Fig. 5 Fig. 6 Fig. 8a a b c d ~1 mm Fig. 7 Fig. 8b ~1 mm Figure 5a) Available keepers implanted...
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Fig. 5 Fig. 6 Fig. 8a a b c d ~1 mm Fig. 7 Fig. 8b ~1 mm Figure 5a) Available keepers implanted directly into the mastoid bone. 5b) Single inner-thread
Fig. 5 Fig. 6 Fig. 8a a b c d ~1 mm Fig. 7 Fig. 8b ~1 mm Figure
5a) Available keepers implanted directly into the mastoid bone. 5b)
Single inner-thread female post. 5c) Single turn screw cap
replacing one magnetic cap. 5d) Two magnetic posts included to
supplement new post design. 6) Fixation mechanism with ear
detached. 7) Fixation mechanism with ear attached. Center post
allows for axial translation. 8) Detailed view of novel fixation
components. Introduction limits for the magnetic attachment system
(Fig 4b). This test failed to provide reliable results because the
recording software begins after the bending arm applies a minimum
force; the prosthesis would detach consistently before the
initiating five N load was supplied. Qualitative testing was done
by the client to determine the bond strength with stainless steel
in the prosthetic ear. Testing was done with a primer from Nusil
Technologies. Bond strength was compared to steel placed in silica
with and without the primer. Once the steel was placed in the
silica, the samples were heated for one hour and allowed to cool.
The bond was found to be of sufficient strength, showing failure in
the bulk silica and not at the bond interface. These initial
designs falter with their inclusion of moving parts. Figure 3
displays various designs which use a clipping mechanism to attach
the prosthesis. Figures 3b and 3c show a flexible, clipping post,
which latches around a disk descending from the prosthesis. The
small scale of these moving parts provoked significant concerns
with fragility. After considering the excessive wear resulting from
day to day use, a system with static parts became a major design
requirement. The current attachment systems fasten three titanium
keepers directly into the mastoid bone. A post is screwed into each
of the keepers (Fig. 1a). Figure 1a uses three magnetic posts to
fasten the prosthetic ear. For a clipping attachment, a bar is
placed over three posts that align with clips in the prosthesis
(Fig. 1b). Prosthetic Ear Fixation Mechanism Joseph Hippensteel,
Steve Noel, Ashley Phillips, Evan Rogers, and Cullen Rotroff
Department of Biomedical Engineering, University of Wisconsin
Client: Greg G. Gion, BA, BS, MMS, CCA Advisor: Willis Tompkins,
Ph.D, Dept. of Biomedical Engineering Future Work Any mechanism
that can endure more than 5 N of normal or shear force without
detachment is an improvement; however, product life and user safety
require further consideration and long term testing. The addition
of the threaded safety lock introduces new, small parts. It will be
vital to determine the life span of the abutment's inner thread. To
determine this, various simulations of unexpected normal and shear
forces on the prosthesis will be conducted in addition to
continuous attachment and removal simulations. The device should
operate properly for a minimum of ~1,000 attachment cycles without
destructive wear. By increasing the amount of force the prosthesis
can endure without detaching, the device effectively increases
force transferred to the user's mastoid. With the fixture anchored
in the mastoid, it is imperative that a minimum injury force is
identified. If the prosthesis withstands and transfers the force
from subsequent impacts to the patient in such a way that causes
injury, the design must be modified to include a breakaway force.
The breakaway force would be a prosthesis detachment mechanism that
engages well before a damaging force could be transferred to the
mastoid bone. Abstract There are currently two mechanisms in use to
attach prosthetic ears. A magnetic mechanism provides an easy, yet
precarious, attachment. A clipping mechanism affords a stable
attachment but requires excessive force, inducing eventual wear on
the prosthesis. A new fixation mechanism was desired that provides
secure fixation with minimal attachment force. A new method was
developed that combined two magnetic place holders with a threaded
locking-post. Figure 1a) Illustration of keeper and post. 1b)
Clipping mechanism Fig. 1a Fig. 1b Problem Statement A new
prosthetic ear attachment system is desired. It should prevent
accidental detachment, while limiting the force required for
attachment and removal; the improved attachment method should
minimize abrasion to the coating of the ear. The system should be
indiscrete and accessible for cleaning and maintenance when the
prosthesis is detached. The posts must be retrofitted to fit the
current keeper design. Lastly, the system should be easily
manufactured, facilitating a manageable incorporation into
mass-production. Figure 2) Patient with a prosthetic ear. Figure
3a) This initial design attached the prosthesis to the post with a
spring loaded clip. 3b) Normal force opening clip mechanism. 3c)
Clip in locked position. Preliminary Designs The magnetic system
minimizes prosthesis abrasion with a nominal attachment force, but
its fixation is unstable. Conversely, the clipping mechanism
provides a secure fixation but requires a large attachment and
detachment force, causing prosthesis wear and discoloration. Most
patients needing a prosthetic ear have suffered from trauma or were
born with microtia, a congenital deformity of the outer ear. An
improved quality of life can be obtained with the use of a
prosthetic ear made to mirror the contralateral ear (Fig. 2).
