Lumbar Total Disc Replacement Part I: Rationale, Biomechanics, and Implant Types
Section snippets
Anatomy and physiology of intervertebral disc
The intervertebral disc develops from notochordal remnants and perichordal mesenchyme. Notochordal cells may persist up to the age of 5 years and have been found in the sacral discs of older patients aged 22 to 45 years [1]. Toward the end of the embryonic period, the developing disc has an external fibrous zone, an intermediate fibrocartilaginous zone, and an internal hyaline zone adjacent to the notochord. The peripheral layers of the anulus are embedded in the outer ring of the cartilage
Biomechanics
Given our upright posture and the load our spines must bear as a result, the human disc has evolved to handle compressive loads. The nucleus is designed to absorb compressive loads, redistribute the forces radially, and convert these forces into tensile loads in the anulus fibrosis. When disc hydration is decreased and the nucleus has suboptimal turgor, the motion segment bears the compressive load poorly, leading to shearing forces in the anulus that result in fissures and tears and the
Pathophysiology of degenerative disc disease
Degenerative disc disease is nearly universal in old age and is a common phenomenon in younger patients [20], [21]. As alluded to earlier, the decreased ability of the aging disc to absorb the loads associated with activities of daily living degrades the anulus and puts excess stress on the facets and end plates [22], [23], [24]. In addition to the classic, mechanical view of disc degeneration, the cellular and molecular aspects of the degenerative process and their potential genetic
Nonsurgical treatment
For most patients with back pain and degenerative lumbar disc disease, the treatment is nonsurgical. Most patients improve satisfactorily without surgery [32]. Modalities such as physical therapy, massage, and manipulation, have been shown to be effective [33]. In the small percentage of patients who do not benefit from nonsurgical treatment and remain disabled from their back pain, surgery can be beneficial. In a randomized trial comparing fusion with nonsurgical treatment in patients with
Surgical treatment
The rationale for fusion as a treatment for degenerative disc disease rests on the premise that the pain generated from the painful motion segment is ameliorated by eliminating motion and loading on pathologic disc tissues. Although some surgeons have argued that a posterolateral fusion suffices, most believe that a 360° fusion, which eliminates the disc and provides solid anterior support, represents the “gold standard.”
Surgical outcomes are not uniformly good, however, and the role of surgery
Ideal characteristics
The ideal lumbar TDR would function as a physiologic replacement for the human intervertebral disc. It would assume the role of the nucleus pulposus and anulus fibrosis complex. In preserving the lumbar spine's range of motion, the lumbar TDR would also need to transmit and absorb loads across the disc space between the vertebral bodies. In other words, it should attempt to reproduce the load transmission properties of the disc and its motion characteristics. In addition to providing
Overview of implants types and characteristics
There are currently four lumbar total disc prostheses coming to market in the United States. The SB Charité disc (DePuy Spine/Johnson & Johnson, Raynham, Massachusetts) was approved by the Food and Drug Administration (FDA) in the fall of 2004. The ProDisc (Spine Solutions/Synthes, West Chester, Pennsylvania) has completed its randomized enrollment, is currently in nonrandomized modes, and may be approved by late 2005. The Maverick (Medtronic Sofamor Danek, Memphis, Tennessee) and FlexiCore
Surgical considerations
As is the case with any surgical procedure, it is important to have a clearly defined preoperative plan. For example, the Charité can be templated using plain radiographs and overlays of the implants much like in total joint arthroplasty. For the approach, it is critical to have an access surgeon, usually a general or vascular surgeon. In addition to the instruments required for the minimally invasive approach, it is important to have instruments available for an emergency (eg, in the event of
Postoperative regime
Similar to the patient education programs used for joint arthroplasty, the authors believe that preoperative education is beneficial. Studies have shown that early mobilization is better [43]. Early motion is best limited to flexion, and no rotation should be allowed for at least 3 weeks. At 6 weeks, the patient can be advanced to rotation and side bending. After 6 weeks, the patient can extend past neutral and begin abdominal exercises. Some advocate a soft brace until that time, recommending
Summary
The excitement surrounding TDR surgery must be tempered with prudence. Motion preservation surgery of the spine is still in its early stages, and although the clinical results are promising, the complexity of the functional spinal unit exceeds the designs of the current implants. Exacting indications, careful surgical technique, and thorough follow-up will allow us to improve on this promising beginning.
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