Although threaded titanium cages initially were the only spinal implants that were available, there is now an assortment of cages in different shapes, sizes and materials.
This gives a person about to undergo anterior lumbar interbody fusion (ALIF) surgery more options.
Threaded Titanium Cage Limitations
One limitation with a threaded titanium cage is that the larger the size of the disc space, the more bone that is removed from the subchondral endplates (the strong bone at the bottom and top of the vertebral body). This weakens the bone and leads to an increase in the subsidence rate (where the cages subside into the vertebral body).
One potential way to avoid subsidence is to use a more rectangular cage that sits in between the vertebral endplates and allows the endplates to support the device. Using endplate sparing techniques and devices allows for less risk of subsidence.
Fixation can be achieved by using porous metal that binds to the bony endplates by friction, or ridges/spikes can be substituted for threads to fixate the cage to the bone.
Indications and Contraindications for Anterior Cages
Because an anterior interbody spine fusion surgery relies on the strength of the vertebral body to keep from subsiding, an absolute contraindication to doing an anterior interbody fusion (without posterior supporting instrumentation) is osteoporosis. The cages do not fail by breaking. They fail because the bone in the vertebral endplates may not be strong enough to support the cages. This leads to a failure of the endplates, with the cage subsiding into the vertebral bodies.
In general, anterior cages are not strictly fixation devices for spine fusion. Pedicle screws used with posterior instrumentation systems provide excellent spinal fixation. Anterior intervertebral devices should be thought of as an interference type of fixation. They are implanted in between the vertebral bodies and do not strictly fixate the two vertebral bodies to each other. Until the bone knits them together, the cage is mostly held by an interference fit.
Because of this difference in mechanics, stand alone anterior fixation is best limited to collapsed disc spaces. It works better at L5-S1 where there is little motion. At L4-L5, there is more flexion/extension motion, and this allows more motion through the cages. Lastly, they work better in one-level spine fusions than two-level fusions, and most spine surgeons feel they should not be used as a stand alone device for three level fusions.
A problem with titanium cages has been that it is difficult to assess spine fusions postoperatively because the metal impedes evaluation by x-ray. One solution has been to use radiolucent cages (made of either carbon fiber or PEEK). Postoperatively, the cages allow much better visualization of the healing bone. Unfortunately, they do not adhere to the bony endplates well and are rarely used by themselves. They are usually used in conjunction with either an anterior/posterior fusion or a PLIF and supplemented with pedicle screws.
Although x-ray visualization of the bone fusion is limited with titanium devices, the fusion can readily be imaged with a CT scans. Almost all cages are MRI-compatible for postoperative imaging.
There are many innovations and technical improvements being developed, and although no one cage is the best, there are certain cages that work well for certain indications. As with any other spine fusion procedure, the implant used is largely dictated by what the treating spine surgeon prefers and has had the most success with in the past.
Whether a spine surgeon approaches the disc space from an anterior approach or from one of the posterior approaches (PLIF, TLIF) is largely dependent on how comfortable the surgeon is with the anterior approach and operating around the aorta and vena cava.
Most spine surgeons have not had a great deal of experience doing the procedure by themselves, and not all spine surgeons have access to a skilled vascular surgeon to help them with the approach. Therefore, a posterior approach for lumbar spine fusion surgery may be more practical for many surgeons.
Allograft Bone Grafts in ALIF Surgery
An ALIF spine surgery can also be done with an allograft bone implant. Allograft bone (cadaveric bone) can be milled to a shape like a titanium implant (cylindrical), or more commonly, it is a femoral ring that can be shaped by the physician to fit the disc space.
Generally, allograft bone is not as strong as other implants. In cases where the lower back surgery is being done as an anterior/posterior approach, it may be strong enough, but most spine surgeons are leery about using it as a stand-alone device (e.g. no posterior instrumentation to help support it). Allograft bone tends to cause resorption of the patient's own bone (osteolysis) at the graft/vertebral endplate interface early in the postoperative course, and can lead to further instability.
In the past, the patient's own bone had been used (autologous bone graft). This required a large bone graft to be taken from the patient's iliac crest, and had a fairly high complication rate (such as postoperative chronic pain, infection, pelvic fracture). Also, this bone is not all that strong or supportive as allograft bone and required supplementation with posterior instrumentation.
For the most part, structural autograft implants are generally not used as there are better options. Morselized autograft from the iliac crest may still be used to obtain a fusion, but commercially available bone graft substitutes are gaining popularity.