Quantitative 3-T Magnetic Resonance Imaging After Matrix-Associated Autologous Chondrocyte Implantation With Autologous Bone Grafting of the Knee: The Importance of Subchondral Bone Parameters

2021 ◽  
Vol 49 (2) ◽  
pp. 476-486
Author(s):  
Matthias Jung ◽  
Dimitrios C. Karampinos ◽  
Christian Holwein ◽  
Joachim Suchowierski ◽  
Thierno D. Diallo ◽  
...  
Keyword(s):  
Magnetic Resonance ◽  
Cartilage Repair ◽  
Subchondral Bone ◽  
Quantitative Mri ◽  
Repair Tissue ◽  
Water Fraction ◽  
Qualitative And Quantitative ◽  
Chondrocyte Implantation ◽  
Defect Filling ◽  
Autologous Chondrocyte

Background: Matrix-associated autologous chondrocyte implantation (MACI) with autologous bone grafting (ABG) is an effective surgical treatment for osteochondral defects. Quantitative magnetic resonance imaging (MRI) techniques are increasingly applied as noninvasive biomarkers to assess the biochemical composition of cartilage repair tissue. Purpose: To evaluate the association of quantitative MRI parameters of cartilage repair tissue and subchondral bone marrow with magnetic resonance morphologic and clinical outcomes after MACI with ABG of the knee. Study Design: Case series; Level of evidence, 4. Methods: Qualitative and quantitative 3 T MRI of the knee was performed in 21 patients (16 male) at 2.5 years after MACI with ABG at the medial (18/21) or lateral (3/21) femoral condyle for the treatment of osteochondral defects. Morphologic MRI sequences were assessed using MOCART (magnetic resonance observation of cartilage repair tissue) 2.0 scores. T2 relaxation time measurements for the assessment of cartilage repair tissue (CRT2) were obtained. Single-voxel magnetic resonance spectroscopy was performed in underlying subchondral bone marrow (BM) and at both central femoral condyles. The presence of pain and Tegner scores were noted. Statistical analyses included Student t tests, correlation analyses, and multivariate regression models. Results: The mean defect size was 4.9 ± 1.9 cm2. At a follow-up of 2.5 ± 0.3 years, 9 of 21 patients were asymptomatic. Perfect defect filling was achieved in 66.7% (14/21) of patients. MOCART 2.0 scores (74.1 ± 18.4) did not indicate pain (68.3 ± 19.0 [pain] vs 81.7 ± 15.4 [no pain]; P = .102). However, knee pain was present in 85.7% (6/7) of patients with deep bony defects (odds ratio, 8.0; P = .078). Relative CRT2 was higher in hypertrophic cartilage repair tissue than in repair tissue with normal filling (1.54 ± 0.42 vs 1.13 ± 0.21, respectively; P = .022). The underlying BM edema–like lesion (BMEL) volume was larger in patients with underfilling compared with patients with perfect defect filling (1.87 ± 1.32 vs 0.31 ± 0.51 cm3, respectively; P = .002). Patients with severe pain showed a higher BMEL volume (1.2 ± 1.3 vs 0.2 ± 0.4 cm3, respectively; P = .046) and had a higher BM water fraction (26.0% ± 12.3% vs 8.6% ± 8.1%, respectively; P = .026) than did patients without pain. Conclusion: Qualitative and quantitative MRI parameters including the presence of subchondral defects, CRT2, BMEL volume, and BM water fraction were correlated with cartilage repair tissue quality and clinical symptoms. Therefore, the integrity of subchondral bone was associated with outcomes after osteochondral transplantation.

Correlation between the subchondral bone marrow lesions and cartilage repair tissue after matrix-associated autologous chondrocyte implantation in the knee: a cross-sectional study

2020 ◽  
pp. 028418512096995
Author(s):  
Jialing Lyu ◽  
Yindi Zhang ◽  
Weimin Zhu ◽  
Dingfu Li ◽  
Weiqiang Lin ◽  
...  
Keyword(s):  
Bone Marrow ◽  
Relaxation Time ◽  
Cartilage Repair ◽  
Subchondral Bone ◽  
Autologous Chondrocyte Implantation ◽  
Repair Tissue ◽  
Bone Marrow Lesions ◽  
Chondrocyte Implantation ◽  
And Cartilage ◽  
Autologous Chondrocyte

