Basic ScienceThe effect of interbody fusion cage design on the stability of the instrumented spine in response to cyclic loading: an experimental study
Introduction
Compared with conventional posterolateral fusion methods, the use of lumbar interbody fusion offers the advantage of restoration of the disc space height [1], [2], [3], anterior column support, increased foraminal area, and the restoration of lumbar lordosis, leading to an improvement in sagittal balance and a decreased incidence of “flat back syndrome” [4], [5], [6], [7]. The transforaminal lumbar interbody fusion (TLIF) approach for unilaterally placed interbody cages [8], [9], which was initially described by Harms and Jeszensky [10], was advocated citing the advantages of both minimizing blood loss and the manipulation of the traversing nerve root. These advantages lead to reduced morbidity and postoperative complications, particularly in the older population [11], [12], [13]. Clinically available TLIF cage designs include, but are not limited to the following: (1) a banana shape, placed in the anterior side of the disc space; (2) a bullet or biconvex shape, designed to better fit the concave shape of the superior and inferior end plates; and (3) a straight-shape, aimed at capturing the support of the peripheral areas of the end plate [14], [15]. Titanium and polyetheretherketone (PEEK) are currently the materials of choice for cage construction [16]. Having a material stiffness closely matched to that of cancellous bone [17], [18], PEEK is radioluscent, allowing for radiographic assessment of the degree of fusion of the bone mass. Although titanium cages are associated with increased subsidence [19], their use is associated with higher fusion rates for both lumbar and cervical spines [16], the difference likely due to the PEEK having a less favorable osteogenic surface [20]. There is disagreement in the literature regarding the role of cage geometry [21], [22], [23], [24], [25] and placement [23], [24], [26] on construct stability. The addition of posterior pedicle screw-based instrumentation systems significantly improved construct stability and reduced the incidence of subsidence [27], [28], [29]. Patients, using anterior and posterior fixation, demonstrate fusion rates between 90% and 100% [30]. In effect, the use of posterior instrumentation largely obscured the contribution of cage design to the stability of the instrumented spine [23], [24], [31], [32].
Cage subsidence, describing the decrease in the vertical height of the disc space before complete incorporation of the fusion mass [15], represents the progression of cage settling during the normal healing process. Postoperatively, radiographic studies reported cage subsidence to occur as early as 13 days for femoral ring implants with a mean of 2.75 months (range: 0.25–8 months) for intervertebral cages post surgery [15]. Over time, degradation of the cage-bone interface will negatively affect the construct's mechanical stability, yielding the reduction in the distraction achieved for foraminal size, as well as the correction of sagittal balance [4], [5], [33], which adversely affects the surgical goals fundamental to lumbar interbody fusion [34], [35]. Using three designs of posterior lumbar interbody fusion cages, a biomechanical study in human cadavers aged 24–57 years, Krammer et al. [21] found the cages to show accelerated subsidence during the initial period of the compressive load cycles, with the cages exhibiting continuous increase in subsidence throughout the duration of the test (40,000 cycles). However, the cage design had no significant effect on the rate of cage subsidence in this study. In more severe cases, a fracture of the end plates was reported in clinical follow-up studies [6], [7], [36]. Symptomatic subsidence thus remains a concern, particularly for elderly patients [33], [37], [38], [39], which may delay the onset and ultimate success of the fusion.
