Patent Publication Number: US-2021177652-A1

Title: Adjustable stiffener for surgical instruments

Description:
PRIORITY CLAIM 
     This application claims the benefit of priority of U.S. Provisional Patent Application Ser. No. 62/946,598 titled “ADJUSTABLE STIFFENER FOR SURGICAL INSTRUMENTS,” filed on Dec. 11, 2019, whose inventors are Bill Chen, James Y. Chon and John R. Underwood, which is hereby incorporated by reference in its entirety as though fully and completely set forth herein. 
    
    
     DESCRIPTION OF THE RELATED ART 
     Continuous efforts to minimize the invasiveness of surgical procedures, such as ophthalmic surgical procedures, have led to the development of small-gauge surgical instrumentation for microincision techniques. Small gauge vitrectomy, also known as minimally invasive vitreous surgery (MIVS), is a classic example of one such type of surgical procedure utilizing small-gauge instrumentation. Examples of common ocular conditions that may be treated by minimally invasive vitreous surgery include retinal detachment, macular holes, premacular fibrosis, and vitreous hemorrhages. The benefits associated with modern MIVS as compared to more invasive vitrectomies include access to greater pathology, greater fluidic stability, increased patient comfort, less conjunctival scarring, less postoperative inflammation, and earlier visual recovery, among others. Accordingly, indications for MIVS and other microincision techniques have expanded in recent years. 
     Despite the aforementioned benefits of microincision techniques and their widespread acceptance, there remain numerous challenges with the utilization of small-gauge surgical instruments, particularly in the field of ophthalmology. One commonly noted concern among surgeons is instrument rigidity. The smaller diameter of these microincision instruments, such as vitrectomy probes, causes decreased stiffness thereof, making it difficult for surgeons to control the instruments during certain ocular surgical procedures. With small gauge ophthalmic surgical instruments, for example, the instrument tips can move in unintended directions at the extreme limits of the eye, thus making delicate procedures such as the peeling of membranes from the retinal surface extremely difficult. 
     Accordingly, what is needed in the art are improved methods and apparatus for minimally-invasive ophthalmic surgical procedures. 
     SUMMARY 
     In one embodiment, a surgical instrument is provided with a base unit, a probe, and a stiffener assembly. The base unit is configured to be held by a user. The probe is disposed through a first opening in a distal end of the base unit and has a length parallel to a longitudinal axis thereof. The stiffener assembly includes a stiffener extending through the first opening in the base unit and an actuation mechanism configured to actuate the stiffener along the length of the probe. The stiffener is formed of a hollow tubular member that surrounds at least a portion of the probe and is slidably coupled thereto. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments and are therefore not to be considered limiting of its scope, and may admit to other equally effective embodiments. 
         FIG. 1  illustrates a perspective view of an exemplary instrument according to one embodiment of the present disclosure. 
         FIG. 2A  illustrates a schematic cross-sectional side view of the instrument of  FIG. 1 . 
         FIG. 2B  illustrates another schematic cross-sectional side view of the instrument of  FIG. 1 . 
         FIG. 3  illustrates a perspective view of an exemplary instrument according to one embodiment of the present disclosure. 
         FIG. 4A  illustrates a schematic cross-sectional side view of the instrument of  FIG. 3 . 
         FIG. 4B  illustrates another schematic cross-sectional side view of the instrument of  FIG. 3 . 
         FIG. 5  illustrates a perspective view of an exemplary instrument according to one embodiment of the present disclosure. 
         FIG. 6A  illustrates a schematic cross-sectional side view of the instrument of  FIG. 5 . 
         FIG. 6B  illustrates another schematic cross-sectional side view of the instrument of  FIG. 5 . 
         FIG. 7  illustrates a perspective view of an exemplary instrument according to one embodiment of the present disclosure. 
         FIG. 8A  illustrates a schematic cross-sectional side view of the instrument of  FIG. 7 . 
         FIG. 8B  illustrates another schematic cross-sectional side view of the instrument of  FIG. 7 . 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. 