Background The contribution of the subchondral bone in the development and progression of osteoarthritis (OA) has long been recognized, but its role in cartilage repair procedures has only recently attracted more attention. Purpose To explore the correlation between the cartilage repair tissue (RT) and the subchondral bone marrow lesions (BMLs) after matrix-associated autologous chondrocyte implantation (MACI) in the knee joint. Material and Methods A total of 30 patients who underwent MACI in the knee from January 2015 to June 2018 and follow-up magnetic resonance imaging (MRI) scan were recruited in this study. The MRI results of cartilage RT were evaluated using T2* relaxation time. Subchondral BMLs were also qualitatively evaluated by use of the two-dimensional proton density-weighted fat-suppressed (2D-PD-FS) and three-dimensional dual-echo steady-state (3D-DESS) sequences. Results The univariate analysis displayed a significant negative correlation between subchondral BMLs and cartilage RT ( P < 0.01). In the minimally adjusted model (only age, sex, and body mass index [BMI] adjusted), the results did not show obvious changes (β = –6.54, 95% confidence interval [CI] = –10.99 to –2.09; P = 0.008). After adjustment for the full models (age, sex, BMI, defect size, combined injury, and preoperative duration of symptoms adjusted), the connection was also detected (β = –6.66, 95% CI –11.82 to –1.50; P = 0.019). Conclusion After MACI, the subchondral BMLs are significantly correlated with cartilage RT-T2* relaxation time. The role of subchondral bone in cartilage repair procedures should not be underestimated.

scholarly journals Magnetic Resonance Imaging Parameters at 1 Year Correlate With Clinical Outcomes Up to 17 Years After Autologous Chondrocyte Implantation

2018 ◽  
Vol 6 (8) ◽  
pp. 232596711878828 ◽  
Author(s):  
Helen S. McCarthy ◽  
Iain W. McCall ◽  
John M. Williams ◽  
Claire Mennan ◽  
Marit N. Dugard ◽  
...  
Keyword(s):  
Magnetic Resonance Imaging ◽  
Magnetic Resonance ◽  
Clinical Outcomes ◽  
Cartilage Repair ◽  
Hyaline Cartilage ◽  
Resonance Imaging ◽  
Repair Tissue ◽  
Chondrocyte Implantation ◽  
Autologous Chondrocyte

Background: The ability to predict the long-term success of surgical treatment in orthopaedics is invaluable, particularly in clinical trials. The quality of repair tissue formed 1 year after autologous chondrocyte implantation (ACI) in the knee was analyzed and compared with clinical outcomes over time. Hypothesis: Better quality repair tissue and a better appearance on magnetic resonance imaging (MRI) 1 year after ACI lead to improved longer-term clinical outcomes. Study Design: Cohort study; Level of evidence, 3. Methods: Repair tissue quality was assessed using either MRI (11.5 ± 1.4 [n = 91] or 39.2 ± 18.5 [n = 76] months after ACI) or histology (16.3 ± 11.0 months [n = 102] after ACI). MRI scans were scored using the whole-organ magnetic resonance imaging score (WORMS) and the magnetic resonance observation of cartilage repair tissue (MOCART) score, with additional assessments of subchondral bone marrow and cysts. Histology of repair tissue was performed using the Oswestry cartilage score (OsScore) and the International Cartilage Repair Society (ICRS) II score. Clinical outcomes were assessed using the modified Lysholm score preoperatively, at the time of MRI or biopsy, and at a mean 8.4 ± 3.7 years (maximum, 17.8 years) after ACI. Results: At 12 months, the total MOCART score and some of its individual parameters correlated significantly with clinical outcomes. The degree of defect fill, overall signal intensity, and surface of repair tissue at 12 months also significantly correlated with longer-term outcomes. The presence of cysts or effusion (WORMS) significantly correlated with clinical outcomes at 12 months, while the presence of synovial cysts/bursae preoperatively or the absence of loose bodies at 12 months correlated significantly with long-term clinical outcomes. Thirty percent of repair tissue biopsies contained hyaline cartilage, 65% contained fibrocartilage, and 5% contained fibrous tissue. Despite no correlation between the histological scores and clinical outcomes at the time of biopsy, a lack of hyaline cartilage or poor basal integration was associated with increased pain; adhesions visible on MRI also correlated with significantly better histological scores. Conclusion: These results demonstrate that MRI at 12 months can predict longer-term clinical outcomes after ACI. Further investigation regarding the presence of cysts, effusion, and adhesions and their relationship with histological and clinical outcomes may yield new insights into the mechanisms of cartilage repair and potential sources of pain.

