The approach taken can have significant biomechanical effects.
The spinal vertebral body endplate shapes undulate variably, whereas most Off The Shelf (OTS) fusion implants have flat contact surfaces.
This study explores some potential biomechanical consequences and surgical options for fitting flat sided implants into the curved interbody space.
There are three main surgical techniques used to prepare the interbody space for spinal fusion:
A) Don’t Remodel the Endplate Bone and Implant an OTS Implant
B) Remodel the Endplate Bone to Approximate Flatness and Implant an OTS Implant
C) Don’t Remodel the Endplate Bone and Implant a Patient-Specific Implant that fits the contours of the endplate
Figure 1: Illustrative cross-section of vertebral bones and interbody implant for Option A, B and C
Let’s explore these options to determine the consequences for fusion.
Option A: Don’t Remodel the Endplate Bone and Implant an OTS Implant
Walsh et al., (2020) showed that gaps >1-2mm between bone and implant surfaces can delay early bone on-growth.
Uneven bone shape combined with a flat sided implant naturally leads to reduced contact footprint, which Suh et al., (2017) have shown increases the risk of implant subsidence.
Option B: Remodel the Endplate Bone to Approximate Flatness and Implant an OTS Implant
Some advocate for altering the endplate to accommodate the flat-sided OTS implant, believing this enhances fusion.
Option C: Don’t Remodel the Endplate Bone and Implant a Patient-Specific Implant that fits the contours of the endplate
Others argue that preserving the natural endplate bone while using a patient-specific implant is a valid alternative.
Finite Element Study Comparison
Biomechanical simulations (Finite Element, FE, models) are used here to compare Options A, B and C.
CT scans of a patient's cervical spine were analysed to determine bone and implant stress distribution. The computer models generated were subjected to a load that simulated walking/jogging forces. *
Results and Discussion:
Cooler colours represent regions with lower strain forces, whereas warmer colours indicate areas experiencing higher strain.
Figure 2: Front side view of Finite Element models for Options A, B and C
Figure 3: Figure 2: Top view of the inferior vertebrate of Finite Element models for Options A, B and C
Option A: Non-Remodelled Endplates with an OTS Implant
When an OTS implant is used without modifying the endplate, strain hotspots appear, as expected, due to the mismatch between the flat sides of the implant and the irregular shape of the endplate. The strain hotspots are where the device contacts the endplates.
Option B: Remodelled Endplates with an OTS Implant
The FE findings revealed that surgically remodelling the endplates and using an OTS implant resulted in the highest strain across the endplate.
The strain was predominantly concentrated on the endplate, rather than spread out through the vertebral body, that could lead to implant subsidence into the vertebral body, which directly reduces nerve decompression and construct stability.
This highlights that reshaping the endplate and exposing the trabecular (soft) bone can be detrimental, leading to elevated strain across the endplate and reducing the benefits of the surgical procedure for the patients.
Option C: Non-Remodelled Endplates with a Patient-Specific Implant
The FE showed an even distribution of low strain throughout the vertebrae.
Maintaining the integrity of the original, strong endplate bone reduces potential post-operative complications such as subsidence, loss of neural decompression and adjacent segment disease.
It has been shown in other studies that bone quality and contact surface are the two most important factors to prevent subsidence, with cage stiffness having minimal influence on subsidence (Suh et al., 2017). The present study reinforces Suh et al's findings by demonstrating that maintaining strong endplate bone and ensuring a high contact surface area significantly reduces the occurrence of high peak strains. Using a curette on the endplate is sufficient to stimulate bone fusion.
Analysis:
The high strains in Option B can be attributed to removal of the strong cortical (hard) bone and exposure of underlying cancellous (soft) bone, which would lead to uneven load distribution across the endplate as the implant loads on strong and soft bone at the same time.
Figure 4: Schematic cross-section of endplate bone and implant for Option B and C
Furthermore, with Option B:
1) During surgery it is difficult to match accurately the endplate shape to the implant shape; there is a high possibility of implant-anatomy shape mismatch.
2) It can be difficult to determine through the surgical window whether the remodelled interbody space has maintained alignment within the spine (the surgical remodelling can change endplate angles).
Conclusion
Leaving the endplate bone shape intact and using patient-specific implants results in even strain distribution, preserving the bone's strength and potentially reducing post-operative complications. This approach is likely beneficial as it eliminates the need for extensive surgical reshaping, reducing operating time, complexity and high strain spots.
By understanding these approaches, surgeons can make more informed decisions, enhancing patient outcomes in spinal fusion surgeries.