The Future: Biomedical Cartilage Surgery

Mats Brittberg, M.D.; Ph.D.

Cartilage Research Unit, Department of Orthopaedics, Kungsbacka Hospital, S-434 40 Kungsbacka, Sweden

Introduction:

In the last two decades of the 20 th century, a thrilling new approach to tissue restoration, regenerative biology, has been developed, which in the 21 st century will be developed clinically into regenerative medicine and could also be performed as biomedical surgery. Research in regenerative biology involves cell and molecular biology, developmental cell biology, immunology, and polymer chemistry. This new direction in medicine will use three strategies: transplantation of cells to form new tissue in the transplant site, implantation of bioartificial tissues constructed in vitro, and induction of regeneration in vivo from healthy tissues adjacent to an injury¹¹.

Regarding the future for cartilage repair in the new century, the idea is to transplant stem/progenitor cells, or their differentiated products, into a cartilage lesion site where they may form new tissue, or the cells could be used to construct a bioartificial tissue in vitro to replace the original tissue or organ. Bioartificial tissues are made by seeding stem or differentiated cells into a natural or artificial biomaterial scaffold shaped in the appropriate

form, then implanting the construct in place of the damaged tissue or organ. Theoretically, the use of stem cells is preferable to the use of differentiated cells harvested directly from a donor because stem cells have the potential for unlimited growth and thus supply. Such so called uncommitted cells are capable of a broad range of chondrogeneic expression and could provide a regenerative tissue that recreates the embryonic lineage transitions originally involved in joint tissue formation

Other joints besides the knee joint ?

Most new articular cartilage repair techniques have been used for the largest human articular joint; the knee joint. Certainly, in the future other joints such as the ankle joint, hip-joint, shoulder , elbow and wrist-joint will be new areas to focuse the cartilage resurfacing techniques on. Still, however, we have to receive more basic knowledge about the repair events with the different methods before we start to use the new techniques generally. The different joints have different biomechanical behaviours and also the cartilage in the other joints differs from the knee articular cartilage.

Need for more animal studies ?

Most cartilage repair studies have been performed with rabbits. One may think that the next step is to use a larger animal such as the dog or horse for the further development of the repair methods to be used. Rasanen and Messner⁹ presented a study in which in situ indentation tests were used to map the short term stiffness and thickness of articular cartilage at seven locations (anterior and posterior areas of the medial and lateral femoral condyles, the patellar groove, and the central areas of the medial and lateral tibial plateaus) in nine normal rabbit knee joints. As the rabbit knee frequently is used as an experimental model for cartilage repair, it is important to know that there exist regional variations of the biomechanical properties in normal rabbit knee articular cartilage which could mean the choice of different joint regions for repair studies makes comparisons between methods difficult.

The same authors⁹ also found that in the dog femoral cartilage indentation was stiffer at the anterior than at the posterior regions as opposed to what was found in the rabbit. They concluded that dissimilarities between animal models may be caused by different joint loading characteristics. In rabbits, the repair process in the more anterior femoral areas with less stiff cartilage may not be comparable with repair in more posterior areas where cartilage is stiffer. The above described findings could be of importance when comparing different treatment models and especially when testing a repair-model in a larger animal with different biomechanical characteristics. These findings are also important when to transfer the cartilage repair animal model to be used in the clinical situation.

Do we need to treat cartilage lesions ?

Do we really need to repair the cartilage lesions and which lesions are to be treated ?

In 1995, 680,000 knee arthroscopies were performed in the US¹. A survey conducted by Curl and colleagues of 31,516 knee arthroscopies revealed that chondral lesions were present in 63%, with an average of 2.7 hyaline cartilage lesions per knee³. From those figures it can be estimated that 427,800 arthroscopies are performed on people with hyaline cartilage lesions present. Based on these estimates, a technology superior to microfracture or mosaicplasty in achieving hyaline cartilage repair has a potential market of $300 million to $1 billion in the US alone¹.

As You could see, it is of interest for bioengineering companies to find a reliable cartilage repair method but we have to be very careful as we still do not know the natural course of a cartilage lesion. And what about pain and cartilage defects; are we treating asymptomatic lesions ? That question is especially important to find out and especially important regarding patellar lesions. Neurosensory innervation is important and probably crucial for normal joint function⁵. Lack of neurosensory restoration of certain intra articular structures that have been surgically reconstructed or transplanted may lead to a common mode of structural failure⁵.

