Articular cartilage injury is a prevalent cause of joint pain that significantly affects patients’ function, daily activities, and athletic performance. While articular cartilage defects may not always cause symptoms,1 studies focusing on the knee joint have shown that approximately 60% of knee arthroscopies reveal high-grade cartilage injuries.2 This is concerning since even minor cartilage defects can progress into symptomatic injuries, ultimately impairing patients’ quality of life.3 Despite the rising number of subjects with knee cartilage injuries, their treatment remains challenging as the cartilage tissue has a poor capacity for spontaneous healing.4,5 Therefore, a comprehensive evaluation and treatment of those injuries is crucial to avoid negative outcomes.

Cartilage defect treatment depends on patient related factors such as age, limb alignment, activity level, type of sports and at which level they perform, associated injuries (ligaments and menisci) and defect factors, such as size, location, depth and bony involvement.6 Conservative measures are generally the first-line approach, aiming to reduce inflammation, relieve pain, and restore knee functionality. This may involve analgesic and nonsteroidal anti-inflammatory medications, injections (corticosteroids, hyaluronic acid, and biologics), as well as weight loss and particularly in the patellofemoral joint, physical therapy with a focus on core and hip strength. If conservative management fails, surgical intervention may be necessary.6 Several surgical strategies have been proposed to treat cartilage defects, which can be grouped in palliative, reparative, and restorative techniques. Palliative treatment includes lavage and debridement, abrasion chondroplasty and removal of loose bodies. Reparative strategies are strategies that allow for homologous tissue repair such as microfracture (bone marrow stimulation) with or without associated techniques (so called microfracture +”), such as the augmentation of the microfracture with a collagen membrane to contain the subchondral bleeding (Autologous Matrix Induced Chondrogenesis AMIC) or augmentations using chondral chips, or gels. Restorative procedures include both cell-based restoration and osteochondral transplantation. Cell-based restoration comprises traditionally the autologous chondrocyte implantation (ACI) and now the updated matrix-induced autologous chondrocyte implantation (MACI). Osteochondral procedures include transplantation of either osteochondral autograft transfer (OAT) or osteochondral allograft (OCA) transplantation.7 Cartilage restoration techniques, have become increasingly preferable for full-thickness cartilage defects in the patella due to their superior outcomes compared to other strategies.7 However, while excellent results have been reported for femoral condyle defects treated with most cartilage repair techniques, the patellofemoral joint remains a challenging environment with patient reported outcomes and overall results being reported to be more variable and somewhat less successful.

Patellofemoral cartilage injuries are particularly complex due to the joint's unique anatomy and biomechanics.8 The patellofemoral joint has to tolerate high sheer and compressive loads even during daily activities, such as knee bending, weight bearing, stair ambulation, and squatting, slight malalignment, overuse or muscular deficits can easily lead to cartilage overload causing chondral damage and progressive breakdown. Anatomic alignment is critical in the patellofemoral joint as it is governed both by soft tissue restraints (in the first 30 degrees of flexion) and bony alignment. Abnormal alignment such as patella alta, lateralized tracking due to abnormal tibial tubercle positioning or trochlear dysplasia, influence the patellofemoral biomechanics and can result in secondary chondral damage. Some of these factors may also lead to patellar instability further increasing the risk of patellofemoral cartilage injuries.9

The surgical management of patellofemoral cartilage injuries continues to be a topic of significant discussion. The effectiveness and suitability of each technique can vary depending on individual indications and defect characteristics. This chapter will focus on the articular cartilage treatment techniques available for patellofemoral cartilage defects, discussing aspects of the surgical techniques and their outcomes, and when concomitant surgery should be performed. Novel strategies of cartilage repair are also presented as promising alternatives to address these complex patellofemoral cartilage injuries.