Fronczak, Frank. Personal interview. 1 Mar. 2007. Gion, Greg G.
"Facial and Somato Prosthetics." 3 Mar. 2007. Vistafix Product
Catalog. Colorado: Cochlear Americas. References/Acknowledgements
Testing A strain gauge was used to determine the shear and normal
force limits of the magnetic attachment (Fig 4a). Results in the
shear and normal direction ranged from one to two pounds. The
strain gage only measured one significant figure, yielding
inaccurate results. Consequently, a three point bending machine was
used to verify shear Final Design Our final design utilizes a
modest alteration to the current magnetic attachment system; the
two outer posts shown in figure 5d will remain unchanged magnetic
attachments. The center post, however, is notably altered in design
and function, now serving as a secondary lock; this post provides
little force in normal use but prevents unintended removal of the
prosthesis when exposed to normal or shear forces. Center Post
Design The Center Post was retrofitted to fit with current keeper
design. The design uses an interlocking thread to secure the
prosthesis to the patient. The center post has relatively larger
outer diameter protruding from the mastoid of the subject (Fig 5b).
The protrusion is a hollowed cylinder, with 1 mm thick walls. The
distal face the cylinder wall has a slightly narrower inner
diameter. This inner diameter of the distal face contains a single
thread to permit screw cap entry and exit into the post (Fig 8a).
Screw Cap Design The screw cap design is to attach solely with the
center post. The screw cap is a solid cylinder emerging from the
inner face of the prosthetic ear (Fig. 5c). The cap is tapered at
the bottom two mm of the cylinder. Just above the taper origin, the
cylinder has a single revolution of threading (Fig. 8b). The thread
is positioned such that the screw cap may slide up and down within
the post cylinder once medial to its thread (Fig. 7). To attach the
prosthesis, the user must insert the screw cap into the center post
and then rotate the entire prosthesis one revolution (Fig 9). The
subject can then adjust the prosthesis orientation to align and
attach the two outer magnetic caps with their respective magnetic
posts. The outer caps adhere the prosthesis to the subjects skin
surface, while the inner cap blocks prosthesis detachment. To
remove the prosthesis, the user must simultaneously apply a tensile
normal force in conjunction with a rotational force to bypass the
threading. Fig. 9 Figure 9: Attachment and removal of the
prosthetic ear is achieved through a 360 rotation. The ear is
manipulated such that the magnets maintain tight fixation.
Fabrication Process The process used to fabricate the prototype may
serve as an ideal method for mass production of the new post and
screw cap designs. The keeper will go unchanged in the new system
and requires no production considerations. The threaded post,
however, creates the difficult manufacturing problem of placing a
functional inner thread in a two mm diameter cylinder. To
accomplish this, the outer diameter of the post is cut and its
outer threads are formed. After the cylinders 2.5 mm inner diameter
is bored from the distal surface, the outermost edge of the distal
surface is rolled inward. The elastic modulus of standard steel
allows its thin walls to be curled inward, producing a narrower
diameter at the surface. Finally, the single thread is cut into the
narrowed of the inner cylinder. Definitions Keeper : Initial
attachment piece that is permanently screwed into the patients
mastoid bone. Post : Protruding intermediate piece between the
keeper and screw/cap. Screw/Magnetic cap : Any piece that descends
from the prosthesis and attaches to the post. Figure 4a) Prosthetic
ears and strain gage Figure 4b) Three point bending machine Fig. 3b
Fig. 3c Fig. 3a Fig. 4b Fig. 4a