Patellofemoral knee replacement surgery is performed in the following cases:

1. Patellofemoral arthritis: Osteoarthritis is an age-related condition in which the cartilage that protects the knee bones wears away slowly. When osteoarthritis arthritis is limited to the underside of the kneecap and the trochlear groove, the condition is known as patellofemoral arthritis.
2. Patellofemoral arthritis with trochlear dysplasia: When patellofemoral arthritis is accompanied by a convex, flat, or dome-shaped trochlea, the condition is known as trochlear dysplasia or unstable kneecap.
3. Post-traumatic patellofemoral arthritis: A traumatic injury or fall may lead to patellofemoral arthritis.

The doctor may perform the following diagnostic tests before a patellofemoral knee replacement surgery:

1. Physical examination: The doctor checks the alignment of the knee joint, palpates or feels around the knee joint to check if there is pain, checks the range of motion of the knee, checks for knee stiffness, checks for any problems associated with patellar tracking, checks the quality of ligaments around the knee joint, and the stability of the knee joint.
2. Medical history: The doctor inquires about the medical health of the patient.
3. Blood tests: These tests help in checking the various blood parameters and if there is a presence of any medical disorder.
4. Urine tests: These tests help in checking for the presence of any underlying urinary infections.
5. Chest x-ray: The heart and lung condition can be checked using a chest x-ray.
6. Electrocardiogram (ECG): The electrical activity of the heart can be measured using an ECG.
7. X-rays of the knee joint: The images of the knee joint are obtained and the x-rays taken from different angles help in ensuring that the arthritis is limited to the space between the kneecap and femur. X-rays also help in assessing the overall knee alignment.
8. Magnetic resonance imaging (MRI) scans: These are types of imaging tests that help in obtaining clearer images of the bones and soft tissues in the knee area. 

• The procedure is either performed under general anesthesia (you are put to sleep during the procedure) or spinal anesthesia (you will be awake during the procedure, but your body is numbed from the waist downwards).
• An incision or cut is made by the surgeon at the front of the knee.
• All the three compartments of the knee are checked to confirm that the damaged cartilage is present only between the patella and femur, and that the knee ligaments are intact.
• The surgeon now uses special surgical tools for removing the damaged cartilage and a small amount of bone from the patellofemoral knee compartment.
• A plastic button or cover is used to resurface the backside of the patella.
• The button lines up with a thin metal component that is used to resurface the trochlear groove at the femur bone end.
• These parts are secured to the bone with cement.
• The incision is then closed using stitches or sutures.
• The surgery takes approximately one to two hours to complete.

A patellofemoral knee replacement surgery may have the following side effects:

• Infection
• Bleeding 
• Formation of blood clots
• Poor wound healing
• Nerve damage
• Blood vessel damage 
• Allergic reaction to the anesthesia used
• Knee pain that persists 
• Dislocation of the kneecap
• Instability of the kneecap
• Need for an additional surgery 

The patient is generally discharged from the hospital two or three days after the procedure.
It is normal to have some amount of pain and swelling in the surgical site for a couple of days after the surgery. 
The doctor prescribes certain painkillers to relieve the discomfort.
The patient can use a walker, cane, or crutches for some days after the surgery to help in walking.
Moderate activities like driving and climbing stairs can be resumed two weeks after the procedure.
The patient can resume most of his/her daily activities six weeks after the surgery.
Complete surgical healing may take approximately six to eight weeks.
A physiotherapist will help the patient with specific exercises designed to maintain the range of motion in the knee and restore the strength of the leg muscles.
The outcome of the surgery can be checked by having regular follow-ups with the doctor.