Mechanically compatible fasteners for use with thin or weakened bone sections in the human mandible are being developed to help reduce large strain discontinuities across the bone/implant interface. Materials being considered for these fasteners are a polyetherertherketone (PEEK) resin with continuous quartz or carbon fiber for the screw. The screws were designed to have a shear strength equivalent to that of compact/trabecular bone and to be used with a conventional nut, nut plate, or an expandable shank/blind nut made of a ceramic filled polymer. Physical and finite element models of the mandible were developed in order to help select the best material fastener design. The models replicate the softer inner core of trabecular bone and the hard outer shell of compact bone. The inner core of the physical model consisted of an expanding foam and the hard outer shell consisted of ceramic particles in an epoxy matrix. This model has some of the cutting and drilling attributes of bone and may be appropriate as an educational tool for surgeons and medical students. The finite element model was exercised to establish boundary conditions consistent with the stress profiles associated with mandible bite forces and muscle loads. Work is continuing to compare stress/strain profiles of a reconstructed mandible with the results from the finite element model. When optimized, these design and fastening techniques may be applicable, not only to other skeletal structures, but to any composite structure

Biermann, Paul J.

Ecker, John A.

Roberts, Jack C.

English

NASA Technical Reports Server

TWE DESIGN OF MECHANICALLY COMPATIBLE FASTENERS 
FOR HUMAN MANDIBLE RECONSTRUCTION 
\ 
I 
Jack C. Roberts, Senior Professional Staff 
John A. Ecker, Senior Professional Staff 
Paul J. Biermann, Senior Professional Staff 
The Johns Hopkins University 
Applied Physics Laboratory 
Laurel, Maryland 
ABSTRACT 
Mechanncally compatible fasteners for use with thin or weakened bone sections in the human mandible are being 
developed lo help reduce large strain discontinuities across the bonelimplant interface. Materials being considered 
for these fasteners are a Polyetherertherketone (PEEK) resin with continuous quartz or carbon fiber for the screw. 
'Die screws were designed to have a shear strength equivalent to that of compact/trabecular bone and to be used 
wlUn a conventional nut, nut plate, or an expandable shank/blind nut made of a ceramic filled polymer. Physical 
and finite element models of the mandible were developed in order to help select the best material fastener 
des~gn. The models replicate the softer inner core of trabecular bone and the hard outer shell of compact bone. 
li'he inner core of the physical model consisted of an expanding foam and the hard outer shell consisted of 
ceramic particles in an epoxy matrix. This model has some of the cutting and drilling attributes of bone and may 
be appropriate as an educational tool for surgeons and medical students. The finite element model was exercised 
to establish boundary conditions consistent with the stress profiles associated with mandible bite forces and muscle 
loads. NJork ns continuing to compare stresslstrain profiles of a reconstructed mandible with the results from the 
finite element model. When optimized, these design and fastening techniques may be applicable, not only to other 
skeletal ststictures, but to any composite structure. 
WTRODUCTION 
During malidible reconstructive surgery many problems may be encountered when attaching thin or weakened 
(diseased) sections of bone to one another, or to a replacement material. Some of these problems may stem from 
differences in the mechanical properties of bone and the implant material, or from the fastening method. The 
use of bonding techniques may result in premature failure of the interface due to the reduced or weakened bone 
section at the interface. The use of mechanical fastening techniques could result in shearout of the thinlweak 
bone due to the higher stiffness of conventional fastener materials, as well as large strain discontinuities across 
the interfacial boundary between different materials. 
Atrophy, due to stress shielding of underlying bone in metal fixation devices, is said to be the most important 
reason for the removal of rigid metallic plates and screws. Several investigators have proposed the use of 
bodegradable materials with the same properties as bone to be used in place of metal fixation devices (1-3). 
These broclegradable materials could be used in situations where the fixation device is no longer needed after the 