Arthroscopic Matrix-Encapsulated Autologous Chondrocyte Implantation: A Pilot Multicenter Investigation in Latin America

Cartilage ◽  
2020 ◽  
pp. 194760352091863
Author(s):  
Enrique Villalobos ◽  
Antonio Madrazo-Ibarra ◽  
Valentín Martínez ◽  
Anell Olivos-Meza ◽  
Cristina Velasquillo ◽  
...  
Keyword(s):  
Magnetic Resonance ◽  
Cell Viability ◽  
Cartilage Repair ◽  
Autologous Chondrocyte Implantation ◽  
T2 Mapping ◽  
Second Look ◽  
Arthroscopic Evaluation ◽  
Chondrocyte Implantation ◽  
Autologous Chondrocyte

Objective. To evaluate minimum biosecurity parameters (MBP) for arthroscopic matrix-encapsulated autologous chondrocyte implantation (AMECI) based on patients’ clinical outcomes, magnetic resonance imaging (MRI) T2-mapping, Magnetic Resonance Observation of Cartilage Repair Tissue (MOCART) score, and International Cartilage Repair Society (ICRS) second-look arthroscopic evaluation, laying the basis for a future multicenter study. Design. Pilot clinical study. We analyzed the logistics to perform AMECI to treat focal chondral lesions in different hospitals following strict biosecurity parameters related to tissue and construct transportation, chondrocyte isolation, and cell expansion. Patient progress was analyzed with patient-reported outcome measures, MRI T2-mapping, MOCART, and ICRS arthroscopic second-look evaluation. Results. Thirty-five lesions in 30 patients treated in 7 different hospitals were evaluated. Cell viability before implantation was >90%. Cell viability in construct remnants was 87% ± 11% at 24 hours, 75% ± 17.1% at 48 hours, and 60% ± 8% at 72 hours after implantation. Mean final follow-up was 37 months (12-72 months). Patients showed statistically significant improvement in all clinical scores and MOCART evaluations. MRI T2-mapping evaluation showed significant decrease in relaxation time from 61.2 ± 14.3 to 42.9 ± 7.2 ms ( P < 0.05). Arthroscopic second-look evaluation showed grade II “near normal” tissue in 83% of patients. Two treatment failures were documented. Conclusions. It was feasible to perform AMECI in 7 different institutions in a large metropolitan area following our biosecurity measures without any implant-related complication. Treated patients showed improvement in clinical, MRI T2-mapping, and MOCART scores, as well as a low failure rate and a favorable ICRS arthroscopic evaluation at a mid-term follow-up. Level of Evidence. 2b.

Sporting Activity Is Reduced 11 Years After First-Generation Autologous Chondrocyte Implantation in the Knee Joint

2017 ◽  
Vol 45 (12) ◽  
pp. 2762-2773 ◽  
Author(s):  
Benjamin Erdle ◽  
Simon Herrmann ◽  
Stella Porichis ◽  
Markus Uhl ◽  
Nadir Ghanem ◽  
...  
Keyword(s):  
Magnetic Resonance ◽  
Knee Joint ◽  
Autologous Chondrocyte Implantation ◽  
Repair Tissue ◽  
Sporting Activity ◽  
Chondrocyte Implantation ◽  
Sporting Activities ◽  
Autologous Chondrocyte

