical Procedure
In a total knee replacement, surgeons remove damaged bone and cartilage and replace them with prosthetic components. The standard procedure involves:

- Making an incision on the front of the knee to expose the joint.
- Removing portions of the lower end of the femur and the top of the tibia, usually using a saw or burr.
- Implanting metal alloy and polyethylene components to resurface the surfaces and recreate knee motion. This includes parts to replicate the femur, tibia, and sometimes the patella or kneecap.
- Repairing soft tissues like muscles and tendons to restore stability and alignment.
- Closing the incision with sutures or staples.

Rehabilitation and Recovery
Patients usually stay in the hospital 1-3 days after surgery. Getting up and walking with crutches or a walker begins on the same day or the next to promote healing. Physiotherapy is crucial during the first 6-12 weeks post-operation to:

- Regain leg muscle strength and gradual increase in range of motion. Stretching and leg lifts/extensions are important exercises.
- Improve walking skills and balance with the assistance of a physical therapist.
- Learn proper body mechanics to avoid stress on the new joint from activities like going up/down stairs.
- Control pain and swelling with ice, compression, and sometimes bracing during initial recovery.
- Progress to resuming regular daily tasks, then low-impact exercise in 3-6 months.

Outcomes and Longevity of Implants
With appropriate rehabilitation, most knee replacement patients report a major reduction in pain levels and great improvement in mobility. Individual outcomes depend on factors like age, health status, lifestyle, and how extensively the joint was damaged before surgery.

Modern prosthetic designs have greatly increased durability, lasting 15-30 years on average in younger/healthier individuals. However, the prosthetic components may require revision surgery later on due to infections, loosening, or wear/fracture. While knee replacements can usually relieve arthritis pain for the remainder of a person's life, the new joint will eventually degrade and may need to be replaced again further down the road.

Complications are also possible with any major surgery like infections, blood clots, and stiffness limiting motion, or fractures around the implant site during recovery. Careful screening of candidates and adherence to post-op therapy minimize risks. Overall, knee replacement is a highly successful procedure that enables patients to regain independence with everyday tasks and low-impact activities, improving their quality of life. With continuing advancements in surgical techniques and prosthetic technology, outcomes continue to improve for individuals requiring this treatment.

In summary, knee replacement surgery has evolved tremendously to offer solutions for arthritis and other forms of end-stage knee damage. Rigorous rehabilitation following a newly implanted joint is key to restoring full mobility while protecting the prosthesis long-term. With appropriate postoperative care and activity modification, most individuals benefit greatly from knee replacements through reduced discomfort and enjoying an active lifestyle once again. Through ongoing research, implant longevity and outcomes are expected to further increase for patients relying on this life-changing procedure.