Introduction
Cartilage lesions of the knee occur frequently and represent a major health problem. Consecutive knee arthroscopies showed that up to 63% of the patients with knee-related symptoms suffered from chondral or osteochondral defects [
1,
2]. These defects comprise focal osteochondral or chondral lesions in 67%, osteoarthritic defects in 29%, lesions related to osteochondritis dissecans in 2%, and other defects in 1% of the cases [
3]. Recently, a variety of surgical techniques that aim for resurfacing and regenerating of the articular cartilage have evolved. In the clinical routine, debridement, bone marrow-stimulating techniques, osteochondral autograft transfer, and autologous chondrocyte implantation (ACI) are commonly used cartilage repair techniques [
4‐
8].
The first ACI was performed in 1987, and the clinical study of Brittberg and colleagues [
4] in 1994 represents the starting point of cell-based cartilage repair and regenerative medicine. Up to now, more than 15,000 patients worldwide have been treated with ACI [
9], and various reports documented the clinical effectiveness of implanting autologous culture-expanded chondrocytes for cartilage repair [
10‐
13]. Although there is no significant evidence that ACI produces superior clinical results for the treatment of full-thickness articular cartilage defects compared with other cartilage repair interventions [
14,
15], it is regarded as a second-line treatment for small defects and a first-line treatment for defects larger than 2 to 4 cm
2 [
16].
For ACI, a small partial or full-thickness cartilage biopsy is taken from a less weight-bearing area of the healthy articular cartilage. The chondrocytes are harvested by enzymatic digestion and cells are grown with autologous serum. For chondrocyte implantation, a periosteal flap or a collagen sheet is sutured to the surrounding healthy cartilage rim, creating a reservoir for the injection of the autologous chondrocyte cell suspension. The need for an intact cartilage rim limits the use of ACI to some regions of the knee, and the covering of the chondrocyte suspension with a periosteal flap or a collagen sheet may be insecure (for instance, in degenerative defects that often miss an intact cartilage rim). In addition, potential sources of complications may include periosteal hypertrophy, loosening of the periosteal flap, ablation, and loss of cells into the joint cavity [
17‐
19]. These technical disadvantages of ACI result in re-operations in up to 25% to 36% of the patients [
20,
21]. Therefore, cartilage tissue engineering grafts that address these disadvantages by using three-dimensional scaffolds stabilizing the graft and the regenerative potential of autologous chondrocytes were developed. Meanwhile, clinical results have shown the effectiveness of hyaluronan-based [
22,
23], collagen-based [
24,
25], and resorbable polymer-based [
26] autologous chondrocyte grafts for the repair of cartilage defects.
Currently, ACI is contraindicated in osteoarthritic patients. Nevertheless, preclinical studies suggest that chondrocytes or mesenchymal stem cells from osteoarthritic patients may have the capacity to form cartilage repair tissue and fulfill the prerequisites for use in ACI [
27,
28]. However, for cell-based cartilage therapies in osteoarthritis, it is important to harvest unaffected healthy cartilage biopsies since healthy chondrocytes have been shown to form a cartilage tissue
in vitro that shows better morphology and a higher proteoglycan content than chondrocytes derived from osteoarthritic joints [
29]. Clinically, it has been shown that microfracture treatment of patients with moderate osteoarthritis improved their pain and activity of daily living and significantly widened the joint spaces 1 year after treatment compared with the preoperative situation [
30]. The effectiveness of a second-generation cartilage graft based on hyaluronan has been shown for the treatment of osteoarthritic knees with osteoarthritis not inhibiting the regeneration sequence [
31].
Recently, it was shown that the autologous cartilage graft BioSeed
®-C (BioTissue Technologies GmbH, Freiburg, Germany) based on a bioresorbable two-component gel-polymer scaffold is effective for the treatment of traumatic and focal osteoarthritic cartilage defects of the knee [
26]. The aim of the present study was to evaluate ACI using BioSeed
®-C for the treatment of mild degenerative and focal osteoarthritic defects of the knee. Magnetic resonance imaging (MRI) analysis of the cartilage repair tissue as well as the clinical evaluation of a series of 19 patients with pre-existing osteoarthritic symptoms and a 4-year clinical follow-up document the effectiveness of BioSeed
®-C for the treatment of focal degenerative cartilage defects.
