Microfracture

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CHAPTER 14 Microfracture

Michael J. Medvecky, Peter Yeh, Peter Jokl

Articular cartilage lesions are a commonly encountered condition seen during arthroscopic surgery (Fig. 14-1)¹^,² and may be encountered incidentally in spite of improvements with magnetic resonance imaging (MRI). The limited repair capabilities of articular cartilage chondrocytes and the avascular nature of hyaline cartilage do not allow for any intrinsic healing capacity of full-thickness articular cartilage lesions that do not violate the subchondral bone.³^,⁴ Different techniques have been used in an attempt to create reparative or transfer cartilaginous tissue into the defect with the goal of improved pain relief, activity tolerance, and avoidance of long-term degenerative changes to the lesion or adjacent cartilage.⁵ Multiple procedures have been used in the past several decades to promote reparative “regenerate” cartilaginous tissue ingrowth into the lesion.⁶^–⁸

FIGURE 14-1 Articular cartilage lesions, ICRS grades 2A (A), 3A (B), and 3C (C).

In the mid-1980s, Rodrigo and colleagues ⁹ and Steadman and associates¹⁰ adapted the Pridie drilling technique by using an awl to create perforations within the subchondral bone layer. The technique was refined with the knowledge of the importance of the removal of the calcified cartilage layer, overlying the subchondral bone, to improve adherence to the subchondral bone.¹¹

After penetration of the subchondral bone, bleeding from the bone marrow creates a fibrin clot within the defect. Marrow-derived pluripotential cells progressively differentiate into fibroblasts, osteoblasts, chondroblasts, and chondrocytes, without contribution from the adjacent articular cartilage. Shapiro and coworkers ¹² have demonstrated after cellular labeling via autoradiography that the repopulation is mediated entirely by the proliferation and differentiation of mesenchymal cells from the bone marrow. The collagen matrix formed did not integrate into the adjacent native cartilage and this weak link is theorized to be the potential source of long-term repair tissue degradation. The repair tissue fibrocartilage is also primarily composed of type I collagen, which has decreased resilience and stiffness compared with the type II collagen seen in normal hyaline cartilage.

The risk of articular cartilage defect progression to degenerative arthritis is multifactorial and defect size may play a significant role in this process. A critical size of articular defect that is potentially damaging to adjacent cartilage is not known; the literature shows wide variability in defect size for which treatment had been indicated. Guettler and colleagues ¹³ have evaluated pressure readings from the peripheral rim of articular defects created at various sizes, ranging from 5 to 20 mm. Rim stress concentration was seen in defects of 10 mm and larger, suggesting this as a critical size defect, which may increase the risk of progressive deterioration of the adjacent cartilage.

TREATMENT

Indications and Contraindications

Indications

Microfracture treatment is ideally indicated as a first-line treatment of well-contained Outerbridge grade III or IV cartilage lesions less than 4 cm ². Treatment of larger lesions has been described but most studies included lesions with a mean defect size of less than 4 cm².¹⁴ Unstable chondral flaps that extend to the subchondral bone layer are also lesions amenable to this technique.

Secondary to its technical ease and limited surgical morbidity, microfracture marrow stimulation can be applied to cartilage lesions detected preoperatively by MRI or those found incidentally at the time of arthroscopic evaluation. We have also treated acute traumatic lesions in the polytrauma patient in the appropriate clinical setting (Fig. 14-2).

FIGURE 14-2 Microfracture also has applications for the treatment of acute traumatic lesions. This is evidenced by this patient with ipsilateral femoral shaft and patellar fractures and a traumatic chondral lesion of the lateral femoral condyle. (Note the retrograde femoral rod.)

Patients sustaining acute, traumatic, chondral full-thickness lesions are best treated as soon as is practical logistically because duration of symptoms of less than 12 months has been associated with better postoperative outcome scores and improved microscopic repair cartilage grading.¹⁵^–¹⁷ Patients with chronic or degenerative lesions are initially treated with nonsurgical measures, including activity modification, oral anti-inflammatory medication, physical therapy, dietary modifications, and weight loss, as well as potential articular injections such as corticosteroids or hyaluronic acid. Patients who fail nonoperative treatment are potential candidates for microfracture.

