Chondral lesions of the patellofemoral joint are a common entity that is identified in >33% of patients undergoing arthroscopic surgery.1 After the medial femoral condyle, the patella is the second most common location in the knee for the occurrence of Outerbridge grade III and IV chondral lesions.2,3 Etiologies for patellofemoral chondral lesions include acute traumatic injuries, such as dislocation and subluxation, microtrauma, osteochondritis dissecans, and degenerative changes.
Multiple factors contribute to the increased challenges of performing patellofemoral cartilage restoration procedures compared with procedures in other areas of the knee. First, patellofemoral joint loads may reach 6.5 times body weight, and chondral injuries that alter force distribution may result in even higher loads.4 Force distribution can be further altered by abnormal patellar tilt, malalignment, and maltracking, as well as patellar or trochlear dysplasia.3,5 Second, the complex morphology of the patellofemoral joint and its heterogeneity between patients complicates efforts to restore the native articular surface contour. Third, the patella contains the thickest cartilage in the body, and femoral autograft has structural properties that differ from those of the adjacent native patellar cartilage. Thus, femoral autograft may not adapt well to patellofemoral joint stresses.1,6,7 Finally, whereas most tibiofemoral defects can be managed arthroscopically, it is often necessary to perform an arthrotomy to manage patellofemoral defects.4,7 These factors may contribute to the inferior outcomes after cartilage restoration of the patellofemoral joint compared with outcomes after cartilage restoration of the tibiofemoral joint.1,7-11
Untreated chondral lesions may be a contributing factor in activity-limiting anterior knee pain.4-8,10,12-14 The main goal in managing patellar and trochlear chondral injuries is to restore cartilage surface congruity with sufficient biomechanical properties to alleviate symptoms, facilitate the return to previous level of activity, and improve quality of life.5
The main indications for cartilage restoration are Outerbridge or International Cartilage Repair Society grade III or IV focal chondral or osteochondral defects of the load-bearing articular surface of the patellofemoral joint in patients with symptomatic knee pain in whom a sufficient trial of nonsurgical treatment has been unsuccessful.1,3,4,6,7,10,12,15 General contraindications to cartilage restoration include osteoarthritis of the patellofemoral joint, inflammatory disease, medical contraindications, lower grade lesions, and patient inability to comply with postoperative rehabilitation protocols.8 Additionally, outcomes may be better after management of unipolar lesions and contained lesions than after management of bipolar lesions and uncontained lesions.5,11 In patients with patellofemoral malalignment or maltracking, concomitant procedures, such as lateral lengthening, medial reefing, vastus medialis obliquus advancement, medial patellofemoral ligament reconstruction, trochleoplasty, or advancement and/or medialization of the tibial tuberosity, should be performed to address this pathology; however, these topics are beyond the scope of this article.3,5,6,8,10,12,13,16
In the microfracture technique, an awl is used to penetrate the subchondral bone to facilitate bleeding, clot formation, and migration of marrow-derived mesenchymal stem cells into the defect, thereby promoting fibrocartilage repair.15,17 Microfracture of the patella is associated with unique technical challenges, including a higher degree of difficulty in visualizing and accessing the lesions arthroscopically compared with microfracture of the tibiofemoral joint as well as the need to maintain counterpressure on the anterior aspect of the patella. The articular cartilage lesion is identified, and all loosely attached cartilage surrounding the defect is débrided to the level of subchondral bone to create a perpendicular edge of stable articular cartilage surrounding the defect.17 Using an appropriately angled awl, the surgeon makes multiple holes perpendicular to the subchondral bone surface throughout the defect. These holes are spaced 3 to 4 mm apart and are 3 to 4 mm deep (Figure 1). After microfracture is complete, the irrigation pump is turned off, and bleeding from the subchondral bone is observed.17
Indications for microfracture include full-thickness chondral lesions or unstable cartilage overlying subchondral bone with a postdébridement lesion size <4 cm2; outcomes are less predictable in patients with a postdébridement lesion size ≥4 cm2.15 Microfracture surgery is contraindicated in patients with uncontained chondral lesions. Although good results have been reported after microfracture in the knee overall, no study has specifically investigated microfracture for the management of patellar and/or trochlear lesions15 (Table 1). Patient factors associated with improved results include age <40 years, preoperative symptoms for <12 months, and body mass index <30 kg/m2.15 Postoperative concerns include persistent knee pain and mechanical symptoms, recurrent knee effusions, incomplete defect filling or poor integration with surrounding articular cartilage, and deterioration of functional outcomes necessitating alternative restoration or arthroplasty procedures.14,15,17 Revision rates of approximately 2.5% to 6% at 2 years postoperatively and 9% to 31% at 5 years postoperatively have been reported.15 Although alternative cartilage restoration techniques may be performed if microfracture fails, outcomes of subsequent procedures may be inferior9,14 (Table 1).