fracture has healed. To create a better interfacial bond between implants and bone other investigators have 
explored the use of porous metallic implant materials (4), and to eliminate corrosion porous polysulfone ( 5 )  and 
porous hyddrxyapatite (6,7). Hydroxyapatite was thought to create a better bond with bone because it is one 
of the consiiluents of bone. However, in situations where the implant or reconstructed mandible must remain in 
glace w ~ t h  the fixation device, it is desirable to have a fixation device that has the same properties as bone and 
will have adequate strength at the bonelimplant interface. 
'Ilerefore, the purpose of this study was to design non-metallic fasteners having properties similar to bone for 
a reconamcted mandible or mandibular implant. The fastening method should "design in" the ability of the 
fastener to flex with bone, thus preventing bone atrophy, while providi~lg a continuous load path from bone to 
the replacement. The design of the fasteners will be guided by both a physical model and a finite element 
mode1 of the mandible. 
https://ntrs.nasa.gov/search.jsp?R=19930016380 2020-03-24T07:26:22+00:00Z
MATERIALS AND FASTENER DESIGN 
Materials 
Bone has a hard outer surface (compact bone) and a soft, porous inner core (trabecular bone). Il?-ins makes ~c 
similar to sandwiched composite structures used in the aerospace field. These structures consist of an h~ghlgi 
porous inner core (usually a honeycomb material to carry shear loads) sandwiched between alumrnurn 
glasslepoxy or graphitelepoxy face sheets (to carry the tensile or compressive loads). A comparison of the two 
assembled structures is shown in Figure-1. Because of the similarity in function, the implant designs wflE. use 
fasteners similar to those used with sandwich composites in the aerospace field. In order to size h e  screws en 
any of the following designs, bone properties in shear (8) were used to calculate the necessary screw drarneter 
In order to minimize strain discontinuity between bone and fastener, the fastener will be designed to have elastrc 
properties equivalent to that of bone. Since the fasteners will have a smaller cross sectional area than that of 
bone, the shear strength will need to be higher than that of bone. The screw materials that most closely met 
these criteria were Polyetheretherketone (PEEK) with either continuous carbon or continuous quarts fibersTTh~s 
design methodology may be a better way to select fasteners for any composite structure, not just bone 
Fastener Design 
For ease of assembly, the fastener design selected should allow the surgeon to insert the fastener from the buccal 
(outside) surface of the mandible. To reduce trauma to the patient and prevent possible infection, the fastener 
should be flush with the surface of the replacement mandible or mandibular bone. It may also be advaniageorns 
to have the fastener put the bone into compression to prevent atrophy. All of these design "criteria" need to be 
considered when selecting the optimum fastener design. 
Three fastener designs are shown in Figure-2. Each design relies on a screw and a nut or nut plate lo retann the 
mandibular replacement. The design in Figure-2A, shows a screw assembled from the lingual (~nslde) of the 
mandible into a blind hole in the replacement mandible. The underside of the head of the screw 1s contoured 
so that the assembly load is gradually spread over compact bone on the mandible's buccal (outside) surface The 
prosthesis or implant has internally threaded blind holes. This design requires the use of a screw drlver from 
the lingual side of the mandible. The design in Figure-2B, uses blind nuts installed from the lingual srde of the 
mandible. A screw is inserted through holes in the buccal side of the replacement mandible This des~gn 
requires the blind nuts to be countersunk from the lingual side. The third design (Figure-2C) agair featrrres a 
screw countersunk into the replacement mandible from the buccal side. This design is unique, In that, the nuts 
have left hand threads on the outside and right hand threads on the inside. Thus, during assembly, no 
counterboring is required on the lingual side. The nuts will self lock from the buccal side by the use of a slmpte 
tool. In all of these designs a jig could be used to drill holes into mandibular bone that correspond vvlte? those 
predrilled in the replacement mandibular or nuts. The shear and tensile strength of the screws in any of the 
aforementioned designs should exceed those of bone. The compressive modulus should match that of bone :o 
prevent strain discontinuities between the replacement and bone. 
