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Table of Contents
Year : 2018  |  Volume : 11  |  Issue : 4  |  Page : 159-165

Analyses of undesirable clinical outcomes and supportive biomechanical data for a device are essential for the evolution of disruptive device technology in spine

1 Departments of Bioengineering and Orthopaedic Surgery, Engineering Center for Orthopaedic Research Excellence, Colleges of Engineering and Medicine, The University of Toledo, Toledo, Ohio, USA
2 Department of Orthopedic Surgery, Nihon University School of Medicine, Tokyo, Japan
3 Spine and Scoliosis Specialists, Tampa, Florida, USA

Date of Web Publication9-Nov-2018

Correspondence Address:
Prof. Vijay K Goel
Department of Bioengineering and Orthopaedic Surgery, Engineering Center for Orthopaedic Research Excellence, 5046 NI, MS 303, College of Engineering, The University of Toledo, Toledo, Ohio 43606
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/HMJ.HMJ_81_18

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Assumptions, inevitable part of engineering solution, lead to the disparities between positive biomechanical data and undesirable long- and short-term clinical outcomes for a given surgical procedure or device. Consequently, high-quality biomechanical data may not necessarily lead to desirable clinical outcomes. This review article discusses such issues for devices that provide ‘“stability/enhanced environment for fusion or restore motion”’ for the spinal segment (s) with goal of improving patient satisfaction.

Keywords: Biomechanical data, clinical outcome, spinal device

How to cite this article:
Erbulut DU, Matsumoto K, Agarwal A, Zavatsky J, Goel VK. Analyses of undesirable clinical outcomes and supportive biomechanical data for a device are essential for the evolution of disruptive device technology in spine. Hamdan Med J 2018;11:159-65

How to cite this URL:
Erbulut DU, Matsumoto K, Agarwal A, Zavatsky J, Goel VK. Analyses of undesirable clinical outcomes and supportive biomechanical data for a device are essential for the evolution of disruptive device technology in spine. Hamdan Med J [serial online] 2018 [cited 2022 Jun 28];11:159-65. Available from: http://www.hamdanjournal.org/text.asp?2018/11/4/159/245136

  Introduction Top

Healthcare costs in the area of spinal disorders, such as lower back pain, are second only to cardiac expenses. However, researchers and clinicians from differing fields are working relentlessly to improve clinical outcomes and thus reduce healthcare costs. Improvements and breakthroughs in conservative treatment options, surgical procedures (e.g., from open to minimally invasive surgeries), spinal devices (technology) and cell therapies are some example areas that hold promise. Ultimately, successful long-term outcomes of a procedure (surgery or conservative) are needed from a patient's perspective. In this respect, an awareness of the disparities between undesirable long- and short-term clinical outcomes and good biomechanical data for a given surgical procedure or device requires further examination. The discrepancies between clinical outcomes may be due to the assumptions of engineering studies (e.g., finite element [FE] analyses, in vitro, in vivo studies, etc.) for the real-life problems that enumerate limitations for proper interpretation of the findings. Therefore, high-quality biomechanical data may not actually lead to desirable clinical outcomes. This review article addresses such issues for devices that provide ‘stability/enhanced environment for fusion or restore motion’ for the spinal segment(s) and thus improve patient satisfaction. It is virtually impractical to review the vast amount of literature, including the work accomplished by the senior author (Goel) and his team over the past 35+ years on this topic. Thus, emphasis in this article is placed on the review examples of undesirable clinical outcome reports and biomechanical studies that support the devices used in a surgical procedure predominantly from our labs. Such comparisons should help improve biomechanical protocols, leading to better agreements, with the hope that ultimately these will lead to better clinical outcomes for the patients at large.