     DETAILED DESCRIPTION 
     The present disclosure generally relates to microsurgical instruments having variable stiffness, and more particularly, microsurgical instruments having variable stiffness for ophthalmic surgical procedures. In one embodiment, a surgical instrument includes a probe and a stiffener assembly. The stiffener assembly further includes a stiffener formed of a hollow tubular member substantially surrounding at least a portion of a length of the probe. Actuation of the stiffener along the length of the probe adjusts the stiffness of the probe, thus providing a user better control of the surgical instrument. 
       FIG. 1  illustrates a perspective view of an exemplary instrument  100  according to one embodiment described herein. As depicted in  FIG. 1 , the instrument  100  comprises a probe or needle  110  (referred to hereinafter as a “probe”) and a base unit  120 . The probe  110  includes a proximal portion  112  and a distal portion  114  which terminates distally at the distal end  116 . In some embodiments, the proximal portion  112  extends through a substantial portion of an interior chamber ( 124 , shown in  FIGS. 2A and 2B ) of the base unit  120 . 
     In one example, the probe  110  is an elongated cutting member of a vitrectomy probe. For example, the probe  110  may be inserted into a cannula for performance of vitreous surgery, which may be aspirating or non-aspirating. The probe  110  may comprise a hollow tube having a diameter less than about 20 gauge. For example, the probe  110  has a diameter less than about 23 gauge, such as a diameter less than about 25 gauge. In one embodiment, the probe  110  has a diameter of approximately 27 gauge. In further examples, the probe  110  may include an illumination device, a laser guide, a suction device, forceps, scissors, retractors, or other suitable devices disposed therein or coupled thereto. 
     Generally, the probe  110  is formed of a material suitable for minimally invasive surgical procedures, such as vitreoretinal surgeries that involve removal of the vitreous in the eye, or other surgical procedures. For example, the probe  110  is formed of surgical grade stainless steel, aluminum, or titanium. 
     The probe  110  is partially and longitudinally disposed through a distal end  121  of the base unit  120  adjacent the proximal portion  112  and may be directly or indirectly attached thereto within the interior chamber of the base unit  120  (interior chamber  124 , as discussed below). In one embodiment, the base unit  120  is a handpiece having an outer surface  122  configured to be held by a user, such as a surgeon. For example, the base unit  120  may be contoured to substantially fit the hand of the user. In some embodiments, the outer surface  122  may be textured or have one or more gripping features formed thereon, such as one or more grooves and/or ridges. 
     The base unit  120  may house at least a portion of a drive mechanism operable to reciprocate the probe  110  within and relative to the base unit  120 . In one example, the drive mechanism may be a pneumatic drive mechanism including a diaphragm. The base unit  120  may further provide one or more ports  123  at a proximal end  125  thereof for one or more supply lines to be routed into the interior chamber  124 . For example, the one or more ports  123  may provide a connection between the base unit  120  and a vacuum source for aspiration. In another example, the one or more ports  123  provides a connection to a pneumatic, hydraulic, or electrical power source to operate the drive mechanism, an illumination device, a laser, or other suitable device within or coupled to the base unit  120 . 
     The instrument  100  further includes a stiffener assembly  130  comprising a stiffener  132  slidably coupled to and substantially surrounding at least a portion of the probe  110 . The stiffener  132  is adjustable relative to the probe  110 , enabling a user to position the stiffener  132  (e.g., a distal end of the stiffener  132 ) at different points along a length L (shown in  FIGS. 2A and 2B ) of the probe  110  exterior to the base unit  120 . Accordingly, a user may selectively adjust the level of stiffness of the probe  110  by re-positioning the stiffener  132  relative to the distal end  116 , thereby manipulating the amount of support provided to the probe  110  and stabilizing the instrument  100  during use thereof. 
       FIGS. 2A and 2B  illustrate schematic cross-sectional views of the instrument  100  with the stiffener  132  positioned at different points along a length L of the probe  110 . Therefore,  FIGS. 2A and 2B  are herein described together with  FIG. 1  for clarity. The stiffener  132  is generally a cylindrical and hollow tube substantially surrounding the probe  110  at or near the proximal portion  112 . Similar to the probe  110 , the stiffener  132  is formed of a material suitable for minimally invasive surgical procedures, such as vitreoretinal surgeries and other surgical procedures. In some embodiments, the stiffener  132  is formed of a metallic material, such as surgical grade stainless steel, aluminum, or titanium. In other embodiments, the stiffener  132  is formed of a composite material, such as a polymer composite material or a ceramic composite material. 
