Is titanium alloy screw harmful to human body?

No.
Titanium is biophile. In the human body, can resist the corrosion of secretions and non-toxic, to any sterilization methods are adapted. Therefore, it is widely used in the manufacture of medical instruments, such as artificial hip joint, knee joint, shoulder joint, hypochondriac joint, cranium, active heart flap and bone fixation clip. As new muscle fibers wrap around these "titanium bones," they begin to support the body's normal activities.
Titanium is widely distributed in the human body, with normal amounts of no more than 15mg per 70kg of body weight, and its role is unclear. But titanium has been shown to stimulate phagocytes, boosting immunity.
Advantages of titanium alloy:
1, high strength, titanium alloy density is generally about 4.51g/ cubic centimeter, only 60% of steel, pure titanium density is close to the density of ordinary steel, some high strength titanium alloy exceeds the strength of many alloy structural steel.
2, high thermal strength, the use of temperature is several hundred degrees higher than aluminum alloy, at moderate temperature can still maintain the required strength, can be in 450 ~ 500℃ temperature for a long time working these two types of titanium alloy in the range of 150℃ ~ 500℃ still has a high specific strength, and aluminum alloy at 150℃ specific strength decreased significantly. The operating temperature of titanium alloy can reach 500℃, and that of aluminum alloy is below 200℃.
3, good corrosion resistance, titanium alloy working in the humid atmosphere and seawater medium, its corrosion resistance is far better than stainless steel; Resistance to pitting corrosion, acid corrosion, stress corrosion is particularly strong; Excellent corrosion resistance to alkali, chloride, chlorine, nitric acid, sulfuric acid, etc. But the corrosion resistance of titanium to reducing oxygen and chromium salt media is poor.