Spinal implants can subsidence—but not for the reasons most people assume.
Subsidence occurs when an implanted device sinks into an adjacent vertebral bone, undermining the benefits of spinal fusion surgery and potentially leading to serious complications such as spinal instability, nerve compression, and misalignment.
Although several factors have been identified as contributors to implant subsidence, including bone quality, contact surface and material, there is limited research comparing their relative impact on the risk of subsidence. The interplay between these factors and their combined influence on subsidence remained poorly analysed before a landmark study by Suh et al. 2017 addressed this gap with an in-vitro study, systematically ranking key contributors.
Three factors analysed in the study:
Cage Material Stiffness
Implants are typically made from titanium alloy or PEEK, each with different elasticity and biocompatibility. PEEK, with elasticity closer to bone, was once thought to reduce subsidence by minimising stress on the endplate.
Bone Quality
Some surgeons reshape the bony endplate to fit flat-sided generic implants. This process often involves removing the strong cortical (hard) bone, which serves as the spine's primary load-bearing layer, and exposing the underlying cancellous (soft) bone. This improves fit but weakens the endplate, resulting in uneven resitance to load.
Footprint Contact
The implant's footprint affects load distribution. The footprint can be increased by enlarging the implant or increasing its contact surface area. Larger footprints spread force across the endplate, reducing stress.
Each of these factors contributes to risk of subsidence to varying degrees. Suh et al. 2017 analysed which factors have the greatest impact on subsidence.
What Did Suh et al. Find?
The study revealed that not all factors contribute equally to subsidence. Among the evaluated variables, two stood out as the most influential:
Bone Quality
The strength of the underlying bone was the single most critical determinant of subsidence risk. Strong cortical bone significantly reduced the likelihood of subsidence, while weaker bone made failure more likely.
Footprint Size
Larger footprints proved to be highly effective in distributing load more evenly across the vertebral endplate, dramatically reducing stress and minimising the risk of subsidence.
In contrast, cage material stiffness—whether titanium alloy or PEEK— had negibligable effect on subsidence.
The findings provide critical insights that help surgeons refine their strategies to minimise the risk of subsidence to improve patient outcomes.
How does 3DMorphic implement design features that align with Suh et al.’s findings to prevent subsidence?
Bone Quality
3DMorphic designs patient-specific implants that fit the endplate, eliminating the need for bone remodelling and preserving the strong cortical layer, reducing subsidence risk. An other study by Grant 2000. stated that “Implants should be designed to spare the endplate, since the bone strength decreased significantly when the endplate was removed”.
3DMorphic implants are designed to load the strongest bone (outer rim), promoting bone on-growth and stability. “Implants used between two lumbar vertebrae should ideally engage the peripheral endplate as much as possible...The center of the endplate should be avoided, since this is the weakest area, and is a good area for bone growth” [Grant 2000]
Footprint Contact
3DMorphic implants maximise contact footprint by matching the endplate shape, optimising load distribution. Additionally, the implants are designed to have the optimal size taking into account soft tissure approach limitations. By maximising footprint size, 3DMorphic reduces stress on the endplate, lowering subsidence risk.
In conclusion, smarter implant design leads to better patient outcomes.
References
Suh, P.B., Puttlitz, C., Lewis, C., Bal, B.S. and McGilvray, K., 2017. The effect of cervical interbody cage morphology, material composition, and substrate density on cage subsidence. JAAOS-Journal of the American Academy of Orthopaedic Surgeons, 25(2), pp.160-168.
Grant, J. Pamela Ma Sc,*; Oxland, Thomas R. PhD,* and; Dvorak, Marcel F. MD†. Mapping the Structural Properties of the Lumbosacral Vertebral Endplates. Spine 26(8):p 889-896, April 15, 2001.