Age younger than 40 years, has been associated with better clinical outcome scores and better repair cartilage fills.¹⁸^,¹⁹ Increased body mass index has been demonstrated to be universally correlated with poor outcomes after microfracture treatment, with worst outcomes seen in those with a body mass index (BMI) more than 30 kg/m².¹⁵

These indications need to be compared with the limited knowledge we have concerning the natural history of nonoperative treatment of articular cartilage lesions and the lack of control groups in trials evaluating operative treatment of these lesions.²⁰ Some studies have demonstrated no difference in outcome for nonoperative management of these lesions compared with operative interventions.²¹^,²² Patient selection also needs to be considered in view of the intensive postoperative rehabilitation program that is usually prescribed following microfracture surgery. Weight-bearing limitations in the postoperative rehabilitation program result in the interruption of normal lifestyle and often induce disuse muscle atrophy.

Currently, the methodologic limitations of previous investigations do not allow us to make broad recommendations about differential indications based on an involved compartment. The Most studies fail to differentiate between patellofemoral and tibiofemoral lesions and there has yet to be a study that analyzed the patellofemoral joint in isolation.

When evaluating a patellofemoral lesion, we use radiographic measurement of the lateral patellofemoral and patellar tilt angles for patellar tilt and subluxation, respectively. This information, combined with MRI evaluation of lesion location, is used to decide whether lateral retinacular release should be considered with a lateral patellar facet lesion. We think that an anteromedialization (AMZ) may be considered for secondary or revision treatment in distal pole lesions but do not believe that the added morbidity is warranted in the first-line treatment of these lesions.

Chondral lesions on opposing surfaces (kissing lesions) present a unique clinical dilemma; little information is available to guide treatment. Well-contained lesions with opposing articular surfaces may need to be considered for microfracture treatment; however, any kissing lesions with one or both having a progressive transition to a full-thickness defect or opposing subchondral bone surface should be considered a degenerative lesion and contraindicated for microfracture.

Contraindications

Microfracture treatment is contraindicated for patients unwilling or unable to pursue the recommended postoperative activity modifications and associated rehabilitation protocols. Patient expectations must be discussed preoperatively in those with known acute articular cartilage lesions and patients undergoing treatment for degenerative lesions. The treatment requires a stable, perpendicular peripheral edge of articular cartilage and, therefore, is contraindicated in uncontained chondral lesions or lesions that gradually transition into a full-thickness defect.

Axial malalignment of more than 5 degrees for lesions in the tibiofemoral joint is an indication for simultaneous or staged corrective osteotomy (Fig. 14-3). Not only is joint malalignment detrimental to normal articular cartilage, but uncorrected malalignment can result in significantly increased contact pressure in the affected compartment, resulting in increased load and shear on the treated lesions.²³

FIGURE 14-3 Weight-bearing standing x-rays are obtained for patients under consideration of treatment for articular cartilage disorders. This patient demonstrated asymmetrical genu varum; articular cartilage preservation surgery would be contraindicated prior to valgus producing osteotomy. WBL on right is ∼45%; WBL on left is ∼ 5%.

High-grade ligamentous instability will also result in increased shear force across the treated lesion and is a contraindication to microfracture treatment unless surgically treated simultaneously.²³

Additional contraindications include generalized degenerative joint disease, inflammatory arthropathy, joint infection, and adjacent bony tumor. Meniscal deficiencies should be addressed prior to or at the time of microfracture.

Patient Evaluation Prior to Treatment

The evaluation of the patient for treatment of an articular cartilage lesion requires the assessment of a large number of variables. The history of present illness should include assessment of the mechanism of injury, with documentation of the acuity and cause of the lesion. Patients who have undergone previous nonsurgical and/or surgical treatment need to be questioned about the types of treatments and their response, because these will play a role in future treatment options. Patients previously having undergone surgical treatment should have a careful evaluation of their preoperative imaging, arthroscopic images, and operative reports. Articular cartilage lesions that have undergone previous treatment that violates the subchondral bone layer are unlikely to gain improvement via microfracture.

