Cartilage regeneration revisited: entering of new one-step procedures for chondral cartilage repair

Introduction This article reviews the evolution of cartilage regeneration therapeutic approaches from two-step cell-based autologous chondrocyte implantation procedures to current one-step cell-free scaffold-assisted cartilage repair approaches for chondral cartilage repair. In particular, our research is focused on clinical data about commercially available cell-free implants used in regenerative medicine approaches for the treatment of chondral cartilage defects. Discussion Chondral cartilage lesions do not heal spontaneously and may progress to severe osteoarthritis. For cartilage repair, a variety of surgical techniques have been established over the years. Further research led to the development of current new one-step cell-free scaffold-assisted cartilage repair approaches based on the experience with scaffold materials in previous two-step autologous chondrocyte implantation procedures. Commercially available scaffold-based products for one-step chondral cartilage repair have been recently tested in first case series and showed promising clinical outcome in the short-term follow-up; however, mediumand long-term comparative studies are necessary to evaluate the regenerative potential of this new one-step cartilage repair procedure and to demonstrate its superiority over or adequacy to traditional approaches. Conclusion This critical review summarises the development from two-step cell-based autologous chondrocyte implantation procedures to new onestep cell-free cartilage repair and discusses the first clinical outcome of commercially available cell-free implants. This new approach, based on the principle of cell ingrowths and guidance towards tissue repair, showed promising first clinical results and is considered as an effective and safe treatment option for chondral cartilage repair. Introduction Damaged cartilage has a limited self-healing capacity. Focal cartilage lesions of the knee occur frequently, are mostly located on the femoral condyle and display a major health problem because they may progress to severe osteoarthritis, when untreated. Predisposing factors for the development of cartilage defects are traumas, inflammatory conditions and biomechanics alterations of the knee1. In cartilage repair, a variety of surgical techniques have been established, including bone-marrow– stimulation (e.g. abrasion, drilling and microfracturing), osteochondral autograft transfer and autologous chondrocyte implantation (ACI) with or without the use of scaffold materials2,3. Treatment options have to be chosen individually, depending on the defect size, depth and location of the cartilage lesion. Surgical treatment options for small defects include bone-marrow stimulation techniques as well as osteochondral autograft transplantation4,5. Especially, for the treatment of large full-thickness chondral articular defects and as second-line cartilage treatment option, two-step ACI approaches based on the implantation of in vitro cultured autologous chondrocytes have been developed. The implantation of autologous chondrocytes within the prepared defect area improves the formation of hyaline-like, biomechanical resistant and durable repair tissue in contrast to the outcomes after microfracture procedure, leading to a fibrocartilaginous defect filled with material properties that are inferior to hyaline cartilage6. Further research led to the development of one-step cell-free scaffoldassisted regenerative approaches for ‘in situ’ cartilage repair. This innovative procedure combines the wellknown microfracture technique with the benefits and long-term experience of biomaterials used in earlier ACI procedures. This critical review shows the evolution of different generations for two-step ACI procedures up to the development of the current new one-step scaffold-assisted regenerative procedures and reports on the clinical applications of scaffoldbased products for chondral cartilage repair. In particular, our research is focused on published clinical data for commercially available cell-free implants used for one-step cell-free scaffold-assisted cartilage repair. Discussion The authors have referenced some of their own studies in this review. These referenced studies have been conducted in accordance with the Declaration of Helsinki (1964) and the protocols of these Di ag no si s & Tr ea tm en t * Corresponding author Email: christian.kaps@transtissue.com 1 TransTissue Technologies GmbH, Berlin, Germany 2 Tissue Engineering Laboratory, Department of Rheumatology and Immunology, Charité Campus Mitte, Charité – Universitätsmedizin Berlin, Berlin, Germany 3 Department of Orthopaedics and Trauma Surgery, Martin-Luther-Krankenhaus Berlin, Berlin, Germany