The implant may eventually be destined to fail structurally with acute or repetititive supraphysiological load

The cartilage repair appearance :

For all new techniques it is important to find out the repair characteristics. Our research group in Göteborg, Sweden, perform regular transarthroscopic biopsies of cartilage grafts and typically is the zonal heterogeneity throughout the repair tissue core biopsy^{2, 7}. The repair consists of superiorly placed periosteal membrane and implanted chondrocytes and a full thickness core biopsy will demonstrate a fibrous top-layer representing the remnants of the periosteum, a transitional zone with a tissue produced by periosteal cells and implanted

chondrocytes and finally a third basal zone with a more hyaline appearance (Fig. 1, Fig. 2)). The ideal situation is to also get a hyalin tissue repair appearance in the top-layer. Furthermore, the periosteum has a tendency to hypertrophy and still there is no good technique to attach it transarthroscopically. Much research is now focused on in vitro regeneration of a 3-dimensional cartilage matrix from articular chondrocytes seeded onto a bioresorbable polymeric scaffold. Already,one company has started to clinically use cultured autologous chondrocytes together with a resorbable porcine collagen I/III membrane but no experimental or clinical results published yet with that method. With chondrocytes in for example a polymer scaffold the repair of cartilage defects may more easily be treated transarthroscopically. Diffusion of nutritients is a major factor that limits cell growth and in in vitro studies , cells die in the central part of the scaffold. Still, there is no reliable available cell-scaffold technique where the scaffold is resorbed at the same speed as the new matrix is formed. Furthermore, the ideal scaffold should be highly adhesive to chondrocytes but not to other cell types and it should also provide two- and three dimensional patterns of adhesiveness. The degraded parts of the scaffold should not provoke toxic or allergic reactions.

Much effort is also focused on the adhesion and integration of the induced cartilage repair tissue to the adjacent cartilage. In vitro studies have shown there is 50% detachment of seeded chondrocytes after 25 minutes of cell seeding by gravity and by 2.3 Pa of shear stress after 40 minutes¹⁰. Transferred to the clinical situation, it might be beneficial to allow chondrocytes or other types of chondrogeneic cells to stabilize in the absence of applied load for some time post-surgery while there seem to be an increase in resistance to shear stress-induced cell detachment with increasing seeding duration ¹⁰. What is then ¨some time¨ ? Manolopoulos and co-workers have reported that maximal adherence occured by 24 hours post-transplantation when cultured chondrocytes were transplanted to cartilage explant surfaces both with intact or removed surface layer⁸. However, after such a time for the chondrocytes to adhere, it is important to stimulate the chondrocytes by cyclic loading in order to stimulate aggrecan and collagen synthesis.

Future research is also needed to improve the interface between the neocartilage and native cartilage.

Tissue engineering:

In my opinion, one can expect two directions in the near use of chondrocytes or other chondrogeneic cells for induced cartilage repair. Either, continuous use of implantation of in vitro cultured cells together with a sutured top-membrane consisting of a resorbable material and cells also seeded on the membrane(sandwich-technique) or an in vitro construct of an osteochondral ¨plug¨ (composite graft with cartilaginous and osseous parts) (Fig. 3).

The presuppostion for a a good product is the induction, development and maintenance of differentiation within the tissue during in vitro culture. The development of a tissue depends on a three dimensional arrangement of cells and the formation and synthesis of an appropriate matrix. There are many problems to solve before we can get such products to give a reliable function in the clinical situation.Cells in vitro produce a cartilaginous layer however with collagen layer about one half of native cartilage. The right type of collagen, the major extracellular material that holds together tissues and organs of most complex organisms is needed. Weight bearing , may be a way to stimulate release of transcription factors that could trigger a chondrocyte switch to produce the right amount and type of collagen but to develop an osteochondral graft in vitro without the weight-bearing forces that are required is a difficult task to solve.

The ideal articular cartilage repair technology ought to be cheap, available off-the-shelf, deliverable arthroscopically, give rise to a durable well-integrated repair tissue, result in clinical improvement, eliminate or significantly postpone the need for arthroplasty, and be suited for both focal defects and degenerative joint disease¹. Techniques today available for cartilage repair is still ¨plug-in¨-techniques for local cartilage repair and not possible to be used in osteoarthtritic repairs.However, recent studies has shown that there is a possibility to repair and rebuild osteoarthric cartilage by means of resurfacing the diseased cartilage with genetically modified chondrocytes⁴.

