What is the minimum spacing between anchors?

10 diameters Minimum Anchor Spacing Generally, a spacing of 10 diameters between anchors is appropriate. For wedge and sleeve anchors, the spacing distance may be reduced by 50% provided the shear and tension values are reduced by 40%.

nova-anchor.com - Design Installation Data- Amityville, New York - Nova Fasteners Co.

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Maximum Working Load The maximum safe working load should not exceed 1/4 of the average ultimate values of a specific anchor size. A higher safety factor may be required for applications with cyclic, fatigue or shock loads and should be selected by the designer on that basis. Minimum Embedment For each type of anchor, a minimum depth of embedment is specified. This is minimum depth for an anchor to be installed and still meet minimum pullout values. Anchors installed less than minimum depth will stress the base material above its limits and may cause failure during installation or expansion of the anchor. Embedment depth is a critical factor in determining load capacity of an anchor, and it represents the point at which the anchor's expansion forces are acting on the concrete. The required anchor embedment is the distance from the concrete surface to the bottom of the anchor. Minimum Anchor Spacing For each type of anchor, a spacing distance between anchors is specified. The load bearing capacity of an anchor is influenced by an adjacent anchor. Anchor spacing is the minimum distance between two anchor center lines without an influence on the tensile or shear failure load of either anchor. Generally, a spacing of 10 diameters between anchors is appropriate. For wedge and sleeve anchors, the spacing distance may be reduced by 50% provided the shear and tension values are reduced by 40%. For drop in anchors, the spacing distance may be reduced by 50%, provided the shear and tension values are reduced by 50%. Linear interpolation may be used for intermediate spacings. Spacing reduction factors are cumulative for groups of three or more anchors. Minimum Edge Distance For each type of anchor, a minimum edge distance is specified. Due to reduced material volume, a minimum edge distance must be maintained to prevent the edge from breaking away prematurely when the anchor is installed. Also, a safe edge distance must be maintained in order to achieve the full load bearing capacity of the anchor. A minimum edge distance of 5 diameters is recommended. Drop in anchors should have a minimum edge distance of 8 diameters. For wedge and sleeve anchors, the edge distance may be reduced by 50%, provided the shear and tension values are reduced by 40%. For drop in anchors, the spacing distance may be reduced by 50%, provided the shear and tension values are reduced by 50%. Combined Loading Anchors loaded in tension and shear simultaneously, will generally have ultimate capacities lower than an anchor loaded in tension and shear separately. Combined loads should be calculated on a straight line interaction diagram of pure shear (S) and pure tension (T). S APPLIED T APPLIED

S ALLOWED + T ALLOWED Less than or equal to 1

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Can an anchor fail?

Conclusion. Anchors failed if tension in the repair exceeds the bones capacity to retain the anchor, if the anchor is incompletely deployed or if one anchor is stressed beyond its tension capability.

online.boneandjoint.org.uk - LOSS OF ANCHOR FIXATION AFTER ARTHROSCOPIC ROTATOR CUFF ...

Abstract

Aim

Review causes of anchor fixation failures in patients who underwent arthroscopic rotator cuff repair.

Methods

Between 2003 and 2006, 650 arthroscopic rotator cuff repairs were performed by the senior author. Of these, anchor fixation failure occurred in fifteen patients. A retrospective review was undertaken to find out the reasons for their failure.

Results

There were ten women and five men, age range 46-84 (mean age 64). Thirteen underwent repair with metallic knotless anchors (Arthrocare), and two with 5.5mm biodegradable screw anchors (Arthrotek). Knotless anchors were used to repair six massive, one large, three medium and three small tears. The two patients with biodegradable anchor repair had only small tears, each held with a single anchor. All but one failure was apparent at six weeks. One metallic anchor failed at four months. Twelve knotless anchors failed through pull-out and one broke. Both biodegradable anchors broke at the eyelet.

Discussion

The increasing strength of suture material has shifted the weak point away from the suture-tendon interface towards the anchor-bone interface. Arthroscopic techniques permit a wider age range of patients suitable for surgery, each with varying degrees of osteoporosis in the proximal humerus, increasing risk of anchor pull-out. Multiple anchor insertions to reduce stiff, retracted tears may also lead to weakening of the bone table in the footprint area of the greater tuberosity. Incomplete anchor deployment, commonly at the curved cortical bone edge of greater tuberosity can also lead to failure.

Conclusion

Anchors failed if tension in the repair exceeds the bones capacity to retain the anchor, if the anchor is incompletely deployed or if one anchor is stressed beyond its tension capability. We recommend that consideration is given to spreading the tension of the tissue repair amongst the anchors placed in the greater tuberosity.