Biomechanical tests are inevitable for evaluating the safety and efficacy of new spinal implants prior to clinical trials[1],[2] Biomechanical testing for a spinal device involves multiple steps, including benchtop testing as per the American Society of Testing and Materials (ASTM) and International Organization for Standardization standards [Figure 1]a, cadaver studies [Figure 1]b, FE studies[3] [Figure 1]c, biocompatibility tests, etc. For a new device, in the absence of clinical data, such extensive evaluations justify the manufacturing of devices at a large scale. However, the final safety and efficacy of a new technique or system can only be revealed following clinical trials. Thus, the design and development of a spinal device is an expensive iterative process.
Figure 1: Examples of biomechanical tests. (a) Mechanical setup for total disc replacement testing; (b) Human cadaver specimen with fixation system ready for in vitro test; (c) Lumbar spine-pelvis finite element model

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Spine fusion surgery has evolved from fusion using bone grafts alone, to bone grafts augmented with spinal devices such as screws, rods, plates, cages, and cell-based therapies. Undesirable clinical outcomes, despite supportive biomechanical data, have led engineers and surgeons alike to evaluate alternatives over the years, such as improving testing protocols, motion preservation devices, and total disc replacements. Spinal fusion technique may be the best example to start with to raise the issue of positive biomechanics and poor clinical outcomes. For instance, fusion treatments provide the expected biomechanical fusion outcome immediately after surgery, but unsuccessful clinical outcomes of at least 20% have been reported for these fusion treatments. Conclusively, biomechanical data in the past had revealed that spinal fusion surgeries augmented with devices provide decompression, stability, and an enhanced fusion environment to the index segment(s), but this does not always translate clinically to spinal pain relief and other issues postoperatively in a patient.

  Spinal Fusion Top

Spinal fusion procedures, augmented with humanmade devices, reduce flexibility and may improve load transmission at the index levels. This segmental bracing leads to higher motion and stresses on the adjacent segments, a cause of subsequent degeneration called adjacent segment degeneration (ASD). ASD incidences were overlooked in earlier short-term clinical fusion reports.[4] The incidence became noticeable once long-term clinical outcomes were analysed. At present, spinal fusion is considered to be the most significant contributing factor for ASD. Over time, other clinical issues such as hypertrophic degenerative arthritis of the facet joints, spinal stenosis, degenerative spondylolisthesis or herniated nucleus pulposus may be attributable to abnormal biomechanics at the adjacent levels.[5],[6],[7] In the past, due to limitations of the test protocols and lack of appropriate technology, biomechanical studies were not able to address all of these issues.

Essential biomechanical criteria of a good fusion device, apart from segmental fusion, include bearing segmental load without damage or migration of the graft and potential restoration of the segmental functionality.[8] This brings an essential question to the forefront: if the segmental motion is eliminated following fusion treatment, how do we restore the basic functionality and load transmission of the segment back to normal, or should we abandon this approach and look to procedures that just restore motions and load sharing back to normal? There have been many biomechanical investigations that revealed the adverse effects of a fusion treatment on index and adjacent segment mechanics.[9],[10] For example, our FE study investigated the effects of posterior lumbar fusion scenarios on the sacroiliac joint.[11] The FE model predicted increases in motion and stresses across the sacroiliac joint following posterior lumbar spinal fusion scenarios [Figure 2].
Figure 2: Finite element models of lumbar spine and pelvis with posterior fusion across different levels

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In our more recent FE study, biomechanical relationships between ASD (proximal and distal junctions) and different spinal and pelvic parameters following lumbar arthrodesis were investigated.[12] The von Mises stress on the proximal and distal vertebrae to the fused segment were calculated and compared to investigate proximal junctional kyphosis and distal junctional kyphosis. FE analyses predicted that the von Mises stress on adjacent vertebra increased by up to 196% in the flat back model, and 527% in the kyphotic model, compared to the normal model. The study concluded that restoring the normal lumbar alignment is important when considering L2–L5 fixation in flat back and kyphotic models. Dynamic stabilisation may be an alternate to fusion for the kyphotic patients.

