Patent Publication Number: US-2023149061-A1

Title: Rotational guided growth devices, systems, and methods

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of U.S. patent application Ser. No. 17/525,847 filed on Nov. 12, 2021, entitled “Rotational Guided Growth Devices, Systems, and Methods, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to bone fixation devices, systems, and methods. More specifically, the present disclosure relates to tether assemblies, systems, and methods for surgically changing the rotational alignment of intact bones. 
     BACKGROUND 
     In orthopedics, rotational deformities of the bones of the lower extremities can change the relative orientation between various anatomical features of the hip, knee, and ankle. For example, in the femur, angulation of the femoral neck in the upper femur with respect to the transcondylar axis of the lower femur is referred to as femoral anteversion. In normal human development, femoral anteversion is generally about 11°.  FIG.  1 B  is a superior view of a femur over the bones of a foot, with normal anteversion, enabling proper gait with the foot facing forward. 
     In contrast to normal human development,  FIG.  2 B  illustrates an abnormal femoral anteversion angle of about 41°, or 30° of abnormal anteversion. This abnormal femoral anteversion results in a knee that twists inward relative to the hip, which results in “in-toeing” of the foot. This may predispose a patient to joint injuries at either end of the femur, such as ligament or labrum injuries in the hip joint and patella dislocations or ligament (e.g., anterior cruciate ligament) injuries in the knee joint. Persistent rotational deformity due to anteversion cannot be corrected with a brace or with physical therapy. Rotational deformities are also referred to as torsional deformities. In either case, these terms refer to the orientation of anatomic features relative to the long axis of the bone. 
     The current standard-of-care surgical remedy for anteversion is a rotational osteotomy of the femur. The femur is severed and the superior and inferior segments are re-attached to each other at a relative orientation that provides proper anteversion. This typically requires internal fixation with a large plate or intramedullary rod that is usually removed once the bone has healed after the procedure. 
     Specifically, during a traditional correction procedure for abnormal femoral anteversion, called a femoral de-rotation osteotomy, the surgeon cuts the femur perpendicular to the long axis of the bone, rotates the distal portion outward, typically about 20° to 30°, relative to the proximal end of the bone to achieve the correct rotational alignment, and then reattaches the transected bone portions together. A large bone plate or an intramedullary rod is then implanted to hold the transected bone portions in a corrected rotational alignment. However, this surgery is extremely invasive and associated with many negative side effects. Some of the negative side effects associated with this procedure include: (1) significant pain associated with bone cutting and healing; (2) relatively large incision and resulting large scar with increased risk of surgical site infection and wound complications, (3) delayed walking for weeks and sometimes months after the procedure to protect the bone while it heals; (4) risk of loss of bone fixation or implant failure and subsequent reoperation; (5) risk of delayed bone healing; (6) risk of non-union of the bone; (7) risk of neurovascular injury, etc. Accordingly, improved implant devices, systems, and methods that can alleviate some, or all, of these negative side effects would be desirable. 
     SUMMARY 
     The various implant devices, systems, and methods of the present disclosure have been developed in response to the present state of the art, and in response to the problems and needs in the art that have not yet been fully solved by currently available implant devices, systems, and methods. In some embodiments, the implant devices, systems, and methods of the present disclosure may provide improved rotational correction of the long bones of the lower extremities. 
     In some embodiments, a tether assembly may be attached to a bone to correct a rotational deformity in a bone, such as femoral anteversion. The bone may have a growth plate that separates a first section of the bone from a second section of the bone. The tether assembly may have a tether member with a first end, a second end, and a central portion extending between the first end and the second end. The first end may have a closed outer wall that defines and fully bounds a first aperture. The second end may have an open outer wall that defines and partially bounds a second aperture. The open outer wall may define a slot in communication with the second aperture. The first and second ends may be securable to the first and second sections of the bone via coupling members inserted through the first and second apertures and anchored in the first and second sections, respectively. 
     The tether assembly may further include the first coupling member, which may have a first head and a first shank with a first bone engagement feature configured to retain the first shank in the bone. The tether assembly may further include the second coupling member, which may have a second head and a second shank with a second bone engagement feature configured to retain the second shank in the bone. 
     The second shank may have a second shank width and the second head may have a second head width. The second shank width and/or the second head width may not be smaller than a corresponding portion of the slot through which it must pass in order to exit the second aperture through the slot, such that the second coupling member is movable through the slot only in response to exertion of a threshold level of tension between the second coupling member and the second end. 
     The corresponding portion of the slot may be configured to deform elastically to permit passage of the second shank therethrough in response to exertion of the threshold level of tension. 
     At least one of the first head and the second head may have a spherical surface. The corresponding one of the first aperture and the second aperture may have a complementary spherical surface sized to receive the spherical surface to provide adjustable positioning of the first head or the second head relative to the first aperture or the second aperture. 
