Evolution of plate design and material composition

S8 Injury, Int. J. Care Injured 49S1 (2018) S8–S11 Volume 49 Supplement 1 June 2018 ISSN 0020-1383

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Injury Plating of Fractures: current treatments and complications Guest Editors: Peter Augat and Sune Larsson

j o u r n a l h o m e p a g e : w w w. e l s e v i e r . c o m / l o c a t e / i n j u r y

Evolution of plate design and material composition David J. Haka,*, Rodrigo Banegasa, Kyros Ipaktchia, Cyril Mauffreya a

Denver Health, University of Colorado, Denver, Colorado, USA

K E Y W O R D S

A B S T R A C T

Internal fixation Compression plate Non-locking plate Locking plate Carbon fibre plate

The evolution of plate fixation of fracture was accompanied by advances in metallurgy and improvement in understanding of the requirements for successful fracture healing. Locked internal fixation minimizes biologic damage and when used in conjunction with minimally invasive approaches may optimize fracture healing. Some current metal locked plate constructs may actually be too stiff, and various methods including screw modification, plate hole modification, and changes in plate material composition may provide a solution to optimize fracture healing. This paper reviews the evolution of plate design and describes the early clinical experience with the use of carbon fibre reinforced reinforced polyetheretherketone composite plates. © 2018 Elsevier Ltd. All rights reserved.

Evolution of plate design Open reduction and internal fixation using plates was popularized by the Arbeitsgemeinschaft fur Osteosynthesefragen (AO) group and gained widespread acceptance for the operative fixation of fractures and osteotomies. Their initial plate contained round screw holes and fracture site compression or axial loading was achieved using an external compression device. In 1965 the AO introduced the dynamic compression plate (DCP). The sides of the screw holes in this plate are inclined, and when a screw is inserted eccentrically into the hole, the screw head impacts the angled side causing movement of the plate leading to compression of the fracture. Conventional open reduction and internal fixation usually requires wide surgical exposure to access and directly visualize the fracture. It requires pre-contouring of the plate to match the surface anatomy of the bone. Fixation stability with a conventional non-locked plate relies upon the force of friction between the plate and the bone (Fig. 1). Stability is dependent on achieving and maintaining an adequate frictional force, which can be challenging in osteoporotic bone or in situations where there is delayed healing. Loosening of screws, along with loss of adequate friction between the plate and bone, can lead to fixation failure and nonunion. One disadvantage of conventional plate fixation is the damage to the periosteum beneath the plate. This initially produces necrosis beneath the plate and with time results in localized osteopenia [1]. Focus on minimizing the periosteal damage lead the AO to develop the Limited Contact-Dynamic Compression Plate, which was introduced in 1990, that has an undercut surface compared to the smooth surface of the original Dynamic Compression Plate

* Corresponding author at: Department of Orthopedic Surgery, Denver Health Medical Center, 777 Bannock Street, MC 0188, Denver, Colorado, USA E-mail address: [email protected] (David J. Hak). 0020-1383/© 2018 Elsevier Ltd. All rights reserved.

[2]. Further decrease in the plate “foot print” was achieved with the Point Contact Fixator (PC-Fix) [3] (Fig. 2). The PC-Fix, which was the forerunner of today’s locking plate implants, was designed for fixation of forearm fractures and has small points on the undersurface to limit the plate contact with bone [4]. The screws for this implant were self-tapping and designed to engage only the near cortex so are available in only one length. The screws, like todays locking implant screws, thread into the reciprocal threaded plate holes. The use of monocortical locking screws was continued with the introduction of the Less Invasive Stabilization System (LISS). The LISS plate, which is anatomically shaped, is designed for fixation of distal femur and proximal tibia fractures [5]. Further advancements lead to the development of the Locking Compression Plate (LCP) which was released for clinical application in March 2000 [6]. Locking plates use screws that have threads on the screw head that engage matching threads in the plate holes, creating a fixed angle implant. Stability is achieved by the engagement of the locking screws in the plate and does not rely on compression of the plate to the bone as in conventional plate fixation (Fig. 3). Locking plates offer several advantages over conventional non-locking plates including improved fixation in osteoporotic bone. Locked plates are commonly used in a “bridge plate” function, preserving periosteal and soft tissue blood supply and providing fixed angle stability. Unlike conventional non-locking plates they do not require exact plate contouring to match the bony contours since the plate does not need to sit directly on the bone surface [7]. The use of minimally invasive surgical techniques became popularized simultaneously with the widespread adoption of locking plating techniques [8]. Initially, locking screws were designed to be inserted along a set axis in order to properly engage the plate thread locking mechanism. Locking screws inserted off-axis in these systems showed a significant decrease of failure load [9]. Correct placement of fixed angle locking screws required the use of a drill sleeve correctly fixed

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Fig. 1. Fixation stability with a conventional non-locked plate relies upon the force of friction (green arrows) between the plate and bone.

