Chapter 52 Improving Biodegradable Interference Screw Properties by Combining Polymers

Timo Järvelä, Janne T. Nurmi, Antti Paakkala, Anna-Stina Moisala, Auvo Kaikkonen, Markku Järvinen

Introduction

Interference screws are widely used for graft fixation in anterior cruciate ligament (ACL) reconstruction, and good clinical results have been reported by several investigators.¹^–⁵ In addition to conventional metal screws, biodegradable interference screws are commercially available and have been shown to provide at least as strong graft fixation as metal screws.^6,⁷ In addition, the biodegradable screws do not interfere with imaging techniques and do not need to be removed in revision cases because the implants have either degraded or can simply be drilled through. However, although biodegradable materials have been attractive for many years, they have been linked to limitations such as breakage during insertion due to brittleness of the material,^8,⁹ tissue reactions due to poor material quality or too fast or uncontrolled degradation (e.g., polyglycolic acid),^10,¹¹ or too slow degradation offering no real advantage over metal implants (e.g., poly-L-lactic acid implants have been documented to take more than 4 years to degrade).^10,¹²^–¹⁵ It is obvious that as a result of these observations, the material properties have been identified to play a critical role, and manufacturers have thus been challenged to further develop and optimize the chemical compositions of biodegradable implants. Whether the biodegradable interference screws are actually finally replaced by bone or by some other tissue remains controversial.¹⁶^–²⁰ As a matter of fact, according to a recent study by Tecklenburg et al,²¹ even the recently introduced composite screws containing osteoconductive materials such as hydroxyapatite and tricalcium phosphate do not degrade in 2 years in vivo and thus cannot be replaced by bone. This clearly demonstrates the need for more optimal materials that degrade faster but are still controlled enough not to cause any clinically significant inflammatory or foreign body reactions. In addition, the material should be strong enough not to break during screw insertion and should provide adequate fixation strength during the healing period.

A number of biodegradable polymers have been approved for safe internal use and have been used in surgical applications for the past 30 years, initially as suture materials. Each polymer has its material-specific properties, and an implant created from a single type of polymer is naturally limited by those properties. This explains some of the problems observed with the first-generation biodegradable implants. For example, polyglycolic acid (PGA) is strong but very fast to degrade; poly-L-lactic acid (PLLA) is strong but brittle and slow to degrade; whereas trimethylene carbonate (TMC) is rather weak but elastic like rubber. Copolymer blending is a novel manufacturing method developed in an attempt to combine the desired properties of different polymers and, by doing so, to overcome the limitations of the previous biodegradable implants. By blending different copolymers it is possible to create a library of material recipes from which to select those of the appropriate strength, toughness, and degradation to meet specific clinical requirements. A biodegradable interference screw made of degradable copolymers composed of L-lactic acid, D-lactic acid, and TMC (Inion Hexalon, Inion Oy, Tampere, Finland) (Fig. 52-1) was introduced in 2002 and has since been studied both biomechanically and clinically. According to a recent preclinical sheep study, this copolymer blend fully degrades in 2 years in vivo without causing any clinically significant inflammatory, foreign body, or other tissue reactions.²²

Fig. 52-1 The Inion Hexalon biodegradable interference screw.

Biomechanical Results

Fixation Strength

Fixation strength of the ACL graft is commonly considered to be the weakest link of ACL reconstruction. A three-part biomechanical study was carried out to study the fixation strength of the new biodegradable copolymer interference screw (Inion Hexalon) and to evaluate its suitability for ACL reconstruction by comparing it with the previously clinically used interference screws.²³ In the first part, the initial soft tissue graft fixation strength of the copolymer screw was compared with that of a conventional metal interference screw (Acufex Softsilk). In the second part of the study, the initial soft tissue graft fixation strength of the copolymer screw was compared with that of another biodegradable interference screw (Bionx SmartScrew). In the third part of the study, the initial bone–tendon–bone graft fixation strength of the copolymer screw was compared with that of another commercially available biodegradable interference screw (Linvatec Bioscrew).

Tibial bone tunnels were created in fresh skeletally mature porcine cadaver tibiae. A porcine ACL soft tissue graft model previously described and used by Ishibashi et al ²⁴ and Harding et al²⁵ was used in Parts I and II. Porcine patellar tendons were cut approximately 8 cm distal from their patellar insertion and left attached to the patellae. The free end of each patellar tendon was sutured using the running baseball stitch and thereafter fixed into tibial bone tunnel with an interference screw. In Part III, porcine bone–patellar tendon–bone grafts were prepared by obtaining a tibial bone block. The graft end with the tibial bone block was fixed into the tibial bone tunnel, and the maximum screw insertion torque was determined with a digital torquemeter connected to the screwdriver. The patellae were left intact to enable easy and rigid fixation to the mechanical testing machine (Lloyd LR 5K, J.J. Lloyd Instruments). The biomechanical tests were performed strictly according to the previously described single-cycle load-to-failure protocol of Kousa et al.⁷ The specimens were first subjected to a 50N preload for 1 minute. Thereafter, vertical tensile loading parallel to the long axis of the bone tunnel was performed at a rate of 50 mm/min until failure and the yield load, maximum failure load, and mode of failure were determined.

In Part I (N = 13), the average yield loads for the copolymer screw and metal screws were 491 ± 154N and 418 ± 77N, respectively (P = 0.15). The average maximum failure loads were 548 ± 130N and 453 ± 94N, respectively (P = 0.04). Although the average maximum failure load for the biodegradable screw group was significantly higher than that observed for the metal screw group, no significant difference was found in the more clinically relevant yield load values. The mode of failure was almost entirely graft slippage past the screw in both study groups, although also some graft laceration (partial rupture) and “graft stretching” were observed in the metal screw group, mainly at the screw–graft interface. In Part II (N = 8), the average yield load for the copolymer screw was 501 ± 122N and for the SmartScrew, 386 ± 79N (P = 0.05). The average maximum failure loads were 563 ± 109N and 536 ± 128N, respectively (P = 0.65). The mode of failure was graft slippage past the screw in both study groups. In Part III, the average maximum insertion torque for the copolymer screw (N = 8) was 1.9 ± 0.7 Nm; for the Bioscrew (N = 4), 1.5 ± 0.6 Nm (P = 0.32). The average yield loads for the copolymer screw and Bioscrew were 901 ± 262N and 795 ± 524N, respectively (P = 0.77). The average maximum failure loads were 926 ± 259N and 800 ± 516N, respectively (P = 0.72). All tested specimens in Part III failed by bone block pullout. One Bioscrew broke in Part III during insertion. No copolymer screw breakage was observed in this study.

Based on these biomechanical results, the new biodegradable copolymer screw provides initial fixation strength similar to the other previously used biodegradable and conventional metal interference screws.