An Overview of the Histology of Skeletal Substitute Materials
September 20, 2007 – 12:12 pmOrthopedic and spine surgeons are in frequent need of bone for skeletal reconstruction.The amount of autograft is limited, and conventional allograft has some disadvantages, so surgeons are now using increasing amounts of demineralized allograft and a variety of synthetic materials to replace or ”extend” autograft.
Objective.-To provide an overview of the composition and histology of the materials most likely to be seen by pathologists today.
Data Sources.-The review is based on published literature and the author’s experience with preclinical studies and human biopsies.
Orthopedic and spine surgeons are in frequent need of bone for skeletal reconstruction.The amount of autograft is limited, and conventional allograft has some disadvantages, so surgeons are now using increasing amounts of demineralized allograft and a variety of synthetic materials to replace or ”extend” autograft.
Objective.-To provide an overview of the composition and histology of the materials most likely to be seen by pathologists today.
Data Sources.-The review is based on published literature and the author’s experience with preclinical studies and human biopsies.
Orthopedic surgeons and neurosurgeons involved in skeletal repair and reconstruction often encounter bone defects that are unlikely to heal if treated by fixation alone. Autograft bone has been used for decades to supplement host repair, but the amount of autograft is limited, and morbidity related to autograft harvesting can be considerable.1 Surgeons commonly use allograft, and they recently have sought synthetic materials that could replace or be mixed with bone graft. Skeletal substitute materials are often needed in spinal fusion, filling voids around failed total joint prostheses, in fracture repair or repair of a nonunion, or after excision of a tumor. These bone graft preparations and graft substitutes may be encountered in subsequent biopsy or resection specimens, and it is important for pathologists to recognize these materials and distinguish them from an unexpected inflammatory response, residual or recurrent tumor, or other foreign material. Histology is key to defining the biocompatibility and bioactive properties of these materials. Pathologists can play an important role in helping recognize complications and defining the most appropriate material for a given clinical application. The purpose of this review is to provide an introduction to the histology of bone graft preparations and graft substitute materials, and to briefly describe the properties of each family of materials that might offer advantages for certain types of skeletal defects.
Several important terms are used to describe the properties of bioactive orthopedic materials. A material is described as osteoconductive if, due to its composition, shape, or surface texture, it promotes bone formation along its surface when it is placed in bone. A material is described as osteoinductive if it induces bone to form in an extraskeletal site, such as within skeletal muscle. A material is osteogenic if it causes bone formation because of the implantation of viable cells, such as autograft or aspirated bone marrow osteoprogenitor cells. All of these terms are relative. For example, a coating of hydroxyapatite (HA) on a titanium total joint implant is more osteoconductive than titanium alone, but titanium is more osteoconductive than cobalt chromium alloy or polymethyl methacrylate bone cement. Understanding these bioactive properties is critical to determining the most appropriate clinical application of a skeletal substitute material.2
Skeletal substitute materials, and orthopedic devices in general, usually show the most bone formation (most favorable histologic appearance) when they are harvested as part of a prospective animal study in which optimum surgery has been combined with an appropriate clinical application without complications. Surgical pathologists occasionally have the opportunity to study clinically successful cases that have been biopsied or retrieved at autopsy, 3 but more frequently we see biopsy or resection specimens obtained because of a clinical failure or complication. 4 Therefore, the histology commonly seen in human specimens may not be representative of findings that occur when that material is associated with a clinically successful outcome. Nevertheless, even the evaluation of clinically failed cases can provide useful information about safety and efficacy. The descriptions that follow will include examples of preclinical studies to illustrate the findings that are thought to occur in clinically successful human applications, as well as human specimens, most of which have been identified after biopsy or excision of a clinical failure. This review is not intended to be a comprehensive comparison of available materials, but is instead an introduction to the histologic appearance of skeletal substitute materials most likely to be seen by pathologists as they evaluate biopsies and skeletal resection specimens .
Bone graft materials in general include autograft (autogenous bone and/or aspirated cells), allograft, xenograft, synthetic materials, and combinations thereof. The gold standard for bone grafting is autograft harvested from the iliac crest, but surgeons also can make use of chips of autologous bone collected at the time of another procedure. For example, spine surgeons commonly harvest bits of decorticated transverse process, facet joints, and vertebral bodies at the time of a spinal fusion operation. Similarly, bone from a resected femoral head may be harvested and used to fill an acetabular defect at the time of total joint replacement. Segments of rib or fibula, sometimes with intact blood vessels, are used for some procedures that require immediate structural support. Few studies have attempted to compare the quality of autograft bone harvested from different sites. Cancellous autograft is incorporated into new bone relatively rapidly, but larger segments of cortical autograft-for example, a fibular autograft used to reconstruct a necrotic femoral head-may persist for years.
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