A three-dimensional osteochondral composite scaffold for articular cartilage repair

Abstract

There is a recognized and urgent need for improved treatment of articular cartilage defects. Tissue engineering of cartilage using a cell-scaffold approach has demonstrated potential to offer an alternative and effective method for treating articular defects. We have developed a unique, heterogeneous, osteochondral scaffold using the TheriForm^TM three-dimensional printing process. The material composition, porosity, macroarchitecture, and mechanical properties varied throughout the scaffold structure. The upper, cartilage region was 90% porous and composed of d,l-PLGA/l-PLA, with macroscopic staggered channels to facilitate homogenous cell seeding. The lower, cloverleaf-shaped bone portion was 55% porous and consisted of a l-PLGA/TCP composite, designed to maximize bone ingrowth while maintaining critical mechanical properties. The transition region between these two sections contained a gradient of materials and porosity to prevent delamination. Chondrocytes preferentially attached to the cartilage portion of the device, and biochemical and histological analyses showed that cartilage formed during a 6-week in vitro culture period. The tensile strength of the bone region was similar in magnitude to fresh cancellous human bone, suggesting that these scaffolds have desirable mechanical properties for in vivo applications, including full joint replacement.

Introduction

Over 16 million people in the US suffer from severe joint pain and related dysfunction, such as loss of motion, as a result of injury or osteoarthritis [1], [2]. In particular, loss of function of the knees can severely impact mobility and thus the patient's quality of life. The biological basis of joint problems is the deterioration of articular cartilage [3], which covers the bone at the joint surface and performs many complex functions. Articular cartilage is composed of hyaline cartilage which has unique properties, such as viscoelastic deformation, that allow it to absorb shock, distribute loads, and facilitate stable motion [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Self-repair of hyaline cartilage is limited [14], [15] and the tissue that forms is usually a combination of hyaline and fibrocartilage [16], which does not perform as well as hyaline cartilage and can degrade over time [17].

Current treatments for articular defects have limited success in that they are deficient in long-term repair or have unacceptable side effects. Autograft procedures, such as Mosaicplasty [18] and Osteochondral Autolograft Transfer System (OATS) [19], remove an osteochondral plug from a non-load bearing area and graft it into the defect site. Despite the recent successes this procedure has seen in repairing cartilage lesions, it requires additional time and money to acquire the donor tissue and results in donor site morbidity and pain [20]. Carticel^®, a procedure consisting of injecting cells under a periosteal flap, has also had limited success; however, the procedure lacks inter-patient consistency with some patients maintaining little relief months or years later, and the surgical procedure is technically challenging. Abrasion arthroscopy, subchondral bone drilling and microfracture typically result in fibrocartilage filling the defect site. Allogenic transplantation of osteochondral grafts has had clinical success, but supply is limited and has a risk of infection [16], [21], [22], [23], [24].

Each of the currently used repair modalities has severe limitations [25], [26], [27], [28], [29], and the outcome is generally regarded as inadequate. Tissue engineering of cartilage has great potential in providing the appropriate replacement tissue with features necessary for a successful repair of cartilage to occur. While there has been success in growing cartilage in vitro, success in vivo requires reliable fixation into the joint defect and integration with the subchondral bone. Ultimately, for defects in articular locations with substantial curvature, the tissue-engineered constructs should also have appropriate topography. We propose using a fully resorbable synthetic scaffold, containing a cartilage region and a bone-appropriate region made by the TheriForm^TM three-dimensional printing process, in a cell-scaffold-based tissue engineering approach to repair articular defects [30], [31], [32]. In the TheriForm process, scaffolds are built one thin layer at a time, which allows for the production of multiphasic devices, and has the capability to fabricate devices with biologically and anatomically relevant features. The primary features of these scaffolds are: (1) a highly porous cartilage region to facilitate seeding chondrocytes selectively in this region, (2) staggered channels in the cartilage region to promote homogeneous seeding throughout the 2- mm thickness of the region [33], (3) a cloverleaf bone region to promote bone ingrowth for fixation and integration while maintaining necessary mechanical characteristics, and (4) a transition region with a gradient of materials and pore structure to prevent delamination. Autologous chondrocytes that have been expanded in culture from a small biopsy or expanded allogenic chondrocytes that have been extensively tested for diseases can then be seeded onto the top portion of the scaffold [34], [35]. A significant amount of work has been published on the interactions of chondrocytes and resorbable polymers [36], [37], [38], [39], [40]. The seeded scaffold can then be cultured in vitro until adequate tissue formation has occurred and then implanted into the cartilage defect site [41].

In this paper we describe studies aimed at: (1) the selection of the appropriate polymeric material for the cartilage region, (2) mechanical testing of the bone region including the effect of porosity and polymer/calcium phosphate ratio, (3) prevention of delamination in the transition region, and (4) selection of an appropriate chondrocyte seeding method that results in high matrix deposition in the cartilage region but little in the bone region.