Fig. 2. Cortical contact area (red) of original AO dynamic compression plate (top), AO limited contact dynamic compression plate (middle), and point contact fixator (bottom).

in the threads of the plate hole. Without the proper use of the drill sleeve, the correct screw insertion angle could not be maintained. In certain complex fracture patterns, fragment specific fixation may require orientation of a screw different than that directed by a fixed locking screw axis. Manufacturers developed various alternative locking mechanisms, including newer plate designs that enable more options to strategically place the locking screws within a variable range of axes [10]. The different “polyaxial” locking interface designs that have been developed include those based on tight fit, and frictional connection or a thread in circular lip connection [11]. In general, these systems allow inclination of the screw insertion angle up to 15°, while maintaining a locking strength equivalent to fixed angle locking screws inserted with 0° inclination. Metal plate composition Metal has long been the foundation for orthopedic implants. Metal implants offer the benefits of high strength, high stiffness, ease of machining, and low cost. Additionally, many metals offer good ductility allowing them to be manually bent or contoured intra-operatively to fit individual fracture sites. The use of stainless steel for surgical applications began in 1926 when Strauss patented 18Cr-8Ni stainless steel that contains 2–4% molybdenum and a very low percentage of carbon, having sufficient corrosion resistance for implantation in the human body [12]. Stainless steel became the most frequently used metal for internal fixation devices because of its favorable combination of mechanical properties, corrosion resistance and cost effectiveness compared to other metallic implant materials. The AO group began exploring the use of pure titanium for plate fixation in the late 1960s. Commercially available pure titanium contains varying traces of iron, oxygen, nitrogen, carbon and hydrogen. There are different grades of pure titanium based on the amount of these trace elements, which influence its mechanical properties. In addition to pure titanium, titanium alloys are also used for internal fixation plates. Ti-6AI-4V contains a nominal

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Fig. 3. Fixations stability with a locked plate is achieved by the engagement of the locking screws in the reciprocal threaded plate holes (green arrows) and there is no compression of the plate against bone.

6% aluminum and 4% vanadium. The addition of aluminum and vanadium to commercially pure titanium produces an alloy whose mechanical properties are closer to that of cold-worked stainless steel. Ti-6AL-7Nb which is an alloy containing 6% aluminum and 7% niobium was developed as an alternative to Ti-6AI-4V because of concerns that vanadium has demonstrated cytotoxic outcomes when isolated [13]. Titanium offers several advantages over stainless steel. Compared to stainless steel, titanium and titanium alloys better match the modulus of elasticity of bone. Titanium has greater superior strength under repeated load stresses, making it capable of withstanding higher strains during internal fixation. It is also considered more biocompatible, with excellent corrosion resistance and chemical inertness. The excellent corrosion resistance of titanium and titanium alloys is due to the formation of an adhesive TiO2 oxide layer on their surface. One disadvantage of titanium fracture fixation plates is the problem of cold-welding seen with the removal of locking screw constructs [14,15]. Disadvantages of metal plates Advances in metallurgy, including addition of various surface coatings, have been beneficial in improving orthopedic care. However, disadvantages of metal implants include a limited fatigue life, mismatch of modulus of elasticity with bone leading to stress shielding, potential for generation of wear debris, corrosion, and their radiodensity that can preclude accurate radiographic visualization. While stiff metal plates worked well for direct fracture reductions in which the load was shared by bone, they may not be optimal for certain cases of indirect reduction with relative stability in which you want more flexible fixation to stimulate callus formation. A 20% rate of nonunion was reported in a retrospective review of 86 distal femur fractures treated with locked metal plate fixation. Limited callus formation in these cases suggest that mechanical factors may play a role in the failure of fracture healing [16]. Several strategies to reduce the stiffness of locked-plate constructs have been proposed. One strategy is that of far cortical locking in which locking screws that engage the far cortex of bone have a reduced mid-shaft diameter to bypass the near cortex, allowing for elastic cantilever bending of the screw shaft within the near cortex [17]. A prospective and observational study of 32 consecutive patients with 33 distal femur fractures treated by plate fixation with far cortical locking screws were followed up for a minimum of 1 year with functional and radiographic assessments [18]. Thirty-one fractures were available for follow-up. Thirty of the 31 fractures healed at an average of 15.6 weeks. There were no cases of hardware failure. Two patients underwent revision surgery, one to correct a malrotation at day 5 and one to treat a nonunion at 6 months. The investigators concluded that the absence of implant and fixation failure suggests that dynamic plating of distal femur fractures with far cortical locking screws provides safe and effective fixation.

