Patent Publication Number: US-2021186574-A1

Title: Facet joint replacement devices

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 62/575,196, filed Oct. 20, 2017. The disclosure of the prior application is considered part of (and is incorporated by reference in) the disclosure of this application. 
    
    
     BACKGROUND 
     1. Technical Field 
     This document relates to devices and systems for treating spinal conditions. For example, this document relates to artificial facet joint systems that can be implanted to treat spinal conditions while facilitating substantially normal stability and motions of the spine. 
     2. Background Information 
     Low back pain (LBP) and neck pain are among the most common patient complaints in the primary care setting. Globally, LBP has been ranked as the greatest contributor to worldwide disability with an estimated annual direct medical cost over $100 billion. According to most recent studies, the mean annual prevalence of LBP is 9-13%, with a lifetime risk up to 65%. Since the mean life expectancy continues to rise in the US population, the number of people with LBP will increase substantially. 
     The major cause of LBP and neck pain is spondylosis (i.e., degenerative spinal disease); especially amongst the elderly. Though non-operative management strategies can be efficacious, many patients ultimately require surgical treatment of the affected vertebral level. Traditionally, such operations have involved fusion of the spine, which includes placing screws at the vertebrae and a small rod that connects the screws together. 
     Apart from spondylosis, another widespread application of spinal fusion is for the treatment of vertebral fractures secondary to osteoporosis. Approximately 8 million women and 2 million men suffer from this condition. Another 34 million have low bone mass and are thus at increased risk for osteoporosis. Moreover, an estimated 700,000 of these patients will experience a vertebral compression fracture that will necessitate spinal fusion in order to preserve spinal stability. Current analyses forecast that osteoporosis will continue to increase in prevalence significantly over the next 20 years. 
     Spinal fusion is an option for the management of spondylosis or fractures secondary to osteoporosis. Spinal fusion has the benefit of allowing for removal of degenerative tissue while simultaneously preserving spinal stability. However, it comes at the cost of reduced range of motion. 
     SUMMARY 
     This document provides devices and systems for treating spinal conditions. For example, this document provides artificial facet joint systems that can be implanted to treat spinal conditions while facilitating normal stability and motions of the spine. The systems and methods provided herein can be used to treat spinal conditions such as, but not limited to, spondylosis, spondylolisthesis, cervical or lumbar stenosis, foraminal stenosis, vertebral fractures, and the like. 
     In one aspect, this disclosure is directed to a facet joint replacement device. The facet joint replacement device can include a housing defining an opening, an internal space, and a channel, a first rod slidingly engaged with the opening, the first rod including an internal end portion disposed within the internal space and an external end portion disposed outside of the internal space, the first rod also including a projection extending from the internal end portion and disposed within the channel, and a second rod extending from the housing opposite the opening. In some cases, the first rod and the second rod can be compressed toward each other such that a portion of the external end portion of the first rod enters the internal space. In some cases, the first rod and the second rod can be pulled away from each other such that a portion of the internal end portion of the first rod exits the internal space. In some cases, the first rod can be limited in rotation around its longitudinal axis by the projection disposed in the channel. In some cases, the housing can have a non-circular cross-sectional shape. In some cases, the housing can have an ovular cross-sectional shape. In some cases, the housing can be arced along a longitudinal axis. In some cases, the facet joint replacement device can include an elastic element disposed within the internal space. In some cases, the opening can be ovular. 
     In another aspect, this disclosure is directed to a facet joint replacement device. In some cases, the facet joint replacement device can include a housing defining an opening and an internal space, a first rod slidingly engaged with the opening, the first rod including an internal end portion disposed within the internal space and comprising a head with a head diameter greater than a diameter of other portions of the first rod, the first rod also comprising an external end portion disposed outside of the internal space, and a second rod extending from the housing opposite the opening. In some cases, the first rod and the second rod can be compressed toward each other such that a portion of the external end portion of the first rod enters the internal space. In some cases, the first rod and the second rod can be pulled away from each other such that a portion of the internal end portion of the first rod exits the internal space. In some cases, the housing can have a non-circular cross-sectional shape. In some cases, the housing can have an ovular cross-sectional shape. In some cases, the housing can be arced along a longitudinal axis. In some cases, the facet joint replacement device can include an elastic element disposed within the internal space. In some cases, the elastic element can be disposed between the head and the opening. In some cases, the elastic element can be disposed between the head and a portion of the internal space located near the second rod. In some cases, the elastic element can be coupled to the head. In some cases, an internal diameter of the housing can decrease as the housing extends away from a horizontal axis of the housing. In some cases, the head can be sized such that resistance between the head and the internal diameter of the housing increases as the internal diameter of the housing decreases. 
     In another aspect, this disclosure is directed to a facet joint replacement device. The facet joint replacement device can include a housing defining a first opening and a first internal space, an insert defining a second opening and a second internal space, wherein the insert is configured to be disposed within the first internal space, a first rod slidingly engaged with the first opening, the first rod including an internal end portion disposed within the second internal space and comprising a head with a head diameter greater than a diameter of other portions of the first rod, the first rod also comprising an external end portion disposed outside of the first internal space, and a second rod extending from the housing opposite the first opening. In some cases, the first rod and the second rod can be compressed toward each other such that a portion of the external end portion of the first rod enters the first internal space or the second internal space. In some cases, the first rod and the second rod can be pulled away from each other such that a portion of the internal end portion of the first rod exits the first internal space or the second internal space. In some cases, the housing can have a non-circular cross-sectional shape. In some cases, the housing can have an ovular cross-sectional shape. In some cases, the housing can be arced along a longitudinal axis. In some cases, the insert can be arced along a longitudinal axis. In some cases, the insert can be an elastic element. In some cases, the elastic element can be an elastomer. In some cases, an internal diameter of the insert can decrease as the insert extends away from a horizontal axis of the insert. In some cases, the head can be sized such that resistance between the head and the internal diameter of the insert increases as the internal diameter of the insert decreases. 