Discussion
The autologous gel-polymer cartilage graft, BioSeed
®-C, uses the well-known regenerative capacity of autologous chondrocytes, gel-like matrices for promoting tissue formation, and the initial mechanical stability of resorbable polymer scaffolds for cartilage repair [
39]. The assembly of chondrocytes in polymer-based scaffolds ensures the even distribution of a high number of vital chondrocytes within the graft and has been shown to allow the production of cartilage grafts that develop toward hyaline cartilage
in vitro [
40]. In particular, the embedding of culture-expanded chondrocytes in fibrin and polymer-based scaffolds made of polyglycolic acid or copolymers of polyglycolic and polylactic acid initiates chondrocyte re-differentiation
in vitro and allows for formation of cartilage matrix
in vivo after implantation of the graft [
41,
42]. The formation of cartilage repair tissue after implantation of the polymer-based graft made of non- and cryo-preserved chondrocytes has been shown in the rabbit joint defect model [
43] as well as in a large animal horse model. In Haflinger horses, full-thickness cartilage defects of the fetlock joint were treated with the autologous polymer-based cartilage graft, and formation of a hyaline-like cartilage repair tissue as well as firm bonding of the graft to the adjacent healthy cartilage and to the subchondral bone tissue were evident 1 year after implantation [
44]. Since the chondrocytes are embedded in and protected by the fibrin-polymer matrix, BioSeed
®-C allows for easy handling of the graft during surgery, avoids the use of cover materials like periosteum or collagen sheets, needs no healthy cartilage rim surrounding the defect, and ensures arthroscopical implantation and secure fixation [
34]. In particular, the secure fixation of cartilage grafts is of importance to avoid transplant loosening, debonding of the graft or ablation, and in turn clinical complications and re-operations. As assessed in recent biomechanical studies, stable second-generation cartilage grafts for ACI like BioSeed
®-C allow good anchoring of the graft in the defect by fibrin gluing, chondral or transosseous suturing, and resorbable pin fixation [
45‐
47]. From the cellular point of view, particularly in an osteoarthritic environment, joint homeostasis and the composition of the synovial fluid may be of special importance in cartilage repair. In a goat model, it has been shown that defects showed better repair when the lesion was covered immediately with periosteum compared with defects that were left untreated before transplantation [
48]. Using a chick limb bud assay, synovial fluid from patients with acute traumatic cartilage defects has been shown to stimulate chondrogenesis, but synovial fluid from patients with chronic traumatic defects predominantly inhibited chondrogenic development [
49]. In contrast, synovial fluid obtained from patients suffering from trauma, osteoarthritis, or inflammatory rheumatoid arthritis stimulated the synthesis of proteoglycans in a bovine model, with osteoarthritis and trauma synovial fluid showing a markedly increased synthesis compared with rheumatoid arthritis [
50]. In a rabbit model, it has been shown that synovial fluid promotes and enhances cartilage tissue development from perichondrium [
51]. Although the influence of synovial fluid or components of the fluid on cartilage repair remains unclear and further studies are needed, it is evident that an altered joint environment may influence tissue regeneration. Cartilage repair should be performed as early as possible and an inflammatory environment should be avoided.
From the clinical point of view, the use of autologous chondrocytes in suspension like in first-generation ACI has been shown to be effective for the repair of localized traumatic defects [
10,
11,
52]. Second-generation cartilage grafts using scaffolds based on collagen or hyaluronan for stabilizing autologous chondrocytes are considered to be technically more attractive than first-generation ACI and have been shown to be clinically as effective as 'classical' ACI [
22,
24]. In a recent report of the treatment of traumatic and degenerative defects [
26] and in this case series, we demonstrated the safety and effectiveness of the autologous cartilage gel-polymer graft BioSeed
®-C for the treatment of challenging defects like large focal degenerative full-thickness cartilage lesions of the knee. After 1-year follow-up, mean scores increased significantly between 30% and 51% compared with the preoperative situation, depending on which score was analyzed. These good results lasted and showed significant improvement of the mean scores between 34% and 55% 4 years after implantation of the graft. This indicates a significant decrease in the patients' pain and knee instabilities during activity as well as a significant increase in patients' quality of life. The implantation of BioSeed
®-C in focal osteoarthritic defects showed significant improvement in the KOOS subclasses pain, activity of daily living, and quality of life 1 year and 4 years after implantation of the graft. Obviously, the efficacy of the polymer-based autologous cartilage graft BioSeed
®-C for the repair of osteoarthritic cartilage defects is shown by the improvement of clinical scores, by patients' pain and quality of life as well as by the good filling of the defects with repair tissue as assessed by MRI. This is of special relevance since the BioSeed
®-C-mediated cartilage repair was achieved in a challenging patient cohort with clinically osteoarthritic symptoms and focal cartilage degeneration 1 and 4 years after implantation of the graft. Interestingly, the status of the patient 2 years after ACI is considered an important indicator for the future outcome. During this time, most of the complications of cell-based cartilage repair as well as improvement in clinical scores and subjective patient satisfaction were found. On the other hand, indicators of a worse outcome like multiple surgical procedures, higher age, and large defects correspond to findings published by others [
24,
53]. In general, for measuring and evaluating the clinical outcome of a given treatment strategy, patients' satisfaction and improvement are most important and are best assessed by well-established clinical outcome scores. From the scientific point of view, additional detailed questions arise regarding measurable parameters like the morphology and quality of the formed repair tissue, chondrocyte viability and distribution within the regenerative tissue as well as defect filling and graft integration. These issues can be addressed by non-invasive MRI techniques as shown here or by dGEMRIC (delayed gadolinium-enhanced MRI of cartilage) [
54] as well as by minimally invasive histological evaluation [
13,
26,
55]. These techniques may offer measurable insights in clinical outcome and value of the graft and may open avenues for developing precise indicators for the clinical outcome and prognosis. However, for a comprehensive evaluation of a regenerative cartilage repair therapy, both approaches are mandatory, giving clinical scores for patients' satisfaction and improvement as well as MRI and/or histological analysis for repair tissue evaluation.
