The treatment of acute full thickness chondral damage within the knee is a surgical challenge. Frequently used surgical techniques include chondroplasty, micro-fracture and chondrocyte implantation. These procedures give unpredictable functional outcomes and if the formation of neocartilage is achieved it is predominantly composed of type 1 collagen. The TruFit osteochondral plug was designed to provide a scaffold for cell proliferation into full thickness chondral defects. It is a composite polymer composed of polylactide co-glycolide, calcium sulphate and poly-glycolide fibres. It is composed of 2 layers, one with a similar trabecular network to cancellous bone and a superficial layer designed to simulate articular lining. The TruFit bone plug was analysed using micro-computed tomography. Its morphology characteristics, granulometry, mechanical performance and image guided failure were tested as well as numerical modelling to assess the permeability of TruFit. Morphological parameters of the TruFit bone plug compared favourably with those of human tissue. Under load the scaffold exhibited shear bands throughout the composite leading to a failure mechanism similar to cancellous bone. Stress relaxation rates of the scaffolds were greatly decreased under wet conditions, likely due to plasticisation of the scaffold by water. The biomechanical properties of the TruFit bone plugs are a cause for concern. The Scaffolds mechanical performance under load rapidly deteriorates in wet conditions at body temperature (the natural knee environment). This early failure will lead to defects in the articular surface where the plug has been inserted. Clinical data is sparse. This study correlates with work performed by Dockery et al & Spalding et al. These clinical studies have shown that the TruFit implant shows no evidence of bone ingrowth or osteoconductivity. It provides no subchondral support to neocartilage or tissue that was stimulated to form around the defects and surgical sites.
Novel hydrogel implants, TRUFIT® bone plugs, have been developed by Smith & Nephew to replace worn-out cartilage surfaces, restoring mobility and relieving joint pain. There is limited information, however, on the biomechanical properties of the implants. Therefore, appropriate mechanical testing and modelling must be carried out to assess their mechanical properties for load bearing applications. In this study, compressive properties of TRUFIT® bone and dual layer implants were examined under selected physiological loading conditions. The bone layer of the implant was also modelled using a biphasic poroviscoelastic (BPVE) material constitutive law and the results from the model are compared with those from the experiments. TRUFIT® CB plugs, with diameters of 11 and 5mm, were sectioned to obtain single layer bone and dual layer samples, with an aspect ratio of 0.86. Specimens were tested in confined and unconfined compressions at two constant strain rates of 0.002/sec (walking) and 0.1/sec (impact) [1-3] on a MTS servo-hydraulic test machine equipped with a bionix envirobath. All samples were tested in phosphate buffered saline (PBS) solution at 37 °C. A preload of 0.1 MPa was applied and preconditioning (10 cycles of 0.008 strain) at a constant strain rate of 0.005 sec−1 [4] was used. The compressive modulus was calculated from the slope of the linear part of the stress-strain curve. In addition, whilst stress relaxation tests were performed on the bone samples in unconfined compression up to 5% strain, at a strain rate of 0.01/s (running) [1-2].Introduction
Materials and Methods
We performed a study to evaluate the material properties of a new cylindrical scaffold plug licensed for the treatment of osteochondral defects as prior to the removal of a core of normal femoral condylar bone, it is imperative that the biomechanical properties of replacement implant material are known. TruFit CB plugs (Smith and Nephew) are resorbable material composed of polylactide-co-glycolide (PLG) copolymer, calcium-sulfate, polyglycolide (PGA) fibres and surfactant. The implants are 7mm, 9mm and 11mm cylindrical plugs. The stress/strain relationships of both the dual layer implant and the base layer material were examined. Compressive load testing at selected strain rates was performed in both confined and unconfined models in a substitute body fluid filled chamber. Compressive failure was found to occur between 40–60% strain with maximum stresses at failure for the dual layer implants occurring at 5.5MPa (7mm), 5.8MPa (9mm) and at 8.5MPa (11mm). The mechanical strength under constrained loading conditions is higher than in unconstrained loading (compressive stress required to develop 5 percent strain being 0.6MPa unconfined to 1.1MPa confined for 7mm; 0.6MPa to 1.4MPa for 9mm and 1.0MPa to 3.2MPa for 11mm implants). This demonstrates the importance of a close press fit. The modulus of elasticity was calculated at 50 MPa (7mm), 60 MPa (9mm) and 80 MPa (11mm). The larger the plug size, the higher the strength shown under test conditions at all strain rates. Prior to this study, the material properties of this implant have not been characterized. The Young’s moduli of the implants are in keeping with previous estimated values for successful regeneration of cartilage within a synthetic scaffold. The biomechanical properties described in this study will help to guide surgeons in TruFit CB use and guide the rehabilitation programmes of those patients who have had osteochondral lesions treated with TruFit CB scaffold plugs.
