PDF Machining of Bone and Hard Tissues

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Like wood, bone is a heterogeneous anisotropic material. It consists of a hard, dense outer layer known as cortical bone with a soft, spongy interior known as cancellous bone. The cortical bone outer layer is composed of osteons, the longitudinal building blocks that provide bone its great strength and rigidity. Cancellous bone, on the other hand, is less dense and is composed of porous osseous tissue. Due to the health risks associated with human and animal tissue, it is rarely permitted to machine bone in a typical workshop. It is however vital that engineers can evaluate the suitability of the cutting tools before they are approved for use by surgeons.

To address these issues, biomechanical test blocks are often used to simulate bone. Both of these materials have their limitations. The former is homogenous and hence does not reflect the longitudinal grain structure of cortical bone; it is not advisable to machine the latter in a typical workshop, as this aerosolizes the glass fiber particles, introducing the risk of respiratory inflammation to anyone within close proximity.

The properties of polyurethane foam are poorly matched to those of bone, with the exception of thermal conductivity for high-density cancellous bone Table 1. The properties of glass fiber reinforced epoxy are in fact very well aligned to those of cortical bone, both along and across the grain. This makes it the ideal material for mechanical tests, yet machinability is still problematic due to the aerosolization of glass fiber particles.

RTI machinists face these same issues. However, their machining environment makes the difficulties associated with these issues unusual indeed. After the donor material arrives, it undergoes a rigorous screening process.

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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 L machining centers using dedicated fixturing and high speed steel or carbide tools.

The bone is now able to serve as an inert scaffold that is absorbed into the body over time.

Cervical Bone Plate with Bumotec Mill Turn

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.

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. 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. 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.

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. Other FE models Lee, Rabin et al.

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Toews and colleagues Toews, Bailey et al. Increasing cortical thickness was also positively correlated with increasing mean maximal temperature. Cortical bone is of a finite thickness, which has definite implications for temperature elevation. One would reasonably surmise that increased axial force and feed rate would produce lower temperatures purely as a function of reduced drilling time. Cordioli and others Cordioli and Majzoub demonstrated a clear relationship between drilling depth and maximum temperature in bovine femurs with 2 and 3mm diameter drills operating at rpm with N applied axial load.

Using cadaveric and bovine bone with markedly different cortical thicknesses Hillery and Shuaib Hillery and Shuaib encountered significantly higher temperatures in bovine bone than in human bone whilst keeping the operational and geometric parameters constant. They attributed this result to the difference in mean cortical thickness between the cadaveric 3 to 5mm and bovine 7 to 9 mm samples. Eriksson et al Eriksson, Albrektsson et al. The differences in this study were also attributed to the difference in cortical thickness between species.

We recently tested the cutting efficiency and thermal profile of commercially available 2- and 3-fluted 3.

From Donor To Shop To Surgery

Despite the finding of improved cutting efficiency for the 3-fluted drills, this did not translate into a significant and parallel reduction in maximum cortical temperatures for both 3-fluted drills either in the presence or absence of external cooling. Many investigators have examined the effects of operational drilling parameters on the maximal temperature experienced during drilling of bone.

A large proportion of these studies have assumed that drilling speed remains constant during drilling and this may not be the case. In orthopaedic surgery external irrigation with sterile saline delivered via a syringe or other device is routinely applied during drilling, the efficacy of which has been demonstrated by several authors Matthews and Hirsch ; Jacob and Berry ; Lavelle and Wedgwood ; Krause, Bradbury et al.

Utilising a numerical model, Lee and co-workers Lee, Rabin et al. Closed-loop and open internal cooling systems are available but are primarily limited to orthodontic and dental applications Haider, Watzek et al. Closed-loop cooling systems are those in which coolant courses through tubules and tunnels incorporated into the drill-bit or bur itself and back through a central heat exchanger. Thermal energy generated at the machining face heats the coolant through a mechanism of conduction, thereby preventing an increase in temperature of the bone to above a critical level.

In open cooling systems fluid courses through tubules in the drill but exits through openings at the cutting tip, thereby absorbing heat but also providing some lubrication in the process of cutting.

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This, of course depends on the precise location of the outlet s in relation to the cutting edges. Strictly speaking, however, the application of coolants by external means does not lubricate the cutting process, as it is applied against the direction of swarf flow. Using an ovine model, Haider and colleagues Haider, Watzek et al.

From Donor To Shop To Surgery : Modern Machine Shop

Interestingly, on the basis of histological results at 4 weeks following implantation it was found that externally applied manual cooling versus internal cooling was more beneficial to the biological response implant bone ongrowth at the cancellous site, but was only advantageous at superficial cortex depths. Internal cooling provided a distinct benefit at the deeper drill levels in compact bone.

At later timepoints of 8 and 16 weeks no appreciable differences were observed between the sites as a function of irrigation type. The result obtained from the cancellous site adds strength to the argument that externally applied coolant may limit maximal temperature elevation even in the advanced stage of drilling Lee, Rabin et al.

From Donor To Shop To Surgery

Admittedly, little research has been directed at the temperature elevation in cancellous bone during drilling, with most if not all research having been conducted in a compact bone bed. Pre-drilling and pilot hole creation are other methods which have been advocated to reduce the biological effects of heat generated during drilling Matthews and Hirsch Using dental burs, however, Reinewirtz et al Reingewirtz, Szmukler-Moncler et al. Sequential drilling at larger diameters is also performed to reduce maximal temperatures Bubeck, Garcia-Lopez et al.

go Despite this there are still surgeons who do not routinely inform their patients of intraoperative drill bit failure. There are no reports in the literature of adverse reactions to portions of broken drill-bit which have been left in situ causing morbidities which have necessitated re-operation for removal, which is representative of the biologically inert nature of the materials used in drill-bit manufacture.

A by-product of the drilling process is the generation of heat energy, which causes a transient increase in temperature of the bone and soft tissues as well as the drill-bit itself. Despite an acute awareness of the association between drilling and temperature rises in bone there are few reports in the clinical orthopaedic literature of complications or implant failures which could be attributed to this. One notable exception was a recent report by Berning et al Berning and Fowler presenting a case of a patient having osteomyelitis of the proximal tibia due to thermal necrosis following tracker pin placement in computer-navigated total knee arthroplasty.

In fact, it appears that most of the evidence in the literature pertaining to thermonecrosis could most suitably be described as being anecdotal.