     Along with the probe  110 , the stiffener  132  is disposed through an opening  117  of the distal end  121  and has a proximal end  133  disposed in the interior chamber  124 . The stiffener  132  is sized to possess an axial length sufficient to provide a desired rigidity and stability to the probe  110  while having a portion thereof still remaining in the interior chamber  124  when the stiffener assembly  130  is in a (e.g., fully) protracted position. For example, the stiffener  132  may have an axial length between about 0.25 inches and about 1.75 inches, such as between about 0.30 inches and about 1.50 inches. For example, the stiffener  132  may have an axial length between about 0.50 inches and about 1.25 inches. 
     In one embodiment, stiffener  132  has a uniform outer diameter from the distal end  131  to the proximal end  133 . Having a uniform outer diameter enables a substantial length of the stiffener  132  to be reciprocated through the opening  117  without forming an airgap therebetween. However, other shapes and morphologies of the stiffener  132  are also contemplated. For example, in some embodiments, the stiffener  132  comprises a square, rectangular, or polygonal tube. In further embodiments, the stiffener  132  may have a non-uniform outer diameter. For example, the stiffener  132  may have an outer diameter having one or more dimensions following a step-wise or gradual delta. 
     An inner cavity  135  of the stiffener  132  is sized to accommodate the outer diameter of the probe  110  while also permitting the stiffener  132  to be readily moved along probe  110 . Thus, an inner diameter or width of the stiffener  132  is greater than the outer diameter of the probe  110  and enables a sliding fit. In one embodiment, a radial clearance between the stiffener  132  and the probe  110  is between about 0.00020 inches and about 0.00060 inches, such as between about 0.00025 inches and about 0.00050 inches. For example, the radial clearance between the stiffener  132  and the probe  110  is between about 0.00030 inches and about 0.00040 inches, such as about 0.00035 inches. Further, the inner dimensions of the stiffener  132  may be uniform from the distal end  131  to the proximal end  133  to enable uniform stabilization of the probe  110  throughout the inner cavity of the stiffener  132 . 
     In one embodiment, the stiffener  132  is indirectly coupled to the control element  138  by the coupling arm  134  and the rod  136 . The coupling arm  134  connects the stiffener  132  to the rod  136  and is oriented in a non-parallel fashion therebetween. In some embodiments, the coupling arm  134  is a direct extension of the stiffener  132  and/or the rod  136 . That is, the coupling arm  134  and the stiffener  132  and/or the rod  136  are a single integral component. In other embodiments, the coupling arm  134  and the stiffener  132  and/or the rod  136  are separate components coupled to one another by one or more coupling mechanisms and/or adhesives. For example, as depicted in  FIGS. 2A and 2B , the coupling arm  134  and the rod  136  are coupled together by a pin  137 . In other examples, the coupling arm  134  and the rod  136  may be snap-fit together. 
     The control element  138  may be a button, knob, switch, toggle, or any other suitable device capable of being actuated by a user. As depicted in  FIGS. 2A and 2B , the control element  138  is partially disposed within a linear channel  128  formed in the base unit  120 . The channel  128  runs substantially parallel to the probe  110  and enables bidirectional sliding of the control element  138  along a longitudinal axis X thereof. In one embodiment, the rod  136  is directly coupled to the control element  138  and runs substantially parallel to the probe  110  within the channel  128 . The rod  136  may further be disposed through a second opening  119  formed in the distal end  121  of the base unit  120  in order to connect with the coupling arm  134 . Generally, the rod  136  may be formed of a metallic or composite material. In some embodiments, the rod  136  is formed of stainless steel, aluminum, or titanium. In other embodiments, the rod  136  is formed of a polymer composite material or ceramic composite material. 