Concordant patient demographics that need consideration include age, prior medical and surgical history, occupation, and activity level, including recent activity level and activity goals. An assessment of the willingness to participate in a postoperative rehabilitation program is important if articular cartilage surgery restoration procedures are being considered.

Physical examination includes documentation of height and weight for determination of BMI. Standing alignment and gait assessment should be included in the examination. The varus thrust in stance phase or varus recurvatum gait would be consistent with a double- and triple-varus knee,²⁴ respectively, and should be considered contraindications to treatment of a medial compartment articular cartilage lesion, unless addressed prior to considering microfracture. Ligamentous integrity of the cruciate and collateral ligaments must be determined, because a deficiency results in abnormal knee translation and resultant excessive shear force on the articular cartilage. Assessment of residual loss of motion or muscular weakness is important because these may play a role in abnormal joint forces and resultant symptomatology. Previous incisions should be evaluated for neuroma formation.

Our radiologic evaluation includes weight-bearing images, including anteroposterior (AP), lateral, Rosenberg (posteroanterior taken at 45-degree flexion), and axial patellar views (Laurin) (Fig. 14-4). A full-length hip to ankle weight-bearing alignment film should also be obtained in the patient undergoing assessment of an articular cartilage lesion. The weight-bearing line (WBL) is measured by drawing a straight line from the center of the femoral head to the center of the tibiotalar joint.²⁵ The mechanical axis is measured by the angle created from a line drawn from the center of the femoral head to the midpoint between the tibial spines in comparison to a line from the center of the tibial spines to the center of the tibiotalar joint. Simultaneous or staged corrective osteotomy should be considered for patients who have greater than 5 degrees of tibiofemoral varus or valgus, based on the involved compartment. Patellofemoral alignment is evaluated via the axial patellar view and trochlear and patellar morphology seen by MRI.

FIGURE 14-4 Radiographic evaluation includes weight-bearing AP and Rosenberg views demonstrating mild joint space narrowing on the AP view (A) but symmetrical joint space on the Rosenberg view (B).

MRI is useful for the preoperative assessment of the size, depth, and location of the articular cartilage lesion.²⁰^,²⁶^,²⁷ Several acquisition protocols have been recommended by the International Cartilage Repair Society (ICRS).²⁸ Spoiled gradient-recalled echo (SPGR) and fast spin-echo (FSE) techniques are the most accurate and widely used.²⁹^,³⁰ New pulse sequences and image acquisition methods are being investigated for improved diagnosis of articular cartilage abnormalities, disease progression, and biologic response to treatment strategies.²⁹^–³¹ The status of the ipsilateral meniscus is important to assess, particularly in a patient having undergone prior surgical treatment and partial meniscectomy. Adjacent and opposing articular surfaces also need to be carefully evaluated. The subchondral and metaphyseal bone are assessed for cyst formation, bone loss, or cavitation. Loss of osseous integrity or sphericity may play a role in impaired fibrocartilaginous ingrowth. The peripheral aspect of the lesion is also evaluated to determine whether there is containment of the lesion, because this is important to the success of microfracture treatment.

Treatment Options

Direct arthroscopic evaluation of articular cartilage lesions is considered the most accurate and reliable means of assessing chondral lesions in regard to size, depth, and location. The lesion size is measured with a calibrated probe. These variables are important in conjunction with preoperative imaging for determining the most appropriate treatment option to address the articular cartilage lesion. Many classification systems exist; the most recent Cartilage ICRS system (Fig. 14-5) uses predébridement and postdébridement measurements of lesion size. However, not included in this or other classification systems is documentation of the condition of the subchondral bone.

FIGURE 14-5 ICRS articular cartilage grading system. Normal—grade 0= almost normal, grade 1A = superficial lesions and soft, grade 1B = and/or superficial fissures and cracks; abnormal—grade 2 = lesions extend <50% deep; severely abnormal—grade 3A = extend >50%, grade 3B = down to calcified layer, grade 3C = down to but not through subchondral bone, grade 3D = includes blisters, grade 4a = penetrating subchondral bone but not full diameter of defect, grade 4B = penetrating full diameter of defect.

