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Elastomeric intervertebral disc prosthesis

Elastomeric intervertebral disc prosthesis description/claims

This application is a division of U.S. application Ser. No. 11/334,423 filed Jan. 19, 2006. This application also claims the benefit of U.S. Provisional Application No. 60/644,527, filed Jan. 19, 2005, the entire disclosure of which is incorporated herein by reference, and the benefit of U.S. Provisional Application No. 60/693,430, filed Jun. 24, 2005, the entire disclosure of which is incorporated herein by reference.

1. Field of the Invention

This invention relates to intervertebral disc prostheses and more particularly to intervertebral disc prostheses having rigid endplates and an elastomeric core.

2. Background Art

Low back pain is a very common pathological condition, affecting approximately 80% of the general population at some time. Although most of patients experience the painful symptoms only occasionally and recover fully, approximately 10% of these patients come to experience chronic and disabling low back pain in spite of various medical treatments.

The most common cause of chronic disabling low back pain is degeneration of one or more of the intervertebral discs that are positioned between the vertebrae of the spine and permit the various natural movements of the spinal column. Such degenerative disc disease (DDD) may become intractable to non-surgical treatment and have to be treated by surgical intervention. Spinal fusion has been a traditional and generally effective treatment method for chronic disabling low back pain that is not responding to non-operative treatments. More recently, alternative treatments involving replacement of the entire disc or its nucleus have been developed for treatment of discogenic pain.

The first generation of prostheses for replacement of degenerated intervertebral discs has generally incorporated mutually sliding surfaces of relatively hard materials to provide for the required intervertebral motion in flexion, extension, lateral bending and torsion. Although such prostheses have been found to be helpful, improvements in shock absorption and replication of the natural motion of the intact intervertebral disc have been sought.

Accordingly, subsequently developed prostheses have incorporated elastomeric members in order to provide for the required motion and shock absorption. Such prostheses typically include relatively hard endplates for contacting the endplates of adjacent vertebrae and fixing the prosthesis thereto, together with an elastomeric disc core, positioned between the hard endplates and fastened thereto.

However, in conventional designs of such intervertebral disc prostheses, the bone-contacting members, i.e., rigid endplates, typically have a and shape and size in a horizontal plane that conforms generally to the shape and size of the vertebral endplate; and the elastomeric element, positioned between the prosthesis endplates, also typically has a similar and shape and size. When such a prosthesis is subjected to stresses induced by bending of the spinal column, e.g., flexion, the elastomeric material at the periphery of the prosthesis may be compressed between the hard endplates and caused to bulge outwardly. Such deformation of the elastomeric component in repeated flexion may lead to eventual failure of the prosthesis. In some known prostheses, the outer periphery of the elastomeric core is provided with a concavity of the lateral wall to reduce the fixation stress in the peripheral region where the elastomer interfaces with the rigid, e.g., metal, endplates. However, even such a structure may be subject to eventual failure.

The present invention has been devised in view of the above background.

According to the invention, an intervertebral prosthesis is provided having generally rigid endplates for fixation to the upper and lower vertebrae of a spinal motion segment and an elastomeric core fastened between the endplates wherein at least an antero-posterior dimension of the interface between the core and at least one of the endplates is less than the antero-posterior dimension of the endplate. The lateral dimension of the interface between the core and at least one of the endplates may also be made smaller than the lateral dimension of the endplate.

Accordingly, it is an object of the invention to provide an intervertebral disc prosthesis having rigid endplates and an elastomeric core.

A further object is to provide such an intervertebral disc prosthesis wherein stress between the elastomeric core and the rigid endplates is reduced.

A further object is to provide an intervertebral disc prosthesis which is less prone to failure in use.

A further object is to provide an intervertebral disc prosthesis wherein the resistance to motions in flexion-extension, lateral bending, and torsion may be readily controlled.

According to one of its principal aspects, the present invention provides an intervertebral disc prosthesis for implanting between adjacent vertebrae in a spinal motion segment. The prosthesis comprises an upper rigid prosthesis endplate for fixation to an adjacent upper vertebra, and having a periphery, an antero-posterior dimension, and a transverse dimension; a lower rigid prosthesis endplate for fixation to an adjacent lower vertebra, and having a periphery, an antero-posterior dimension, and a transverse dimension; and an elastomeric core structure located between the prosthesis endplates and attached to the endplates. The elastomeric core structure includes at least one core member and has a total cross-sectional area in a horizontal plane and durometer hardness sufficient to provide sufficient compressive strength to support physiological axial loads.

According to one preferred feature, the elastomeric core structure has at least an average antero-posterior dimension, sufficiently less than at least one of the upper prosthesis endplate antero-posterior dimension and the lower prosthesis endplate antero-posterior dimension, such that the elastomer core does not protrude beyond the periphery of one of the prosthesis endplates during normal flexion and extension of the spinal motion segment.

According to another preferred feature, the core member has an antero-posterior dimension not greater than three times an axial height dimension of the core member.

• Provisional Patent
• Utility Patent

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