Microfracture Before Autologous Chondrocyte Implantation: Are We Burning Our Bridges?The authors of this study offer some information that might suggest bone marrow stimulation before autologous chondrocyte implantation (ACI) may ultimately lead to failure of the ACI procedure.
Bone marrow stimulation techniques are used to treat a knee with a hole or defect in the articular cartilage. Articular cartilage is the rubbery, fibrous cartilage that covers the ends of bones to protect the joint. When the defect goes all the way down to the first layer of bone, it's called a full-thickness defect.
There are different ways to stimulate the bone marrow to produce new chondrocytes (cartilage cells). Drilling into the first layer of bone under the articular cartilage is one. Shaving the surface of the bone called abrasion arthroplasty is another and microfracture is the third method currently in use.
All of these procedures have one thing in common. They are designed to fill the hole or defect with tissue from the bone at the bottom of the defect. This can help set up the repair process needed. A clot forms that eventually turns into fibrocartilage tissue. But the question is: if the surgeon does this repair technique and it fails, can the patient still have a transplantation of cartilage cells called autologous chondrocyte implantation (ACI)?
With ACI, normal, healthy cartilage cells (chondrocytes) are removed from a part of the knee joint that is not weight-bearing. They are taken to a lab where they are multiplied to form enough repair cells to put back in the defect and stimulate growth of the needed fibrocartilage. The cultured chondrocytes are injected underneath a special patch that has been placed over the hole.
The current belief is that marrow stimulation doesn't prevent a successful ACI. They refer to marrow stimulation techniques as non-bridge-burning. The results of this study do NOT support that belief. What is this conclusion based on?
Surgeons from the Cartilage Repair Center at Harvard Medical School compared the results of 111 joints that had been pretreated with marrow stimulation before ACI with 214 (control) patients who just had ACI without marrow stimulation first. Some patients had more than one defect. And some of those patients had pretreatment with marrow stimulation while others did not. That made it possible to use patients as their own internal control to compare what would happen in one patient with both types of treatment.
They found that the failure rate after ACI was higher in patients who had first had marrow stimulation done (26 per cent in the pretreated group compared to eight per cent in the ACI only group). Failure meant the patient still had pain that limited function, MRIs showing that the graft didn't take, and/or surgery had to be done to remove the graft.
Just defining what constituted a failure wasn't really enough. They also had to look at the number of defects, type of defects, and defect size and location. Each of these variables had the potential to affect the outcomes. Likewise, it was necessary to compare each individual type of marrow stimulating technique to see if one was more successful than the others.
As it turns out, simple defects were more likely to be successful. The more complex defects had three times the failure rate of simple defects. The type of marrow stimulation technique didn't seem to make a difference. The outcomes were about the same (equal failure rates) for all three methods (drilling, abrasion, microfracture). They did find that worker's compensation patients had higher failure rates.
The authors concluded that treatment of full-thickness cartilage defects with marrow stimulation techniques can negatively affect later cartilage repair procedures. Patients who developed thickening of the subchondral (first layer of bone under the cartilage), bony overgrowth, and/or the formation of subchondral cysts after bone marrow stimulation were more likely to have a poor outcome.
Deterioration and failure of the bone marrow stimulation may occur because the new tissue forms over a thick, protruding, and stiff subchondral base. Eventually that subchondral plate starts to degenerate.
But this new evidence doesn't mean surgeons shouldn't use marrow stimulation techniques when they are called for. The majority of evidence shows a 60 to 80 per cent success rate with excellent results. Patients recover quickly with few postoperative problems and minimal rehab needed.
It's likely that patient selection is an important factor in limiting failure rates. For example, there's some evidence to suggest that large defects (more than 4 cm-squared) should just be treated first with ACI (i.e., skip the marrow stimulating step). Some experts have suggested that unknown biologic factors might set some patients up for failure no matter what. Maybe failure has nothing to do with pretreatment and everything to do with something else.
That leads us right into the need for future studies to look at the exact cause of failure. Is it really the increased mechanical stiffness of the subchondral plate, defect-specific factors, or patient-specific factors -- or something else altogether? Scientists are just at the beginning of exploring cartilage repair and looking for minimally invasive approaches that won't burn any bridges for later treatment.
Tom Minas, MD, MS, et al. Increased Failure Rate of Autologous Chondrocyte Implantation After Previous Treatment With Marrow Stimulation Techniques. In The American Journal of Sports Medicine. May 2009. Vol. 37. No. 5. Pp. 902-908.
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