Abstract: A dispensing gun includes a housing for holding a cartridge, a push rod engaged through the housing and having a plunger for engaging with the cartridge, an actuating plate engaged onto the push rod, a trigger pivotally secured to the housing and having an actuating pin for operating the actuating plate to move the push rod relative to the housing, a spring member biasing the actuating plate to engage with the actuating pin of the trigger, a latch engaged onto the push rod and biased to prevent the push rod from moving rearwardly relative to the housing when the actuating plate is released by the trigger, and a release member is located between the latch and the actuating pin of the trigger for being moved to release the push rod from the latch.

Abstract: Described herein is a novel drug delivery assembly having particular applicability to the field of orthopedic and surgical medicine. The devices and assemblies described herein enable the efficient application and retention of potentially expensive therapeutic materials to very specific locations, particularly those associated with voids in bone and tissue. One particularly unique aspect of the present invention involves the introduction of constructs which promote the retention of therapeutic material in the target area of application for beneficial use, for example by forming a proximal barrier that prevents leakage of the therapeutic material out of the target area. Additionally, the present invention provides unique devices and methods for surgical introduction of such constructs.

Abstract: A percutaneous path is established into a selected bone having an interior volume occupied, at least in part, by a cancellous bone, e.g., a vertebral body. An expandable mesh structure is introduced into the cancellous bone by deployment of a tool through the percutaneous path into the cancellous bone. The expandable mesh structure is expanded within cancellous bone by conveying a material into the mesh structure. Expansion of the mesh structure can, e.g., compact cancellous bone, and/or form a cavity in cancellous bone, and/or move fractured cortical bone.

Abstract: A system and method is provided for distracting opposite surfaces from the interior of a bone, such as a vertebral body. A working channel cannula provides a working channel through which an inserter and an injection cannula can simultaneously pass. The inserter transports a plurality of wafers into the interior of the bone to form a load-bearing stack bearing against the opposite surfaces. The injection cannula is used to inject a fluent material into and/or around the stack. In certain embodiments, the fluent material is a load-bearing or hardenable material, such as bone cement. In other embodiments, the fluent material can be a BMP, HAP, or other osteo-inductive, osteo-conductive, or pharmaceutical compositions. A syringe containing the fluent material is engaged to the injection cannula and is operable to inject the fluent material into the vertebral body under controlled pressure.

Abstract: In at least one embodiment of the present invention, a device for mixing and dispensing a bone cement mixture is provided. The device comprises an injector housing having a chamber for containing a first and a second bone cement component and a second housing having a chamber for containing a flexible wire mixing element. A plunger actuates within the chamber of the second housing to transfer the wire mixing element to the injector housing for mixing the first and second bone cement components to form the bone cement mixture. The plunger rotates the wire mixing element to mix the bone cement mixture. Thereafter, the wire mixing element is withdrawn from the injector housing and the injector housing is assembled with an injector handle. A plunger coupled to the injector handle advances through the injector chamber to dispense the bone cement mixture from the device.

Abstract: A bone tap may be used to deliver and/or aspirate fluid to and/or from a bone. The bone tap may include a passage. A distal portion of the bone tap may be threaded. In some embodiments, openings in a distal portion of the bone tap may communicate with the passage. The bone tap may be driven into bone, and material may enter or exit the bone through the openings and passage.

Abstract: A stand for use with a syringe is structured to engage a portion of the syringe in order to maintain the syringe in an upright position. The stand may include an aperture structured to engage the nozzle or the plunger of the syringe. Another embodiment of the stand may include a protrusion structured to engage the proximal end of the syringe, or a clip structured to engage a handle of the syringe. The stand may include a package portion having compartments for holding various components, such as components of a bone cement preparation kit. The proximal and distal portions of the barrel of the syringe may be separable to provide an opening into the barrel through which an injectable material may be inserted. The plunger of the syringe may also include a plunger rod removably affixed to the plunger tip.

Abstract: A method for stabilizing fractured bone structure having a fractured height includes expanding the bone structure to a restored height greater than the fractured height by transitioning an expandable member inserted into the bone structure from a contracted state to an expanded state. A curable material is delivered into a cavity formed in the bone structure and allowed to harden while the expandable member maintains the bone structure at the restored height. The expandable member is then removed from the bone structure.