     In another aspect, this disclosure is directed to a method of treating a spinal condition. The method can include removing facet joint bone matter of adjacent vertebrae, installing two cortical screws or lateral mass screws in each vertebrae of the adjacent vertebrae, and coupling two facet joint replacement devices to the cortical screws or lateral mass screws. In some cases, each facet joint replacement device can include a housing defining an opening and an internal space, a first rod slidingly engaged with the opening, the first rod including an internal end portion disposed within the internal space and an external end portion disposed outside of the internal space, and a second rod extending from the housing opposite the opening. 
     Particular embodiments of the subject matter described in this document can be implemented to realize one or more of the following advantages. In some embodiments, spinal conditions, such as spondylosis, vertebral fractures, and the like, can be treated using the devices and methods provided herein. The artificial facet joint devices and systems provided herein maintain the natural facet joint center of rotation. Hence, the devices and systems can be implanted to treat spinal conditions while advantageously facilitating normal stability and motions of the spine. The devices provided herein can utilize lateral mass screws (cervical spine) or cortical bone trajectory screws (thoraco-lumbar spine) rather than traditional pedicle screws. The use of such cortical screws is advantageous because they can be anchored to solid bone and preserve motion in the center of the vertebral body. Further, the use of cortical screws with the devices provided herein enables the artificial facet joints to be located near to where the natural facet joint was previously anatomically located (prior to its removal for installation of the artificial facet joint). Hence, a substantially naturally-behaving (from a biomechanical standpoint) artificial facet joint can be attained using the artificial facet joint systems provided herein. Moreover, in some embodiments, various spinal conditions can be advantageously treated in a minimally invasive fashion using the systems and methods provided herein. Such minimally invasive techniques can reduce recovery times, patient discomfort, and treatment costs. 
     Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Although methods and materials similar or equivalent to those described herein can be used to practice the invention, suitable methods and materials are described herein. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods and examples are illustrative only and not intended to be limiting. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a lateral view of a portion of a spine that has had facet joints surgically removed in preparation for the installation of artificial facet joints. 
         FIG. 2  is a posterior view of a portion of a spine in which an example artificial facet joint system has been installed in accordance with some embodiments provided herein. 
         FIG. 3  is an isometric view of an example artificial facet joint device in accordance with some embodiments provided herein. 
         FIG. 4  is a longitudinal cross-sectional view of the artificial facet joint device of  FIG. 3 . 
         FIG. 5  is a transverse cross-sectional view of the artificial facet joint device of  FIG. 4 . 
         FIG. 6  is an isometric view of an example artificial facet joint device, in accordance with some embodiments provided herein. 
         FIG. 7  is a longitudinal cross-sectional view of the artificial facet joint device of  FIG. 6 . 
         FIG. 8  is a longitudinal cross-sectional view of the artificial facet joint device of  FIG. 7 , in flexion. 
         FIG. 9  is a longitudinal cross-sectional view of the artificial facet joint device of  FIG. 7 , in extension. 
         FIG. 10  is a longitudinal cross-sectional view of a second embodiment of the artificial facet joint device of  FIG. 6 . 
         FIG. 11  is a longitudinal cross-sectional view of the artificial facet joint device of  FIG. 10 , in flexion. 
         FIG. 12  is a longitudinal cross-sectional view of the artificial facet joint device of  FIG. 10 , in extension. 
         FIG. 13  is an isometric view of an example sleeve removal tool, in accordance with some embodiments provided herein. 
         FIG. 14  is an isometric view of the sleeve removal tool of  FIG. 13  being used with the artificial facet joint device of  FIG. 6 , in accordance with some embodiments provided herein. 
     
    
    
     Like reference numbers represent corresponding parts throughout. 
     DETAILED DESCRIPTION 
     This document provides devices and systems for treating spinal conditions. For example, this document provides artificial facet joint systems that can be implanted to treat spinal conditions while facilitating normal stability and motions of the spine (including limitations to such motions). The systems and methods provided herein can be used to treat spinal conditions such as, but not limited to, spondylosis, spondylolisthesis, spinal stenosis, foraminal stenosis, vertebral fractures, osteoporosis, and the like. 
     The facet joints play a pivotal role in spinal biomechanics. In the lumbar and cervical spine, the major role of the facet joint is to allow for flexion and extension. To a lesser degree, lateral bending and axial rotation also occur about this joint. In some cases, the facet joint can carry up to 25% of a spinal axial compression load. 
     In view of the facts above, artificial facet joint devices would preferentially allow for the following motions: flexion, extension, lateral bending, axial compression, axial tension, and axial rotation. Moreover, it is desirable to use a device that can achieve such motions while also maintaining the natural facet joint center of rotation and natural limitations on such motions. 
     This disclosure describes facet joint replacement (FJR) systems that would allow surgeons to perform an inferior facetectomy (a common procedure in both the cervical and lumbar spine) or similar surgical technique in which the natural facet joints would be removed and effectively replaced with an artificial facet joint. Such artificial FJR systems will preserve to a first order approximation the natural biomechanics of the facet joint. 
