Cracked Theory for Taking Screws from the Spine
Doctors sometimes use screws and metal implants to hold bones together. This is a useful treatment approach for fractures of the spine bones. There are many possible problems with the use of these devices.

Once the bone has healed, the implants are generally left in. This is because there is an equal risk of problems when putting them in and when taking them out. There can be infection, metal toxicity, corrosion, tumor formation, or loss of bone around the implant.

Doctors may consider removing the screws if the patient complains of pain or discomfort. Physicians in Switzerland advise caution in making this decision. Two cases of screw removal resulted in fracture of the spine two or three days later.

Both patients had severe osteoporosis (brittle bones). Both had worse pain after the screws were taken out than before the operation to remove them. Other reports say that mild back pain may continue in patients who have the implants successfully removed.

Some doctors feel that spinal implants shouldn't be removed without good cause. Mild pain or local discomfort isn't enough of a reason. Loose or broken screws should be removed or repaired. Imaging studies may be used to make this decision. If there is significant bone disease such as osteoporosis, the implants should probably remain untouched.
References
Beat Waelchli, et al. Vertebral Body Compression Fracture After Removal of Pedicle Screws: A Report of Two Cases. In European Spine Journal. October 2002. Vol. 11. No. 5. Pp. 504-506.