Staples for bone fixation

Details Staples for bone fixation Staples for bone fixation

2001-02-28

2004-02-03

Lewis, Ralph A. (Department: 3732)

Surgery

Instruments

Orthopedic instrumentation

C606S075000

Reexamination Certificate

active

06685708

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to staples for bone fixation, formed of shape-memory-alloys (SMA) and other biocompatible metals and alloys. The present invention relates in particular to SMA staples of adjustable length spans.
BACKGROUND OF THE INVENTION
Titanium-nickel, shape-memory alloys are biocompatible and resistant to corrosion; therefore, they are suitable for medical applications. These alloys have different phase structures, hence, different mechanical properties, at different temperatures. Information about shape memory alloys may be found, for example, on web site www.nitinol.com, by Nitinol Devices & components, copyright 1998, and in Conference information of “The Third International conference on Shape Memory and Superelastic Technologies Engineering and Biomedical Applications,” held in Pacific Grove, Calif. during Apr. 30-May 4, 2000.
FIGS. 1A and 1B
, together, schematically illustrate a typical temperature hysteresis, typical elastic stresses, es, in phase transitions, and typical stress-strain curves for a shape-memory alloy in the austenitic and martensitic phases. At a low temperature, the alloy is martensitic, and is soft and plastic, having a low es. At a high temperature, the alloy is austenitic and tough, having a high es. When a martensitic alloy is heated to a temperature A
s
, the austenitic phase begins to form. Above a temperature A
f
, the alloy is fully austenitic. Likewise, as an austenitic alloy is cooled to a temperature M
s
, the martensitic phase begins to form. Below a temperature M
f
, the alloy is fully martensitic.
The temperature-dependent phase structure gives rise to shape memory. At the fully austenitic phase, under proper heat treatment and working conditions, an SMA element can be given a physical shape and “pre-programmed” to memorize that shape and resume it, whenever in the austenitic phase. The “memorized” SMA element may then be cooled to a martensitic phase and plastically deformed in the martensitic phase. But when heated back to the austenitic phase is will resume its memorized shape. The transformation temperature between the phases is noted as TTR.
The reason for the shape memory is found in the phase structure of the alloy. Most metals deform by atomic slip. Dislocations and atomic planes slide over one another and assume a new crystal position. In the new position, the crystal has no memory of its order prior to the deformation. With increased deformation, there is generally a work-hardening effect, in which the increased tangle of dislocations makes additional deformation more difficult. This is the case even when the increased deformation is in the direction of restoring the crystal to its original shape. However, for shape memory alloys, both transitions between the austenitic and martensitic phases and deformation in the martensitic phase change lattice angles in the crystal, uniformly for the whole crystal. The original austenitic lattice structure is “remembered” and can be restored.
FIG. 1C
schematically illustrates typical phase structures of a shape-memory alloy, as functions of temperature and deformation, as follows:
in the austenitic phase, the crystal has a cubic structure, and the atoms in the lattice are arranged generally at right angles to each other;
when the austenitic crystal is cooled to a martensitic phase, a twinned lattice structure is formed;
when the twinned martensitic crystal is deformed by an amount no greater than &dgr;, the twinned structure is “stretched” so that the atoms in the lattice are arranged generally at oblique angles to each other, wherein the oblique angles are determined by the amount of deformation; and
when the deformed martensitic crystal is heated, the crystal resumes its cubic structure, wherein, again, the atoms in the lattice are arranged generally at right angles to each other.
Another property that can be imparted to SMA elements, under proper heat treatment and working conditions, is super-elasticity, or Stress-Induced Martensite (SIM). With this property, a fully austenitic SMA element, at a temperature above A
f
, will become martensitic and plastic under high stress, and deform under the stress. When the stress is removed, the SMA element will return to the austenitic phase and to its memorized shape in the austenitic phase. Super-elasticity is also referred to as rubber-band like property, because the SMA element behaves like a rubber band or a spring, deforming under stress and resuming its original shape when the stress is removed. However, this property is present only above the temperature A
f
, and only when it is specifically imparted to an SMA element, by proper heat treatment and working conditions.
FIG. 1D
schematically illustrates a typical cyclic transformation of a super elastic alloy, at a constant temperature above the temperature A
f
. The transformation between the austenitic phase and a stress-induced martensitic phase is brought about by stress and is eliminated when the stress is removed.
It should be emphasized that both full shape memory and stress-induced superelasticity occur as long as the deformation is no greater than &dgr;, and with greater deformations the crystal structure will be damaged.
Staples and clamps for bone fixation of fractures, formed of shape-memory alloys, are known. They are easily inserted in a martensitic phase, when deformed to an open, straightedge state, and they resume a closed, clamped state in the body, thus forming a closure on the fracture.
Basically, there are two approaches to working with SMA elements for bone fixation. In accordance with the first approach, the elements are fully martensitic at room temperature and are deformed and inserted into the bone when at room temperature. After insertion, the elements are locally heated to about 42-45° C., a temperature above A
f
, and transform to the austenitic shape, resuming their memorized austenitic shape. The staples then cool down to body temperature, which is generally below A
f
, although still above M
s
. Thus, in the body, the SMA elements remain austenitic and retain their austenitic shape. The advantage of this approach is that the SMA elements need not be cooled in order to remain in the martensitic phase, prior to insertion. The disadvantages, however, are that the mechanical properties of the SMA elements are not uniquely defined at body temperature, and that the SMA elements are not super-elastic in the body.
In accordance with the second approach, A
f
is designed below body temperature. The SMA elements are cooled to 0-5° C., or lower, to a temperature below their M
f
temperature, for deformation and insertion into the bone. Upon insertion, the elements are naturally heated to body temperature, by contact with the body only. Since body temperature is above A
f
, the elements transform to the austenitic phase and resume their memorized austenitic shape. The advantages of this approach are that, in the body, the SMA elements are fully austenitic, their mechanical properties are defined, and if properly heat-treated, they are super-elastic. The disadvantage, however, is that plastic deformation in the martensitic phase must be performed after the elements are cooled, and the deformed SMA elements must remain cooled during procedure manipulation and insertion.
The publication, “Use of TiNiCo Shape-Memory Clamps in the Surgical Treatment of Mandibular Fractures,” by Drugacz J., et al., American Association of Oral and Maxillofacial Surgeons, 0278-2391/95/5306-0006, describes a study in which clamps made of Ti
50
Ni
48.7
Co
1.3
, memorized to resume their shape at body temperature, were used to fix mandibular fractures. Seventy-seven patients with mandibular single or multiple fractures were treated, using 124 clamps. In 72 of the 75 patients, the treatment progressed satisfactorily, and only in five cases, infections occurred. The study concluded that the application of shape-memory clamps for surgical treatment of mandibular fractures facilitated treatment and ensured stable fixation of the bone fragments. Ther

Affiliated with

Also associated with

Rating

Staples for bone fixation does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Staples for bone fixation, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Staples for bone fixation will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3282573