New direct fixation implant for upper-leg prostheses.
Reduced bone failure risk and periprosthetic bone resorption.
P.K. Tomaszewski1, MSc
N. Verdonschot2,3, PhD
S.K. Bulstra4, MD, PhD
G.J. Verkerke1,3, PhD
1
Department of Biomedical Engineering, UMC Groningen, University of Groningen
Orthopaedics Research Lab, RUMC Nijmegen
3
Department of Biomechanical Engineering, University of Twente, Enschede
4
Department of Orthopaedics, UMC Groningen, University of Groningen
2
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Osseointegrated trans-femoral artificial limb fixation
Each year around 600 patients undergo transfemoral amputation in the Netherlands 1. The
conventional prosthetic limb attachment is realized by a stump fitting socket to which the
artificial limb is fixed. Decades of development of socket technology has led to a more
optimal coupling between stump and socket. However, performance of this fixation
method is often reported as unsatisfactory 2. Pain, soft tissue irritation and breakdown 3
and lack of appropriate control of the prosthetic limb 4 arise due to the fact that the soft
tissues of the residual limb are not appropriately suited for body weight support.
Moreover, the fixation is affected by volumetric variations of the stump due to swelling.
An alternative solution to conventional stump-socket prosthetic limb attachment is offered
by direct skeletal fixation (Figure1).
Figure 1.
Schematic representation of direct limb fixation.
Direct attachment of an artificial limb to the skeletal system allows overcoming the
conventional socket system problems and provides a better control of the prosthetic limb
including sensory feedback from the ground surface 5. Finally, the range of the hip joint
motion is unrestricted and sitting comfort is increased relative to socket prostheses 2.
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Design of the new intramedullary direct fixation implant
Figure 2.
Components of the new implant.
Key features of the new implant1:
 Short stem length.
 Low amplitude sliding motion between inner and outer part (stem and bushing).
 Collared stem for distal loading of the bone remnant.
 Outer part of the implant (bushing) which is in direct contact with bone is of the
elasticity comparable with the cortical bone.
Materials:
 Titanium alloy: Ti6Al4V – high strength bio-compatible alloy.
 PEEK: PEEK Motis (Invibio Ltd.)- carbon fibre rainforced bio-polymer.
 Dual Titanium and Hydroxyapatite coating: coating enabling osseointegration
(Eurocoating Spa).
 Silicon rubber: bio-compatible silicone rubber MED-1511 (Nusil Corp.), strong
adhesion to PEEK polymers.
1
Patent application: WO2011037458 (A1).
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Implant-bone load transfer
Implant-bone load transfer determines
the the initial and long-term stability of
every intramedullary stem. It is mostly
depandent on the relative stiffnes of the
implant and the cortical bone.
In case of standard metallic implants the
stifness of the prosthesis is 10 (Ti and Tialloys) to 20 (CoCrMo-alloys) times
higher than the stiffness of the cortex.
This
causes
non-uniform
stress
distribution in the cortex, namely
induces high stress peaks around the
tips of implant and elsewhere bone
remains
almost
non-loaded
(phenomenon termed in the literature
as “stress shielding”).
he high stress peaks might cause bone
overlaod and damage (peri-implant bone
fractures). In the long-term non-uniform
Figure 3. Implant-bone load transfer for a standard
(non-physiological) stress distribution
stiff stem and the new implant.
around an implant often results in a
progressive loss of bone mineral density
(BMD). This reduces prosthesis support, decreases bone strength and ultimately leads to
implant loosening.
The collared stem, which is able to slide in the surrounding bushing, enables distal load
transfer to the bone (Figure 2). The elastic outer part of the new implant reduces stress
peaks by redistributing the stresses more uniformly over a larger interface surface.
Finite element analysis
Finite element (FE) models were created to represent an intact femur and an femur
amputated at the metaphyseal level and implanted with an OPRA (Integrum, Sweden), ISP
Endo/Exo (Eska Implants, Germany) and the new prosthesis.
The geometry of the bone was determined from CT scans of a male femur bone with
normal bone mineral density. The implants with the same outer diameter of the
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intramedullary part were assumed to be in close contact to the bone. Two loading cases
from a normal walking cycle were considered 6.
Figure 4.
OPRA
ISP
Schematic representation of the
analyzed implants.
new
Periprosthetic bone stress distribution
Equivalent von Mises stress [MPa]
The equivalent von Mises stresses in the diaphysis of the intact femur was uniformly
distributed along the cortex. The stress pattern in the bone changed considerably after
implantation of the OPRA and the ISP implant, however only a minor change was observed
for the new implant.
intact bone
Figure 5.
OPRA
ISP
new
Equivalent von Mises stress distribution in the intact bone (left) and bones around considered
stems.
After introduction of the OPRA and the ISP implants the high stress concentration in the
proximal region (close to the implant tip) was found and much lower stresses were present
in the distal part 7.
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Similarly as in the intact bone, the stresses around the new implant were uniformly
distributed along the cortex (Figure 5).
Periprosthetic bone failure risk
Equivalent von Mises stress /strength
The bone failure risk was defined as a ratio of the equivalent stress and local bone
strength. The highest failure risk was always found in the bone region in contact with the
proximal end of the implants. None of the implants showed direct bone failure for the
normal walking activities.
Intact bone
Figure 5.
OPRA
ISP
new
Periprosthetic bone failure risk during normal walking activity.