Osteochondral Autograft Transplantation
In osteochondral autograft transplantation (OAT), healthy, intact hyaline cartilage is harvested from a non–weight-bearing portion of the knee joint and then is used to repair full-thickness chondral defects.13 The biomechanical properties of healthy hyaline cartilage are superior to those of microfracture-induced fibrocartilage.7
Chondral lesions are identified arthroscopically, then débrided in the same manner as previously discussed for microfracture, after which the lesions are measured. Depending on their location, trochlear lesions may be managed using an all-arthroscopic technique. Patellar lesions, however, must be managed via arthrotomy to permit instrumentation perpendicular to the articular surface. If an arthrotomy is performed, donor plugs may be harvested from the non–weight-bearing periphery of the femoral condyles. Less commonly, donor plugs are harvested from the intercondylar notch using an all-arthroscopic approach.4,6,7 The harvester is positioned perpendicular to the chondral surface, impacted to a depth of approximately 10 mm, and then carefully removed with the donor plug intact. The recipient site is prepared to accept an appropriate-sized osteochondral plug by either drilling (in the case of the patella) or impaction of a corresponding recipient core harvester. The use of powered drills may facilitate the creation of more perpendicular drill tunnels, which is critically important to recipient site preparation. The graft is inserted into the recipient socket and gently impacted to match the height of the surrounding cartilage (Figure 2). The graft should not be left proud because it is unlikely to settle over time, and incongruity of the repaired recipient site can permanently alter joint biomechanics and accelerate graft failure.6,13 The use of multiple smaller grafts may improve contouring at the recipient site but comes at the expense of decreased coverage of the repair area.6,7,13 Plug diameter varies depending on the size of the defect, but typically ranges in size from 5 to 10 mm; up to 16 plugs can be used.4,6,7,13,18-20
The graft harvest should be limited to approximately 3 to 4 cm2 in size. Although up to 8 to 9 cm2 of graft can be harvested, donor sites of this size place the patient at increased risk for donor site morbidity.7 Typically, donor sites fill in with fibrocartilage; however, if >5 cm2 of graft is harvested and the harvest site is neither grafted nor filled with biosynthetic plugs, the patient is at risk for failure of the donor surface to reconstitute.13 The authors of one article reported giant cell inflammatory reactions in two patients after implantation of biosynthetic plugs.21 Both patients underwent revision surgery. In a different study, donor site symptoms resolved within 6 weeks postoperatively in 95% of patients who underwent OAT from the non–weight-bearing femoral condyle margin to manage chondral defects outside the knee.7 The preference of the senior author (R.H.B.) is to harvest graft measuring ≤2.5 to 3 cm2 in total area to minimize donor site morbidity and avoid the routine use of biosynthetic graft to fill donor holes.
