Anterior column spinal implants in the thoracolumbar spine may be broadly classified as extracolumnar or intracolumnar.
Extracolumnar implants
- consist of vertebral body screws connected to a longitudinal member (plate or a rod) located on the external aspect of the vertebral body spanning one or more adjacent vertebral motion segments.
- Screws enter the lateral aspect of the vertebral body in a coronal plane trajectory (except at L5/S1 where placed in anteroposterior direction due to vessels):
- Anterior plate systems:
- Pros
- Indication include tumors, burst fractures, and degenerative spinal disorders requiring anterior fusion over one or two levels;
- Cons (not to used)
- When significant coronal or sagittal plane deformity exists (as deformity must be corrected before plate application)
- When multiple anterior vertebral segments require fixation.
- See
- Anterior rod systems:
- Pros
- Help correct deformity
- In short-segment spinal problems with deformity, anterior rod systems permit corrective forces to be applied directly to spinal segments by distraction, thereby restoring spinal alignment (and facilitating placement of intracolumnar implant).
- Subsequent compression of the anterior graft or cage restores anterior load sharing and enhances arthrodesis.
- In long-segment spinal problems (e.g. scoliosis) single or double rod systems can be used.
Intracolumnar implants
- consist of implants that reside within the contour of the vertebral bodies that are capable of bearing loads:
- Autografts
- Pros
- Highest rate of fusion success
- Cons
- Significant donor site morbidity.
- Allografts
- Pros
- Good early strength
- Avoids donor site morbidity
- Cons
- Slower/lower fusion rate compared with autograft and donor tissue
- infectious risk.
- Cage devices
- Pros
- possess excellent strength
- provide the advantage of mechanical interdigitation with vertebral bone (reducing dislodgement)
- Can be filled with cancellous autograft, allograft, or biologic agents (e.g. bone morphogenetic proteins) to promote fusion.
- Cons
- Cage devices may subside into the vertebral bodies (loss of anterior column height)
- radiographic assessment of fusion can be difficult in their presence.
- Subtype
- Static
- Cage dimensions determined prior to implantation
- Expandable
- Possess capacity for expansion following implantation to optimize stability
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