Background: Little is known about long-term sporting activity after periosteal autologous chondrocyte implantation (ACI-P) and its correlation to clinical, morphological, and ultrastructural cartilage characteristics on magnetic resonance imaging (MRI). Purpose: To evaluate long-term sporting activity after ACI-P and to correlate with clinical and MRI findings. Study Design: Case series; Level of evidence, 4. Methods: Patients who underwent ACI-P for isolated cartilage defects of the knee joint between 1997 and 2001 were analyzed for sporting ability for 3 different time points: lifetime until the onset of pain, the year before ACI-P, and 11 years (range, 9.0-13.4 years) postoperatively. Sporting activity was assessed and patients’ level of activity scaled using standardized questionnaires. MRI scans of the affected knee joint at follow-up were analyzed using the MOCART (magnetic resonance observation of cartilage repair tissue) score and T2 mapping. Results: Seventy of 86 patients (81% follow-up rate) consisting of 25 female and 45 male patients, with a mean age of 33.3 ± 10.2 years at the time of surgery, mean defect size of 6.5 ± 4.0 cm2, and 1.17 treated defects per patient, agreed to participate in the study at a mean 10.9 ± 1.1 years after ACI-P. Fifty-nine patients (69% of total; 84% of follow-up) agreed to MRI, allowing the complete evaluation of 71 transplant sites. Before the onset of symptoms (lifetime), 95.7% of patients played a mean 6.0 sporting activities at a competitive level. In the year before ACI-P, 81.4% of patients played a mean 3.4 sporting activities in 2.4 sessions during 5.4 hours per week at a recreational level. At follow-up, 82.9% of the patients played a mean 3.0 sporting activities in 1.8 sessions during 3.0 hours per week at a recreational level. In contrast to objective factors, 65.6% of the patients felt that their subjective sporting ability had improved or strongly improved after ACI-P, whereas 12.9% felt that their situation had declined or strongly declined, and 21.4% stated that their sporting ability had undergone no change because of surgery. Factors of sporting activity correlated significantly with clinical long-term outcomes. MRI analysis with a mean repair tissue T2 relaxation time of 35.2 milliseconds and mean MOCART score of 44.9 showed no conclusive significant correlation to sporting activity. Level of performance was the only sporting activity factor to show a weak correlation with subgroups of the MOCART score. Conclusion: The premorbid level of sporting and recreational activities cannot be achieved 11 years after ACI-P. The MRI results determined at this time point did not conclusively correlate with long-term sporting activity.

Cartilage T2 Relaxation Times and Subchondral Trabecular Bone Parameters Predict Morphological Outcome After Matrix-Associated Autologous Chondrocyte Implantation With Autologous Bone Grafting

2020 ◽  
Vol 48 (14) ◽  
pp. 3573-3585
Author(s):  
Alexandra S. Gersing ◽  
Christian Holwein ◽  
Joachim Suchowierski ◽  
Georg Feuerriegel ◽  
Florian T. Gassert ◽  
...  
Keyword(s):  
Mr Imaging ◽  
Trabecular Bone ◽  
Cartilage Repair ◽  
Subchondral Bone ◽  
Clinical Symptoms ◽  
Repair Tissue ◽  
Bone Parameters ◽  
Bone Fraction ◽  
Chondrocyte Implantation ◽  
Trabecular Number

Background: Quantitative magnetic resonance (MR) imaging techniques are established for evaluation of cartilage composition and trabecular bone microstructure at the knee. It remains unclear whether quantitative MR parameters predict the midterm morphological outcome after matrix-associated chondrocyte implantation (MACI) with autologous bone grafting (ABG). Purpose: To assess longitudinal changes and associations of the biochemical composition of cartilage repair tissue, the subchondral bone architecture, and morphological knee joint abnormalities on 3-T MR imaging after MACI with ABG at the knee. Study Design: Case series; Level of evidence, 4. Methods: Knees of 18 patients (28.7 ± 8.4 years [mean ± SD]; 5 women) were examined preoperatively and 3, 6, 12, and 24 months after MACI and ABG using 3-T MR imaging. Cartilage composition was assessed using T2 relaxation time measurements. Subchondral bone microstructure was quantified using a 3-dimensional phase-cycled balanced steady-state free precision sequence. Trabecular bone parameters were calculated using a dual threshold algorithm (apparent bone fraction, apparent trabecular number, and apparent trabecular separation). Morphological abnormalities were assessed using the MOCART (magnetic resonace observation of cartilage repair tissue) score, the WORMS (Whole-Organ Magnetic Resonance Imaging Score), and the CROAKS (Cartilage Repair Osteoarthritis Knee Score). Clinical symptoms were assessed using the Tegner activity and Lysholm knee scores. Statistical analyses were performed by using multiple linear regression analysis. Results: Total WORMS ( P = .02) and MOCART ( P = .001) scores significantly improved over 24 months after MACI. Clinical symptoms were significantly associated with the presence of bone marrow edema pattern abnormalities 24 months after surgery ( P = .035). Overall there was a good to excellent radiological outcome found after 24 months (MOCART score, 88.8 ± 10.1). Cartilage repair T2 values significantly decreased between 12 and 24 months after MACI ( P = .009). Lower global T2 values after 3 months were significantly associated with better MOCART scores after 24 months ( P = .04). Moreover, trabecular bone parameters after 3 months were significantly associated with the total WORMS after 24 months (apparent bone fraction, P = .048; apparent trabecular number, P = .013; apparent trabecular separation, P = .013). Conclusion: After MACI with ABG, early postoperative quantitative assessment of biochemical composition of cartilage and microstructure of subchondral bone may predict the outcome after 24 months. The perioperative global joint cartilage matrix quality is essential for proper proliferation of the repair tissue, reflected by MOCART scores. The subchondral bone quality of the ABG site is essential for proper maturation of the cartilage repair tissue, reflected by cartilage T2 values.

