The Spine Journal 14 (2014) 1010–1016
Basic Science
Biomechanical analysis of the upper thoracic spine after decompressive procedures Andrew T. Healy, MDa,*, Daniel Lubelski, BAb, Prasath Mageswaran, PhDc, Deb A. Bhowmick, MDd, Adam J. Bartsch, PhD, PEc, Edward C. Benzel, MDa, Thomas E. Mroz, MDa a
Department of Neurosurgery, Neurological Institute, Cleveland Clinic, 9500 Euclid Ave, S4, Cleveland, OH 44195, USA b Cleveland Clinic Lerner College of Medicine, 9500 Euclid Ave, NA21, Cleveland, OH 44195, USA c Head, Neck & Spine Research Laboratory, 1730 W. 25th St, Lutheran Hospital, 2C, Cleveland Clinic, Cleveland, OH 44195, USA d Department of Neurosurgery, University of North Carolina, 170 Manning Dr, Campus Box 7060, Chapel Hill, NC 27599, USA Received 30 May 2013; revised 2 October 2013; accepted 21 November 2013
Abstract
BACKGROUND CONTEXT: Decompressive procedures such as laminectomy, facetectomy, and costotransversectomy are routinely performed for various pathologies in the thoracic spine. The thoracic spine is unique, in part, because of the sternocostovertebral articulations that provide additional strength to the region relative to the cervical and lumbar spines. During decompressive surgeries, stability is compromised at a presently unknown point. PURPOSE: To evaluate thoracic spinal stability after common surgical decompressive procedures in thoracic spines with intact sternocostovertebral articulations. STUDY DESIGN: Biomechanical cadaveric study. METHODS: Fresh-frozen human cadaveric spine specimens with intact rib cages, C7–L1 (n59), were used. An industrial robot tested all spines in axial rotation (AR), lateral bending (LB), and flexion-extension (FE) by applying pure moments (65 Nm). The specimens were first tested in their intact state and then tested after each of the following sequential surgical decompressive procedures at T4–T5 consisting of laminectomy; unilateral facetectomy; unilateral costotransversectomy, and subsequently instrumented fusion from T3–T7. RESULTS: We found that in all three planes of motion, the sequential decompressive procedures caused no statistically significant change in motion between T3–T7 or T1–T12 when compared with intact. In comparing between intact and instrumented specimens, our study found that instrumentation reduced global range of motion (ROM) between T1–T12 by 16.3% (p5.001), 12% (p5.002), and 18.4% (p5.0004) for AR, FE, and LB, respectively. Age showed a negative correlation with motion in FE (r50.78, p5.01) and AR (r50.7, p5.04). CONCLUSIONS: Thoracic spine stability was not significantly affected by sequential decompressive procedures in thoracic segments at the level of the true ribs in all three planes of motion in intact thoracic specimens. Age appeared to negatively correlate with ROM of the specimen. Our study suggests that thoracic spinal stability is maintained immediately after unilateral decompression at the level of the true ribs. These preliminary observations, however, do not depict the long-
FDA device/drug status: Not applicable. Author disclosures: ATH: Nothing to disclose. DL: Nothing to disclose. PM: Nothing to disclose. DAB: Nothing to disclose. AJB: Nothing to disclose. ECB: Royalties: Elsevier Pub (B), Thieme Pub (B); Stock Ownership: Axiomed (F), Orthomems (E), Turning Point (B); Consulting: Axiomed (None); Speaking and/or Teaching Arrangements: Multiple (None); Trips/Travel: Multiple (None); Grants: OREF (F, Paid directly to institution), Rawlings (F, Paid directly to institution). TEM: Stock Ownership: PearlDiver Inc (None); Consulting: Globus Medical (B); Speaking and/or Teaching Arrangements: AO Spine (B). 1529-9430/$ - see front matter Ó 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.spinee.2013.11.035
The disclosure key can be found on the Table of Contents and at www. TheSpineJournalOnline.com. This study was supported in part by funds received through the Cleveland Clinic Stanley Zielony Spinal Research & Education Fund, the Neurosurgery Research and Education Foundation (NREF), and the Cleveland Clinic Research Programs Committee. * Corresponding author. Department of Neurosurgery, Neurological Institute, Cleveland Clinic, 9500 Euclid Ave, S4, Cleveland, OH 44118, USA. Tel.: (216) 444-2200; fax: (216) 636-0454. E-mail address: [email protected] (A.T. Healy)
A.T. Healy et al. / The Spine Journal 14 (2014) 1010–1016
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term sequelae of such procedures and warrant further investigation. Ó 2014 Elsevier Inc. All rights reserved. Keywords:
Thoracic; Spine; Decompression; Costotransversectomy; Biomechanics; Cadaver
Introduction Decompressive procedures of the thoracic spine (ie, laminectomy, facetectomy, and costotransversectomy) are performed for numerous pathologies, including disc herniation, infection, tumor, and trauma. Unlike the cervical and lumbar spines, however, there is little evidence that quantifies the biomechanical consequences of these types of procedures in the thoracic region. The thoracic spine is unique, in part, because of the stenocostovertebral articulations that afford increased stiffness and stability relative to the cervical and lumbar spines. The intervertebral discs, supra/interspinous ligaments (SIL), and rib cage contribute to a majority of the stability of the thoracic spine [1–5]. The SIL mainly contributes to the flexion range of motion (ROM) and may be specifically important at levels adjacent to long constructs [4]. The rib cage, or ‘‘the fourth column,’’ [6] has been shown to contribute up to 78% of the thoracic spinal stability [2]. Specifically, it limits the ROM of the thoracic spine by 40% in flexion-extension (FE), 35% in lateral bending (LB), and 31% in axial rotation (AR) [5]. During decompressive surgeries in the thoracic spine, stability is compromised at a presently unknown threshold. The decision regarding the use of an instrumented fusion in such cases can substantially affect the patient’s outcome by either providing insufficient stability or causing unnecessary additional surgery (ie, instrumented fusion). Cost considerations are also a concern, and it is possible that costly procedures are currently being performed on the thoracic spine, absent of data that proves their mechanical usefulness. Presently, there exist no guidelines delineating indications for a fusion procedure after the various thoracic spine decompression procedures. The purpose of the present study was to define the angular ROM in the thoracic spine after laminectomy, laminectomy with unilateral facetectomy, laminectomy with unilateral costotransversectomy, and the subsequent addition of pedicle screw instrumentation. With this information, we intended to provide an algorithm for the use of fusion after thoracic decompressive surgery.
