Poly (lactic acid)-based biomaterials for orthopaedic regenerative engineering

Abstract

Regenerative engineering converges tissue engineering, advanced materials science, stem cell science, and developmental biology to regenerate complex tissues such as whole limbs. Regenerative engineering scaffolds provide mechanical support and nanoscale control over architecture, topography, and biochemical cues to influence cellular outcome. In this regard, poly (lactic acid) (PLA)-based biomaterials may be considered as a gold standard for many orthopaedic regenerative engineering applications because of their versatility in fabrication, biodegradability, and compatibility with biomolecules and cells. Here we discuss recent developments in PLA-based biomaterials with respect to processability and current applications in the clinical and research settings for bone, ligament, meniscus, and cartilage regeneration.

Keywords: Bone; Cartillage; Growth factors; Ligament; Meniscus regeneration; Poly (lactic acid); Regenerative Engineering; Small molecules.

Figure 1. Representative PLA based medical devices currently employed in orthopaedic and dental applications – Biotrak® pins and screws

(a) Mini screw used for fixation in treating osteochondral defects and in foot, ankle, and hand surgeries, (b) standard screw used for treating osteochondral defects, osteotomies, and navicular fractures, (c) helical nail used for fixing radial styloid fractures and hammertoes, and (d) pin used for the repair of ulnar styloid fracture. Images reprinted with permission from Acumed LLC.

Figure 2

(a) The hierarchical organization of bone structure. (b) Internal structure of the bone is seen with osteons running parallel to the bone structure and centrally-running blood vessels for nutrient and waste transport. (c) Microstructure of the osteon is seen with the constituents of bone extracellular matrix (ECM). (d) Nanostructure of ECM consisting of collagen molecules nucleated with n-HA crystals. Image reproduced with permission from ref [128],copyright Elsevier (2008).

Figure 3. Scaffolds reported for bone tissue regeneration applications

(i) SEM micrographs of (i) PLGA/HA composite obtained from solvent casting/particulate leaching method, (ii) representative electrospun nanofibrous scaffolds, (iii a–b) porosity and pore diameter of electrospun dependence of PLLA nanofibers on the added PEG content, (iii c–e) combined polarized bright field and fluorescent imaging indicate the cellular infiltration through the scaffolds, (iii f) cellular infiltration observed for scaffolds made with varying PEG content at various time points. Higher cellular infiltration is observed in scaffolds containing higher PEG contents even at shorter time points, while scaffolds with lower PEG content exhibit poor cellular infiltration at shorter time points, (iii g) quantification of cellular infiltration through scaffolds made with varying PEG content (iv) PLGA based microspheres: (a). before and (b) 7-days after in-vitro cell culture with osteoblast cells (MG-63), (v) Top (a) and cross sectional view (c) of PLA based scaffolds obtained by nozzle deposition system (3-D printing process). Image (i) reprinted with permission from [137], copyright Elsevier 2006. Image (ii) reproduced with permission from ref [168], copyright American Chemical Society 2015. Images (iii) reproduced with permission from ref [172]. Images (iv) and (v) reprinted with permission from refs [197, 198], copyright Elsevier 2010 and 2013.

Figure 3. Scaffolds reported for bone tissue regeneration applications

(i) SEM micrographs of (i) PLGA/HA composite obtained from solvent casting/particulate leaching method, (ii) representative electrospun nanofibrous scaffolds, (iii a–b) porosity and pore diameter of electrospun dependence of PLLA nanofibers on the added PEG content, (iii c–e) combined polarized bright field and fluorescent imaging indicate the cellular infiltration through the scaffolds, (iii f) cellular infiltration observed for scaffolds made with varying PEG content at various time points. Higher cellular infiltration is observed in scaffolds containing higher PEG contents even at shorter time points, while scaffolds with lower PEG content exhibit poor cellular infiltration at shorter time points, (iii g) quantification of cellular infiltration through scaffolds made with varying PEG content (iv) PLGA based microspheres: (a). before and (b) 7-days after in-vitro cell culture with osteoblast cells (MG-63), (v) Top (a) and cross sectional view (c) of PLA based scaffolds obtained by nozzle deposition system (3-D printing process). Image (i) reprinted with permission from [137], copyright Elsevier 2006. Image (ii) reproduced with permission from ref [168], copyright American Chemical Society 2015. Images (iii) reproduced with permission from ref [172]. Images (iv) and (v) reprinted with permission from refs [197, 198], copyright Elsevier 2010 and 2013.