Despite the promising clinical results with some of the new articular cartilage repair methods, there remains a considerable distance between where the technology is and where we want it to be in clinical usage¹. However, with the new interdisciplinary tissue engineering groups formed over the world where the cell biologists, engineers and surgeons work closely together with the construction of artificial connective, epithelial and neuronal tissues using living cells and different kinds of biomaterials hopefully we will able to produce tissues that can used as real spare parts instead of destroyed human tissues. This new concept could be called biomedical surgery.

Paul Ficat, famous french cartilage researcher summarize very well how to handle the cartilage and cartilage injuries in the best way⁶:

Le Cartilage:

- Il faut le preserver par un diagnostic précoce et un traitement d'arret

- Le reconstituer par la chirurgie regeneratrice

- Le conserver et le soulager par la chirurgie conservatrice

References:

1. Bobic V. Conference report. Tissue repair techniques of the future:Options for articular cartilage injury. Techvest, LLC First Annual Conference on Tissue Reapir, replacement and regeneration. October 27-28, 1999, New York, USA. http://orthopedics.medscape.com/Medscape/OrthoSportsMed/journal/2000/v04.n01/mos0114.bobi/mos0114.bobi-02.html

2. Brittberg M, Lindahl A, Nilsson A, Ohlsson C, Isaksson O, Peterson L. Treatment of deep cartilage defects in the knee with autologous chondrocyte transplantation. New England Journal of Medicine 1994; 331:889-895.

3. Curl WW, Krome J, Gordon ES, Rushing J, Smith BP, Poehling GG. Cartilage injuries: a review of 31,516 knee arthroscopies. Arthroscopy 1997; 13: 456-460

4. Doherty PJ, Zhang H, Tremblay L, Manolopoulos V, Marshall KW. Resurfacing of articular cartilage explants with genetically-modified human chondrocytes in vitro. Osteoarthritis Cartilage 1998 May;6(3):153-159

5. Dye SF, Vaupel GL. The pathophysiology of patellofemoral pain. Sports Med Arthroscopy Rev 1994; 2: 203-210

6. Ficat P. Perspectives therapeutiques dans les lesions degeneartives du cartilage. In:Ficat P. Cartilage et arthrose. Exploration fonctionele, pathologie et thérapeutique. Masson ,Paris 1979. 107-112.

7. Gillogly SD. Autologous chondrocyte implantation: Current state-of-the-art. In: Knieinstabilität-knorpelschaden. Ed: Imhoff AB, Burkart A. Steinkopff Verlag, Darmstadt, Germany 1998:60-66.

8. Manolopoulos V, Wayne Marshall K, Zhang H, Trogadis J, Tremblay L, Hoherty PJ. Factors affecting the efficacy of bovine chondrocyte transplantation in vitro. 1999; 7: 453-460.

9. Rasanen T, Messner K: Regional variations of indentation stiffness and thickness of normal rabbit knee articular cartilage. J Biomed Mater Res 31: 519-524, 1996

10. Schinagl RM, Kurtis MS, Ellis KD, Chien S, Sah RL. Effect of seeding duration on the strength of chondrocyte adhesion to articular cartilage. J Orthop Res 1999; 17: 121-129

11. Stocum DL. Review.Regenerative biology and medicine in the 21st century. E-biomed 2000; 1: 17-20.

Legends to the figures:

Fig. 1:

A cartilage repair tissue appearance after chondrocyte-periosteum grafting. The top layer represents the repair tissue and an eventual hypertrophy from the periosteum, mostly fibrous appearance followed by a transitional repair tissue with contribution from both periosteal cells and chondrocytes. Finally a third area with more hyalinlike appearance.

Fig. 2:

The ideal cartilage repair situation after chondrocyte implantation in combination with a cell-seeded membrane. Less risk for a fibrous part compared to the situation seen in Fig. 1 with the periosteum.

Fig. 3:

The potential future situation with on top the in vitro reconstructed osteochondral repair plug and the lower drawing representing the combination of a ¨double-sandwich¨cell-seeded resorbable membrane and additional implanted committed chondrocytes or mesenchymal stem cells.