     The central portion may have a contoured shape created by projecting an elongate area defined on a sagittal plane onto a medial epicondylar bone surface or a lateral epicondylar bone surface of a pediatric distal femur. The elongate area may have a long axis and a short axis orthogonal to the long axis. 
     The long axis as measured on the sagittal plane may be positioned at an angle relative to a transverse plane. The angle may be within the range of 30° to 60°. 
     The slot may be oriented nonperpendicular to a longitudinal length of the central portion. 
     The slot may be oriented at an angle relative to the longitudinal length. The angle may be within the range of 30° to 80°. 
     In some embodiments, a tether assembly may be attachable to a bone to correct a rotational deformity in a bone, such as femoral anteversion. The bone may have a growth plate that separates a first section of the bone from a second section of the bone. The tether assembly may have a first coupling member with a first head and a first shank with a first bone engagement feature configured to retain the first shank in the bone. The tether assembly may further have a second coupling member with a second head and a second shank with a second bone engagement feature configured to retain the second shank in the bone. The tether assembly may further have a tether member with a first end, a second end, and a central portion extending between the first end and the second end. The first end may be configured to engage the first head to nonreleasably secure the first end to the first section of the bone. The second end may be configured to engage the second head to releasably secure the second end to the second section of the bone such that, in response to exertion of a threshold level of tension between the second coupling member and the second end, the second end is released from the second section of the bone. 
     The first end may have a fully-bounded first aperture. The second end may have a partially-bounded second aperture that is accessible via a slot. 
     The second shank may have a second shank width that is not smaller than a slot width of the slot and is movable through the slot in response to exertion of the threshold level of tension. 
     The slot may be configured to deform elastically to permit passage of the second shank therethrough in response to exertion of the threshold level of tension. 
     The slot may be oriented nonperpendicular to a longitudinal length of the central portion. 
     At least one of the first head and the second head may have a spherical surface. The corresponding one of the first end and the second end may have a complementary spherical surface sized to receive the spherical surface to provide adjustable positioning of the first head or the second head relative to the first end or the second end. 
     The central portion may have a contoured shape created by projecting an elongate area defined on a sagittal plane onto a medial epicondylar bone surface or a lateral epicondylar bone surface of a pediatric distal femur. The elongate area may have a long axis and a short axis orthogonal to the long axis. 
     In some embodiments, a method may be used to perform rotational deformity correction on a bone with a growth plate that separates a first section of the bone from a second section of the bone. The method may include positioning a tether member of a tether assembly on the bone. The tether assembly may include a first coupling member with a first head and a first shank with a first bone engagement feature configured to retain the first shank in the bone, a second coupling member with a second head and a second shank with a second bone engagement feature configured to retain the second shank in the bone, and the tether member. The tether member may have a first end, a second end, and a central portion extending between the first end and the second end. The method may further include, with the first coupling member, securing the first end of the tether member to the first section of the bone, and, with the second coupling member, releasably securing the second end of the tether member to the second section of the bone such that, in response to exertion of a threshold level of tension between the second coupling member and the second end, the second end is released from the second section of the bone. 
     The first end may have a fully-bounded first aperture. The second end may have a partially-bounded second aperture that is accessible via a slot. Securing the first end to the first section may include inserting the first shank through the first aperture and anchoring the first shank to the first section. Releasably securing the second end to the second section may include inserting the second shank through the second aperture and anchoring the second shank to the second section. 
     The slot may be oriented nonperpendicular to a longitudinal length of the central portion. 
     Each of the first head and the second head may have a spherical surface. Each of the first end and the second end may have a complementary spherical surface. Securing the first end to the first section may include receiving the spherical surface of the first head in the complementary spherical surface of the first end to provide adjustable positioning of the first head relative to the first end. Securing the second end to the second section may include receiving the spherical surface of the second head in the complementary spherical surface of the second end to provide adjustable positioning of the second head relative to the second end. 
     These and other features and advantages of the present disclosure will become more fully apparent from the following description and appended claims or may be learned by the practice of the devices, systems, and methods set forth hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present disclosure will become more fully apparent from the following description taken in conjunction with the accompanying drawings. Understanding that these drawings depict only exemplary embodiments and are, therefore, not to be considered limiting of the scope of the present disclosure, the exemplary embodiments of the present disclosure will be described with additional specificity and detail through use of the accompanying drawings in which: 
         FIG.  1 A  is a posterior view of lower skeletal extremities with normal anteversion. 
         FIG.  1 B  is a superior view of the right lower extremity in  FIG.  1 A . 
         FIG.  2 A  is a posterior view of the lower skeletal extremities with abnormal anteversion. 
         FIG.  2 B  is a superior view of the right lower extremity in  FIG.  2 A . 
         FIG.  3 A  is a medial view of a right pediatric knee with a tether assembly according to one embodiment attached to the femur, at the time of surgery. 
         FIG.  3 B  is an inferior view of the femur of  FIG.  3 A . 
