Spinal instrumentation utilizes surgical procedures to implant titanium, titanium-alloy, stainless steel, or non-metallic devices into the spine. Instrumentation provides a permanent solution to spinal instability. Medical implants are specially designed and come in many shapes and sizes. Typically these include rods, hooks, braided cable, plates, screws, and interbody cages. Cages are simply structures that support bones (either between bones or in place of them) while new bone growth occurs through and around them.
Spinal fusion is a process using bone graft to cause two opposing bony surfaces to grow together. In medical terminology, this is called arthrodesis. Bone graft can be taken from the patient (termed autologous bone) during the primary surgical procedure or harvested from other individuals (termed allograft bone). Another option for some patients is bone morphogenetic protein (BMP). BMP stimulates the body to make bone.
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Instrumentation maintains spinal stability while facilitating the process of fusion. These procedures are used to restore stability to the spine, correct deformity (such as scoliosis
), and bridge space created by the removal of a spinal element (eg, intervertebral disc).
Both procedures immobilize the involved spinal level(s). This does not necessarily mean the patient is unable to move (eg, bend over). Many patients state they actually feel more mobile because their pain has been reduced or eliminated.
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An Old Concept Made New
Spinal instrumentation and fusion are not new surgical concepts. Dr. Paul Harrington developed spinal instrumentation in the late 1950s.
During this time, many children with polio developed spinal deformities. In an attempt to treat these children, Dr. Harrington developed the first spinal instrumentation system (Harrington Instrumentation). Rods were secured to the spine at two ends using hooks. The position of the spine was adjusted using a tackling type of device. Through Dr. Harrington's experience, fusion was discovered to be a necessary adjunct to instrumentation. Today, fusion remains an integral part of procedures utilizing instrumentation.
With instrumentation, there is less need for rigid external bracing. Much like a cast stabilizes a broken bone to heal, instrumentation stabilizes the 2 bony components of a fusion while they heal. The hardware basically functions like an internal brace. In fact, most instrumented spinal fusions are so stable that bracing is only used for comfort.
Instrumentation placed without fusion can result in hardware failure. All metal fatigues with repetitive stress. Continual stress on an implant, unsecured by a solid bone growth, can lead to screw pullout, or even fracture of the metal. This can result in broken screws, rods, and even complete breakdown of the construct. Consequently, a solid bony fusion is crucial to the proper healing of a spinal fusion.
Factors such as osteoporosis and smoking are known to impair bone healing and reduce the success of fusion. These patients are more likely to have a pseudofusion, which can result in continued pain at the surgical site and hardware failure.
|Examples of Different Types of Spinal Instrumentation
Technology and Technique Progress
During the 1960s, instrumentation became more mainstream as doctors, who saw the benefits to patients, found almost 50 ways to modify Harrington's original system. Bone screws and threaded cabling were developed. In the 1970s, Dr. Eduardo Luque was using smooth, bendable rods and wire to stabilize the spine.
Moving into the 1980s, instrumentation evolved into a three-dimensional approach to spinal correction. Rods, hooks, and screws were streamlined to meet individual patient needs with less demand on the surgeon to customize implants on the spot.
Today and Tomorrow
Spinal instrumentation continues to develop as technology advances the machining, biomechanics, and usability of these implants.
Areas of development include smaller, lower profile instrumentation to reduce patient discomfort, implants that can be placed through minimally invasive approaches, and bioabsorbable implants that can dissolve after the bony fusion has occurred.
In some cases, rigid titanium or metal implants are too strong and can erode into bone. Consequently, some implants are now made out of polymers that more closely resemble the characterics of bone.
Another area of research is applying spinal instrumentation to motion-preserving (or non-fusion) surgeries, such as artificial disc replacement, laminoplasty, and dynamic stabilization.