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Another strategy to improve fracture healing is the concept of a dynamic plate in which the locking holes are elastically suspended within the plate by means of a silicone envelope that controls the amount of permissible axial motion [19]. In a sheep osteotomy model, animals treated with this implant showed six times more callus area by week 9 (P<0.001) compared to those treated with standard compression plates and were over twice as strong as the standard compression plate group recovering on average 64% of their native strength by week 9 (P=0.008). Alternatives to metal plates Carbon fibre reinforced epoxy resin plates were initially studied in both animal models and humans in the early 1980s [20]. The investigators aim in exploring this implant material was to eliminate the known problems of stress shielding produced by metal implants and to provide semi-rigid fixation that permits normal use of the limb but with sufficient flexibility to allow movement at the fracture site, and hence promote rapid bony union by the development of external bridging callus. The clinical study included 20 patients (average age 21 years) that sustained closed displaced transverse or short oblique mid-shaft tibial fractures. The first 7 cases were treated with a carbon fibre reinforced epoxy resin plate designed with a DCP style plate hole. Three of these patients complained of aching pain and one developed a hypertrophic nonunion. The investigators felt that the DCP hole weakened the plate so the next 13 were treated with a stiffer round hole plate and all of these went on to union. Three patients developed deep infections but still healed their fracture. All plates were removed between 27 and 57 weeks following surgery, and none had developed osteopenia allowing all patients to return immediately to their normal life style after plate removal. Disadvantages of these plates were their cost, several times that of stainless steel, and the fact that they could not be contoured to bone limiting their use in some anatomical situations. This same group of investigators reported on their use of this semi-rigid plate in 19 more challenging fractures including open fractures and fractures complicated by either infection, nonunion, or comminution [21]. Radiological union with satisfactory clinical results was achieved in 18 of 19 patients by 40 weeks postoperatively. By the late 1990s, polyetheretherketone (PEEK) had emerged as the leading high-performance thermoplastic candidate for replacing metal implant components, especially in orthopedics and trauma. PEEK intervertebral spinal cage implants have been in use since 1999. Carbon fibre reinforced polyetheretherketone (CF-PEEK) is a composite material that is composed of carbon fibres within a polymer matrix. This material has very high fatigue strength and is even more flexible than titanium, better matching the modulus of elasticity of bone. CF-PEEK has greater strength and fatigue properties than stainless steel implants. The modulus of elastic of PEEK is approximately 3.5 GPa, which better matches that of cortical (12–20 GPa) and cancellous bone (while variable, roughly 1 GPa). In contrast, titanium has a modulus of elasticity of 100–110 GPA. Modulus mismatch of metal implants can lead to altered loading, stress shielding, and detrimental periprosthetic bone remodeling [22]. Another important benefit of PEEK and other carbon fibre reinforced materials is its ability to permit improved radiographic assessment of bone healing post-operatively. The material is radiolucent allowing improved standard radiographic imaging to assess healing. The lack of artifact also permits unhindered CT and MRI imaging [23]. Carbofix Orthopaedics (Herzeliya, Israel) initially developed carbon fibre reinforced PEEK (CFR-PEEK) plates for use in the proximal humerus and distal radius, along with a straight plate for use in the diaphysis (Fig. 4). Subsequently they have developed

Fig. 4. A proximal humerus fracture treated with a carbon-fibre reinforced PEEK plate. Radiographic metallic wires within the implant indicate the borders of the plate.