While MRI is a valuable tool for diagnosing knee cartilage injuries, recent studies have highlighted its relatively low sensitivity and specificity in thoroughly assessing chondral defects.10 In particular, MRI tends to underestimate the extent of cartilage lesions in the knee, making it insufficient for definitive conclusions regarding the optimal treatment strategy when a detailed evaluation is required.11 In this regard, an important consideration during the management of cartilage defects is the potential benefits of conducting an initial arthroscopic evaluation prior to cartilage transplantation. Although single-stage procedures are appealing, staging arthroscopy can provide surgeons with the opportunity to directly visualize and assess the size, location and severity of lesions, remove unstable cartilage and loose bodies, and identify other relevant injuries.12 This approach can prevent aggressive interventions or guide the need for concomitant procedures in advance of the cartilage transplantation. While the effectiveness of chondroplasty for treating cartilage injuries prior to the cartilage restoration procedure remains a matter of debate,13 certain patients may benefit from an initial staging evaluation and chondroplasty. A recent study has demonstrated that patients' age, sex, BMI, and lesion etiology did not influence the need for conversion to cartilage transplantation after chondroplasty.14 On the other hand, patients who had low preoperative values of Knee Injury and Osteoarthritis Outcome Score for pain and Veterans Rand 12-Item Health Survey Physical score, along with a high preoperative AMADEUS grade, a score system based on the cartilage defect size, depth and morphology, and subchondral bone quality, were more likely to require cartilage transplantation within 6 months following staging chondroplasty.14 Considering that staging arthroscopy has been proven to alter the surgical plan in over one third of cases, and chondroplasty has shown potential for improving symptoms in some patients,12,14 recognizing that poorer preoperative pain and function scores are associated with higher likelihood of requiring transplantation post-chondroplasty can be valuable during the management of cartilage defects.

Cartilage defects in the patellofemoral joint have a strong correlation with patellar dislocations. Previous research indicates that approximately 95% of patellar dislocation cases result in cartilage defects.15 Given that various anatomical abnormalities contribute to patellar instability, including damage to soft tissue restraints, such as the medial patellofemoral ligament (MPFL), patella alta, increased tibial tuberosity-trochlear groove (TT-TG) distance, and trochlear dysplasia, it is reasonable to consider that these factors are also associated with cartilage injuries or worse outcomes following cartilage restoration procedures.16 Interestingly, even in the absence of patellar instability, previous studies have shown a significant association between patellofemoral cartilage injury and patella alta, patellar tilt, as well as trochlear dysplasia.17 This highlights the critical importance of addressing anatomical abnormalities in the patellofemoral joint.18

Cartilage restoration procedures can be combined with patellar stability surgery to achieve optimal outcomes. That is, in subjects with anatomical abnormalities predisposing patellar instability, addressing these underlying factors is essential to prevent recurrent chondral injuries and provide a stable environment for cartilage restoration healing. The surgical procedure to be performed will depend on the anatomical abnormality identified. In this regard, reconstruction of MPFL is usually indicated when patellar instability is detected, given its importance as a resistance to lateral displacement of the patella.19 Patients with increased TT-TG may undergo medialization tibial tubercle osteotomy (TTO) in order to realign the patella into the trochlea and prevent its dislocation. Concomitantly, tibial tubercle can be anteriorized, which will decrease the load on the patella and improve the contact forces to the trochlea. A recent cadaveric study evaluating the biomechanical effects of TTO in the setting of patellofemoral dysplasia has found that anteromedialization TTO resulted in more favorable contact forces in the patellofemoral joint than TTO medialization only.20 This would increase the chance of better outcomes after cartilage restoration. Likewise, patella alta can be safely addressed by TTO with distalization to normalize the patellar height and improve patellofemoral alignment.21

While the real impact of realignment procedures on the outcomes of patellofemoral cartilage lesions remains unclear,22 previous research suggests positive effects when ACI or MACI are combined with TTO.23, 24, 25, 26, 27 In addition, it is worth noting that a systematic review has highlighted reservations regarding the influence of concurrent realignment procedures on the improved functional outcomes associated with OCA transplantation for patellofemoral injuries.28 This raises the question of when patellar realignment alone would effectively alleviate patients symptoms and avoid the need for cartilage procedures. Previous studies evaluating the association between anteromedialization TTO alone and patellofemoral cartilage lesions revealed that distal and lateral facet chondral lesions were more likely to exhibit good to excellent clinical outcomes, while lesions in the medial facet, central and proximal or diffuse disease were more likely to yield poorer outcomes.29 Lesions located in the central area of the trochlea were associated with medial patellar lesions and also showed worse results.29 In these cases, the combining cartilage restoration with TTO would result in better outcomes, as indeed previously demonstrated.24,30

Chondroplasty, or cartilage debridement, is a common procedure for treating both partial and full-thickness cartilage injuries. Because it does not stimulate cartilage tissue production, it is usually restricted for smaller cartilage defects. Its overall goal is to create a lesion with stable walls. Overly aggressive debridement should be avoided. Clinical outcomes for chondroplasty in the patellofemoral joint are limited, but studies suggest that patients with lower preoperative functional scores may benefit more from the procedure.31 Chondroplasty can also be done prior to cartilage repair. This use as the first stage before cartilage restoration will be further discussed later in this chapter.

