12.16 Absorbable implants for fracture fixation

When internal fixation is required for the treatment of a fracture, the implants used become unnecessary, and may even be harmful, as soon as there is secure union between the fragments. In contrast with metallic devices, absorbable implants leave no hardware in the tissues. Absorbable implants are radiolucent and do not interfere with magnetic resonance imaging (MRI). Metallic appliances sometimes require removal. In addition, they are associated with some other well-known minor disadvantages, such as excessive rigidity and corrosion. Hence efforts have been made during the past three decades to develop absorbable implants for fracture fixation. In this context, the terms absorbable, resorbable, and biodegradable are used interchangeably. A section on the use of calcium phosphate as bone void filler is also included.

Despite the obvious potential advantages of biodegradable internal fixation devices, such implants did not exist in clinical practice until the mid-1980s, when they were used first in maxillofacial surgery and then in orthopaedic fracture surgery. Once the physicochemical and technical problems associated with the construction and production of the implants were solved, development was rapid.

Many organic macromolecular compounds are degradable and resorbable in living tissues, but few have the chemical and physical properties necessary for an internal fracture fixation device (Box 12.16.1). Most clinical experience to date has been with implants made from polyglycolic acid (PGA), polylactic acid (PLA), and polyparadioxanone, since these were the first materials used in clinical practice. More recently, other compounds, such as glycolide-trimethylene carbonate copolymer, have been used.

High-molecular-weight PGA and PLA are produced by ring-opening polymerization of the corresponding cyclic diester, glycolide, or lactide. Therefore the polymerization products are often called polyglycolide and polylactide respectively. These homopolymers, as well as copolymers of polyglycolide and polylactide in various ratios, have been used for manufacturing internal fixation devices. Because of the asymmetry of the lactic acid molecule, it occurs in two stereoisomeric forms. Consequently, PLA may be modified in stereoisomeric terms into polylevolactic acid (PLLA) and a stereocopolymer of polydextro- and polylevolactic acids (PDLLA). Both PGA and PLA are α-hydroxy polyesters, but PGA is hydrophilic whereas PLA is hydrophobic because of its methyl groups. This difference influences the degradation rate of these polymers.

Because of their thermal behaviour, several of the synthetic biodegradable polymers are difficult to shape into complex designs such as screws and plates. For example, PLLA is relatively brittle and rigid in room temperature. Also, the strict demands on the initial mechanical strength of the devices made by the clinical applications set limits on the geometric shaping of the implants.

Internal fixation implants which are now commercially available for orthopaedic applications include cylindrical pins, rods, screws, plugs, staples, anchors, tacks, arrows, and cords. The screw profiles are best suited for fixation of cancellous bone fragments. Staples, anchors, tacks, and cords are mainly intended to fix or reconstruct ligaments and joint capsules. Small PLLA plates are in clinical use principally in maxillofacial surgery but they are also suitable for certain small-fragment fractures of the extremities. Degradable cages have been developed for spinal surgery.

The degradation of absorbable polymers occurs principally by a simple hydrolytic reaction and, to a lesser extent, through non-specific enzymatic action (Figure 12.16.1), with the main route of final elimination being respiration. Despite these similarities in metabolism, the rates of degradation of different polymers vary greatly. As seen in experimental studies using light microscopy, PGA totally disappears from the tissues within 36 weeks, whereas PLLA still persists after 4.5 years. Also in clinical use, macroscopic remnants of PLLA devices have been found to be present in the ankle after 4 years and in the maxillofacial region after 5 years. The degradation rates of copolymers depend on the ratios of the constituent polymers.

In animal experiments, the polymer is seen to become gradually invaded by connective tissue as the degradation proceeds (Figure 12.16.2). The degree of final restoration of the original tissue architecture within the implant track varies greatly for reasons that are not yet fully understood.

