Initial Fixation Strength of Bio-absorbable Magnesium Screw

Transcription

1 Initial Fixation Strength of Bio-absorbable Magnesium Screw Joon Kyu Lee, MD, PhD, Sahnghoon Lee, Sang Cheol Seong, Myung Chul Lee, MD, PhD. Seoul National University College of Medicine, Seoul, Korea, Republic of. Disclosures: J. Lee: None. S. Lee: None. S. Seong: None. M. Lee: None. Introduction: Interference screw is used for the graft fixation during anterior cruciate ligament (ACL) reconstruction. The materials usually used for the interference screw is metal due to great mechanical property as an implant. However, there is more than ten-fold difference in elastic modulus between metal and bone, which may lead to stress shielding. There is also a need for second operation to remove metal screw after healing. As an alternative, various kinds of bio-absorbable screws are developed. Bio-absorbable screws are degraded in vivo after certain amount of time and then either absorbed or secreted. Therefore, inflammation or foreign body reaction by the metal implant can be avoided and there is no need for the second operation. The bio-absorbable screws used these days are made of polylactic or polyglycolic acid (PLA or PGA) or a polymer of these materials. However, the use of these bio-absorbable screws is somewhat limited due to the low mechanical strength of the material and the acidity when degraded. Magnesium is one of the important minerals that compose the human body. It has relatively low elastic modulus compared to metals, therefore it can prevent stress shielding occurring after metal implant use. Additionally, it has higher strength, lighter weight and better processability. The magnesium has its own shortcomings. It can erode fast in vivo which could lead to strength failure. To improve rate of degradation and improve strength in vivo, several magnesium alloys were developed. Surface coating of the magnesium is getting attention as the method to control the rate of the degradation. The purpose of this study was to evaluate the initial fixation strength of the newly developed bio-absorbable magnesium (hydroxyapatite coated) screw. Methods: There were 6 types of screws compared in this study; Double thread magnesium screw (Group Mg2), Single thread magnesium screw (Group Mg1), Double thread titanium screw (Group Ti2), Single thread titanium screw (Group Ti1), Bioabsorbable screw (PLDLA-TCP screw; Group Bio), Metal screw (Linvatec, titanium alloy; Group Metal). Load to failure test, cyclic loading test were performed. Failure modes were also checked. Five porcine knees were used for each group. The porcine knees were obtained on the day that they were butchered. All soft tissues except for patellar tendon were removed. Bone mineral density of the lateral femoral condyle and the patella tip of every specimen were checked. Patellar tendons were carefully dissected in continuity with the patellar bone plug, leaving the tibial insertion site intact. Patellar bone plug was prepared in bullet shape which was 8mm in diameter and 2.5cm in length. Tip of the bone plug was reduced to a smaller diameter to achieve an easy entrance into the femoral tunnel (Fig. 1). The femoral tunnel was created at the origin of the femoral attachment site, 9mm in diameter and 2.5cm in length. Prepared patellar bone plug was inserted into the femoral tunnel with cancellous surface oriented 10 or 2 o clock and flush with intra-articular edge of the tunnel. A guide wire was inserted along the cancellous surface of bone plug and parallel to the long axis of the tunnel. A screw was inserted over the guide wire and advanced until flushed with the edge of bone block (Fig. 2). Then, the specimen was potted in a zig with PMMA resin and the femoral tunnel oriented vertical to the ground for mechanical testing (Fig. 3). Load was applied vertical to the ground and parallel to the long axis of the femoral tunnel. Cyclic loading test: 500 cycles with a frequency of 0.2Hz between 0 and 2mm of displacement were performed. Loads during the cycles were recorded according the load-displacement curve. The maximum loads at the first and last cycles were measured. Unloading was calculated by subtraction the latter from the former. Loads to failure test: Vertical tensile loads were applied at a rate of 50mm/min until failure. Load-displacement curve was obtained. Linear load was defined as the load at the point of first significant failure following the initial toe region. Ultimate failure load was defined as the maximum load in the curve. And the stiffness was defined as the slope of the curve in the linear region. Failure modes were inspected visually. Figure 1. Prepared bone-patellar-bone graft of the specimen Figure 2. Specimen after bone plug and screw insertion to the femoral tunnel Figure. 3. Specimen potted in a zig with PMMA for mechanical testing Results: Unloading of the Groups Mg1 and Mg2 were in between of the unloading of Groups Bio and Metal (Table 1). Linear load, ultimate failure load and the stiffness were higher in Groups Mg1 and Mg2 compared to other groups (Table 2). However, there were no significant differences among the groups. Bone mineral densities of the femur and patella of the specimen were comparable among the groups (Table 3). There also were no significant differences in failure modes among the groups (Table 4). Discussion: This study demonstrated that the newly developed bio-absorbable magnesium screw showed comparable initial fixation strength similar to widely used bio-absorbable and metal interference screws. Significance: The newly developed magnesium screw showed adequate initial fixation strength.

2 Acknowledgments: This study was supported by a grant of the Korean Health Technology R&D Project, Ministry for Health, Welfare & Family Affairs, Republic of Korea. (A100451). References: 1) Lee M et al.; KSSTA ) Weiler A et al.; AJSM

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7 ORS 2014 Annual Meeting Poster No: 0305

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