Excellent histological results in terms of articular cartilage regeneration after spheroid-based autologous chondrocyte implantation (ACI)

Hyaline cartilage has a unique capacity to answer pressure transformations and is thus critical for the proper function of the musculoskeletal system [11]. However, it has limited regeneration capacity after trauma, leading to degenerative changes of the traumatized cartilage. These changes represent a risk factor for early development of osteoarthritis, resulting in the complete destruction of the joint.

Different strategies have been developed to treat cartilage lesions, such as mosaic arthroplasty, microfracture, autologous matrix-induced chondrogenesis (AMIC) and autologous chondrocyte implantation (ACI) [10, 12]. These strategies were shown to significantly reduce relevant clinical burdens of traumatic cartilage lesions, such as pain and immobility. Moreover, the necessity of total joint replacement could be postponed.

Currently, several Matrix-based ACI (MACI) are in clinical use [1, 9, 18, 19]. However, until today it is not yet clear, which is the optimal one, especially with respect to hyaline cartilage regeneration [4]. Bentley et al. compared ACI with mosaic arthroplasty and revealed excellent clinical, radiological, and histological results [6]. Zeifang et al. compared ACI with MACI and reported no significant differences regarding the IKDC-score, Tegner-activity and the SF-36, but a significantly better result in the Lysholm-score for the ACI-P [20]. A trial comparing periosteal flap covered (ACI-P) and collagen membrane-covered ACI (ACI-C) could not detect histological differences in Safranin-O staining between ACI-P and ACI-C [13, 15]. However, a recent trial reports a high number of failures in osteoarthritic knees and postulate to better profile patients before undergoing the MACI [3].

The spheroid-based ACI represents a relatively new technique. It requires two surgical interventions, one for harvesting chondrocytes and one for the implantation of the generated chondrocyte spheroids [2, 14]. The spheroids are implanted into the defect after debridement and adhere within 20 min at the implantation site. The safety and efficacy of this technique could be demonstrated by clinical studies [5, 17]. However, until today there are no publications regarding the histological outcome after this treatment. In the present case study, we show for the first time histological and immunohistological results of regenerated hyaline cartilage-like tissue after spheroid-based ACT. The present results are of clinical importance as the regenerated cartilage tissue clearly resembles normal hyaline cartilage tissue, which gives relevant implications for its functional properties.

Ethical approval for the re-evaluation was given by the Institutional Review Board of the University Regensburg (ID-number: 19-1558-104).

Samples were collected from five patients (one biopsy from each patient) treated with spheroid-based ACI (Codon, Teltow, Germany) due to traumatic lesions followed by re-arthroscopy. All defects were localized on the medial femur condyle. Implantation was performed via mini-arthrotomy (Fig. 1). Re-arthroscopy was necessary on the basis of clinical indications. In three patients a lesion of the medial meniscus, in one patient arthrofibrosis and in one patient persisting pain of unknown origin. Therefore, time-points of re-arthroscopy were not homogenous in the present report. Every patient agreed with a transplant-biopsy in written form to ensure histological analyses of a potential cartilage-hypertrophy versus hypertrophy of the subchondral bone. Biopsies were taken from the central part of the implant using a 2 mm Jamshidi-needle. Specimens were immediately fixed in formalin. The patient characteristics are summarized in Table 1.

The present study demonstrates for the first time that the regenerated cartilage tissue after spheroid-based ACI is histologically, histochemically and immunohistochemically very similar to hyaline articular cartilage. The typical layer-based architecture with a deep, middle and apical zone was identified in three patients. In addition, these three patients demonstrated a typical apical zone with two to three layers of flattened chondrocytes and a decrease of alcian blue-positive glycosaminoglycans, which is well known from initial articular cartilage. Specimens with high amounts of glycosaminoglycans, high expression of collagen type II and cells with chondroid phenotype, but without the typical layer-based architecture was defined as “very similar to hyaline articular cartilage”, which was present already 5 ½ months after ACI and almost of similar structure in the specimen which was taken 16 months after ACI. These findings implicate a good regenerative potential of the spheroids, making them an attractive alternative to other methods where changes in the regenerative tissue and long-term failures have been described [3, 6]. Long-term follow-up studies should clarify if spheroid-based ACI also reveals a good long-term performance with respect to proper cartilage regeneration and good long-term preservation of the regenerated tissue. For that purpose, histological analyses are the method of choice, since clinical or radiological examinations were not able to verify the quality of the regenerated tissue [7, 16, 20]. However, further long-term follow-up trials are mandatory for a better understanding of the cellular mechanisms involved in cartilage regeneration and preservation, as well as to give a proper base for the comparison with single-stage techniques as e.g. microfracture, which may be easier to perform [18]. From the clinical point of view, a recent study demonstrated a substantial improvement of various clinical outcomes parameters after 36 months after matrix-associated, spheroid-based ACI compared to microfracturing [14]. Nevertheless, the ACI should also prove, that the regenerated tissue is indeed a noticeable improvement in tissue quality which is mandatory for long-term function.

In this study biopsies taken between 5 ½ and 16 months after ACI clearly demonstrated the regeneration of predominantly hyaline cartilage-tissue. Nevertheless, the limited number of specimens without a homogenous study cohort is a major limitation of the present case-study. Further clinical trials with a larger number of patients have to be performed to consolidate these results and to generate statistically significant data [3]. The indication for the biopsy in this report was given by a potential hypertrophy of the subchondral bone, which had to be excluded by histomorphological analyses.

The present study represents the first histological and immunohistological analysis of spheroid-based ACI. It shows that spheroid-based ACI yields excellent histological results in terms of regeneration with predominantly hyaline articular cartilage.