Like any other medical implant, an absorbable fracture fixation implant must be free of infectious, toxic, immunological, teratogenic, and cancerogenic hazards. The results of biocompatibility studies in test animals cannot be directly extrapolated to humans. The only adverse reaction so far recorded for these implants in clinical use is a local transient inflammatory foreign-body reaction. Such a reaction has occurred in 2–25% of the patients depending on the type of implant used and the fracture site operated on. These reactions are discussed in detail later in this chapter.

The mechanical properties of absorbable implants must be discussed in terms of initial strength, strength retention during degradation, and elasticity. The initial strength is influenced more by the manufacturing technique of the implant than by the polymer utilized. Simple melt-moulding or extrusion of synthetic biodegradable polymers into pins results in weaker implants than when certain special manufacturing techniques are used. A reinforcing technique using high pressures and temperatures gives high-strength composite implants with fibres embedded within a matrix of the same biodegradable polymer. The initial flexural strength of a 4.5-mm diameter PLLA pin manufactured by a fibre-reinforcing technique is 245MPa. On the whole, the mechanical properties of absorbable materials are very different from those of stainless steel. Indeed, a direct comparison seems meaningless, since absorbable implants were not developed to mimic metallic ones.

The fixation properties of several types of absorbable implants have been tested under conditions simulating fractures in humans. In a study on the distal radius, satisfactory fixation was reported with PLA rods of diameter 2.7mm. A PLLA screw of diameter 6.3mm was found to be as good as a conventional metal screw for fixing a bone–patellar-tendon–bone graft for the anterior cruciate ligament in a bovine experimental model.

The strength retention of an absorbable implant is determined by its degradation rate. As already discussed earlier, the degradation rate is influenced by the micro- and macrostructural properties of the implant as well as by environmental factors. A rough estimate is that half of the initial bending and shear strengths is lost within 12 weeks when the implants are made of PLLA.

Some of the mechanical properties of biodegradable implants can be expressed in terms of Young’s modulus of elasticity. The modulus of elasticity of absorbable polymers is much less than that of stainless steel. Rather, it is close to that of cortical bone and only slightly higher than that of cancellous bone. The ultimate mode of failure of an implant can be ductile or brittle.

In the upper extremity, absorbable implants have been used in the internal fixation of small-fragment fractures from the lateral clavicle to the phalanges. Other applications have been reported in addition to fractures (Box 12.16.2). Absorbable tacks, staples and anchors are used in reconstructive procedures on the ligaments around the shoulder joint.

The most common clinical applications for absorbable internal fixation devices in the upper extremity are displaced fractures of the humeral capitellum, the olecranon, the radial head, and the metacarpal bones. Exact reduction is necessary to secure the fixation when pins are used (Figures 12.16.3 and 12.16.4). The pins are inserted through the articular surfaces of the humeral capitellum (Figure 12.16.3) and the radial head (Figures 12.16.5 and 12.16.6). In fractures of the olecranon screws may also be used. Absorbable pins are used in the fixation of displaced fractures of the distal part of the radius in countries where percutaneous pinning is a popular method of treatment of these fractures. Unlike K-wires, absorbable pins cannot be mounted on a chuck and be directly driven through bone fragments; a hole of the appropriate diameter must first be drilled for the implant. As a rule, postoperative plaster cast immobilization is used. In fractures of the metacarpal bones, small plates can be used.

The spectrum of the clinical applications of absorbable implants in the lower extremity (Box 12.16.3) is as broad as that in the upper extremity. With the exception of diaphyseal fractures of the femur and tibia, absorbable implants have been used in the internal fixation of almost all kinds of fracture occurring in the lower extremity.

The most common fracture type for which biodegradable implants have been used is displaced malleolar fracture of the ankle (Figure 12.16.7). The operative approach in ankle fractures, including disruptions of the syndesmosis, is similar to that used with metallic screws. However, a torque-limiting screwdriver is required to decrease the risk of screw breakage during insertion. After fixation, protruding screw heads can easily be cut off with an oscillating saw. Postoperative plaster cast immobilization has been used in the majority of the clinical studies reported.

