Effect of two-level total disc replacement on cervical spine kinematics.

STUDY DESIGN: Biomechanical study using human cadaver spines. OBJECTIVE: To characterize kinematics of cervical spines implanted with total disc replacement (TDR) at 2-levels referencing the implanted and adjacent levels. SUMMARY OF BACKGROUND DATA: Cervical TDR is an appealing alternative to fusion particularly when treating multilevel disease, where the advantages of maintaining motion and reducing adjacent level stresses with TDR are compelling. To our knowledge there are no biomechanical studies evaluating multilevel cervical TDR. METHODS: Six human cadaveric cervical spine specimens (C3-C7, age: 57 +/- 12 years) were tested (i) intact, (ii) after TDR (Discover, DePuy, Raynham, MA) at C5-C6, and (iii) after additional TDR at C6-C7. Specimens were subjected to flexion/extension, lateral bending and axial rotation (+/-1.5 Nm). Segmental range of motion (ROM) was measured using optoelectronic instrumentation and fluoroscopy. RESULTS: Insertion of TDR at C5-C6 increased flexion/extension ROM of the implanted segment compared with intact (8.6 +/- 1.0 vs. 12.3 +/- 3.3 degrees , P < 0.025). The TDR maintained ROM to intact levels in lateral bending (7.4 +/- 2.6 vs 6.0 +/- 1.6, P > 0.025) and axial rotation (5.5 +/- 1.9 vs. 6.0 +/- 2.9, P > 0.025). The TDR at C5-C6 did not affect ROM at the adjacent levels. Implantation of a second TDR at C6-C7 maintained the ROM at that segment to intact values in flexion/extension (9.6 +/- 4.3 vs. 11.2 +/- 5.5, P > 0.025), lateral bending (6.1 +/- 4.0 vs. 4.1 +/- 2.1, P > 0.025), and axial rotation (6.7 +/- 3.6 vs. 5.5 +/- 3.7, P > 0.025). The second TDR at C6-C7 did not affect the ROM of the prosthesis implanted at C5-C6. Two-level TDR at C5-C6-C7 did not affect the ROM at C4-C5 in flexion/extension or axial rotation, however, in lateral bending a small increase occurred (8.9 +/- 3.6 vs. 10.1 +/- 4.5, P < 0.025). CONCLUSION: Cervical TDR at 2 levels can provide near-normal mobility at both levels without destabilizing the implanted segments or affecting adjacent segment motions. These observations lend support to the notion that single or multilevel cervical TDR may be advantageous when compared to fusion.

The fundamentals of biotribology and its application to spine arthroplasty.

The biological effect of wear of articulating surfaces is a continued concern with large joint replacements and, likewise, of interest for total disc replacements. There are a number of important biotribological testing parameters that can greatly affect the outcome of a wear study in addition to the implant design and material selection. The current ASTM and ISO wear testing standards/guides for spine arthroplasty leave many choices as testing parameters. These factors include but are not limited to the sequence of kinematics and load, phasing, type of lubricant, and specimen preparation (sterilization and artificial aging). The spinal community should critically assess wear studies and be cognizant of the influence of the selected parameters on the test results.

Effect of uncovertebral joint excision on the motion response of the cervical spine after total disc replacement.

STUDY DESIGN: In vitro biomechanical study. OBJECTIVE: To quantify the effects of uncinatectomy on cervical motion after total disc replacement (TDR). SUMMARY OF BACKGROUND DATA: The effect of uncinatectomy on TDR motion is unknown. Partial uncinatectomy may be required to decompress the foramen; however, the residual uncinates can potentially limit TDR motion and serve as a source of progressive spondylosis. Complete resection of the uncinates may decrease this risk yet endanger destabilizing the segment. METHODS: Seven human cervical spines (C3-C7) (age, 63.4 +/- 6.9 years) were tested first intact and then after implantation of a metal-on-polyethylene ball-and-socket semiconstrained prosthesis at C5-C6. Following this, gradually increased uncinatectomy was performed in the following order: 1) right partial-posteromedial (two thirds), 2) right complete, and 3) bilateral complete resection. Specimens were tested in flexion-extension, lateral bending, and axial rotation (+/-1.5 Nm). Flexion-extension was tested under 150 N follower preload. RESULTS: TDR without uncinatectomy increased C5-C6 flexion-extension range of motion from 8.4 degrees +/- 3.5 degrees to 11.6 degrees +/- 3.4 degrees, but statistical significance was not reached (P > 0.05). Lateral bending decreased from 6.2 degrees +/- 2.2 degrees to 3.1 degrees +/- 1.4 degrees, with a trend for statistical significance (P = 0.07). Axial rotation decreased from 5.5 degrees +/- 2.4 degrees to 4.3 degrees +/- 1.4 degrees after the implantation (P > 0.05). Both right partial and right complete uncinatectomy resulted in nearly symmetrical restoration of lateral bending to intact values and significantly increased flexion-extension compared with intact (P < or = 0.05); however, axial rotation still did not differ from intact (P > 0.05). Complete bilateral resection also restored lateral bending to intact values (7.3 degrees +/- 2.7 degrees, P > 0.05); however, it resulted in significant increase in range of motion in flexion-extension (14.1 degrees +/- 3.0 degrees, P < or = 0.05) and axial rotation (8.7 degrees +/- 2.4 degrees, P < or = 0.05). CONCLUSION: Unilateral complete or even partial uncinatectomy can normalize lateral bending after TDR. Bilateral complete uncinatectomy is not necessary to restore lateral bending and may result in significantly increased range of motion in flexion-extension and axial rotation compared with intact values.