Microfracture is a surgical technique used to treat full-thickness cartilage defects by creating small holes within the subchondral bone. This promotes bleeding, clot formation, and mesenchymal cell migration that will eventually facilitate repair into fibrocartilage. However, performing microfracture on the patella presents challenges given the difficult arthroscopic visualization and access. The procedure involves identifying the cartilage defect, removing loose cartilage, and creating multiple holes with an awl perpendicular to the subchondral bone.32 Microfracture is indicated for smaller chondral lesions, with better outcomes potentially in younger patients.33 Possible postoperative concerns include pain, mechanical symptoms, and incomplete defect healing. If microfracture fails, additional cartilage restoration procedures may be necessary, although their outcomes may be inferior after microfractures.34

Another procedure that has been investigated in the management of patellofemoral cartilage defects is the autologous matrix-induced chondrogenesis (AMIC). AMIC involves the use of a collagen membrane along with microfracturing. The membrane contains the post-microfracture clot and contains the potential prochondrogenic factors that come from the subchondral bone, in an attempt to optimize the healing response. A recent study evaluating the effect of AMIC in patellofemoral cartilage injuries, showed that when combined to corrective surgery for patellar instability, AMIC resulted in satisfactory clinical outcomes, with a mean Kujala score increasing from 63.5 preoperatively to 72.2 postoperatively, and a failure rate of 12.9% after an average of 4 years follow-up.35

Autologous chondrocyte implantation and MACI are usually indicated for symptomatic partial and full-thickness cartilage defects that do not extend through the subchondral bone. Because they rely on autologous chondrocytes obtained from the patient, these techniques are a two-stage process.36,37 In the first stage, knee arthroscopy is performed for diagnosis, better characterization of the cartilage defect, and for cartilage biopsy. The small pieces of cartilage (usually 10×2 mm piece of cartilage) are harvested from non-weight bearing areas of the knee and sent for culture and expansion of chondrocytes.37 In a second stage, usually 5 to 9 weeks post-initial arthroscopy, arthrotomy is commonly necessary to prepare the chondral defect for membrane implantation. Adequate debridement of the chondral lesion is performed, removing all unstable cartilage and scar fibrocartilage tissue that surrounds the defect. It is important to avoid violating the subchondral bone and halt any significant subchondral bleeding during the preparation Additionally, the lesion's edges should be perpendicular and be lined with healthy cartilage to assure a good rim integration of the MACI tissue. After preparing the defect, a collagen membrane precisely shaped to fit and cover the chondral defect is implanted. Traditionally the membrane was sutured over the defect and the articular chondrocytes were injected beneath the membrane. The more updated technique allows for securing the membrane that is seeded with articular chondrocytes into the bottom of the defect using a thin layer of fibrin glue (Figure 1). After the membrane is fixed with fibrin glue in some cases stay sutures can be used to assure positioning of the membrane in partially uncontained areas on the patella or trochlea. Full knee flexion-extension movements are performed to confirm the stability of the membrane covering the lesion. Given the fact that the membrane can be cut in different shapes according to the defect, this technique is excellent to address patellofemoral cartilage defects. Excellent results have been reported for ACI and MACI of the patellofemoral joint at short-, mid- and long-term follow-up.38, 39, 40 A previous multicenter study has found that 84% of patients who underwent ACI for patellofemoral defects presented good to excellent outcomes after at least 4 years follow-up.39 After 15 years of follow-up, patellofemoral ACI demonstrated 83% of good to excellent results, with a failure rate of 10%.40

ACI/MACI has been used to treat patellofemoral defects since the late 90’s. Brittberg and Peterson, described their early results in the patellofemoral joint with rather high failure rates. They then reported that the addition of alignment correction using a tibial tubercle osteotomy with anteriorization and correction of a pathologic TT-TG in combination with the ACI/MACI was needed to achieve overall satisfactory and successful results.16,41,42 This has since been confirmed many times in the literature demonstrating excellent results in the treatment of patellofemoral defects.23,24,43, 44, 45, 46 Although bipolar defects have worse outcomes than unifocal defects regardless of the restorative technique, ACI/MACI still shows successful treatment outcomes and remains superior to other reported results such as osteochondral allograft transplantations.24,47