Absorbable implants are used in many non-traumatic orthopaedic disorders in the lower extremity (Table 12.16.3). The two most common of these are the use of biodegradable interference screws to secure a tendon graft in reconstructive surgery of the anterior cruciate ligament, and the fixation of a torn meniscus by using a small absorbable arrow. Fixation of an osteotomy of the first metatarsal bone for hallux valgus (Figure 12.16.8) has been one of the more popular applications of absorbable implants.

The psychological advantages of avoiding implant removal procedures would seem to be of particular value in children. Indeed, absorbable pins have been used in the fixation of many kinds of displaced small-fragment fractures in children (Tables 12.16.2 and 12.16.3). Experimental studies have shown that as long as absorbable implants piercing the growth plate occupy 3% or less of the cross-section of the plate, no growth disturbance will occur. If this limit is observed, transphyseal insertion of absorbable pins through growth plates should be safe. A transphyseal fixation is not necessary in all fracture types in children that require internal fixation, since in many the fixation can be done proximally or distally to the growth plate. The most convenient way of inserting small polymeric pins of diameter 1.1–2.0mm is first to fix the fracture temporarily with one or two K-wires and then to replace the wires one after another with an absorbable pin (Figure 12.16.9). Of course, multiple tentative K-wire drillings through the physeal plates should be avoided. Transphyseal fixations should preferably be done using implants with a shorter degradation time than those made of PLLA. Pins made of PGA or polyparadioxanone are more suitable. The diameter must be chosen according to the size of the fragments and the estimated area of the growth plate.

The future applications of biodegradable implants will be influenced by the development of the devices, the inventiveness of orthopaedic surgeons, and the accumulating experience of the advantages and disadvantages of the implants. An expanding field is the use of absorbable implants in many kinds of arthroscopic procedures of the shoulder and the knee. Degradable cages for fusion procedures of the spine are under development.

The most obvious advantage of absorbable implants is that no removal procedure has to be considered after fracture union, osteotomy, or arthrodesis.

The fact that these implants are radiolucent and do not interfere with MRI is of particular value in applications at the shoulder and the knee joints. With regard to operative techniques, an advantage of absorbable pins is that they can be inserted through articular cartilage and left in place with less concern than when using metallic devices. However, because of their long degradation time, PLLA implants should not be left protruding in joint cavities.

A disadvantage of biodegradable implants is their loss of mechanical strength with time which makes the use of postoperative plaster immobilization advisable in many fracture types in order to minimize the risk of resdisplacement. Disadvantages that will probably resolve with time include the current limited assortment and the price of absorbable fracture fixation devices. At the time of writing, the cost of an absorbable screw is approximately five to eight times that of a metallic screw.

In labral and capsular stabilization procedures of the glenohumeral joint, the results of the use of absorbable tacks seem promising. Among the fractures in the upper extremity, a clear contraindication appears to be displaced fracture of the supracondylar humerus in adults, which is a fracture type with mechanical demands that are too high for the absorbable devices.

In contrast, displaced small-fragment fractures at the elbow joint seem to be a rewarding field for absorbable implants. Good results have been presented in fractures of the humeral capitellum and of the distal radius, the mechanical reliability of PGA rods was found to be good in a randomized study. Displaced fractures of the metacarpal bones can be successfully managed by small PLLA plates.

Acetabular osteotomies performed for dysplasia have been successfully fixed using PLA screws in a series of 28 patients. The fixation of osteochondral flake fractures as well as fragments loosened by osteochondritis dissecans in the knee joint are established and generally accepted indications for absorbable implants. Absorbable interference screws are also widely used to fix the graft in reconstruction of the anterior cruciate ligament. However, the synovial tissues are at risk of developing aseptic inflammatory reactions to absorbable polymers.

The literature on displaced malleolar fractures of the ankle includes more patients than any other application. A fracture redisplacement requiring reoperation has occurred in approximately 1%. In two randomized studies comparing absorbable pins and screws with metallic implants no differences were found in the results of treatment.