     During use, the rod  136  transfers motion of the control element  138  to the coupling arm  134 , and thus, the stiffener  132 . Accordingly, sliding of the control element  138  within the channel  128  results in sliding of the stiffener  132  along the length L of the probe  110 . In some embodiments, the stiffener  132  is adjustable up to a distance of about 15 mm along the length L of the probe  110 , such as a distance up to about 10 mm along the length L of the probe  110 . For example, the stiffener  132  is adjustable up to a distance of about 5 mm along the length L of the probe  110 . 
     In one embodiment, the channel  128  comprises a track having one or more protrusions  139  disposed at preset locations along a length of the channel  128  upon which the control element  138  may be secured. For example, the control element  138  may have a groove disposed on a lower or oblique surface thereof and matching the morphology of the one or more protrusions  139 . Thus, the control element  138  may be locked upon a protrusion  139  by sliding the control element  138  adjacent thereto and engaging the groove with the protrusion  139 . As a result, the one or more protrusions  139  may be utilized to provide predetermined levels of rigidity to the probe  110 . That is, the one or more protrusions  139  may be located at preset increments along the length of the channel  128  corresponding to predetermined levels of rigidity provided to the probe  110 . 
     In another embodiment, the channel  128  comprises a track with substantially planar surfaces upon which the control element  138  may be slidably and dynamically actuated by the user, providing greater flexibility and freedom to the user in determining a desired position of the stiffener  132  relative to the probe  110 . Accordingly, the user may set the control element  138  at a desired position by simply controlling the control element  138  with their thumb. 
       FIGS. 2A and 2B  illustrate the channel  128  having three protrusions  139   a - 139   c  disposed therein. Generally, sliding the stiffener  132  towards the distal end  116  of the probe  110  increases the rigidity of the probe  110 . In  FIG. 2A , the stiffener assembly  130  is disposed in a fully retracted position where the control element  138  is locked in place over the protrusion  139   a . Accordingly, a majority of the stiffener  132  is retracted within the base unit  120 , providing decreased stability and rigidity to the probe  110 . In  FIG. 2B , the stiffener assembly  130  is disposed in a protracted position wherein the control element  138  is locked in place over the protrusion  139   b . Accordingly, a greater portion of the stiffener  132  is protracted over the proximal portion  112  of the probe  110 , providing increased stability and rigidity to the probe  110 . 
     Although the stiffener assembly  130  is depicted and described as having the control element  138 , the coupling arm  134 , and the rod  136 , these elements comprise only one embodiment of an actuation mechanism for a stiffener and thus should not be considered limiting thereof. Additional embodiments and configurations of actuation mechanisms for a stiffener are further described below. 
       FIG. 3  illustrates a perspective view of another exemplary instrument  300  having a stiffener assembly  330 . The instrument  300  is substantially similar to the instrument  100 , except for the structure and actuating mechanism of the stiffener assembly  330 . As depicted in  FIG. 3 , the stiffener assembly  330  includes a pinion  338  operatively engaged with a proximal end (e.g., proximal end  333 , discussed below) of a stiffener  332  within the interior chamber  124  (shown in  FIGS. 4A and 4B ) to actuate the stiffener  332  along the probe  110 . 
       FIGS. 4A and 4B  illustrate schematic cross-sectional views of the exemplary instrument  300  with the stiffener  332  positioned at different points along the length L of the probe  110 . Therefore,  FIGS. 4A and 4B  are herein described together with  FIG. 3  for clarity. 
     As described above, the stiffener assembly  330  includes the stiffener  332  and the pinion  338 . Similar to the stiffener  132 , the stiffener  332  is substantially a hollow tube slidably mounted to and surrounding the probe  110 . Along with the probe  110 , the stiffener  332  is disposed through the opening  117  in the base unit  120  and extends into the interior chamber  124  thereof. Unlike the stiffener  132 , however, the stiffener  332  includes the proximal end  333  having a rack  336  formed thereon and engaged with the pinion  338 . In one embodiment, the proximal end  333  is integrally coupled to a distal end  331  thereof. In another embodiment, the proximal end  333  is removably coupled to the distal end  331  via any suitable coupling mechanism and/or adhesive. The stiffener  332 , including the proximal end  333 , is sized to possess an axial length sufficient to provide a desired rigidity and stability to the probe  110  when the stiffener assembly  330  is in a (e.g., fully) protracted position. For example, the stiffener  332  may have an axial length between about 0.25 inches and about 1.75 inches, such as between about 0.30 inches and about 1.50 inches. For example, the stiffener  132  may have an axial length between about 0.50 inches and about 1.25 inches. 
