To the Editor:
I read with considerable interest Tokuhashi et als article Subsidence of Metal Interbody Cage After Posterior Lumbar Interbody Fusion With Pedicle Screw Fixation (www.orthosupersite.com/view.asp?rID=38061) in the April 2009 issue of Orthopedics.
The article made no mention of the pedicle screws as a contributor and/or consequence of subsidence. The pedicle screw constructs function as a load-sharing implant, intended to resist flexion moments and anterior column subsidence, was not elaborated on. It would seem that subsidence could not occur without at least 1 of potentially 3 concomitant events (or combinations thereof) occurring: (1) pedicle screw migration within the pedicles and vertebral bodies, (2) deformation of the connecting rod, and (3) failure or slippage of the pedicle screws polyaxial (ball-in-socket) joint.
Screw migration would be relatively overt on radiographs and exhibited by a windshield wiper pattern adjacent to the pedicle screws. I doubt this would have escaped the authors observations. Although connecting rod deformation is possible, based on my clinical experience in bending connecting rods and more than a casual understanding of static and dynamic mechanical testing of pedicle screw constructs (I have designed and submitted American Society for Testing and Materials-tested pedicle screws for 510K designation), this is far less likely than slippage occurring at the polyaxial screw receiver elementshaft articulation.
I have long suspected that polyaxial screws would fail to maintain intraoperative sagittal plane stability in constructs having less-than-optimal anterior column load-bearing capacity. Studies by Fogel et al1 and Stanford et al2 disclosed the limitation of polyaxial screws as a whole in resisting flexion moment loads. Fogel et al1 found in static loading (single event challenge and not the more demanding fatigue or dynamic cyclical testing) in 9 different polyaxial screw designs (including the Xia [Stryker, Mahwah, New Jersey], 1 of the 2 screws used in this study) that
the polyaxial head coupling to the screw was the first failure point
Stanford et al2 concluded, The static compression bending yield loads of the designs tested [6 different polyaxial screw designs, including the Xia] barely exceeded the expected in vivo compression bending loads
. Ball-in-cup locking mechanism of the rod-screw link appeared vulnerable to fatigue failure.
These observations have not to my knowledge been examined and documented in clinical applications of polyaxial screws, yet in a number of papers such as Tokuhashi et als, interbody implant subsidence is reported in conjunction with the polyaxial screw usage. Prior to the nearly ubiquitous and universal usage of polyaxial screws in preference to monoaxial or fixed pedicle screws, overt screw failure or breakage was not uncommonly observed. It is reasonable to presume that this form of failure is less evident in polyaxial screws due to the less overt and relatively invisible failure of the ball-in-socket joint locking mechanism.
The clinical significance of this hidden flaw should not be speculatively viewed as a beneficial attribute, as Fogel et al1 concluded, stating that the coupling mechanisms failure may be a protective feature, preventing pedicle screw or rod breakage; nor should one underestimate the adverse clinical effects of loss of lordosis, particularly in fusions involving the lower lumbar motion segments of L4-L5 and L5-S1 (where two-thirds of global lumbar lordosis resides).
This article indicated that the intervertebral angle tended to be closer to 0° in all 3 groups, and the mean was 2.8°±3.9° at final examination
. I respectfully take exception to this articles subjective conclusions regarding the changes in intervertebral angle or loss of lordosis, as reflected in the text
that a good intervertebral angle was maintained. Normal values for intervertebral angles at L4-L5 should be in the range of 10° to 12° and at L3-L4 in the range of 5° to 8° (when the lordotic morphology of the superior vertebral body is not included in the endplate-to-endplate measurement, as was the case in this study).
The significance of this study may have been minimized in its conclusions, as the effects of hypolordosis (as a consequence of anterior column implant subsidence and presumed polyaxial pedicle screw slippage failure) on adjacent segments is more than a theoretical contributor to adjacent segment degeneration. Displacement of the supported bodys center of mass (less pelvis and lower extremities) anteriorly leads to compensatory mechanisms, including hyperextension of supradjacent levels and increased postural extensor muscle activity.
Umehara et al3 concluded that the hypolordosis in the instrumented segments caused a significant change in the mechanical loading of the adjacent segment.
These biomechanical effects of postoperative sagittal malalignment on the loading of the adjacent segment may contribute to the degenerative changes at the junctional level reported as long-term consequences of lumbar fusion. The anterior displacement reduces the mechanical advantage of the normal erector spinae extensor musculotendinous lever arm mechanisms, resulting in increased intradiscal pressure of adjacent motion segments.4
Clinical corroboration of adjacent segment degeneration as a consequence of postoperative segmental hypolordosis was initially provided by Djurasovic et al5,6 and confirmed by Lee et al.7 Both of these retrospective clinical studies examined several potential risk factors in 51 and 55 patients, respectively, for development of adjacent segment degeneration, and both concluded that monosegmental or oligosegmental fusion of the lumbar spine segment(s) in hypolordotic position(s) was the only significant risk factor in the development of adjacent segment degeneration.
The absence of a relationship between the final Japanese Orthopaedic Association score and the degree of cage subsidence suggests the relative insensitivity of this outcome instrument to capture and quantify these adverse biomechanical consequences rather than a reassurance, as these observers noted, of its clinical insignificance.
James F. Marino, MD
La Jolla, California
- Fogel GR, Reitman CA, Liu W, Esses SI. Physical characteristics of polyaxial-headed pedicle screws and biomechanical comparison of load with their failure. Spine (Phila Pa 1976). 2003; 28(5):470-473.
