fusion that is incorporated into the native bone in the spine is very similar to the pro-cess of bone healing after fracture. 2. The process has three stages. a. Early inflammatory stage (weeks 1–2)(1) A micro-hematoma forms in the bone.(2) Inflammatory changes occur at the area of the graft.(3) Inflammation promotes granulation.(4) The use of antiinflammatory medi-cations or cytotoxic drugs during this period may inhibit bone healing and fusion formation. (5) With vascular ingrowth progres-sion, a collagen matrix is laid down and a soft callus forms (Pilitsis, Lucas, & Rengachary, 2002).b. Repair stage (weeks 2–6)(1) Development of vascular and capil-lary supply to the new bone.(2) Collagen and callus bone forms.(3) The bone formed during this period is very weak for 4–6 weeks, requir-ing either internal fixation (instru-mentation) or bracing.(4) Use of nicotine during this period can inhibit capillary growth and result in failure of the bone to heal.c. Late remodeling stage (slow process, requiring months to years)(1) Restoration of the original bone shape, structure, and mechanical strength occurs.(2) As the patient places axial forces on the new bone, additional bone forms in the areas of stress. Conversely, the areas that do not receive stress will resorb.E. Wolff’s lawWolff’s law states that bone placed under com-pressive stress is remodeled. Bone is formed where stresses require its presence and resorbed where stresses do not require it (Kalfas, 2001). Thus, when a bone graft is placed, it needs mechanical compressive stress so that new bone will be formed, thereby “healing” the bone.F. Limitations to proper bone healingA number of factors may negatively affect proper bone healing: anti-inflammatory, cyto-toxic, and steroid medications during the early inflammatory stage; nicotine use; radiation; and systemic illnesses (e.g., diabetes mellitu
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