Likewise, the development of the circular BAK cage as a way to reduce the amount of bone graft requirements for the fusion of the segment, and reduce subsidence across the fused level, was considered a breakthrough in fusion technology. Biomechanical studies supported this notion. However, long-term clinical follow-ups showed that the circular cage can migrate in to the spinal canal and subsidence remained an issue. Biomechanical studies at that time did not predict cage migration and/or subsidence. This led to protocol improvements and design of newer cages; for example, from circular to rectangular to truss shapes, from titanium to polyetheretherketone (PEEK) material, cage shapes/designs suitable for use with minimally invasive surgical approaches, etc. [Figure 3].[13],[14],[15],[16],[17]
Figure 3: (a) Examples of different types of cages are shown. (a) BAK cage placed in anteroposterior and lateral configurations to avoid its migration in to the canal. However, migration was still an issue even in the lateral configuration, although biomechanically it was a sound design. (b) The shape was changed to a rectangular cage and subsequent other shape designs are shown as well. (c) The shape was changed to a rectangular cage and subsequent other shape designs are shown as well. The latest concepts are a Truss (4 WEB) design and cages suitable for the minimal surgical approach. (d) The shape was changed to a rectangular cage and subsequent other shape designs are shown as well. (e) The latest concepts are a Truss (4 WEB) design and cages suitable for the minimal surgical approach

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However, ASD still is persistent and needs an alternative approach. Biomechanical studies may have suggested a way to overcome the ASD incidence following spinal fusion.

For example, our in vitro study using 12 L2–S1 fresh frozen specimens instrumented with either titanium rods or PEEK rods, investigated the effects of these materials on the kinematics of the adjacent level.[18] Study revealed that semi-rigid PEEK rods provide similar superior adjacent level motion to titanium rod in flexion and extension. On the other hand, we know that a dynamic posterior stabilisation (PDS) system, which can allow posterior band stiffness closer to the normal spine, may protect adjacent levels from abnormal motion.[19] These studies suggest that the field of spinal implant development is moving toward dynamic concepts to address the ASD.

  Dynamic Stabilisation Top

Dynesys (Zimmer Spine, Minneapolis, MN, USA) was introduced a decade ago and is now one of the most widely implanted PDS system. The first biomechanical report was presented by the inventor of Dynesys in 1999.[20] In this cadaver study, range of motion (ROM) at the index level was measured under flexion and extension moments with compressive, and anterior shear loads. The effect of posterior implants on lumbar motion was emphasised. They suggested Dynesys was an efficient implant that behaves as an additional biomechanical support to the index segment.

The first series of clinical results about Dynesys were positive.[21],[22] They found that Dynesys was able to compensate initial morphologic changes and resist further segmental degeneration. The inventor reported pre- and post-operative pain, function and radiologic data on a consecutive series of 73 patients who underwent Dynesys implantation. During the follow-up time-range between 11.2–79.1 months no screw breakage was observed. Dynesys was presented as a safe and effective alternative implant for unstable lumbar conditions. However, Sengupta et al.[23] analysed their data, and questioned the aetiology of the good clinical outcome reports. Was the success primarily due to the Dynesys instrumentation or the decompression? Other studies supported the finding that half of the Dynesys cases without accompanying decompression were not clinically successful.[24]

Clinical reports have provided no support for the impression that Dynesys system is better than other fusion systems. The system acts like a rigid fixation and does not address the adjacent-segment degeneration problem.[25],[26] Furthermore, clinical outcomes agreed with biomechanical studies that the system limits extension motion, which may be the reason for the poor clinical outcomes in terms of screw loosening or breakages, relatively common complications with Dynesys.[24] Conclusively, stand-alone use of Dynesys without fusion is considered an off-label use and Food and Drug Administration (FDA) approval includes only usage at the adjacent level of fusion.