Introduction
Damaged cartilage has a limited self-healing capacity.Focal cartilage lesions of the knee occur frequently, are mostly located on the femoral condyle and display a major health problem because they may progress to severe osteoarthritis, when untreated.Predisposing factors for the development of cartilage defects are traumas, inflammatory conditions and biomechanics alterations of the knee 1 .
In cartilage repair, a variety of surgical techniques have been established, including bone-marrowstimulation (e.g.abrasion, drilling and microfracturing), osteochondral autograft transfer and autologous chondrocyte implantation (ACI) with or without the use of scaffold materials 2,3 .Treatment options have to be chosen individually, depending on the defect size, depth and location of the cartilage lesion.Surgical treatment options for small defects include bone-marrow stimulation techniques as well as osteochondral autograft transplantation 4,5 .Especially, for the treatment of large full-thickness chondral articular defects and as second-line cartilage treatment option, two-step ACI approaches based on the implantation of in vitro cultured autologous chondrocytes have been developed.The implantation of autologous chondrocytes within the prepared defect area improves the formation of hyaline-like, biomechanical resistant and durable repair tissue in contrast to the outcomes after microfracture procedure, leading to a fibrocartilaginous defect filled with material properties that are inferior to hyaline cartilage 6 .
Further research led to the development of one-step cell-free scaffoldassisted regenerative approaches for 'in situ' cartilage repair.This innovative procedure combines the wellknown microfracture technique with the benefits and long-term experience of biomaterials used in earlier ACI procedures.
This critical review shows the evolution of different generations for two-step ACI procedures up to the development of the current new one-step scaffold-assisted regenerative procedures and reports on the clinical applications of scaffoldbased products for chondral cartilage repair.In particular, our research is focused on published clinical data for commercially available cell-free implants used for one-step cell-free scaffold-assisted cartilage repair.

Discussion
The authors have referenced some of their own studies in this review.These referenced studies have been conducted in accordance with the Declaration of Helsinki (1964) and the protocols of these

Diagnosis & Treatment
studies have been approved by the relevant ethics committees related to the institution in which they were performed.All human subjects, in these referenced studies, gave informed consent to participate in these studies.

