Canter pulled a stool close to an operating microscope equipped with a camera. He moved aside a tray of fractured ribs and thyroid cartilage that he must have been working on before I arrived.
‘This guy was kicked in the throat, among other things,’ he absently said as he pulled on surgical gloves.
‘Such a nice world we live in,’ I commented.
Canter opened the Ziploc bag containing the segment of right femur. Because he could not fit it on the microscope’s stage without cutting a section that was thin enough to mount, he had me hold the two-inch length of bone against the table’s edge. Then he bent a twenty-five-power fiber optics light close to one of the sawn surfaces.
‘Definitely a Stryker saw,’ he said as he peered into the lenses. ‘You got to have a fast-moving, reciprocating motion to create a polish like this. It almost looks like polished stone. See?’
He moved aside and I looked. The bone was slightly beveled, like water frozen in gentle ripples, and it shone. Unlike other power saws, the Stryker had an oscillating blade that did not move very far. It did not cut skin, only the hard surface it was pressed against, like bone or a cast an orthopedist cut from a mending limb.
‘Obviously,’ I said, ‘the transverse cuts across the midshaft are mine. From removing marrow for DNA.’
‘But the knife marks aren’t.’
‘No. Absolutely not.’
‘Well, we’re probably not going to have much luck with them.’
Knives basically covered their own tracks, unless the victim’s bone or cartilage was stabbed or hacked.
‘But the good news is, we got a few false starts, a wider kerf and TPI,’ he said, adjusting the microscope’s focus as I continued holding the bone.
I had known nothing about saws until I began spending so much time with Canter. Bone is an excellent surface for tool marks, and when saw teeth cut into it, a groove or kerf is formed. By microscopically examining the walls and floor of a kerf, one can determine exit chipping on the side where the saw exited bone. Determining the characteristics of the individual teeth, the number of teeth per inch (TPI), the spacing of them and the striae, can reveal the shape of the blade.
Canter angled the optic light to sharpen the striations and defects.
‘You can see the curve of the blade.’ He pointed to several false starts on the shaft, where someone had pushed the saw blade into the bone, and then tried again in another spot.
‘Not mine,’ I said. ‘Or at least I hope I’m more adept than that.’
‘Since this also is the end where most of the knife cuts are, I’m going to agree that it wasn’t you. Whoever did this had to cut first with something else, since an oscillating blade won’t cut flesh.’
‘What about the saw blade?’ I asked, for I knew what I used in the morgue.
‘Teeth are large, seventeen per inch. So this is going to be a round autopsy blade. Let’s turn it over.’
I did, and he directed the light at the other end, where there were no false starts. The surface was polished and beveled like the other one, but not identical to Canter’s discerning eye.
‘Power autopsy saw with a large, sectioning blade,’ he said. ‘Multidirectional cut since the radius of the blade’s too small to cut through the whole bone in one stroke. So, whoever did this just changed directions, going as it from different angles, with a great deal of skill. We have slight bending of the kerfs. Minimal exit chipping. Again denoting great skill with a saw. I’m going to bump up the power some and see if we can accentuate the harmonics.’
He referred to the distance between saw teeth.
‘Tooth distance is point-oh-six. Sixteen teeth per inch,’ he counted. ‘Direction is push-pull, tooth-type chisel. I’m voting this is yours.’
‘You caught me,’ I said with relief. ‘Guilty as charged.’
‘I would think so.’ He was still looking. ‘I wouldn’t think you use a round blade for anything.’
The large, round autopsy blades were heavy and continuous rolling, and destroyed more bone. Generally, this was a utility blade used in labs or in doctors’ offices to saw off casts.