     Referring to  FIG. 1 , a portion of a spine  10  can be prepared to receive an artificial FJR system as provided herein. In this example, the spinal portion  10  has undergone an inferior facetectomy. In some cases, other similar surgical techniques in which the facet joints, or portions thereof, are removed can be performed in preparation to receive an artificial FJR system as provided herein. 
     Spinal portion  10  includes natural facet joints  12 . However, in this example spinal portion  10  has facet joints removed in region  14 . The removal of the natural facet joint in region  14  can be performed in preparation for the installation of an artificial FJR system as provided herein. In some cases, the removal of the natural facet joint in region  14  can be performed in a minimally-invasive manner. 
     Referring to  FIG. 2 , an example artificial FJR system  100  can be installed in a spinal portion  20  to substitute for the function of a removed natural facet joint. Artificial FJR system  100  can be implanted to treat spinal conditions while facilitating substantially normal stability and motions of the spine  20 . In some cases, in addition to installing artificial FJR system  100 , one or more prosthetic elements that simulate intervertebral discs can be installed between adjacent vertebrae. 
     In some cases, artificial FJR system  100  can allow for the following motions: flexion, extension, lateral bending, axial compression, axial tension, and rotation. In addition, artificial FJR system  100  can facilitate such motions while also substantially maintaining the natural facet joint center of rotation. 
     In some embodiments, artificial FJR system  100  is structurally designed to allow limited amounts of displacement related to the aforementioned motions. Moreover, in some embodiments the resistance provided by artificial FJR system  100  to displacements from such motions can be linear, or non-linear. For example, in some embodiments non-linear resistance to displacements can be provided by artificial FJR system  100  such that the resistance substantially increases as the structural limitations on the displacements are neared. 
     In the depicted embodiment, artificial FJR system  100  includes a first pair of cortical trajectory screws (cortical screws)  110   a  and  110   a ′, a first facet joint replacement device  120   a,  a second pair of cortical screws  110   b  and  110   b ′, and a second facet joint replacement device  120   b.  First facet joint replacement device  120   a  is fixedly joined to, and extending between, cortical screw  110   a  and cortical screw  110   a ′. Second facet joint replacement device  120   b  is fixedly joined to, and extending between, cortical screw  110   b  and cortical screw  110   b′.    
     In some cases, cortical screws  110   a,    110   a ′,  110   b,  and  110   b ′ can be the same as, or similar to, the types of cortical screws that are installed along with rods as part of a spinal fusion system. The cortical screws  110   a,    110   a ′,  110   b,  and  110   b ′ may also be referred to as cortical bone trajectory screws (for thoracolumbar applications) and lateral mass screws (for cervical applications). One example of such cortical screws (for thoracolumbar) is the VIPER® Cortical Fix Screw from DePuy Synthes (a Johnson &amp; Johnson company), which can be used for this application. One example of lateral mass screws (for cervical applications) is the MOUNTAINEER® OCT Spinal System also from DePuy Synthes. Other companies make similar screws. Such cortical screws  110   a,    110   a ′,  110   b,  and  110   b ′ can be installed (i.e., affixed to vertebrae) using a minimally-invasive surgical procedure in some cases. Other types of fixation devices can additionally or alternatively be used. 
     Facet joint replacement devices  120   a  and  120   b  extend between first pair of cortical screws  110   a  and  110   a ′ and second pair of cortical screws  110   b  and  110   b ′ respectively. Facet joint replacement devices  120   a  and  120   b  thereby replace the function of the natural facet joints that have been removed to make room to install artificial FJR system  100 . As described further below, facet joint replacement devices  120   a  and  120   b  are designed with multiple degrees of freedom, and with certain physical constraints on those degrees of freedom, so as to mimic the functions and range of motion of natural facet joints. 
     In some embodiments, one or more laterally-extending stabilizing members (not shown) can be installed between facet joint replacement devices  120   a  and  120   b.  The stabilizing members can be completely rigid or can be somewhat flexible. In some embodiments, a single stabilizing member is included that extends between mid-body portions of facet joint replacement devices  120   a  and  120   b.  In some embodiments, such stabilizing members are installed in an x-pattern (i.e., a first stabilizing member extends between cortical screws  110   a  and  110   b ′, and a second stabilizing member extends between  110   b  and  110   a ′). 
     Referring to  FIGS. 3-5 , a facet joint replacement device  200  is an example of the types of facet joint replacement devices that can be used with artificial FJR system  100 . Facet joint replacement device  200  can function as a piston/cylinder mechanism. Facet joint replacement device  200  can include a first rod  210   a,  a second rod  210   b,  a housing  220 , and an opening  222 . 
     First rod  210   a  and second rod  210   b  can couple to one of cortical screws  110   a,    110   a ′,  110   b,  or  110   b ′. First rod  210   a  can include a first projection  212   a  and a second projection  212   b.  First and second projections  212   a  and  212   b  can be located opposite one another. In some cases, first and second projections  212   a  and  212   b  can be located on a distal portion of first rod  210   a.  In some cases, first and second projections  212   a  and  212   b  can extend perpendicular to first rod  210   a.  In some cases, first rod  210  can include a cavity  214 . In some cases, cavity  214  can receive an elastomeric member. In some cases, the elastomeric member can cushion contact between first rod  210   a  (e.g., rod projections  212   a  and  212   b ) and second rod  210   b.    
