Original Article
Biomechanical Study: Determining the Optimum Insertion Angle for Screw-In Suture Anchors—Is Deadman's Angle Correct?

https://doi.org/10.1016/j.arthro.2014.07.010Get rights and content

Purpose

To assess the effect of the insertion angle and the angle of applied load on the pullout strength of screw-in suture anchors.

Methods

Screw-in metallic suture anchors were inserted into a 10–lb/cu ft synthetic cancellous bone block at 30°, 45°, 60°, and 90° to the surface. The suture pull angle was then varied in 30° increments between 0° and 180°. Five constructs were tested to failure (anchor pullout) for each combination of angles using a Zwick tensile testing machine (Zwick Roell, Ulm, Germany).

Results

There were a total of 25 combinations. The greatest pullout strength was seen with a suture anchor inserted at 90° to the bone block with a pull angle of 90° to the bone (mean, 306 N; standard deviation [SD], 9 N). The weakest pullout strength was seen with a suture anchor inserted at 30° with the angle of pull at 120° (i.e., opposite to the direction of insertion of the anchor) (mean, 97 N; SD, 11 N). A simulated deadman's angle of 45° with an angle of pull of 150° produced a pullout strength of 127 N (SD, 4 N). The pullout strengths for each insertion angle were greatest when the angle of pull was similar to the angle of insertion (P < .0001, repeated-measures analysis of variance).

Conclusions

The angle of applied load to a suture anchor and the insertion angle significantly influence the biomechanical pullout strength of screw-in suture anchors. The insertion angle of the suture anchor should replicate the angle of applied load to ensure the optimum pullout strength.

Clinical Relevance

The screw-in anchor insertion angle and angle of applied load may have an influence on pullout strength.

Section snippets

Methods

To provide a uniform material, we decided to perform the biomechanical tests using solid rigid polyurethane foam blocks (Sawbones, Limhamn, Sweden). This is a recognized alternative test medium that has similar properties to cancellous bone. The density of the material used was 10 lb/cu ft. We used 5.5-mm Twinfix screw-in anchors (Smith & Nephew, Andover, MA) according to the manufacturer's guidelines. The suture material was replaced with No. 5 FiberWire (Arthrex, Naples, FL) to ensure that

Results

All constructs were loaded to failure. In every experiment the metal anchor pulled out of the synthetic bone block. There was a statistically significant difference in failure load among all constructs (P < .0001, repeated-measures ANOVA test). The mean pullout strengths for all constructs are shown in Table 1. An angle of insertion of 90° and applied load of 90° produced the strongest construct, failing at a mean of 306 N (standard deviation [SD], 9 N). The strongest pullout strengths for each

Discussion

We have shown that with a contemporary screw-in anchor, the ultimate pullout strength is greatest when it is inserted at the same angle as or similar angle to the angle of applied load. Do these findings have a clinical impact? Rotator cuff repairs with suture anchors can fail at 3 interfaces: the anchor can pull out of the bone, the eyelet or suture material can break, and the suture can cut out through tendon. It is likely that all 3 are linked. The focus of research has moved among these

Conclusions

The angle of applied load to a suture anchor and the insertion angle significantly influence the biomechanical pullout strength of screw-in suture anchors. The insertion angle of the suture anchor should replicate the angle of applied load to ensure the optimum pullout strength.

References (19)

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Cited by (35)

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    This ‘deadman angle’ describes both the angle at which the anchor is inserted and the angle that the suture makes with the direction of pull of the rotator cuff. Traditionally, an angle less than 45° has been advocated, but recent evidence suggests that a more vertical entry point corresponding to the angle of applied load more reliably optimises pullout strength.27–30 Suture anchors constitute a strong and stable form of fixation between the torn rotator cuff and its bony footprint.

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    Since the deadman theory proposed that the ideal insertion angle of suture anchors was 45° to the bone surface, most surgeons believe that inserting suture anchors at this angle will result in the strongest pullout strength. However, several recent studies did not demonstrate the strongest ultimate load to failure for CSA inserted at 45°.10,12,13,30 Itoi et al.30 compared the pullout strength of 3 different insertion angles (45°, 90°, and 135°) in both finite element analysis and biomechanical study with a synthetic bone analog model.

  • Effect of anchor threads on the pullout strength: A biomechanical study

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    After the introduction of this theory, many researchers and surgeons use 45° as a standard angle of anchor insertion.6,8–10,12,13 However, biomechanical studies using cadavers and synthetic bones revealed that the pullout strength of the anchor inserted at 90° to the bone surface was greater than that of the anchor inserted at 45°.14–16 The greatest difference between the commonly-accepted intuitional understanding of tent peg insertion and the outcome of these biomechanical studies is the friction between the bone (ground) and the anchor (peg).

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The authors report that they have no conflicts of interest in the authorship and publication of this article.

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