     The rack  336  includes a first plurality of linear gear teeth  334  formed on an outer surface of the proximal end  333  and operatively engaged with a second plurality of teeth  335  formed on the pinion  338 . A linear pitch between each of the plurality of linear gear teeth  334  is dependent on a diameter of the pinion  338 . In one example, the pitch between each of the plurality of linear gear teeth  334  is between about 0.025 inches and about 0.25 inches, such as between about 0.05 inches and about 0.20 inches. For example, the pitch between each of the plurality of linear gear teeth  334  is between about 0.075 inches and about 0.15 inches, such as between about 0.090 inches and about 0.10 inches. Generally, the rack  336  is formed of a metallic or composite material. In some embodiments, the rack  336  is formed of stainless steel, aluminum, or titanium. In other embodiments, the rack  336  is formed of a polymer composite material or ceramic composite material. 
     The pinion  338  is disposed in a recess  337  (e.g., opening) formed in the outer surface  122  of the base unit  120  such that a first portion of the pinion  338  protrudes from the recess  337  towards an exterior of the base unit  120  and is diametrically opposed to a second portion of the pinion  338  engaged with the rack  336  within the interior chamber  124 . Similar to the rack  336 , the pinion  338  is formed of a metallic or composite material, such as stainless steel, aluminum, titanium, polymer composite, or ceramic composite. The recess  337  may be formed in any suitable location along the outer surface  122 . For example, the recess  337  may be disposed adjacent either the distal end  121  or the proximal end  125  of the base unit. In other embodiments, the recess  337  may be more centrally disposed between the distal end  121  and the proximal end  125 . 
     In one embodiment, the pinion  338  is rotatably supported within the recess  337  by a pin  339  rotatably coupled to the base unit  120 . Accordingly, rotation of the pinion  338  about an axis Z normal to the longitudinal axis X linearly actuates the stiffener  332  along the length L of the probe  110  in a first or second direction, X 1  and X 2 , respectively. For example, as depicted in  FIG. 4A and 4B , rotation of the pinion  338  in a first rotational direction Y 1  actuates the stiffener  332  in the first linear direction X 1  along the probe  110 , thus protracting the stiffener  332  from within the interior chamber  124  of the base unit  120  and increasing the rigidity of the probe  110 . Conversely, rotation of the pinion  338  in a second rotational direction Y 2  actuates the stiffener  332  in the second linear direction X 2  along the probe  110 , thus retracting the stiffener  332  into the base unit  120  and reducing the rigidity of the probe  110 . In some embodiments, the stiffener  332  is adjustable up to a distance of about 15 mm along the length L of the probe  110 , such as a distance up to about 10 mm along the length L of the probe  110 . For example, the stiffener  332  is adjustable up to a distance of about 5 mm along the length L of the probe  110 . 
     Although the stiffener assembly  330  is depicted and described as having the pinion  338  and the rack  336 , these elements comprise only one embodiment of an actuation mechanism for a stiffener and thus should not be considered limiting thereof. Additional embodiments and configurations of actuation mechanisms for a stiffener are further described throughout this application. 
       FIG. 5  illustrates a perspective view of another exemplary instrument  500  according to one embodiment described herein. The instrument  500  is substantially similar to the instruments  100  and  300 , except for the structure and actuating mechanism of stiffener assembly  530 . As depicted in  FIG. 5 , the stiffener assembly  530  includes a rotatable distal end  538  movingly coupled to the stiffener  532  to actuate the stiffener  532  along the probe  110 . 
       FIGS. 6A and 6B  illustrate schematic cross-sectional views of the exemplary instrument  500  with the stiffener  532  positioned at different points along the length L of the probe  110 . Therefore,  FIGS. 6A and 6B  are herein described together with  FIG. 5  for clarity. 
