What's New in Cartilage Repair

An interview with Dr. Riley J. Williams, III

Articular Cartilage: What Is It?

Whether the cause is trauma or disease, damage to articular cartilage - the smooth cushion that lines the end of the bones where they meet at the joints - can cause significant pain and weakness.

“Because of the body’s limited ability to regenerate articular cartilage cells, injuries to this highly specialized tissue can result in significant and progressive damage to the joint surface,” says Riley J. Williams, MD, III, associate attending orthopedic surgeon at HSS. If left untreated, these localized areas of cartilage damage and loss may rapidly spread to involve the entire joint surface, an end result known as osteoarthritis of the joint.

To better understand these injuries and their treatment, it may be helpful to describe the structure of articular cartilage. Articular cartilage lines the ends of our joint surfaces and is composed of cells called chondrocytes with a matrix or scaffolding made of collagen and proteins. In healthy joints, this unique and durable material allows bones to move against one another with minimal friction.

Image: healthy knee through arthroscope Image - arthroscopic view of joint cartilage defect
Figure 1 (Left): View of a normal knee joint through an arthroscope, showing smooth articular cartilage and the meniscus. Figure 2 (right): Arthroscopic view of a joint surface showing a full thickness defect of articular cartilage exposing underlying subchondral bone.

When areas of cartilage are worn away or torn away, exposing underlying (subchondral) bone, treatment is designed to fill in the missing area or defect with healthy articular cartilage and provide new protection for the joint surface. Orthopedists categorize the severity of cartilage injury using the Outerbridge Classification system, a scale from 0 to 4, with 0 being normal and grade 4 being complete exposure of the subchondral bone. Most of the patients treated at the Institute for Cartilage Repair have a grade 3 or 4 injury.

Most symptomatic cartilage lesions are at least 2 cm in diameter, but may often be as small as 1 cm in diameter or as large as 4 cm in size. These areas of cartilage loss interfere with normal joint mechanics, and result in pain and poor joint function in affected individuals.


In addition to patient history and physical evaluation, orthopedic surgeons rely on Magnetic Resonance Imaging (MRI) to assess the nature of a cartilage injury and formulate a treatment strategy. At HSS, sophisticated MRI techniques provide images in which cartilage is clearly distinguished from bone. As recently as a decade ago, most imaging modalities did not provide information on the structure and state of articular cartilage. As a result, surgeons had to rely on direct observation during an operation rather than obtaining it in advance.

This often resulted in patients receiving suboptimal treatment, as the treatment of these undetected lesions had not been discussed with patients prior to their operative procedure.

Fortunately, surgeons at HSS have benefitted from the efforts of their colleagues in the Department of Radiology in this area. Led by Hollis Potter, MD, MRI technology has been at the forefront of articular cartilage imaging for many years. This advanced MRI technology has been tremendously useful to orthopedic surgeons in detecting articular cartilage lesions before surgery and objectively assessing articular cartilage repair methods in treated patients following surgery over time.

“The use of cartilage sensitive MRI is a de facto part of my postoperative regimen in patients treated for articular lesions,” says Dr. Williams. “These images allow me to objectively assess the success of my surgical efforts in creating cartilage-repair material in an area which was devoid of any cartilage before the surgery. I can correlate what I see on these MRIs and what I observe clinically in the patient’s recovery and use this information to appropriately adjust the patient’s postoperative care and rehabilitation.”

Image - osteochondral allograft
Figure 3: MRI showing a healed osteochondral allograft

Treatment Options

Treatment recommendations for articular cartilage injury are based on a variety of factors, including the patient’s age, patient activity level, overall condition of the joint, the size and location of the lesion, the patient’s ability to participate in rehabilitation, and whether the patient is to return to sports.

Hospital for Special Surgery cares for many professional and collegiate teams, and as a result, surgeons at the Institute are very focused on return to sport as an endpoint in assessing their success in the area of cartilage repair. Treatment options vary from patient to patient, depending on the situation.

Palliative Care

In some cases, the orthopedic surgeon will recommend palliative care, in which he or she removes the loose fragments of cartilage or meniscus that might cause a mechanical block and joint pain.

“While palliative care does not address the issue of articular cartilage fill or repair as an objective of the surgery, we often recommend this as a short-term approach for athletes who are in-season and face time pressures to return to sport as soon as possible,” states Dr. Williams. “Such procedures are usually minimally invasive, and athletes can return to sport within four to six weeks.” For these patients, the goal is not to repair the injury, but to reduce pain and improve function. These patients will often require another procedure at a later time to address the cartilage lesion formally.

In contrast to palliative care, other techniques seek to stimulate cartilage regeneration and/or replace lost cartilage with cartilage transplants or synthetic materials.


The first efforts to address cartilage injury focused on getting the body to produce new articular cartilage as a response to injury to the subchondral bone; a form of the technique, microfracture, is still widely used and is, in fact, the current standard of care for the treatment of cartilage injury in the United States.

With microfracture, the surgeon creates tiny holes or fractures in the underlying subchondral bone in the damaged area. The holes allow clots to form (in response to the injury); over time, the clots change into a cartilage tissue.

Image - arthroscopic view showing articular cartilage surface following microfracture
Figure 4: Arthroscopic view showing the articular cartilage surface following microfracture
*Note the tiny holes in cartilage.

While microfracture has resulted in the generation of new cartilage tissue in thousands of patients, orthopedic surgeons and researchers have continued to look for methods that would result in more durable repair. “Although a number of studies at HSS have demonstrated the efficacy of microfracture,” Dr. Williams notes, “among elite athletes we have found that only 40% were able to return to sport after the surgery.”