Specific indications for OAT include relatively large osteochondral lesions of >1 cm2 in size but <4 to 8 cm2. Lesions <2 cm2 are associated with the best outcomes.22 Patients who are treated with a single plug,4 patients who do not require concomitant realignment procedures,6 and patients who have isolated traumatic chondral lesions are among those with improved outcomes.13 Inferior outcomes may occur in patients who have both medial and lateral patellar facet lesions;4 who are aged ≥50 years;7 who have larger surface area lesions;4,6,13,22 and who require at least six grafts, which places them at increased risk for donor site morbidity.13 Complication and failure rates, which are relatively low in comparison with other cartilage restoration techniques, include stiffness requiring manipulation under anesthesia (3% to 9%) and graft failure (zero to 8%)4,6,18,20 (Table 2).
Autologous Chondrocyte Implantation
Autologous chondrocyte implantation (ACI) is the most well-established and widely used biologic cell transplantation technique. ACI is a two-stage procedure. In the first stage, 100 to 300 mg of healthy articular cartilage is harvested from the non–weight-bearing portion of the femoral condyles or intercondylar notch, and the cartilage is sent for culture and expansion of donor chondrocytes. In the second stage, the expanded chondrocytes are re-implanted within the prepared defect site during a second procedure 3 to 8 weeks later.1,5 The first-generation technique consisted of injection of cultured chondrocytes into the defect beneath a collagen membrane or periosteal patch.3,12,18 Although this method is effective, it necessitates water-tight adhesion of the patch to surrounding cartilage and may be complicated by periosteal hypertrophy.3 The second-generation technique, which is also referred to as matrix-induced autologous chondrocyte implantation or matrix-induced autologous chondrocyte transplantation, consists of seeding cultured chondrocytes onto three-dimensional scaffolds before implantation.1,12 The third-generation technique, which is similar to the second-generation technique, consists of implantation of cultured chondrocytes within three-dimensional chondroinductive or chondroconductive matrices.1,10,12
Outcomes are better in patients who have isolated trochlear defects than in those who have patellar defects.1 Patients treated with concomitant proximal or distal realignment surgery tend to have good outcomes, and patients with unipolar lesions have better outcomes than those with bipolar lesions.3,5,10,12 Revision rates are relatively high, ranging from zero to 72%; complications include arthrofibrosis (8% to 18%) and periosteal hypertrophy or extrusion (6% to 32%)3,5,9,10,16 (Table 3). In a systematic review, Trinh et al10 reported an overall revision rate of 16% and an overall complication rate of 15% for all patients undergoing isolated patellar or trochlear ACI.
Other Techniques and Salvage Options
Osteochondral allograft has been used as a salvage procedure for chondral lesions that are too large to be managed using other cartilage restoration procedures in patients who are poor candidates for arthroplasty procedures.2,13,16 The technique for osteochondral allograft is similar to OAT, albeit with larger recipient sites into which matching plugs prepared from cadaver tissue are placed. Although concerns of disease transmission exist, the more substantial concerns are long-term chondrocyte viability and graft resorption, with outcomes inferior to those in the tibiofemoral joint.2 In a cohort of patients undergoing osteochondral allograft to the trochlea, Cameron et al23 reported a 100% graft survivorship rate at 5-year follow-up, a 91.7% graft survivorship rate at 10-year follow-up, a 21% revision rate, and an 89% patient satisfaction rate. In a cohort of patients who underwent osteochondral allograft to the patella, Gracitelli et al24 reported a 78.1% graft survivorship rate at 5- and 10-year follow-up, a 55.8% graft survivorship rate at 15-year follow-up, a 61% revision rate, and an 89% overall patient satisfaction rate.
Additional salvage options include patellofemoral arthroplasty and total knee arthroplasty.2,5,16 In a systematic review of large patellofemoral cartilage lesions measuring ≥4 cm2 in patients aged ≤50 years, Noyes and Barber-Westin16 reported a failure rate of zero to 24% after patellofemoral arthroplasty and a mean failure rate of 32% after osteochondral allograft, with no benefit achieved in an average of 22% and 53% of patients, respectively.
