$372 Journal of Biomechanics 2006, Vol. 39 (Suppl 1)

nevertheless with significantly inferior results for bisegmental interventions at 12- and 24-month follow-up and considerably higher complication rate. Whilst elderly patients were still highly satisfied with postoperative outcome, better functional outcome is to be expected for younger patients. Conclusion: Present data suggest beneficial clinical results of TDR for treat- ment of DDD in a highly selected group of patients. Results are significantly dependent on preoperative diagnosis and patient selection, number of replaced segments and age of the patient at the time of operation. Due to significantly varying outcomes, indications for disc replacement must be defined precisely.

6334 Tu, 11:20-11:40 (P20) Biomechanical research considerations for total disc replacement T.D. Brown 1 , R.M. Hall 2, H.M. Mayer 3, J. Fisher 2, S. Mendoza 1 , E. Ingham 2. 1University of Iowa, USA, 2University of Leeds, UK, 3Spine Center Munich, Germany

There is rapidly growing clinical interest in intervertebral total disc replacement (TDR) arthroplasty. This surgical option's rationale - mobility preservation - holds the attraction of forestalling the adjacent segment degeneration that is often associated with fusion as a treatment for end-stage disc degeneration. Clinical experience with this class of implants is thus far restricted to short- and intermediate-term studies. The results have generally been encouraging, but the sub-set of cases, where these implants have failed, has revealed a number of unique challenges for the biomechanics research community. In the very early phase, sub-optimal surgical placement of the implant can lead to aberrant loading conditions that result in dissociation of the implant components, a potentially life-threatening complication owing to the close proximity of great vessels. Less acutely, there can be failure of the implant to successfully anchor in the host bony bed of the vertebral bodies. Depending on design, surgical placement, and host bone quality, subsidence of the implant can sometimes occur. Stable and successfully incorporated implants can pose other biomechanical challenges, since many designs involve alteration of the physiologic kinematics of the motion segment. The sequellae can cause ab- normal load partitioning between the structures regionally, particularly the facet joints. Some designs are vulnerable to hardware failure from untoward kinetic events such as impingement. Longer-term, these is the potential for wear and osteolysis, a consideration which arises from the parallel of some designs with total hip and knee replacements, in terms of bearing surface materials and articulation tribology. Since TDR recipients are typically a decade or more younger than total hip or knee replacement patients, the demands upon device longevity are correspondingly higher. Many new TDR designs are in various stages of development and pre-clinical evaluation, and consensus standards for performance evaluation are in their infancy. These many considerations entail a large need for systematic biomechanical study, to hopefully facilitate smooth evolution toward predictably successful long-term function.

4585 Tu, 11:40-12:00 (P20) Wear testing of spinal disc implants C. Kaddick. EndeLab GmbH, Resenheim, Germany

Advancements in the spinal implant marketplace have lead to a demand for test procedures that evaluate implant biomechanical performance parameters. One treatment for spinal disc degeneration, motion preservation by intervertebral disc (IVD) replacement, is quickly growing in popularity, and multiple design evolutions are already underway. The goal of IVD replacement is to replace the painful and degenerated IVD with a new bearing unit to enable pain free motion. Two normative bodies, ISO and ASTM, have developed two sepa- rate IVD replacement testing protocols with two differing biomechanical input parameter sets. The goal of in-vitro implant testing is to realistically simulate in-vivo loads and motions as well as the surrounding fluid under laboratory conditions. Currently, there are open questions about which biomechanical input parameters should be selected for realistic spinal implant simulation. Current simulation methods draw upon lessons learned from hip and knee joint testing, as well as published knowledge about the spinal range of mo- tion and loading environment. The principle of the biomechanical simulation has proven to distinguish between promising developments and prototypes requiring improvement. So far more than 1.6 billion wear cycles have been performed on spinal disc simulators according to ISO/DIS 18192, as well as with custom motion and loading input parameters. Further test method developments should be based on a better understanding of IVD replacement patient daily living activities and comparisons between simulated and retrieved prostheses as they become available.