         FIG.  4 A  is a medial view of a right pediatric knee with the tether assembly of  FIG.  3 A , at a first period after the surgery. 
         FIG.  4 B  is an inferior view of the femur of  FIG.  4 A . 
         FIG.  5 A  is a medial view of a right pediatric knee with the tether assembly of  FIG.  3 A , at a second period after the surgery. 
         FIG.  5 B  is an inferior view of the femur of  FIG.  5 A . 
         FIG.  6 A  is a top view of the tether assembly of  FIG.  3 A . 
         FIG.  6 B  is a side view of the tether assembly of  FIG.  3 A , showing the tether member with multiple potential orientations of each of the coupling members. 
         FIG.  6 C  is a close-up perspective view of the tether member and one coupling member of the tether assembly of  FIG.  3 A . 
         FIG.  7 A  is a top view of the tether member of the tether assembly of  FIG.  3 A . 
         FIG.  7 B  is a side elevation, partial section view of the tether member of  FIG.  3 A . 
         FIG.  8 A  is a side elevation view of a coupling member of the tether assembly of  FIG.  3 A . 
         FIG.  8 B  is a side elevation, section view of the coupling member of  FIG.  8 A . 
         FIG.  9    is a table showing a relationship between correction angle, femur width, start angle, and treatment time for plate lengths of 20 mm, 26 mm and 32 mm, respectively. 
     
    
    
     It is to be understood that the drawings are for purposes of illustrating the concepts of the present disclosure and may be drawn to scale, or may include variations from scale drawings. Furthermore, the drawings illustrate exemplary embodiments and do not represent limitations to the scope of the present disclosure. 
     DETAILED DESCRIPTION 
     Exemplary embodiments of the present disclosure will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. It will be readily understood that the components of the present disclosure, as generally described and illustrated in the drawings, could be arranged, and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the implants, systems, and methods, as represented in the drawings, is not intended to limit the scope of the present disclosure, but is merely representative of exemplary embodiments of the present disclosure. 
     The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. While the various aspects of the embodiments are presented in the drawings, the drawings are not necessarily drawn to scale unless specifically indicated. 
     The following examples have been included to provide guidance to one of ordinary skill in the art for practicing representative embodiments of the presently disclosed subject matter. In light of the present disclosure and the general level of skill in the art, those of skill in the art can appreciate that the following examples are intended to be exemplary only and that numerous changes, modifications, and alterations can be employed without departing from the scope of the presently disclosed subject matter. 
     It will be understood that any feature of any embodiment described or contemplated herein may be combined with any other embodiment that is described or contemplated herein without departing from the spirit or scope of the present disclosure. 
       FIG.  1 A  is a posterior view (i.e. a view from a posterior viewpoint) of the lower skeletal extremities showing normal alignment of the left extremity  20  and the right extremity  22  in the pediatric population.  FIG.  1 B  is a superior view of the right extremity  22 , showing the anteversion of the femoral neck  30  and femoral head  32 . It can be appreciated in  FIG.  1 B  that the foot  34  is well aligned with the knee  36  and the hip  38 . Anteversion is the angle, or anteversion angle  40 , measured between a line  42  tangent to the posterior condyles of the distal femur, also referred to herein as the transcondylar axis, and a line  44  that bisects the femoral neck and head. In  FIG.  1 B , the anteversion angle  40  is 11°, which is considered to be within the normal range in the general pediatric population. 
       FIG.  2 A  is a posterior view of the left extremity  20  and the right extremity  22  showing an abnormal alignment of the left extremity  20  and the right extremity  22  in the pediatric population.  FIG.  2 B  is a superior view of the right extremity  22 , showing the anteversion of the femoral neck  30  and femoral head  32 . It can be appreciated in  FIG.  2 B  that the foot  34  and the knee  36  are turned inward relative to the hip  38 . In  FIG.  2 B , the anteversion angle  50  is 41° (measured between the line  44  that bisects the femoral neck and head and a line  46  tangent to the posterior condyles of the distal femur), which is outside the normal range in the general pediatric population. This abnormal anteversion is observed as “in-toeing” of the feet, causing the left foot to be over rotated in the clockwise direction and the foot  34  to be over rotated in the counterclockwise direction, as viewed when looking down at the feet from a standing position. Abnormal anteversion is also referred to as a rotational or torsional deformity, as it represents an angular abnormality as viewed along the long axis of the bone. It can cause “in-toeing” as mentioned above, or splay the foot outward. Either can beneficially be corrected via the present disclosure. Although anteversion is used as a specific example, those of skill in the art will recognize that the techniques, implants, and principles taught by this disclose may be applied to other rotational deformities in femurs and/or other bones. 
       FIG.  2 A  provides an example of a bilateral femoral rotational deformity, which is common when femoral rotational deformity is present; however, unilateral femoral rotational deformities are also occasionally present in the pediatric population. Although the preceding discussion is limited to the femur, it is understood that rotational defects can exist in other bones of the extremities, such as the tibia, and that the devices, systems and methods presented herein are equally applicable to other extremity bones, including but not limited to any other bones of the legs, feet, arms or hands. 
