The Spine Journal 9 (2009) 607–608

Interbody device footprint and endplate engagement characteristics:biomechanical implications

Edward C. Benzel, MD*Cleveland Clinic Foundation, Orthopedic Surgery, S80, 9500 Euclid Avenue, Cleveland, OH 44195, USA

Received 13 April 2009; accepted 17 April 2009

COMMENTARY ON: Buttermann GR, Beau

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Author disclosure

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doi:10.1016/j.spinee.2

bien BP, Freeman AL, et al. Interbody device endplate engagementeffects on motion segment biomechanics. Spine J 2009;9:564–73 (this issue).

The assessment of the efficacy of anterior lumbar inter-body fusion (ALIF) constructs has been insufficiently stud-ied to date. In order to appropriately address this deficit, atleast four variables must be considered: (1) surface area ofcontact of the fusion substrate (eg, bone graft) with the end-plate; (2) surface are of contact of the ALIF construct withthe endplate; (3) ALIF construct/endplate interface integ-rity; and (4) fusion substrate (eg, bone graft) and endplatepreparation. Variables (1) and (2) are in conflict with eachother. When one is large, the other is small. The larger thesurface area of the ALIF construct/endplate contact, thesmaller the surface area of contact of the fusion substratewith the endplate. ALIF construct/endplate interface integ-rity varies, depending on intrinsic factors such as bone den-sity and location of the implant contact with the endplate.For example, the perimeter of the endplate is much morerobust regarding subsidence resistance than the central por-tion of the endplate. Finally, endplate preparation and bonefusion substrate choice and preparation cannot be empha-sized enough.

The article by Butterman et al. [1] addresses many of theaforementioned variables. Motion segment stability aug-mentation, as achieved after the placement of spine fixationdevices, is integral to the enhancement of spinal fusion. It,henceforth, is paramount to determine the factors that pos-itively affect such stability, including implant design

ticle: 10.1016/j.spinee.2009.03.014.

status: not applicable.

s: ECB (royalties from Depuy Spine; stockholder at

ioMed; private investor at AxioMed; consultant for

r for DePuy; member of scientific advisory board

urning Point).

uthor. Cleveland Clinic Foundation, Orthopedic Sur-

d Avenue, Cleveland, OH 44195, USA.

enzele@ccf.org (E.C. Benzel)

front matter � 2009 Elsevier Inc. All rights reserved.

009.04.013

characteristics. Butterman et al. [1] pursue the stabilizationenhancing characteristics of ALIF constructs for the en-hancement of lumbar fusion. The strategy they used as-sessed three very different devices. These devicesparticularly differ from an ‘‘end plate engagement’’ per-spective. A polymeric interbody device without spikes, aninterbody metal device with spikes, and a dual threadedmetal interbody device were compared in a biomechanicalflexibility test in human cadavers. Ventral plates and dorsalpedicle screw fixation devices were subsequently added andtested.

The devices with spikes or threads increased disc spaceheight more than the polymeric devicedmost likely relatedto insertional technique variations. All devices reducedrange of motion and neutral zone size in lateral bending.Devices with spikes or threads performed better in flexionand extension than the polymeric device. The spiked deviceperformed the best in axial rotation. Dorsal and ventral fix-ations provided additive stability over the interbody devicealone. Pedicle fixation provided greater stability than theventral plate in lateral bending.

Butterman et al. [1] concluded that the incorporation ofspikes or threads on an ALIF construct provides greatermotion segment rigidity and that spikes increase rigidityin torsion. They, henceforth, concluded that such torsionresistance may be beneficial clinically.

Butterman and colleagues’ study [1] is typical of modernday biomechanical studies. It, as well as many of its con-temporary publications, exposes the flaws associated withthis type of analysis. Such studies determine, in a limitedmanner, the stability and stiffness characteristics of a mo-tion segment as might be observed on the first day after sur-gery. The conclusions, however, cannot be extended beyondthis time frame in the clinical situation. Subsidence of theimplant into the end plate, loss of tensioning of the support-ing ligaments such as the annulus fibrosus, and the additive

608 E.C. Benzel / The Spine Journal 9 (2009) 607–608

effects of the resultant motion on the creation of furthermotion are results that have immense clinical impact inthe days that follow insertion.

Threaded interbody fusion cages, which performed rela-tively well in this study, are known to have poor subsidenceresistance characteristics. This is related to their suboptimalsurface area of contact at the plane of the endplate. Agreater end plate ‘‘foot print’’ would minimize such. Endplate ‘‘footprint’’ size, however, is often not studied. Flex-ibility studies, as used here, by Butterman and colleaguesassess the positive ligament tensioning characteristics ofsuch implants, whereas ignoring the negative subsidencecharacteristics. In this vein, the authors suggest thata 100,000 cycle biomechanical study be performed. Sucha cyclical testing paradigm would allow an assessment ofthe long-range effects created by repetitive loading of themotion segment. Such a study approaches the practicallimit of cadaveric studies for a multitude of reasons (eg,difficulty with specimen decomposition, absence of physi-ological response to surgery in a cadaveric model, and soforth). It, however, would relatively accurately mimic themechanics and mechanical effects of loading at roughly 2months. This is a reasonable time frame in which to assessimplant bone interface integrity.

If the devices studied here were exposed to cyclical test-ing, as alluded to above, the results may be very different.Spikes and threads may provide little value in such a studyand in the long run clinically. If an implant does not ‘‘hold’’sufficiently during a 2-month time frame, fusion will likelynot occur in the clinical situation.

In my opinion, ‘‘endplate foot print’’ is much more im-portant than short spikes and threads. The resistance to sub-sidence and the maintenance of some ‘‘threshold’’ level ofmotion segment stability and stiffness in all planes overtime is most likely what really counts clinically.

Torsional stiffness of an ALIF implant is perhaps of lim-ited clinical relevance. We must remember that intrinsictorsional stability exists in the lumbar spine, predominantlyrelated to the relative sagittal orientation of the lumbar fac-et joints. The employment of spikes that would erode theend plate under cyclical loading, would most likely providelittle clinical/mechanical value.

Finally, the addition of ventral or dorsal fixation supple-ments makes intuitive sense. The authors have corroboratedsuch. The fact that pedicle fixation performed better thanventral plating also makes intuitive sense by virtue of thegreater extent of fixation and their more lateral placement(compared with the ventral plate).

In summary, the results of this and similar biomechani-cal flexibility studies should be carefully scrutinized re-garding their true clinical value. They make far too manyassumptionsdso many that clinical relevance is near nil.We as clinicians must liberally use common sense andlogic. We must not rely on ‘‘evidence’’ that is irrelevantto the clinical situation at hand. This is not to say that stud-ies such as that presented here do not provide value, be-cause they do. We must, however, seek to design studiesthat provide greater relevance. We also must remain vigi-lant regarding the deployment of the information and ‘‘con-clusions’’ derived from such studies. In fact, Butterman andColleagues [1] have emphasized this.

Reference

[1] Butterman GR, Beaubien BP, Freeman AF, et al. Interbody device

endplate engagement effects on motion segment biomechanics. Spine

2009;9:564–73.

doi:10.1016/j.spinee.2009.04.013