An Improved Puncture Access Tip-Retraction Mechanism (2013)

At the instant an instrument punctures tissue, delay in the physician's reaction causes a short period of unbalanced force and subsequent device acceleration further into the patient. This “over-puncture” may lead to potentially catastrophic adjacent tissue damage. Surgical puncture devices requiring a higher insertion force experience a greater acceleration at puncture and result in up to twice as many patient injuries [1]. A novel force-controlled tip retraction mechanism has been previously proposed. This mechanism employs a sharp tip to minimize insertion force and subsequent acceleration upon puncture. In addition, the mechanism reacts to the sudden drop in resistive force upon puncture and retracts the device tip, actively opposing its forward acceleration.The original mechanism demonstrated potential to decrease the risk of over-puncture in a variety of access procedures, and future development steps were determined. The flexural mechanism required further conceptualization and iteration as well as design for manufacture. In addition, with the original mechanism, the user has to keep applying force to expose the blade until the blade is pressed against tissue. The functionality of the original mechanism depends on the user knowing when to stop applying force, which is something that has to be practiced or taught.The proposed improved mechanism adds “pre-loading” capability to the original technology, such that the physician does not have to know when or how to apply or relieve force during tissue puncture to ensure proper device function. In both the original and proposed mechanisms, a direction-biased two-force input flexural “double-knuckle linkage” converts axial force applied to the instrument tip into radial force applied to a static wall or ground in the device casing. Figure 1 shows two configurations of the proposed mechanism with the linkage shaded. Note that an axial force applied to the tip of the device is transferred through the lower arms of the mechanism and applied to the device wall in a substantially radial direction. This radial force creates a frictional lock between the mechanism and the wall, keeping the instrument tip exposed as long as it continues to press against tissue. When the device tip punctures tissue, the force applied to the tip becomes zero and the frictional lock disengages, allowing a spring to retract the mechanism and actively oppose the forward acceleration of the tip.In the proposed improved device, the walls contain a securing feature and are compliant in the radial direction with respect to the device main (puncture) axis. When the mechanism is initially advanced to expose the device tip, it will engage the securing feature on the walls. The securing feature may be a snap, latch, or friction-inducing surface, such that once the mechanism engages the securing feature, the user can release the mechanism and it will remain in the advanced position with the tip exposed. Figure 1 shows a possible mechanism embodiment in the initial state as well as the pre-loaded state with the securing features engaged. This embodiment employs cantilever beam walls with a snap securing feature. Depending on the specific embodiment of the feature, the walls may remain deflected when engaged with the mechanism, as in the case of a frictional clamp.Once the device is pre-loaded, the physician presses it against tissue to begin puncturing. As the insertion force increases, the mechanism will apply radial force to the walls, causing them to deflect outwards. At a certain level of insertion force before the tip has punctured tissue, the securing feature will disengage the mechanism. However, the mechanism does not retract because force is still applied to the device tip, maintaining the frictional lock with the wall. When the tip punctures tissue and the frictional lock is released, the mechanism will retract since it is no longer engaged with the securing feature. If for some reason the user releases insertion force before the device has fully punctured tissue, the mechanism may retract the device tip. In this case, the user can re-advance the tip to the pre-loaded position and continue puncturing tissue.At any given time, the only forces that may be acting on the mechanism in the direction of puncture (axial direction) are Fs, the retracting spring force, acting away from tissue; Ft, the tissue resistance force applied to the tip, acting away from tissue, and equal and opposite to the user-applied “puncture force”; Ff, the locking friction force applied to the mechanism by the device walls, acting towards tissue; and Fhold, the force applied to the mechanism by the securing feature to retain it in the pre-loaded position, acting towards tissue. During puncture, the blade will remain exposed as long as(1)Fs+Ft&lt;Ff+FholdIt is important to note that Ff and Fhold are both functions of Fs and Ft, and that the specific relations are determined by the geometry of the mechanism. In general, as Ft increases, Ff will increase and Fhold will decrease.In order for the mechanism to function correctly, it is critical that the securing feature always disengages the mechanism before puncture; if not the mechanism cannot retract the tip upon puncture. To ensure this order of events, it is necessary to know Ftpm, the minimum value of Ft at which the device tip will puncture tissue. This value can be determined experimentally and an appropriate safety factor applied. Given this minimum value and the relationship between Ft and Fhold, the compliance and sizing of various components can be chosen to ensure that the pre-load will always disengage at a value of Ft less than Ftpm.In addition to disengaging the mechanism prior to puncture, it is critical that the securing feature does not snap back upon puncture and re-engage the mechanism before it can retract. Several methods are proposed to avert this risk. If some part of the compliant walls is made viscoelastic, the walls will be slow to return to their original position and will not be able to re-engage the mechanism before it has retracted. In addition, an element of axial compliance may be introduced in the mechanism between the mechanism's securing feature and the remaining distal portion of the mechanism. When the pre-load disengages during puncture, this compliance allows the retraction spring to retract the securing feature to a position where it cannot be re-engaged, while the rest of the mechanism remains static until puncture. Finally, the mechanism components may be designed such that their dynamic behavior upon release makes it impossible for the securing feature to re-engage the mechanism.As the walls deflect outwards during puncture, the forces applied between the mechanism and wall at the friction contact points may change direction and magnitude. In order to maintain successful device function, this effect must be accounted for such that the mechanism does not retract prematurely. In the embodiment shown in Figure 1, the contact point is close to the fixed end of the cantilever walls, ensuring that angular and radial deflection are minimal. Additional countermeasures include rough surface treatments or an angled surface section on the wall at the contact points.In addition to the added pre-load functionality, the flexural linkage was redesigned for performance and manufacturability. In the original mechanism, the linkage was composed of pin-jointed non-compliant links. In the first iteration of the flexural mechanism, the shape of the non-compliant mechanism was kept relatively constant and the pin joints were replaced by corner-filleted flexure hinges. This flexure design offered very low stiffness for the relatively small angular deflections applied at the hinge points. However, the highly localized deformation caused stress concentrations at the hinge elements. As a result, the flexural hinges often failed in testing. In the proposed flexural mechanism, deformation is more generalized, minimizing stress concentrations in the structure. In addition, the two distal-most hinge elements, which carry significant compressive loads, are replaced by seated joints.Like the original mechanism, the proposed design employs a sharp, bladed instrument tip to reduce the required insertion force for the device and further reduce acceleration upon puncture.Due to the variety of potential applications and the many possible configurations of the proposed mechanism, a modular prototype allowing iterative design and testing was constructed. The prototype was assembled in layers of laser-cut acrylic parts: the base layer provides structure and pins for aligning the remaining layers; the bearing layer contains a linear bearing and fixation points for the mechanism and retraction spring; the mechanism layer contains the mechanism and walls; and the transparent top layer constrains the underlying components. By removing the top layer, various flexure and wall embodiments could be rapidly interchanged and tested. With this prototype, the pre-loading functionality and other previously described modifications were successfully demonstrated.In addition, a miniaturized prototype for use in hypodermic needle puncture procedures was constructed for proof of concept. This device, excluding the hypodermic needle, occupies a space of only 1" × 1" × 3/8", and demonstrates the significant scalability of the proposed design. Figure 2 shows the various prototypes constructed in the course of development.The proposed improved mechanism adds significant functionality, usability, and commercial feasibility to the original design. The technology remains widely scalable to a variety of medical and other applications. Once a specific application and embodiment have been chosen, detailed analysis of mechanism components is essential to ensuring proper function.Several prototypes of the mechanism for specific medical applications, including an epidural anesthesia needle as well as a bone drilling tool, are currently in development.Patents are pending on this device.