       FIG.  3 A  is a medial view of the distal portion of a femur  58  showing a physis  60 , also referred to as a growth plate, and showing an embodiment of the present invention. The femur  58  may have an epiphyseal section  62  distal to the physis  60 , and a metaphyseal section  64  proximal to the physis  60 . The physis  60  may separate the epiphyseal section  62  from the metaphyseal section  64 . The femur  58  may be a pediatric femur, which may continue to grow longitudinally from the physis  60 . 
     Coupled to the distal portion of the femur  58  is a tether assembly  100 , which may include a tether member  102 , also referred to herein as a “bone plate,” and two coupling members  104 , which are also referred to herein as “bone screws.” The tether member  102  may have a first end  110 , a second end  112 , and a central portion  114  extending between the first end  110  and the second end  112 . 
     The tether member  102  may be fabricated from any of a variety of rigid biocompatible materials, such as but not limited to: stainless steel, titanium and its alloys, nickel titanium alloy, polyetheretherketone (PEEK), carbon fiber reinforced PEEK, biodegradable polymers such as poly-L-lactic acid (PLLA), and combinations of the foregoing. Alternatively, the tether member  102  may be formed of flexible biocompatible textiles, such as those used for sutures or surgical meshes. Alternatively, the tether member  102  may be a hybrid construct in which the central portion  114  is comprised of a flexible textile material and the first end  110  and the second end  112  are comprised of a rigid material. Alternatively, the tether member  102  may be formed of a composite material using any of the aforementioned polymers as a matrix and any of the aforementioned textiles as fiber reinforcement of the matrix. Composite material may be customized to provide high stiffness and strength in the direction of highest tensile stresses, such as along a longitudinal axis of the tether member  102 , but provide more flexibility and compliance in other directions, to allow the tether member  102  to better conform to the contours of the bone surface of the distal portion of the femur  58 . In some embodiments, the tether member  102  may be resorbable. 
     The coupling members  104  may be fabricated from any of the rigid biocompatible materials listed above for the tether member  102 , so long as the material used for the coupling members  104  is electrochemically and mechanically compatible with the material used for the tether member  102 . The coupling members  104  may be type of bone screws known in the orthopedic arts. In alternative embodiments (not shown), coupling members may include staples, suture anchors, pins, tacks and/or other bone fastening devices known to those skilled in the art. Coupling members may also be made resorbable if desired. 
     The tether member  102  shown in  FIG.  3 A  may be coupled to the medial side of the distal portion of the femur  58 , which may be a right femur. The first end  110  and the second end  112  of the tether member  102  may each be secured to the femur  58  by one of the coupling members  104 . 
     The central portion  114  of the tether member  102  may be elongated in shape, with a length, extending along a longitudinal axis  70  passing through the first end  110  and the second end  112 , that is greater than its width transverse to the longitudinal axis. The first end  110  may have a closed outer wall  120  and a first aperture  122  fully bounded by the closed outer wall  120 . The second end  112  may have an open outer wall  124  and a second aperture  126  bounded by the open outer wall  124 . The open outer wall  124  may define an opening, or slot  128 , through which the associated one of the coupling members  104  may be removed from the second aperture  126  along a direction  130  that is generally in-plane with, and nonperpendicular to, the tether member  102 . By contrast, the associated one of the coupling members  104  retained within the first aperture  122  may only be withdrawable from the first aperture  122  along a direction generally perpendicular to the tether member  102  (i.e., out of the page, in the view of  FIG.  3 A ). 
     As shown, the slot  128  may oriented nonparallel and/or nonperpendicular to the length of the central portion  114  of the tether member  102 . Thus, an angle  132  may exist between the direction  130  and the longitudinal axis  70  of the tether member  102 . The angle  132  may be a constant for all patients, or may be selected on a patient-specific basis to control the desired amount of anteversion correction. The angle  132  may be greater than 0° and less than 90°. Further, the angle  132  may be greater than 20° and less than 70°. Yet further, the angle  132  may be greater than 30° and less than 50°. Still further, the angle  132  may be greater than 35° and less than 45°. In some embodiments, the angle  132  may be about 40°. 
     As will be described in greater detail below, the slot  128  may enable the second end  112  to be releasably secured to the epiphyseal section  62 . “Releasable” securement means the securement of two items together in a surgical setting such that they can be detached from each other by time or by the body after the surgery is complete, rather than requiring another surgical intervention to effect release. By contrast, “nonreleasable” securement means the securement of two items together in a surgical setting such that they cannot generally be detached from each other without another surgical intervention. The first end  110  may be nonreleasably secured to the metaphyseal section  64 . 
