It's no surprise that surgically implanting devices into the human spine calls for cool heads and steady hands. Most spinal surgeries are done close to nerves so a millimeter one way or another is crucial. Some sources say up to 25% of implants are misaligned in scoliosis operations. Misalignments are associated with an increased risk of neural and vascular complications, as well as injuries to the spinal meninx.

High-precision guidance for drilling during spinal surgeries may avoid those downsides, thanks to a miniature hexapod. SpineAssist, from Mazor Surgical Technologies, Norcross, Ga., (mazorst.com) has six degrees of freedom, letting doctors stabilize spinal fusions with implants in open and minimally invasive operations. The pod has rigid-bone-fixation clamps and platforms, and surgical-planning software that imports x-ray and CT images.

Spinal fusion is often used to straighten crooked spines and counteract progressive deformations from diseases such as scoliosis. It can also support weakened or damaged spines, or counteract and alleviate pain from pinched and degenerated nerves.

The robot assists this way: After examining CT scans and planning the surgery, doctors would rigidly attach the hexapod's platform to the patient's spine and calibrate its position by aligning it based on x-rays and CT scans. Surgeons attach the SpineAssist robot to the platform and position the robot's guide arm based on the presurgical plan, and drill.

Minimally invasive surgery means smaller incisions, less scarring, and fewer risks of infections and hemorrhages. Post-operative pain and trauma are lower, and hospital stays and recovery time are shorter. The SpineAssist also requires fewer x-rays than other procedures, so surgeons and patients are exposed to less radiation.

The hexapod robot is about the size of soda can, (50-mm dia. ×80-mm high) and weighs 250 g. Its work volume is several cubic centimeters and depends on the guide arm. Accuracy and precision is less than 100 µm and 10 µm, respectively.

The hexapod is constructed of precise miniature spindles. Electronic controls from Faulhaber Clearwater, Fla., (micromo.com), drives linear actuators powered by six brushless dc drives from the company's Smoovy line of motors. Each actuator has an LVDT sensor, and a seventh monitors the performance of the other six.

Building the hexapod was challenging because of tight tolerances on its small dimensions and the precision needed for actuator ball joints. Cooperation between Micro Precision Systems AG in Switzerland (smoovy.com) and Mazor Surgical Tech, was initially limited to the Smoovy dc motor developed by MPS, but the company is now responsible for production, assembly, and quality control of the entire robot.