Many PDS systems have been used as an alternative technique to fusion to overcome/reduce the incidences of ASD.[27] Graft system was one of those and aimed to prevent abnormal flexion and restore segmental lordosis. Initial clinical results of the system were promising.[28] However, later reports indicated system failure related to transferring the load to the posterior annulus. In addition, higher rates of revision surgery in Graf patients compared to fusion patients was reported.[29] Another example is the interspinous process (ISP) device as a dynamic system and was designed for treatment of spinal pathologies such as spinal stenosis or facet arthritis [Figure 4].
Figure 4: Examples of different types of interspinous devices

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However, lack of design considerations of the device has been reported after the clinical and biomechanical investigations.[30],[31],[32] In our FE study, the biomechanical effects of an implanted ISP device on load shearing at the index and adjacent segments were evaluated by using a hybrid protocol.[33] The maximum von Mises stress in the implanted spinous process increased and facet loads also increased at both the superior and inferior adjacent levels with extension. It is worth noting that biomechanical data of such devices did support its effectiveness, but clinical outcomes are not satisfactory. Alternatives are currently being pursued, like the posterior pedicle screw-based dynamic systems.

Unlike the fusion augmenting devices, the dynamic systems must stay functional for a long period of time since in a patient these are expected to restore and maintain motion back to normal values. Thus, these devices must withstand cyclic loading over a longer period of time, which usually leads to fatigue failure or screw loosening.[25],[34],[35] As a result, most of the dynamic stabilisation devices, particularly pedicle screw-based systems, are designed to be stiffer to withstand cyclic loading.[19] However, stiffer dynamic implants may not provide load shearing functionality.[35],[36] Studies suggest that axial stiffness of 45 N/mm and bending stiffness of 30 N/mm may be considered as desirable stiffness parameters for a posterior dynamic system.[37] Biomechanical studies support the development of such systems, but clinically these devices have not been accepted.

Once long-term clinical outcomes were analysed, it became evident that screw loosening and implant failure are more likely to occur following very stiff dynamic system implantation. Load sharing capabilities of an implant would create a desirable condition to prevent implant failure. To facilitate this aspect, posterior non-fusion hinged-screw (HS) systems were designed to provide lesser stress shielding due its non-rigidity characteristic. The HS head provides around ±15 degrees of rotation horizontal to the sagittal plane [Figure 5]. These screws are used for discogenic pain treatments unless spinal corrections, especially in the sagittal plane, are necessary to treat pain. Biomechanical and clinical findings showed that load reduction in the pedicle screw decreased the possible loosening.[38] In our FE study, von Mises stresses in the screw were higher with a standard screw than with a HS system.[19] This showed that a greater load transfer would occur through the rigid screw. Similar findings were reported by Goel et al.,[39] in which a HS system allowed an increased rate and degree of bony fusion by relatively enhanced load sharing capabilities.
Figure 5: (a) The graphical representation of an implanted hinged-screw and the rigid screw. (b) Safinaz (Medikon, AS) screws. The screw head allows about 15° of rotation on the sagittal plane and the rod interconnecting the screw head at adjacent levels

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The next step in the evolution of spinal devices was the development of total disc replacement (TDR) systems that mimic the motion of a normal intervertebral disc.[40],[41] For example, instead of cervical discectomy and fusion, TDRs seem favourable for certain cervical spine treatments.[42] Biomechanical studies showed that centre of rotation (COR) of the segment was controlled by facet joints.[43] Therefore, COR should be considered for TDR design to protect facet joints. Clinical studies showed that most of the patients with neck pain displayed abnormal instantaneous COR.[44] Consequently, biomechanical studies have emerged to investigate sliding articulation feature of a TDR.[42],[45] COR is a function of both translation and rotation of the segment. Studies revealed that TDR with a fixed COR may cause facet load increase at the index segment. Facet joint stress was eliminated when an artificial disc had a sliding articulation feature to allow translation in the sagittal plane.[46],[47] Despite the biomechanical efficacy of the metal-polymer discs, clinical outcome has not been encouraging. These studies still report the presence of ASD. Thus, the researcher moved to the development of polymer disc replacements that mimic natural disc in all aspects.