A relatively new technique with limited data in the patellofemoral joint involves the use of particulated juvenile cartilage allograft (DeNovo NT Natural Tissue Graft; Zimmer Biomet), which may have indications similar to ACI and a theoretical advantage over ACI because, as allograft, there is no limit to the amount of implantable material available for use in a single-stage procedure.2 Additional studies are needed to better determine indications and assess outcomes after these procedures.
Postoperative rehabilitation is similar irrespective of the restoration procedure. Continuous passive motion and cryotherapy are begun immediately postoperatively. Weight bearing is allowed within 1 to 2 weeks postoperatively with a hinged knee brace locked in extension for 4 to 8 weeks postoperatively to reduce shear on the repair site with ambulation.17
Significant improvement in mean Lysholm score at 2-year14 and 7-year17 follow-up has been reported for microfracture in all compartments of the knee (P < 0.001 and P < 0.05, respectively). These studies included patients who underwent patellar and/or trochlear microfracture, but results were not specified by lesion location. No study to date has provided outcome data specifically for patellar or trochlear lesions after microfracture. MRI evaluation after patellar OAT demonstrated 67% to 100% cartilage repair fill at a mean follow-up of 28.7 months in one study6 and 100% osseous integration at 1-year follow-up in a different study.4 Ebert et al12 reported MRI findings of 40.4% complete graft infill in patients who had undergone ACI. They anecdotally noted that trochlear lesions are more forgiving than patellar lesions but demonstrated no clinical or radiologic differences in outcomes. Using a first-generation ACI technique, Farr8 also reported that outcomes were similar for the patella and the trochlea. Conversely, Filardo et al1 demonstrated improved outcomes, and Nawaz et al9 demonstrated improved graft survival after ACI to manage trochlear lesions compared with patellar lesions. Recent studies using fresh allograft also demonstrated improved outcomes for isolated trochlear lesions compared with patellar lesions.23,24 It is unclear why treatment outcomes may be better for trochlear lesions than for patellar lesions, but the difference may in part be the result of better vascularity of the trochlea or the increased difficulty in accessing patellar lesions. To our knowledge, no data exist comparing the outcomes of trochlear and patellar lesions after microfracture or OAT. Because the data suggest better outcomes after management of trochlear lesions than patellar lesions, it may be better to consider them as separate entities in future studies.1,5,9,11,23,24
Several methods have been described for the surgical management of patellofemoral chondral lesions, and the heterogeneity of outcome measures used makes comparison of techniques somewhat difficult. Regardless of the method used, patellar malalignment and/or maltracking should be managed before or during the surgeon’s cartilage restoration procedure of choice to optimize outcomes and reduce the likelihood of treatment failure and revision. Microfracture is a good first-line cartilage restoration technique for the treatment of small, contained chondral lesions, and even if the treatment fails, the patient may be a candidate for an alternative cartilage restorative procedure. OAT is associated with lower comparative revision rates, complication rates, and cost, as well as equivalent or better patient outcome scores compared with osteochondral allograft; thus, OAT is a good option for managing larger lesions and patients in whom microfracture has been unsuccessful. ACI is an option for large lesions (>4 cm2). Osteochondral allograft is a good option for managing defects >2 to 4 cm2 or after failed OAT or ACI. Further studies specific to management of the patella and trochlea are needed to better understand the optimal indications for and outcomes of patellofemoral cartilage restoration surgery.
Evidence-based Medicine: Levels of evidence are described in the table of contents. In this article, reference 18 is a level I study. References 1 and 5 are level II studies. References 3, 4, 6-17, and 19-24 are level IV studies. Reference 2 is level V expert opinion.
References printed in bold type are those published within the past 5 years.
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2. Strauss EJ, Galos DK: The evaluation and management of cartilage lesions affecting the patellofemoral joint. Curr Rev Musculoskelet Med 2013;6(2):141-149.23392780
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Keywords:Copyright 2017 by the American Academy of Orthopaedic Surgeons.
patella; trochlea; articular cartilage; microfracture; osteochondral autograft transplantation; OAT; autologous chondrocyte implantation; ACI