Abstract: A needle assembly for dispensing a bone cement mixture is provided. The needle assembly includes a cannula defining an inner lumen, the cannula having an outlet end for dispensing the bone cement mixture from the assembly. The outlet end has at least one tip moveable between an expanded position and a collapsed position. The tip is configured to cut the bone cement mixture in the collapsed position when the cannula is rotated. The needle assembly also has an inner member disposed within the inner lumen of the cannula, which is movable between an extended position and a retracted position. The inner member is operable to hold the tip in the expanded position when the inner member is in the extended position. The tip automatically collapses into the collapsed position when the inner member is in the retracted position. A method of introducing a bone cement mixture into a damaged bone of a patient is also disclosed.

Abstract: A cavity creator including a tube with a lumen. A tip able to penetrate bone extends forward from the distal end of the tube. A blade is moveably attached to the outer tube proximal to the tip so as to move between retracted and extended states. The tube has a discharge port adjacent the blade. The tip is used to position the upper tube in bone. The blade is extended to form a cavity in the bone. Once the cavity is created filler material can be introduced into the cavity by flowing the material through the tube lumen.

Abstract: An apparatus and methods for delivery of reinforcing materials to a weakened or fractured bone is disclosed. An apparatus for delivering a reinforcing mixture to a bone including a tube having a proximal end, a distal end, and a longitudinal axis therebetween, wherein the tube has at least one inner lumen capable of allowing a bone reinforcing mixture to pass therethrough; a balloon engaging the tube wherein the balloon expands from a substantially deflated state to a substantially inflated state upon the bone reinforcing mixture entering the balloon; and at least one light guide extending through the tube into the balloon to guide a light into the balloon.

Abstract: Apparatus and method to dispense PMMA bone cement including a temperature controller, a disposable cement cartridge and a means for extracting cement from the cement cartridge so that the temperature of the extracted cement is first cooled to discourage polymerization and to prolong working time, then warmed so as to control viscosity of the cement flowing into the desired bone repair location. A first embodiment includes a spiral cement cartridge along with a means for extracting cement therefrom. A second embodiment includes a linear disposable cement cartridge along with a means for extracting cement therefrom. A color matching mechanism identifies cement temperature and viscosity during dispensation. A manual cement dispensing method includes a calibrated hand crank mechanism for causing calibrated delivery of cement. An automatic cement dispensing method uses a stepper motor and computer programmed means for causing calibrated flow of cement.

Abstract: A ribbon device and method of using the same can be employed to remove extravagated bone cement during an implantation procedure while minimizing or eliminating the use of mechanical force, heat, or potentially injurious chemical removal agents. The ribbon device bonds chemically or mechanically with substantially unset extravagated bone cement, resulting in removal of the extravagated bone cement when the device is used according to the methods of this invention. The ribbon device is formed of biocompatible mesh or weave materials tending to bond with bone cement chemically or mechanically, such as surgical grade fabrics, surgical tape, or mersilene tape. The ribbon device and method of its use can be used in conjunction with implants including tibial and femoral implants.

Abstract: An adapter attached between a syringe containing a bone void filler and the head of a cannulated surgical screw allows the cannulated surgical screw to be used as a port to inject bone void filler into a bone void during the course of a surgical repair which would use a surgical screw to attach broken or separated bone fragments providing a more secure bone anchor matrix within which the surgical screw is set.

Abstract: In at least one embodiment of the present invention, a method for stabilizing a collapsed vertebra of a patient is provided. The method comprises introducing a supercooled liquid into the collapsed vertebra. A solid structure that supports the collapsed vertebra is then form by solidifying the supercooled liquid.

Abstract: Systems and methods for repairing a weakened or fractured bone are disclosed herein. A system for repairing a fractured bone includes a delivery catheter having an elongated shaft with a proximal end, a distal end, and a longitudinal axis therebetween, wherein the delivery catheter has an inner void for passage of bone cement components and an inner lumen for passage of a mixing element; an internal bone fixation device releasably engaging the distal end of the delivery catheter, wherein the internal bone fixation device moves from a first compact state to a second expanded state when the bone cement components are delivered to the internal bone fixation device and mixed within the internal bone fixation device; and a spinning mechanism engaged to a rotatable shaft, wherein the spinning mechanism is inserted into the internal bone fixation device.