Our research group simulated a FE model of a cervical spine with TDR at C4–C5 segment to investigate the biomechanical effects of the novel elastomer TDR, and then compared it to an intact spine.[48] An experimentally validated, ligamentous, three-dimensional, FE model of C3–C7 cervical spinal model was used for this study [Figure 6]a. To simulate the total discectomy procedure, the entire nucleus, all ligaments, and anterior part of the annulus were removed from the disc space of the C4–C5 segment. A computer-aided design model of the FREEFORM cervical TDR [Figure 6]b was placed in the opening and endplates were affixed to the adjacent vertebral endplates. A compressive follower load of 75 N and a pure bending moment of 1.5 Nm were applied to the intact and instrumented model to simulate physiological motions. The ROM, intradiscal pressure and facet loads were computed and compared for segments of intact and instrumented spines.
Figure 6: (a) Finite element model of the instrumented cervical spine. (b) FREEFORM Cervical total disc replacement

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[Figure 7] shows the ROM for index and superior adjacent levels. TDR device increased flexion and left/right bending up to 7% compared to an intact spine. The instrumented cervical FE model predicted motion increase in extension (28%) and axial rotation (23%). The motion at the superior adjacent segment was unaffected in any planes of motion, except in extension (13% decrease) following disc replacement. The intradiscal pressure at the adjacent segment remained essentially unchanged following TDR except in extension, where there was a slight increase. There was no significant change in facet loads following instrumentation.
Figure 7: Range of motion for intact and instrumented spines at index and adjacent levels

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The FE study predicted that the cervical TDR device preserved the ROM, especially in flexion and lateral bending. In addition, the device may minimise the risk of adjacent segment disc and facet degeneration. Furthermore, the FREEFORM device did not alter intradiscal pressure or facet joints stress at the adjacent segments. Therefore, the device may lower the long-term risk of ASD incidences.

Similar analyses in the lumbar region have shown that metal-on-metal TDRs restore spinal motion to normal values but clinical literature is not that encouraging; reports of ASD appear. The surgeons in the USA do not use these for the patients. As a result, the third generation of the TDRs has been developed. These are polymer designs. More recently, our group has developed a ligamentous FE model of the L1–S1 spinal segment with and without a polymer artificial disc placed across L4–L5 level. The data show that the motion and load sharing are restored back to normal. Clinical data generated thus far in Europe are encouraging.[49]

As previously stated, biomechanical studies support the development of artificial discs for the lumbar region but clinical data does not.[50] In response to this discrepancy, engineers have designed a hybrid system [Figure 8]. A polymer TDR is used to replace the disc, but artificial facets are used to replace the posterior elements–thus, a truly 360° replacement. We developed and analysed an anterior TDR and posterior facet replacement system [Figure 8]a, [Figure 8]b, [Figure 8]c. Biomechanical studies support the development of such systems, but clinical data are lacking. Similar analyses were accomplished for a bilateral posterior novel TDR and facet replacement technology [Figure 8]d. Biomechanical data supported the development of such a concept, but clinical data are lacking.
Figure 8: (a and b) Different types of anterior polymer total disc replacements. (c) facet replacements. Disc design (a or b) was added with facet technology and (c) to produce 360° replacement. (d) Similar approach is shown with a bilateral posterior disc and a facet system

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In order to address the disparities between biomechanical studies and clinical outcome data, our group designed and developed a polymer disc in which piezo electric sensors were placed to record forces across the disc during activities of daily living in a patient [Figure 9]. Biomechanical studies were undertaken to show the safety and efficacy of the design, but we were unable to receive FDA approval for implantation in a patient. Such initiatives are essential to decrease the gap between biomechanical studies and clinical outcomes.
Figure 9: Theken eDISC total disc replacement

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Biomechanical data are essential to the research process. At the same time, understanding the limitations of biomechanical data as it translates to clinical outcomes is paramount to the iterative process in implant design. Knowing that beneficial biomechanical data may not actually yield good clinical outcomes is critical. Biomechanical data can also be utilised to analyse poor clinical outcomes to understand reasons for failure. Optimising this process may help to address poor clinical outcomes by again engaging biomechanical work as an iterative process.


The review work was supported in part by National Science Foundation Industry/University Cooperative Research Center at the University of California at San Francisco, CA, and The University of Toledo, Toledo, OH.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9]


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