carbon fibre reinforced PEEK plates for use in the distal fibula and distal femur. Lima (Udina, Italy) has developed carbon fibre reinforced PEEK plates for use in the proximal humerus and distal radius. Clinical experience with CFR-PEEK plates In a multicenter study, 182 patients with proximal humeral fractures were treated with a CFR-PEEK Plate (Diphos H; Lima Corporate, Udine, Italy), and 160 patients were followed clinically and radiographically for 2 years or longer [24]. At 2 years postoperatively the mean Constant score was 76 and mean DASH score was 28 suggesting that the cohort experiences some residual upper extremity disability. A total of 23 (14%) complications occurred including 2 nonunions (one septic and one aseptic), 13 cases of humeral head necrosis (9 partial and 4 massive), and 8 cases of screw penetration of the humeral head. The investigators concluded that the CFR-PEEK plates proved as reliable as metal plates in the treatment of proximal humeral fractures. They noted the advantages of better visualization of fracture reduction during intraoperative fluoroscopic assessment and easy hardware removal due to the absence of screw-plate cold fusion. In another study twenty-nine patients (mean age, 66 years) with a 3- or 4-part proximal humerus fracture were treated with a CFRPEEK plate (DiPhos-H; Lima Corporate, Udine, Italy) and followed prospectively [25]. Their results were compared with a matched group of patients treated with a conventional locked plate. Patients treated with the CFR-PEEK plate achieved significantly better results with regard to their Constant-Murley score and Oxford shoulder score (P<0.05) compared to those treated with a locked metal plate. The investigators concluded that fixation of proximal humerus fractures with a CFR-PEEK plate provides satisfying clinical and radiographic results after 2 years of follow-up, and that the results are comparable to those achieved with conventional locked metal plates. In a matched case-control study, the results at 1 year follow-up of 21 patients (mean age, 66.8±9.9 years) with displaced proximal humeral fractures treated with a carbon fibre reinforced PEEK plate (PEEK Power Humeral Fracture Plate; Arthrex, Naples, FL, USA) were compared with the results of 21 patients (mean age,

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67.4±9.7 years) treated with a conventional titanium locked plating [26]. While there were no significant differences in the functional outcomes between patients treated with the CF-PEEK plate and patients treated with titanium plates, patients treated with titanium plates were significantly more likely to require revision surgery related to articular screw perforations (P=0.048). The investigators concluded that treatment with the CFR-PEEK plate resulted in good to excellent 1-year functional outcomes which were similar to outcomes of conventional titanium plates, but that the stiffer locked titanium plates were associated with a higher risk of articular screw perforations than the more elastic CFR-PEEK plate. Investigators reported on 40 AO type B and C distal radius fractures treated with a CFR-PEEK volar plate (DiPHOS-RM; Lima Udine, Italy). All fractures healed, with radiographic union at an average of 6 weeks [27]. There were no cases of hardware breakage or loss of the reduction at 12 months follow-up. The final Disabilities of Arm, Shoulder and Hand score was 6.0 points, and the average grip strength was 92 % of the contralateral limb. The investigators concluded that at early follow-up the CFR-PEEK plate showed good clinical results and allowed maintenance of reduction in complex distal radius fractures. While the early clinical results of CFR-PEEK plates are promising, further clinical research will be required to identify the actual benefits and preferred indications for the use CFR-PEEK implants in fracture repair. Their future use may be especially advantageous for fractures of the distal femur, since certain current plate fixation constructs may be too stiff to promote callus formation [16]. Conclusion There have been many innovations in plate fixation of fractures during the past several decades, all aiming to improve fracture healing and decrease complications. How best to achieve the ideal plate fixation construct remains an unanswered question, in part due to the variability and complexity of individual fractures which each have a unique pattern. Ongoing basic science and clinical research will further assess these innovations as we strive to optimize techniques for plate fixation of fractures. Disclosure The authors’ institution has received a research grant from Carbofix for a clinical study of their implants. DJH has served as a paid consultant for Invibio, which produces PEEK. Acknowledgment The authors of this manuscript express their thanks to the Osteosynthesis and Trauma Care Foundation for the sponsorship of the publication of this Supplement in Injury. References [1] Perren SM, Cordey J, Rahn BA, Gautier E, Schneider E. Early temporary porosis of bone induced by internal fixation implants: a reaction to necrosis, not to stress protection. Clin Orthop 1988;232:139–151. [2] Perren SM, Klaue K, Pohler 0, Predieri M, Steinemann S,Gautier E.The limited contact dynamic compression plate (LC-DCP). Arch Orthop Trauma Surg 1990;109:304–310.

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