Microfracture and Augments

10.1055/b-0034-92481

Microfracture and Augments

Andreas H. Gomoll

Microfracture was originally developed by Steadman in the 1980s in response to perceived limitations of the then commonly used marrow stimulation techniques (MSTs), abrasion arthroplasty and subchondral (Pridie) drilling—namely, destabilization of the subchondral plate and heat necrosis, respectively. Microfracture was quickly adopted for the treatment of cartilage defects, first in the knee, followed by the ankle, shoulder, elbow, and hip. The technique aims to induce the formation of a reparative tissue by the creation of channels in the subchondral plate, allowing the migration of mesenchymal stem cells (MSCs) from the subchondral marrow space into the defect. Here, they differentiate and produce a fibrocartilaginous tissue to fill the defect. The underlying biology is discussed in greater detail by Dr. Fortier and co-authors in chapter 6.

This chapter will review the indications, clinical application, rehabilitation, and outcomes of the standard microfracture procedure. In addition, it will present an overview of new technologies currently under development that aim to augment microfracture through the use of bio materials and growth factors in hopes of improving outcomes and broadening the indications.

The Microfracture Procedure

Indications

The following indications are based on findings from multiple studies discussed further in the Results section. Microfracture is primarily indicated for the treatment of full-thickness articular cartilage defects without significant bone loss (Outerbridge Grade 3 and 4; International Cartilage Research Society [ICRS] Grade 3) measuring less than 2 to 4 cm² on the femoral condyles. Articular comorbidities such as malalignment and meniscal deficiency do not represent a contraindication provided they are corrected in a staged or concomitant fashion. Elevated body mass index (BMI) over 30 kg/m², defect size larger than 2 to 4 cm², defect location in the patellofemoral compartment or on the tibial plateau, and age older than 40 years are associated with worse outcomes.

Technique

Microfracture is generally performed as an all-arthroscopic procedure utilizing standard anteromedial and anterolateral portals. Rarely, accessory portals may become necessary for optimal access. When performed together with other intra-articular procedures such as anterior cruciate ligament reconstruction or meniscal repair, microfracture should be performed last to preserve the developing blood clot that could otherwise be irrigated away by the arthroscopic fluid. The use of a tourniquet is optional.

Arthroscopic view of a femoral condyle cartilage defect.

Same defect after debridement of degenerated tissue and the layer of calcified cartilage with creation of stable shoulders.

Once the entire joint has been carefully evaluated and any articular comorbidities have been addressed, the cartilage defect ( Fig. 7.1 ) is prepared. First, all degenerated cartilage is removed with a sharp curet, including any areas of surrounding cartilage that is delaminated. Vertical shoulders of stable cartilage are thus created. Next, the layer of calcified cartilage is removed with the curet; however, avoid excessive force that can injure the subchondral plate ( Fig. 7.2 ). Generally, a motorized shaver can assist in removing larger flaps but is inadequate to appropriately prepare the defect by itself. Microfracture awls are available with different angled tips; depending on the location of the defect, the awl providing perpendicular alignment of the tip to the defect surface should be chosen. If the angle of placement is too oblique, furrows are created rather than holes, with increased damage to the subchondral plate. Microfracture holes are now created, starting at the periphery to improve edge integration ( Fig. 7.3 ). The holes should be placed ∼ 3 to 4 mm apart to prevent holes from becoming confluent and destabilizing the subchondral plate. At the end, the tourniquet should be deflated or the pump pressure lowered to observe fat droplets and bone marrow from each hole; otherwise, individual holes can be revisited and deepened with the awl. The use of intraarticular drains should be avoided because removal of the intra-articular hematoma would be counterproductive to the formation of the desired marrow clot.

Rehabilitation

The postoperative rehabilitation is a critical and inherent component of the micro-fracture procedure, and its contribution to the overall success cannot be overemphasized.1–3 Weightbearing restrictions are tailored to the individual patient: small and well-shouldered defects (< 1 cm²) require less protection than larger defects with compromised shoulders. Generally, patients are kept touch-down weightbearing on two crutches for 4 to 8 weeks. A continuous passive motion machine is started on postoperative day 1 and continued for 6 weeks, 6 to 8 hours per day, increasing range of motion (ROM) as tolerated. Quadriceps isometrics and straight-leg raises can be started immediately, adding resistance bands and mini-squats at 2 months. More aggressive weight training is delayed until 4 months postoperatively. Impact sports, especially those involving cutting or pivoting, should not be resumed until 6 to 9 months after the procedure, and only once swelling has resolved and adequate muscle strength and proprioception have returned.

