Abstract
Introduction
Direct skeletal attachment of prosthetic limbs, commonly known as osseointegration (“OI”), is being investigated by our team with the goal of safely introducing this technology into the United States for human use. OI technology allows for anchorage of prosthetic devices directly to bone using an intramedullary stem. For OI to be effective and secure, bone ingrowth and remodeling around the implant must be achieved. Physicians need an effective way to measure bone remodeling in order to make informed decisions on prescribed loading. This work describes methodology that was developed that utilizes computed tomography (CT) imaging as a tool for analyzing bone remodeling around an osseointegrated implant.
Method
A subject implanted with a new Percutaneous Osseointegrated Prosthesis (POP) (DJO Surgical, Austin, TX) had CTs taken of their residual femur at 6-weeks and 12-months post-op in a FDA Early Feasibility Study with Institutional Review Board approval. Three-dimensional models of the femur were created from dicom files of the CT slices using Mimics (v21.0, Materialise, Leuven, Belgium). Each scan was segmented into four objects: cortical bone, medullary cavity, total volume (cortical bone plus the medullary cavity) and endoprosthetic stem (Fig. 1).
Following segmentation, models were uploaded to 3-Matic Research (v13.0, Materialise, Leuven, Blegium) in STL format for alignment to a common world coordinate system (Fig. 2). A common origin was set by taking the average distance between planes of the femoral head and the greater trochanter. Once aligned to the coordinate system, biomechanical length (BML) was calculated from the proximal origin to the distal end of the amputated femur.
BML and STLs of the aligned medullary cavity and femur volume were entered into custom Matlab code designed to measure cortical and medullary morphology in transverse cross sections of the femur. Morphology data from 6-weeks and 12-month time points were compared in order to determine if bone remodeling around the POP implant could be detected using these methods.
Results
Comparing longitudinal data from post-operative visits suggests that important indicators of bone remodeling around the device could be detected (Fig 3). One year after implantation of the POP device the medullary perimeter and area had minimal % differences (−1.5 and 2.2) from the 6-week visit, validating that consistent alignment of the femoral model was achieved between scans from different time points.
The cortical area, cortical perimeter, and cortical thickness around the POP implant showed positive percent changes at the 12-months of 19.44%, 4.04% and 14.36% respectively, with the largest increases observed at the the distal end for each parameter. These increases in cortex morphology values indicate bone changes were identified around the endoprosthetic stem of the implant.
Discussion/Conclusion
This pilot study utilized CT imaging as a tool for analyzing bone remodeling around a new osseointegrated device. These methods can be performed quickly and accurately and have the potential for use in monitoring bone remodeling. CT scans from additional subjects are being analyzed to further validate and optimize these methods for clinical use.
This study described an investigational device, limited by federal law to investigational use. No long-term data exists about its performance.
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