Methods Specimen preparation Nine (n59) fresh-frozen human cadaveric spine specimens, spanning C7–L1, were used that included the sternum, ribs, and all articulations intact. Computed tomography and dual-energy X-ray absorptiometry scans of each specimen were carried out to determine preexisting spinal
pathology or fusion and the bone mineral density (BMD) of each specimen before biomechanical testing. Specimens with previous spinal surgery, spinal implants, soft-tissue abnormalities, or fractures were excluded from this study. Specimens with kyphosis (more than 65 ) or severe scoliosis were excluded from the sample. The average age of the specimens was 59 years (69.5 years) with an average height of 170 cm (68.6 cm) and an average BMD of 0.941 g/cm2 (60.11 g/cm2). There were five female and four male specimens. The existing conditions, in addition to the documented cause of death of each specimen, are listed in Table 1. One patient had been diagnosed with rheumatoid arthritis; however, no gross or radiographic changes characteristic of rheumatoid arthritis were found with this specimen. Before testing, the specimens were removed from a 20 C freezer, thawed, and the surrounding musculature was meticulously dissected, leaving all ligamentous and articular attachments preserved. Custom-designed spinal fixtures were used to secure the spine cranially and caudally onto a robotic spine testing system. The cranial (C7–T1) and caudal (T12–L1) levels were mounted onto the custom test fixtures using pedicle screws and rods. The test setup was further secured using wood screws inserted into the cranial and caudal vertebral bodies and embedded in Cereband, a liquid metal alloy (HiTech Alloys, Squamish, WA, USA). Multidirectional biomechanical testing An industrial robot (KUKA, GmbH, Augsburg, Germany) capable of motion in six axes was used as the spine testing apparatus for implementing in vitro multidirectional flexibility tests. It was used to apply pure moments on the spinal segments through custom-designed mounting fixtures (Fig. 1). Multidirectional testing was carried out in three orthogonal directions. These three test directions corresponded to FE, bilateral LB, and bilateral AR of the thoracic spine. The specimens were unconstrained so as to allow for natural coupled motion of the spine. A six-axes, force-moment sensor (GAMMA; ATI, Apex, NC, USA) was used to measure the applied load and provide feedback for the robot. The sensor also measured the off-axis forces and moments to provide feedback to ensure that a pure moment was being applied along the primary axis of motion of the spine. Three-dimensional motion was monitored continuously using an optoelectronic camera system (Optotrak Certus; Northern Digital, Inc., Waterloo, ON, Canada) at a rate of 20 Hz. The camera system measured the vertebral motion by tracking the relative motion between the infrared markers placed on rigid body
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Table 1 Specimen demographics and average bone density from DEXA scans Weight (Ibs)
Gender
Smoker
Cause of death
Conditions
Average bone density (g/cm2)
50 4 50 7 60 1 50 5 50 7
114 190 150 150 220
M M M F F
Y Y Y Y Y
COPD MI Cirrhosis Alzheimer’s DM
0.777 1.019 1.1 0.979 0.962
63 52 45
50 5 50 2 50 9
111 143 119
F F F
N Y N
Cholangiocarcinoma COPD Glioblastoma
68
50 10
140
M
N
Renal failure
PNA, CAD, MI, GIB HTN, CAD CKD, CVA, IVDA OA, CVA, CAD, COPD, DM CHF, pericarditis, CVA, ESRD, OA Sepsis, perforated viscus CHF, SVT, DM, RA, OA Seizures, GBM, chemotherapy Dialysis, MI, DM, OA
Specimen
Age (y)
1 2 3 4 5
68 67 55 65 51
6 7 8 9
Height
0.862 1.059 0.92 0.789
DEXA, dual-energy X-ray absorptiometry; M, male; F, female; Y, yes; N, no; OA, osteoarthritis; CVA, cerebral vascular disease; CAD, coronary artery disease; COPD, chronic obstructive pulmonary disease; DM, diabetes mellitus; CHF, congestive heart failure; SVT, supraventricular tachycardia; RA, rheumatoid arthritis; ESRD, end-stage renal disease; MI, myocardial infarction; PNA, pneumonia; GIB, gastrointestinal bleed; HTN, hypertension; CKD, chronic kidney disease; IVDA, intravenous drug abuse; GBM, glioblastoma multiforme.
vertebral segments. This system has a measurement accuracy of 60.1 mm in translation and 60.1 in rotation. For this study, motion markers were attached to the spinous processes of T1, T3, T7, and T12. Posttest analyses were used to determine the ROM between segments. After testing the first four specimens, it was felt the addition of optoelectric sensors to measure intrinsic ROM between T3 and
T7 would better depict changes across the levels of decompression. This was carried out in the final five specimens. On the day of testing, the specimen was mounted on the robotic testing fixtures. The caudal spinal fixture was attached to a base pedestal, while the cranial fixture was attached to the force-moment sensor fixed to the robotic arm. The spine’s posture was adjusted to neutral and this position was recorded by the test system. Nondestructive flexibility testing was then performed on each specimen. The specimens were preconditioned to minimize any viscoelastic effects by being subjected to three cycles of FE, LB, and AR at an applied pure moment of 65 Nm, while continuously minimizing all other off-axis loads. The mounted spine was kept moist during testing by periodically lightly spraying the exposed tissues with saline solution. Before the start of a new test condition or sequence, the spine was returned to its initial neutral position. Test conditions
Fig. 1. Robotic spine testing system. Pictured in the figure is the Kuka robot (Augsburg, Germany) with complete thoracic torso fitted with custom fixture at the Spine Research Lab, Cleveland, OH, USA.
Flexibility tests were conducted on each intact specimen and then repeated again after each sequential surgical decompressive procedure performed at T4–T5 and instrumented fusion spanning at T3–T7. The sequence of surgical procedures (Fig. 2) was as follows, (1) laminectomy; (2) unilateral facetectomy; (3) unilateral costotransversectomy; and (4) instrumented pedicle screw fixation. All spinal implants for simulated fusion and associated surgical tools were supplied by Medtronic (Medtronic, Inc., Minneapolis, MN, USA). Pedicle screws (size: 5.035 mm) were placed in the bilateral pedicles of the two supraadjacent and subadjacent levels: T3, T4, T6, and T7. Pedicle screws were placed using the free-hand technique [7]. The entry point was decorticated with a high-speed drill and the appropriate trajectory through cancellous bone was determined with a Medtronic ‘‘pedicle finder’’ blunt probe. This trajectory was tapped with a 4.0 mm cortical tap and adequate pedicle placement was confirmed with a sound probe. Pedicle screws were joined by a 5.5 mm
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Fig. 2. Surgical procedures—(A) laminectomy, (B) unilateral facetectomy, (C) unilateral costotraversectomy, and (D) instrumented fusion (T3–T7).
titanium rod. The surgical procedures (1–4) were performed by a fellowship-trained spine surgeon and a neurosurgical resident (TEM and ATH).