     In some cases, first rod  210   a  and second rod  210   b  can extend along a single shared axis. In some cases, first rod  210   a  and second rod  210   b  extend along different axes. In some cases, second rod  210   b  can be coupled to, or made unitarily with, housing  220 . In some cases, first rod  210   a  and/or second rod  210   b  can be linear. In some cases, first rod  210   a  and/or second rod  210   b  can be non-linear. In some cases, first rod  210   a  and/or second rod  210   b  can be arced to replicate a natural trajectory of a healthy facet joint in extension and flexion. In some cases, the cross-sectional shape of rods  210   a  and  210   b  are circular. In some cases, rods  210   a  and  210   b  can have other cross-sectional shapes such as, but not limited to, ovular, elliptical, polygonal, and the like. 
     Housing  220  can be an elongate member that defines an internal space  224 . End portion of rod  210   a  can be movably disposed within the internal space  224  of housing  220 . In some cases, facet joint replacement device  200  can be installed such that the rod  210   a  and internal space  224  are in a neutral position with one another (i.e., no forces acting between the internal space  224  or the rod  210   a ). 
     Housing  220 , and other portions of facet joint replacement device  200 , can be made of various biocompatible metallic or polymeric materials such as, but not limited to, stainless steel, titanium, titanium alloys, cobalt-chrome, nitinol, polyetheretherketone (PEEK), polytetrafluoroethylene (PTFE), perfluoroalkoxy alkanes (PFA), polyvinylidene difluoride (PVDF), and the like, and combinations thereof. In some cases, housing  220  can be made by a molding process. In some cases, housing  220  can be made by a machining process. In some cases, housing  220  and/or one or more other components of facet joint replacement device  200  can be made by a 3D printing method. In some such cases, the design of the component(s) can be customized for a particular patient&#39;s spinal anatomy as defined by MR or CT imaging modalities. 
     In some cases, housing  220  can be cylindrical. In some cases, an exterior diameter of housing  220  can follow an arc that replicates a natural trajectory of a healthy facet joint in extension and flexion. In some cases, an internal diameter of housing  220  can follow an arc that replicates a natural trajectory of a healthy facet joint in extension and flexion. In some cases, housing  220  can have an angled top and/or an angled bottom for the internal space  224  such that when projections  212   a  and  212   b  abut the top and/or bottom of the internal space  224 , even contact is made between the projections  212   a  and  212   b  of first rod  210   a  and housing  220 . In some cases, when projections  212   a  and  212   b  come into contact with housing  220  during extension and flexion, projections  212   a  and  212   b  can limit range of movement. 
     Housing  220  can have various cross-sectional shapes. In some cases, the cross-sectional shape of the internal space  224  defined by housing  220  is circular, ovular, elliptical, polygonal, and the like. The cross-sectional shape of the internal space can be selected to allow for a certain amount of free play between rod  210   a  and an internal wall of housing  220 . In some cases, housing  220  can be designed to have some degree of compliance such that housing  220  can bend or flex while in use. In some cases, housing  220  can be designed to be rigid such that housing  220  substantially does not bend or flex while in use. 
     Housing  220  can include a first channel  226   a  and second channel  226   b.  In some cases, first channel  226   a  and second channel  226   b  can extend longitudinally along the internal space  224 . In some cases, first channel  226   a  and second channel  226   b  can align with projections  212   a  and  212   b.  In some cases, channels  226   a  and  226   b  can be slightly larger than projections  212   a  and  212   b  such that channels  226   a  and  226   b  can allow a limited amount of side bending and/or axial rotation of the facet joint replacement device  200 . In some cases, a size of channels  226   a  and  226   b  can determine how much motion is permitted. 
     In some cases, housing  220  can include a cavity  228  in an internal portion of housing  220 . In some cases, cavity  228  can receive an elastomeric member. In some cases, the elastomeric member can cushion contact between first rod  210   a  (e.g., rod projections  212   a  and  212   b ) and a bottom surface of the internal space  224 . In some cases, housing  220  can include a projection  230 . Projection  230  can provide a visual marker to aid a clinician in placing the facet joint replacement device  200  in the correct orientation. 
     Housing  220  can define opening  222 . Opening  222  can be elliptical such that a greater range of motion is permitted in one direction than a second direction. In some cases, opening  222  can be shaped and sized to allow first rod  210   a  to follow the arc of housing  220 . In some cases, opening  222  can limit motion in lateral bending and/or axial rotation. In some case, opening  222  can be angled to guide the first rod  210   a  in spine extension and/or flexion. In some cases, the opening  222  can be off-center to allow greater motion in a first direction along a first axis than a second direction along the first axis. In some cases, opening  222  can define an angle. In some cases, the angle can be unique for different locations of the opening  222  such that when movement causes the first rod  210   a  comes into contact with a wall of the opening  222 , an even contact surface is encountered. 
     In some cases, housing  220  can include one or more inserts. In some cases, an insert can be located at a top portion of the internal space  224  of housing  220 . In some cases, an insert can be located at a bottom portion of the internal space  224  of housing  220 . In some cases, the insert can be a spring. In some cases, the insert can be an elastic element. In some cases, the elastic element can be made of various materials and can be constructed in various manners. In some cases, the elastic element can be made of a mesh composite. In some embodiments, the elastic element can be an elastic polymeric element. In some cases, the elastic element can be made of a variety of different types of materials and combinations of materials. For example, elastic element can be made of polymeric materials such as, but not limited to, polyurethane, polyethylene, silicone, and other biocompatible elastomers. In some cases, the elastic element can be made of metallic materials such as, but not limited to, nitinol, stainless steel, titanium, titanium alloys, cobalt-chrome, and the like. In some cases, elastic element is solid, but in other cases, elastic element can be constructed for enhanced compliance and/or elasticity. For example, in some embodiments elastic elements, or portions thereof, can be molded, laser-cut, machined, braided, woven, etc. so that the compliance and/or elasticity of elastic element can be enhanced. In some cases, the insert can provide a soft stop at a maximum range of motion in extension and/or flexion. 
