Guest Editorial: Dr. Slavitt is Chief of the Division of Podiatry, Department of Orthopedics, Northwest Hospital Center, Baltimore, MD. He is the Residency Director for the Baltimore VA Medical Center, Northwest Hospital rotation, Board Certified, American Board of Podiatric Surgery.
Compression Staples for Bone Fixation in Podiatric Surgery	by Jerome A. Slavitt DPM, FACFAS Chief of the Division of Podiatry, Department of Orthopedics Northwest Hospital Center Baltimore, Maryland

The use of staples for osseous fixation has been used in the United States since 1906. These internal fixative devices were developed to assist in improving reduction and maintaining stabilization across a fracture or osteotomy site. Early staples were labeled as mechanical or pre-compression devices. They maintained a permanent form using compression or reduction proportional to the application of force provided by a particular instrument. The next generation of staples was introduced into the United States in 2001, the true compression or "shape memory alloy (SMA)" staple. This was developed by the Naval Ordnance Laboratory by combining nickel and titanium. Hence, the acronym Nitinol. These are manufactured with specific shape and stiffness designed to specific temperature requirements.

Compression across an osteotomy site potentiates an increase in friction and counteracts shearing forces. It insures good approximation of bone fragments with dynamic compression and has shown to require in some cases, less operating room time when compared to bone screws. Due to irregularities in osseous anatomy, stable fixation is dependent on staple dimensions, e.g. leg length, thickness, and width. Additionally, it behooves the surgeon to have a non-tension osteotomy with correct angle of staple insertion, adequate bone density along with consideration of power versus hand driven instrumentation.

Heat activated compression staples, Nitinol, behave differently than standard metals when used for stable fixation. When heated, this metal contracts and produces 100 times greater thermal movement compared to standard metals. Shape memory staples facilitate the storing of energy in the implant to impart residual compression. In order for successful bone healing to occur across the osteotomy site, frictional forces maintained by compression must exceed deforming forces, i.e. non-tension osteotomy. Staples provide a high degree of stability and subsequent compression which yields the likelihood of successful fusion.

Major staple providers such as BioPro, BME, Telos Medical, Integra, and DePuy, all present with the same general shape. All staples are manufactured by one of three companies, two in the United States and one in Europe, and are marketed by one of the aforementioned companies and advertised trade names. Shape memory staples possess transitional temperatures ranging from 25°C to 50°C or 77°F 125.6°F. Each company dictates their specific staple designs and their own activation temperature. With the exception of temperature variations all compression staples have the same physical shape characteristics. Temperature design characteristics are seen in Table 1.

These are the peak activation temperatures yielding specific pounds of force at that specific temperature. It must be understood that compression forces at peak temperature compression activation are not the same forces at body temperature. Additionally, manufacturers of the OSStaple, Memodyne, and Aeon require activation instrumentation, whereas BioPro and Depuy rely only on body temperature for activation ( Figure 1).

Staples can be used for all types of fixation including forefoot, midfoot, and rear foot procedures. Nitinol (nickel and titanium) chemical components provide excellent trabecular response in regards to bone healing as compared to stainless steel. Overall bone quality is less of an issue when using Nitinol. These staples are at their optimum superelastic behavior at body temperature.

Key point: do not rely on compression staples to approximate the osteotomy site, but rather to maintain the coaptated positions of the bones using the staples compression force. As previously stated, staple activation temperatures in relation to force are designed differently by each company (Table 2).

Therefore, a maximum compression force at 131°F will not be the same compression force at 98°F. Perfect osteotomies are those which maintain their apposition prior to fixation. Tension across the osteotomy site which may yield too gapping, should be avoided. Additionally, full bone to bone contact should be inherent across the entire osteotomy site. Very often osteotomies may take on a teardrop presentation at the hinge area due to abnormal positioning of the bone saw or repetitive cuts through the same osteotomy site.

Let's examine a first metatarsal base osteotomy with the apex medial and base proximal. The teardrop affect presents as a rounded, enlarged, or gapped area at the medial segment with closely approximated bone laterally. This is developed because the proximal and distal cuts of the wedge osteotomy although separate, meet at the medial hinge area thereby causing additional cuts from the osteotomy blade. When the osteotomy is approximated the lateral cortex appears coaptated end to end and the medial hinge area shows a slightly enlarged area of bone removal due to several passes in the same plane from the saw blade. This is where the concept of "feathering" is used for correction. Feathering provides complete approximation of the osteotomy sites. To perform feathering correctly, the osteotomy site is approximated and the saw blade is introduced at the medial hinge area applying the saw blade pressure across the osteotomy from medial to lateral, dorsal to plantar, remodeling the lateral section to cause uniform bone to bone contact across the site. After the initial pass, the osteotomy is again visualized for complete approximation. Additional feathering may be required. A conjoined Temple University study was performed showing the pounds of force at the activation temperature as compared to pounds of force with temperature stabilization at 98°F., body temperature.

