<unk>  is an abnormal response of the immune system to foreign biomaterials and alloplastic materials. It is most discussed in the context of complications from breast implants and artificial joint prosthetics.
Capsules of tightly-woven collagen fibers normally form as an immune response around a foreign body (eg. breast implants, pacemakers, orthopedic joint prosthetics), tending to wall it off.<unk>  occurs when the capsule tightens and squeezes the implant. This<unk>  is a complication that can be very painful and distort the appearance of the implanted breast or limit the range of motion of a artificial joint.  The exact cause of<unk>  is not known.  However, some factors include bacterial contamination, silicone breast implant rupture or leakage, and hematoma.<unk>  may happen again after additional surgery.
Methods which have reduced<unk>  include submuscular breast implant placement, using textured or polyurethane-coated implants, limiting handling of the implants and skin contact prior to insertion and irrigation with triple-antibiotic solutions.
Correction of<unk>  may require surgical removal or release of the capsule, or removal and possible replacement of the implant itself.  Closed capsulotomy (disrupting the capsule via external manipulation), a once common maneuver for treating hard capsules, has been discouraged as it can cause implant rupture.  Nonsurgical methods of treating capsules include massage, external ultrasound, treatment with leukotriene pathway inhibitors (Accolate, Singulair), and pulsed electromagnetic field therapy.
There are four grades of breast<unk>  - Baker grades I through IV. The Baker grading is as follows:
Grade I 	the breast is normally soft and looks natural
Grade II 	the breast is a little firm but looks normal
Grade III 	the breast is firm and looks abnormal
Grade IV 	the breast is hard, painful, and looks abnormal.
Capsular contracture
Periosteal reaction
Disc slip
Pseudarthrosis
Osteolysis
Stasis 
Injection fibrosis
Spinal cord compression