It's that painful pop or snap that more than 80,000 Americans hear every year.
ACL tears often mean surgery to repair cartilage and months of rehab, but even that doesn't always ease the pain.
Now, the answer for knee injuries may be growing in the lab.
Lisa Groom takes on the San Francisco hills with confidence.
But not long ago, a knee injury while playing tennis kicked her off the court and onto the couch.
"I hit the shot and won the game, and I collapsed onto the ground. I felt my tibia push out the side of my leg totally," Lisa recalls.
She tore her ACL and destroyed her knee cartilage. The damage meant months of pain.
"It can overtake you," she says. "I would be awake all night on and off feeling it."
The tissue can't re-grow itself, meaning treatment options are limited.
"Cartilage cells are very lazy cells," explains Dr. Benjamin Ma, chief of sports medicine and shoulder surgery at the University of California.
Traditional procedures remove damaged cartilage but can't replace it. Dr. Ma is helping develop new surgery that can replace cartilage with a patch made of a patient's own cells.
Surgeons remove a sample of the patient's knee cartilage through a ten to 15-minute outpatient procedure. That cartilage is then sent to the lab, attached to a 3D scaffold made of collagen, and grown for eight to ten weeks.
Then, the cartilage "patch" is implanted into the patient's knee.
"You're actually putting articular cartilage back into the knee, and you're not taking it from somewhere else," Dr. Ma explains.
It's a more natural way of healing injuries that means patients can give their knees a new start.
Following the neo-cart procedure, patients have to keep off their leg for six weeks.
Unfortunately, the procedure doesn't work for arthritis.
Dr. Ma says he plans to grow larger pieces of cartilage so doctors could potentially heal an entirely worn out knee.
MEDICAL BREAKTHROUGHS RESEARCH SUMMARY
TOPIC: GROWING NEW KNEES
REPORT: MB #2984
Articular cartilage, also known as hyaline cartilage, is a smooth, white tissue that covers the ends of bones where they come together. The tissue allows bones to glide over each other with little friction.
Articular cartilage is made up of the protein collagen and has no direct blood supply, which means if it is damaged, it is extremely slow if not impossible to heal.
The cartilage can be damaged by either normal wear and tear -- which is, in most cases, arthritis -- or injury.
CAUSES OF INJURY:
Damage to the articular cartilage often takes place in conjunction with other knee injuries, including injuries to the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL), meniscus tears and dislocation of the kneecap.
Another cause of injury to the articular cartilage of the knee is osteoarthritis. The condition is characterized by progressive loss of cartilage and destruction of bone underneath the cartilage. Also known as degenerative bone disease, the risk of osteoarthritis can be worsened by high-impact sports, abnormal joint anatomy, joint instability and inadequate muscle strength or endurance.
The American Orthopaedic Society for Sports Medicine says in many cases of articular cartilage damage in the knee, a patient will experience swelling and vague pain. The patient might also describe the knee as "locking" or "catching." With arthritis, the patient often presents with stiffness, decreased range of motion, joint and and/or swelling.
Unfortunately, injuries to the cartilage that don't extend to the bone won't normally heal on their own, but the knee is the most common area for cartilage restoration, according to the American Academy of Orthopaedic Surgeons.
Injuries that are smaller than 2 cm have the best treatment options and prognosis. Those treatment options include surgeries like drilling, microfracture and abrasion arthroplasty, which all increase blood flow to the damaged area to encourage the growth of new cartilage; transplantation of cartilage from another part of the knee or a cadaver; transplantation of cartilage cells that are grown in a lab; rehabilitation; and lifestyle modification.
Many surgeries performed to restore articular cartilage can be done arthroscopically. During an arthroscopic procedure, a surgeon makes three small puncture incisions around your knee rather than one large incision.
CULTIVATING NEW CARTILAGE:
One procedure in phase III of clinical trials involves taking a biopsy of knee cartilage from a patient and growing it in a laboratory.
Once the cartilage cells are multiplied in the lab, they are kept in a high-pressure tissue engineering processor that mimics the natural conditions of a knee in motion.
Those cells form a 3-D piece of new cartilage and are implanted into the injured area of the patient's knee. After a few months, that new piece of tissue is integrated into the surrounding tissue, relieving pain and restoring motion.