     Use of a slotted aperture is only one mechanism capable of providing releasable securement. In alternative embodiments, releasable securement may be provided by making a tether member or coupling member weak enough to break under the desired conditions. For example, the tether member  102  could be modified to make the central portion  114  much thinner, and thus breakable under tension. Alternatively, one of the coupling members  104  could be modified to have a breakaway head or the like. 
     Returning to  FIG.  3 A , the longitudinal axis  70  may bisect the central portion  114  of the tether member  102 . The tether member  102  may further have a third aperture  140  positioned approximately at the mid-point of the longitudinal axis  70 . The third aperture  140  may be used to position the tether member  102  centrally over the physis  60  by aligning the third aperture  140  with the physis  60 . This may be done visually by a surgeon installing the tether member  102 , for example, by ensuring that the physis  60  is visible through the third aperture  140  when the tether member  102  is seated on the distal end of the femur  58 . Additionally or alternatively, a guidewire or other instrument may be registered on or near the physis  60  and inserted through the third aperture  140  and into contact with the physis  60  to guide placement of the tether member  102  such that the third aperture  140  is placed on or near the physis  60 . 
     The longitudinal axis  70  may be at an alignment angle α to a transverse axis  72  that is aligned with an anatomic transverse plane that is perpendicular to the longitudinal axis  74  of the femur  58 . Angle α is referred herein as the “initial alignment angle.” 
     Once the tether member  102  has been properly positioned on the femur  58 , one of the coupling members  104  may be placed through the first aperture  122  to couple the tether member  102  to the anterior portion of the metaphyseal section  64  of the femur  58 , proximal to the physis  60 . Another of the coupling members  104  may be placed through the second aperture  126  to couple the tether member  102  to the posterior portion of the epiphyseal section  62 , distal to the physis  60  and posterior to the first end  110 . The coupling members  104  may advantageously be placed a minimum distance of 6 mm to 8 mm away from the central portion of the physis  60  to ensure that the coupling members  104  do not impede or interfere with the natural growth emanating from the physis  60 . In alternative embodiments, the tether member  102  may be reversed, such that the first end  110  is secured to the metaphyseal section  64  and the second end  112  is secured to the epiphyseal section  62 . 
       FIG.  3 B  shows the femur  58  of  FIG.  3 A  without the tether assembly  100 . It can be appreciated in  FIG.  3 A  that the femur  58  has abnormal anteversion; the degree of abnormality is shown as the angle Θ in  FIG.  3 B . This angle Θ is the amount of anteversion in excess of the normal anteversion angle of 11°. The angle Θ may be the anteversion angle  50  of  FIG.  2 B  minus the anteversion angle  40  (11°) of  FIG.  1 B . It is desirable to correct the rotational deformity by rotating the distal end of the femur  58  by 8° relative to the proximal end of the femur  58 , so that the posterior condyles of the femur  58  are restored to a normal alignment with the femoral neck and head of the femur  58 . To ensure that the induced rotational change in the femur  58  is radially symmetric, a second tether assembly (not shown) may advantageously be placed on a second side of the femur  58 . In the case of the distal end of the femur  58 , the second tether assembly may be placed on the lateral side of the femur  58 , opposite and radially symmetrical to the placement of the tether assembly  100  on the medial side as shown in  FIGS.  3 A,  4 A and  5 A . 
     Those of skill in the art will recognize that the use of two tether assemblies is optional. In some embodiments, only a single tether assembly may be used. A single tether assembly may be placed on the lateral, medial, anterior, or posterior sides of the femur  58 , or even on the postero-lateral, antero-lateral, postero-medial, or antero-medial sides of the femur  58 . In alternative embodiments, more than two tether assemblies may be used. In such cases, the tether assemblies may optionally be arranged and oriented in radially-symmetrical fashion about the distal end of the femur  58 , and may be placed on any of the sides of the femur  58  set forth above. In further alternative embodiments, two tether assemblies may be used, and may be arranged differently than described above. For example, the tether assemblies need not necessarily be placed on the lateral and medial sides of the femur  58 , but may be placed on any of the sides set forth above. Again, radial symmetry is optional. 
       FIGS.  3 A and  3 B  illustrate the femur  58  and the tether assembly  100  at the time of a surgery in which the tether assembly  100  (and optionally one or more additional tether assemblies) are initially installed.  FIGS.  4 A and  4 B  Illustrate the femur  58  and tether assembly  100  shown in  FIGS.  3 A and  3 B  after a first period of time following the surgery. During the first period of time, natural growth of the femur  58  may occur, increasing the longitudinal spacing between the epiphyseal section  62  and the metaphyseal section  64 . Due to the constraint of the tether assembly  100  (and optional one or more additional tether assemblies), some of the longitudinal growth of the femur  58  may be converted to a relative rotation, about the longitudinal axis  74 , between the epiphyseal section  62  and the metaphyseal section  64 , thereby reducing and eventually eliminating the abnormal anteversion of 8° that was initially present in the femur  58 . The relative rotation between the epiphyseal section  62  and the metaphyseal section  64  may cause the tether member  102  to rotate from the initial alignment angle α (as shown in  FIGS.  3 A and  3 B ), relative to the transverse axis  72 , to a terminal intermediate alignment angle β (as shown in  FIGS.  4 A and  4 B ). 
