➢ Microfracture is a treatment option for symptomatic, full-thickness cartilage defects.
➢ Microfracture is most likely to be successful when performed in nonobese patients under the age of thirty years for small (<2 to 4-cm2) femoral condylar defects that have been symptomatic for a short time (less than twelve to twenty-four months).
➢ Microfracture has acceptable short-term clinical results, but results can be expected to decline over time.
➢ Long-term studies that compare microfracture with advanced cartilage restoration techniques are required to ascertain whether these newer techniques provide longer-lasting results.
Articular cartilage has a complex structure and function, and its degradation is implicated in the osteoarthritic degeneration of synovial joints1-3. While there is evidence that articular cartilage is maintained through a complex interplay among chondrocytes, water, and matrix macromolecules, this maintenance capacity does not extend to repair, as there is ample evidence that demonstrates that the reparative capacity of cartilage is limited4,5. Over the past several decades, the field of articular cartilage restoration has grown tremendously as investigators and practitioners continue to learn about cartilage biomechanics, physiology, and repair.
Cartilage degradation encompasses a spectrum of disease, from cartilage fibrillation and softening to joint-space narrowing with associated subchondral sclerosis, cystic change, and osteophyte formation that typify end-stage osteoarthritis2. While osteoarthritis has many causes, much attention has been focused on addressing focal, full-thickness cartilage defects2. The natural history of these cartilage defects is not well defined, but there is evidence that defects may progress to degeneration of the entire joint over the long term6. Messner and Maletius reported that, of twenty-eight young athletes with arthroscopically diagnosed cartilage defects, seven reported fair to poor knee function and twelve had joint-space narrowing in the affected compartment after fourteen years6. Shelbourne et al. reported that patients treated with ACL (anterior cruciate ligament) reconstruction who had untreated focal chondral defects had lower subjective outcome scores compared with patients without defects at an average of six years postoperatively7. Focal defects are thought to be associated with pain, swelling, and functional limitations in some patients2.
Focal full-thickness cartilage defects are common findings during knee arthroscopy. In a prospective study of 1000 knee arthroscopies, 7.1% of patients under the age of fifty years were found to have an ICRS (International Cartilage Repair Society) grade-III or IV lesion that measured >1 cm2 in size8. In a similar series of 993 consecutive knee arthroscopies, 11% of patients had a full-thickness cartilage defect9. In a retrospective review of over 25,000 knee arthroscopies, 7% of patients under the age of fifty years were found to have one to three Outerbridge grade-III or IV cartilage lesions10. Furthermore, specific patient populations may be at even higher risk for the development of cartilage defects. In a systematic review, athletes were found to have a higher prevalence of full-thickness cartilage defects at the time of arthroscopy compared with the general population11.
Current treatment options for symptomatic focal cartilage defects of the knee fall into three categories: mesenchymal stem cell (MSC) stimulation (microfracture, drilling, abrasion chondroplasty), substitution options (osteochondral autograft transfer system [OATS], osteochondral allograft), and cell-based, biologic replacement options (autologous chondrocyte implantation [ACI], stem cell therapy, tissue engineering)12. Early MSC stimulation strategies involved abrasion chondroplasty, consisting of debridement followed by subchondral bone penetration or drilling to stimulate a reparative response. Techniques of this type were initially described by Pridie and others, but each of these techniques has limitations13,14. More recently, Steadman et al. described a modified technique, which is termed microfracture15. Microfracture is a commonly employed surgical treatment and is accepted as a safe, relatively inexpensive, minimally invasive first-line treatment for small, contained cartilage defects16,17.
The purpose of this article is to review microfracture as a treatment for focal, full-thickness cartilage defects. Topics addressed include the basic science of microfracture, its indications and contraindications, as well as technical considerations, reported outcomes, and future directions.
The premise of microfracture is that disruption of blood vessels in the subchondral bone causes bleeding within a cartilage defect that leads to fibrin clot formation18. It is suggested that if this clot is protected from loading, undifferentiated MSCs from the bone marrow are able to migrate into the defect, proliferate, and differentiate into fibrochondrocytes, which are chondrocyte-like in morphology19. The fibrochondrocytes then synthesize a fibrocartilaginous matrix, which fills the defect. Despite limited evidence for its efficacy in humans, Steadman et al. emphasize the need for continuous passive motion (CPM) after microfracture15,20, citing Salter’s thesis work that describes the benefits of CPM in the healing of full-thickness cartilage defects in rabbits21.
