Every chip is precious at Regeneration Technologies, Inc. Rather than billets of metal alloy, the material RTI uses to produce its precision machined grafts is retrieved from donated human tissue. Located in Alachua, Florida, RTI processes bone, cartilage, tendons and ligaments to make implants used to repair and promote the healing of a variety of bone and tissue defects, for procedures including musculoskeletal reconstructions, mending bone fractures and repairing spinal vertebrae. These implants of human tissue from one person to another are called "allografts."

RTI aims to reduce surgery time and improve convenience for surgeons. These aims are best accomplished by using human tissue (and soon, bovine tissue) that is shaped appropriately for the surgical procedure. The shaping is accomplished through CNC machining.

RTI applies CNC machining primarily to spinal allografts, including blocks, wedges, threaded dowels and rings used in cervical and lumbar procedures. The company's Cornerstone ASR Cortical-Cancellous Block provides an example. This "engineered allograft" is multilayer part made up of two cortical bone wedges (the outer hard part of bones) around a central, faster-healing cancellous bone section (the inner spongy part of bones). The three pieces are held together using press-fit bone pins.

While the material and products are unusual, RTI's machine shop is similar in many ways to more traditional shops. The company meets production deadlines, accommodates design changes, solves fixturing problems, provides prototypes and addresses a host of other concerns common to production.

Many of the differences that do exist are a mailer of degree. For example, every machine shop is familiar with the importance of process work flow, the need for work area cleanliness and the challenges posed by batch-to-batch material variations. RTI machinists face these same issues. However, their machining environment makes the difficulties associated with these issues unusual indeed.

Work Flow

After the donor material arrives, it undergoes a rigorous screening process. The tissue is removed from the bone using a wire brush wheel or scalpel. The bone (such as a femur) is then sectioned into allograft blanks using a band saw. This process requires a fair amount of skill, with the experienced band saw operator planning the cuts in order to maximize the yield from a given bone.

The blanks are then machined using CNC lathes and mills. Pins are turned and sized using OmniTurn GT-Jr lathes, while milling is performed on Fadal 904-1L machining centers using dedicated fixturing and high speed steel or carbide tools. After any required assembly steps, the bone is sterilized using RTI's "BioCleanse" process, which eliminates any bacteria, fungi, spores and viruses (and also leaves the bone appearing perfectly white). The bone is now able to serve as an inert scaffold that is absorbed into the body over time. In fact, once the surrounding bone has had a chance to grow into the grafted bone (many months after surgery), the implant is nearly impossible to detect in an X-ray. Lyophilization (similar to freeze drying) follows BioCleanse, and this process allows the final parts to be stored at the hospital without refrigeration. Packaging and inspection complete the production process.

Work Area Cleanliness

Protective clothing and related precautions also take on a different dimension for RTI machinists. Employees wear personal protection equipment, protecting both the bone from the machinist and (potentially) the machinist from the bone. Also, neither personnel nor equipment enter or leave the controlled production area "core" without cleaning or sterilization. All tooling is first treated with disinfectant, then it is wiped with isopropyl alcohol and dried before it can be taken into the clean area. For cutting fluid, RTI uses only isopropyl alcohol, a choice that is able to serve as a disinfectant, a lubricant assisting in chip removal and a drying agent.

Finally, for safety purposes, machinists are instructed to treat all tissue as though it were infected. If an injury were to occur in the core, then the machinist would be escorted out for medical care and tested for infection.

Batch-To-Batch Material Variations

For a more conventional shop, variations in metallic stock can include differences in machinability and mechanical properties, as well as machined part distortions related to residual stresses.

In a similar manner, RTI's donors differ in age and sex, leading to variations in density, elastic modulus and yield strength. Even for one bone from a single donor, the properties can vary from one point to another. RTI addresses this problem by weighing the tissue prior to machining. This gives an indication of the bone density, which has been shown to be closely related to the material strength. If a minimum weight is not met, then the tissue is rejected.