Abstract: The invention features delayed-setting calcium phosphate pastes which are useful for the preparation of delivery vehicles for biologically active agents, useful for the treatment of orthopedic conditions and can be stored for long periods without prematurely setting.

Abstract: A method of compacting and stabilizing bone in the spine using an expandable screw and bone filler. The method comprises placing an expanding screw through a pedicle and into a fractured vertebral body using established techniques; and expanding the screw within the body to create a cavity by tamping bone around the expanded aspect of the screw. The screw can be expanded and rotated about its rotational-axis to tamp bone adjacent to the expanded screw. After compaction, the screw is removed and the cavity is filled with bone filler.

Abstract: An apparatus and method is provided for distracting, in a given direction, and supporting opposing vertebral bodies. A plurality of wafers are consecutively inserted between the vertebral bodies to create a column of wafers. The column of wafers is oriented between the vertebral bodies so as to expand in the given direction as the wafers are consecutively added to the column.

Abstract: Instruments and methods for reducing and stabilizing bone fractures are presented, which include providing a cavity in bone, such as a vertebra, wherein the cavity is proximate to an endplate of the bone, such as a vertebra endplate, and wherein the cavity extends radially from a passage into the cavity. The methods further include providing a device, such as an inflatable device, into the cavity and manipulating the device for expansion. The cavity may be expanded thereby reducing and/or stabilizing the fracture. The fracture may be further reduced and/or stabilized by adding a material into the cavity.

Abstract: Methods and devices are shown for forming polymer fasteners into bone by expelling the polymer from a cannula. Devices and methods shown allow a user to form multiple fasteners of various sizes without re-loading a device. Devices and methods shown further provide temperature profiles during fastener formation that reduce or eliminate thermal necrosis. Devices and methods shown further provide fasteners with increased strength.

Abstract: A system and method is provided for distracting opposite surfaces from the interior of a bone, such as a vertebral body. A working channel cannula provides a working channel through which an inserter and an injection cannula can simultaneously pass. The inserter transports a plurality of wafers into the interior of the bone to form a load-bearing stack bearing against the opposite surfaces. The injection cannula is used to inject a fluent material into and/or around the stack. In certain embodiments, the fluent material is a load-bearing or hardenable material, such as bone cement. In other embodiments, the fluent material can be a BMP, HAP, or other osteo-inductive, osteo-conductive, or pharmaceutical compositions. A syringe containing the fluent material is engaged to the injection cannula and is operable to inject the fluent material into the vertebral body under controlled pressure.

Abstract: The present invention relates in certain embodiments to medical devices for treating osteoplasty procedures such as vertebral compression fractures. More particularly, embodiments of the invention relate to instruments and methods for controllably restoring vertebral body height by controlling the geometry of fill material introduced into cancellous bone. A method of treating bone includes injecting a volume of fill material into a bone and selectively modifying a viscosity of the bone filler to control the direction of flow of the fill material within the bone. A system for treating bone using this method includes an introducer for delivering fill material into the bone and an energy source selectively coupleable to the fill material to alter the viscosity of the fill material via an energy emitter in the introducer.

Abstract: Provided is a bone expanding framework, which can secures complete reduction of a compressed bone tissue because elevation of the bone tissue is increased by a desired height without shrinking through a characteristic in which a spiral leaf spring having superelasticity is self-set in bone. The framework for expanding bone includes a spiral leaf spring which has a shape in which a thin and long band formed of a metal material is wound in a spiral shape, is inserted into the bone, and is expanded in the bone to reduce elevation of the compressed bone.

Abstract: A shroud for protecting exterior portions of a prosthetic implant from adhesive and a method for utilizing the same. The shroud includes a hub, a rim, and at least one bridge connecting the hub and the rim. The rim of the shroud may be sized to cover substantially the same area as a guard of the implant. The method of utilizing the shroud involves securing the shroud to the prosthetic implant. A quantity of adhesive is provided, and the prosthetic implant is secured thereto. The shroud is removed from the implant, and then the guard is secured to the implant.

Abstract: The risk of bone cement extravasation can be reduced by delivering a calcium-dependent polymerizing sealant into a bone structure prior to delivery of bone cement into that structure. The polymerization of the sealant in response to the calcium within the bone structure can fill cracks and any other potential cement leakage paths, thereby minimizing the potential for subsequent extravasation. The benefits of the use of a calcium-dependent polymerizing sealant can be provided in any procedure involving the use of bone cement, such as spinal fixation, vertebroplasty, and kyphoplasty, among others.