Data analysis Global (T1–T12) and intrinsic intersegmental ROM (T3–T7) was obtained for each specimen at the peak moment (5 Nm) in the three primary planes of motion for each test condition. Each specimen thus, acted as its own control and, thereby, interspecimen variability was addressed. The nine specimens were tested in the intact state and after the surgical procedures described previously, carried out at the level of T4–T5. We measured global ROM between T1 and T12 in all nine specimens and, additionally, the intrinsic ROM across T3–T7 in five specimens. Statistical analysis was performed using Minitab 16 (Minitab, Inc., State College, PA, USA). A repeated measures analysis of variance was used to analyze the global and intrinsic ROM between test conditions with a 95% level of significance. Posthoc Tukey-Kramer analysis (p!.05 was considered statistically significant) was used for multiple comparisons of the ROM between conditions.
Results We found that interspecimen analysis of both global and intrinsic ROM in the native state showed variability between the specimens (p!.05, Fig. 3). The following preprocedural variables: gender, smoker, weight, height, and BMD did not significantly correlate with motion in the intact state in all planes of motion. However, age showed a negative correlation with motion in FE (r50.78, p5.01) and AR (r50.7, p5.04). Tables 2 and 3 show the mean global and intrinsic ROM with standard deviation values for each test condition in FE, LB, and AR, respectively. A graphical representation of the mean intrinsic ROM is shown in Fig. 4. We found that neither the global ROM, nor the intrinsic ROM exhibited statistically significant increases after laminectomy, unilateral facetectomy, and costotransversectomy, respectively, compared with the intact state. There was an increase in motion in FE from 10.6 to 11.8 (p5.9) across T3–T7 and from 26.9 to 29.2 (p5.09) across T1–T12, when comparing intact with costotransversectomy, respectively. The imaging of more mobile specimens within this cohort, however, were qualitatively observed to be less fused in terms of the absence of
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A.T. Healy et al. / The Spine Journal 14 (2014) 1010–1016 Table 3 Mean intrinsic ROM with standard deviation values for FE, LB, and AR Intrinsic ROM (n55)
AR ( )
FE ( )
LB ( )
Intact Laminectomy
11.1564.34 10.6064.94 (p5.98) 10.7964.78 (p5.99) 10.9464.50 (p5.99) 1.9762.74 (p5.00) 82.33%
10.6265.27 11.4166.02 (p5.99) 11.7866.00 (p5.95) 11.8465.94 (p5.94) 0.72160.54 (p5.00) 93.20%
17.9667.09 16.4368.04 (p5.94) 16.7367.33 (p5.97) 16.7667.14 (p5.98) 1.6560.37 (p5.00) 90.82%
Facetectomy Costotransversectomy Fusion Percentage decrease from intact to fusion
ROM, range of motion; FE, flexion-extension; LB, lateral bending; AR, axial rotation. Note: p Values: between group comparison with respect to intact only.
T3–T7) decreased by 82%, 93%, and 91%, whereas the global ROM (from T1–T12) decreased by 16.3%, 12.0%, and 18.4% in AR, FE, and LB, respectively. Discussion
Fig. 3. Distribution of global ROM values for (Top) each specimen in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) and (Bottom) mean FE, LB, and AR for all specimens after laminectomy, facetectomy, and costotransversectomy. ROM, range of motion.
ankylosis across disc spaces, facet joints, and spinous processes relative to the less mobile specimens. Motion in AR, LB, and FE were all significantly decreased after pedicle screw fixation (T3–T7) for both intrinsic and global ROM values. The intrinsic ROM (from Table 2 Mean global ROM with standard deviation values for FE, LB, and AR Global ROM (n59)
AR ( )
FE ( )
LB ( )
Intact Laminectomy
43.69616.87 43.90619.36 (p5.99) 44.30619.71 (p5.99) 44.13619.54 (p5.99) 36.59615.36 (p5.00) 16.25%
26.90610.00 28.7869.43 (p5.24) 29.1269.30 (p5.12) 29.2169.41 (p5.094) 23.6867.12 (p5.01) 11.97%
42.06619.04 40.84618.08 (p5.99) 43.21617.30 (p5.99) 43.47617.10 (p5.98) 34.31612.56 (p5.00) 18.43%
Facetectomy Costotransversectomy Fusion Percentage decrease from intact to fusion
ROM, range of motion; FE, flexion-extension; LB, lateral bending; AR, axial rotation. Note: p Values: between group comparison with respect to intact only.
Previous studies have analyzed the contribution of various components of the thoracic spine to its overall stability [1,2,8]. The bony articulations account for only a minority of the thoracic spinal column stability, whereas the intervertebral discs, SIL, and rib cage contribute a majority of the stability [2–5]. Horton et al. [3] performed bilateral facetectomy at four consecutive levels in the upper thoracic spine in specimens with intact rib cage and the ROM was increased by only 12%. Andriacchi et al. [9] demonstrated, via computer modeling studies in the 1970s, that the rib cage contributed an additional 27% of flexion stiffness and 132% of extension stiffness. Unfortunately, only a small proportion of biomechanical studies of the thoracic spine include the rib cage [2,3,5,10]. Brasiliense et al. [2] used cadaveric models to determine that the rib cage added 181% increase in flexion stiffness and 702% in extension. Therefore, in the setting of an intact rib cage, it is conceivable that a complete facetectomy and even costotransversectomy would not compromise spinal stability. Feiertag et al. [10] showed that complete facetectomy and further segmental rib head resection did not significantly increase ROM when anterior column (disc space) and lamina/SIL were left intact. However, Oda et al. [11] demonstrated that in the setting of laminectomy, bilateral medial facetectomy significantly increased ROM even with presence of dorsal ribs and intervertebral disc. Moreover, resection of the bilateral lateral facet complex led to further destabilization and the authors commented that the lateral facet complex should be spared when attempting to preserve stability [11]. This study, however, did not include the complete rib cage with sternocostal joints that confer much of the added stability afforded by the rib cage [3,5]. In contrast to the aforementioned in vitro studies, a
A.T. Healy et al. / The Spine Journal 14 (2014) 1010–1016
Fig. 4. Distribution of intrinsic ROM values for (Top) each specimen in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) and (Bottom) mean FE, LB, and AR for all specimens after laminectomy, facetectomy, and costotransversectomy. ROM, range of motion.
