Set screw with strain gauges
Embodiments pertaining to a medical construct are disclosed. The medical construct may include a set screw extending along a longitudinal axis between a proximal portion and a distal portion, for example. In various embodiments, the set screw may have an outside thread form and an interior socket, for example. In various embodiments, the socket may extend parallel to the longitudinal axis and have a circular lower surface, for example. In various embodiments, a first strain gauge, a second strain gauge, a third strain gauge, a fourth strain gauge, and a fifth strain gauge may be provided. In at least some embodiments, each of the first, second, third, and fourth strain gauges may be symmetrically disposed with respect to the lower surface, for example. Additionally, in at least some embodiments, the fifth strain gauge is disposed within a center portion of the lower surface, for example.
1 . A medical construct comprising:
a set screw extending along a longitudinal axis between a proximal portion and a distal portion, wherein the set screw is configured to engage with a threaded portion of a tulip head and fix a spinal rod relative to the tulip head, the set screw comprising:
an outside thread form;
a tip portion extending outward from a distal end of the distal portion;
a socket disposed within an interior of the set screw relative to the outside thread form, the socket extending parallel to the longitudinal axis and having a lower surface; and
a plurality of strain gauges disposed on the lower surface, each of the plurality of strain gauges being configured to measure a force applied to the tip portion of the set screw by the spinal rod when the set screw engages with the threaded portion of the tulip head and fixes the spinal rod relative to the tulip head,
wherein each of the plurality of strain gauges are symmetrically disposed with respect to the lower surface.
2 . The medical construct recited in claim 1 , wherein the lower surface comprises an edge portion defined by a radius of a circle.
3 . The medical construct recited in claim 2 , wherein each of the plurality of strain gauges comprises an outer curved edge approximating a curvature of the edge portion of the lower surface.
4 . The medical construct recited in claim 3 , wherein the plurality of strain gauges includes a first strain gauge, a second strain gauge, a third strain gauge, and a fourth strain gauge.
5 . The medical construct recited in claim 4 , wherein:
the first strain gauge is disposed in a first quadrant of the lower surface;
the second strain gauge is disposed in a second quadrant of the lower surface;
the third strain gauge is disposed in a third quadrant of the lower surface; and
the fourth strain gauge is disposed in a fourth quadrant of the lower surface.
6 . The medical construct recited in claim 1 , wherein the lower surface extends in a direction that is perpendicular to the longitudinal axis.
7 . The medical construct recited in claim 6 , wherein the lower surface comprises a curved edge portion defined by a radius of a circle.
8 . The medical construct recited in claim 7 , wherein the plurality of strain gauges includes a first strain gauge, a second strain gauge, a third strain gauge, and a fourth strain gauge.
9 . The medical construct recited in claim 8 , wherein:
the first strain gauge comprises a first outer curved edge approximating a curvature of the curved edge portion of the lower surface;
the second strain gauge comprises a second outer curved edge approximating the curvature of the curved edge portion of the lower surface;
the third strain gauge comprises a third outer curved edge approximating the curvature of the curved edge portion of the lower surface; and
the fourth strain gauge comprises a fourth outer curved edge approximating the curvature of the curved edge portion of the lower surface.
10 . The medical construct recited in claim 9 , wherein:
a first distance between the first outer curved edge of the first strain gauge and the curved edge portion of the lower surface remains the same along a length of the first outer curved edge;
a second distance between the second outer curved edge of the second strain gauge and the curved edge portion of the lower surface remains the same along a length of the second outer curved edge;
a third distance between the third outer curved edge of the third strain gauge and the curved edge portion of the lower surface remains the same along a length of the third outer curved edge; and
a fourth distance between the fourth outer curved edge of the fourth strain gauge and the curved edge portion of the lower surface remains the same along a length of the fourth outer curved edge.
11 . The medical construct recited in claim 10 , wherein the first distance, the second distance, the third distance, and the fourth distance are the same.
12 . The medical construct recited in claim 1 , wherein the plurality of strain gauges are interconnected.
13 . The medical construct recited in claim 1 , wherein at least one of the plurality of strain gauges is chosen from the group consisting of: a linear strain gauge, a rosette foil strain gauge, a torsion strain gauge, a double linear foil strain gauge, a full bridge strain gauge, and a chain foil strain gauge.
14 . A medical construct comprising:
a set screw extending along a longitudinal axis between a proximal portion and a distal portion, wherein the set screw is configured to engage with a threaded portion of a tulip head and fix a spinal rod relative to the tulip head, the set screw comprising:
an outside thread form;
a tip portion extending outward from a distal end of the distal portion;
an interior socket extending parallel to the longitudinal axis and having a circular lower surface; and
a first strain gauge, a second strain gauge, a third strain gauge, a fourth strain gauge, and a fifth strain gauge;
wherein:
each of the first, second, third, and fourth strain gauges are configured to measure a force applied to the tip portion of the set screw by the spinal rod when the set screw engages with the threaded portion of the tulip head and fixes the spinal rod relative to the tulip head,
each of the first, second, third, and fourth strain gauges are symmetrically disposed with respect to the circular lower surface, and
the fifth strain gauge is disposed within a center portion of the circular lower surface.
15 . The medical construct recited in claim 14 , wherein each of the first, second, third, and fourth strain gauges are symmetrically disposed within a respective quadrant of the circular lower surface.
16 . The medical construct recited in claim 15 , wherein the tip portion comprises a hemispherical tip portion at the distal end that is coaxially aligned with the longitudinal axis.
17 . The medical construct recited in claim 16 , wherein the fifth strain gauge is circular and a center point of the fifth strain gauge is coaxially aligned with the longitudinal axis.
18 . The medical construct recited in claim 14 , wherein:
the circular lower surface comprises an outer edge portion having a consistent curvature,
the first strain gauge comprises a first outer curved edge approximating the consistent curvature of the outer edge portion of the circular lower surface;
the second strain gauge comprises a second outer curved edge approximating the consistent curvature of the outer edge portion of the circular lower surface;
the third strain gauge comprises a third outer curved edge approximating the consistent curvature of the outer edge portion of the circular lower surface; and
the fourth strain gauge comprises a fourth outer curved edge approximating the consistent curvature of the outer edge portion of the circular lower surface.
19 . The medical construct recited in claim 18 , wherein the first strain gauge, the second strain gauge, the third strain gauge, and the fourth strain gauge have the same size and shape.
20 . The medical construct recited in claim 19 , wherein:
the outside thread form of the set screw is configured to be inserted within and mate with an internal thread form of the tulip head, the tulip head comprising a receiving cavity configured to receive the spinal rod,
a lower portion of the tulip head is configured to couple to a pedicle screw,
the set screw is configured to secure the spinal rod within the receiving cavity,
the first strain gauge, second strain gauge, third strain gauge, fourth strain gauge, and fifth strain gauge are each configured to provide an independent measurement of the force applied to the tip portion of the set screw, and
each independent measurement is indicative of an average load between the spinal rod and the tip portion of the set screw.