**1. Introduction**

Fractured bones require stabilisation to allow healing to occur. For complex fractures, and in anatomical areas not conducive to external fixation, internal fracture fixation plates are required. Currently, titanium alloys are the most commonly used materials for these plates due to their strength and stiffness being high enough to protect the damaged bone during healing. However, once healing is complete, these high mechanical properties can become problematic, in particular, disrupting the normal remodelling processes of the bone, leading to a reduction in bone quality under the plates, termed stress shielding [1,2]. In fact, stress shielding can become so sufficiently severe that a significant number of revision operations are required annually to remove the fracture plates—along with the associated costs to health care funders and health risks for patients [3].

To reduce the incidence of post-operative stress-shielding research has focused on developing alternative materials for use in fracture plates. In particular, attractive materials are polymers that dissolve, or resorb, in aqueous environments, such as those based on polylactic acid (PLA) and polyglycolic acid (PGA) [4]. This resorption behaviour offers the potential advantage of the fracture plate disappearing once healing is complete. By combining PLA and PGA as copolymers or blends, and by controlling the concentrations of each component, materials with a range of mechanical properties have been developed, with differences in dissolution times, ranging from months to years [5]. Consequently, fracture plates have been developed that are suitable for use in a number of anatomical areas (e.g., the craniofacial region). However, the inferior mechanical properties of these

**Citation:** Naseem, R.; Tzivelekis, C.; German, M.J.; Gentile, P.; Ferreira, A.M.; Dalgarno, K. Strategies for Enhancing Polyester-Based Materials for Bone Fixation Applications. *Molecules* **2021**, *26*, 992. https:// doi.org/10.3390/molecules26040992

Academic Editor: Marek Brzezi ´nski

Received: 21 December 2020 Accepted: 9 February 2021 Published: 13 February 2021

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polymers, in respect to titanium alloys and the bone means that applications for these polymeric fracture plates are limited [4]. The plates often need to be much thicker than Ti-alloy plates designed for similar applications due to the lower stiffness of the polymer plates being insufficient to protect the healing bone without the increased polymer bulk. Additionally, these plates have several (potential) post-operative complications (e.g., acid degradation products can cause inflammation) [6–9]. Considerable research has been conducted to improve these polymers, to extend the range of clinical applications of these materials and reduce the incidence of post-operative complications.

This review describes the properties of PLA and PGA homopolymers, their copolymers, and blends, together with a review of other polymers that have been added to improve properties. It also describes the methods used to improve the fracture plate's mechanical and physical properties, by producing composite materials, which are often made using the addition of glasses and ceramics. Further, we discuss attempts to improve the biocompatibility of these materials by adding bioactive components or functionalisation of already existing components of the polymer glass composites.