     Upon reaching the terminal alignment of @°, the changed alignment (relative to the transverse plane) of the slot in the tether member is such that the slot  128  of the second aperture  126  is oriented to permit withdrawal of the associated one of the coupling members  104  from the second aperture  126  in response to continued longitudinal growth of the femur  58 . Thus, any further longitudinal growth of the femur  58  may cause the associated one of the coupling members  104  to move along the direction  130  to escape the second aperture  126 . This may release the tether member  102  from attachment to the epiphyseal section  62 , thereby permitting the femur  58  to elongate without further rotational adjustment. 
     The femur  58  shown in  FIGS.  5 A and  5 B  illustrates the femur  58  of  FIGS.  3 A,  3 B,  4 A and  4 B  after a second period following the surgery, where the second period is greater than the first period. During the interval of time after the first period until the end of the second period, additional longitudinal growth of the femur  58  has occurred, causing the one of the coupling members  104  previously captured in the second aperture  126  to traverse the length of the slot  128  of the second aperture  126  and to translate outside the outer perimeter of the second end  112  of the tether member  102 . Thus, the amount of rotational correction as measured by the angle Θ can be “programmed” into the surgical technique by selecting the right combination of α, β, and the distance  150  between the first aperture  122  and the second aperture  126  of the tether member  102  (referred to herein as “plate length”) for a given diametrical width of a distal femur. The foregoing list of dimensional parameters have analytical geometry relationships that can be expressed in equation form to determine the right parameter values to achieve a target rotational correction angle Θ. 
     It can be appreciated that the femur in  FIG.  5 B  has an unchanged rotational alignment when compared to the femur in  FIG.  4 B , as the one of the coupling members  104  formerly captured in the second aperture  126  has escaped the tether member  102 , and thus the constraint that forced the prior rotational change is no longer in effect. Furthermore, it may be advantageous to provide an “automated” removal of the constraint imposed by the tether member  102  once the rotational deformity in the femur  58  is corrected and before the tether member  102  migrates to a more vertical alignment with respect to the transverse plane. If the tether member  102  were to continue to constrain the distance between the coupling members  104  as it achieved vertical alignment, the tether member  102  would arrest further longitudinal growth of the femur  58 . Indeed, such a constraint is known in the clinical literature as “shutting down the growth plate,” a condition that permanently disables the ability of the growth plate, or physis  60 , to generate new bone to continue the natural growth of the bone. Such a clinical condition can be very deleterious to the child, as it could lead to leg length discrepancies or failure to achieve normal height in adulthood. 
       FIG.  6 A  is a top view of the tether assembly  100  of  FIG.  3 A , in isolation. The coupling members  104  are positioned in the first aperture  122  and the second aperture  126 .  FIG.  6 B  is a side view of the tether assembly  100  of  FIG.  3 A , illustrating how the coupling members  104  can articulate with the first aperture  122  and the second aperture  126  to allow a multitude of relative orientations between the tether member  102  and the coupling members  104 . The aforementioned articulation may be achieved by having spherical surfaces  160  on the coupling members  104  and complementary spherical surfaces  162  on the first aperture  122  and the second aperture  126 . The phrase “spherical surface” will be understood to require not an entire sphere, but any three-dimensional portion of a concave or convex spherical shape. 
     Each of the complementary spherical surfaces  162  may be a concave spherical segment defined between two spaced-apart parallel planes. One of these planes may be defined by the top surface of the first end  110  or the second end  112 , as applicable, and the other may pass through the space between this top surface and the associated bottom surface. The second aperture  126  may further be bounded by a plane positioned at an angle to the two parallel planes to create the slot  128 . A close-up view of the slot  128  is provided in  FIG.  6 C . 
     It is noted that the relative alignment of the mid-range articulation position  170  of each of the coupling members  104  relative to the tether member  102  is at a divergent angle. This is to help ensure that the coupling members  104  are directed away from the physis  60  when the tether member  102  is attached to the femur  58 , as placement of the coupling members  104  into the physis  60  could inhibit the bone growth from the physis  60 . While  FIG.  6 B  shows the vertical range for orienting the coupling members  104  relative to the tether member  102 , due to the spherical articulation described above, a similar range of motion may be present in all planes containing the axis  172  of each of the coupling members  104  located at its mid-range articulation position  170 . 
     It can be further appreciated in  FIGS.  6 A and  6 B  that the tether member  102  has a three-dimensional contour. This contour may be selected to match that of the medial or lateral epicondylar bone surface, on which the tether member  102  is to be attached. Since the tether member  102  is to rotate on the associated bone surface in the course of correcting the anteversion of the bone, the tether member  102  may be contoured to match a portion of the bone that is angled between the initial and final alignment angles (for example, angled between α as shown in  FIG.  3 A  and β as shown in  FIG.  4 A ). The contour that is to be matched may thus be the portion of bone that will be overlaid by the tether member  102  partway through the anteversion correction process. 