Results have been satisfactory in fixation of osteotomies of the first metatarsal bone for hallux valgus as well as in fusion of the first metatarsophalangeal joint in rheumatoid arthritis.

Among a variety of displaced fractures in 71 children with a mean age of 9.8 years treated using PGA pins, mechanical failure and severe redisplacement occurred in three out of 14 supracondylar fractures of the humerus. In another study of 50 different fractures, two cases with an osteolytic non-union of the radial head were observed. In a randomized study of 24 children with displaced fractures at the elbow joint, small-diameter PGA pins were found to be as effective as K-wires for fixation. This study did not include patients with supracondylar fractures of the humerus. Growth disturbances have not been reported.

Owing to the ductile mode of failure of fibre-reinforced absorbable implants, they lose their shear strength more slowly than their bending strength. Therefore angulation rather than lateral displacement between fixed fracture fragments is likely to occur when moderate overloading occurs.

An aseptic inflammatory reaction to biodegradable polymers occurs. The reaction presents clinically as a local painful fluctuant erythematous swelling approximately 0.5cm in diameter. Unless promptly aspirated or drained, it results in a discharging sinus. The bacterial cultures are negative but a secondary infection may ensue. The reactions occur in the final liquefaction phase of the degradation process of the polymer in question. Accordingly, they are seen on average 12 weeks postoperatively with implants made of PGA, but after 2–5 years with PLLA implants. The reactions usually subside within 4 weeks but occasionally show a protracted course. In biopsy specimens the lesions have the histopathological characteristics of a non-specific foreign-body reaction. Polymeric debris is seen lying intracellularly within abundant phagocytically active macrophages. Osteolytic changes may be seen on plain radiographs.

The incidence for PLA (<2%) seems to be lower than for PGA, at least as far as screws and ankle fractures are concerned. In contrast, use of PLA plates to fix mandibular fractures has resulted in a considerably higher rate of local inflammatory reactions (up to 40%).

It is often difficult to achieve stable fixation and retention of a comminuted cancellous bone fracture associated with a significant metaphyseal defect. Augmentation by using calcium phosphate synthetic substitutes may then be helpful. Since these materials are resorbable, they are included in this chapter, although implants made of calcium phosphate actually are not fracture fixation devices. Calcium phosphate is available in a variety of different forms, including ceramic blocks, granules, powders and cements. Such devices provide an osteoconductive matrix for host osteogenic cells, but they are not osteoinductive unless specific osteoinductive substances are added. Calcium phosphate bone void filler appliances are relatively brittle and have little tensile strength. Their rate of integration depends on their crystalline size and stoichiometry.

One of the available resorbable ceramics is tricalcium phosphate. Another calcium phosphate-based bone substitute, synthetic coralline or cancellous hydroxyapatite is manufactured as a ceramic through a sintering process. Hydroxyapatite is not commonly used alone as an osteoconductive bone substitute because of its slow resorption and high brittleness. Tricalcium phosphate is less brittle and has a faster resorption. It has not yet been possible to exactly determine the resorption rate of calcium phosphate in humans. The resorption process occurs by dissolution and osteoclast activity.

Calcium phosphate can be manufactured as a cement by adding an aqueous solution to dissolve the calcium. An advantage of cements over blocks or granules is the ability to custom-fill the metaphyseal bone defects. Injectable cement can, however, be extruded beyond the boundaries of the defect, thus potentially disturbing or even damaging the surrounding tissues. If calcium phosphate migrates into a joint, it will not dissolve and become resorbed.

The ability of calcium phosphate bone substitutes to act as bone void filler in cancellous bone has been documented experimentally and clinically.

When judiciously used, absorbable implants provide a valuable extension of current internal fixation devices. In certain intra-articular fracture types they are already superior to metallic fixation devices. Avoiding hardware removals may provide economic benefits. So far, a methodical clinical use of absorbable implants has a history of only two decades. Although absorbable implants never will become universal for fracture fixation, these devices will undoubtedly be used in those specific clinical applications, in which they possess clear advantages over metallic devices.