number of clinical situations require laminectomy with unilateral facetectomy or even costotransversectomy, disrupting SIL, unilateral facet, and the costovertebral joint, but sparing the anterior column to reach ventral intra- or extradural pathology. Segmental disruption from costotransversectomy has not been investigated with an intact rib cage to date. In our study, laminectomy with or without unilateral facetectomy or costotransversectomy in the thoracic spine (at the level of the true ribs) did not statistically change the motion of the thoracic spine in AR, FE, or LB. We found the interspecimen ROM of the specimens, in the intact state, to be variable in all three modalities (Fig. 3). Specimen-specific data were analyzed and we found a statistically significant association between age and ROM. Specifically, two of our three most mobile specimens were noted to be the youngest specimens in our study (45 and 51 years old). Anecdotally, these specimens did not show significant ankylosis on preprocedural computed tomography imaging. More rigorous grading of bridging ossification may potentially predict specimen mobility and more
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importantly, may predict the patients that will tolerate decompression without instrumented fusion. Further investigation will need to address these findings. Our decompressive procedures disrupted adjacent ligamentous integrity only when called for. The laminectomy was carried out just medial to the facet joint and care was taken not to disrupt overlying soft tissue and facet capsule. Laminectomy was performed in a clinically relevant fashion, taking the lamina of both T4 and T5 and the adjacent SIL. Previous studies have shown that the SIL contributes significantly to the flexion ROM of the thoracic spine [1], however, there were no significant changes in ROM after our procedure in specimens that included the entire thoracic spine and rib cage with intact stenocostovertebral articulations. Of note, the global ROM group (n59) had a greater increase in FE mobility (26.9 to 28.8 ; p5.2) compared with the change in the intrinsic group (10.6 to 11.4 ; p5.98). This was felt to potentially reflect the presence of a more mobile specimen in the global ROM group that was not included in the intrinsic group (Specimen #2). The ROM of Specimen #2 was found to increase by 49%, 123%, 133% in LB after laminectomy, facetectomy, and costotransversectomy, respectively. This specimen was one of our oldest specimens; thus, going against the group trend of a negative correlation of motion to age. Qualitatively, we observed that the preprocedural imaging of this specimen indicated that the spine was ‘‘unfused’’ in terms of lacking bridging osteophytes and spondylosis. This specimen may be an outlier whose data may be the result of technical error or tissue degradation/deterioration during the thawing process; however, if accurate, these values may reflect destabilization and determining whether the imaging of this specimen could predict its hypermobility is of great clinical interest. Oda et al. [11] performed laminectomy, then bilateral medial facetectomy, and then bilateral complete facetectomy. In this scenario, bilateral medial facetectomy significantly increased the ROM. The authors commented that the lateral facet complex should be spared when attempting to preserve stability when bilateral facet joints are compromised. This study included 5 cm of dorsal rib, leaving costovertebral joints intact; however, the sternocostal articulations were not. Even without supporting rib cage articulations, Deniz et al. [8] showed that a complete unilateral facetectomy added only a negligible amount of motion when compared with a partial facetectomy in the setting of discectomy. Pedicle screw fixation with dorsal instrumentation reduced ROM of the thoracic spine by 16.3%, 12.0%, and 18.4% in AR, FE, and LB, respectively. It is conceivable that motion may be more dramatically affected when evaluating lower thoracic vertebrae at the level of false or floating ribs. In future investigations, we intend to study the effect of fusing the lower thoracic segments on global ROM of the thoracic spine. The limitations of our study include the use of cadaveric specimens. Although specimen processing is intended to be
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uniform and previous studies have validated the preservation of osseous and ligamentous integrity [3,5], the laxity found in Specimen #2 may reflect an anomalous component of the tissue handling process. A larger number of specimens may be useful to better define this variability. A larger number of specimens could also help us analyze whether age or preprocedural imaging correlate to in vitro biomechanics (ie, specimen laxity) or predispose to specimen instability. Another limitation is the relatively restricted nature of the population that this study addresses. It is specific to the thoracic spine at the level of the true ribs in patients with an intact anterior column. Costotransversectomy is commonly performed for ventral pathology that is often in the form of vertebral body destructive lesions (ie, tumor, trauma, and osteomyelitis). However, surgeons often use lateral decompression, in the form of complete facetectomy or costotransversectomy, to reach ventral intra- or extradural pathology. Perhaps these findings will encourage wider decompression if it provides the surgeon with greater exposure and less risk of injury to the thoracic spinal cord. Finally, the nature of such a ROM testing does not necessarily rule out long-term morbidity including pain secondary to pathologic motion, vulnerability to catastrophic failure, or progressive deformity. When compared with procedures performed routinely in the clinical setting, however, the change in flexion ROM that we have found with unilateral costotransversectomy (8.5%) is substantially lower compared with that seen in biomechanical evaluations after multilevel lumbar laminectomy (32%) [12]. The concern associated with added destabilization from unilateral facet or costovertebral resection is not supported by our data.
Conclusion Our study suggests that laminectomy, unilateral facetectomy, and unilateral costotransversectomy at the level of the true ribs did not significantly alter the ROM in our cadaveric model, suggesting that such procedures may not require instrumentation; however, long-term consequences cannot be ruled out and require future investigation. Age appeared to negatively correlate with specimen ROM; however, other demographic variables such as gender, height,
weight, BMD, or smoking history were not found to predict the ROM.
Acknowledgments The authors would like to thank Robb Colbrunn, PhD, and Tara Bonner, MS, of the Cleveland Clinic Biorobotics Team for their tremendous support and technical expertise throughout the biomechanical testing process. References [1] Anderson AL, McIff TE, Asher MA, et al. The effect of posterior thoracic spine anatomical structures on motion segment flexion stiffness. Spine 2009;34:441–6. [2] Brasiliense LBC, Lazaro BCR, Reyes PM, et al. Biomechanical contribution of the rib cage to thoracic stability. Spine 2011;36:E1686–93. [3] Horton WC, Kraiwattanapong C, Akamaru T, et al. The role of the sternum, costosternal articulations, intervertebral disc, and facets in thoracic sagittal plane biomechanics: a comparison of three different sequences of surgical release. Spine 2005;30:2014–23. [4] Kretzer RM, Hu N, Umekoji H, et al. The effect of spinal instrumentation on kinematics at the cervicothoracic junction: emphasis on soft-tissue response in an in vitro human cadaveric model. J Neurosurg Spine 2010;13:435–42. [5] Watkins R 4th, Watkins R 3rd, Williams L, et al. Stability provided by the sternum and rib cage in the thoracic spine. Spine 2005;30: 1283–6. [6] Berg EE. The sternal-rib complex. A possible fourth column in thoracic spine fractures. Spine 1993;18:1916–9. [7] Kim YJ, Lenke LG. Thoracic pedicle screw placement: free-hand technique. Neurol India 2005;53:512–9. [8] Deniz FE, Brasiliense LBC, Lazaro BCR, et al. Biomechanical evaluation of posterior thoracic transpedicular discectomy. J Neurosurg Spine 2010;13:253–9. [9] Andriacchi T, Schultz A, Belytschko T, Galante J. A model for studies of mechanical interactions between the human spine and rib cage. J Biomech 1974;7:497–507. [10] Feiertag MA, Horton WC, Norman JT, et al. The effect of different surgical releases on thoracic spinal motion. A cadaveric study. Spine 1995;20:1604–11. [11] Oda I, Abumi K, Cunningham BW, et al. An in vitro human cadaveric study investigating the biomechanical properties of the thoracic spine. Spine 2002;27:E64–70. [12] Lee MJ, Bransford RJ, Bellabarba C, et al. The effect of bilateral laminotomy versus laminectomy on the motion and stiffness of the human lumbar spine: a biomechanical comparison. Spine 2010;35: 1789–93.