     Referring to  FIG. 6 , a facet joint replacement device  300  is another example of the types of facet joint replacement devices that can be used with artificial FJR system  100 . Facet joint replacement device  300  can function as a piston/cylinder mechanism. Facet joint replacement device  300  can include a first rod  310   a,  a second rod  310   b,  a housing  320 , and an opening  322 . 
     First rod  310   a  and second rod  310   b  can couple to one of cortical screws  110   a,    110   a ′,  110   b,  or  110   b ′. In some cases, first rod  310   a  and second rod  310   b  can extend along a single axis. In some cases, first rod  310   a  and second rod  310   b  extend along different axes. In some cases, second rod  310   b  can be coupled to housing  320 . In some cases, first rod  310   a  and/or second rod  310   b  can be linear. In some cases, first rod  310   a  and/or second rod  310   b  can be non-linear. In some cases, first rod  310   a  and/or second rod  310   b  can be arced to replicate a natural trajectory of a healthy facet joint in extension and flexion. In some cases, the cross-sectional shape of rods  310   a  and  310   b  are circular. In some cases, rods  310   a  and  310   b  can have other cross-sectional shapes such as, but not limited to, ovular, elliptical, polygonal, and the like. 
     Housing  320 , and the other portions of facet joint replacement device  300 , can be made of various biocompatible metallic or polymeric materials such as, but not limited to, stainless steel, titanium, titanium alloys, cobalt-chrome, nitinol, polyetheretherketone (PEEK), polytetrafluoroethylene (PTFE), perfluoroalkoxy alkanes (PFA), polyvinylidene difluoride (PVDF), and the like, and combinations thereof. In some cases, housing  320  can be made by a molding process. In some cases, housing  320  can be made by a machining process. In some cases, housing  320  and/or one or more other components of facet joint replacement device  300  can be made by a 3D printing method. In some such cases, the design of the component(s) can be customized for a particular patient&#39;s spinal anatomy as defined by MR or CT imaging modalities. In some cases, housing  320  can be cylindrical. In some cases, an exterior diameter of housing  320  can follow an arc that replicates a natural trajectory of a healthy facet joint in extension and flexion. In some cases, an internal diameter of housing  320  can follow an arc that replicates a natural trajectory of a healthy facet joint in extension and flexion. 
     Housing  320  can define opening  322 . Opening  322  can be elliptical such that a greater range of motion is permitted in one direction than a second direction. In some cases, opening  322  can be shaped and sized to allow first rod  310   a  to follow an arced path defined by housing  320 . In some cases, opening  322  can limit motion in lateral bending and/or axial rotation. In some case, opening  322  can be angled to guide the first rod  310   a  in spine extension and/or flexion. In some cases, the opening  322  can be off-center to allow greater motion in a first direction along a first axis than a second direction along the first axis. In some cases, opening  322  can define an angle. In some cases, the angle can be unique for different locations of the opening  322  such that when movement causes the first rod  310   a  comes into contact with a wall of the opening  322 , an even contact surface is encountered. 
     Housing  320  can include a sleeve  326 . Sleeve  326  can be shaped to surround the housing  320 . In some cases, sleeve  326  can be arced to follow the curvature of housing  320 . In some cases, an outer diameter of sleeve  326  can be substantially similar to an outer diameter of housing  320 . In some cases, the outer diameter of the sleeve  326  and the outer diameter of the housing  320  being similar can reduce sharp or uneven edges between sleeve  326  and housing  320 . In some cases, sleeve  326  can extend along a portion of housing  320 . In some cases, sleeve  326  can secure multiple pieces of housing  320  together, such as a top piece and a bottom piece of housing  320 . In some cases, housing  320  can be configured as a single unit. In some cases, sleeve  326  can be secured in place via a ring  324 . 
     In some cases, housing  320  can include a recess  328 . In some cases, housing  320  can include multiple recesses  328 . In some cases, recess  328  can guide a sleeve removal tool  400 , shown in  FIGS. 13 and 14 , along housing  320 . In some cases, sleeve  326  can be snuggly fit on housing  320  such that a sleeve removal tool is needed to remove sleeve  326 . 
     In some cases, housing  320  can include a divot  330 . In some cases, divot  330  can be used to identify a posterior side of facet joint replacement device  300 . In some cases, such as when housing  320  is cylindrical, divot  330  can be used to as an indication of the interior curvature of facet joint replacement device  300 . In some cases, facet joint replacement device  330  must be mounted on cortical screws in a specific orientation so that the rotation of first rod  310   a  takes place in the sagittal plane and the flexion and extension angles are in the correct directions. In some cases, divot  330  can be a physical divot or another type of marker. In some cases, the other type of marker can be a laser printed marker, a printed marker, or other marking on the surface of housing  320 . 
     Referring to  FIGS. 7-9 , a facet joint replacement device  302  is an example of the types of facet joint replacement devices that can be used with artificial FJR system  100 . Facet joint replacement device  302  can function as a piston/cylinder mechanism and can be substantially similar to facet joint replacement device  300 .  FIGS. 7-9  show cross section A-A of  FIG. 6 . 
     First rod  310   a  can include a head  312 . In some cases, head  312  can be located on a distal portion of first rod  310   a.  In some cases, head  312  can be rounded. In some cases, head  312  can be mushroom shaped. In some cases, head  312  can be rounded on a distal portion and flat on a proximal portion. In some cases, head  312  can extend perpendicular to the first rod  310   a.    