Osteochondral Autograft Transplantation Surgery (OATS)

During the 1990s, orthopedists began using plugs or cylinders of bone and articular cartilage taken from a healthy area of the patient’s knee (one that does not have to bear much weight) and transplanting them into the damaged area of the knee. This method, called Osteochondral Autograft Transplantation Surgery (OATS), allows placement of mature, durable articular cartilage in the damaged area.  Placement of these bone-cartilage cylinders, known as autografts, can be used to resurface an area of the knee that lacks a cartilage cover.

Image - joint surface following Osteochondral Allograft Transplantation Surgery
Figure 5: Arthroscopic view of joint surface following Osteochondral Autograft Transplantation Surgery (OATS), showing implanted bone-cartilage cylinders

In a study conducted at HSS, investigators who compared patients treated with microfracture to those treated with osteochondral autograft transplantation found that, four years after surgery, individuals in the latter group were able to be more active for a longer period of time.

Cell-based Repair

One of the first techniques to offer an alternative to microfracture and OATS emerged in the United States in 1995. This method involved removing some of the patient’s cartilage cells or chondrocytes, growing them in a culture in a laboratory setting, then returning the larger quantity of cells to the injured area to fill in the joint surface defect. This multi-step process requires at least two surgeries: one to harvest the patient’s cells and another to reimplant them into the articular cartilage defect. 

This technology has continued to develop. Researchers at HSS are currently conducting two FDA-sponsored trials testing cell-based repair methods: the Neocart implant, manufactured by Histogenics, and the De Novo ET implant from Zimmer.

Image - articular surface following Neocart cartilage cells implantation
Figure 6: Articular surface following Neocart cartilage cells implantation

Cartilage and Cartilage/Bone Transplantation

Patients with bigger defects or who have failed other types of surgery may be eligible for allograft transplants of donor bone and cartilage (Figure 3). In this method, cartilage defects are treated with tissue that is donated for transplantation, much like a liver or heart transplant.

Historically, this method was thought to be appropriate only when other means of repair were unsuccessful. However, a study scheduled for publication in The Journal of Sports Medicine shows that modern transplant surgery developed at HSS results in a 90% return to sport among patients undergoing the procedure. “It’s been my observation over the past decade that cartilage transplantation is one of the most reliably successful cartilage treatment methods that I’ve had at my disposal, and I look forward to changing the perception that this is a salvage, or ‘last option’ procedure.” says Dr. Williams.

Image - joint surface after osteochondral allograft transplantation
Figure 7: Joint surface after osteochondral allograft transplantation

Looking to the Future

For a significant number of patients, available treatments provide satisfactory relief of pain and other symptoms relating to their cartilage injury. However, a new generation of therapeutic options is under development and in clinical trials at HSS and other select centers in the United States. These include techniques that use both patient and donor cells, as well as synthetic matrix or scaffold materials, to trigger growth of tissue that more closely resembles the patient’s original articular cartilage and promotes faster recovery, permitting earlier post-operative weight bearing and a return to sports.

Cell and Scaffold Technology

One such technique that came into use around 2005 involves the placement into the articular cartilage defect of two to five plugs of synthetic material that mimic natural bone and cartilage and serve as a scaffold. The patient’s own cells can then migrate to the area of damage and facilitate a repair by growing inside the scaffold. More than three hundred patients who have had the procedure at HSS have been tracked, and early results suggest that the method may be superior to microfracture over the long term.

Image - Joint surface after placement of two synthetic plugs to serve as scaffolds for cartilage growth and repair
Figure 8: Joint surface after placement of two synthetic plugs to serve as scaffolds for cartilage growth and repair

Clinical Trials at HSS

Orthopedic surgeons at HSS have taken a leading role in evaluating available treatments as well as researching new therapeutic options through clinical trials at the Hospital’s Institute for Cartilage Repair. The Institute’s Cartilage Repair Registry, a data collection system, was established and designed to track the progress of patient volunteers.

Clinical data from more than 1400 patients treated for articular cartilage injury have been collected and orthopedic sports medicine specialists at HSS continue to enroll new patients who are tracked prospectively from diagnosis through treatment and follow-up care to assess final outcome. After treatment, patients provide information online at regular intervals to enable long-term follow-up on their progress and to determine long-term results of treatment.

“Very few centers in the country have such extensive patient centered prospective studies of this condition,” explains Dr. Williams. “This method allows us to objectively assess outcomes. We’re not just relying on a subjective impression of how well the patient has done.” Findings from the registry have allowed HSS orthopedists to make important connections between factors such as patient weight and activity and success of treatment, and the relationship between duration of symptoms and treatment outcome.  

“Information derived from the HSS registry provides us with objective findings about the various methods of cartilage repair,” he continues. “This allows us to adjust our treatment recommendations accordingly.” When considering treatment for cartilage injury Dr. Williams offers patients two pieces of advice: seek intervention early to achieve a better outcome and choose an institution where many cartilage repair procedures are performed.

If you would like more information about treatment of cartilage injuries at HSS including information about enrollment in ongoing clinical studies, please visit the Physician Referral Service or call 1.877.606.1555.

Summary by Nancy Novick


Headshot of Riley J. Williams III, MD
Riley J. Williams III, MD
Director of the Institute for Cartilage Repair, Hospital for Special Surgery
Attending Orthopedic Surgeon, Hospital for Special Surgery

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