Improving Biodegradable Interference Screw Properties by Combining Polymers

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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.

Torsional Strength

Screw breakage due to applied torsional forces during screw insertion rather than postoperative failure of graft fixation is the most common failure mode of biodegradable interference screws. The torsional strength of the interference screw is largely determined by the design of the screwdriver recess (socket) and the material of the screw. To test the torsional strength of the new biodegradable copolymer screw, a torsional strength study was performed according to the testing protocol of Costi et al.^8,²⁶ Six 7- × 20-mm copolymer interference screws (Inion Hexalon) were mounted in a 10-mm layer of polyurethane resin, leaving the proximal 10 mm of the screws unembedded. This mounting reproduced the failure scenario observed in vivo, in which only part of the screw length has been inserted and becomes jammed in bone. Torque was applied manually with a digital electronic torque meter (Torqueleader TSD 350, MHH Engineering) mounted on the screwdriver. The same person applied torque in all cases in an attempt to provide a constant rate of application as well as compression on the screw. Care was taken to ensure that the application of torque was performed without associated bending or excessive compression. The maximum insertion torque was recorded, and the mode of failure was visually observed. In addition, to further investigate the failure of the screw, one screw was fixed into the 7- × 20-mm screw cavity of the injection mold and torque was applied manually with a presettable torque wrench until failure.

A desirable outcome of screw advancement through the polyurethane resin, rather than a failure of the screw or instrument, occurred with all test samples. The mean maximum insertion torque measured during screw penetration into the resin was 2.4 ± 0.3 Nm. When the screw was fixed into the injection mold, no failure was observed at torque values between 0 and 5 Nm. When clinically irrelevant torque of more than 5 Nm was applied, the screwdriver shaft failed by rotational bending approximately 20 mm from the tip of the driver.

Costi et al ⁸ previously tested 12 different biodegradable interference screws using the same protocol. In their study, the only screws observed to continue screwing into the resin with no subsequent failure were the majority of the 7-mm PLLA Linvatec Bioscrews. In our study, all tested Inion Hexalon copolymer screws could be advanced through the resin without failure. In our additional test in which the screw was fixed into its injection mold to determine the ultimate failure point, the failure occurred first after a torque of more than 5 Nm was applied, again not by screw breakage but by bending of the metallic screwdriver shaft. Based on the previous observations made by Costi et al,⁸ this failure torque is above the clinically relevant insertion torques and the failure torques of most commercially available biodegradable interference screws.

Strength Retention

To investigate the effect of hydrolytic degradation on the mechanical properties of the Inion Hexalon copolymer screws over time, screw compression tests were performed after 24 hours and 4, 8, and 12 weeks of incubation of 6- × 20-mm and 7- × 20-mm screws in phosphate buffer solution at 37° C (N = 4/time point).²⁷ In the compression test, each screw was set flat between the compression plates and loaded with a constant speed of 5 mm/min until failure (Zwick Z020, Zwick GmbH, Ulm, Germany). In the compression test, both screws retained more than 80% of their initial mechanical strength as long as 12 weeks.

Clinical Results

Clinical Experience

In our clinical work, we have used these new biodegradable interference screws made of degradable copolymers composed of L-lactic acid, D-lactic acid, and TMC (Inion Hexalon) for ACL reconstruction for more than 4 years. During this period, more than 400 of these screws have been inserted to patients, and only one screw breakage has occurred during screw insertion. In this particular case, the screwdriver broke first, which was the reason for the screw breakage. These screws can be used both with single-bundle and double-bundle technique when performing ACL reconstruction.^28,²⁹

Prospective Randomized Clinical Trial

We have done a prospective randomized clinical trial using either biodegradable screw or metallic screw in fixation of the ACL reconstruction with a hamstring autograft.²⁹ In this study, 55 patients were randomized to either metallic interference screw (Timoni, Finland) (N = 26) or biodegradable screw fixation (Inion Hexalon) (N = 29) in ACL reconstruction with hamstring tendons. The evaluation methods were clinical examination, KT-1000 arthrometer (MEDmetric Corporation, San Diego, CA) measurements,³⁰ radiographic evaluation, MRI, and International Knee Documentation Committee (IKDC)³¹ as well as Lysholm³² knee scores. There were no differences between the study groups preoperatively. For the minimum of 1-year follow-up (range 12–19 months), 23 patients of the metallic interference screw group and 26 patients of the biodegradable screw group were available (90%). The evaluation methods disclosed no statistical differences between the groups at the follow-up examinations. However, the results were significantly better at the follow-up than preoperatively, in both groups. Kaeding et al³³ have reported similar results in their prospective randomized study comparing biodegradable and titanium interference screw in fixation of the bone–patellar tendon–bone autograft for the ACL reconstruction.