Abstract: Methods of employing bone defect filling, e.g., orthopedic cements, such as calcium phosphate cements and the like, are provided. A feature of the subject methods is that vibration is employed in conjunction with the use of the cement, e.g., in preparation of the cement, in preparation of the target site, in delivery of the cement to the target site, and/or following delivery of the cement to the target site. Also provided are devices, systems and kits that find use in practicing the subject methods. The subject methods, devices and systems find use in a variety of different applications.

Abstract: A method and apparatus for forming any size antibiotic impregnated bone cement (AIBC) intramedullary (IM) nail. The cement nail is formed around a base support or base IM nail. The apparatus includes a molded plastic tube device to form the shape of the cement IM nail and plastic spacers which position the molded plastic tube around the base support or base IM nail for formation of the cement nail. The AIBC material may be any of the known antibiotic loaded/impregnated bone cements that are commercially available.

Abstract: Erosion of elements of the ossicular chain in the middle ear, resulting in hearing loss, is corrected by creating a bridge between the eroded elements, employing a bone cement formed as a paste of a self-hardening material, which is molded into an elongated bridge, secured at its two ends to the elements to be bridged, and supported in its desired position until the material hardens to fuse the two elements, at which time the support may be removed. The material is preferably hydroxyapatite cement, bone source, or similar material, such as ionomeric bone cement. These materials strongly bond to the contacting bone and may stimulate bone in-growth, resulting in osseointegration. The apparatus for forming the bridge consists of a reservoir of the bridge material in liquid or paste form connected to a bridge supporting tube. The tube is preferably placed in contact with the two elements to be bridged and the bone source material is injected into the tube using a manually actuated plunger until the tube is full.

Abstract: The present invention relates to a method of implanting a prosthesis device in a subject. In one embodiment, this method involves: (i) providing a prosthesis device to be attached to an exposed surface of a bone; (ii) applying bone cement in liquid form to a bone-implant interface region, the bone-implant interface region including a region between the exposed surface of the bone and an outer surface of the prosthesis device; and (iii) contacting the bone cement to the exposed surface of the bone and the outer surface of the prosthesis device under conditions effective to allow the bone cement to cure, thereby resulting in attachment of the prosthesis device to the bone of the subject. The present invention also relates to a kit for implanting a prosthesis device in a subject, a method of performing a joint arthroplasty surgical procedure, and other methods of using bone cement in liquid form.

Abstract: This relates to an improved delivery system for accurately loading and controlling the delivery of flowable material to a patient. Particularly, the system may be used in the injection of hard tissue implant materials such as PMMA under pressures up to about 4000 psi. The system includes an applicator with a first column having an implant material introduction section adapted to provide for effective loading of the implant material and a second column housing a piston. The introduction section has a larger size than that of a vessel section or bore in which the requisite pressure seal between the piston and bore wall is formed. The first column may include an introduction section flared open to an included larger funnel-like opening or a separate funnel may be used that interfaces with the introduction section to facilitate the introduction of implant material.

Abstract: Methods of employing bone defect filling, e.g., orthopedic cements, in conjunction with hard tissue securing devices, e.g., screws, plates or rods, are provided. A feature of the subject methods is that the cement is introduced to a target bone site through a passageway of the securing device while a vibratory force is applied to the securing device. Also provided are systems and kits that find use in practicing the subject methods. The subject methods, devices and systems find use in a variety of different applications.

Abstract: A radiation shield for use in treating spinal metastatic disease is formed of a body that is configured to surround the spinal cord and includes a section that can be axially extended for increasing the surface area of the shield.

Abstract: The present invention relates to the use of composite materials that can be prepared and delivered to an implant area. The composite materials may be a biocomposite remodeling bone cement (BRBC) material. In one embodiment of the invention the material is introduced in an interface between a first bone and a first joint implant component.

Abstract: Described are syringe devices useful in delivering medical materials internally in patients. In certain embodiments, syringe devices of the invention include widening internal lumen to assist in effective delivery of substances therethrough, and/or curved barrel portions with curved lumens therein. In still further disclosed embodiments, syringe devices include adaptations to prevent compression of compressible carrier materials to be delivered therethrough, curved barrel portions with flexible plunger arms to track the curved barrel portions, barrel portions controllable in shape such as to introduce and remove curves, visible markings to denote the direction of deflection of syringe barrel portions, imagable markers located on the syringe barrel, a cooperating plunger apparatus, or both; or combinations of some or all of these features.

