Bone Mechanics: What Is the Force Sense Behind a Master Surgeon's "God Hands"?

Naoki Takano (Professor, Department of Mechanical Engineering)

The spine is composed of stacked vertebral bodies (Fig. 1) and intervertebral discs. The surface of the bone is made of dense cortical bone, but its interior consists of porous cancellous bone, which is described as sponge-like (as shown in Fig. 2), and its cavities contain vital bone marrow. When observed with a micro-CT scanner, cancellous bone, as shown in Figure 2, has a three-dimensional network structure of interconnected trabeculae, each approximately 150 μm (*) thick.

(*) μm (micrometer) is one-thousandth of a millimeter.

Figure 2 shows the lumbar cancellous bone of a patient with osteoporosis, a condition where bones become porous and prone to fracture. In this example, the bone density in the cancellous bone region is only about 7%. One of the roles of bone is to support body weight and withstand the forces applied during movement. Similarly, even at the micrometer scale, the trabeculae bear a portion of the load. Figure 3 shows the calculation results, with the trabeculae responsible for supporting body weight highlighted in blue.

The decrease in bone strength in the zero-gravity environment of space became a topic of discussion during the long-term stays of Japanese astronauts on the space station. However, the evaluation of "bone quality," which cannot be explained by bone density alone, has become a challenge. Medical-engineering collaborative research on bone that incorporates mechanics is leading to advancements in areas such as predicting fracture risk in osteoporosis and evaluating the effectiveness of preventive and therapeutic drugs.

Additionally, as is evident from orthodontic treatment, the jawbone is also deeply related to mechanics. As shown in Figure 4, cancellous bone also exists in the jawbone. Figure 5 shows the results of a calculation of how forces are transmitted from a dental implant—a treatment for tooth loss—to the cancellous bone. These results have provided new insights into the role of the trabeculae surrounding the implant and bone metabolism.

Now, placing a dental implant requires drilling into the cancellous bone of the jaw. Care must be taken not to damage blood vessels and nerves within or around the jawbone. In surgeries that depend on "bone quality," which cannot be determined from X-rays or CT scans alone, the repulsive force felt by the surgeon's hand becomes the decisive factor. It is very difficult for a master surgeon with "God hands" to express in words and teach the force sense they feel during surgery. Therefore, we are also developing a drilling simulator that predicts the repulsive force during drilling through calculations, allowing users to experience the sensation of "God hands." If trainees can learn from master surgeons through simulated experiences using this simulator, we can expect significant progress toward ensuring medical safety.