1Department of Biomedical Engineering, UMC Groningen, University of Groningen
2Orthopaedics Research Lab, RUMC Nijmegen
3Department of Biomechanical Engineering, University of Twente, Enschede
4Department ofOrthopaedics, UMC Groningen, University of Groningen 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).
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.
Design of the new intramedullary direct fixation implant
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.
Titanium alloy: Ti6Al4V – high strength bio-compatible alloy.
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 Ti-alloys) 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”).
Implant-bone load transfer for a standard stiff stem and the new implant.
he high stress peaks might cause bone overlaod and damage (peri-implant bone fractures). In the long-term non-uniform (non-physiological) stress distribution 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 intramedullary part were assumed to be in close contact to the bone. Two loading cases from a normal walking cycle were considered 6.
OPRA ISP new
Schematic representation of the analyzed implants.
Periprosthetic bone stress distribution
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.
Equivalent von Mises stress [MPa]
intact bone 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.
Similarly as in the intact bone, the stresses around the new implant were uniformly distributed along the cortex (Figure 5.).
Periprosthetic bone failure risk
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.
Equivalent von Mises stress /strength
Intact bone 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.
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.
Periprosthetic bone remodeling presented in form of DXA radiographs.
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.
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 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 (2). 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.
Left: Implants used in the experiment: the new implant (left) and the standard titanium implant -OPRA (right). Right: Experimental setup.
Experimental and FE simulated strains presented as a percentage of physiological strains.
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.
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 (4). 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.
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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