SCLEROTIC BONE AND IMPLANT FAILURE

Sclerotic Bone Failure And How It Relates To Dental Implantology

All non-biocompatible bone graft materials such as freeze dried bone allografts and low temperature bone xenografts (Bio-Oss) produce sclerotic bone. Sclerotic bone is also produced throughout the skeleton as a result of inflammation. The source of the inflammatory process in joints is most commonly mechanical abrasion or compression producing an inflammatory response resulting in pathologic bone defined as sclerotic bone. Sclerotic bone is high in mineral content but lacks the mechanical strength of normal bone. In addition sclerotic bone is produced by osteoblasts that are abnormal in morphology, physiology and function. The bone is called sclerotic because the word sclerosis refers to a tissue that is unable to adapt. This inability to adapt is evident in the absence of osteoblasts and osteoclasts in sclerotic bone that allows bone to remodel and repair and adapt to changing loads. Allografts and low temperature xenografts produce sclerotic bone as a result of an immune response to a foreign body. The process sclerotic bone formation as a result of an immune response has been termed by our group as antigenic ossification.

Implants placed in sites grafted with allografts or low temperature xenografts appear to function normally until complete failure of the grafted site. While this might seem like an abrupt occurrence research in the failure of sclerotic bone in joints indicates that the failure of sclerotic bone is a gradual process that builds until a complete clinical failure occurs. The following images document the process of sclerotic bone failure in the joints of race horses and it is proposed that the same process occurs in the failure of implants placed in sclerotic sites grafted with allografts or low temperature xenografts.

Radiographs of 4 mm condylar slices used for SEM. These show the spectrum of changes studied. A: Minimal/mild sclerosis. B: Severe sclerosis. C: Severe sclerosis with focal radiolucency/rarefaction. D: Advanced lesion with subchondral bone failure consisting of collapse with microfracture in more dense bone (arrow).

Unaffected subchondral bone site from a sample in the group with mild sclerosis.

Bone with microfracture lines and early collapse in a sample from the group with severe sclerosis and focal rarefaction. Note multiple cracks and fragmentation of bone with formation of gaps within fracture lines.

Higher magnification of a microfracture gap with entrapped vessel. Note smooth edges on ground fragments indicating chronic wear on the surfaces.

An interesting finding of the physical analysis found that when sclerotic bone was loaded it did not absorb the load but transmitted the load to the underlying normal trabecular bone.

In conclusion failure of allografts and low temperature xenografts appear to develop over time. First starting with microfractures that are unable to be repaired. The microfractures develop into the fragmentation of the sclerotic bone with formation of gaps within fracture lines. The bone in the area of the gaps grind upon each other until complete collapse of the sclerotic bone occurs as seen as radiographic rarefaction and the implant fails. These images were taken from J Musculoskelet Neuronal Interact 2006; 6(3):251-257 Subchondral bone failure in overload arthrosis: A scanning electron microscopic study in horses R.W. Norrdin, S.M. Stover with permission.