Generations of two-step ACI procedures
The classic first generation ACI is based on the implantation of a suspension of cultured chondrocytes underneath a sealed periosteal cover in a two-step procedure 7 .Since Brittberg et al. introduced this technique in 1987, more than 15,000 patients have been treated with ACI worldwide 8 .The first step includes arthroscopic cartilage biopsy harvest from a minor weight-bearing area of the injured knee followed by in vitro cell expansion under good manufacturing practice (GMP) conditions to a defined cell number (Figure 1).The second operation includes arthrotomy, preparation of the defect, periosteal harvest, suturing the periosteum over the defect, application of fibrin glue sealant and the injection of the cell suspension underneath the periosteal flap (Figure 2).Autologous chondrocytes in a cell suspension are offered by different manufacturers (Carticel ® , Genzyme, USA; ChondroCelect ® , Tigenix, Belgium; Novocart, B. Braun-Tetec, Germany).First-generation ACI has inherent disadvantages, including periosteal complications such as periost hypertrophy, periosteal graft detachment and delamination as well as loss of cells into the joint cavity [9][10][11] leading to a revision surgery rate of up to 25% to 40% 12,13 .
Second-generation ACI uses a collagen membrane rather than a periosteal flap to cover the cartilage defect before cell injection.The use of a bi-layer type-I/type-III collagen membrane (Chondro-Gide TM , Geistlich, Switzerland) reduces the length and number of incisions, surgical morbidity and the risk of hypertrophy compared with periosteal flap suturing 12 .The use of first and second-generation ACI is limited and may be difficult in knee defect locations lacking a stable and intact cartilage rim as known from post-traumatic and/or focal degenerative cartilage defects.
Further development of tissue engineering/regenerative medicine led to the third-generation of ACI to overcome these limitations.Thereby, three-dimensional scaffolds are seeded with in vitro cultured chondrocytes and subsequently implanted into the lesion area to generate new functional cartilage repair tissue.After debridement of the defect, the cartilage graft is cut to the defect size and implanted without the use of a periosteal or collagen cover flap (Figure 3).The use of three-dimensional scaffolds ensures a homogeneous cell distribution and avoids the risk of chondrocyte leakage from the liquid cell suspension.The easy handling of the tissue-engineered graft has surgical advantages and allows for a minimally invasive and faster surgical procedure by avoiding additional periost harvest and defect covering 14,15 .
Many biomaterials have been tested for scaffold-based chondrocyte implantation.Therein, various hydrogels were used as three-dimensional cell carriers for autologous chondrocytes.Among these geltype scaffolds, a three-dimensional type-I collagen gel (CaReS TM , Arthro Kinetics, Germany) is clinically used, where isolated chondrocytes are immediately cultivated in the collagen gel without expansion in monolayer culture 16 .Other hydrogelbased technologies use either a mixture of autologous chondrocytes with a hydrogel composed of agarose and alginate (Cartipatch; Tissue Bank of France, France), which involves the injection of a cell suspension-fibrin mixture into the cartilage defect (Chondron; Sewon Cellontech Co. Ltd., Korea), or use autologous chondrocytes cultured on an atelocollagen gel for cartilage repair 16 .
Other technologies using scaffolds based on collagen, hyaluronan and resorbable polymers have been shown to be clinically efficient for the repair of cartilage defects 16 : collagen membranes (ACI-maix/MACI ® , Matricel, Germany) and hyaluronanbased scaffolds (Hyalograft C TM ;  Fidia Advanced Biopolymers, Italy) have been used in combination with autologous chondrocytes in clinical practice since more than a decade 16 .Other clinically used biomaterials for cartilage repair are, for example, three-dimensional collagen-chondroitin sulphate scaffolds with embedded autologous chondrocytes (Novocart 3D, B. Braun-Tetec, Germany) or cell-seeded type I collagen scaffolds produced in a bioreactor (NeoCart, Histogenics Corporation, USA) 16 .Among resorbable polymer scaffolds, BioSeed ® -C (BioTissue Technologies GmbH, Germany), a two-component scaffold using a porous gel-like matrix composed of fibrin and a textile polyglycolic acid-based felt-like scaffold, is one of the most widely used cartilage grafts, which combines autologous chondrocytes with an initially mechanically stable polymer scaffold, allowing stable, subchondral and containment-independent fixation with resorbable nails or anchors for the first time.These characteristics have led to the possibility to treat degenerative defects also for the first time, showing to be clinically efficient for cartilage repair of those indications 17 .
Although the scaffold-based chondrocyte implantation is already a clinically effective procedure for cartilage repair, there still remain some disadvantages.This method is a two-step operation, which needs the removal of a healthy cartilage biopsy first and second, an implantation of the cultivated scaffold or matrix seeded with chondrocytes.The procedure may increase donorsite morbidity, the creation of a new defect at an undamaged cartilage zone has to be accepted and a potentially longer rehabilitation time must be tolerated.Furthermore, the timeintensive cultivation period for the autologous chondrocytes under GMP conditions, requiring for example up to 4 weeks, is cost-intensive, may have a potential risk leading to degeneration/de-differentiation of the cells and can result-depending on the degree of sub-culturing and de-differentiation-in an efficiency loss of cartilage regeneration 18 .