Historically, the Outerbridge system (Box 14-1) was the most commonly used classification system for articular cartilage lesions. A limitation of this classification is that the intermediate grades (grades II and III) are only differentiated based on size (more or less than 0.5 inch in diameter) and there is no quantification of the depth of the lesion. Lesions with exposure of subchondral bone are considered to be Outerbridge grade IV.³²^,³³

Box 14-1 Outerbridge Classification

• Grade 0: normal cartilage

• Grade I: cartilage with softening and swelling

• Grade II: a partial-thickness defect with fissures on the surface that do not reach subchondral bone or exceed 1.5 cm in diameter

• Grade III: fissuring to the level of subchondral bone in an area with a diameter > 1.5 cm

• Grade IV: exposed subchondral bone

Arthroscopic Technique

We perform the microfracture procedure with the patient in the supine position using a well-padded pneumatic tourniquet on the patient’s upper thigh, an arthroscopic leg holder, and general or regional anesthesia. A complete arthroscopic examination of the knee is performed using standard medial and lateral anterior portals. The articular surfaces are observed and palpated with an arthroscopic probe. The status and integrity of the menisci are also evaluated and documented. The ligamentous integrity of the knee is verified, particularly the lateral ligaments, using the arthroscopic gap test, with the knee positioned at 0 and 30 degrees of flexion (Fig. 14-6).

FIGURE 14-6 Arthroscopic gap test performed at 30 degrees flexion to verify functional integrity of the lateral collateral ligament.

Accessory portals are occasionally used pending lesion location. If additional pathology is present that requires treatment, this is performed prior to the microfracture procedure to minimize the amount of time that the microfractured bone is exposed to elevated intra-articular fluid pressure and to allow for adequate clot formation.

The articular defect is assessed for size and location (Fig. 14-7). Associated articular flaps are removed and the margins are débrided back to a peripheral rim with edges perpendicular to the subchondral bone using a curette or arthroscopic knife (Fig. 14-8). Debris is evacuated with the arthroscopic shaver. After débridement, the size of the lesion is again measured with a calibrated probe and recorded. In addition, the arc of motion in which the lesion articulates with the opposing joint surface is recorded. The range of motion of contact will be used for planning the postoperative rehabilitation program (Fig. 14-9).

FIGURE 14-7 The lesion is evaluated for size, depth, and margin integrity. A, B.

(Adapted from Mithoefer K, Williams RJ 3rd, Warren RF, et al. The microfracture technique for the treatment of articular cartilage lesions in the knee. A prospective cohort study. J Bone Joint Surg Am. 2005;87:1911-1920.)

FIGURE 14-8 The margins are débrided to a peripheral rim with edges perpendicular to the subchondral bone using a curette or arthroscopic knife.

FIGURE 14-9 The lesion is evaluated for range of motion where it articulates with the opposing joint surface. This information is documented and used to guide the postoperative physical therapy program.

A curette is used to remove the calcified cartilage layer that is just superficial to the subchondral bone layer (Fig. 14-10). Débridement of this calcified cartilage layer has been shown to improve the bonding of the mesenchymal clot to the subchondral bone.²³^,³⁴ Care is also taken to avoid excessive débridement and subsequent thinning of the subchondral bone; this could stimulate subchondral bone overgrowth, with subsequent relative thinning of the overlying repair cartilage layer and, therefore, subsequent inferior biologic and biomechanical repair tissue quality.¹⁵^,³⁵ Arthroscopic shaver use is minimized to avoid thinning of the subchondral bone.

FIGURE 14-10 Careful débridement of the calcified cartilage layer is performed to avoid excessive thinning or cavitation, which can stimulate subchondral bone hypertrophy and subsequent thinning of the overlying repair cartilage layer.

