The Effect of Humeral Elevation Angle on the Pullout Strength of Suture Anchors

INTRODUCTION: Rotator cuff injuries account for more than 4.5 million physician visits annually in the U.S. Injuries to the rotator cuff are typically treated arthroscopically by using implanted suture anchors for the repair. Common suture anchor designs include variations in thread design and the material used. The primary objective of this study was to understand and model the effects of frontal plane humeral elevation and anchor type on the pullout strength of common suture anchors. We examined both titanium and resorbable suture anchors in this study. Our hypothesis was that the humeral elevation angle in the frontal plane will have a greater effect on pullout strength compared to the thread design or material choice. METHODS: This study was performed on polyurethane foam proximal humeral models (Model 1051, Pacific Research Labs, Vashon Island, WA) for the elevation angle study and 5 lb-ft and 10 lb-ft polyruethane foam blocks (Model 1522-23, and 1522-01. Pacific Research Labs, Vashon Island, WA) for the thread form study. To evaluate suture anchor thread form and material choice, we used a 5mm PLA (poly lactic acid) biodegradable anchor (Mitek SpiralokTM) and 6.5mm titanium anchor (Smith & Nephew TwinfixTM). Ten tests were performed for each anchor type for a total of twenty tests. The anchor was threaded such that the axis of the screw was perpendicular to the block surface and top of the anchor was flush with the polyurethane block. The anchor was pulled out along the axis of the screw until failure in a materials testing system at 0.15 mm/s (Inspec 2200, Instron Corp, Norwood, MA). To evaluate the affect of elevation angle and thread form, we used the 6.5mm Smith & Nephew Twinfix , 5mm Mitek Fastin RC , and 3.5mm Smith & Nephew Twinfix suture anchors to examine the effect of humeral angle on the pullout strength. Custom fixturing was designed to allow for the tendon pull to be set to various angles in a materials testing system (Bose SmartTest SP, Eden Prairie, MN). Three humeral angles were 120o, 150o, and 180o measured with respect to the horizontal. For the case where the angle is 180o, the line of action of the force is aligned with the humeral shaft. A design of experiments was performed to populate a runs order of three different anchors and three different frontal elevation angles. The suture anchor was inserted into the humeral head greater tuberosity at an angle of 45o to the axis of the humerus and threaded to a depth such that the top of the anchor was flush with the surface of the humerus. The JMP Statistical software (SAS Institute, Cary, NC) was used for the analysis of data for this study. For the uniaxial tests, the suture anchors were grouped according to type and the polyurethane blocks were grouped according to density. A student’s t test was performed to see significant pullout force differences between block density and suture anchors. An F test was used to see the probability that the two distributions being examined are statistically similar. F ratios were calculated to examine the contribution of each source (anchor, angle, density, and interactions) on the variance seen in the model. Tukey’s HSD (Honesty Significant Difference) analysis was also used to compare all possible pairs of means for the three anchors of the angle evaluation. RESULTS SECTION: The simple pullout analysis of the Mitek and Smith and Nephew screws led to a model for pullout force as a function of thread diameter in mm and polyurethane foam (Table 1). The regression model R was 0.98 with both coefficients p-values <0.0001. A regression analysis was performed for the pullout force as a function of the humeral angle and the anchor diameter (Figure 1). The model R was 0.92 and the coefficients p-values <0.0001. DISCUSSION: It was our hypothesis that thread design and material would not significantly affect the pullout strength of suture anchors. Table 1 confirms that the density of block and diameter of the anchor are significant. Surprisingly, our model predictions fit the data very well and agree with the literature values. The model even agrees with the 1⁄4 -20 screws data from Nien and coworkers. This shows that the holding power of a suture anchor is really dependent on the overall size, rather than the thread design or material. Table 1. Pullout data for three screws in three different polyurethane foams. The data on the 1⁄4 20 screws were taken from the literature. Screw Type Pullout Force (N) Model