Cage subsidence may result from the combined effect of biological remodeling at the cage-bone interface, the creep-based deformation of the end plate, and the increasing degree of micro-fractures [40] at the cage-bone interface. Contributing factors include bone quality, cage geometry, cage placement within the intervertebral space, preparation of the end plate for implantation [41], [42], the use of osteobiologics [43], and iatrogenic end plate violation [44]. In the lumbar spine, the density and thickness of the end plate was found to increase from the center to the periphery [45], resulting in a corresponding increase in strength in the periphery end plate compared to its center [46]. The posterior portion of the end plate, and for the disc joint, its inferior end plate, were similarly reported to be stronger compared with the anterior portion and superior end plate, respectively [46]. These anatomical differences corresponded to clinical radiographic findings of higher subsidence at the superior end plate [24], with reduced subsidence rates noted for cages spanning the apophyseal region of the end plate [44], [47]. Although the rate of creep of the vertebral bone (ie, the rate of its deformation under constant load) and the occurrence of micro-fractures within the bone are strongly associated with bone density and its stiffness [40], [48], the effect of bone density on cage subsidence remains unclear. Biomechanical studies reported that higher bone density was associated with either higher bone-cage interface failure load and lower displacement to failure [49], [50] or with an unpredictable effect on cage subsidence [21]. As a corollary, radiographic studies showed no significant association with the clinically observed subsidence [44]. Thus, the effect of cage placement and design and the interaction of these variables with the density of the subchondral vertebral bone in affecting the time-dependent subsidence of intervertebral cages remain unclear.
This cadaver-based in vitro study investigated the effect of compressive cyclic loading, simulating a loading regime during the first few weeks post surgery [51], [52], on cage subsidence and the stiffness of the cage-bone interface of three different interbody cage constructs. The constructs investigated were the bilateral linear (BLL) cages, the anterior conformal shaped (ACS) cages, and the unilateral oblique linear (UOL) cages. We further assessed the impact of vertebral bone density. We hypothesize that the peripheral location of the cage and the higher bone density will correlate with a decrease in subsidence.
Section snippets
Test groups
Three PEEK-based cage constructs, the double BLL, the single ACS cage, and the single UOL, were compared in this study. Fig. 1 presents an instrumented functional spinal unit (FSU) for each cage design.
Specimens
Nine cadaveric thoracolumbar (T12–L4) human spines were obtained fresh-frozen from donors aged 43–72 years (Table 1). Each spine was radiographed (Faxitron model 43855A, Hewlett-Packard, McMinnville, OR, USA) to exclude levels with preexisting fractures, lytic or blastic defects, and evidence of
Results
All of the tests were completed to the prescribed number of load cycles. Under the applied load, the instrumented FSUs exhibited continual increase in measured displacement, with the rate of increase being highest during the initial load cycles, 0–1000 (Table 2). Independent of cage design, the instrumented segments' compressive stiffness decreased significantly during the first 500 cycles (p<.01) (Fig. 5). Although stiffness continued to decrease between consecutive sample intervals thereafter
Discussion
Posterior lumbar interbody fusion and TLIF are widely used for achieving spinal fusion in the vertebral interspace [3], [5], [6], [7]. Postoperatively, daily activities impose complex mechanical loading on the vertebral interspace. This loading critically affects the establishment and integration of the bone fusion mass between the adjacent vertebrae [60]. Technological advancement in surgical techniques cage design, and in the materials used for fabrication of these cages have improved
Conclusion
We conclude that the choice of implant should be motivated primarily by minimizing the technical challenge and morbidity of cage insertion at the time of placement. Our findings further suggest that with further development of volumetric assessment of bone density, such assessment may aid in deciding where to best place the cage to maximize stability. Unilateral cage insertion can be used as a primary treatment strategy with the understanding that the cage should cross the midline and overlie
Acknowledgment
The support of DePuy Synthes for this study is gratefully acknowledged.
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Cited by (0)
FDA device/drug status: Approved (CONCORDE Inline Lumbar Interbody System; LEOPARD Lumbar interbody fusion cage; Brantigan interbody fusion cage).
Author disclosures: RNA: Nothing to disclose. RA: Grant: DePuy Spine (E, Paid directly to institution/employer), in relation to the submitted work. MWG: Grant: DePuy Spine (E, Paid directly to institution/employer), pertaining to the submitted work. Royalties: Biomet (F), DePuy Spine (B); Fellowship Support: NREF (E, Paid directly to institution/employer), outside the submitted work.
The disclosure key can be found on the Table of Contents and at www.TheSpineJournalOnline.com.
Neither the authors nor their family members have received any financial gain, consultancy, nor lecturing fees related to this research and submitted work.
This research project was supported by DePuy Synthes ($60,521).