Determining which patients and cartilage defects are best treated with microfracture is difficult. The literature has shown that outcomes vary depending on various patient and defect characteristics. Steadman et al. originally suggested that microfracture is indicated for the treatment of posttraumatic, full-thickness articular cartilage defects of the knee. They also suggested that degenerative cartilage lesions could be treated with microfracture20. As results of microfracture have been reported over the years, indications for its use have narrowed, as there are certain populations and defect characteristics for which the chance of success is greater.
Defect size has been shown to affect outcomes after microfracture and should influence treatment decisions. It is clear that large lesions of >4 cm2 have worse outcomes after microfracture compared with other techniques, but the literature is less clear regarding lesions of 2 to 4 cm2. These differences are likely due to multiple factors, including the defect location, local mechanical environment, and size of the defect relative to the size of the condyle16,22-24. Mithoefer et al. suggested that microfracture should only be used when the defect size is no larger than 2 × 2 cm (4 cm2), but they found that return to high-impact sports was more likely with a <2-cm2 defect25,26. In their series, athletes with a lesion size of <2 cm2 had a significantly higher rate of return to high-impact sports (64%) compared with those with lesions larger than this (22%, p = 0.04)26. In a randomized controlled trial by Gudas et al. that compared mosaic-type OATS with microfracture, patients in the microfracture group with lesions in the central part of the medial femoral condyle that were larger than 2 cm2 had significantly worse clinical results compared with patients who had lesions in other areas of the joint27. In their ten-year follow-up study, the investigators reported that patients with lesion sizes of <2 cm2 had a significantly higher rate of return to sports compared with patients with lesions larger than this (p = 0.04)28. In a randomized trial involving microfracture and ACI, Knutsen et al. found that patients in the microfracture group with a lesion smaller than 4 cm2 had significantly better clinical results compared with patients in the microfracture group with lesions larger than 4 cm2 (p < 0.003)29. Basad et al. reported the results of a randomized controlled trial comparing matrix-induced ACI (MACI) and microfracture, which suggested that MACI yielded better results than microfracture for defects >4 cm2 in size24.
The minimum defect size for which microfracture should be used has not been clearly defined. Guettler et al. demonstrated that increased rim stress occurred in defects ≥10 mm in diameter but not in smaller defects30. For that reason, some surgeons may choose to leave asymptomatic defects smaller than 1 cm2 untreated. While microfracture may be appropriate for symptomatic lesions smaller than 1 cm2, definitive conclusions about the decision to treat asymptomatic defects smaller than 1 cm2 cannot be made at this time.
Age is another factor that affects the results of microfracture. Overall, after microfracture, younger patients have better clinical outcomes and MRI (magnetic resonance imaging)-detectable defect fill compared with older patients27-29,31-33. This finding is likely due to the greater prevalence of early degenerative changes in older patients. Steadman et al. followed their original series of patients and reported the long-term results of a cohort of patients who were less than forty-five years old at the time of surgery31. They reported improvements in function compared with the preoperative level; however, no comparisons were made with patients older than forty-five years. At ten years of follow-up, Gudas et al. found that patients under the age of thirty years with small lesions had better clinical outcomes after microfracture compared with patients over the age of thirty with small lesions27,28. Knutsen et al. also found that patients with large lesions who underwent microfracture had better clinical outcomes if they were under the age of thirty years29,32. In a prospective case series of patients treated with microfracture, Kreuz et al. found that patients less than forty years old had significantly better results and better MRI-detectable fill compared with older patients (p < 0.01)33. Patients over the age of forty years also showed significant clinical deterioration between eighteen and thirty-six months postoperatively (p < 0.05), whereas younger patients did not show the same pattern of deterioration (p > 0.1).
Defect age also likely plays a role in the determination of success after microfracture. Utilizing data from a prospective database, de Windt et al. were able to show that patients treated with either microfracture or ACI with a defect age of less than twenty-four months were more likely (odds ratio, 1.8 to 4.0) than patients with an older defect to report a KOOS (Knee injury and Osteoarthritis Outcome Score) result that was similar to age-matched controls34. Mithoefer et al., in a prospective cohort study, demonstrated that patients with a preoperative symptom duration of more than twelve months were less likely to have improvement in the score for activities of daily living35. Mithoefer et al. found that the rate of successful return to sports increased from 44% to 67% if microfracture was performed within one year after cartilage injury26. In the same study, it was found that return to high-impact sports was more likely after microfracture if there had been no prior surgical intervention26.