     The central portion  114  of the tether member  102  may have a central bend  180  that exists as part of this contouring. The third aperture  140  may pass through the central bend  180 . Further, the central portion  114  may have a central twist such that the first end  110  and the second end  112  are not in the same plane. As a result, the second aperture  126  may have an axis  176  that is not parallel to the axis  174  of the first aperture  122  (as shown in  FIG.  7 B ). 
     In some embodiments, this contour may be created by projecting an elongate area defined on a sagittal plane onto one of the medial and lateral epicondylar bone surfaces of a representative pediatric distal femur, such as the femur  58 . The elongate area may have a long axis and a short axis orthogonal to the long axis. The long axis, measured on the sagittal plane, may be positioned at an angle relative to a transverse plane, wherein the angle is less than 70°. The long axis is generally between 20° and 70°, preferably between 30° and 60°, and more preferably between 40° and 50°. The long axis may be at about 45°. 
       FIG.  7 A  shows another top view of an embodiment of the tether member  102  of the tether assembly  100  of  FIG.  3 A , and  FIG.  7 B  is a side elevation, partial sectional view of the tether member  102 . As shown, the slot  128  may have side walls  190  and a slot axis  192  that is located between the side walls  190  and bisects the side walls  190 . The section view shows the portion of the complementary spherical surface  162  of the second aperture  126  and also shows the side wall  190  on one slide of the slot  128 . A portion of the slot  128  has a slot width  194  that is smaller than the diameter  196  of the second aperture. 
       FIG.  8 A  is a side elevation view of one of the coupling members  104  of the tether assembly  100  of  FIG.  3 A .  FIG.  8 B  is a side elevation, section view of one of the coupling members  104 . Each of the coupling members  104  may be a bone screw or other bone fastening device of any type known in the orthopedic arts. As shown, each of the coupling members  104  may have a head  200  and a shank  202  extending from the head  200 . The shank  202  may have a plurality of bone engagement features extending therefrom. As embodied in  FIG.  8 B , the bone engagement features may be screw threads  204 . 
     The shank  202  may have a shank diameter  206  that is larger than the slot width  194  and smaller than the diameter  196  of the second aperture  126 . The spherical surfaces  160  of the coupling members  104  may be on the head  200  of each of the coupling members  104 . The spherical surfaces  160  may mate with the complementary spherical surface  162  of the first aperture  122  and the second aperture  126  to enable polyaxial articulation as set forth above. The relative sizing between the shank diameter  206  and the diameter  196  of the second aperture  126  may enable the spherical articulation between the second end  112  of the tether member  102  and the associated one of the coupling members  104  as demonstrated in  FIG.  6 B . 
     The shank  202  may be positioned in the second aperture  126  such that the shank diameter  206  is aligned with the slot  128  as shown in  FIG.  6 C . Thus, the shank  202  can pass through the slot  128 , as described above. However, the relative sizing between the shank diameter  206  and the slot width  194  may be selected such that a threshold force must be applied between the tether member  102  and the shank  202  before the shank  202  passes through the slot  128 . Specifically, the relatively smaller size of the slot width  194  relative to the shank diameter  206  may be selected such that when a threshold force is applied to the shank  202  in a direction aligned with the slot axis  192 , the shank  202  will elastically deform (i.e., the resulting strain is below the yield strain of the material) the side walls  190  of the slot  128 , thus permitting the shank  202  to escape from the second aperture  126  via the slot  128 . 
     Because escapement of the shank  202  from the tether member  102  induces fully recoverable strain in the tether member  102 , the shank  202  can be reengaged with the second aperture  126  with no loss in the threshold force required to induce another escapement. Furthermore, the threshold force may be selected such that it induces a corresponding restraining force on the physis  60  that is below the force that would induce growth from the physis  60  to “shut down.” Alternatively, another embodiment (not shown) may have an escapement configuration that causes the slot side walls to permanently deform upon escapement of the coupling member from the tether member. 
     In addition to or in the alternative to interference between the shank  202  and the slot  128 , the head  200  may interfere with passage of the head  200  through the slot  128 . Specifically, the head  200  may have a head width  208 , shown in  FIG.  8 A , at a portion of the head  200  that also passes through the slot  128 , in addition to the shank  202 . As more clearly shown in  FIG.  7 B , the slot  128  may have a shank-contacting portion  210  that will lie adjacent to the surface of the femur  58 , and a head-contacting portion  212  further from the femur  58 . 
