Fixation Strength

A three-part biomechanical study was carried out to study the fixation strength of the biodegradable copolymer interference screw (Inion Hexalon) and to evaluate its suitability for ACL reconstruction by comparing it with the previously clinically used interference screws. 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. and Harding et al. 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 the 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 torque meter 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. 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/minute 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 491N (SD 154N) and 418 N (SD 77N), respectively ( P = .15). The average maximum failure loads were 548N (SD 130N) and 453N (SD 94N), respectively ( P = .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 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 501N (SD 122N) and for the SmartScrew, 386N (SD 79N; P = .05). The average maximum failure loads were 563N (SD 109N) and 536N (SD 128N), respectively ( P = .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 Nm (SD 0.7 Nm) and for the Bioscrew ( N = 4), 1.5 Nm (SD 0.6 Nm; P = .32). The average yield loads for the copolymer screw and Bioscrew were 901N (SD 262N) and 795N (SD 524N), respectively ( P = .77). The average maximum failure loads were 926N (SD 259N) and 800 N (SD 516N), respectively ( P = .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. 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 (SD 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. 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., 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). In the compression test, each screw was set flat between the compression plates and loaded with a constant speed of 5 mm/minute until failure (Zwick Z020, Zwick GmbH, Ulm, Germany). In the compression test, both screws retained more than 80% of their initial mechanical strength for as long as 12 weeks.

Fixation Strength

A three-part biomechanical study was carried out to study the fixation strength of the biodegradable copolymer interference screw (Inion Hexalon) and to evaluate its suitability for ACL reconstruction by comparing it with the previously clinically used interference screws. 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. and Harding et al. 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 the 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 torque meter 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. 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/minute 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 491N (SD 154N) and 418 N (SD 77N), respectively ( P = .15). The average maximum failure loads were 548N (SD 130N) and 453N (SD 94N), respectively ( P = .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 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 501N (SD 122N) and for the SmartScrew, 386N (SD 79N; P = .05). The average maximum failure loads were 563N (SD 109N) and 536N (SD 128N), respectively ( P = .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 Nm (SD 0.7 Nm) and for the Bioscrew ( N = 4), 1.5 Nm (SD 0.6 Nm; P = .32). The average yield loads for the copolymer screw and Bioscrew were 901N (SD 262N) and 795N (SD 524N), respectively ( P = .77). The average maximum failure loads were 926N (SD 259N) and 800 N (SD 516N), respectively ( P = .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. 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 (SD 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. 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., 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). In the compression test, each screw was set flat between the compression plates and loaded with a constant speed of 5 mm/minute until failure (Zwick Z020, Zwick GmbH, Ulm, Germany). In the compression test, both screws retained more than 80% of their initial mechanical strength for as long as 12 weeks.