After implantation of BioSeed
®-C, 9 out of 19 patients were subjected to second-look arthroscopy due to symptoms like grinding, catching, pain, or swelling. This re-intervention rate, though for diagnostic purposes, is relatively high and may be related to this challenging patient cohort with a considerable number of concomitant surgeries like high tibial osteotomy and anterior cruciate ligament reconstruction. However, only 4 out of the 19 patients (21%) had to undergo revision surgery or re-operation. This is in concordance with other studies reporting rates of revision surgery of between 0% [
56] and 25% [
20]. Two patients were treated with an abrasion chondroplasty for newly formed cartilage lesions beside the transplanted area. Two additional patients out of the 19 patients received a total knee endoprosthesis and were classified as 'graft failure', although there is no evidence that the prosthesis was indicated because of a failure of BioSeed
®-C. This failure rate of 10% corresponds to earlier findings describing rates of failure in ACI with other implants of between 5% [
12] and 13% [
20]. These patients with focal degenerative defects treated with the chondrocyte graft were young and in the mid to long term may have no treatment option other than total knee arthroplasty. Alternate treatment strategies such as ACI may lead to several re-interventions and dissatisfied patients but may postpone total knee arthrosplasty as possibly the only option for these patients and are therefore considered a good treatment for focal degenerative cartilage defects. However, long-term studies are needed to evaluate whether cell-based cartilage grafts prevent total joint replacement in osteoarthritis. After 6 months, there was no difference in the scores of these 2 patients with an endoprosthesis and the rest of the study group. After 12 months, these two patients with a 'graft failure' had not yet improved and revealed worse results compared with the other patients. After 12 months or after 48 months, neither of the 2 patients reached 55 points in the Lysholm score or 45 points in the IKDC score, indicating that there might be a threshold for long-term graft survival and successful tissue regeneration. Even if graft regeneration takes a long time (from 2 to 3 years after surgery), a continuous improvement should be detected 6 to 12 months after surgery. In this context, the lack of clinical improvement, combined with insufficient MRI results, may be signs of long-term graft failure. In general, re-operations and graft failure after implantation of chondrocytes in a first-generation ACI procedure are caused by problems associated with the periosteal flap [
12,
18,
57]. This inherent technical disadvantage of the original ACI procedure does not occur when using second-generation ACI grafts like BioSeed
®-C that are void of any cover materials. In addition, the easy and secure fixation of BioSeed
®-C along with the lack of any covering may reduce the operating time and may result in a less invasive procedure since there is no need to harvest periosteum from the tibia. Limitations of the study are the small number of patients and the lack of a control group. Since this observational case report study was initiated first to gain insights in the safety and effectiveness of treating consecutive cartilage defects with BioSeed
®-C, the study is further limited by the lack of predefined primary outcome goals. The study was not performed in a randomized controlled manner in comparison with an appropriate control group or with an alternate treatment option like microfracturing. Furthermore, in young patients, degeneration of cartilage can be observed after trauma whereas well-defined osteoarthritis occurs predominantly in older patients. Therefore, first- or second-generation ACI will not be recommended unrestrictedly for the treatment of focal osteoarthritic cartilage defects. However, this is the first study showing long-term results using the second-generation ACI graft BioSeed
®-C in patients with osteoarthritic and/or degenerative changes of the knee. The study presents a continuous objective patient evaluation including clinical scores and control MRI over the course of a period of 4 years.
Competing interests
CK is an employee of TransTissue Technologies GmbH, which is a subsidiary of BioTissue Technologies GmbH (Freiburg, Germany). BioTissue Technologies GmbH produces and distributes BioSeed®-C. CE works as a consultant for BioTissue Technologies GmbH. All other authors declare that they have no competing interests.
Authors' contributions
PCK helped to carry out the data evaluation, to draft the manuscript, and to perform the patient data collection and participated in data evaluation. CK helped to carry out the data evaluation and to draft the manuscript. SM and CO helped to perform the patient data collection and participated in data evaluation. CE conceived the study, participated in its design and coordination, performed the surgical procedures, and was involved in the patient data collection and interpretation. All authors read and approved the final manuscript.