Biomechanics of the posterior lumbar articulating elements.

The clinical success of lumbar spinal fusion varies considerably, depending on techniques and indications. Although spinal fusion generally helps to eliminate certain types of pain, it may also decrease function by limiting patient mobility. Furthermore, spinal fusion may increase stresses on adjacent nonfused motion segments, accelerating the natural degeneration process at adjacent discs. Additionally, pseudarthrosis, that is, incomplete or ineffective fusion, may result in an absence of pain relief. Finally, the recuperation time after a fusion procedure can be lengthy. The era of disc replacement is in its third decade, and this procedure has demonstrated promise in relieving back pain through preservation of motion. Total joint replacement with facet arthroplasty of the lumbar spine is a new concept in the field of spinal surgery. The devices used are intended to replace either the entire functional spinal unit (FSU) or just the facets. These devices provide dynamic stabilization for the functional spinal segment as an adjunct to disc replacement or laminectomy and facetectomy performed for neural decompression. The major role of facet replacement is to augment the instabilities created by the surgical decompression or to address chronic instability. Additionally, facet joint replacement devices can be used to replace the painful facet joints, restore stability, and/or to salvage a failed disc or nucleus prosthesis without losing motion. In this paper the authors review and discuss the role of the lumbar facet joints as part of the three-joint complex and discuss their role in intersegmental motion load transfer and multidirectional flexibility in a lumbar FSU.

In vitro wear assessment of the Charité Artificial Disc according to ASTM recommendations.

STUDY DESIGN: Biomechanical laboratory research. OBJECTIVE: To evaluate the potential for Ultra High Molecular Weight Polyethylene (UHMWPE) wear debris from the Charité Artificial Disc. SUMMARY OF BACKGROUND DATA: Cases of osteolysis from artificial discs are extremely rare, but hip and knee studies demonstrate the osteolytic potential and clinical concern of UHMWPE wear debris. Standards for testing artificial discs continue to evolve, and there are few detailed reports of artificial disc wear characterizations. METHODS: Implant assemblies were tested to 10 million cycles of +/- 7.5 degrees flexion-extension or +/- 7.5 degrees left/right lateral bending, both with +/- 2 degrees axial rotation and 900 N to 1,850 N cyclic compression. Cores were weighed, measured, and photographed. Soak and loaded soak controls were used. Wear debris was analyzed via scanning electron microscopy and particle counters. RESULTS: The average total wear of the implants was 0.11 and 0.13 mg per million cycles, before and after accounting for serum absorption, respectively. Total height loss was approximately 0.2 mm. Wear debris ranged from submicron to > 10 microm in size. CONCLUSIONS: Under these test conditions, the Charité Artificial Disc produced minimal wear debris. Debris size and morphology tended to be similar to other CoCr-UHMWPE joints. More testing is necessary to evaluate the implants under a spectrum of loading conditions.

Test protocols for evaluation of spinal implants.

Prior to implantation, medical devices are subjected to rigorous testing to ensure safety and efficacy. A full battery of testing protocols for implantable spinal devices may include many steps. Testing for biocompatibility is a necessary first step. On selection of the material, evaluation protocols should address both the biomechanical and clinical performance of the device. Before and during mechanical testing, finite element modeling can be used to optimize the design, predict performance, and, to some extent, predict durability and efficacy of the device. Following bench-type evaluations, the biomechanical characteristics of the device (e.g., motion, load-sharing, and intradiscal pressure) can be evaluated with use of fresh human cadaveric spines. The information gained from cadaveric testing may be supplemented by the finite element model-based analyses. Upon the successful completion of these tests, studies that make use of an animal model are performed to assess the structure, function, histology, and biomechanics of the device in situ and as a final step before clinical investigations are initiated. The protocols that are presently being used for the testing of spinal devices reflect the basic and applied research experience of the last three decades in the field of orthopaedic biomechanics in general and the spine in particular. The innovation within the spinal implant industry (e.g., fusion devices in the past versus motion-preservation devices at present) suggests that test protocols represent a dynamic process that must keep pace with changing expectations. Apart from randomized clinical trials, no single test can fully evaluate all of the characteristics of a device. Due to the inherent limitations of each test, data must be viewed in a proper context. Finally, a case is made for the medical community to converge toward standardized test protocols that will enable us to compare the vast number of currently available devices, whether on the market or still under development, in a systematic, laboratory-independent manner.