Basic Science
Biomechanical analysis of the upper thoracic spine after decompressive procedures Andrew T. Healy, MDa,*, Daniel Lubelski, BAb, Prasath Mageswaran, PhDc, Deb A. Bhowmick, MDd, Adam J. Bartsch, PhD, PEc, Edward C. Benzel, MDa, Thomas E. Mroz, MDa a
Department of Neurosurgery, Neurological Institute, Cleveland Clinic, 9500 Euclid Ave, S4, Cleveland, OH 44195, USA b Cleveland Clinic Lerner College of Medicine, 9500 Euclid Ave, NA21, Cleveland, OH 44195, USA c Head, Neck & Spine Research Laboratory, 1730 W. 25th St, Lutheran Hospital, 2C, Cleveland Clinic, Cleveland, OH 44195, USA d Department of Neurosurgery, University of North Carolina, 170 Manning Dr, Campus Box 7060, Chapel Hill, NC 27599, USA Received 30 May 2013; revised 2 October 2013; accepted 21 November 2013
Abstract
BACKGROUND CONTEXT: Decompressive procedures such as laminectomy, facetectomy, and costotransversectomy are routinely performed for various pathologies in the thoracic spine. The thoracic spine is unique, in part, because of the sternocostovertebral articulations that provide additional strength to the region relative to the cervical and lumbar spines. During decompressive surgeries, stability is compromised at a presently unknown point. PURPOSE: To evaluate thoracic spinal stability after common surgical decompressive procedures in thoracic spines with intact sternocostovertebral articulations. STUDY DESIGN: Biomechanical cadaveric study. METHODS: Fresh-frozen human cadaveric spine specimens with intact rib cages, C7–L1 (n59), were used. An industrial robot tested all spines in axial rotation (AR), lateral bending (LB), and flexion-extension (FE) by applying pure moments (65 Nm). The specimens were first tested in their intact state and then tested after each of the following sequential surgical decompressive procedures at T4–T5 consisting of laminectomy; unilateral facetectomy; unilateral costotransversectomy, and subsequently instrumented fusion from T3–T7. RESULTS: We found that in all three planes of motion, the sequential decompressive procedures caused no statistically significant change in motion between T3–T7 or T1–T12 when compared with intact. In comparing between intact and instrumented specimens, our study found that instrumentation reduced global range of motion (ROM) between T1–T12 by 16.3% (p5.001), 12% (p5.002), and 18.4% (p5.0004) for AR, FE, and LB, respectively. Age showed a negative correlation with motion in FE (r50.78, p5.01) and AR (r50.7, p5.04). CONCLUSIONS: Thoracic spine stability was not significantly affected by sequential decompressive procedures in thoracic segments at the level of the true ribs in all three planes of motion in intact thoracic specimens. Age appeared to negatively correlate with ROM of the specimen. Our study suggests that thoracic spinal stability is maintained immediately after unilateral decompression at the level of the true ribs. These preliminary observations, however, do not depict the long-
FDA device/drug status: Not applicable. Author disclosures: ATH: Nothing to disclose. DL: Nothing to disclose. PM: Nothing to disclose. DAB: Nothing to disclose. AJB: Nothing to disclose. ECB: Royalties: Elsevier Pub (B), Thieme Pub (B); Stock Ownership: Axiomed (F), Orthomems (E), Turning Point (B); Consulting: Axiomed (None); Speaking and/or Teaching Arrangements: Multiple (None); Trips/Travel: Multiple (None); Grants: OREF (F, Paid directly to institution), Rawlings (F, Paid directly to institution). TEM: Stock Ownership: PearlDiver Inc (None); Consulting: Globus Medical (B); Speaking and/or Teaching Arrangements: AO Spine (B). 1529-9430/$ - see front matter Ó 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.spinee.2013.11.035
The disclosure key can be found on the Table of Contents and at www. TheSpineJournalOnline.com. This study was supported in part by funds received through the Cleveland Clinic Stanley Zielony Spinal Research & Education Fund, the Neurosurgery Research and Education Foundation (NREF), and the Cleveland Clinic Research Programs Committee. * Corresponding author. Department of Neurosurgery, Neurological Institute, Cleveland Clinic, 9500 Euclid Ave, S4, Cleveland, OH 44118, USA. Tel.: (216) 444-2200; fax: (216) 636-0454. E-mail address: [email protected] (A.T. Healy)
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term sequelae of such procedures and warrant further investigation. Ó 2014 Elsevier Inc. All rights reserved. Keywords:
Thoracic; Spine; Decompression; Costotransversectomy; Biomechanics; Cadaver
Introduction Decompressive procedures of the thoracic spine (ie, laminectomy, facetectomy, and costotransversectomy) are performed for numerous pathologies, including disc herniation, infection, tumor, and trauma. Unlike the cervical and lumbar spines, however, there is little evidence that quantifies the biomechanical consequences of these types of procedures in the thoracic region. The thoracic spine is unique, in part, because of the stenocostovertebral articulations that afford increased stiffness and stability relative to the cervical and lumbar spines. The intervertebral discs, supra/interspinous ligaments (SIL), and rib cage contribute to a majority of the stability of the thoracic spine [1–5]. The SIL mainly contributes to the flexion range of motion (ROM) and may be specifically important at levels adjacent to long constructs [4]. The rib cage, or ‘‘the fourth column,’’ [6] has been shown to contribute up to 78% of the thoracic spinal stability [2]. Specifically, it limits the ROM of the thoracic spine by 40% in flexion-extension (FE), 35% in lateral bending (LB), and 31% in axial rotation (AR) [5]. During decompressive surgeries in the thoracic spine, stability is compromised at a presently unknown threshold. The decision regarding the use of an instrumented fusion in such cases can substantially affect the patient’s outcome by either providing insufficient stability or causing unnecessary additional surgery (ie, instrumented fusion). Cost considerations are also a concern, and it is possible that costly procedures are currently being performed on the thoracic spine, absent of data that proves their mechanical usefulness. Presently, there exist no guidelines delineating indications for a fusion procedure after the various thoracic spine decompression procedures. The purpose of the present study was to define the angular ROM in the thoracic spine after laminectomy, laminectomy with unilateral facetectomy, laminectomy with unilateral costotransversectomy, and the subsequent addition of pedicle screw instrumentation. With this information, we intended to provide an algorithm for the use of fusion after thoracic decompressive surgery.