Costal Chondrocyte–Derived Pellet-Type Autologous Chondrocyte Implantation for Treatment of Articular Cartilage Defect

2020 ◽  
Vol 48 (5) ◽  
pp. 1236-1245 ◽  
Author(s):  
Kyoung-Ho Yoon ◽  
Jae-Young Park ◽  
Jin-Yeon Lee ◽  
EunAh Lee ◽  
Jungsun Lee ◽  
...  
Keyword(s):  
Articular Cartilage ◽  
Magnetic Resonance ◽  
Cartilage Repair ◽  
Therapeutic Option ◽  
Cartilage Defects ◽  
Resonance Imaging ◽  
Defect Filling ◽  
Costal Chondrocyte ◽  
Autologous Chondrocyte

Background: Because articular chondrocyte-based autologous chondrocyte implantations (ACIs) have restrictively restored articular cartilage defects, alternative cell sources as a new therapeutic option for cartilage repair have been introduced. Purpose: To assess whether implantation of a costal chondrocyte–derived pellet-type (CCP) ACI allows safe, functional, and structural restoration of full-thickness cartilage defects in the knee. Study Design: Case series; Level of evidence, 4. Methods: In this first-in-human study, 7 patients with symptomatic, full-thickness cartilage lesions were enrolled. The chondrocytes isolated from the patients’ costal cartilage were expanded, followed by 3-dimensional pellet culture to prepare the CCP-ACI. Implantation of the pellets was performed via minimal arthrotomy and secured with a fibrin sealant. Clinical scores, including the International Knee Documentation Committee (IKDC) subjective, Lysholm, and Tegner activity scores, were estimated preoperatively and at 1, 2, and 5 years postoperatively. High-resolution magnetic resonance imaging was also performed to evaluate cartilage repair as well as to calculate the MOCART (magnetic resonance observation of cartilage repair tissue) score. Results: The costal chondrocytes of all patients formed homogeneous-sized pellets, which showed the characteristics of the hyaline cartilaginous tissue with lacunae-occupied chondrocytes surrounded by glycosaminoglycan and type II collagen-rich extracellular matrix. There were no treatment-related serious adverse events during the 5-year follow-up period. Significant improvements were seen in all clinical scores from preoperative baseline to the 5-year follow-up (IKDC subjective score, 34.67 to 75.86; Lysholm score, 34.00 to 85.33; Tegner activity score, 1.17 to 4.67; and MOCART score, 28.33 to 83.33). Two patients had complete defect filling on magnetic resonance imaging evaluation at 1 year. Moreover, at 5 years postoperatively, complete defect filling was observed in 4 patients, and hypertrophy or incomplete defect filling (50%-100%) was observed in 2 patients. Conclusion: The overall results of this clinical study suggest that CCP-ACI can emerge as a promising therapeutic option for articular cartilage repair with good clinical outcomes and structural regeneration and with stable results at midterm follow-up. Registration: NCT03517046 ( ClinicalTrials.gov identifier)

scholarly journals Prospective Long-term Follow-up of Autologous Chondrocyte Implantation With Periosteum Versus Matrix-Associated Autologous Chondrocyte Implantation: A Randomized Clinical Trial

2020 ◽  
Vol 48 (9) ◽  
pp. 2230-2241
Author(s):  
Alexander Barié ◽  
Patrizia Kruck ◽  
Reza Sorbi ◽  
Christoph Rehnitz ◽  
Doris Oberle ◽  
...  
Keyword(s):  
Magnetic Resonance ◽  
Autologous Chondrocyte Implantation ◽  
Cartilage Defects ◽  
Repair Tissue ◽  
Sf 36 ◽  
Chondrocyte Implantation ◽  
Autologous Chondrocyte ◽  
Time Point

