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- The procedure is designed to treat patients with posttraumatic lesions of the knee that have become full-thickness chondral defects. The general sign that microfracture is appropriate is the full-thickness loss of articular cartilage in either a weight bearing area between the femur and tibia or in an area of contact between the patella and trochlear groove. Another indication is unstable cartilage lying over the subchondral bone. Degenerative changes in a knee that has proper axial alignment also could indicate that microfracture is needed. (21)
- The first step to microfracture knee surgery is a 10-point diagnostic arthroscopy to carefully examine the posterior aspects of the medial and lateral femoral condyle. Proper alignment will be checked for during this exploratory procedure. If any changes to the articular surfaces are noticed, a probe can be used to determine the cartilage quality. Unstable parts of the cartilage are removed with an arthroscopic shaver or curet. A curet is also used to remove the calcified cartilage layer from the base of the defect. Specifically in chronic degenerative lesions with a thickened subchondral plate, a burr is utilized to remove the sclerotic bone until visual punctate bleeding. (7, 13, 19)
- It is important to use a curet rather than a shaver as it is hard to control the amount of bone removed with the shaver. The subchondral bone is more likely to be damaged with the shaver. (7, 13, 19)
- Once the bone or damaged cartilage is exposed, perpendicular margins are created at the border of the defect and the healthy articular cartilage. A surgical awl is used to make multiple small holes spaced 1 to 5 mm apart (depending on the literature) and 2 to 4 mm deep in the exposed subchondral bone of the chondral defect. The amount of perforations for each lesion depends on the size of the defect. (1, 7, 13, 15, 23)
Figure 2- Grade IV lesion being microfractured
Permission granted by Masoud Riyami (19)
- After the holes are completed, a rough surface is produced for the adherence of the blood clot that contains the marrow-based mesenchymal stem cells and various cytokines from the subchondral bone. The fibrin clot must form at the base of the prepared chondral lesion. It is important that the most external parts of the lesion are penetrated by the awl to help the healing of the mixed fibrocartilage repair tissue to the surrounding articular surface. The stem cells will proliferate and differentiate into chondrocytes. The regenerated chondrocytes produce a cartilaginous repair tissue that fills the defect. (1, 7, 13, 15, 22, 23)
- During the procedure fat is seen coming from the subchondral marrow. After the area has been microfractured, the arthroscopic pump is turned off reducing the fluid pressure in the joint. At this point marrow bleeding is seen flowing from the microfractures and filling the defect. (1, 7)
- The repair cartilage volume plays an important role for the durability of functional improvement after microfracture. Deterioration of knee function occurred primarily in patients who lacked repair cartilage fill when evaluated with a second-look arthroscopic evaluation. (15)
- Fracture healing and bone repair rely on the capacity to amass enough cells at the lesion site to form a repair blastema. The mesenchymal stem cells play an important role in the process of healing. In this process of healing an osseous defect, bone formation occurs by a direct conversion of stem cells into osteoblasts and chondrocytes. When the stem cells differentiate, they recreate the spatial orientation of the tissue, cartilage at the surface, and bone underneath. (2, 3, 9)
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