A spine of a vertebrate animal, including a human, has a space between adjacent vertebrae, referred to as the “inter-body space” of the spine. This space is occupied by a disk including an annulus of firm material, surrounding a moist, mushy central material that is called the nucleus. This space separates and cushions adjacent vertebrae. Medical conditions of degenerative disk disease that cause severe back pain include herniated nucleus pulposis and collapse disk space that cause compression of spinal cord roots. Intervertebral spinal fusion is a surgical technique that often relieves back pain arising from these medical conditions, by removing an affected disk, fusing two adjacent vertebral bodies together and by inserting one or more implants that allow bone to grow between the two fused intervertebal bodies where a disk has been removed. In the field of prosthetic implants, the materials used include bone grafts (autografts, allografts and xenografts) and implants produced by non-bone materials, such as stainless steel, titanium, ceramics and plastics. Success or failure of a bone graft or non-bone implant depends in part on whether the insert remains at the implant location, whether the implant is cellularized to induce new bone formation, and whether the insert can withstand the resulting mechanical load and remain stable. Inter-body fusion between two adjacent vertebral bodies in a space occupied by a disk is recognized and encouraged for good biomechanical, neurophysiological and anatomical practices.
Lumbar interbody fusion is performed from an anterior or posterior location, although transversely oriented positions are used as well. In posterior lumbar interbody fusion (PLIF), surgeons have traditionally used an open approach to perform a spinal fusion procedure. This involves making an incision along a center line on the back, stripping bands of back muscles from the spine, and pulling or retracting the muscles to each side of the opening so that the spine and vertebra can be viewed and accessed for device implantation. The main benefit of a PLIF procedure is the degree of exposure and accessibility provided for the surgeon. However, several studies have shown that extensive surgical exposure for extended time periods (hours) can seriously injure or degrade the major back muscles and can cause post-surgical pain. Also, two implant spacers usually need to be surgically inserted from two posterior portals.
Transforaminal lumbar interbody fusion (TLIF) is an improvement of the PLIF procedure in which the bone spacer is inserted from a unilateral approach posteriolaterally, without having to forcefully retract the nerve roots. TLIF minimizes the amount of surgical dissection required to access the intervertebral space and preserves more of the posterior elements. In a TLIF procedure, several traditional PLIF implant devices, such as threaded titanium or stainless steel cages, allograft wedges and rings have been used, with mixed results. However, these devices are not easily inserted into the affected disk space and may produce temporary or permanent nerve damage. Use of a threaded titanium or stainless steel cage requires drilling and cutting into vertebral endplates.
U.S. Pat. No. 5,769,897, issued to Harle, discloses improvements in artificial bone material, such as bio-ceramics, that can be used to form segments of or entire artificial bones for implantation. According to the '897 patent, synthetic bone has two components: (1) a first component to sustain the required mechanical strength and (2) a second component to enhance bio-integration with natural bone tissue. As illustrated in FIG. 1A, the first component has at least one accessible void.
U.S. Pat. No. 5,888,227, issued to Cottle, discloses an inter-vertebral implant having a frame-line cage that encloses a cavity and has perforated cover and face bases as bone contacting surfaces, illustrated herein in FIG. 1B. These background references indicate the significance of osteo-induction for new bone formation after implantation but require inclusion of an artificial substance (metal, bio-ceramic, etc.) to provide the required mechanical strength and stability. However, an implant system based on natural bone substances may not withstand the estimated 500-1500 Newtons/cm2 pressure to which portions of a spine are subjected.
U.S. Pat. No. 6,458,158, issued to Anderson et al, discloses a composite bone graft, including a cancellous (porous) bone located between two cortical bones, with one or more cortical bone pins to hold a three-piece structure together, as illustrated in FIG. 1C. The cancellous bone is positioned to promote osteo-inductions for bone growth. One problem with provision of a plurality of perforation channels is that these channels may reduce mechanical strength of the resulting structure.
U.S. Patent Application US2002/0029084, filed by Paul, discloses a bone fusion implant for repair or replacement of bone and including at least two bone fragments (male and female), joined together by an interlocking arrangement, as illustrated in FIG. 1D. This approach appears to provide the required mechanical strength but does not appear to encourage osteo-induction at the interface of the bone fragments; the bone fragments remain separate, with little or no prospect of fusion.
What is needed is a bone implant system that uses single or multiple segments without use of bone pins, that accommodates multiple perforation channels and voids to promote autograft, allograft and/or xenograft osteo-induction, without loss of the required mechanical strength and stability. A need also exists for an improved transforaminal lumbar interbody fusion (TLIF) implant system that allows adequate host-graft interface with multiple segments of a bone implant system, and which accommodates perforations and voids to promote osteo-induction.
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