Commercially available awls are used to create multiple perforations in the subchondral bone approximately 3 to 4 mm apart (Fig. 14-11).¹⁰ The peripheral margin of the lesion is treated first, progressing toward the center of the lesion. Adequate spacing between the perforations is important to minimize the risk of subchondral bone collapse and subsequent cavitation. Perforation of the subchondral bone is performed with the awl tip aligned perpendicular to the subchondral bone layer. The knee is flexed or extended to optimize instrument positioning (Fig. 14-12). Microfracture awls are available in different tip angulations (30, 45, and 90 degrees) with the 30- or 45-degree awl used most commonly (Fig. 14-13). Care is taken to avoid skiving of the awl during impact. Skiving can create a large area of subchondral bone disruption or can break into an adjacent hole. Skiving can be avoided by toeing in the instrument tip and by impaction as close to perpendicular as possible by instrument angulation or knee position.

FIGURE 14-11 A 3- to 4-mm-wide bone bridge is maintained between perforations to maintain biomechanical integrity.

FIGURE 14-12 Initiation of the penetrations is performed at the peripheral edge of the lesion and progresses inward. Impaction at a perpendicular angle to the subchondral bone avoids skiving and disruption of a larger area of subchondral bone.

FIGURE 14-13 Microfracture awls are produced by several surgical companies and made in a variety of angles. They allow for controlled bone perforation and avoid heat necrosis that could occur with motorized drilling. A,; B,.

Patellar lesions more frequently require the use of the 90-degree awl. Patellar perforations are also more easily created by impacting the side of the handle rather than the end. Distal patellar pole lesions may also be more readily assessed via higher flexion angles and the 30-degree awl. Proximal pole lesions may be more readily accessed via suprapatellar accessory portals and the 90-degree awl. Counterpressure on the anterior aspect of the patella is needed for most patellar lesions. A miniarthrotomy may be needed for patellar lesions if access is difficult to achieve.

Osseous debris is removed from the microfracture perforations with an arthroscopic shaver. Confirmation of adequate subchondral penetration is accomplished by observation of the release of blood and marrow fat droplets after lowering the arthroscopic intra-articular pump pressure. A drain is not used so as not to remove the mesenchymal clot and hematoma from within the joint (Fig. 14-14). Postoperative intraarticular anesthetic (1% lidocaine) is used without epinephrine. We have also more recently discontinued the use of bupivacaine secondary to reports of chondrotoxicity.³⁶^–³⁸ A compression dressing is used as well as cryotherapy for control of postoperative swelling and inflammation.

FIGURE 14-14 The arthroscopic pump pressure is lowered and release of marrow fat droplets and blood is verified.

PEARLS& PITFALLS

PITFALLS

• Avoid articular cartilage surgery in an unstable or malaligned knee.

• Avoid skiving, which can create cavitary defects on the subchondral bone.

• Carefully débride calcified cartilage layer but avoid thinning the subchondral bone.

• Premature weight bearing, particularly in tibiofemoral lesions, can lead to the displacement of the mesenchymal clot or may lessen the adherence to the subchondral bone or peripheral integration.

POSTOPERATIVE REHABILITATION

Systematic analysis of the literature has demonstrated the variability of rehabilitation protocols after microfracture treatment.¹⁴ Limited data exist on rehabilitation protocols after microfracture or the effect of continuous passive range of motion (CPM) and weight-bearing status. Basic science studies have demonstrated beneficial effects of continuous motion on cartilage nutrition, metabolism, and marrow stimulation procedures. In his animal model, Salter⁹ has demonstrated improved metaplasia of repair tissue within created articular cartilage defects when CPM was used compared with those who did not undergo CPM. Rodrigo and associates⁹ have also demonstrated an improved visual grading of cartilage repair tissue quality in those patients who used CPM compared with patients who were unable to comply with the postoperative CPM machine (55% vs. 85%). However, Marder and coworkers³⁹ have found no significant difference in outcome with or without the use of CPM or protective weight bearing in lesions smaller than 1 cm².

Tibiofemoral Lesions.