Methods Specimen preparation Nine (n59) fresh-frozen human cadaveric spine specimens, spanning C7–L1, were used that included the sternum, ribs, and all articulations intact. Computed tomography and dual-energy X-ray absorptiometry scans of each specimen were carried out to determine preexisting spinal
pathology or fusion and the bone mineral density (BMD) of each specimen before biomechanical testing. Specimens with previous spinal surgery, spinal implants, soft-tissue abnormalities, or fractures were excluded from this study. Specimens with kyphosis (more than 65 ) or severe scoliosis were excluded from the sample. The average age of the specimens was 59 years (69.5 years) with an average height of 170 cm (68.6 cm) and an average BMD of 0.941 g/cm2 (60.11 g/cm2). There were five female and four male specimens. The existing conditions, in addition to the documented cause of death of each specimen, are listed in Table 1. One patient had been diagnosed with rheumatoid arthritis; however, no gross or radiographic changes characteristic of rheumatoid arthritis were found with this specimen. Before testing, the specimens were removed from a 20 C freezer, thawed, and the surrounding musculature was meticulously dissected, leaving all ligamentous and articular attachments preserved. Custom-designed spinal fixtures were used to secure the spine cranially and caudally onto a robotic spine testing system. The cranial (C7–T1) and caudal (T12–L1) levels were mounted onto the custom test fixtures using pedicle screws and rods. The test setup was further secured using wood screws inserted into the cranial and caudal vertebral bodies and embedded in Cereband, a liquid metal alloy (HiTech Alloys, Squamish, WA, USA). Multidirectional biomechanical testing An industrial robot (KUKA, GmbH, Augsburg, Germany) capable of motion in six axes was used as the spine testing apparatus for implementing in vitro multidirectional flexibility tests. It was used to apply pure moments on the spinal segments through custom-designed mounting fixtures (Fig. 1). Multidirectional testing was carried out in three orthogonal directions. These three test directions corresponded to FE, bilateral LB, and bilateral AR of the thoracic spine. The specimens were unconstrained so as to allow for natural coupled motion of the spine. A six-axes, force-moment sensor (GAMMA; ATI, Apex, NC, USA) was used to measure the applied load and provide feedback for the robot. The sensor also measured the off-axis forces and moments to provide feedback to ensure that a pure moment was being applied along the primary axis of motion of the spine. Three-dimensional motion was monitored continuously using an optoelectronic camera system (Optotrak Certus; Northern Digital, Inc., Waterloo, ON, Canada) at a rate of 20 Hz. The camera system measured the vertebral motion by tracking the relative motion between the infrared markers placed on rigid body
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Table 1 Specimen demographics and average bone density from DEXA scans Weight (Ibs)
Gender
Smoker
Cause of death
Conditions
Average bone density (g/cm2)
50 4 50 7 60 1 50 5 50 7
114 190 150 150 220
M M M F F
Y Y Y Y Y
COPD MI Cirrhosis Alzheimer’s DM
0.777 1.019 1.1 0.979 0.962
63 52 45
50 5 50 2 50 9
111 143 119
F F F
N Y N
Cholangiocarcinoma COPD Glioblastoma
68
50 10
140
M
N
Renal failure
PNA, CAD, MI, GIB HTN, CAD CKD, CVA, IVDA OA, CVA, CAD, COPD, DM CHF, pericarditis, CVA, ESRD, OA Sepsis, perforated viscus CHF, SVT, DM, RA, OA Seizures, GBM, chemotherapy Dialysis, MI, DM, OA
Specimen
Age (y)
1 2 3 4 5
68 67 55 65 51
6 7 8 9
Height
0.862 1.059 0.92 0.789
DEXA, dual-energy X-ray absorptiometry; M, male; F, female; Y, yes; N, no; OA, osteoarthritis; CVA, cerebral vascular disease; CAD, coronary artery disease; COPD, chronic obstructive pulmonary disease; DM, diabetes mellitus; CHF, congestive heart failure; SVT, supraventricular tachycardia; RA, rheumatoid arthritis; ESRD, end-stage renal disease; MI, myocardial infarction; PNA, pneumonia; GIB, gastrointestinal bleed; HTN, hypertension; CKD, chronic kidney disease; IVDA, intravenous drug abuse; GBM, glioblastoma multiforme.
vertebral segments. This system has a measurement accuracy of 60.1 mm in translation and 60.1 in rotation. For this study, motion markers were attached to the spinous processes of T1, T3, T7, and T12. Posttest analyses were used to determine the ROM between segments. After testing the first four specimens, it was felt the addition of optoelectric sensors to measure intrinsic ROM between T3 and
T7 would better depict changes across the levels of decompression. This was carried out in the final five specimens. On the day of testing, the specimen was mounted on the robotic testing fixtures. The caudal spinal fixture was attached to a base pedestal, while the cranial fixture was attached to the force-moment sensor fixed to the robotic arm. The spine’s posture was adjusted to neutral and this position was recorded by the test system. Nondestructive flexibility testing was then performed on each specimen. The specimens were preconditioned to minimize any viscoelastic effects by being subjected to three cycles of FE, LB, and AR at an applied pure moment of 65 Nm, while continuously minimizing all other off-axis loads. The mounted spine was kept moist during testing by periodically lightly spraying the exposed tissues with saline solution. Before the start of a new test condition or sequence, the spine was returned to its initial neutral position. Test conditions
Fig. 1. Robotic spine testing system. Pictured in the figure is the Kuka robot (Augsburg, Germany) with complete thoracic torso fitted with custom fixture at the Spine Research Lab, Cleveland, OH, USA.
Flexibility tests were conducted on each intact specimen and then repeated again after each sequential surgical decompressive procedure performed at T4–T5 and instrumented fusion spanning at T3–T7. The sequence of surgical procedures (Fig. 2) was as follows, (1) laminectomy; (2) unilateral facetectomy; (3) unilateral costotransversectomy; and (4) instrumented pedicle screw fixation. All spinal implants for simulated fusion and associated surgical tools were supplied by Medtronic (Medtronic, Inc., Minneapolis, MN, USA). Pedicle screws (size: 5.035 mm) were placed in the bilateral pedicles of the two supraadjacent and subadjacent levels: T3, T4, T6, and T7. Pedicle screws were placed using the free-hand technique [7]. The entry point was decorticated with a high-speed drill and the appropriate trajectory through cancellous bone was determined with a Medtronic ‘‘pedicle finder’’ blunt probe. This trajectory was tapped with a 4.0 mm cortical tap and adequate pedicle placement was confirmed with a sound probe. Pedicle screws were joined by a 5.5 mm
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Fig. 2. Surgical procedures—(A) laminectomy, (B) unilateral facetectomy, (C) unilateral costotraversectomy, and (D) instrumented fusion (T3–T7).
titanium rod. The surgical procedures (1–4) were performed by a fellowship-trained spine surgeon and a neurosurgical resident (TEM and ATH).