Defect location reportedly affects the outcomes of microfracture, with better results reported when treated defects are on the femoral condyles. In a prospective case series, Kreuz et al. found that patients treated with microfracture for a defect on one of the femoral condyles had significantly greater improvement at thirty-six months of follow-up compared with patients with defects on the trochlea, tibia, or patella (mean Cincinnati score improvement [and standard deviation] of 12.34 ± 0.87 points compared with 1.06 ± 1.29, 1.45 ± 0.98, and 1.45 ± 0.98 points, respectively; p < 0.05)36. Patients with defects on the trochlea, tibia, or patella demonstrated significant deterioration in their ICRS and Cincinnati scores between eighteen and thirty-six months that was not demonstrated in patients with condylar defects (p < 0.05)36. In another series, de Windt et al. showed that medial femoral condylar defects had better results with microfracture than lateral femoral condylar defects did34.
Body mass index (BMI) also likely influences the results of microfracture. Mithoefer et al. reported in a prospective cohort study that BMI was inversely correlated with repair fill on MRI, the score for activities of daily living, and the SF-36 (Short Form-36) physical component subscore35. Patients with a BMI of >30 kg/m2 had significantly lower outcome scores and subjective ratings compared with patients with a BMI of ≤30 kg/m2 (p < 0.05).
The following have been deemed contraindications to microfracture: uncorrected axial malalignment, a partial-thickness defect, a defect that lacks a perpendicular rim of intact cartilage (a noncontained lesion), global osteoarthritis, systemic immune-mediated arthritis, or an unwillingness or inability to follow the rehabilitation protocol (particularly weight-bearing restrictions)20. Given the poor results of microfracture for defects of >4 cm2, microfracture for a defect of this size is probably contraindicated24,25,29.
After performing diagnostic arthroscopy, the defect along with unstable surrounding cartilage is debrided with a curet, ensuring a stable perpendicular rim and complete removal of the calcified cartilage layer. An arthroscopic awl is then utilized to make multiple holes in the subchondral bone plate that are 3 to 4 mm apart. The hole depth should be sufficient (2 to 4 mm) to see marrow droplets emerge. Steadman et al. recommended the use of an awl over a drill, because they believed that an awl avoided the theoretical risk of heat necrosis and allowed a more consistent hole depth15. Recent studies that have investigated the use of a drill will be discussed later in this paper.
Steadman et al. describe the rehabilitation after microfracture as crucial20. Their patients are counseled that improvements may be expected between six and twenty-four months after microfracture. Patients with femoral condylar defects treated with microfracture are prescribed CPM for six to eight hours per day in a range that is gradually increased until full passive motion is achieved. Crutch-assisted toe-touch weight-bearing is recommended for six to eight weeks, with patients with smaller lesions allowed to walk earlier than those with larger defects. The immediate postoperative rehabilitation program consists of limited strength training that adheres to weight-bearing restrictions and is gradually progressed to more intensive resistance training. After completion of an appropriate return-to-sport program, athletes are allowed to return to high-impact sports after sixteen to twenty-four weeks. Patients with patellofemoral defects treated with microfracture are prescribed a hinged knee brace set to allow 0° to 20° of flexion. The brace is removed for CPM use (as described above) but is worn otherwise. Weight-bearing is allowed in the brace. After eight weeks, the brace is gradually unlocked and then discontinued20.
Unfortunately, multiple different regimens of CPM, weight-bearing, and rehabilitation have been reported, and a common protocol does not exist38. Several studies have compared early and delayed rehabilitation after other cartilage restoration procedures39-43; however, to our knowledge only one comparative study has investigated CPM use and weight-bearing after microfracture. Marder et al., in a retrospective cohort study of fifty patients, analyzed patients with small (<2-cm2) defects on the femoral condyles treated with microfracture followed by CPM and non-weight-bearing compared with patients treated without CPM who were allowed early weight-bearing on the basis of comfort44. They found no difference in clinical results between the two groups. Despite basic-science studies supporting the importance of CPM for cartilage health, there is a lack of consensus regarding how CPM should be administered following microfracture and other forms of cartilage restoration surgery38,45.
Initial outcome studies reporting on the results of microfracture were published nearly twenty years ago, and an eleven-year follow-up of those patients has also been reported15,31. Average scores for pain, swelling, and function improved compared with the preoperative scores (p < 0.05 for each)31. The majority (fifty-nine) of the seventy-one patients reported increased function and reduced pain compared with preoperatively. However, to our knowledge no comparative studies with a control group have been reported, which makes it difficult to know how much microfracture affects the natural history of these lesions in the long term. Despite the initial enthusiasm regarding the microfracture technique, the literature has since shown mixed results, especially when compared with other techniques.
