Assessment of bioactivity of the implant using SPECT-CT
During our investigations, we attempted to demonstrate the migration of bone cells along the surfaces of the glass-ceramic or biotitanium implants using imaging investigations. Unfortunately, standard CT or MRI were not able to provide this precise information. The CT scans were limited by screw artefacts, and the MRI scans were generally unable to detect changes in bone. In an attempt to resolve these problems, we utilized SPECT-CT, a method that provides up-to-date computed tomography (CT) and gamma camera (SPECT), to detect the activity of the osteoblasts on the body of the titanium implant applied into the interbody space. The computed tomography (CT) can precisely display the anatomic structure of the investigated tissue, and the gamma camera investigation (SPECT) can yield a functional view of the metabolic process in the patient’s body, but without its precise localization or other anatomical details. Thereby, the combination of these investigations provided more complete information on the precise place of the metabolic process as well as its dynamics. In our study, the metabolic process included the activity of the osteoblasts on the surface of the bioactive implant, as applied by the PLIF method.
In 2009, we performed this type of investigation in four patients after surgery for the primary instability of the lower lumbar spine segment using the PLIF operation technique with Implaspin. The study was conducted before the surgery as well as two and six months after the intervention, and we assessed the anatomical changes and metabolic activity at the location where the implants were applied by using the combined scans. The investigation provided preoperative signs of instability localized to the affected space in the area of the disc in all four patients. We detected a hyperintense signal at the operated segment two to three months after the surgery, which was a sign of osteoblast activity on the surface of the implant. We also observed a decrease of this activity (hypointense signal) six months after the surgery as well as a change on the surface of the implant using the combined CT scans.
Fig. 13. Implants applied into the spaces L3/4 and L4/5 on SPECT CT. The figure shows the surface of the implant with the bone tissue (grey-black colour) and the titanium screw in the body of the L3 (white colour).
Fig. 14. Implants on a SPECT-CT scan. The bone growth at the border of bone tissue (grey) and Implaspin (white) is visible in the space L4/5.
According to our method, hypointensity signified the completion of the osteoblastic activity. The changes on the CT scans were completed by conducting a measurement using Haunsfield’s units (metal – about 2000 HU; bone tissue 100-300 HU), which provided evidence that the implant was overgrown by bone tissue. This kind of image detects the primary successful binding of the implant via activation of the osteoblasts by its specially adapted bioactive surface (figure 13 and 14). Using this combined imaging technique in all four patients, we demonstrated the migration of bone cells along Implaspin wall and the formation of fusion without the addition of another material, such as autografts or TCP, six months after the surgery. Therefore, the successful fusion was indirectly confirmed using the SPECT-CT improving the postoperative clinical findings.
Conclusion
The development of both the material and the shapes of implants continues to progress. Currently, the primary focus of this development is to produce an implant that forms a firm fusion as soon as possible and to ensure the formation of new bone due to its material composition. The current implants for PLIF combine two separate components, including a solid cage shape and osseoconductive material (i.e., TCP, BMP) that ensures the activity of osteoblasts and the formation of the interbody fusion. To date, none of the materials for PLIF available on the market optimally meet both characteristics (see Table 5).
|
Optimal parameters of the implant |
Metal (Titanium, steel) |
PEEK or +PEEK carbon fibres |
Glass-ceramic |
Resorbable implants -polylactides |
Bioactive titanium (LASAK Ltd.) |
|
1. Firm structural support (load resistance immediately after implantation) |
+ |
+ |
+/- |
+/- |
+ |
|
2. Osseoconductivity, bioactivity – ability to bind with a bone, support of fusion without addition of other material (bone, TCP, etc.) |
- |
- |
+ |
- |
+ |
|
3. Possibility of radiographic assessment of the bone fusion progression |
+ |
+ |
+ |
+ |
+ |
|
4. Biomechanical properties (elasticity modulus similar to bone) |
- |
+ |
- |
 |
- |
Table 5. Parameters of the implant according to the type of material.
During this investigation, our goal was to develop an implant that would combine both of these components in one unit, ultimately maintaining the strength and bioactive properties present in two-component implants. At the end of the 1990s, we were close to the development of such material due to the implant BAS-0.However, due to these experiences, we and other technicians successfully designed an implant that meets our original conception. This implant is currently used in clinical practice, and experimental studies have confirmed its supposed properties. The combination of the implant’s strength and shape with bioactivity enables the smooth application and restoration of anatomy, thereby providing a perfect fixation of the operated segment and stimulating growth of osteoblasts and their migration along its surface. Our original implant Implaspin combines the osteoconductive and osteoplastic properties of the glass-ceramic with the strength of titanium, which was the aim of our research. Thanks to these properties, this implant represents a quality alternative to implants constructed from other materials dedicated to PLIF (see Table 5).
