Cannulated continuous compression screw
A bone screw may be insertable into a bone. The bone screw may have a distal member with bone-engaging threads, a proximal member configured to slidably engage the distal member, and a tension member with a proximal end coupled to the proximal member, and a distal end coupled to the distal member such that, in response to motion of the distal member away from the proximal member, the tension member elongates and urges the distal member to move toward the proximal member.
1. A bone screw insertable into a bone, the bone screw comprising:
a distal member comprising bone-engaging threads;
a proximal member configured to slidably engage the distal member; and
a tension member coupled to the proximal member and the distal member such that, in response to motion of the distal member away from the proximal member, the tension member elongates and urges the distal member to move toward the proximal member;
wherein:
the proximal member is further configured to cooperate with the distal member and/or the tension member to define a length limiting mechanism configured to prevent the bone screw from decreasing below a minimum length and from exceeding a maximum length that is greater than the minimum length;
the tension member is at least partially formed of a superelastic material having an upper plateau stress and a lower plateau stress;
the tension member imparts a first compressive force between the proximal member and the distal member when the bone screw is at the minimum length and a second compression force between the proximal member and the distal member when the bone screw is at the maximum length; and
the maximum length is selected such that, at the maximum length, the tension member is at the upper plateau stress;
the minimum length is selected such that, at the minimum length, the tension member is at the lower plateau stress;
the length limiting mechanism comprises a distal shoulder, a proximal shoulder, and a stop feature, each of which is on the proximal member, the tension member, and/or the distal member; and
the stop feature is positioned between the distal shoulder and the proximal shoulder to define the minimum length and the maximum length by abutting the distal shoulder and the proximal shoulder.
2. The bone screw of claim 1 , wherein the tension member further comprises a shaft extending from a proximal end to a distal end of the tension member, the shaft comprising a cannula sized to receive a guidewire.
3. The bone screw of claim 1 , wherein the proximal member comprises a shank and a head comprising head threading with a first pitch different from a second pitch of the bone-engaging threads.
4. The bone screw of claim 1 , wherein:
the proximal member and the distal member define a longitudinal axis; and
the proximal member comprises a shank and a head comprising a proximal-most surface that is nonperpendicular to the longitudinal axis.
5. The bone screw of claim 1 , wherein:
the proximal member and the distal member define a longitudinal axis;
one of the distal member and the tension member comprises a distal engagement surface facing away from the longitudinal axis;
the proximal member comprises a proximal engagement surface facing toward the longitudinal axis; and
the distal engagement surface is positioned to abut the proximal engagement surface to transmit bending loads from the distal member to the proximal member without transmission of torque about the longitudinal axis between the distal engagement surface and the proximal engagement surface.
6. The bone screw of claim 1 , wherein the tension member is configured such that, at the minimum length of the bone screw, the tension member is pre-stretched so that the tension member is under tension to urge the bone screw to shorten.
7. The bone screw of claim 1 , wherein:
the tension member is further configured to have the upper plateau stress at a first temperature and to have a second lower plateau stress at a second temperature higher than the first temperature; and
the upper plateau stress is similar in value to the second lower plateau stress.
8. A bone screw insertable into a bone, the bone screw comprising:
a distal member comprising bone-engaging threads;
a proximal member configured to slidably engage the distal member and to cooperate with the distal member to define a length limiting mechanism configured to prevent the bone screw from decreasing below a minimum length and from exceeding a maximum length that is greater than the minimum length; and
a tension member coupled to the proximal member and the distal member such that, in response to motion of the distal member away from the proximal member, the tension member elongates and urges the distal member to move toward the proximal member;
wherein:
the tension member is at least partially formed of a superelastic material having an upper plateau stress at a first temperature and a lower plateau stress at a second temperature;
the bone screw is at the minimum length when the superelastic material is at the lower plateau stress; and
the bone screw is at the maximum length when the superelastic material is at the upper plateau stress.
9. The bone screw of claim 8 , wherein the tension member further comprises a shaft extending from a proximal end to a distal end of the tension member, the shaft comprising a cannula sized to receive a guidewire.
10. The bone screw of claim 8 , wherein the proximal member comprises a shank and a head comprising head threading with a first pitch different from a second pitch of the bone-engaging threads.
11. The bone screw of claim 8 , wherein:
the proximal member and the distal member define a longitudinal axis; and
the proximal member comprises a shank and a head comprising a proximal-most surface that is nonperpendicular to the longitudinal axis.
12. The bone screw of claim 8 , wherein:
the proximal member and the distal member define a longitudinal axis;
the distal member comprises a distal engagement surface facing away from the longitudinal axis;
the proximal member comprises a proximal engagement surface facing toward the longitudinal axis; and
the distal engagement surface is positioned to abut the proximal engagement surface to transmit bending loads from the distal member to the proximal member without transmission of torque about the longitudinal axis between the distal engagement surface and the proximal engagement surface.
13. The bone screw of claim 8 , wherein the tension member is configured such that, at the minimum length of the bone screw, the tension member is pre-stretched so that the tension member is under tension to urge the bone screw to shorten.
14. The bone screw of claim 8 , wherein the second temperature is an internal body temperature of a patient.