Evaluation of insertion of torque and pull-out strength of mini-screws according to different thickness of artificial cortical bone / 대한치과교정학회지

OBJECTIVE: The purpose of this study was to evaluate the mechanical performance of mini-screws during insertion into artificial bone with use of the driving torque tester (Biomaterials Korea, Seoul, Korea), as well as testing of Pull-out Strength (POS). METHODS: Experimental bone blocks with different cortical bone thickness were used as specimens. Three modules of commercially available drill-free type mini-screws (Type A; pure cylindrical type, Biomaterials Korea, Seoul, Korea, Type B; partially cylindrical type, Jeil Medical, Seoul, Korea, Type C; combination type of cylindrical and tapered portions, Ortholution, Seoul, Korea), were used. RESULTS: Difference in the cortical bone thickness had little effect on the maximum insertion torque (MIT) in Type A mini-screws. But in Type B and C, MIT increased as the cortical bone thickness increased. MIT of Type C was highest in all situations, then Type B and Type A in order. Type C showed lower POS than Type A or B in all situations. There were statistically significant correlations between cortical bone thickness and MIT, and POS for each type of the mini-screws. CONCLUSION: Since different screw designs showed different insertion torques with increases in cortical bone thickness, the best suitable screw design should be selected according to the different cortical thicknesses at the implant sites

A Biomechanical Study on the Pull-Out Strength of Pedicle Screw Augmented with Polymethylmethacrylate(PMMA)- Cadaveric Study -

OBJECTIVE: The authors evaluate the differences in the pedicle screw pull-out strength between an unaugmented screw and a screw augmented by high pressurized PMMA in an embalmed human lumbar cadaveric spine. METHODS: Pedicle screws were inserted in the 50 pedicles of cadaver lumbar spine after measuring the bone mineral density(BMD) of the specimens. The pedicle screw was inserted without augmentation on the left pedicle and with augmentation by a vertebral reconstruction with high pressurized PMMA on the right side. The pedicle screw was inserted before the hardening of bone cement on the right side. A computed tomographic scan was done to evaluate the location of the PMMA and PMMA/ screw interface. After that, a screw pullout strength test was performed. RESULTS: There were positive correlations between the pull-out strength and BMD in the control group (r=0.7339). The average pull-out strength of the augmented group was 168% greater than that of the control group(p<0.05). The pull-out strength of the augmented group was correlated with in the volume of the PMMA. CONCLUSION: A pedicle screw that is augmented with PMMA will provide a more rigid fixation than that of an unaugmented screw, especially in the low BMD group of the specimens.

Pull - out Strength of Different Multiple Suturing Method of the Cruciate Ligament / 대한정형외과학회잡지

The aim of this study was to measure the immediate pull-out strengths by increasing the number of suture loops and to compare the immediate pull-out strengths of three different suturing techniques. In one group, the number of suture loops increased from 2 to 9 and the suturing technique of multiple loops through the proximal ligament stumps was used. The other group, the three different techniques were transverse, vertical, and criss-cross suture. The ultimate strength for the 2 loops was 16.62 (+/-6.7)N, for the 3 loops was 34.45 (+/-12.5)N, for the 4 loops was 54.80 (+/-17.7)N, for the 5 loops was 74.30 (+/-21.4)N, for the 6 loops was 102.49 (+/-13.5)N, for the 7 loops was 105.05 (+/-24.8)N, for the 8 loops was 129.50 (+/-76.1)N, for the 9 loops was 229.50 (+/-48.7)N. The transverse suture had a mean ultimate strength of 47.38 (+/-14.8)N, where-as the vertical suture failed at 76.94 (+/-26.4)N and the criss-cross suture at 101.82 (+/-25.7)N. Consequently, their strength can be improved by increasing the number of loops and we suggest that the criss-cross suture technique is reliable method for clinical use, because of high failure strength and wider apposition area of bone to ligament.

A study on changes of the Vertebral Pedicles and Mechanical Strengths after Screw Insertion / 대한정형외과학회잡지

Spinal fixation using pedicle screws has recently been the focus of increased attention, but the adequate size of pedicle screw and maximum percentage fill as related to the pedicle diameter and are not well known. The objects of this study were to determine the ideal ratio among pedicle, drill and screw diameter, and to determine the maximum percentage fill of the screw without significant decrease of pull-out strength. The materials used for the experiments were 376 thoracic pedicles obtained from the 38 young pigs, and the diameters of pedicles ranged from 3.0 to 8.5mm. After 40% to 100% drilling as compared to pedicle diameter, screws were inserted carefully, and measurements were taken of the outer pedicle changes and pull-out strengths, and adequate drill and screw sizes as related to the diameters of given pedicles were determined. It was found that pull-out strength was the strongest after 60% drill, and the larger the drill diameter, the smaller the holding power, and the larger the screw diameter, the greater the holding power. Maximum pull-out strength was seen at 80-90% fill with 60% drill. After sequentially drilling each pedicle with increasingly larger drill bits, larger screws could be inserted with pedicle changes such as expansion, cutout, split fracture, and comminuted fracture. after larger drilling up to 100%, pedicle screws with diameters smaller than 115% of measured pedicle diameters could be safly inserted without fracture and significant decrease of pull-out strength. It is concluded that effective percentages of drill and screw diameters to the pedicle diameter are 60% and 80-90% respectively, and pedicle screw up to 115% of measured pedicle diameter can be safely inserted into pedicle without significant decrease of pull-out strength. It is thought that fresh pedicle has elasticity and larger screw can be inserted to the pedicle with strong holding after larger drilling.