Osteochondral autograft transplantation (OAT) and osteochondral allograft (OCA) transplantation enable the direct transfer of hyaline cartilage to the cartilage defect in a single-stage procedure. During OAT, osteochondral plugs are harvested from non-weight-bearing areas of the patient's knee, such as the medial or lateral margins of the trochlea, posterior femoral condyles, or intercondylar notch, and subsequently transplanting to the corresponding defect site.48 Due to the cartilage bone composite structure, OAT is preferably used for full-thickness cartilage defects that extend through the subchondral bone plate. Since autologous tissue is used in this procedure, the risk of immunologic complications is eliminated. However, several technical concerns have been raised over the years. There is an associated donor site morbidity when larger or multiple plugs are used, hence there are limitations on the size of the defect that can be effectively treated. OAT is typically limited for small osteochondral lesions measuring up to 2 to 3 cm2 in size.49 Additionally there is a potential for a chondral mismatch due to different chondral thickness between the patella and the donor area. Patellar cartilage is very thick (4-8mm), and the donor area cartilage phase is usually between 2-4 mm.50 This raises concern that a resultant elevation of the subchondral bone barrier may serve as a stress riser for earlier chondral breakdown. During OAT, typically a set of harvesting devices is used to prepare the recipient and the donor site and allow for the harvest of the osteochondral plugs ensuring that the size of the transferred plug fits perfectly. This step is crucial as achieving a precise fitting of the graft is essential for creating a smooth articular surface in the patellofemoral joint, that will ultimately influence the surgical results as previous studies have demonstrated that graft prominence exceeding 1 mm is associated with inferior postoperative outcomes.51 This technique can be very difficult to perform on the patella due to the very hard subchondral bone in the patella. In those cases, the technique may have to be modified and a drill may have to be used for the subchondral bone preparation in the patella.

Fresh osteochondral allograft (OCA) transplantation is challenging when performed the in patellofemoral joint as the success of OCA transplantation depends on achieving a perfect match between the donor graft and the recipient area (Figure 2).52 Therefore, careful patient selection and appropriate stratification is crucial when considering OCA transplantation for patellofemoral injuries. Technically, after exposing the defect by arthrotomy, a meticulous assessment of the articular cartilage affected has to be done. Typically, we use a circular sizer to assess the size of the graft needed. A central reaming pin is inserted through the sizer and using a Cloward-type drill the defect is reamed out to a depth of about 6mm. Again, this is harder to do in the patella than in the trochlea. It is imperative that the area of the chondral defect has to be matched with the donor graft in order to adjust the often biconcave (trochlea) or biconvex(patella) anatomic dimension of the graft and its recipient area.52 These anatomical characteristics become critical during the graft harvesting from the donor and the preparation of the recipient bed. In particular, the coronal orientation of the graft plays a crucial role in the patellofemoral joint.52 One of the primary challenges during OCA transplantation in the patellofemoral joint is regarding the graft sizing. Sizing the trochlea and the patella for cartilage transplantation currently relies on standardized X-rays and magnetic resonance imaging (MRI) measurements. However, due to considerable anatomical variations, these measurements only offer estimations, and intraoperative visual assessment is often necessary to ensure optimal fitting of the graft. When preparing the graft for trochlear defects, single or double grafts can be harvested, but it must be done perpendicular to the cartilage surface. For central trochlear or patellar defects, achieving precise alignment of the cutting saw can be particularly difficult due to their double contour, and requires careful estimation of the perpendicular alignment. In those cases, measurements of the cartilage in the graft should be taken at all four corners to ensure a proper fit in the defect. Therefore, unlike defects in the tibiofemoral joint, there is limited room for coronal plane rotation.52 Jigs are available to help with the appropriate orientation of the drills preparing the donor and recipient site.

In terms of outcomes, overall, OAT appears to be a safe and reliable strategy for treating patellofemoral cartilage injuries, although less favorable outcomes and high failure rates have been reported for patellar lesions.53, 54, 55 Better clinical results and graft integration were observed for patients with small patellar cartilage defects (<2.5 cm2) and minimal or no patellofemoral malalignment.56 For OCA transplantation, initial studies reported mixed results, with some patients experiencing significant improvements in functional scores, while others required revision procedures or total knee replacements within a few years.57, 58, 59 However, more recent research has demonstrated consistent long-term results comparable to other procedures, providing support for the use of OCA in the patellofemoral joint. For instance, a previous study reported a graft survival rate of 78.1% at 5 and 10 years and 55.8% at 15 years, accompanied by substantial pain reduction and high patient satisfaction.60 Another study focused on isolated trochlear defects found higher than 90% survivorship of the graft at 10 years follow-up, suggesting the successful treatment of trochlear defects with OCA.61 These findings emphasize the potential of OCA as a viable approach to address osteochondral lesions in the patellofemoral joint.