Overall, despite initial improvements in patient outcomes, the results of microfracture appear to deteriorate over time46,47. In an attempt to prevent the gradual deterioration that is inherent with the fibrocartilaginous result of microfracture, advanced cartilage restoration techniques have been devised to restore an articular surface that more closely resembles native hyaline cartilage12. Magnussen et al. performed a systematic review of six studies that compared those other cartilage restoration techniques (ACI, MACI, and OATS) with microfracture and reported that no technique consistently had superior results compared with the others at short-term (two-year) follow-up48. Those authors did report that outcomes for microfracture tended to be worse in lesions >2 cm2 in size.
Mithoefer et al. performed a comprehensive systematic analysis of the clinical efficacy of microfracture that included twenty-eight studies46. They compiled an overview of clinical results after microfracture in studies with short-term and long-term follow-up46. In studies with follow-up of twenty-four months or less, clinical improvement rates were found to be between 75% and 100% across studies. In studies with follow-up longer than twenty-four months, clinical improvement rates were lower and ranged between 67% and 86%. Additionally, functional deterioration was reported after twenty-four months in 47% to 80% of patients, although they remained improved relative to the preoperative status. The complication risk was reportedly low (0% to 13%), with complications including arthralgia, effusion, crepitation, arthrofibrosis, infection, and deep vein thrombosis. The rate of failure, as defined by the need for further surgery, varied considerably between randomized studies and case series, but was found to be as high as 23% to 31% at two to five years postoperatively46.
Goyal et al. recently performed a systematic review of fifteen Level-I and II studies that investigated microfracture, all of which lacked an untreated control group47. The included studies compared microfracture with procedures such as ACI and OATS. Heterogeneity in many different study characteristics prevented direct comparison among the various studies or a meta-analysis. Of the fifteen studies, thirteen reported inferior outcomes with microfracture compared with OATS and various ACI techniques. Conversely, only two studies reported no difference when microfracture was compared with OATS and ACI47. Despite finding that the majority of these Level-I and II studies favored newer cartilage restoration techniques, the authors concluded that further research was needed before definitively stating that microfracture is inferior to these techniques. They concluded that the use of microfracture for the treatment of small lesions in patients with low functional demands resulted in good short-term clinical outcomes. However, they cautioned that treatment failure and symptom recurrence could be expected after five years, regardless of lesion size, likely as a result of the inferior wear characteristics and durability of fibrocartilage compared with articular cartilage.
Adjuncts and modifications to microfracture have been proposed and are currently under investigation. Autologous matrix-induced chondrogenesis involves augmentation of a microfractured defect with a matrix of collagen-I and III. Anders et al. recently published a randomized controlled trial that compared this technique with microfracture alone49. At one and two years postoperatively, there were no significant differences between the groups. The authors acknowledged that further long-term study is required before conclusions about this technique can be made.
The use of an awl was compared with the use of a fluted drill with irrigation by Chen et al. in rabbit studies50,51. Distinct differences in bone compaction around the holes were observed in comparisons of microfracture with an awl to a depth of 2 mm and drilling to 2 mm, but these differences did not affect outcomes. Drilling to 6 mm instead of 2 mm did improve repair tissue quantity and quality50,51. To our knowledge, no human clinical studies have been completed that compare awl use with drilling, or shallow with deep drilling techniques. Given the findings of the rabbit studies by Chen et al., other authors have recommended a subchondral needling procedure that may lead to narrower, deeper holes52. It remains to be determined how this method compares with microfracture performed with a traditional awl.
Because of its relative low cost and minimal invasiveness, microfracture remains one of the treatment options for symptomatic, traumatic, full-thickness cartilage defects. It is more likely to be successful when performed as a primary procedure for nonobese patients under the age of thirty to forty years with small (<2 to 4-cm2) femoral condylar defects that have been symptomatic for a short time (less than twelve to twenty-four months). Microfracture is likely to produce acceptable short-term clinical results; however, results can be expected to decline over time. High-quality, long-term studies that compare microfracture with advanced cartilage restoration techniques are required to ascertain whether those newer techniques provide longer-lasting results.
Investigation performed at The Ohio State University Wexner Medical Center, Columbus, Ohio
Disclosure: No external funding was received for this study. On the Disclosure of Potential Conflicts of Interest forms, which are provided with the online version of the article, one or more of the authors checked “yes” to indicate that the author (or the author’s institution) had a relevant financial relationship in the biomedical arena outside the submitted work.
- Copyright © 2016 by The Journal of Bone and Joint Surgery, Incorporated