     In some cases, head  312  can include an insert  314 . In some cases, the insert  314  can be a spring. In some cases, the insert  314  can be an elastic element. In some cases, the elastic element can be made of various materials and can be constructed in various manners. In some cases, the elastic element can be made of a mesh composite. In some embodiments, the elastic element can be an elastic polymeric element. In some cases, the elastic element can be made of a variety of different types of materials and combinations of materials. For example, elastic element can be made of polymeric materials such as, but not limited to, polyurethane, polyethylene, silicone, and other biocompatible elastomers. In some cases, the elastic element can be made of metallic materials such as, but not limited to, nitinol, stainless steel, titanium, titanium alloys, cobalt-chrome, and the like. In some cases, elastic element is solid, but in other cases, elastic element can be constructed for enhanced compliance and/or elasticity. For example, in some embodiments elastic elements, or portions thereof, can be molded, laser-cut, machined, braided, woven, etc. so that the compliance and/or elasticity of elastic element can be enhanced. In some cases, the insert  314  can provide a soft stop at a maximum range of motion in extension and/or flexion. In some cases, the insert  314  can be coupled to head  312 , such that insert  314  and head  312  move in unison. In some cases, insert  314  can be donut shaped to surround rod  310   a  above head  312 . 
     Housing  320  can be an elongate member that defines an internal space  340 . End portion of rod  310   a  can be movably disposed within the internal space  340  of housing  320 . In some cases, facet joint replacement device  302  can be installed such that the rod  310   a  and internal space  340  are in a neutral position with one another (i.e., no forces acting between the internal space  340  or the rod  310   a ). In some cases, head  312  can be movably disposed within the internal space  340  of housing  320 . In some cases, insert  314  can be coupled to housing  320  in an upper portion of the internal space  340  and can abut head  312  during motion. In some cases, housing  320  can have an angled top and/or an angled bottom for the internal space  340  such that when head  312  and/or insert  314  abut the top and/or bottom of the internal space  340 , even contact is made between the head  312  and/or insert  314  of first rod  310   a  and housing  320 . In some cases, when head  312  and/or insert  314  come into contact with housing  320  during extension and flexion, head  312  and/or insert  314  can limit range of movement. In some cases, an internal diameter of housing  320  defining internal space  340  can follow an arc that replicates a natural trajectory of a healthy facet joint in extension and flexion. 
     Housing  320  can have various cross-sectional shapes. In some cases, the cross-sectional shape of the internal space  340  defined by housing  320  is circular, ovular, elliptical, polygonal, and the like. The cross-sectional shape of the internal space  340  can be selected to allow for a certain amount of free play between rod  310   a  and an internal wall of housing  320 . In some cases, housing  320  can be designed to have some degree of compliance such that housing  320  can bend or flex while in use. In some cases, housing  320  can be designed to be rigid such that housing  320  substantially does not bend or flex while in use. 
     In some cases, housing  320  can include one or more inserts. In some cases, an insert can be located at a top portion of the internal space  340  of housing  320 , as discussed above. In some cases, an insert  316  can be located at a bottom portion of the internal space  340  of housing  320 . In some cases, the insert  316  can be a spring. In some cases, the insert  316  can be an elastic element. In some cases, the elastic element can be made of various materials and can be constructed in various manners. In some cases, the elastic element can be made of a mesh composite. In some embodiments, the elastic element can be an elastic polymeric element. In some cases, the elastic element can be made of a variety of different types of materials and combinations of materials. For example, elastic element can be made of polymeric materials such as, but not limited to, polyurethane, polyethylene, silicone, and other biocompatible elastomers. In some cases, the elastic element can be made of metallic materials such as, but not limited to, nitinol, stainless steel, titanium, titanium alloys, cobalt-chrome, and the like. In some cases, elastic element is solid, but in other cases, elastic element can be constructed for enhanced compliance and/or elasticity. For example, in some embodiments elastic elements, or portions thereof, can be molded, laser-cut, machined, braided, woven, etc. so that the compliance and/or elasticity of elastic element can be enhanced. In some cases, the insert can provide a soft stop at a maximum range of motion in extension caused by forces  308   a  and/or  308   b  (as shown in  FIG. 9 ) and/or flexion caused by forces  306   a  and/or  306   b  (as shown in  FIG. 8 ). In some cases, insert  316  can include channels cut-out of the insert  316  to allow deformation and/or specific a specific amount of resistance. In some cases, insert  316  can extend across internal space  340 , leaving a gap between insert  316  and a bottom of the internal space  340 . In some cases, insert  316  can rest on a bottom of the internal space  340  and insert  316  can be configured to follow the shape and/or dimensions of the bottom of the internal space  340 . 
     In some cases, internal space  340  can be defined by two or more distinct portions. In some cases, the interior of housing  320  can have an upper portion  342  and a lower portion  344  that conjoin at an inflection region or bend  346  such that internal space establishes a natural trajectory of healthy facet movement. In some cases, upper portion  342  and lower portion  344  can have a diameter that decreases as portions  342  and  344  extend away from bend  346 . In some cases, the diameters of upper portion  342  and lower portion  344  can change uniformly. In some cases, the diameters of upper portion  342  and lower portion  344  can change uniquely for each of the upper portion  342  and lower portion  344 . In some cases, bend  346  can be a single area where upper portion  342  and lower portion  344  meet. In some cases, bend  346  can be a separate portion with a uniform diameter along bend  346 . In some cases, sections of upper portion  342  and/or lower portion  344  can have a consistent diameter before the diameter decreases. In some cases, bend  346  can be oriented in such a way that the movement of first rod  310   a  follows a natural trajectory of healthy facet movement, which is opposite in direction to the curvature of the lumbar spine vertebrae structure ( FIG. 1 ) in some embodiments. In some cases, bend  346  can have a diameter to allow for a certain amount of free play between rod  310   a  and an internal wall of housing  320 . In some cases, the decrease in diameter of upper portion  342  and/or lower portion  344  can cause an increase of resistance as head  312  and/or first rod  310   a  moves further along upper portion  342  and/or lower portion  344 . 