Figure 4. Mechanical properties, cell viability, and gene expression (Runx2) of PLLA-PCL scaffolds containing BNNT (2 and 5 wt %)

(A) Nano-indentation experiments demonstrate significant increases in modulus values in composites reinforced with BNNTs. The effect is more pronounced in composites containing 5 wt% BNNT. (B and C) quantification and fluorescent images of human osteoblast cells seeded on PLLA-PCL composites with and without BNNTs obtained by live-dead cell assay. Both quantified chart as well as fluorescent image indicate cytocompatibility of cells in scaffolds containing BNNTs; no statistical difference was observed in scaffolds with varying BNNT wt%. Finally, (D) several fold increases in Runx2 expression: a key regulator of osteoblastic differentiation. Images adapted with permission from ref [207], copyright Elsevier Ltd 2010.

Figure 5. Surface roughness, mechanical properties, cell viability and proliferation, and degradation induced pH changes observed in MgO/HA reinforced PLLA composites

(A) Effect of MgO addition causing surface roughness at nanoscale level, while having minimal effect (B) at microscale level. (C) Lower elongation at failure and higher modulus (D) of composites containing higher loading of MgO indicating higher stiffness, while neat PLLA and 20% HA containing samples exhibit a ductile-like failure. (E) Higher cell viability and proliferation (F) observed in scaffolds containing higher MgO content (20% or 10%MgO/10%HA) compared to neat PLLA scaffolds. (F) pH changes in the cell culture media caused by degrading MgO NPs. While neat PLLA, plain media, and HA containing samples caused the media to turn acidic, MgO containing scaffolds caused sharp decrease at shorter times followed by a marginal increase at longer time points indicating low cytotoxic effects. Images adapted with permission from ref [210] copyright Elsevier Ltd 2015.

Figure 6

A schematic representation of the ACL progressing to a complete tear (b) partial tear of the ACL caused by an injury, (c) gradually progressing to a complete tear, (D) three stages of behavior encountered by the ligament under mechanical strain. Under strain, there is a toe region where applied strain is not translated to stress because of the straightening of the crimp fibers, while in the linear region, after crimp straightening, applied strain is directly proportional to the stress (Hookean limit). Finally, beyond linear region, ligament yields to undergo rupture. Image (6A–C) modified and reprinted with permission from ref [244], copyright Elsevier (2014). Image 6D reprinted with permission from ref [33]

Figure 7. Reported scaffolds for ligament-cartilage-bone regeneration

(i) 3-D braided biomimetic scaffold with (a) femoral, (b) intra-articular, and (c) tibial bony ends. The bony ends have higher orientation and the ligament region has lower fiber orientation closely mimicking the physical nature of the ligament, (ii) Triphasic scaffold with each phase mimicking a region (a) ligament, (b) cartilage, and (c) bone, of the ACL. (iii) SEM micrographs of the three phases in the triphasic scaffolds: (a) braided and PLGA fibers sintered with the neighboring phase, (b). Sintered PLGA microspheres, (c). PLGA microspheres with encapsulated bioglass to promote bone formation, (iv). FTIR-imaging of the mineralized and non-mineralized regions of the cartilage region of the ACL (variation of proteoglycan content in this noted). Images (i), (ii) and (iii), modified and reprinted with permission from refs [268, 269] copyright Elsevier (2005), (2015). Image (iv) reprinted with permission from PLOS One (2013) [270].

Figure 7. Reported scaffolds for ligament-cartilage-bone regeneration

(i) 3-D braided biomimetic scaffold with (a) femoral, (b) intra-articular, and (c) tibial bony ends. The bony ends have higher orientation and the ligament region has lower fiber orientation closely mimicking the physical nature of the ligament, (ii) Triphasic scaffold with each phase mimicking a region (a) ligament, (b) cartilage, and (c) bone, of the ACL. (iii) SEM micrographs of the three phases in the triphasic scaffolds: (a) braided and PLGA fibers sintered with the neighboring phase, (b). Sintered PLGA microspheres, (c). PLGA microspheres with encapsulated bioglass to promote bone formation, (iv). FTIR-imaging of the mineralized and non-mineralized regions of the cartilage region of the ACL (variation of proteoglycan content in this noted). Images (i), (ii) and (iii), modified and reprinted with permission from refs [268, 269] copyright Elsevier (2005), (2015). Image (iv) reprinted with permission from PLOS One (2013) [270].