Background: Matrix-associated autologous chondrocyte implantation (MACI) is a further development of the original autologous chondrocyte implantation periosteal flap technique (ACI-P) for the treatment of articular cartilage defects. Purpose: We aimed to establish whether MACI or ACI-P provides superior long-term outcomes in terms of patient satisfaction, clinical assessment, and magnetic resonance imaging (MRI) evaluation. Study Design: Randomized controlled trial; Level of evidence, 2. Methods: A total of 21 patients with cartilage defects at the femoral condyle were randomized to MACI (n = 11) or ACI-P (n = 10) between the years 2004 and 2006. Patients were assessed for subjective International Knee Documentation Committee (IKDC) score, Lysholm and Gillquist score, Tegner Activity Score, and 36-Item Short Form Health Survey (SF-36) preoperatively (T0), at 1 and 2 years postoperatively (T1, T2), and at the final follow-up 8 to 11 years after surgery (T3). Onset of osteoarthritis was determined using the Kellgren-Lawrence score and Magnetic Resonance Observation of Cartilage Repair Tissue (MOCART) score, and delayed gadolinium-enhanced MRI of cartilage was used to evaluate the cartilage. Adverse events were recorded to assess safety. Results: There were 16 patients (MACI, n = 9; ACI-P, n = 7) who were reassessed on average 9.6 years after surgery (76% follow-up rate). The Lysholm and Gillquist score improved in both groups after surgery and remained elevated but reached statistical significance only in ACI-P at T1 and T2. IKDC scores increased significantly at all postoperative evaluation time points in ACI-P. In MACI, IKDC scores showed a significant increase at T1 and T3 when compared with T0. In the majority of the patients (10/16; MACI, 5/9; ACI-P, 5/7) a complete defect filling was present at the final follow-up as shown by the MOCART score, and 1 patient in the ACI-P group displayed hypertrophy of the repair tissue, which represents 6% of the whole study group and 14.3% of the ACI-P group. Besides higher SF-36 vitality scores in ACI-P at T3, no significant differences were seen in clinical scores and MRI scores between the 2 methods at any time point. Revision rate was 33.3% in MACI and 28.6% in ACI-P at the last follow-up. Conclusion: Our long-term results suggest that first- and third-generation ACI methods are equally effective treatments for isolated full-thickness cartilage defects of the knee. With the number of participants available, no significant difference was noted between MACI and ACI-P at any time point. Interpretation of our data has to be performed with caution due to the small sample size, which was further limited by a loss to follow-up of 24%.

scholarly journals Biomechanical Changes of Repair Tissue after Autologous Chondrocyte Implantation at Long-Term Follow-Up

Cartilage ◽  
2020 ◽  
pp. 194760352092143
Author(s):  
Teemu Paatela ◽  
Anna Vasara ◽  
Heikki Nurmi ◽  
Hannu Kautiainen ◽  
Jukka S. Jurvelin ◽  
...  
Keyword(s):  
Cartilage Repair ◽  
Autologous Chondrocyte Implantation ◽  
Maturation Process ◽  
Tissue Stiffness ◽  
Repair Tissue ◽  
Long Term Follow Up ◽  
Chondrocyte Implantation ◽  
Autologous Chondrocyte

Objective. This study aims to describe biomechanical maturation process of repair tissue after cartilage repair with autologous chondrocyte implantation (ACI) at long-term follow-up. Design. After ACI, 40 patients underwent altogether 60 arthroscopic biomechanical measurements of the repair tissue at various time points during an up to 11-year follow-up period. Of these patients, 30 patients had full-thickness cartilage lesions and 10 had an osteochondritis dissecans (OCD) defect. The mean lesion area was 6.5 cm2 (SD 3.2). A relative indentation stiffness value for each individually measured lesion was calculated as a ratio of repair tissue and surrounding cartilage indentation value to enable interindividual comparison. Results. Repair tissue stiffness improved during approximately 5 years after surgery. Most of the increase in stiffness occurred during the first 2 years. The curvilinear correlation between relative stiffness values and the follow-up time was 0.31 (95% CI 0.07-0.52), P = 0.017. The interindividual variation of the stiffness was high. Lesion properties or demographic factors showed no significant correlation to biomechanical outcome. The overall postoperative average relative stiffness was 0.75 (SD 0.47). Conclusions. Our clinical study describes a biomechanical maturation process of cartilage repair that may continue even longer than expected. A substantial increase in tissue stiffness proceeds for the first two years postoperatively. Minor progression proceeds for even longer. In some repairs, the biomechanical result was equal to native cartilage, suggesting hyaline-type repair. The variation in biomechanical results suggests substantial inconsistency in the structural outcome following ACI.

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