Results

Several authors have reported on the outcome of microfracture using both case series and randomized controlled trials (RCTs). Steadman′s group has the largest experience with microfracture and has published extensively on this technique in various sub-populations. They demonstrated symptom improvement in 80% of cases in a minimum 7-year follow-up study of patients younger than 45 years without concomitant intraarticular comorbidities; at 3 years postoperatively 16% rated themselves unchanged and 4% considered their symptoms worse than preoperatively.4 A more challenging group of patients with degenerative defects reported improvements in pain and function; 13 of 81 (16%) patients required repeat surgery for lysis of adhesions, and 5 (6%) patients underwent repeat microfracture or revision to arthroplasty at an average of 23 months postoperatively.5 In their experience, results were not affected by lesion size or location.4 Other groups have reported good and excellent results in 60 to 80% of patients4,6–9 but have recommended more narrow indications, most reporting worse outcomes in defects larger than 2 to 4 cm².7,9–12 The treatment of patellofemoral lesions with micro-fracture has been associated with worse outcomes than the treatment of femoral condyles.11 Microfracture in patients older than 35 to 40 years resulted in worse outcomes than in younger patients.4,9,10,12,13 The influence of defect chronicity appears controversial, with Steadman et al reporting no effect, while Mithoefer et al demonstrated better outcomes with lesions less than 1 year old.4,6 Finally, BMI over 25 to 30 kg/m² appears negatively correlated with outcomes.6,10

When comparing microfracture to other procedures, Knutsen et al demonstrated overall comparable results to autologous chondrocyte implantation (ACI) for various lesions sizes in an RCT,12 while Saris et al showed better histological and functional outcomes with ACI.14,15 Coleman et al reported a trial of microfracture versus ACI, showing 44 versus 22% increased Cincinnati scores, respectively. Magnetic resonance imaging (MRI) scores were better for ACI; however, this did not correlate with functional outcomes.16 Basad et al specifically focused on size-related outcomes in an RCT of micro-fracture versus ACI in defects larger than 4 cm², which demonstrated better results for ACI.17 Kon et al presented results from a cohort study comparing micro fracture to ACI with comparable results at 2 years, but worse results for microfracture at 5 years.8 Gudas et al compared micro fracture to osteochondral autograft transfer (OAT) in two RCTs. One demonstrated better arthroscopic, histo-logic, and MRI appearance, and higher return to play with OAT than microfracture (93 vs. 52%, respectively) in athletes.9 Gudas et al′s second RCT randomized patients with osteochondritis dissecans lesions to OAT versus microfracture, showing better outcome with OAT at 4 years (83 vs. 63%, respectively).18

Several studies used MRI to evaluate the quality of the repair tissue at follow-up, reporting good and excellent fill in approximately half of patients or fewer.6,9,19,20 Poor fill on MRI correlated with worsening symptoms after an initial period of improvement.6 When serial MRIs were performed, quality improved in the early postoperative period up to 2 years.6

Complications

Significant surgical complications are rare with this arthroscopic procedure. Depending on lesion size and location, patients can experience catching until the defect has filled with repair tissue. Over the mid to long term, microfracture has been shown to result in the formation of intralesional osteophytes, subchondral sclerosis, and cysts in up to 50% of patients ( Fig. 7.4 ).6,9,20 The potential influence of these subchondral changes on subsequent revision surgery with ACI has been reported in several studies. Some investigators reported no negative influence of prior microfracture in subanalyses of studies designed for general outcomes after ACI.21,22 Conversely, two publications specifically tailored to investigate this question reported failure rates of ACI after prior marrow stimulation that were up to three times the failure rates seen in ACI in not previously treated defects.23,24

Augmentation Techniques

Although microfracture provides good short-term outcomes for many patients, it results in a fibrocartilaginous, rather than hyaline-like, repair tissue.25 Mid- to long-term studies have demonstrated gradual decreases in functional outcomes after 24 to 36 months, potentially due to tissue degradation over time.8,11 Increased interest in augmenting the body′s own reparative response to improve quality and functional outcomes has led to the development of various biomaterials for implantation in conjunction with MSTs. Several of these treatment approaches will be discussed briefly below.

Drilling

Pridie developed subchondral drilling as treatment for cartilage defects in the 1950s and reported patient satisfaction of 77%; 64% of the knees were rated as good.26 Utilizing an open arthrotomy and postoperative immobilization to perform the drilling, however, resulted in a high number of patients with stiffness, and heat necrosis of the subchondral bone was a concern. The rise of arthroscopic instrumentation facilitated the development of microfracture, addressing both the need for an open approach as well as any concern for heat necrosis.

More recently, several studies have pointed to certain benefits of drilling over micro-fracture, leading to a possible renaissance of this procedure. Since the current drilling technique is performed arthroscopically in an aqueous environment, heat necrosis is of lesser or no concern. Animal models have demonstrated that drilling with actual drill bits (rather than smooth K-wires) results in better marrow clot formation. This has been explained by the deeper channels created with drilling rather than microfracture, which can access more of the subchondral marrow space. Also, while microfracture compacts the bone around the hole and potentially seals off the marrow space, drilling removes bone and allows for more blood flow into the defect. A rabbit model demonstrated more complete fill and higher-quality tissue after subchondral drilling than micro-fracture.27,28 The same group also demonstrated better repair of the subchondral bone with deeper drilling rather than the more shallow micro fracture, with less incidence of cystic and sclerotic abnormalities.28,29 Even though many of the following techniques have been tested with microfracture, drilling could be substituted, and additional studies should confirm its benefits.