Currently, microfracture is the most popular technique of cartilage repair/restoration in the United States. Other less commonly performed techniques include: autograft plugs (for example, OATs and mosaicplasty), allograft plugs or shells and autologous cultured chondrocyte implantation (ACI, which in the US, is Carticel by Genzyme).
The microfracture concept is based on earlier work that allowed blood elements within the bone marrow, which include marrow cells (adult stem cells) to exit the bone (through drill holes or by abrading the bone (abrasionoplasty) and form a clot in the cartilage defect.
The cartilage lesion is prepared to have clean 90 degree (vertical) walls or normal (or near normal) cartilage that will contain the clot. The base of the lesion is cleared of all debris and the thin layer of calcified cartilage is removed, which exposes the bone that is under (i.e., sub) the cartilage (chondral layer) in preparation for the microfracture. This bone at the base of the lesion is called the subchondral bone plate. Is is firm but less than a few millimeters thick. Deep to this layer is the cancellous or spongey bone that contains blood and marrow elements.
To perform the microfracture, a small "pic" or "awl" (similar to an ice pick) is used to create small holes. As the 1- 1.5 mm awl is pushed (or hammered) through the subcondral bone, small fracture (hence the term "microfracture") holes are formed. These small holes are equally spaced throughout the area of the exposed bone at 3-4 mm intervals to maintain the subchondral bone strength between them. This microfracture environment may inititate a fracture healing response, allows a rough surface for the bone clot to adhere and allows the ingress of cells from the bone marrow.
Gentle continuous or cyclic knee range of motion and avoidance of weight bearing over 6 to 8 weeks is note in most studies to be very important. The goal is that over this time period the clot will change into scar cartilage (also known as fibrocartilage).
If the scar cartilage is durable, patients will experience decreased pain and improved function. The originator of this innovative approach, Dr. Steadman, has reported approximately 80-85% of patients do quite well, 10-15% may have little improvement and 5% may not improve. There are various patient and knee factors that may influence these outcomes.
If the scar cartilage is not durable, then over time, the patient will experience recurrence of pain as in the small subgroups Dr. Steadman reported. In this latter group, the time course for "failure" is variable, but reports in the Orthopaedic literature (see the Kreuz and Zazlav article reviews on this site) typically note this may occur in the range of 18 to 24 months.
If failures do occur there are several options for salvage detailed at these web sites: