Suture/Eyelet Orientation Effects On Bone Anchor Failure Strengths: Improving Surgical Repair of Tendons Torn Off Bone.

Philip J. Eichhorn [1]

Bone anchors are tiny metal screws with a hole in the end for connecting sutures from torn-off tendons or ligaments being reattached to bone (heel, shoulder) during surgical repair. Strength of the repair is critical both to prevent failure of the repair (requiring repeat surgery) and to allow early vigorous physical therapy. This study evaluated failure of suture-anchor combinations as a function of anchor design and orientation during use. Four brands of bone anchors (Peba, SuperQuick, Fastin, and Mainstay) were evaluated using the heavy suture employed in real surgery, n=232 (looped through the eyelets of anchors inserted into plastic that simulates bone), which were pulled at three angles of orientation (0[degrees], 90[degrees], and 90/90[degrees]). Using a computerized screw-type load frame, pull was applied to simulate a patient putting weight on a repaired ankle. Suture failure strength varied significantly among samples for two anchor brands and also as a function of pull angle: suture in Fastin and S uperQuick at 90[degrees] and 90[degrees]/90[degrees] broke easily. Strength in the Mainstay anchor depended on eyelet orientation: weak at 90[degrees], but strongest of all tested at 90[degrees]/90[degrees]. Suture in the Peba anchor had good strength at both orientations. Sutures are used at 90[degrees] and 90[degrees]/90[degrees] in anchors in surgery. Therefore, the Peba anchor and, particularly, the Mainstay used at 90[degrees]/90[degrees] have the best repair strength. Use of these results by surgeons can improve repair outcome with bone anchors. Also, the results suggest further work and, ultimately, modifications that should improve suture-anchor function.

Tendons and ligaments torn off their attachments to bone (avulsed) in an injury must be surgically repaired. Bone anchors are used by surgeons (often, but not exclusively, orthopedists) to reattach these tendons or ligaments to bone. Thick surgical sutures are threaded through the end of the ripped tendon or ligament and then tied down to the eyelet hole of a small metal bone anchor which is firmly inserted and fixed in the bone. A torn Achilles tendon in the heel is the most common repair (Maniscalco et al., 1998; Weintraub et al., 1998) and there is occasional use in the shoulder (Wetzler et al., 1996). The strongest repair possible (Barber et al., 1995; Barber et al., 1996) is sought for two main reasons: to decrease the likelihood of failure (breaking of the repair) prior to complete healing, and to allow for earlier initiation of physical therapy activities that will shorten the patient's overall recovery period. In Achilles tendon repairs especially (Figure 1), there has been a problem with breaking of the suture at the point it is threaded through the bone anchor eyelet. This causes failure of the repair and need for repeat surgery (Watson et al., 1995).

There are different types of bone anchors which may be placed in different orientations during surgery. The objectives of this study are: 1. to evaluate mechanically (Komi, 1990) the failure strength of the combination of standard surgical suture with each of four commonly used bone anchors as a function of anchor design, suture pull angle, and anchor eyelet rotational orientation; 2. if possible, to make recommendations as to which ones have the best engineering designs for their intended function; and, 3. to analyze how anchors of this type are best used and how they might be improved.

Hypothesis. All the bone anchors at each of the relevant pull angles would cause suture failure at the same adequate pull strength, indicating they are all the same and are appropriate in this function.

MATERIALS AND METHODS

Four commercially available commonly used bone anchors (Figure 2) were evaluated: Peba (Orthopedic Biosystems, Ltd.), SuperQuick and Fastin (Mitek Surgical Products, Inc.), and Mainstay (Howmedica, Inc.). All tests were run with #2 braided polyester suture (Ethibond Excel, Johnson & Johnson, Inc.). This is the suture used in actual surgery for repair of a ruptured Achilles tendon. Mechanical stress (pull on the sutures threaded through the eyelets of the bone anchors) was applied with a MTS screw-type load frame at a rate of 1.0 mm/sec. Bone anchors were inserted into high-density polyethylene blocks commonly used in orthopedic surgery research to simulate the characteristics of bone. Suture was looped through the anchor eyelet and clamped to the load frame crosshead (Figure 3). The suture was pulled parallel (0[degrees]) or perpendicular to the long axis of the anchor. The 0[degrees] pulls were done as controls and to test the uniformity of the suture-anchor combinations. The perpendicular pulls simulated t he angle created in actual surgical repair of an avulsed Achilles tendon (Figure 1). When pulling perpendicular to the anchor long axis, the anchor eyelet orientation was set at either parallel (90[degrees]) or perpendicular (90[degrees]/90[degrees]) to the suture pull line (Figure 4). The strength of the common type of surgical knot (without any bone anchor) was also evaluated. For each "pull" (trial of a piece of suture pulled until it broke), a continuous record of the force applied in Newtons and the breaking strength was captured and stored by the computer connected to and controlling the load frame. For each individual anchor placed in the plastic, four pulls were performed (with three exceptions where n=8-10, done for testing of precision of the system). There were three orientations of each of four types of anchor. This set of experiments was repeated four times for each anchor type using four different individual anchors in each orientation (48 anchor experiments which, with the knot trials, yielded a total of 232 pulls). Statistical significance between experimental sets was determined by an analysis of variance and comparison of pairs of means by post hoc Tukey test and Z test with significant of p[less than]0.05 in all cases (using the "SPSS" statistical analysis software).

RESULTS

A tabular summary of the failure strength of suture in anchor eyelets is given in Table 1. The degree of variation among samples of each brand of bone anchor was noted by Tukey test comparison of the means (n=4 [or more in certain cases] trials for each of four samples of each anchor) of 0[degrees] pulls. Peba and SuperQuick showed no significant variance among samples while Fastin (range 171 to 197 N) and Mainstay (range 163 to 206 N) each showed moderate but statistically significant variation among the four samples.

Trends were noted regarding suture failure strength as a function of angle of pull on the suture in the anchor eyelet (Figure 5). Suture in Fastin and SuperQuick failed at much lower strengths compared to 0[degrees] when strained at 90[degrees] or 90/90[degrees]. [Note that these values were less than the 115 N for suture breaking at the knots, so the suture would fail at the eyelet of these anchors before any knots would break.] Suture failure in the Mainstay anchor was highly dependent upon eyelet orientation. With an orientation of 90[degrees] in the Mainstay, a very significant decrease (60% less than at 0[degrees]) in failure strength was seen, making this the lowest strength (easiest breaking) in the entire set of measurements. However, in marked contrast, in an orientation of 90[degrees]/90[degrees], only a slight decrease in failure strength was measured compared to that at 0[degrees], making the suture in this anchor and eyelet orientation for surgery the strongest of all tested. The Peba anchor was the most robust overall because at both 90[degrees] and 90/90[degrees], there was comparatively little decrease in suture failure strength compared to that at 0[degrees], indicating a minimal effect of eyelet orientation on suture performance with this anchor.

The surgical knots in suture alone broke at an average pull force of 115 N. Failure strength of the suture at the anchor eyelets was compared to the strength of knots in the suture (Figures 6 and 7). No sutures in Peba failed at strengths lower than knot strength. At both 90[degrees] and 90/90[degrees], all suture in Fastin failed before the knots and, likewise, all in SuperQuick at 90[degrees] and 75% at 90/90[degrees] failed before the knots. In Mainstay, all at 90[degrees] failed before the knots and none at 90/90[degrees] failed at strengths lower than knot failure strength.

DISCUSSION

The hypothesis that at the relevant pull angles of 90[degrees] and 90/90[degrees], all the bone anchors would be the same for suture failure strength in the anchor eyelet is rejected because there are statistically significant differences.

The 0[degrees] control pulls tested precision of manufacturing, and Peba and SuperQuick showed no variation in samples of those anchors and the small variations seen with Fastin and Mainstay seem likely not to be clinically significant, but additional testing on this point could be done.

There were clear differences among anchor brands at the 90[degrees] and 90/90[degrees] pull angles that are used in actual surgery and with the suture used in real operations (thicker suture would likely be stronger, but the knots could cause an even bigger ball of scar tissue, interfering with the repair and also causing pain to the patient). Also, the specific failure strength of the suture in each anchor's eyelet is important compared to the strength of the knot, which fails at a pull force of 115 Newtons. Sutures in the Fastin and SuperQuick anchors fail at strengths of just over 100 N, less than that of the knots, suggesting use of these two anchors may be less effective or desirable in repair surgery. Also, suture in the Mainstay at 90 [degrees] would fail well before the knots (73 vs. 115 N), making that configuration potentially much less useful in surgery. In contrast, suture in the Peba at both 90 [degrees] and 90/90[degrees] and in the Mainstay at 90/90[degrees] is significantly stronger than the surgical knots, suggesting that these anchors at these angles are good for use in surgery for repair of an avulsed Achilles tendon and would not cause the suture to fail at the anchor eyelet.

Whether orientation of the bone anchor eyelet is a factor in suture failure strength at the anchor eyelets was a key question of this study. Often, apparently believing it makes no difference, surgeons have not noted or adjusted the exact angle of the anchor eyelet when doing these repairs. The results obtained in this study clearly show that for the Mainstay anchor, eyelet orientation is critical. With the eyelet at 90[degrees], the suture breaks at the lowest pull force of the entire study (73 N), but with the eyelet at 90/90[degrees], the suture is the strongest in the study (165 N). The extra notches in the top of the Mainstay anchor eyelet clearly are made so that when it is turned to 90/90 [degrees], there are smooth rather than sharp edges directly under the suture. Therefore, if Mainstay is used, it should definitely be in the 90/90[degrees] orientation. When surgeons may not be fully attentive to the precise angle of the eyelet or it is not a consideration, the Peba may be a good choice of anchor bec ause suture at both 900 and 90/900 was almost as strong (153 and 147 N, but still statistically significantly less) as Mainstay at 90/90[degrees], suggesting that the eyelet edges are not too sharp at either angle (consistent with the appearance of the anchor eyelet shown in Figure 2).

Future research in this area will involve microphotography and high-speed video to help identify exactly how the suture breaks at the eyelet. Efforts could then be made to engineer an even better bone anchor eyelet that will be less likely to cause suture failure at the anchor eyelet. Also, companion experiments will be conducted to improve the surgical knots for the suture by designing a knot that is at least as strong as the suture in the anchor eyelet but still not too bulky. Combining both sets of experiments should help improve the strength of the surgical repair of an Achilles tendon torn off the heel bone. This should allow more secure healing and earlier, more active physical therapy that should result in speedier recovery from this severe orthopedic injury that may side-line high-performance athletes for extended periods, possibly up to a year, or even threaten their entire careers.

Summary. For simulated Achilles tendon repair, there is significant difference in the failure strength of suture in eyelets of anchors placed in bone based on brand of anchor and the angle of eyelet orientation. Suture in the Mainstay anchor used at 90/90[degrees] has the greatest strength, but the anchor must be at that specific angle. Suture in the Peba anchor at both 900 and 90/90[degrees] was slightly less strong, but almost as good, and the issue of attention to exact eyelet angle is not a factor, making it possibly easier for surgeons to use this anchor. Additional research to improve bone anchor design and implementation is planned.

ACKNOWLEDGMENTS

These experiments were supported in part by a generous research grant from the Mississippi Junior Academy of Sciences. Additional support was provided by Thom Tarquinio, M.D., of the Department of Orthopedic Surgery at the University of Mississippi Medical Center.

This work was performed in the Biomaterials Laboratory of the University of Mississippi Medical Center and I thank the staff for allowing me to share their facilities. In particular, I thank tremendously my tireless and always supportive research mentor, Doug Parsell, Ph.D., of the Biomaterials Laboratory. He is a constantly inspiring teacher and I look forward to continuing my work with him.

Philip Eichhorn received the Mississippi Junior Academy of Sciences Clyde Sheeley Award for 2000. Special thanks to Howard Computers in Laurel, Mississippi, for underwriting the publication of this research paper.

(1.) Correspondence to: 150 Burnham Rd., Brandon, MS 39042

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