The microfracture procedure is a common surgical technique used to treat damage to the articular cartilage in joints, most often performed in the knee. It is considered a marrow stimulation procedure, designed to encourage the body’s own healing response to fill a cartilage defect. It involves creating tiny perforations in the underlying bone to release healing components into the damaged area. The procedure is typically minimally invasive, utilizing an arthroscope, and aims to restore a smooth joint surface to reduce pain and improve function.
Why Articular Cartilage Needs Repair
Articular cartilage is the smooth tissue covering the ends of bones where they meet to form a joint. This specialized tissue allows bones to glide over one another with minimal friction and acts as a shock absorber during movement. Unlike most other tissues, mature articular cartilage does not have a direct blood supply (avascularity). Because it lacks blood vessels, it also lacks the necessary cells and growth factors to initiate self-repair when damaged.
When a full-thickness defect occurs, the exposed bone surface can rub against the opposing bone, leading to pain, swelling, and mechanical symptoms like catching or locking. Since the damage will not heal on its own, the lesion can progress over time and potentially lead to osteoarthritis. This lack of intrinsic healing capacity creates the need for surgical interventions, such as microfracture, to stimulate biological repair.
The Surgical Procedure
The microfracture technique is performed arthroscopically, meaning the surgeon works through small incisions using a camera and specialized instruments. The first step involves debriding the damaged area, where the surgeon removes unstable or fragmented cartilage to create a clean, stable border around the defect. This preparation ensures the surrounding healthy cartilage is preserved and establishes an edge to contain the repair tissue.
Once the bone is exposed at the base of the lesion, a specialized pointed instrument called an awl is used to create multiple small holes in the subchondral bone plate. These perforations, or microfractures, are typically placed about three to four millimeters apart and penetrate the hard layer of bone. The holes reach the underlying bone marrow, which is rich in blood and multipotent mesenchymal stem cells.
As the marrow seeps out of the microfractures, it mixes with the joint fluid to form a clot, often referred to as a “superclot,” over the defect site. This clot is a temporary scaffold containing the stem cells and growth factors needed for healing. Over several weeks, this clot will differentiate and mature into a repair tissue that covers the joint surface.
Post-Operative Rehabilitation
The success of the microfracture procedure relies on a structured and lengthy post-operative rehabilitation protocol. The initial phase requires a strict period of non-weight-bearing, typically lasting six to eight weeks. This restriction is crucial because the newly formed blood clot is delicate, and applying mechanical load would crush the clot, preventing the formation of repair tissue.
Patients are often instructed to use a Continuous Passive Motion (CPM) machine for several hours each day during the initial weeks. The CPM machine gently bends and straightens the joint without engaging the muscles, which helps circulate joint fluid to nourish the healing area and prevents scar tissue from forming. This gentle movement encourages the stem cells to mature into a more resilient repair tissue.
Physical therapy begins almost immediately and progresses slowly, focusing on regaining range of motion and strengthening the surrounding muscles without loading the healing site. After the initial non-weight-bearing period, the patient gradually progresses to partial and then full weight-bearing activities. High-impact activities and return to sports are significantly delayed, often for six to nine months, to allow the new repair tissue time to mature and withstand greater forces.
Understanding Fibrocartilage
The new tissue that develops from the blood clot is not the same as the original articular cartilage, which is known as hyaline cartilage. Instead, the microfracture procedure stimulates the growth of fibrocartilage. The key difference lies in their composition: native hyaline cartilage is primarily composed of Type II collagen, giving it smooth, elastic, and pressure-resistant properties.
Fibrocartilage is a less organized tissue that contains a higher proportion of Type I collagen, the same type found in scar tissue. This makes the new repair tissue stiffer and less elastic than the original hyaline cartilage. While fibrocartilage provides a functional surface to cover the defect and reduce friction, it is biologically inferior to native tissue. The long-term concern is that fibrocartilage may wear down faster than hyaline cartilage, potentially leading to a decline in clinical results.