New one-step scaff old-assisted regenerative procedures
To overcome these limitations, a new concept for 'in situ' cartilage repair, based on the use of cell-free scaffolds for cell ingrowths and guidance towards tissue repair, was devel-oped.This procedure is an innovative treatment combining the well-known microfracture technique with the benefits and long-term experience of known biomaterials.
Behrens et al. first introduced this new matrix-coupled microfracture with a collagen matrix implant to cover cartilage defects 19 .Thereby microfracturing of the subchondral bone for the recruitment of human mesenchymal stem cells (MSCs) from the bone marrow blood was combined with a cell-free scaffold, which was fixated with fibrin glue.By covering the defect area, endogenous progenitor or stem cells that float into the cartilage defect are kept in the scaffold/matrix and the resulting blood clot is held in the defect zone.The resorbable scaffold thereby builds up a natural 3D environment, which optimises cell migration and ingrowths, leads to a homogeneous cell arrangement and supports differentiation into cartilaginous repair tissue (Figure 4).This new cell-free one-step approach has several advantages as compared with previous cell-based techniques.The scaffolds are offthe-shelf products, which are storable and available on demand, allow for a single-stage surgical step with reduced surgery time and avoid donor-site morbidity and costs for cell expansion.
However, at this time there is limited clinical data available on the outcome of the one-step scaffoldassisted regenerative procedures for the repair of chondral knee defects.Different commercially available cell-free medical devices based on collagen (Chondro-Gide TM, Geistlich, Switzerland; CaReS-1S TM , Arthro Kinetics, Germany; MeRG ® , Bioteck, Italy), different hydrogels (BST-Cargel™, Piramal Healthcare, Canada; GelrinC TM , Regentis Biomaterials, Israel) and resorbable synthetic polymers (chondrotissue ® , BioTissue AG, Switzerland) have been recently tested for one-step chondral cartilage repair and showed first clinical outcomes.One of these products is a bilayer matrix of porcine type I and III collagen with one compact and one porous side (Chondro-Gide TM , Geistlich, Switzerland) to cover chondral cartilage defects after microfracturing.Clinical outcome was recently shown in several case series, including up to 38 patients with a follow-up time of maximum 51 months.Therein, results of patients showed a significant improvement in clinical scores (International Knee Documentation Comitee (IKDC), Lysholm, Tegner, Visual Analog Scale (VAS) pain score) and patient satisfaction with the treatment procedure, whereas magnet resonance imaging showed an incomplete and inhomogeneous repair tissue formation and defect filling rates between 50% and 58.8% 1,20,21 .Recently, a controlled, randomised trial was initiated, comparing the use of Chondro-Gide TM in second-generation ACI and in a one-step cell-free scaffoldassisted approach for the repair of symptomatic knee cartilage defects.Both techniques use the Chondro-Gide TM membrane in an arthrotomic approach to cover the defects of 40 patients each 22 .So far no preliminary or clinical results are available for this randomised trial.
Another collagen-based product for one-step scaffold-assisted cartilage repair is a round type I collagen gel-matrix derived from rat tails for deep chondral defects (CaReS-1S TM , Arthro Kinetics, Germany), which is implanted into the debrided defect in a press-fit manner.Clinical results were shown in first case series for up to 15 patients and with 24-36 months follow-up 23,24 .After 24 months, magnetic resonance imaging showed complete filling with a mainly smooth surface, complete integration, homogenous repair tissue structure and nearly normal signal intensity.Histological examination of one specimen at 42 months after implantation revealed type II collagen positive repair tissue 23 .
Functional, clinical and subjective assessment sho wed significant improvement when compared with the preoperative values 24 .First analysis of preliminary results of an ongoing multi-centre study including 37 patients with 3-24 months follow-up showed identical or better IKDC scores for the one-step cell-free product when compared with those of a multi-centre study with the twostep cell-based product CaReS ® 2S.
Another collagen product, consisting of a microfibrillar equine type I collagen membrane (MeRG ® , Bioteck, Italy) was described in a first case presentation.Therein, a 37-year-old man with a 3 cm 2 cartilage lesion of the medial condyle was treated with covered microfracture and bone marrow concentrate for arthroscopic knee cartilage repair 25 .Magnetic resonance imaging results at 12 months postoperatively showed good defect filling with a tissue signal similar to that of surrounding tissue.In a follow-up of 24 months, the patient remained asymptomatic.
As an alternative to collagen-based matrix, different hydrogels are now being clinically tested for one-step scaffold-assisted cartilage repair.In a mini-arthrotomic approach, these gels are mixed with whole blood or bone marrow released after microfracturing to fill the chondral cartilage defect.In this group, a chitosan-glycerol phosphate/blood implant (BST-Cargel™, Piramal Healthcare, Canada) was clinically tested.First clinical results are available for a randomised, comparative multi-centre clinical trial evaluating BST-CarGel™ and microfracture at 12 months for cartilage repair.Therein, BST-CarGel ® treatment achieved significantly better results compared with microfracture in defect filling and in the quality of the repair tissue 26 .
Other commercially available hydrogels are mixed compositions of polyethylene glycol diacrylate with fibrinogen (GelrinC TM , Regentis Biomaterials, Israel).Thereby, a gel is inserted as a liquid to fill the cartilage defect and is then converted into a solid through exposure to ultra-violet light.First short-time results of a pilot clinical study including 15 patients with focal cartilage defects showed significantly higher levels of tissue fill and reduced pain levels compared with microfracture controls and an improvement in patients' knee function after 6 months 27 .A multi-centre study is currently recruiting patients to evaluate the safety and effectiveness of the hydrogel in the treatment of articular cartilage lesions 28 .
Newer one-step scaffold-based cartilage repair approaches favour the use of stable textile polyglycolicacid-hyaluronan implants (chondrotissue ® , BioTissue AG, Switzerland) to cover the defect after microfracturing or bone marrow stimulation.This scaffold incorporates biological factors, such as hyaluronic acid, and can be used in combination with for example human serum or platelet rich plasma, to recruit MSCs into the scaffold and guide them towards cartilage repair.In contrast to other unstable biomaterials, the textile scaffold provides a mechanically stable formulation, which can be stably fixated into the defect by suturing, trans-osseous fixation, pin fixation or fibrin glue and even allows for the implantation into partly un-shouldered degenerative defects.First pilot studies have shown that covering of microfractured cartilage defects with the chondrotissue ® cartilage implant is safe, improves the patients' situation and leads to complete defect filling with histologically confirmed hyaline-like cartilaginous repair tissue formation in a follow-up period of up to 2 years [29][30][31][32][33] .Preliminary clinical results of a randomised, comparative multicenter clinical trial showed that the chondrotissue ® treatment for cartilage repair significantly improves the patients' situation as assessed by VAS, Knee Injury and Osteoarthritis Outcome Score (KOOS) and IKDC score, while there is no significant Licensee OA Publishing London 2013.Creative Commons Attribution Licence (CC-BY) FOR CITATION PURPOSES: Freymann U, Petersen W, Kaps C. Cartilage regeneration revisited: entering of new one-step procedures for chondral cartilage repair.OA Orthopaedics 2013 Jun 05;1(1):6.improvement after microfracture treatment in patients with 12-24 months follow-up 34 .