Our postoperative weight-bearing protocol is predicated on lesion location. Patients with tibiofemoral lesions are advised to maintain foot-flat touchdown weight bearing (approximately 25%) for 6 to 8 weeks. CPM is initiated in the recovery room and its use is encouraged for 6 to 8 hours daily for approximately 6 weeks. The ROM initiated is typically from 0 to 60 degrees and is progressively increased in 10-degree increments until full passive ROM is achieved. The rate is set at one cycle/min but can be altered based on patient comfort. If a CPM unit is not covered by insurance carriers and is cost-prohibitive for patient rental, we instruct the patient to undergo approximately 500 cycles of passive ROM in three sessions daily. This is most easily performed by the patient in a closed-chain fashion doing rolling chair seated flexion (Fig. 14-15).

FIGURE 14-15 Rolling chair, closed-chain seated flexion to allow for controlled passive range of motion. A,; B,; C,.

We usually encourage the patient to perform home exercises for the first 1 to 2 weeks after surgery, with an emphasis on rest, elevation, and cryotherapy to lessen the postsurgical effusion.⁴⁰

Stationary bicycling without resistance and aquatic non–weight-bearing exercises may be initiated within 1 to 2 weeks post-operatively. Resistance exercises are started at approximately 8 weeks after surgery, avoiding the lesion contact ROM that was determined intra-operatively. Running, pivoting, and jumping exercises are restricted until approximately 6 to 8 months postoperatively, pending lesion site and location. Competitive level athletic activity is usually restricted until approximately 8 to 12 months postoperatively.

Patellar or Trochlear Lesion Microfracture.

The rehabilitation protocol following patellar or trochlear lesion microfracture differs from the above tibiofemoral protocol.¹⁰^,⁴¹^,⁴² The patients are initiated at partial weight bearing in full extension using a knee immobilizer. They are progressed to full weight bearing more rapidly based on their quadriceps reactivation and control. However, ambulation is performed in the knee immobilizer to allow for limited flexion of less than 20 degrees. This is done to eliminate the shear force across the patellofemoral region that would occur with concentric and eccentric quadriceps contraction during ambulation. CPM is also used for patellofemoral lesions, initiated in low ranges of motion (0 to 20 degrees), progressing toward full motion without restriction.

The beneficial effects of joint motion on the healing of articular cartilage lesions are well documented. These include enhanced articular cartilage nutrition and metabolic activity and stimulation of pluripotential mesenchymal cells to differentiate into fibrocartilage. Pressure-dependent matrix flow from surrounding articular surfaces may influence chondrocyte metaplasia.⁴³

Repair Cartilage Fill

Previous experimental studies using an equine model have demonstrated the volume of repaired tissue averaged approximately 64% (Fig. 14-16). A similar finding was seen by Mithoefer and colleagues ¹⁵ in that most patients demonstrated a persistent depression of the defect; however, all patients with a fill volume of more than two thirds of the defect demonstrated significant improvement in knee function after the microfracture procedure. They also demonstrated persistent gaps between the native and repair cartilage in 92% of patients. This observation was also seen in previous clinical laboratory studies and in alternative articular cartilage repair strategies, including autologous chondrocyte implantation and mosaicplasty.⁴⁴^,⁴⁵ The failure of peripheral integration is thought to increase the shear stresses on the lesion, potentially leading to micromotion and degeneration.¹²

FIGURE 14-16 MRI sagittal proton density high-resolution images of medial femoral condyle full-thickness lesion. A, Patient with chronic anterior cruciate ligament–deficient knee. B, Image taken at 1-year follow-up demonstrates partial fill of defect, with apparent peripheral integration and no subchondral bone hypertrophy.

COMPLICATIONS

Complications related to the surgical procedure have been rarely reported.⁹^,⁴⁴^–⁴⁸ Postoperative hemarthrosis or transient swelling is controlled with a postoperative RICE protocol (rest, ice, compression, elevation).⁸ We try to minimize the use of anti-inflammatory medication in the early postoperative course, if possible. We use active, passive, and pharmacologic adjuncts to minimize postoperative deep venous thrombosis (DVT) risk (e.g., muscle isometrics of the quadriceps, hamstrings, and gastrocnemius-soleus, compression stockings, and aspirin).^{39, 44,45,47} Supplemental pharmacologic DVT prophylaxis is used on a patient-specific basis.