Data analysis Global (T1–T12) and intrinsic intersegmental ROM (T3–T7) was obtained for each specimen at the peak moment (5 Nm) in the three primary planes of motion for each test condition. Each specimen thus, acted as its own control and, thereby, interspecimen variability was addressed. The nine specimens were tested in the intact state and after the surgical procedures described previously, carried out at the level of T4–T5. We measured global ROM between T1 and T12 in all nine specimens and, additionally, the intrinsic ROM across T3–T7 in five specimens. Statistical analysis was performed using Minitab 16 (Minitab, Inc., State College, PA, USA). A repeated measures analysis of variance was used to analyze the global and intrinsic ROM between test conditions with a 95% level of significance. Posthoc Tukey-Kramer analysis (p!.05 was considered statistically significant) was used for multiple comparisons of the ROM between conditions.
Results We found that interspecimen analysis of both global and intrinsic ROM in the native state showed variability between the specimens (p!.05, Fig. 3). The following preprocedural variables: gender, smoker, weight, height, and BMD did not significantly correlate with motion in the intact state in all planes of motion. However, age showed a negative correlation with motion in FE (r50.78, p5.01) and AR (r50.7, p5.04). Tables 2 and 3 show the mean global and intrinsic ROM with standard deviation values for each test condition in FE, LB, and AR, respectively. A graphical representation of the mean intrinsic ROM is shown in Fig. 4. We found that neither the global ROM, nor the intrinsic ROM exhibited statistically significant increases after laminectomy, unilateral facetectomy, and costotransversectomy, respectively, compared with the intact state. There was an increase in motion in FE from 10.6 to 11.8 (p5.9) across T3–T7 and from 26.9 to 29.2 (p5.09) across T1–T12, when comparing intact with costotransversectomy, respectively. The imaging of more mobile specimens within this cohort, however, were qualitatively observed to be less fused in terms of the absence of
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A.T. Healy et al. / The Spine Journal 14 (2014) 1010–1016 Table 3 Mean intrinsic ROM with standard deviation values for FE, LB, and AR Intrinsic ROM (n55)
AR ( )
FE ( )
LB ( )
Intact Laminectomy
11.1564.34 10.6064.94 (p5.98) 10.7964.78 (p5.99) 10.9464.50 (p5.99) 1.9762.74 (p5.00) 82.33%
10.6265.27 11.4166.02 (p5.99) 11.7866.00 (p5.95) 11.8465.94 (p5.94) 0.72160.54 (p5.00) 93.20%
17.9667.09 16.4368.04 (p5.94) 16.7367.33 (p5.97) 16.7667.14 (p5.98) 1.6560.37 (p5.00) 90.82%
Facetectomy Costotransversectomy Fusion Percentage decrease from intact to fusion
ROM, range of motion; FE, flexion-extension; LB, lateral bending; AR, axial rotation. Note: p Values: between group comparison with respect to intact only.
T3–T7) decreased by 82%, 93%, and 91%, whereas the global ROM (from T1–T12) decreased by 16.3%, 12.0%, and 18.4% in AR, FE, and LB, respectively. Discussion
Fig. 3. Distribution of global ROM values for (Top) each specimen in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) and (Bottom) mean FE, LB, and AR for all specimens after laminectomy, facetectomy, and costotransversectomy. ROM, range of motion.
ankylosis across disc spaces, facet joints, and spinous processes relative to the less mobile specimens. Motion in AR, LB, and FE were all significantly decreased after pedicle screw fixation (T3–T7) for both intrinsic and global ROM values. The intrinsic ROM (from Table 2 Mean global ROM with standard deviation values for FE, LB, and AR Global ROM (n59)
AR ( )
FE ( )
LB ( )
Intact Laminectomy
43.69616.87 43.90619.36 (p5.99) 44.30619.71 (p5.99) 44.13619.54 (p5.99) 36.59615.36 (p5.00) 16.25%
26.90610.00 28.7869.43 (p5.24) 29.1269.30 (p5.12) 29.2169.41 (p5.094) 23.6867.12 (p5.01) 11.97%
42.06619.04 40.84618.08 (p5.99) 43.21617.30 (p5.99) 43.47617.10 (p5.98) 34.31612.56 (p5.00) 18.43%
Facetectomy Costotransversectomy Fusion Percentage decrease from intact to fusion
ROM, range of motion; FE, flexion-extension; LB, lateral bending; AR, axial rotation. Note: p Values: between group comparison with respect to intact only.
Previous studies have analyzed the contribution of various components of the thoracic spine to its overall stability [1,2,8]. The bony articulations account for only a minority of the thoracic spinal column stability, whereas the intervertebral discs, SIL, and rib cage contribute a majority of the stability [2–5]. Horton et al. [3] performed bilateral facetectomy at four consecutive levels in the upper thoracic spine in specimens with intact rib cage and the ROM was increased by only 12%. Andriacchi et al. [9] demonstrated, via computer modeling studies in the 1970s, that the rib cage contributed an additional 27% of flexion stiffness and 132% of extension stiffness. Unfortunately, only a small proportion of biomechanical studies of the thoracic spine include the rib cage [2,3,5,10]. Brasiliense et al. [2] used cadaveric models to determine that the rib cage added 181% increase in flexion stiffness and 702% in extension. Therefore, in the setting of an intact rib cage, it is conceivable that a complete facetectomy and even costotransversectomy would not compromise spinal stability. Feiertag et al. [10] showed that complete facetectomy and further segmental rib head resection did not significantly increase ROM when anterior column (disc space) and lamina/SIL were left intact. However, Oda et al. [11] demonstrated that in the setting of laminectomy, bilateral medial facetectomy significantly increased ROM even with presence of dorsal ribs and intervertebral disc. Moreover, resection of the bilateral lateral facet complex led to further destabilization and the authors commented that the lateral facet complex should be spared when attempting to preserve stability [11]. This study, however, did not include the complete rib cage with sternocostal joints that confer much of the added stability afforded by the rib cage [3,5]. In contrast to the aforementioned in vitro studies, a
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Fig. 4. Distribution of intrinsic ROM values for (Top) each specimen in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) and (Bottom) mean FE, LB, and AR for all specimens after laminectomy, facetectomy, and costotransversectomy. ROM, range of motion.