     The shank-contacting portion  210  may provide interference with the shank  202  as described above. However, in some embodiments, given the polyaxially-adjustable adjustability of the orientation of the shank  202  relative to the tether member  102 , interference between the shank  202  and the shank-contacting portion  210  may provide relatively unpredictable pullout force. Specifically, if the shank  202  is angularly displaced from perpendicularity with the tether member  102 , the pullout force may be higher than if the shank  202  is perpendicular to the tether member  102 . 
     Accordingly, it may be beneficial to have interference between the head  200  and the head-contacting portion  212 . Thus, the head width  208  may be equal to or larger than the width of the head-contacting portion  212 , at the depth at which the head  200  is to contact the head-contacting portion  212 .  FIG.  7 A  illustrates that the second aperture  126  may sweep across more than 180°. Thus, in order for the head  200  to escape the second aperture  126  and enter the slot  128 , the head  200  may have to push hard enough on the adjoining shoulders  214  of the head-contacting portion  212  to push them apart, thereby increasing the width of the head-contacting portion  212  of the slot  128 . The head  200  may then enter and pass through the slot  128 , permitting the second end  112  to disengage from the corresponding one of the coupling members  104  as described above. 
       FIG.  9    is a table  250  illustrating the start angle α (in degrees) and approximate treatment time (in months) that correspond to a rotational correction angle Θ (in degrees), an average distal femur width (in mm) for a given length plate (in mm). The plate length provided in the table  250  is the distance  150  between a center of the first aperture  122  and a center of the second aperture  126 , as shown in  FIG.  5 A . 
     All of the values in the table  250  are based on a constant end angle β of 70°. Using analytical geometry, similar tables can be developed for other values of end angle, start angle, treatment time, femur width, rotational correction angle and plate lengths as needed. For more severe deformities than provided in the table  250 , multiple treatments may be applied to the same patient. For example, for a patient having 35° of rotational deformity and a 54 mm femur width, the surgeon can apply the tether member  102  at a start angle of 43° to achieve 20° of rotational correction in approximately 7 months. Then in a subsequent surgical procedure, the surgeon can remove and reapply the coupling members  104  and the tether member  102  (or a tether member  102  with a different size and/or contour) at a start angle of 52° to achieve another 15° of rotational correction in approximately 5 months. Thus, the patient undergoes a total of 35° of rotational correction in approximately 12 months. 
     The foregoing disclosure describes only selected embodiments encompassed within the scope of the disclosure. Those of skill in the art will recognize that the principles taught herein may be applied to generate many alternative concepts. For example, various clips, clasps, staples, plates, screws, and/or other fastening systems may be used to secure two sections of a bone together on either side of a growth plate. Such fastening systems may be made deliberately releasable, through detachable connections and/or breakable components, to effect release when the desired anteversion correction has been obtained. 
     Reference throughout this specification to “an embodiment” or “the embodiment” means that a particular feature, structure, or characteristic described in connection with that embodiment is included in at least one embodiment. Thus, the quoted phrases, or variations thereof, as recited throughout this specification are not necessarily all referring to the same embodiment. 
     Similarly, it should be appreciated that in the above description of embodiments, various features are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the present disclosure. This method of disclosure, however, is not to be interpreted as reflecting an intention that any embodiment requires more features than those expressly recited in that embodiment. Rather, inventive aspects lie in a combination of fewer than all features of any single foregoing disclosed embodiment. 
     As used herein, the term “proximal” means a location at the end of a part that faces a user when the user is installing the part. The term “distal” means a location at the opposite end of the proximal end. For example, when a user installs a bone screw into a material with a driver, the end of the bone screw engaged with the driver is the proximal end, and the tip of the bone screw that first engages the material is the distal end. The term “cannulated” means having a central bore extending along a longitudinal axis of a part between a proximal end and a distal end of the part. 
     Recitation of the term “first” with respect to a feature or element does not necessarily imply the existence of a second or additional such feature or element. Elements recited in means-plus-function format are intended to be construed in accordance with 35 U.S.C. § 112(f). It will be apparent to those having skill in the art that changes may be made to the details of the above-described embodiments without departing from the underlying principles set forth herein. 
     The phrases “connected to,” “coupled to” and “in communication with” refer to any form of interaction between two or more entities, including mechanical, electrical, magnetic, electromagnetic, fluid, and thermal interaction. Two components may be functionally coupled to each other even though they are not in direct contact with each other. The term “coupled” can include components that are coupled to each other via integral formation, as well as components that are removably and/or non-removably coupled with each other. The term “abutting” refers to items that may be in direct physical contact with each other, although the items may not necessarily be attached together. The phrase “fluid communication” refers to two or more features that are connected such that a fluid within one feature is able to pass into another feature. As defined herein the term “substantially” means within +/−20% of a target value, measurement, or desired characteristic. 
     While specific embodiments and applications of the present disclosure have been illustrated and described, it is to be understood that the scope of this disclosure is not limited to the precise configuration and components disclosed herein. Various modifications, changes, and variations which will be apparent to those skilled in the art may be made in the arrangement, operation, and details of the devices, systems, and methods disclosed herein.