     As described above, the stiffener assembly  530  includes the stiffener  532  and the rotatable distal end  538 . The distal end  538  is rotatably coupled to the base unit  120  and configured to rotate about the longitudinal axis X through the opening  537 . The distal end  538  is typically formed of a metallic or composite material. In some embodiments, the distal end  538  is formed of stainless steel, aluminum, or titanium. In other embodiments, the distal end  538  is formed of a polymer composite material or ceramic composite material. 
     Similar to the stiffeners  132  and  332 , the stiffener  532  is generally a hollow tube slidably mounted to and substantially surrounding the probe  110  adjacent the proximal portion  112 . Along with the probe  110 , the stiffener  532  is disposed through the opening  537  in the distal end  538  and extends into the interior chamber  124  thereof. The stiffener  532  is sized to possess an axial length sufficient to provide a desired rigidity and stability to the probe  110  while having a portion thereof still extending through the opening  537  when the stiffener assembly  530  is in a (e.g., fully) protracted position. For example, the stiffener  532  may have an axial length between about 0.25 inches and about 1.75 inches, such as between about 0.30 inches and about 1.50 inches. For example, the stiffener  132  may have an axial length between about 0.50 inches and about 1.25 inches. 
     The stiffener  532  has one or more features  535  formed on an exterior surface  534  thereof. In one embodiment, the features  535  include a spiraling thread. In another embodiment, the features  535  include one or more protrusions and/or grooves formed on the exterior surface  534 . The features  535  of the stiffener  532  are operatively engaged with one or more features  539  formed on an interior surface of the opening  537 . Similar to the features  535 , the features  539  may include protrusions, grooves, and/or a spiraling thread. However, at least one of the opening  537  and the exterior surface  534  has a spiraling thread formed thereon. Generally, the features  535  of the stiffener  532  are female mating features and the features  539  of the opening  537  are male mating features. However, it is also contemplated that the features  535  may be male mating features and the features  539  may be female mating features. 
     Accordingly, rotation of the distal end  538  about the longitudinal axis X linearly actuates the stiffener  532  along the length L of the probe  110  in a first or second direction X 1  and X 2 , respectively. For example, rotation of the distal end  538  in a first rotational direction around the longitudinal axis X may actuate the stiffener  532  in the first linear direction X 1  along the probe  110 , thus protracting the stiffener  532  from the interior chamber  124  of the base unit  120  and increasing the rigidity of the probe  110 . Conversely, rotation of the distal end  538  in a second rotational direction around the longitudinal axis X may actuate the stiffener  532  in the second linear direction X 2  along the probe  110 , thus retracting the stiffener  532  into the base unit  120  and reducing the rigidity of the probe  110 . In some embodiments, the stiffener  532  is adjustable up to a distance of about 15 mm along the length L of the probe  110 , such as a distance up to about 10 mm along the length L of the probe  110 . For example, the stiffener  532  is adjustable up to a distance of about 5 mm along the length L of the probe  110 . Note that, in the embodiments described herein, at least a portion (e.g., distal portion  114 ) of probe  110  is inserted into a patient&#39;s eye through an insertion cannula. However, the remainder (e.g., proximal portion  112 ) of the probe remains outside of the eye and the insertion cannula. When in a (e.g., fully) protracted state, the stiffeners described herein cover the portion of the probe that remains outside of the eye and the insertion cannula (or the hub of the insertion cannula). 
     Although the stiffener assembly  530  is depicted and described as having the rotatable distal end  538 , this element comprises only one embodiment of an actuation mechanism for a stiffener and thus should not be considered limiting thereof. Additional embodiments and configurations of actuation mechanisms for a stiffener are further described throughout this application. 
       FIG. 7  illustrates a perspective view of another exemplary instrument  700  according to one embodiment described herein. The instrument  700  is substantially similar to the instruments  100 ,  300 , and  500 , except for the structure and actuating mechanism of stiffener assembly  730  (shown in  FIGS. 8A and 8B ). The stiffener assembly  730  is a self-adjusting stiffener assembly and includes a stiffener  732  coupled to a biasing device  738 .  FIGS. 8A and 8B  illustrate schematic cross-sectional views of the instrument  700  with the stiffener  732  positioned at different points along the length L of the probe  110 , and thus, are herein described together with  FIG. 7  for clarity. 
