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Smart Toe Implant

Hammertoe deformities occur on the feet of people with various foot types and anatomical structures. For a digital deformity to develop, several factors must exist. There must be abnormal pull of the intrinsic and extrinsic musculature of the foot, reactive forces between the foot and the environment are abnormal, and alterations in the function of the digit. While methods for fixing the deformities are varied, it is common for these problems to lead to ambulatory problems and painful syndromes.

Biomechanics

Normally during the gait cycle, muscles in the foot and the extrinsic muscles of the leg fire in a particular order to stabilize the toes. Since the concept of hammertoe occurs mostly in the sagittal plane, it is important to understand how the normal toe functions. During the gait cycle, the muscles that affect the position of the digit include the Extensor Digitorium Longus and Brevis, Flexor Digitorium Longus and Brevis, Dorsal and Plantar Interossei, Lumbricales, and the Quadratus Plantae. In order for the digit to be a rigid beam during stance phase, the interossei muscles must fire first to stabilize the base of the proximal phalanx against the ground before the long and short flexors fire. When this fails to happen, digital deformities develop.1

A hammertoe is where you have MPJ extension, PIPJ Flexion, and DIPJ extension. Hammertoe deformities can be divided into three main types, Flexor Stabilization (the most common), flexor substitution, and extensor substitution. Flexor stabilization occurs in a pronated foot late in the gait cycle when the flexor digitorium longus gains mechanical advantage over the intrinsic interossei by firing before the interossei have time to stabilize the proximal phalanx. Flexor substitution occurs in the supinate or cavus foot type in the late stages of the gait cycle where the extrinsic flexor tendons gain mechanical advantage over the interossei muscles, usually associated with a weak triceps surae. Extensor substitution occurs in the swing phase of gait where the extensor digitorium longus gains mechanical advantage over the lumbrical muscles.2

Historical Perspective

Multiple procedures have been described for the correction of hammertoes. They include tenotomies and capsulotomies, arthroplasties, syndactylization, and arthrodeses. Based on the type of deformity and its severity, a surgeon must choose the appropriate procedure for each individual patient. When an arthodesis is the procedure of choice, the Smart toe implant provides a new alternative to the traditional k-wire fixation, or newer technologies that have been developed.

In 1910, Soule first described the proximal interphalangeal joint arthrodesis of the lesser digits. The head of the proximal phalanx and base of the middle phalanx were resected via a plantar incision. Buried cat-gut sutures were used to reapproximate the subcutaneous structures and a plaster of Paris bandage maintained the digits in extension for 6 weeks. A Kirschner wire was first used by Taylor and Sheffield in 1940 to improve the stability of the fusion site for earlier mobilization and ambulation. Similarly, in 1941, Selig used a Kirschner wire across the arthrodesis site while adding a bend at the distal end of the wire to prevent pin migration. In 1931, Higgs 4 developed the spike-in-hole arthrodesis procedure, which involves resection of the articular surfaces of the proximal interphalangeal joint. A sharp cone or "spike" was constructed out of the proximal phalangeal head, which was then inserted into a hole bored in the marrow cavity of the base of the middle phalanx. In 1938, Young improved this procedure by converting the spike into a truncated cone and maintaining the integrity of the central dorsal cortex. In 1983, Schlefman et al., modified this technique into what is known today as the peg-in-hole arthrodesis. The authors fashioned a peg out of the dorsal cortex of the proximal phalangeal head, drilled a corresponding hole in the medullary canal of the middle phalangeal base, and used Kirschner wire fixation.3

Why Use Smart Toe

Advantages of the Smart Toe implant are that the implant is one piece, no connection or failure point exisits. There is no post operative implant exposure. Uneffected joints are not violated during the implantation of this implant. There is an active compressive force that is distributed through the implant that resists rotation. The procedure is simple with no need for implant removal, or changes in normal post operative care. The implant is composed of Nitinol, (an acronym for NIckel TItanium Naval Ordnance Laboratory) is a family of intermetallic materials, which contain a nearly equal mixture of nickel (55 wt. %) and titanium. Other elements can be added to adjust or "tune" the material properties. Nitinol exhibits unique behavior. The two terms used to describe this behavior are "Shape Memory" and "Superelasticity".4

Implantation

Technique for implantation of the smart toe is similar to these procedures. A dorsal incision is made over the digit, and normal dissection is undertaken. Resection of the head of the proximal phalanx and base of the middle phalanx is performed on a slightly oblique manner. Use the specific broaches to prepare (Fig. 1) the medullary canal (the starter awl or drill may be used to prepare a pilot hole) Remove the implant from the freezer: the work time before expansion is approximately 1.5 to 2 minutes. Using the provided forceps, start the implantation in proximal bone, and then place in distal bone by flexion/ distraction of the PIP joint (Fig. 3). The implant should be approximately 75% in the proximal phalanx and 25% in the distal phalanx.5

Conclusion

Usage of the newer technologies for hammertoe arthrodesis will aid the foot surgeons in improved clinical outcomes. The Smart Toe implant utilizes new materials in a biomechanically ideal way to apply compression across the fusion site while avoiding the complications and technically involved nature of other products. Since no connection is needed, it avoids disconnection or implant failure. Since there is no aspect that is absorbed, early failure of the device is unlikely, and there is no need to remove the device as it does not cross unaffected joints. Applying compression across the arthrodesis site will reduce nonunion risks and the locking nature of the material insures equal and symmetrical compression.

 
     
     
   
     
 
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