Oral Presentations

5991 Tu, 12:00-12:20 (P20) The biomechanical properties and biological ingrowth characteristics of porous coated motion cervical disc arthroplasty B.W. Cunningham. Department of Orthopaedic Surgery, Johns Hopkins University; Union Memorial Hospital, Baltimore, Maryland USA

Introduction: The current study was undertaken to investigate the biome- chanical and biologic in-growth characteristics of the Porous Coated Motion TM

cervical disc prosthesis following a six and twelve-month implant duration using an in-vivo caprine model. Methods: Twelve mature Nubian goats were divided into two groups based on post-operative survival periods of six (n=6) and twelve-months (n=6). Using an anterior surgical approach, a complete diskectomy was performed at C3-C4, followed by implantation of the Porous Coated Motion TM device. Functional outcomes were based on computed tomography, biomechanical testing, undecalcifed histology, histomorphometry and immunocytochemical analyses. Results: There was no evidence of prosthesis loosening, neurologic or vas- cular complications. CT scans demonstrated the ability to image the cervical spinal canal for the presence of compressive pathology in the area of the CoCrMo prosthesis. Multi-directional flexibility testing indicated no differences in range of intervertebral motion between the disc prosthesis and non-operative controls (n=7) under axial rotation or lateral bending (p>0.05). Flexion- extension produced significantly more motion for the intact spine compared to the cervical disc prosthesis (p<0.05). Based on immunohistochemical and histologic analysis, there was no evidence of particulate debris or pro- inflammatory cytokines within the local tissues overlying the operative site or systemic tissues. Moreover, the spinal cord at the operative levels indicated no evidence of cord lesions, inflammatory reaction, wear particles or significant pathologic changes. Histomorphometric analysis at the metal-bone interface indicated the mean trabecular ingrowth of 40±24% at six and 58% ± 28 at twelve months. Conclusion: All twelve goats undergoing cervical disc replacement had no evidence of implant loosening or inflammatory reactions from particulate wear debris. Segmental intervertebral motion was preserved based on multi- directional flexibility testing. The TiCaP porous ingrowth surface provided some immediate advantages for endplate osseointegration as there was no evidence of implant subluxation, despite immediate post-operative unrestricted cervical activity. Following cervical disc replacement, histological osseointegration at the implant-bone interface is possible, while preserving segmental motion.

18.7. Car Racing and Craniovertebral Trauma 7708 Tu, 14:00-14:15 (P23) Development of airbag and HANS Head And Neck Support for FIA Formula One H. Gramling 1 , R. Hubbard 2. 1FIA Institute for Motor Sport Safety, Paris, France, 2Michigan State University, East Lansing, Michigan, USA

After the Senna accident in 1994 FIA conducted a safety program including the investigation of airbags. They asked Mercedes Benz to conduct this project and Hubert Gramling was announced as project leader. Looking at other approaches than the airbag he found the HANS device to be a promising alternative. FIA and Mercedes Benz decided to investigate both. This presen- tation shows the results of the investigation. It includes the project background, investigated concepts, the test environment, the comparison of HANS and airbag, real crash data and videos that illustrate the differences between HANS and airbag. The conclusion is that airbag and HANS are equivalent from their performance with advantages for the HANS due to its simplicity.

7707 Tu, 14:15-14:30 (P23) Assessment of the injury risk from airbag-helmet interaction H. Gramling. FIA Institute for Motor Sport Safety, Paris, France

During the development of the airbag for Formula One a strong interaction of the chin protector with the airbag was observed. Calculated forces were great enough that breaking the jaw could not be ruled out. With this information the Sports Car Club of America (SCCA) considered to recommend open face helmets or disabling the airbag to reduce this risk. The presentation reports the results from static out of position testing by HMS Motorsport and GM and dynamic tests by the FIA Institute for Motor Sport Safety all at Delphi. Furthermore field experience is reported. All testing and field experience showed that the problem of violent airbag-helmet interaction is limited to the fast deploying Formula One airbag. With standard car airbags no injury risk could be found.