Suture slippage in knotless suture anchors as a potential failure mechanism in rotator cuff repair.

PURPOSE: To quantify the strength of suture fixation of knotless suture anchors in relation to the anchors' pullout strength and to compare these results with the static friction between different sutures and anchor materials. METHODS: Suture slippage within the anchor and pullout strength of 4 different knotless suture anchor models were assessed in a bovine bone model. Furthermore, the peak force before onset of slippage of different sutures trapped between increasingly loaded 4-mm rods made of commonly used anchor material (polyetheretherketone, poly-L-lactide acid, metal) was assessed. RESULTS: In all but 1 of the tested anchors, there was a relevantly lower load needed for slippage of the sutures than to pull out the anchor from bone. The mean load to anchor pullout ranged between 156 and 269 N. The load to suture slippage ranged between 66 and 109 N. All sutures were better held between the metal rods (mean, 21; 95% confidence interval [CI], 19.2 to 23.3) than with polyetheretherketone rods (mean, 17; 95% CI, 15.7 to 18.1) or poly-L-lactide acid rods (mean, 18; 95% CI, 17.6 to 18.4). CONCLUSIONS: In the case of suture anchors that hold the sutures by clamping, the hold of the suture in the anchor may be far lower than the pullout strength of the anchor from bone, because the sutures just slip out from the anchor through the clamping mechanism. This is well explained by the low static friction achieved between the tested sutures and the test rods made of anchor materials. CLINICAL RELEVANCE: The use of knotless suture anchors appears quick and easy to perform; however, most of the anchor systems could not even reach half of the anchor pullout strength from bone before suture slippage occurred.

Controlled release of tetracycline from biodegradable beta-tricalcium phosphate composites.

For use in the prevention of bone infections, a novel controlled release system composed of beta-tricalcium phosphate (TCP) granules with biodegradable coatings incorporating the antibiotic drug tetracycline (TC) was developed. Six formulations using poly(D,L-lactide) and poly(D,L-lactide-co-glycolide) as coating materials to incorporate TC were prepared and tested in vitro and in vivo. Release of TC from TCP composites was dependent on the biodegradability of the used polymers and on physical-chemical interactions of TC with the polymer materials. Three characteristic release profiles were obtained: slow release lasting up to 67 days, intermediate release with 60% of the total dose released up to day 20, and fast release with a high initial burst and 90% of TC released within 4 days. Even though TC decomposition products had formed during in vitro release, no cytotoxic effects on osteoblast-like cells were observed. The biological activity of TC after incorporation into PL(G)A films was confirmed using a TC-repressible promoter system in genetically engineered Chinese Hamster Ovary (CHO) cells. TC-loaded TCP composites implanted in ovine cancellous bone defects showed good biocompatibility and new bone formation in the histological evaluation. No differences in the cellular reactions were seen between antibiotic-loaded composites and the control group. These experimental results indicate the potential of coated TCP composites to be used as local carrier system for controlled TC delivery with different release kinetics and good in vitro and in vivo biocompatibility.

Influence of test temperature and test speed on the mechanical strength of absorbable suture anchors.

PURPOSE: Absorbable implant materials offer various advantages but are mechanically far weaker than metals. Despite known temperature dependence of the biomechanical properties of these materials, mechanical testing has almost exclusively been performed at room temperature in the literature. In this study, the difference in mechanical performance at room and body temperature was assessed in vitro at different test speeds. TYPE OF STUDY: Biomechanical bench study. METHODS: Five absorbable suture anchor models were held in a metallic holder and loaded under tension using 0.5-mm steel wires until failure. Testing temperature was 20 degrees C +/- 1 degrees C or 37 degrees C +/- 1 degrees C, test speed was 50 mm/min or 5 mm/min. Tensile load at failure and failure mode were recorded. To test creep behavior, a constant load of 100 N was applied, and time to failure was recorded at both temperatures. RESULTS: Both raising the temperature and decreasing test speed significantly (P <.0001) impaired the mechanical performance of the tested implants. Increase of temperature (20 degrees C to 37 degrees C) resulted in a decrease of the maximal failure strength by up to 40% and decreased time to failure by up to 98% under static load. At 37 degrees, decreasing the test speed from 50 to 5 mm/min lowered the load to failure by up to 18%. Failure of the anchors always occurred by eyelet cutout of the wire. CONCLUSIONS: The lower the test speed, the higher is the influence of the testing temperature. Testing of implants at room temperature instead of body temperature may falsely improve test results by a factor of up to 50 under static load. Therefore, testing absorbable implants at body temperature seems mandatory, preferably at slow test speeds.

Mechanical testing of absorbable suture anchors.

