Synchrotron X-ray Tomography Study of Bone-implant Integration

Abstract

Implant loosening is a recurring problem in the field of orthopedics, typically resulting in a physical and financial burden for the patient. Proper integration of orthopedic implants within host bones is a fundamental requirement for the successful function and limited risk of failure of implants which, in-vivo, must support various loading conditions. This implant fixation is highly dependent on the amount and quality of the bone formed around the implant post-surgery.

Previous studies have used mechanical loading, combined with lab or medical source X-ray imaging at macro- and micro-scale, as well as digital volume correlation (DVC), to successfully characterize the structural and mechanical properties of the bone-implant interface of small animals ex-vivo. Nevertheless, these studies had limitations due to imaging artifacts, low image resolution and small sample sizes.

In this study, both the number of specimens and the image resolution were increased compared to previous studies. Here, 21 rat tibiae were implanted with polyether ether ketone (PEEK) implants and imaged at high resolution using X-ray synchrotron micro-Computed Tomography (micro-CT) during in situ pull-out of the implants. The PEEK implants were filled with a Calcium sulfate-hydroxyapatite (CaS/HA) based bone cement or with a combination of CaS/HA and bioactive molecules (zoledronic acid (ZA) and recombinant human bone morphogenic protein-2 (rhBMP-2)) known to promote bone formation. The experiments were conducted prior to the start of this degree project. The focus of this degree project lay on processing and analyzing the collected data. The quality of the bone-implant interface was quantitatively assessed based on the mechanical properties obtained from the pull-out experiment. Quantitative evaluation was achieved by using the CT data to visualize and quantify the bone formation around the implants. A qualitative assessment of the interface was also performed, analyzing the internal strain distribution through DVC analysis of micro-CT images. Additionally, the use of DVC as a tool to identify cracks and deformations in the peri-implant bone, in loadsteps prior to failure, was investigated.

Combining PEEK implants with bioactive molecules improved the overall mechanical resistance of the bone-implant interface. Bone volume fraction increased significantly by 250.4%. Furthermore, DVC served as useful tool in visualizing the strain distributions in bone specimens under loading, successfully indicating local regions of deformation and fracturing within the loaded samples, prior to complete failure. Both untreated and treated samples showed comparable modes of failure; the bone failing at the threads and the thin trabecular bone surrounding the implant.

Improving newly formed peri-implant bone quality and quantity could lead to an increased quality of life for patients and a decrease of the financial burden caused by orthopedic surgery.

Popular Abstract

Placing drug treatments in orthopedic implants can improve the peri-implant bone quality and quantity.

In the field of orthopedic surgery, implant loosening is a commonly arising problem which often leads to revision surgeries. These surgeries cause a physical, psychological, and financial burden for the patients and their caretakers. To have the orthopedic implants function long-term under various conditions inside the human body, proper integration of the implant into the host bone is required. This integration relies on the quality and quantity of the newly formed trabecular bone around the implant. In this project, it has been assessed whether drug treatments consisting of bioactive molecules improved the quality and quantity of the peri-implant bone.

For this study, 21 polyether ether ketone implants were implanted in the proximal tibiae of rats. Some implants were treated by filling the hollow core of the implants with bioactive molecules, known to promote bone formation. Implants were left to integrate for 6 weeks. After which, the tibiae containing the implants were harvested and subjected to mechanical testing, i.e., incremental pull-out testing. The implants were pulled out of the host bone in a stepwise manner and imaged after every loading step. The images of the bone-implant interface were acquired using synchrotron source computed tomography and resulted in images with a $\mu m$ resolution. Computed tomography is an X-ray imaging technique used to obtain three-dimensional medical images of parts of the body.

Based on the mechanical tests performed on the bone-implant interfaces, the mechanical properties of the newly generated bone could be evaluated. The images acquired during the testing were used for two different analyses. A first analysis assessed the quantity of bone formed within a defined volume around the implant. A second analysis evaluated the internal strain distribution within the newly formed bone. To that end, digital volume correlation was used. This is a non-destructive experimental technique, which refers to tracking translations of small sub-volumes between two subsequent 3D images.

Failure of the bone-implant interface occurred in similar ways for both treatment groups, and the location of high strains leading to bone damage could be detected using digital volume correlation. The results of the various analyzes showed that there was a trend towards improved mechanical properties, as well as bone quantity for the peri-implant bone of the treated implants. This hinted towards improved implant integration due to the treatment.

Translating these observations to human orthopedic applications will help improve peri-implant bone quality and increase its quantity. This could lead to a reduced number of reoperations, thereby increasing the quality of life of patients and decreasing the financial burden of undergoing orthopedic surgery.

• author: Lohéac, Ornella
• supervisor: Hanna Isaksson; Elin Törnquist; Joeri Kok
• organization: Department of Biomedical Engineering
• alternative title: Implantat-integration i ben studerat med synkrotron-tomografi
• course: BMEM01 20211
• year: 2021
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Bone-implant, Digital volume correlation, Micro-computed tomography, PEEK, Synchrotron
• language: English
• additional info: 2021-15
• id: 9063722
• date added to LUP: 2021-10-01 10:46:21
• date last changed: 2022-08-26 03:41:14