Cage Subsidence in Spinal Surgery
Cage subsidence is the sinking or migration of an interbody cage (or other implant with higher rigidity) into the adjacent vertebral body (which has lower rigidity), resulting in loss of disc height and three-dimensional changes in spinal alignment. 1
Biomechanical Definition
Cage subsidence occurs when a rigid structure—such as an interbody cage, spacer, or graft—penetrates into the softer vertebral bone due to:
- Load pressure applied to the construct
- Difference in elasticity modulus between the implant (higher) and vertebral body (lower)
- Surface area of the graft-vertebral interface (smaller contact area increases subsidence risk)
- Quality of the endplate and underlying bone 1
The magnitude of subsidence is directly proportional to the applied load and the difference in material stiffness, but inversely proportional to the contact surface area.
Clinical Significance
Subsidence is recognized as a form of instrumentation failure in the context of osteoporosis and poor bone quality 2. The Congress of Neurological Surgeons guidelines specifically identify cage subsidence as one of the adverse events predicted by poor bone mineral density (T-score < -2.5 on DEXA, HU < 97.9 on CT) 2.
Consequences of Excessive Subsidence:
- Loss of intervertebral disc height
- Local and segmental kyphotic deformity
- Destabilization of fixation hardware (screw loosening, altered screw angulation, screw pullout, or breakage) 1
- Lower fusion rates when associated with screw loosening (71.4% vs 93.9%, p=0.038) 2
- Potential return of symptoms due to foraminal height loss 3
Measurement Variability
A critical pitfall in understanding cage subsidence is the lack of standardized measurement methodology across the literature 4, 3. Different studies use varying definitions:
Common Measurement Methods:
For TLIF/PLIF approaches:
- Cage migration into vertebral bodies (most common)
- Loss of disc height over time
- Most frequent threshold: ≥2mm or >2mm 4
For ALIF/OLIF/LLIF approaches:
- ALIF: Loss of disc height (threshold >2mm most common)
- OLIF: Total cage migration (threshold >2mm)
- LLIF: Grading scales for disc height loss 3
Absolute subsidence measurement: Can be calculated using the known length of the implanted hardware (such as an anterior plate) as a radiographic reference 1
Risk Factors
Based on Level II evidence, subsidence risk is significantly increased by:
- Osteoporosis (T-score < -2.5): 65.4% subsidence rate vs 17.6% in non-osteoporotic patients (p<0.001) 2
- Low body mass index 2
- Narrower implant width 5
- Surgical level (C5-C7 higher risk in cervical spine) 6
- Stand-alone cages vs cage-plate constructs 6
Clinical Context
Subsidence is inherent to the interbody fusion process 1. Some degree of settling is expected and does not necessarily indicate failure. However, excessive subsidence (typically defined as >2-3mm or >25% disc height loss) becomes clinically significant when it:
- Compromises neural foraminal height
- Creates biomechanical instability
- Threatens hardware integrity
- Impairs fusion progression
Key clinical pearl: Endplate preservation during surgery and use of dynamic plating systems can help control subsidence and reduce complications 1. In osteoporotic patients, preoperative optimization with teriparatide or bisphosphonates should be considered to reduce subsidence risk 2.