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Hardware for Back Surgery

Jan. 12, 2006
Biomedical engineers are refining the bone screws and other mechanical devices used in spinal fusion.

Mark Bender
Senior Project Engineer
Vertebron Inc.
Stratford, Conn.

Back pain will afflict 80% of the U.S. population at some time in their lives. And lower-back pain is the most common disability among people under the age of 45. For the worst cases, those with fractured vertebrae, congenital deformities, or nerve damage, the treatment of choice is back surgery. In all, $8 billion was spent in the U.S. on back surgery last year. And one of the most common back surgeries is spinal fusion. There were 440,000 such operations done last year.

The good news is that doctors have been getting better at performing spinal fusions, especially on vertebrae in the thoracic and lumbar regions of the spine, the most commonly done fusions. This is due to years of clinical experience, new imaging and surgical technologies, and perhaps most importantly, improvements made to the hardware that gets implanted. Though the hardware seems relatively simple — screws, rods, and clamps — significant advances have helped propel fusion success rates from 60 to 90% over the last two decades. During that time, for example, the breakage rate for hardware has decreased significantly from 10 to 0.1%. Infection rates and the potential for nerve damage have also plummeted, as well as recovery times.

You first have to understand a little spinal anatomy before you can appreciate the work done by biomedical companies designing and manufacturing fusion implant hardware. The spinal column consists of a series of bones or vertebrae separated by thin discs of soft tissue referred to as intervertebral discs. The bones protect the spinal chord and nerves branch off from the chord between each pair of vertebrae. Healthy discs act as lubricating cushions, absorbing energy, and letting the spinal column twist, flex, and bend without adjacent backbones touching each other.

When there is a problem, such as a slipped disc, and the doctor determines spinal fusion is the best course of action, a surgeon removes much of the disc between two adjacent vertebrae being extremely careful not to knick or sever a nerve. He also prepares the site to accept grafts. These specially prepared bone fragments come either from the patient (autograft) or a donor ( allograft) and are packed between the vertebrae and the backbone. In some cases, a thin disc made of bone is slipped between them, replacing the disc.

Vertebron supplies surgeons with allografts made of bone taken from cadavers. The bone is shaped into a crescent and elliptical grooves are cut into the top and bottom faces. This gives the vertebrae above and below it a better grip on the disc and promotes better bone growth and adhesion. After being shaped, grafts are processed to prevent disease transmission, then freeze-dried and packaged. The entire package is sterilized with a low dose of gamma irradiation. They are rehydrated just prior to surgery.

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The surgeon inserts a pair of pedicle screws carrying rod clamps (also called tulips for their shape) into the pedicles of each vertebra. A pair of rods is fixed in the clamps so the rods run parallel to the spine and on both sides of it. The doctor then fixes the rods in place. After a few months, the original bone and grafts grow together or fuse. This limits the patient's spine in terms of how much it can bend or twist, but relieves the patient's pain. And the procedure only restricts movement between two or three vertebrae, so it doesn't immobilize the entire spine. The rods and screws, after holding the vertebrae in position and bracing the spine, are usually kept in place, though they serve no further purpose. In about 5 to 10% of the cases, however, the hardware causes the patient discomfort and is usually removed.

Single-level fusion involves two vertebrae and is the most common fusion procedure done. Two-level fusion, almost as common, involves three vertebrae and sometimes includes a cross bar between titanium rods for additional stability. Single-level surgery costs approximately $55,000, with hardware accounting for $4,000 to $8,000 of it. Individual pedicle-screw assemblies, for example, cost about $1,000.

To keep the bones in position, implanted hardware must be strong enough to handle the loads. Loads vary with the size and age of the patient, and condition of the bones. The hardware must also be as small as possible to minimize the effects it has on nearby muscle, bone, and soft tissue. If it is too large, it irritates the tissue which could lead to hardware removal. There are ergonomic concerns as well. The surgeon, for example, must be able to insert, handle, and install the hardware. And as with all implants, especially long-term ones, the hardware must be made of materials the body can withstand.

Meeting strength and biocompatibility issues leaves engineers with two choices for spinal implants, stainless steel or titanium. Titanium is by far the metal of choice. It is strong, lightweight, weighing 56% as much as steel, and it is one of the few materials that bone grows into and on. This adds to the structural integrity of the screws. However, it could be problematic if the surgeon has to remove the screws.

Titanium, like all metals, has the drawback in that it is not translucent to X-rays or MRI scans. So once installed, it can blur or hide anatomical changes. But unlike steel, titanium is non-ferrous, so magnets used in MRI machines will not exert a force on them. At Vertebron, the entire screw assembly and rods are titanium.

Pedicle screws hold the rod clamp or tulip in place. They range from 4.5 to 8.5 mm in diameter and are 25 to 60-mm long. Doctors determine which size to use depending on the person's age and anatomy, as well as the condition of the bone. The screws have self-tapping threads, but doctors can predrill holes, a decision they make based on experience and bone quality. Taps are usually include in a kit that contains all installation instruments. Like most instrumentation kits, Vertebron's are reusable and can be sterilized in an autoclave.

Each screw is slipped through tulip or locking clamp and fastened to the pedicles (outgrowths toward the posterior of each vertebra). To give the doctor flexibility in positioning the screw, the screw and tulip must have as much angulation as possible. Angulation is a measure of how much the screw can pivot in the tulip. It gives doctors options for implanting hardware and bone grafts rather than having the hardware completely dictate placement. Vertebron screws have between 74 and 84° of angulation. The smaller screws have the most angulation. Most pedicle screws from other companies are limited to 50 or 60°.

Engineers at Vertebron also round the bottom or bone side of the tulip to match the anatomy. This lets doctors position the clamp and rod as close to the vertebrae as possible. In effect, this reduces the distance from the center of the rod to the load, i.e., the spine. With Vertebron screw systems, that distance is 10.4 mm.

The titanium rod, the simplest component, has a hex-end fitting machined in one end. The surgeon places the rod in the tulips attached to two pedicle screws, then attaches locking caps to both tulips. The hex fittings let the surgeon turn the rod, which can be curved, to match the anatomy of the spine.

The locking caps initially screw into the tulips with a quarter turn, engaging a pair of wings on the cap. This prevents the surgeon from cross threading the cap into the tulip. At this stage, the locking caps hold the rod in place, but the surgeon can still adjust the rod's orientation. Setscrews in the locking caps firmly fix the rod in place. The wings on Vertebron's locking cap have an inverted buttress taper. This directs all forces generated by the setscrew downward which pulls the tulip head around the rod and prevents head splaying. Splaying can cause the implant to fail. The implant system also includes a cap inserter. When positioned on the tulip, which can be done by feel, the surgeon can attach the locking cap without having to see the tulip or cap. This simplifies the surgery. Another feature that makes the surgery easier is that Vertebron setscrews need only 65 lb-in. of torque to lock in place. Most others need between-90 and 125 lb-in.

It is difficult to update medical devices because the FDA demands that significant changes require extensive requalification. But designers will continue to lower manufacturing costs for spinal implants by reexamining individual parts and assemblies to see if any areas can be modified. They know that incremental decreases in the size or profile will enhance the overall design.

Although spinal fusion is the current treatment of choice for many patients, there is significant research into motion-preservation devices. These devices are intended to preserve the natural motion of the spine while still alleviating the pain and replacing or repairing intervertebral discs.

Vertebron Inc.,

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