Laminectomy defects heal in a fashion that is similar to that which occurs in long-bone fractures: organization of a hematoma that fills the defect and formation of a fibrous callus or scar with a varying extent of metaplasia of this callus into the cartilage and bone. This normal consequence of vertebral healing is called a “laminectomy membrane,” “epidural fibrosis,” or “postlaminectomy scar.”

In human patients, perineural fibrosis developing after laminectomy for lumbar disc disease is a common cause of postoperative pain or neurological deficits [1]. In dogs, the potential for postoperative compression of the spinal cord is the major limiting factor for the extent of bone removal during laminectomy. A variety of methods for preventing epidural fibrosis have been studied, including free or pedicle fat grafts, absorbable gelatin films and sponges, and cellulose products. Despite numerous studies, there is no clear consensus on which technique consistently reduces epidural fibrosis in dogs and human patients. This chapter reviews the formation of laminectomy membrane and discusses some of the materials used to cover the laminectomy after thoracolumbar disc surgery. We also address the topical use of analgesics in conjunction with disc surgery.

Healing of laminectomies

Healing of a laminectomy starts with the formation of a hematoma that completely fills the laminectomy defect and is in contact with the adjacent epaxial muscles. By 1 week after surgery, the hematoma organizes by infiltration of fibroblasts from the adjacent epaxial muscles. A thick fibrous scar gradually forms, starting from the epaxial muscles and extending over the dura mater and nerve roots. By 2 weeks, this fibrous connective tissue completely bridges the defect, resulting in a dense, tough membrane called the laminectomy membrane [2]. Woven bone starts forming at the cut surfaces of the vertebral bone, and by 4 weeks, most of the woven bone has been replaced by lamellar bone. Between 8 and 16 weeks, there is further maturation of connective tissue, and bony proliferation varies from minimal to completely bridging the defect [3].

It is important to realize that all laminectomies heal by the formation of a laminectomy membrane. Three factors are responsible for the extent of laminectomy membrane: the destruction of epidural fat, epidural hematoma, and the invasion of muscle fibers from the epaxial muscles into the defect. The extent of peridural fibrosis is directly proportional to the size of the laminectomy defect [2]. Whether the laminectomy membrane compresses the spinal cord is related to the location and degree of bone removal. For example, with an extensive laminectomy called a Funkquist type A laminectomy, the lamina, articular facets, and pedicles are removed bilaterally down to a level at the middle of the spinal cord. With the Funkquist type A laminectomy, the spinal cord often becomes compressed by the laminectomy membrane. This complication is prevented by a Funkquist type B laminectomy, which preserves the facets and lamina/pedicles to a level dorsal to the cord. This prevents late spinal cord compression but severely limits exposure [3]. A modification to the type B laminectomy has been described where the inner cortical bone of the pedicles and medial articular facet are removed, thus still preventing secondary compression from laminectomy membrane but improving exposure and room to manipulate surgical instruments in the vertebral canal [4]. Neural compression due to laminectomy membrane is uncommon with hemilaminectomy or similar procedures such as pediculectomy [5]. Brown et al. found that 3 of 187 (1.6%) dogs with thoracolumbar disc surgery developed spinal cord compression from a laminectomy membrane after hemilaminectomy and 3/3 had resolution of signs with removal of the laminectomy membrane [6]. However, the precise incidence of this complication is unknown because many dogs with persistent deficits after surgery do not undergo follow-up imaging to document the presence or causes of persistent or recurrent spinal cord compression.