Mechanical characteristics of three staples commonly used in foot surgery

Bone staples are an accepted method of fixation in foot surgery. They reduce operating time and trauma in surgical procedures. A variety of memory staples are available but their properties compared to standard staples are not known. We carried out a study comparing two popular types of memory staples and a standard stainless steel staple.

Standardised bone models of metatarsals made from Tufnol tubes were osteotomised and stabilised using one of three types of bone staples, two types of memory staple (Memory staple and heat-activated Memoclip) or a standard stainless steel staple (Richards). Constructs were loaded in bending and torsion on a material testing machine. The moment and torque to achieve 10 degree of bending or torsion and permanent angulation of the osteotomised bones were assessed.

The Richards staple was found to provide a four times larger resistance to bending and torsion than the two memory staples. However, it was permanently deformed after bending. The Memory and Memoclip staples were equal in their stiffness. In addition, angulation of bones fixed with the Memoclip was elastic, preventing any permanent deformation.

The Richards staple was stiffer, although the permanent deformation of this staple is a disadvantage. Memoclip staples exhibit lower but adequate stiffnesss when compared to the standard Richards staple and are not permanently deformed after bending. The Memoclip staples were easier to handle. The results will enable surgeons to determine the optimal staple for foot and ankle procedures.

Bone staples are widely used in foot and ankle surgery. They are considered to be an effective internal fixation method1. The cited advantages of bone staples include easier fixation onto bones, reduced surgical time and trauma. This in turn leads to improved healing and reduced post-operative pain2.

Various types of bone staples are available, differing in their shape and physical properties. Recently, bone staples have been introduced based on shape-memory materials. For all staples, manufacturers cite advantages in terms of ease of use and provide compression strength based on physical properties of the metals used to produce the staples. However, little is known about the actual fixation capabilities of staples used in foot and ankle surgery and how these compare between staples. It is unknown how the fixation capabilities of staples based on memory material compare to those of conventional metals.

The aim of this study was to compare two types of staples based on memory metal and one conventional staple, all three commonly used in foot and ankle surgery. Specifically, we addressed the question regarding the bending stiffness, torsional stiffness, and permanent deformation of these staples. The null hypothesis was that there was no difference in stiffness between the staples.

This study was conducted in the Laboratory for Biomechanics at The Robert Jones and Agnes Hunt Orthopaedic Hospital in Oswestry after approval from the review board.


Figure 1. Memory (left) & mechanical (right) staples.

Three types of staples were used in this study, two of which were made of nickel-titanium (nitinol) memory material. All the staples had a bridge dimension of 14 mm and leg dimensions of 12 mm. The first of these, Memoclip (Heritage, United Kingdom), is heat activated (Figure 1). When the staple is exposed to a temperature above 50 degrees Centigrade, these staples deform their limbs and generate compression. The second of these, the Memory staple (Depuy, United Kingdom), is normally stored at 0 - 4 degrees Centigrade. When the staple is brought to body temperature (37 degrees Centigrade), the limbs deform and cause compression of bone. The third staple (Richards staple, Smith & Nephew, United Kingdom) is made of stainless steel (Figure 1). Compression is achieved by the mechanical properties of the staple. No further compression is achieved once the staple is inserted into bone.

Specimen preparation

Figure 2. Tufnol tubes used.
Figure 3. Materials testing machine.

Standardised metatarsal bone models made of Tufnol tubes (10 mm diameter) were used (Figure 2). We used Tufnol tubes for our study as they exhibit similar properties to bone and are commonly used in biomechanical studies3,4.

The Tufnol tubes were 15 cm in length and they were osteotomised in the centre with a saw. The osteotomised tubes were stabilised using a single staple applied perpendicular to the osteotomy. Fixation of the near and far cortices (bicortical) was done for each specimen. 5 specimens were constructed for each staple type (n = 5 for each staple type).

Mechanical testing

The prepared specimens were mounted on a materials testing machine (ESH Ltd, Brierly Hills, United Kingdom) (Figure 3). Each construct was tested in four point bending and torsional stress (Figure 4). Load displacement curve and stiffness of fixation of each staple were measured. A constant temperature of 37 degrees Centigrade was maintained throughout the experiment using a thermostat controlled fan heater.

Figure 4. Specimen undergoing four point bending stress.
Figure 5. Deformation of construct on four point bending stress.

Outcome measures and statistical analysis

The primary outcome measure was the moment to achieve 10 degree of bending or torsion at the osteotomy site of the specimen. A 10 degree deformation of the construct was considered as mechanical failure of the staple as in a clinical situation 10 degree angulation at a fracture or osteotomy after fixation would be considered as an implant failure (Figure 5).

One-way Analysis of Variance (ANOVA) was used for statistical analysis of the results and the Tukey test was used for pairwise comparison between the staples. p &lt 0.05 was considered to be statistically significant.

Bending stress
In four point bending, there was a non-linear relation between the bending angle and the bending moment. The average moment to achieve 10 degree of bend at the osteotomy site of the specimens was as follows: Richards staple: 1.9

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