Considerably lower bone failure risk around the new implant was found in comparison to
the currently used devices (Figure 5).
Periprosthetic bone remodeling
Usually intramedullary amputation prostheses are implanted in patients that have used
standard socket prosthesis before 8, 9. Hence, the bone has not been loaded to
physiological levels for some time, resulting in a average overall bone loss of 30% 10.
Another clinical study 11 has described considerable bone resorption around the distal end
of the femur and bone deposition at the proximal end of the OPRA implant.
The long-term bone turnover after implantation can be quantitatively predicted by an
adaptive bone-remodeling simulation, which combines bone remodeling theory with finite
element (FE) analysis.
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The results showed high bone resorption around the distal end of the femur around both
the OPRA and the ISP stems. Furthermore a considerable bone deposition at the proximal
tip of all the implants was observed 12.
OPRA,
clinical X-ray
Figure 6.
Periprosthetic bone remodeling presented in form of DXA radiographs.
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60 months 'post-socket' implantation
10%
OPRA
0%
BMD change
-10%
ISP
-20%
new
-30%
-40%
-50%
-60%
-70%
-80%
-90%
1
2
3
4
5
6
7
Zone
Figure 7.
Bone remodeling calculated in 7 zones around the implants.
In case of both the OPRA and the ISP implants, only a short-term periprosthetic bone
densification was found and afterwards the progressive bone resorption was observed.
Figure 8.
Total bone mineral content (BMC) change around the implants.
The new implant is predicted to minimize the long-term periprosthetic bone resorption
(Figure 6-8).
In-vitro testing
Bone resorption is recognized as a risk to stability of the orthopaedic implants 13, 14 and is
directly related to non-physiological load transfer characteristic to metallic implants. The
new implant was designed to overcome this issue and its improved performance over
existing devices was previously predicted by finite element analyses. To confirm these
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results we compared a standard titanium implant (OPRA system, Integrum, Sweden) and
the new direct fixation implant on cortical strains in the femur by in-vitro measurements
(Figure 9). The measurements were in agreement with numerical predictions and showed
that the new implant can provide more physiological load in the bone as compared with
the standard implant.
Figure 9. Left: Implants used in the experiment: the new implant (left) and the standard titanium implant OPRA (right). Right: Experimental setup.
Figure 10. Experimental and FE simulated strains presented as a percentage of physiological strains.
Conclusion
The results show that the new design provides close to physiological distribution of
stresses in the bone and lower bone failure risk for the normal walking as compared to
the OPRA and the ISP implants. The bone remodeling simulations did not reveal any
overall bone loss around the new design, as opposed to the OPRA and the ISP implants,
which induce considerable bone loss in the distal end of the femur. This positive outcome
shows that the presented concept has a potential to considerably improve safety of the
rehabilitation with the direct fixation implants.
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Current work
In-vivo study
The experiment aims to demonstrate the in-vivo applicability of the new composite
amputation prosthesis and to compare periprosthetic bone remodeling around a standard
titanium screw implant and the newly developed composite system.
Both implants are placed intramedullary in one femur of the goat after a bone defect at
the midshaft is created (Figure 11). The position (proximal/distal) of both stems is
alternated within the experimental group of 10 animals, so that the influence of the
position is eliminated. Both stems are sized based on a pre-operative radiographs.
The in-vivo experiment is expected to confirm favorable results of previously conducted
computer modeling on long-term bone changes around the direct fixation prosthesis and
prove safety of the new implant prior to future clinical trials.
Figure 11.
Left: Scheme of the animal mode; Right: Implants adjusted for goat experiment.
Bone remodeling response in transfemoral amputatees with an osseo-integrated
prosthesis
The objective is to measure periprosthetic bone mineral density changes in order to assure
a long-term stability of the osseo-integrated prosthesis.
The bone mineral density around implant is measured by means of Dual X-Ray
Absorptiometry (DXA). The DXA scanning is done postoperatively and at three months, one
year, and two years after the operation. Bone density is measured in seven zones defined
around the implant (corresponding to Gruen zones defined around hip stems).
Measurements made at three, six months and two years are compared with pre-operative
results.
The study population consists of all the patients, who undergo the implantation of the ISP
implant in the St. Radboud UMC in Nijmegen in years 2010 and 2011.
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Literature:
1
Rommers GM. Epidemiologie van amputaties aan de onderste extremiteit. In: Geertzen JHB, Rietman J.S.,
editor. Amputatie en prosthesiologie van de onderste extremiteit: Lemma; 2008. p. 53.
2
Hagberg K, Branemark R, Gunterberg B, Rydevik B. Osseointegrated trans-femoral amputation prostheses:
prospective results of general and condition-specific quality of life in 18 patients at 2-year follow-up.
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3
Sullivan J, Uden M, Robinson KP, Sooriakumaran S. Rehabilitation of the trans-femoral amputee with an
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4
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11
Xu W, Robinson K. X-ray image review of the bone remodeling around an osseointegrated trans-femoral
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12
Tomaszewski PK, Verdonschot, N., Bulstra, S.K., Rietman, J.S., Verkerke, G.J. Simulated bone remodeling
around two types of osseointegrated implants for direct fixation of upper-leg prostheses.
. Submitted to J Biomech. 2010.
13
Spittlehouse AJ, Smith TW, Eastell R. Bone loss around 2 different types of hip prostheses. Journal of
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