Marrow Stimulation Techniques
These are a group of arthroscopic procedures in the category of “marrow stimulation,” whose purpose is to violate the subchondral boney surface underlying the cartilage lesion, which allows the marrow elements to leech into the damaged area.
These procedures are generally reserved for “full-thickness” cartilage lesions in which the depth of the cartilage lesion is down to areas of exposed bone (ICRS grade 4). When the subchondral plate is violated, bleeding bone is encountered with the theoretical release of adult pluripotential stem cells into the defect. These stem cells, adherent to a fibrin clot within the defect, differentiate and fill the defect. Histological specimens have repeatedly been shown to contain a mixture of chondrocytes (normal cartilage tissue) and fibroblasts (scar tissue) that create a predominantly fibrocartilaginous repair tissue. The mechanical properties and durability of fibrocartilage are inferior to native hyaline cartilage, and long-term outcomes are mixed. Johnson first described abrasion chondroplasty in 1983, which involves burring the bone surface until it bleeds. This procedure has been generally supplanted by drilling chondroplasty or microfracture. This procedure involves drilling many small holes into an area where full thickness cartilage degeneration has occurred and raw, subchondral bone is exposed.
The intention of this procedure is to stimulate the body to fill in the full thickness cartilage defect with a layer of fibrocartilage. Although fibrocartilage is not the same as native articular cartilage, it may have acceptable material properties. This procedure is often performed as an initial procedure if full thickness cartilage defects are encountered. Steadman popularized the microfracture technique for full-thickness lesions of the knee by which a small awl is used to create the holes in the sub-chondral bone. In microfracture, the theory of fibrocartilage creation via stem cells is similar to that of abrasion arthroplasty, i.e., to obtain a healthy bleeding bony base for the fibrin clot to adhere. Microfracture is thought to generate less heat than drilling techniques, eliminating the possibility of thermal damage. In addition, there is no loss of subchondral bone as in abrasion chondroplasty.
The technique is preferably used for small to mid-sized full-thickness cartilage lesions. Although there is no consensus as to the size limit of the lesion, generally less than 4 square cm is thought to be favorable for this procedure. As with most cartilage procedures, the more extensive the damage, a less favorable outcome is to be expected. Single rather than multiple lesions are preferable. The predictability of this procedure increases in knees with normal alignment, acute rather than chronic damage, low BMI, minimal radiographic evidence of degeneration with good preservation of the joint space. Patients who are elderly with more extensive degenerative disease do less predictably well. Partial-thickness lesions are treated with debridement chondroplasty and generally not converted to full-thickness lesions. Lesion that are “well-shouldered” or have a steep, normal surrounding cartilage rim have a superior outcome. This perpendicular rim provides an area of adherence for the fibrin clot that will form in the area, and protects the forming tissue as it matures.
These are arthroscopic procedures. The introduction of instrumentation and preparation is similar to that of a debridement chondroplasty as discussed in the above section. Drilling chondroplasties are performed with a motored drill and a thin sharpened wire (K wire). The surfaced is perforated multiple times in a dense-packed configuration until blood and fat droplets are seen emanating through the created holes. The microfrature technique is performed with a mallet and small awl. In both techniques the bone surface perceives itself as injured and initiates a healing response.
The recovery process varies according to the location and extent of damage. Steadman described the use of continuous passive machine (CPM) 6 to 8 hours per day until full passive range of motion is achieved. Crutches are used, as weight-bearing is limited for 6-8 weeks. Physical therapy is started early in the process. At 8 weeks, patients are fully weight-bearing and more aggressive strength programs are implemented. Free weight can be started with caution at 16 weeks, and return to sport is implemented when full range of motion and strength have returned, but not before 4-6 months.
Woodson et al. noted the following in a review of the literature. Steadman’s results on microfracture at 3 to 5 year follow-up showed 75% of patients as improved, 20% unchanged, and 5% worse. Two thirds of patients were able to perform activities of daily living and more patients could perform strenuous work than before the procedure. In a follow-up to his original work, Steadman and colleagues stated that patients treated with CPM for 6-8 weeks improved compared with those who did not use CPM.
Several groups of investigators have looked at the results of microfracture in an older subgroup of patients with osteoarthritis (40-70 years of age). These patients had normal alignment and Outerbridge grade IV lesions and demonstrated improvement. Williams and colleagues prospectively looked at 34 patients who underwent microfracture and found that improvement in results can be anticipated up to 2 years.
Drilling chondroplasty and microfracture have a defined role in the treatment of full-thickness chondral lesions of the knee joint, though the durability of fibrocartilage repair has yet to be fully defined. In most cases, repair cartilage or fibrocartilage is inferior to native hyaline articular cartilage in numerous ways. Recent concern has been raised that boney overgrowth is a potential concern and diminishes the outcome of subsequent procedures, like ACI (below).