Conclusion
Regenerative medicine led to the development of a variety of cartilage repair treatment options.Cell-based approaches include an initial invasive biopsy harvest to obtain the cells and in vitro proliferation and possibly de-differentiation of the cells before implantation.Newer one-step scaffold-based procedures for cartilage repair have been recently developed to simplify and further improve regenerative techniques.The new one-step treatment option includes the attraction of MSCs to the site of the cartilage defect and thereby overcomes the necessity of in vitro proliferation and differentiation of cells before transplantation.This one-step approach also avoids donor-site morbidity, reduces costs, surgical steps and potentially patients' rehabilitation time.The use of biomaterials for the treatment of chondral knee cartilage defects is strongly increasing, since the properties of the used polymers are varied, allow for modification and can be adapted to the surgical need.
Clinical applications are described for different types of commercially available cell-free implants, including collagen variations, hydrogels and synthetic textile.For the abovementioned products, first promising clinical results showed a reduction in pain, a functional improvement of the knee and the formation of hyaline-like repair tissue.By the application of cell-free scaffolds for defect covering after bone-marrow stimulation, the newly formed tissue was shown to be structured similar to native cartilage tissue and to be superior to the repair tissue formed after microfracture alone.
Difficulties in comparing various studies arise from the different study populations, follow-up times, evaluation systems, scaffold fixation methods, surgical approaches and postoperative rehabilitation procedures.However, evaluation of current clinical outcome is based on first case series with short-to mid-term follow-up and preliminary results of ongoing controlled, randomised clinical studies.Thus, well-designed mid-to long-term comparative studies are needed to evaluate the potential of these new regenerative one-step cartilage repair approaches and to show its superiority over or adequacy to traditional two-step ACI approaches.

Figure 1 :
Figure 1: Cartilage biopsy harvest and in vitro cell expansion in monolayer cultures.

Figure 2 :
Figure 2: Principle of autologous chondrocyte implantation (ACI): a harvested periosteum flap (first-generation ACI) or collagen membrane (second-generation ACI) is sutured over the debrided cartilage defect and the cell suspension is injected underneath.

Figure 3 :
Figure 3: Surgical steps of third-generation scaffold-based ACI including the transplantation of a chondrocyte-seeded scaffold into the debrided defect.

Figure 4 :
Figure 4: Principle of one-step scaffold-assisted cartilage repair including bonemarrow stimulation and additional implantation of the cell-free scaffold for a tissue-guided cartilage repair.