An alternative design that relies on an interference fit between a screw and a self-clinching expansion nut ratha 
than tension as in the previous designs, is the shown in Figure-3. The self-clinching expansion nut wouEd be 
made of ceramic particulate filled polymer. The nut has scores or flutes on the outer surface to allow E C  to 
separate under load. An oversized screw is inserted into an undersized tapped hole in the nut. Tbe wail as sn~ed 
such that the interference fit of the screw and cylinder causes the cylinder walls to expand and split The wail 
segments are pushed outward and trapped between the fastener and compact/trabecular bone. The polymer used 
in this cylinder could be a thermoset or thermoplastic resin filled with enough ceramic to create a somewhat 
brittle material, but not brittle enough to crush under the screw compressive load. If a thermoplasl~c 1s rased, 
enough filler would have to be added to prevent creep under compressive load. The outside surface of the 
cylinder could be tapered in such a way as to optimize the pressure profile on the outer surface of the fastener 
The neck of the fastener could be sized to allow the driver head to be torqued off when the proper preload 
torque is reached. The entire installation requires only boring holes and no fastener is required on the X~ngual 
side of the mandible. The fastener design from Figure-2B or 2C is shown in Figure-4 as an example of a partpal 
replacement mandible. 
1 CPN800A-06-03 (PEEKLong Carbon), CPN800J-06-03 (PEEKLong Carbon), Cherry 'kx'extron. Santa 
Ana, CA. 
PWUSICAL AND FINITE ELEMENT MODELS 
A physical model of the mandible has been developed and fabricated to help with selection of the optimal 
fastening technique. The fabrication process involves a two step molding process. The first mold is used to 
fabricate the replacement for trabecular bone. Once solidified, this structure is positioned inside the second mold 
where a substitute for compact bone is formed over it. Once completed the external geometry of the model, 
formed by the mold, duplicates that of a human mandible. After evaluating many combinations, the materials 
that gave cutting and drilling properties similar to bone, were a foamable polymer for trabecular bone, and a 
ceramic I-3ied epoxy for compact bone. The replacement mandible is shown in Figure-5 and a section through 
the replacement mandible is compared to a section through a human mandible in Figure-6. This physical model 
was drnlled and cut using medical drills and saws,i.e., a Synthes drill2 and a standard Micro-E sagittal saw. The 
replacement mandible in a dry state, cut and burned like bone . However, while under irrigation the saw blade 
bound in the material like it would in human bone. The replacement mandible drilled and tapped similarly to 
bone. 
In addition to its use as a tool in the selection of the best fastening technique, the replacement mandible could 
also, and perhaps more importantly, be used as a training tool for surgeons and medical students. 
Finite Eleinent Model 
PDA-PATWAN3 was used to create a finite element model of 112 the mandible as shown in Figure-7. This model 
conslsls of 7560 nodal points and 6716 solid isoparametric hexagonal elements with symmetry boundary conditions 
applied at h e  mld-plane and fixed in the condylar region. A very refined mesh was used along the buccalllingual 
side In order to provide for easy modifications for incorporation of fastener devices. Both trabecular and compact 
bone properties were used in the model as taken from Ref. (9). Figure-8 shows stress contours in the mandible 
model for a static analysis incorporating a first molar point load of 250 N (1,100 lb) and appropriate muscle 
forces as taken from Ref. (10). MSC/NASTRAN4 and COSMOS/M5 will be used to exercise this model with 
different loading, boundary conditions and alternate material properties. The results will be compared to the 
pnys~cal model under similar loads. Once the finite element model has been verified, it can be used to determine 
the optamal fastening technique. 
SUMMARY AND CONCLUSIONS 
Several fastener designs have been proposed as fastening devices for a replacement mandible. The first design 
simply relies on a composite screw inserted into the human mandible on the lingual surface and threaded into 
a blmd hole In the replacement mandible. The second design consists of a composite screw countersunk into a 
rcplacemeni mandible on the buccal surface and threaded into a nut countersunk into the human mandible. The 
trnra des~gra relies on right hand threads on the inside and left hand threads on the outside of a nut on the lingual 