number of clinical situations require laminectomy with unilateral facetectomy or even costotransversectomy, disrupting SIL, unilateral facet, and the costovertebral joint, but sparing the anterior column to reach ventral intra- or extradural pathology. Segmental disruption from costotransversectomy has not been investigated with an intact rib cage to date. In our study, laminectomy with or without unilateral facetectomy or costotransversectomy in the thoracic spine (at the level of the true ribs) did not statistically change the motion of the thoracic spine in AR, FE, or LB. We found the interspecimen ROM of the specimens, in the intact state, to be variable in all three modalities (Fig. 3). Specimen-specific data were analyzed and we found a statistically significant association between age and ROM. Specifically, two of our three most mobile specimens were noted to be the youngest specimens in our study (45 and 51 years old). Anecdotally, these specimens did not show significant ankylosis on preprocedural computed tomography imaging. More rigorous grading of bridging ossification may potentially predict specimen mobility and more
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importantly, may predict the patients that will tolerate decompression without instrumented fusion. Further investigation will need to address these findings. Our decompressive procedures disrupted adjacent ligamentous integrity only when called for. The laminectomy was carried out just medial to the facet joint and care was taken not to disrupt overlying soft tissue and facet capsule. Laminectomy was performed in a clinically relevant fashion, taking the lamina of both T4 and T5 and the adjacent SIL. Previous studies have shown that the SIL contributes significantly to the flexion ROM of the thoracic spine [1], however, there were no significant changes in ROM after our procedure in specimens that included the entire thoracic spine and rib cage with intact stenocostovertebral articulations. Of note, the global ROM group (n59) had a greater increase in FE mobility (26.9 to 28.8 ; p5.2) compared with the change in the intrinsic group (10.6 to 11.4 ; p5.98). This was felt to potentially reflect the presence of a more mobile specimen in the global ROM group that was not included in the intrinsic group (Specimen #2). The ROM of Specimen #2 was found to increase by 49%, 123%, 133% in LB after laminectomy, facetectomy, and costotransversectomy, respectively. This specimen was one of our oldest specimens; thus, going against the group trend of a negative correlation of motion to age. Qualitatively, we observed that the preprocedural imaging of this specimen indicated that the spine was ‘‘unfused’’ in terms of lacking bridging osteophytes and spondylosis. This specimen may be an outlier whose data may be the result of technical error or tissue degradation/deterioration during the thawing process; however, if accurate, these values may reflect destabilization and determining whether the imaging of this specimen could predict its hypermobility is of great clinical interest. Oda et al. [11] performed laminectomy, then bilateral medial facetectomy, and then bilateral complete facetectomy. In this scenario, bilateral medial facetectomy significantly increased the ROM. The authors commented that the lateral facet complex should be spared when attempting to preserve stability when bilateral facet joints are compromised. This study included 5 cm of dorsal rib, leaving costovertebral joints intact; however, the sternocostal articulations were not. Even without supporting rib cage articulations, Deniz et al. [8] showed that a complete unilateral facetectomy added only a negligible amount of motion when compared with a partial facetectomy in the setting of discectomy. Pedicle screw fixation with dorsal instrumentation reduced ROM of the thoracic spine by 16.3%, 12.0%, and 18.4% in AR, FE, and LB, respectively. It is conceivable that motion may be more dramatically affected when evaluating lower thoracic vertebrae at the level of false or floating ribs. In future investigations, we intend to study the effect of fusing the lower thoracic segments on global ROM of the thoracic spine. The limitations of our study include the use of cadaveric specimens. Although specimen processing is intended to be
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uniform and previous studies have validated the preservation of osseous and ligamentous integrity [3,5], the laxity found in Specimen #2 may reflect an anomalous component of the tissue handling process. A larger number of specimens may be useful to better define this variability. A larger number of specimens could also help us analyze whether age or preprocedural imaging correlate to in vitro biomechanics (ie, specimen laxity) or predispose to specimen instability. Another limitation is the relatively restricted nature of the population that this study addresses. It is specific to the thoracic spine at the level of the true ribs in patients with an intact anterior column. Costotransversectomy is commonly performed for ventral pathology that is often in the form of vertebral body destructive lesions (ie, tumor, trauma, and osteomyelitis). However, surgeons often use lateral decompression, in the form of complete facetectomy or costotransversectomy, to reach ventral intra- or extradural pathology. Perhaps these findings will encourage wider decompression if it provides the surgeon with greater exposure and less risk of injury to the thoracic spinal cord. Finally, the nature of such a ROM testing does not necessarily rule out long-term morbidity including pain secondary to pathologic motion, vulnerability to catastrophic failure, or progressive deformity. When compared with procedures performed routinely in the clinical setting, however, the change in flexion ROM that we have found with unilateral costotransversectomy (8.5%) is substantially lower compared with that seen in biomechanical evaluations after multilevel lumbar laminectomy (32%) [12]. The concern associated with added destabilization from unilateral facet or costovertebral resection is not supported by our data.
Conclusion Our study suggests that laminectomy, unilateral facetectomy, and unilateral costotransversectomy at the level of the true ribs did not significantly alter the ROM in our cadaveric model, suggesting that such procedures may not require instrumentation; however, long-term consequences cannot be ruled out and require future investigation. Age appeared to negatively correlate with specimen ROM; however, other demographic variables such as gender, height,
weight, BMD, or smoking history were not found to predict the ROM.
Acknowledgments The authors would like to thank Robb Colbrunn, PhD, and Tara Bonner, MS, of the Cleveland Clinic Biorobotics Team for their tremendous support and technical expertise throughout the biomechanical testing process. References [1] Anderson AL, McIff TE, Asher MA, et al. The effect of posterior thoracic spine anatomical structures on motion segment flexion stiffness. Spine 2009;34:441–6. [2] Brasiliense LBC, Lazaro BCR, Reyes PM, et al. Biomechanical contribution of the rib cage to thoracic stability. Spine 2011;36:E1686–93. [3] Horton WC, Kraiwattanapong C, Akamaru T, et al. The role of the sternum, costosternal articulations, intervertebral disc, and facets in thoracic sagittal plane biomechanics: a comparison of three different sequences of surgical release. Spine 2005;30:2014–23. [4] Kretzer RM, Hu N, Umekoji H, et al. The effect of spinal instrumentation on kinematics at the cervicothoracic junction: emphasis on soft-tissue response in an in vitro human cadaveric model. J Neurosurg Spine 2010;13:435–42. [5] Watkins R 4th, Watkins R 3rd, Williams L, et al. Stability provided by the sternum and rib cage in the thoracic spine. Spine 2005;30: 1283–6. [6] Berg EE. The sternal-rib complex. A possible fourth column in thoracic spine fractures. Spine 1993;18:1916–9. [7] Kim YJ, Lenke LG. Thoracic pedicle screw placement: free-hand technique. Neurol India 2005;53:512–9. [8] Deniz FE, Brasiliense LBC, Lazaro BCR, et al. Biomechanical evaluation of posterior thoracic transpedicular discectomy. J Neurosurg Spine 2010;13:253–9. [9] Andriacchi T, Schultz A, Belytschko T, Galante J. A model for studies of mechanical interactions between the human spine and rib cage. J Biomech 1974;7:497–507. [10] Feiertag MA, Horton WC, Norman JT, et al. The effect of different surgical releases on thoracic spinal motion. A cadaveric study. Spine 1995;20:1604–11. [11] Oda I, Abumi K, Cunningham BW, et al. An in vitro human cadaveric study investigating the biomechanical properties of the thoracic spine. Spine 2002;27:E64–70. [12] Lee MJ, Bransford RJ, Bellabarba C, et al. The effect of bilateral laminotomy versus laminectomy on the motion and stiffness of the human lumbar spine: a biomechanical comparison. Spine 2010;35: 1789–93.