     Housing  320  can include a first portion  324   a  and second portion  326   a.  In some cases, first portion  324   a  and second portion  326   a  can be indents on an exterior of housing  320 . In some cases, first portion  324   a  can aid in securing ring  324 . In some cases, second portion  326   a  can aid in securing sleeve  326 . In some cases, first portion  324   a  can have a depth greater than second portion  326   a.  In some cases, an exterior diameter of housing  320  can follow an arc that replicates a natural trajectory of a healthy facet joint in extension and flexion. In some cases, housing  320  can have a uniform exterior, such that portions  324   a  and  326   a  are not included (e.g., when housing  320  does not include ring  324  and/or sleeve  326 ). In some cases, portions  324   a  and  326   a  can be two separate pieces of housing  320 . In some cases, sleeve  326  and ring  324  can hold portions  324   a  and  326   a  of housing  320  together. In some cases, portions  324   a  and  326   a  can be used to allow housing  320  to receive head  312 . 
     Referring to  FIGS. 10-12 , a facet joint replacement device  304  is an example of the types of facet joint replacement devices that can be used with artificial FJR system  100 . Facet joint replacement device  304  can function as a piston/cylinder mechanism and can be substantially similar to facet joint replacement device  300 .  FIGS. 10-12  show a second embodiment of cross section A-A of  FIG. 6 . 
     First rod  310   a  can include a head  312 . In some cases, head  312  can be located on a distal portion of first rod  310   a.  In some cases, head  312  can be rounded. In some cases, head  312  can be mushroom shaped. In some cases, head  312  can be rounded on a distal portion and flat on a proximal portion. In some cases, head  312  can extend perpendicular to the first rod  310   a.    
     Housing  320  can be an elongate member that defines an internal space  340 . End portion of rod  310   a  can be movably disposed within the internal space  340  of housing  320 . In some cases, facet joint replacement device  304  can be installed such that the rod  310   a  and internal space  340  are in a neutral position with one another (i.e., no forces acting between the internal space  340  or the rod  310   a ). In some cases, head  312  can be movably disposed within the internal space  340  of housing  320 . 
     Housing  320  can have various cross-sectional shapes. In some cases, the cross-sectional shape of the internal space  340  defined by housing  320  is circular, ovular, elliptical, polygonal, and the like. The cross-sectional shape of the internal space  340  can be selected to allow for a certain amount of free play between rod  310   a  and an internal wall of housing  320 . In some cases, housing  320  can be designed to have some degree of compliance such that housing  320  can bend or flex while in use. In some cases, housing  320  can be designed to be rigid such that housing  320  substantially does not bend or flex while in use. 
     In some cases, internal space  340  can be defined by two or more distinct portions. In some cases, the interior of housing  320  can have an upper portion  342  and a lower portion  344 , separated by a bend  346 . In some cases, upper portion  342  and lower portion  344  can have a diameter that decreases as portions  342  and  344  extend away from bend  346 . In some cases, the diameters of upper portion  342  and lower portion  344  can change uniformly. In some cases, the diameters of upper portion  342  and lower portion  344  can change uniquely for each of the upper portion  342  and lower portion  344 . In some cases, bend  346  can be a single area where upper portion  342  and lower portion  344  meet. In some cases, bend  346  can be a separate portion with a uniform diameter along bend  346 . In some cases, sections of upper portion  342  and/or lower portion  344  can have a consistent diameter before the diameter decreases. In some cases, bend  346  can be located at a point such that the movement of first rod  310   a  follows a natural trajectory of healthy facet movement. In some cases, bend  346  can have a diameter to allow for a certain amount of free play between rod  310   a  and an internal wall of housing  320 . In some cases, the decrease in diameter of upper portion  342  and/or lower portion  344  can cause an increase of resistance as head  312  and/or first rod  310   a  moves further along upper portion  342  and/or lower portion  344 . 
     In some cases, housing  320  can have an angled top and/or an angled bottom for the internal space  340  such that when head  312  abuts the top and/or bottom of the internal space  340 , even contact is made between the head  312  of first rod  310   a  and housing  320 . In some cases, when head  312  comes into contact with housing  320  during extension and flexion, head  312  can limit range of movement. In some cases, an internal diameter of housing  320  defining internal space  340  can follow an arc that replicates a natural trajectory of a healthy facet joint in extension and flexion. 
     In some cases, housing  320  can include an insert  350 . In some cases, housing  320  can be configured to allow an insert  350  to be inserted and/or removed. In some cases, insert  350  can be interchangeable with another insert  350 . In some cases, insert  350  can have an exterior diameter, or diameters, that align with the internal diameter of housing  320 . In some cases, insert  350  can have a bottom and side. In some cases, insert  350  can define an opening that can align with opening  322 . In some cases, the opening of insert  350  can be larger than the opening  322 , such that the opening of insert  350  does not limit range of motion. In some cases, insert  350  can include a top that defines an opening, such that when head  312  moves upward (e.g., during flexion, as shown in  FIG. 11 ), the head  312  abuts a portion of insert  350 . In some cases, insert  350  defines an internal space. In some cases, head  312  can be sized to fit inside the internal space defined by the insert  350 . 