During the follow-up of our study, three revision ACL reconstructions had to be performed (two in the biodegradable screw group and one in the metallic screw group) because of new knee trauma. No other complications were found with these patients. The revision in the biodegradable screw group 8 months after the primary operation showed that the biodegradable screw was already soft. The other revision performed 18 months after primary surgery showed that the biodegradable screw was almost totally absorbed. The revision ACL reconstructions with these patients were easy to perform because we did not have to remove the screws at all. In the case in which the screw was soft but not totally absorbed yet, we simply drilled through it and created a 1 mm wider tunnel for the new graft. In addition, with another patient, the second-look arthroscopy showed that the biodegradable screw was totally absorbed 2 years after the primary operation (Fig. 52-2).

Fig. 52-2 A, The Inion Hexalon biodegradable interference screw at the operation during the screw insertion into the femoral tunnel. B, The same knee 2 years after the anterior cruciate ligament (ACL) reconstruction. The screw has totally absorbed, and the ACL graft is intact. However, there is some tunnel enlargement in front of the graft.

Magnetic Resonance Imaging

Sixteen patients (10 patients in the biodegradable screw group and six in the metallic screw group) of our prospective randomized study have been evaluated by MRI examination at a mean follow-up of 27 months (range 24–31 months). According to this evaluation, we have found that all the biodegradable screws (Inion Hexalon) were absorbed totally at the follow-up (Fig. 52-3). The MRI images appear to show that the bone tunnels are filled with fibrous tissue with signal intensity similar to that of the intraarticular ACL graft. However, because no histological analysis could be carried out in these human patients, no final conclusions regarding the tissue type that finally replaces the screw can be drawn at this point. The follow-up of these patients has been planned to continue for a minimum of 5 years postoperatively.

Fig. 52-3 A, Magnetic resonance imaging (MRI) changes 4 months after anterior cruciate ligament (ACL) reconstruction with the Inion Hexalon biodegradable interference screw fixation on the femoral side. The screw is still visible (white circle in front of the hamstring graft). B, MRI changes 24 months after ACL reconstruction with the Inion Hexalon biodegradable interference screw fixation on the femoral side. The screw has absorbed, but there is some tunnel enlargement in front of the hamstring graft (the graft is the black circle in the posterior part of the femoral tunnel).

The fact that the screws used in our study had been absorbed in 2 years is contradictory to the finding in the previous studies of Ma et al ¹⁴ and Radford et al.¹⁵ They found that the biodegradable screws they used did not absorb in even 2 to 4 years. The explanation for this difference seems logical: the materials of these screws are different. In our study, we used biodegradable screws made of copolymers composed of L-lactic acid, D-lactic acid, and TMC, whereas in the studies of Ma et al¹⁴ and Radford et al,¹⁵ PLLA interference screws were used.

However, two of our patients in the biodegradable screw group had some tunnel enlargement or cyst of the tunnel at the 2-year follow-up. One was in the tibial side, and another was in the femoral side (see Fig. 52-3, B). In these cases, the enlargement was only 2 to 3 mm when the width of the normal tunnel was compared. With all patients, the mean widths of the femoral and tibial tunnels were 10 mm (range 7–12 mm) and 10 mm (range 8–14 mm), respectively. No difference was found between the biodegradable and metallic screw groups. Previously in the literature, tunnel enlargement has been reported after using biodegradable fixation methods, as well as after using other fixation methods such as metallic screws and especially Endobutton fixation.^14,³³^–³⁵ However, the clinical importance of the tunnel enlargement still remains controversial. Theoretically, if the tunnel enlargement were large, it could be a problem when performing the revision ACL reconstruction. However, with our patients, the tunnel enlargement was so minimal that no problem would be expected later in the event that revision ACL surgery is needed. In fact, there were no difficulties in performing the revision ACL reconstruction with the two patients who underwent a revision ACL surgery in the biodegradable screw group of our study.