Abstract: A spinal platform for delivering a therapeutic agent to a spinal cord target includes a brace member for attachment to a plurality of bony structures surrounding at least a portion of the spinal cord target, a carriage member operatively coupled to and movable along the brace member, and a universal joint for adjusting the coronal or sagittal angle of a surgical instrument. The brace member includes at least one longitudinal rail and a plurality of attachment members. The plurality of attachment members is operably connected to the at least one longitudinal rail. The at least one longitudinal rail includes a proximal end portion, a distal end portion, and a middle portion extending between the proximal and distal end portions. The universal joint is seated within a portion of the carriage member and is capable of delivering the therapeutic agent to the spinal cord target.

Abstract: The invention relates to a dental implant method using a tissue punch, sinus lift drill (SLD) and hydraulic effect which places a bone graft and embed a dental implant at a maxillary sinus area where there is not sufficient bone to embed a dental implant. The method includes: removing mucoperiosteum having a size of an dental implant to be implanted at a alveolar crest area using a tissue punch, drilling a maxillary sinus floor easily and safely without causing damage to a maxillary sinus membrane by inserting a sinus lift drill which has a protruding member that is elastically installed in the body so as to protrude from and retract in the longitudinal direction of the body, elevating a maxillary sinus membrane by the hydraulic effect (injecting graft adjacent to the drilled maxillary sinus floor); and installing the implant in an inside portion of the drilled maxillary sinus floor.

Abstract: The disclosure relates in certain embodiments to medical devices, systems and methods for use in osteoplasty procedures, such as vertebral compression fractures. One system for delivering a bone fill material to a bone includes an elongated introducer configured for insertion into a bone and having a channel sized to allow a flow of bone fill material therethrough. The introducer has at least one outlet opening in communication with the channel for delivering the bone fill material into the bone. A thermal energy emitter is coupled to the introducer and configured to apply thermal energy to the bone fill material flowing through the introducer. A sensor and a computer controller can be used to measure data about the bone fill material and to control certain aspects of the flow of bone fill material. A hydraulic pressure source may also be operatively coupled to the introducer and configured to apply a force on the bone fill material to provide a pressurized flow of bone fill material through the introducer.

Abstract: A remotely-activated injection device for use in vertebroplasty is provided to inject a flourescent probe material into a patient. The injection device includes a pump defining an injection chamber having an exit opening; an actuator; and a cable having a first end coupled to the actuator, and a second end remotely engaging the pump. The actuator remotely controls the pump by responsive movement of the cable to thereby cause injection of a flourescent probe material from the injection chamber of the pump through the exit opening to the patient.

Abstract: Systems and methods for repairing a weakened or fractured bone are disclosed herein. A system for repairing a fractured bone includes a delivery catheter having an elongated shaft with a proximal end, a distal end, and a longitudinal axis therebetween, wherein the delivery catheter has an inner void for passage of bone cement components and an inner lumen for passage of a mixing element; an internal bone fixation device releasably engaging the distal end of the delivery catheter, wherein the internal bone fixation device moves from a first compact state to a second expanded state when the bone cement components are delivered to the internal bone fixation device and mixed within the internal bone fixation device; and a spinning mechanism engaged to a rotatable shaft, wherein the spinning mechanism is inserted into the internal bone fixation device.

Abstract: An ultrasonic handpiece is used for augmenting a bone screw. The handpieces comprise a housing, an ultrasonic converter shiftingly accommodated in the housing, and a sonotrode connected to one end of the ultrasonic converter. The sonotrode protrudes from the housing into the bone screw. An adjuster wheel is adjustably connected to the housing. A spring is provided the ends of which are supported by the ultrasonic converter and the adjuster wheel, respectively. The ultrasonic converter is preloaded in the direction away from the adjuster wheel by setting the adjuster wheel, thereby the sonotrode is urged with a selected force into the bone screw and the augmentation material is pressed out of the bone screw and into the bone.