s~arhce of a human mandible to secure a composite screw whose head is countersunk and locked into the 
rcplxement mandible. The last design relies on the interference fit between an oversized composite screw and 
an undersized hole in a ceramic filled epoxy self-clinching expansion nut to force the ceramic to expand outward 
Into h e  human mandible. These designs must be analytically modeled and experimentally verified before a final 
dev~ce 1s selected. 
Synthes Ltd. USA, Wayne, PA. 
3 PDA-PATRAN finite element pre-and post-processor code, PDA Engineering, Costa Mesa ,CA. 
4 MSCINASTRAN finite element solver, MacNeal-Schwendler Corp., Los Angles, CA. 
COSMOSM finite element software, Structural Research and Analysis Corp., Santa Monica, CA. 
A physical model of the mandible was fabricated that included both trabecular and compact bone substitutes. 
An expanding foam was used to represent trabecular bone and a ceramic filled epoxy was used to simulate 
compact bone. When cut and drilled, this material acted similarly to bone. Besides being used to rest the 
fastener designs, this substitute mandible could be of benefit to surgeons and medical students training in 
orthopedic surgery. 
The mandible has been modeled using the finite element technique. Several different loading conditions will be 
applied to both the physical and finite element models and their results compared. After the finite element model 
is verified, it will be used to evaluate stress states in each of the fastening techniques. Once optimized for the 
specific design criteria, the selected fastening technique should not be limited to the mandible or other skeletal 
structures, but also to composite structures in general. 
REFERENCES 
Lewis, D.H., Dunn, R.L. and Casper, R.A., "Development Of Biodegradable Implants For Use In 
Maxiflofacial Snrgery", Southern Research Institute, SORI-EAS-82-507, June 1982. 
Bos, R.R.M, Rozema, F.R., Boering, G., Nijenhuis, A.J., Penning, A.J. and Jansen, H.W.B., "Bone- 
Plates And Screws Of Bioabsorable Poly (L-Lactide) - An Animal Pilot Study", British Journal of Oral 
and Maxillofacial Surgery, 27(6), 1898, 467-476. 
bay,  A.C., "Development Of A Moldable, Resorbable Appliance For Use In Maxillofacial Surgery", 
Southern Research Institute, SRI-APC-91-571, June 1991. 
Cook, S.D., Klawitter, J.J. and Weinstein, A.M., "Model For The Implant-Bone Interface Characteristics 
Of Porous Dental Implants", J. Dental Research, August, 1982, 1006-1009. 
Ballintyn, N.J. and Spector, M., "Porous Polysulfone As An Attachment Vehicle For Orthopedic And 
Dental Implants", Biomat., Med. Dev., Art. Org., Z(l), 1979, 23-29. 
Schliephake, W. and Neukarn, F.W., "Bone Replacement With Porous Hydroxyapatite Blocks and Titanium 
Screw Implants: An Experimental Study", J. Oral Maxillofacial Surgery, 49(2), 1991, 151-156. 
Denissen, H.W., Kalk, W., de Nieuport, H.M., Maltha, J.C. and van de Hooff, A., "Mandibular Bone 
Response To Plasma-Sprayed Coatings Of Hydroxyapatite", Int. J. Prosth., 2(l), 1990, 53-58. 
Reilly, D.T. and Burstein,A.H.,"The Mechanical Properties of Cortical Bone", J. Bone and Joint Surgery, 
56A(5), July, 1974, 1001-1022. 
- 
Wart, R.T., Hennebed, V.V., Thonogreda N., VanBuskirk, W.C. and Anderson, R.C., "Modeling The 
Biomechanics of The Mandible: A Three-Dimensio~lal Finite Element Study", J Biomechanics, 25(3), 
1992, 261-286. 
Waskell, B., Day, M. and Tetz, J.,"Computer-Aided Modeling In The Assessment Of The Biomechanical 
Determinants Of Diverse Skeletal Patterns", Am. J. Orthodontics, 89(5), 1986, 363-382. 
I- Compact Bone 
Trabecular Bone 7 
Fastener 
Bone Architecture and Current 
Fastening Technique 
Thin Face Skin 
7 Honevcomb 
Aerospace Fastener for Honeycomb Panels 
FIGURE-1 Comparison of Fastening in bone to 
That in Aerospace Composites 
Lingual Side 
Buccal Side 
Lingual Side 
Buccal Side 
Lingual Side 
Buccal Side 
FIGURE-2 Proposed Fastener Designs for Replacement Mandible 
Mandible 
Replacement 
Replacement 
Mandible 1 
r Driver Head 
Composite I 
Compact 
Bone 
Bone 
FIGURE-3 Alternative Fastener Design That Relies 
on an Interference Fit 
FIGURE - 4 Partial Replacement Mandible With 
PEEK Composite Fasteners 
Figure - 5 Replacement Mandible 
Figure - 6 Section Through Replacement Mandible Compared to 
Section Through Human Mandible 
Pivot 
Point 
Figure - 7 Finite Element Model of 112 The 
Human Mandible 
Figure - 8 Stress Contours in a Human Mandible Under a 250 N 
(1 , I  00 Ib) Point Molar Load 


The Design of Mechanically Compatible Fasteners for Human Mandible Reconstruction

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