     Similar to the stiffeners  132 ,  332 , and  532  described above, the stiffener  732  is generally a hollow tube slidably mounted to and substantially surrounding the probe  110  at the proximal portion  112 . The stiffener  732  is disposed through the opening  117  in the base unit  120  and extends into the interior chamber  124  thereof. In one embodiment, the stiffener  732  includes an annular flange (e.g., flange  736 ) disposed at a proximal end (e.g., proximal end  733 ) within the interior chamber  124 . In other embodiments, the flange  736  is disposed more axially along a length of the stiffener  732 . The flange  736  is configured to prevent the stiffener  732  from completely sliding through the opening  117  and out of the base unit  120 . Thus, the flange  736  acts as an anchor in one capacity. In some embodiments, the flange  736  further provides a coupling surface between the stiffener  732  and the biasing device  738 . 
     The biasing device  738  applies a biasing force against the stiffener  732  in a distal direction to urge the stiffener  732  towards a protracted position P along the length L of the probe  110 . Thus, without an application of a force in an opposite, proximal direction, the stiffener  732  is constantly disposed in the protracted position P. During use, the probe  110  may be inserted into an insertion cannula with a hub (e.g., including a valve), at a desired depth along the length L selected by the user. Upon a distal end  731  of the stiffener  732  reaching the hub of the insertion cannula, the user may further press the instrument  700  towards the hub to drive the probe  110  deeper therein. Application of a force against the hub greater than that of the force provided by the biasing device  738  will cause the stiffener  732  to retract into the base unit  120  (shown in  FIG. 8B ), allowing a greater portion of the probe  110  to enter the eye. Accordingly, a maximum amount of support is constantly applied by the stiffener  732  to the probe  110  while the probe  110  is the only component of the instrument  700  to enter the cannula and the eye. Thus, no manual adjustment is necessary to adjust the position of the stiffener  732 , and an optimal rigidity or stiffness is provided to the probe  110  at all times. 
     In some embodiments, the stiffener  732  is adjustable up to a distance of about 10 mm along the length L of the probe  110 , such as a distance up to about 6 mm along the length L of the probe  110 . For example, the stiffener  732  is adjustable up to a distance of about 3 mm along the length L of the probe  110 . 
     In one embodiment, the biasing device  738  is actuated by a spring  739 , such as a compression spring. For example, the biasing device  738  may be actuated by a coil or helical spring. In other examples, the biasing device  738  may include spring configurations other than coils. In one embodiment, the biasing device  738  is actuated by a compressible and expandable polymeric or elastomeric material. In yet another embodiment, the biasing device is actuated by a pneumatic or hydraulic piston. 
     Although the stiffener assembly  730  is depicted and described as having the biasing device  738 , this element comprises only one embodiment of an actuation mechanism for a stiffener and thus should not be considered limiting thereof. Additional embodiments and configurations of actuation mechanisms for a stiffener are further described throughout this application. 
     In summary, embodiments of the present disclosure include structures and mechanisms for adjusting the stiffness of microsurgical instruments, such as small-gauge instruments for minimally-invasive ophthalmologic operations. The instruments described above include embodiments wherein a user, such as a surgeon, may adjust the stiffness of the instruments during use thereof. Accordingly, the described embodiments enable a surgeon to access a wider range of tissues with a single instrument, thus expanding the applicability of smaller gauge instruments to a greater range of indications. 
     In one example, the described embodiments enable a surgeon to dynamically adjust the stiffness and length of a vitrectomy probe to access all areas of a vitreous cavity during a single procedure. The adjustment of the probe may be carried out prior to insertion of the probe into the eye or after the probe has already been inserted therein. Thus, the described embodiments may be utilized to facilitate access to the posterior segment of an eye during vitreous surgeries while retaining the benefits of smaller gauge probes, such as increased patient comfort, less conjunctival scarring, less postoperative inflammation, and faster healing time. Although vitreous surgery is discussed as an example of a surgical procedure that may benefit from the described embodiments, the advantages of an instrument with adjustable stiffness may benefit other surgical procedures as well. 
     While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.