When using anatomical finite element models of the femur for different levels of bone resection with an osseointegrated intraosseous transcutaneous amputation prosthesis, stress concentrations on the stem/bone interface are seen at the resected, distal end and adjacent to the implant tip, with the highest stress due to bending load, according to a study published in Medical Engineering and Physics.
For amputees with long residual bone, a cylindrical model of bone and implant can be used to estimate the stresses due to bending and axial loads by taking the thickness of the bone from a radiograph in the medial-lateral direction. However, amputees with a short residual limb should not be fitted with a straight implant, but a design that facilitates transmission of the stress to the cortical bone since the implant is in contact with weaker cancellous bone.
“Finite element analysis (FEA) has been used for analyzing the stresses in and around the complex geometries of orthopaedic implants. In order to develop safety features and protect the bone from dangerous loads it is important to know the stress distribution, concentration and shielding around the implant for the attachment of a transfemoral prosthesis,” the researchers wrote. “In this contribution the stress distribution in the femur has been calculated for three anatomical FEA models with different levels of bone resection to inform the choice of the activation loads for the fail-safe device. Results from the anatomical FEA models based on the CT scan of a healthy femur were compared with homogeneous, cylindrical FEA models to assess a simpler option to predict stress levels for a patient.”
Stress concentrations
To create a 3-D image of the bone, a human cadaveric femur was scanned every 2 mm from the femoral head to the lesser trochanter and then at 5-mm intervals through the rest of the bone. From this scan, researchers calculated stress distribution in the femur and at the implant-bone interface using finite element analysis for the 3-D geometry and inhomogeneous, anisotropic material properties. Three levels of bone resection were modeled, representing a long residual limb, a short residual limb and a resection level between the two.
Researchers found stress concentrations at the distal end of the bone and adjacent to the implant tip, while stress shielding was found adjacent to the implant. When using the correct choice of bone diameter as measured from a radiograph, a cylindrical finite element model can predict stress distribution in the femur distal to the epiphysis, where the femur geometry is close to cylindrical, according to study results. As the femur geometry diverges significantly from a cylinder, study results showed stress decreases proximal to the lesser trochanter, and when a collared implant is employed, the stress concentration at the distal, resected end of the bone is removed.
“Gradual loading of the implant is required to prepare the bone for weight bearing. Proposals have been made for strengthening the bone, including gradual increase in axial load bearing using a set of scales. More recently an investigation into the stress in bone around the implant for the Osseointegrated Prosthesis for the Rehabilitation of Amputees system has concluded that gradual axial loading is insufficient to strengthen the bone in preparation for the high loads experienced near the implant tip and the distal end of the bone, and that bending and torsional loading should be prescribed in the rehabilitation program,” the researchers stated. “This is in line with the findings presented here where the most significant stress concentration was found under a bending load at the proximal end of the implant.”
Precautions
For this study, researchers based calculations on bone of normal strength and not on the significantly less dense and smaller diameter femoral bone that is normal for transfemoral amputees who have used a socket prosthesis or no prosthesis for some time. The researchers believe an intraosseous transcutaneous amputation prosthesis user’s bone will increase in density and volume as the prosthesis is loaded, but further studies are necessary to monitor the development of the bone to measure the rate of increase in bone density.
Researchers noted they represented femur models using perfect integration between the bone and implant. This is unlikely to occur in the clinical setting and would have to be taken into account in future studies. — by Casey Tingle
Disclosure: Newcombe and Dewar hold a patent for prosthetic limb attachment. Newcombe received funding from Stanmore Implants.