Tests were performed using the Fisher Scientific HFG-45 Hand Held Force Gauge and the Fisher Scientific Digital Thermometer (Figure 2). The test was performed by mounting the staple in the force gauge and then immersing in a water bath which reached the activation temperature as set down by each manufacturer. That temperature measurement was recorded along with the pounds of force. The water was then cooled to body temperature, 98.6°F and the final resting force was measured. All results were graphed. With the DePuy staple, once removed from the freezer, compression began almost immediately, 62°F. You better work quickly!

So, how does one chose the appropriate stable manufacturer taking product reps out of the equation? Is a staple in fact the right choice for fixation in your selected type of osteotomy? I personally do not recommend using staples for lesser digital arthrodesis due to the smaller bone areas involved along with skin thinness in these locations. Additionally, in other parts of the foot, placement of staples may cause some irritation due to tendon location, lack of subcutaneous tissue as a natural buffer, and specific anatomical contours of bone which may not allow the staple to lie in a parallel plane consistent with the cortical area.

Since all staples are manufactured essentially the same with their shape physical characteristics, your choice should be based on the temperature property selected by a manufacturer for activation and subsequent resting temperature (98°F) compression force. I have had the opportunity to use the majority of staples on the market today and I am comfortable using the BioPro Compression Staple for several reasons. Certain manufacturers require activation using ancillary devices to achieve temperature and compression. BME requires the use of external actuator with a wand that must be sterilized prior to use. This device may require Bio-Med Testing, an additional charge to the hospital or surgery center. Memodyne and Integra use a form of cautery device to activate their staple. The BioPro and Depuy staples are activated by body temperature alone using no ancillary actuator devices. Compression is hastened after implantation by irrigating the staple area with 100°F saline solution or lactated Ringer's. More importantly, the compression obtained on the operating table in pounds of force will be the same after closure. Therefore, based on the staple manufacturers activation temperature and corresponding force at that temperature, when the staple drops to or reaches body temperature, the compression force will have reduced. Based on the quality of the osteotomy and tension, once the staple reaches 98°F there may be some separation. I like the idea, "What you see is what you get." This phrase relates to the BioPro and Depuy staples.

In order to determine the correct staple size, the manufacturers provided a clear plastic overlay of their staples with measured size for leg lengths and diameters (see Figure 3). After taking the appropriate AP and lateral x-ray, the plastic overlay is placed over the film on the view box or digital screening and the appropriate staple size selection is made. For example, a first metatarsal base wedge osteotomy will incorporate a staple with a long and short arm, based on the anatomical contour of the first metatarsal. The long arm will be on the proximal side of the osteotomy and short arm on the distal side of the osteotomy. It is my recommendation to choose the longest leg length possible without meeting or penetrating the plantar cortex.

Figure 3.

Surgical Technique: First MTP fusion (BioPro System)

1. Remove the articular surfaces with the use of power equipment. Articular surfaces at other locations may be removed by curetting or finestrating. (see Figure 4)
2. Temporarily fixate the osteotomy site.
3. Performing the appropriate drill holes may be accomplished by your choice of using the single hole drill guide or adjustable drill guide. Both will be presented.

1. Using the single hole drill guide:


1. Using the appropriate color-coded drill and guide, drill the first hole approximately 0.5 cm +/- from the osteotomy site at the proximal end (from dorsal to plantar, perpendicular to the long axis). (see Figure 5)
2. Place the small leg of the color-coded guide in the proximal hole, drill the distal hole. (see Figure 6)
3. Retrieve the appropriate size staple with the staple holder taking care not to touch the staple with your hands (due to body heat activation), seat the staple in the appropriate holes and finalize using the correct size staple impactor. (see Figure 7)

Figure 4.

Figure 5.

Figure 6.

Figure 7.

b. Using the adjustable drill guide:
1. Set the number on adjustable drill guide corresponding to the staple number. (see Figure 8)
2. Using the appropriate color-coded drill bit, drill the proximal hole first.
3. Insert the anchor pin into the proximal hole for stability and drill the distal hole. (see Figure 9)
4. Repeat step 3a
   
   

Figure 8.

Figure 9.

The ultimate success of any bone surgery is the correct non-tension bone to bone apposition with stabilization of the fusion site prior to the use of any fixative device. The surgeon's comfort zone using the product is also important. With all the types of fixation devices available to the surgeon how does one choose? If your choice is staples here are some advantages.

1. Surgical technique is relatively simple
2. Instrumentation is usually color coded
3. Provides excellent compression
4. Bio-compatible materials
5. Better for more questionable bone stock
6. Hospital cost reduction (BioPro, Depuy, no Med-Testing)
7. No extra sterilization or ancillary actuators (BioPro, Depuy)

If you have never used staples before it’s time to get your hands wet. As a resident, this is the time for you to be exposed to as many types of fixations and staples play a prominent role in podiatric surgery.