Abstract: Instrumentation and method for treatment of a spinal structure, comprising an elongate member including a deformable distal portion having an initial configuration for placement within the spinal structure and a deformed configuration wherein the distal portion is outwardly deformed to form a cavity in the spinal structure. The elongate member comprises a cannula member and an actuator member that is removably positioned within the cannula member and configured to transition the deformable distal portion from the initial configuration toward the deformed configuration. The actuator member is selectively removed from the cannula member to provide a passageway for delivery of a material into the cavity formed in the spinal structure.

Abstract: An assembly for use in fixing a first component of a joint prosthesis, having a bone facing surface and an exposed surface which faces towards a second component of the joint prosthesis when implanted on a patient's bone. The assembly includes a skirt which can be temporarily fixed to the edge of the first component extending along the edge thereof from a remote portion of the edge to an accessible portion of the edge. The skirt provides at least one opening through which cement located between the bone facing surface of the component and the surface of the bone can be displaced. The opening is located so that cement is prevented from being displaced at the remote portion of the edge and is instead displaced at the accessible portion of the edge.

Abstract: A method and apparatus for fluidly coupling a reservoir containing a flowable biomaterial to a mold. The flow of the flowable biomaterial into the mold is controlled in accordance with a first operating parameter. At least one injection condition is monitored. The flow of the flowable biomaterial is controlled in accordance with a second operating parameter in response to one or more of the injection conditions reaching a threshold level. The second operating parameter is maintained during at least a portion of the curing of the flowable biomaterial. In some embodiments, the second operating parameter may optionally include permitting a portion of the flowable biomaterial to be expelled from the mold.

Abstract: Featured are a biocompatible, injectable, self-setting, cohesive, bone-bonding and remodeling calcium phosphate composite material and its use in methods of repairing defective bone, e.g., in vertebroplasty augmentation and kyphoplasty.

Abstract: The invention concerns a process for treating of vertebral body (70) with the following steps: introducing a trocar (10) and a trocar jacket (20) into the vertebral body (70), removing the trocar (10), introducing the same or a different biopsy and cement cannula (30) into the trocar jacket, extracting spongiosa (74) into the biopsy and cement cannula (30), removing the biopsy and cement cannula (30), filling a biopsy and cement cannula (30) with contact cement (60), introducing the biopsy and cement cannula (30) into the trocar jacket (20), and introducing the bone cement (60) into the vertebral body (70) by introduction of a plunger (40) into the biopsy and cement cannula (30). The invention further concerns a device for treatment of vertebral bodies with at least one biopsy and cement cannula, which are fillable with bone cement, and a plunger, which is introducible into the biopsy and cement cannula, in order to expel bone cement out of the biopsy and cement cannula.

Abstract: Methods and apparatus for treating bones, including, in one or more embodiments, methods and apparatus for treatment of vertebral fractures that include a containment assembly for cement containment and/or a balloon assembly for maintaining vertebral height. Methods for treating a bone comprising: inserting a cannula having an inner lumen into the bone; creating a cavity in the bone; providing a containment assembly, wherein the containment assembly may comprise a tubular member having a proximal end and a distal end, a containment jacket disposed on the distal end of the tubular member, and a guide wire disposed through the tubular member with the guide extending into the containment jacket; inserting the containment jacket through the inner lumen of the cannula and into the cavity; and introducing a filler material into the containment jacket.

Abstract: The invention relates to dry free-flowing filler (1) for forming a support structure in a bone cavity (4a). The filler comprises a plurality of biocompatible support bodies (2) which are resistant to deformation or fracture under the physiological loads usually occurring in the bone cavity (4a). Said support bodies (2) have a size between 2 mm and 10 mm and an axis (A) and two opposite abutments (2g) spaced apart in the direction of the axis (A).

Abstract: The present invention relates to an expandable metal balloon that may be used for the treatment of diseased or injured bone tissues, and a method of using the same. The metal balloon is inserted into the interior space of a cancellous bone tissue, and is filled with a suitable material to provide internal structural support to the bone.

Abstract: The present invention discloses a technique for forming a hardened orthopaedic paste in a bone cavity, which involves a forced-feeding balloon rupture mechanism. This mechanism includes continuously or intermittently injecting a liquid or gas into a perforated balloon containing a hardened orthopaedic paste therein in a bone cavity until the perforated balloon is dilated to exceed a critical size, and thus ruptures.