     In some cases, the internal space defined by the insert  350  can be defined by two or more distinct portions. In some cases, the interior of insert  350  can have an upper portion  352  and a lower portion  354 , separated by a bend  356 . In some cases, upper portion  352  and lower portion  354  can have a diameter that decreases as portions  352  and  354  extend away from bend  356 . In some cases, the diameters of upper portion  352  and lower portion  354  can change uniformly. In some cases, the diameters of upper portion  352  and lower portion  354  can change uniquely for each of the upper portion  352  and lower portion  354 . In some cases, bend  356  can be a single area where upper portion  352  and lower portion  354  meet. In some cases, bend  356  can be a separate portion with a uniform diameter along bend  356 . In some cases, sections of upper portion  352  and/or lower portion  354  can have a consistent diameter before the diameter decreases. In some cases, bend  356  can be located at a point such that the movement of first rod  310   a  follows a natural trajectory of healthy facet movement. In some cases, bend  356  can have a diameter to allow for a certain amount of free play between rod  310   a  and an internal wall of insert  350 . In some cases, the decrease in diameter of upper portion  352  and/or lower portion  354  can cause an increase of resistance as head  312  and/or first rod  310   a  moves further along upper portion  352  and/or lower portion  354 . 
     In some cases, insert  350  can have an angled internal bottom for the internal space such that when head  312  abuts the bottom of the internal space defined by the insert  350 , even contact is made between the head  312  of first rod  310   a  and insert  350 . In some cases, when head  312  comes into contact with insert  350  during extension and flexion, head  312  can limit range of movement. In some cases, an internal diameter of insert  350 , defining the insert internal space, can follow an arc that replicates a natural trajectory of a healthy facet joint in extension and flexion. 
     In some cases, the insert  350  can be an elastic element. In some cases, the elastic element can be made of various materials and can be constructed in various manners. In some cases, the elastic element can be made of a mesh composite. In some embodiments, the elastic element can be an elastic polymeric element. In some cases, the elastic element can be made of a variety of different types of materials and combinations of materials. For example, elastic element can be made of polymeric materials such as, but not limited to, polyurethane, polyethylene, silicone, and other biocompatible elastomers. In some cases, the elastic element can be made of metallic materials such as, but not limited to, nitinol, stainless steel, titanium, titanium alloys, cobalt-chrome, and the like. In some cases, elastic element is solid, but in other cases, elastic element can be constructed for enhanced compliance and/or elasticity. For example, in some embodiments elastic elements, or portions thereof, can be molded, laser-cut, machined, braided, woven, etc. so that the compliance and/or elasticity of elastic element can be enhanced. In some cases, the insert  350  can provide a soft stop at a maximum range of motion in extension caused by forces  308   a  and/or  308   b  (as shown in  FIG. 12 ) and/or flexion caused by forces  306   a  and/or  306   b  (as shown in  FIG. 11 ). In some cases, insert  350  can include channels cut-out of the insert  350  to allow deformation and/or specific a specific amount of resistance. 
     Housing  320  can include a first portion  324   a  and second portion  326   a.  In some cases, first portion  324   a  and second portion  326   a  can be indents on an exterior of housing  320 . In some cases, first portion  324   a  can aid in securing ring  324 . In some cases, second portion  326   a  can aid in securing sleeve  326 . In some cases, first portion  324   a  can have a depth greater than second portion  326   a.  In some cases, an exterior diameter of housing  320  can follow an arc that replicates a natural trajectory of a healthy facet joint in extension and flexion. In some cases, portions  324   a  and  326   a  can be two separate pieces of housing  320 . In some cases, sleeve  326  and ring  324  can hold portions  324   a  and  326   a  of housing  320  together. In some cases, portions  324   a  and  326   a  can be used to allow housing  320  to receive head  312 . 
     Referring to  FIGS. 13 and 14 , a sleeve removal tool  400  can include housing  402 . Housing  402  can define an internal chamber  404 . In some cases, sleeve removal tool  400  can include one or more protrusions  406   a  and  406   b.    
     In some cases, internal chamber  404  can extend through an entire length of housing  402 . In some cases, internal chamber  404  can extend a portion of a length of housing  402 . In some cases, internal chamber  404  can have a diameter that is substantially equal to or slightly larger than an outer diameter of sleeve  326 , such that internal chamber  404  can receive facet joint replacement device  300 . 
     In some cases, protrusions  406   a  and  406   b  can extend into internal chamber  404 . In some cases, protrusions  406   a  and  406   b  can be sized and shaped such that recess  328  can receive one of protrusions  406   a  or  406   b.    
     In some cases, sleeve removal tool  400  can be used to remove sleeve  326  from facet joint replacement device  300 . For example, sleeve removal tool  400  can be pushed upward and protrusions  406   a  and  406   b  can be aligned with recess  328 , such that a top of sleeve removal tool  400  abuts a bottom of sleeve  328 . Sleeve removal tool  400  can then be pushed up, or tapped, to push sleeve  326  up housing  320 , such that sleeve  326  can be removed. 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described herein as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system modules and components in the embodiments described herein should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single product or packaged into multiple products. 
     Particular embodiments of the subject matter have been described. Other embodiments are within the scope of the following claims. For example, the actions recited in the claims can be performed in a different order and still achieve desirable results. As one example, the process depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results. In certain implementations, multitasking and parallel processing may be advantageous.