- Stanford RE, Loefler AH, Stanford PM, Walsh WR. Multiaxial pedicle screw designs: static and dynamic mechanical testing. Spine (Phila Pa 1976). 2004; 29(4):367-375.
- Umehara S, Zindrick MR, Patwardhan AG, et al. The biomechanical effect of postoperative hypolordosis in instrumented lumbar fusion on instrumented and adjacent spinal segments. Spine (Phila Pa 1976). 2000; 25(13):1617-1624.
- Tveit P, Daggfeldt K, Hetland S, Thorstensson A. Erector spinae lever arm length variations with changes in spinal curvature. Spine (Phila Pa 1976). 1994; 19(2):199-204.
- Djurasovic MO, Carreon YL, Glassman SD, Dimar, JR II, Johnson JR. Risk factors for adjacent level degeneration. Spine J. 2005; 5(4):S3.
- Djurasovic MO, Carreon LY, Glassman SD, Dimar JR II, Puno RM, Johnson JR. Sagittal alignment as a risk factor for adjacent level degeneration: a case-control study. Orthopedics. 2008; 31(6):546.
- Lee JC, Soh JW, Kim YI, Shin BJ. Analysis of risk factors of adjacent segment degeneration after fusion using pedicle screws for degenerative lumbar disease. Spine J. 2008; 8(Suppl 5):20S-21S.
In subsidence of the interbody cage, 3 factors should be considered: patient characteristics, implant quality, and surgical procedure techniques. Patient characteristics include bone strength, body weight, and grade of instability of the stabilized disks, among others. Problems involving implants include cage material and hardness, stability of pedicle screw fixation, and screw migration, which was pointed out by Dr Marino. As for the surgical procedure techniques, the quantity of endplate excision in the intervertebral disk and the grade of correction may affect subsidence of the intervertebral cage.
There have been questions about the course of cage subsidence, but few reports with long-term follow-up. Furthermore, the greatest effect on cage subsidence remains unknown, and further detailed examination is necessary.
The pedicle screw fixation system used in our study was Diapason (Stryker, Mahwah, New Jersey) and Xia (Stryker). Xia is a so-called polyaxial screw system, and Diapason is a semirigid type of ring-rod system. In other words, both systems had mechanical drawbacks in the screwrod junction, as Dr Marino pointed out.
However, there is no confirmation that patients have had a loosened junction between the screw and rod because no patients have shown a changed distance between fixed screws. Of course, screw migration, as Dr Marino pointed out, occurs since there have been changes in the intervertebral disk height. It is recognized that this phenomenon is mainly correction loss as a result of the impossibility of maintaining the correct position regardless of pedicle screw fixation. The effect of the screwbone interface is considered greater than the effect of the screwrod junction in this phenomenon.
We also examined the radiolucent zone around the screw, the so-called clear zone around the screwbone interface.1 Patients with nonunion were excluded, and no patients had a radiolucent zone around the pedicle screw. However, screw migration occurred because there were changes in the disk height even if there was no clear zone around the screw.
Many articles have described correction loss after pedicle screw fixation, but no articles have mentioned screw migration. The inevitable correction loss after instrumentation has been considered a natural result, but the mechanism and bone-implant interface have been not clarified; although screw migration is also related to correction loss, it is difficult to measure the extent of screw migration.
On radiographic evaluation, it is necessary to take images using reproducible radiography and to set up secure reference points; therefore, doubts exist about reproducible high quantitative evaluation. It is believed that the effect of cage subsidence is greater than the effect of the screwrod junction in screw migration. Also, it is thought that patient characteristics such as bone strength, the bonescrew interface, and the extent of correction are more important than the effect of the screwrod junction; however, we have no evidence concerning these factors.
Our article does not mention adjacent segment degeneration and accompanying disorders. Ten years postoperatively, the findings of adjacent disk degeneration on radiographs were shown in almost all patients, but this may not be necessarily symptomatic.
Over <8 years postoperatively on average, the possibility that the Japanese Orthopaedic Association score does not reflect slight changes in their symptoms is high. During this time, the preoperative symptoms had not persisted and there was no significant difference in the clinical outcome due to the extent of cage subsidence; however, the effect of the adjacent disk by kyphotic fusion or highly rigid fusion has been pointed out, according to Dr Marino.
In our study, a wedge-shaped cage could not achieve lordotic union and reach 0° straight union even if kyphotic union could be prevented to some extent.
Unfortunately, for posterior lumbar interbody fusion with 0° angle union, so-called straight union, the effect of adjacent segment degeneration is great and is certain to be a risk factor for adjacent segment disorders.2,3
The natural history of the coexistence of an organism and metal implants also remains unknown, and further detailed, careful follow-up is necessary.
Yasuaki Tokuhashi, MD
- Tokuhashi Y, Matsuzaki H, Oda H, Uei H. Clinical course and significance of the clear zone around the pedicle screws in the lumbar degenerative disease. Spine (Phila Pa 1976). 2008; 33(8):903-908.
- Oda I, Cunningham BW, Buckley RA, et al. Does spinal kyphotic deformity influence the biomechanical characteristics of the adjacent motion segments? An in vivo animal model. Spine (Phila Pa 1976). 1999; 24(20):2139-2146.
- Oda I, Abumi K, Yu BS, Sudo H, Minami A. Types of spinal instability that require interbody support in posterior lumbar reconstruction: an in vitro biomechanical investigation. Spine (Phila Pa 1976). 2003; 28(14):1573-1580.