PURPOSE: Absorbable suture anchors offer great advantages but are made of mechanically weak material. The weakest link in the fixation of soft tissue to bone may therefore be the anchor itself. In this study, several commercially available anchors were mechanically tested in vitro. TYPE OF STUDY: Biomechanical bench study. METHODS: Twelve absorbable suture anchor models were implanted into an artificial test bone according to the recommended technique. Testing temperature was 37 degrees C +/- 1 degrees C. The anchors were loaded with an Instron testing machine with the suture material (USP No. 2, Ethibond, Ethicon, Somerville, NJ) in line with the anchor axis, with and without previous abrasion of the suture at the eyelet. Tensile load at failure and failure mode were recorded. To test creep behavior, a permanent load of 100 N was applied to the anchors, and time to failure was recorded. Suture anchor weight and crystallinity were analyzed. RESULTS: Mean failure load on tensile testing using a cross-head speed of 60 mm/min ranged from 124 to 244 N. Failure modes were eyelet failure in 5 cases, suture failure in 6 cases, and anchor pullout in 1 case. In creep testing, eyelet failure occurred in 8 anchor models after a mean duration of 0.5 to 99 hours; 3 anchor models remained intact after 300 hours, and 1 anchor model failed by pullout of the test sample. Crystallinity ranged from 0% (amorphous) to 57.2%; anchor weight ranged from 0.036 to 0.161 g. Mechanical properties did not correlate with crystallinity but with anchor weight. Abrasion of the suture material at the eyelet had little effect on failure load. CONCLUSIONS: At 37 degrees C, structural failure (breaking) of absorbable suture anchors may occur if loaded to the mechanical limit. Absorbable anchors are particularly sensitive to static, long-term loading.

Non-destructive three-dimensional evaluation of a polymer sponge by micro-tomography using synchrotron radiation.

X-ray micro-tomography, a non-destructive technique is used to uncover the complex 3-D micro-architecture of a degradable polymer sponge designed for bone augmentation. The measurements performed at HASYLAB at DESY are based on a synchrotron radiation source resulting in a spatial resolution of about 5.4 microm. In the present communication we report the quantitative analysis of the porosity and of the pore architecture. First, we elucidate that synchrotron radiation at the photon energy of 9 keV has an appropriate cross section for this low-weight material. Modifications in sponge micro-architecture during measurement are not detected. Second, the treatment of the data, an amount of 2.5 Gbyte to generate binary data is described. We compare the 3-D with the 2-D analysis in a quantitative manner. The obtained values for the mean distance to material within the sponge calculated from 2-D and 3-D data of the whole tomogram differ significantly: 12.5 microm for 3-D and 17.6 microm for 2-D analysis. If the pores exhibit a spherical shape as frequently found, the derived mean pore diameter, however, is overestimated only by 6% in the 2-D image analysis with respect to the 3-D evaluation. This approach can be applied to different porous biomaterials and composites even in a hydrated state close to physiological conditions, where any surface preparation artifact is avoided.

pH-stabilization of predegraded PDLLA by an admixture of water-soluble sodiumhydrogenphosphate--results of an in vitro- and in vivo-study.

Aim of the study was to examine if the addition of buffering sodiumhydrogenphosphate to poly(D,L)lactide(PDLLA) would stabilize the pH-value in the in vivo environment of implanted material and whether this improves its biocompatibility. The material was predegraded just to the point of viscous disintegration to test the PDLLA in the moment of its most aggressive effect on the surrounding tissue. Racemic amorphous PDLLA was injection-molded with or without the admixture of 1 mol NaP per 100 mol lactate, the degradation product of PDLLA (=1 mol%) to form 20mm x 3 mm x 2mm rods. Predegradation was performed by storing the rods at 55 degrees C for 14 days, just to the point of beginning dissolution. Predegraded PDLLA or PDLLA + NaP samples were used for in vitro incubation tests, as well as for the in vivo study, where the rods were implanted into the spinal muscles of 30 male Wistar rats. Repeatedly, measurements of the pH-value were made in the incubation solutions in vitro. The surrounding tissue of the implanted samples as well as the normal contralateral muscle tissue was checked for its pH-value in a group of 3 rats, respectively, anaesthesized at various time intervals after implantation. After these measurements the implants and their surrounding tissues were excised for histological examination. In Ringer's solution pH-values dropped immediately within the first week of incubation of both predegraded materials reaching -4 pH units after 4 weeks in the PDLLA containing medium, after 6 weeks in the PDLLA + NaP containing medium. Soerensen buffer slowed the pH decrease with significant differences between the material groups up to the 28th week. In vivo, the pH of the surrounding tissue was influenced by the implanted PDLLA material up to the 4th week, while the admixture of NaP resulted in a significant pH stabilization. A higher quantity of macrophages and giant cells were seen between the 2nd and 6th week after the implantation in the environment of pure PDLLA compared with PDLLA + NaP. Complete resorption of predegraded pure PDLLA or PDLLA + NaP from the extracellular space was reached 28 weeks postimplantation in vivo. Thus, sodiumhydrogenphosphate improves the biocompatibility of degrading PDLLA at the point of viscous disintegration by stabilizing the pH-value in the environment of the implants for several weeks and reducing adverse tissue reactions.