Bone marrow aspirate concentrate (BMAC) is a promising alternative strategy for cartilage repair. BMAC contains pluripotent mesenchymal stem cells and growth factors that potentially regenerate cartilage tissue. In comparison to traditional techniques, one advantage of BMAC is that it prevents the damage of subchondral bone that follows microfracture procedures. In addition, BMAC avoids the need for an initial surgery to obtain cartilage biopsy and subsequent cultivation of chondrocyte cells. This not only simplifies the procedure but may also reduce the overall cost involved.62 In the BMAC technique, the bone marrow aspirate, usually obtained from the iliac crest, is processed using specific commercially-available systems and implanted into the cartilage defect. A membrane is then applied over the defect to stabilize the BMAC. Studies have shown that patients who underwent BMAC implantation presented improved mid-term follow-up scores of knee function, in particular those with single or small lesions.62,63 Moreover, MRI scans and histological analyses demonstrated hyaline-like tissue covering the cartilage defects, which was associated with the positive clinical outcomes.62,63

BST-CarGel® is a chitosan/glycerol copolymer hydrogel that has shown promise in treating chondral defects after microfracture procedures. In this treatment approach, blood is obtained from the patient and combined with the gel, resulting in a semi-solid consistency. The gel plays a crucial role in retaining the mesenchymal blood clot, its cellular components, and growth factors to contribute to the stimulation of chondrogenesis. The application of BST-CarGel® is typically performed via arthroscopy. Following the microfracture procedure, the prepared blood-gel mixture is applied to each microfracture hole and the surrounding defect. This allows for the gel to interact with the damaged area, improving the healing process. BST-CarGel® has demonstrated effectiveness in the treatment of cartilage defects in a mid-term follow-up. After 5 years, BST-CarGel® significantly improved the quantity and quality of tissue repair in comparison to microfracture alone as well as improved the functional scores overtime.64,65 While this technique is frequently used in some areas of the world, to date there is no larger case series or randomized clinical trial that has been published on the use of this technology in the patellofemoral joint.

Gel-type autologous chondrocyte implantation (ChondrolTM) and hydrogel-based autologous chondrocyte implantation (Novocart®) are both products intended to facilitate the attachment and distribution of chondrocytes in a cartilage defect. By mixing cultured autologous chondrocytes with fibrin glue or a polyethylene glycol-crosslinked, these procedures avoid the need for using collagen membranes, a material that can detach after implantation. It also evades the repeated manipulation of membranes, which can impair the chondrocyte viability. Overall, patients treated with Chondrol have presented positive effects in terms of pain reduction and knee function improvement after at least two years follow-up.66 Similarly, after two years of follow-up, patients with large cartilage defects (i.e., larger than 4 cm2) treated with hydrogel-based autologous chondrocyte implantation presented clinical improvements in knee function, and MRI findings consistent with enhanced maturation, reorganization and tissue integration.67, 68, 69

Alginate-based biocompatible scaffold products use human mature allogeneic chondrocytes covered by a periosteal flap to ensure adequate cartilage restoration. While the evidence for knee cartilage repair is still incipient, a previous case series study evaluating the effect of alginate beads containing mature allogeneic chondrocytes in knee cartilage defects has demonstrated safety and improvements in the clinical scores after 24 months follow-up.70

Particulated articular cartilage, obtained through autograft or juvenile allografts, is a promising strategy for chondral lesion restoration. Autograft procedures involve harvesting cartilage from non-weight-bearing areas of the same knee, mechanically mincing it, and re-implanting it into the lesion. Similarly, juvenile allografts are minced into small pieces and stored in vials for transplantation. Although the evidence is limited, clinical studies have demonstrated promising results following particulate cartilage implantation, especially for lesions located on the patella and trochlea.71,72 Figure 3 demonstrates a bipolar trochlear and patellar cartilage defects treated with particulated juvenile articular cartilage allograft.