Figure 8. Schematic of the reported multiphasic scaffold comprising phases for ligament, cartilage, and bone regeneration

The ligament phase of this scaffold consisted of braided PLGA scaffold, with phase A consisting of PLGA microspheres to promote formation of non-calcified cartilage, and a minor constituent of bioglass added to PLGA microspheres in Phase B to facilitate calcification of cartilage tissue. Phase C consisted of PLGA microspheres with higher concentration of bioglass than Phase B for the promotion of bone regeneration (b) ligament-cartilage-bone interface in native ACL tissue. Images reprinted with permission from ref [280]. Copyright Elsevier 2015.

Figure 9. A schematic representation of the synovial joint

Articular cartilage is illustrated with three zones (deep, middle, and superficial zones), flattened chondrocytes, synovial membrane and subchondral bone.

Figure 10. Arthrotomic implantation of BioSeed^®-C

(a) BioSeed^®-C was armed in each corner with resorbable threads secured by threefold knots. (b) In every corner of the defect, k-wires were drilled using the inside-out technique. (c) Guiding threads were pulled through the femoral bone using the k-wires, and the knots were guided into the subchondral bone. (d) The knots serve as anchors, seizing the subchondral bone and securely fixing the graft. Images adapted with permission from Kreuz et al. [312]. Copyright Biomed Central 2009.

Fig 11. Physical composition of the knee joint with individual components. Processing techniques and scaffolds prepared from those techniques

(A) Smooth surface of articular surface is shown with middle and deep zones showing some degree of vascularity. Also shown is a layer of fibrocartilage in the center, progressively forming a bony layer culminating with cancellous bone. (B–C) shows biphasic scaffolds, most widely studied scaffolds for engineering osteochondral regeneration. (D–E) As biphasic scaffolds have been found sub-optimal for regenerating interface between the chondral and osteochondral regions, a modified biphasic scaffolds (multiphasic scaffolds) are current choice of scaffolds that has a separate region between chondral and osteochondral regions that modulates fibrocartilage layer. (F) Shown is sample representation of scaffold intended to regenerating bony region. (G) One way to counter the drawbacks of biphasic scaffold is by utilizing a biphasic scaffold with gradient pore size thereby providing optimal growth of appropriate cells in three different regions (cartilage, calcified cartilage, and subchondral bone). (H–I) schematic of 3-D printing and (melt) electrospinning techniques that can be combined or utilized along to fabricate custom-made scaffolds ideal for osteochondral tissue regeneration. (J–I) Morphological and reconstructed image of goat femoral head. (L–N) 3-D scaffolds developed by 3-D printing demonstrating the possibility of utilizing this technique to fabricate scaffolds for osteochondral regeneration. Images 11A and C; 11B,D,E, and F; G; H and I; J–N adapted and reprinted with permission from ref [, –333]. Copyright Springer 2014, Plos One 2014, Elsevier Ltd 2008, 2013.

Fig 11. Physical composition of the knee joint with individual components. Processing techniques and scaffolds prepared from those techniques

(A) Smooth surface of articular surface is shown with middle and deep zones showing some degree of vascularity. Also shown is a layer of fibrocartilage in the center, progressively forming a bony layer culminating with cancellous bone. (B–C) shows biphasic scaffolds, most widely studied scaffolds for engineering osteochondral regeneration. (D–E) As biphasic scaffolds have been found sub-optimal for regenerating interface between the chondral and osteochondral regions, a modified biphasic scaffolds (multiphasic scaffolds) are current choice of scaffolds that has a separate region between chondral and osteochondral regions that modulates fibrocartilage layer. (F) Shown is sample representation of scaffold intended to regenerating bony region. (G) One way to counter the drawbacks of biphasic scaffold is by utilizing a biphasic scaffold with gradient pore size thereby providing optimal growth of appropriate cells in three different regions (cartilage, calcified cartilage, and subchondral bone). (H–I) schematic of 3-D printing and (melt) electrospinning techniques that can be combined or utilized along to fabricate custom-made scaffolds ideal for osteochondral tissue regeneration. (J–I) Morphological and reconstructed image of goat femoral head. (L–N) 3-D scaffolds developed by 3-D printing demonstrating the possibility of utilizing this technique to fabricate scaffolds for osteochondral regeneration. Images 11A and C; 11B,D,E, and F; G; H and I; J–N adapted and reprinted with permission from ref [, –333]. Copyright Springer 2014, Plos One 2014, Elsevier Ltd 2008, 2013.