lateral mass plating

The Spine Journal 1 (2001) 166–170

Biomechanical comparison of anterior cervical plating and combined anterior/lateral mass plating Mark S. Adams, MD, Neil R. Crawford, PhD,* Robert H. Chamberlain, BSE, Volker K.H. Sonntag, MD, Curtis A. Dickman, MD Spinal Biomechanics Research Laboratory, Barrow Neurological Institute, 350 W. Thomas Road, Phoenix, AZ 85013, USA Received 11 January 2001; revised 14 March 2001; accepted 9 April 2001

Abstract

Background context: Previous studies showed anterior plates of older design to be inadequate for stabilizing the cervical spine in all loading directions. No studies have investigated enhancement in stability obtained by combining anterior and posterior plates. Purpose: To determine which modes of loading are stabilized by anterior plating after a cervical burst fracture and to determine whether adding posterior plating further significantly stabilizes the construct. Study design/setting: A repeated-measures in vitro biomechanical flexibility experiment was performed to investigate how surgical destabilization and subsequent addition of hardware components alter spinal stability. Patient sample: Six human cadaveric specimens were studied. Outcome measures: Angular range of motion (ROM) and neutral zone (NZ) were quantified during flexion, extension, lateral bending, and axial rotation. Methods: Nonconstraining, nondestructive torques were applied while recording three-dimensional motion optoelectronically. Specimens were tested intact, destabilized by simulated burst fracture with posterior distraction, plated anteriorly with a unicortical locking system, and plated with a combined anterior/posterior construct. Results: The anterior plate significantly (p.05) reduced the ROM relative to normal in all modes of loading and significantly reduced the NZ in flexion and extension. Addition of the posterior plates further significantly reduced the ROM in all modes of loading and reduced the NZ in lateral bending. Conclusions: Anterior plating systems are capable of substantially stabilizing the cervical spine in all modes of loading after a burst fracture. The combined approach adds significant stability over anterior plating alone in treating this injury but may be unnecessary clinically. Further study is needed to assess the added clinical benefits of the combined approach and associated risks. © 2001 Elsevier Science Inc. All rights reserved.

Keywords:

Burst fracture; Anterior plate; Lateral mass plate; Biomechanics; Combined anterior and posterior; Cervical spine; Stability

Introduction The treatment of cervical burst fractures with distraction injury but no bony injury of the posterior elements is controversial. Some surgeons believe that these injuries can be treated using anterior grafting and plating alone. Others beMedtronic Sofamor Danek provided instrumentation and funding for this study. * Corresponding author. Neuroscience Publications, Barrow Neurological Institute, 350 W. Thomas Road, Phoenix, AZ 85013, USA. Tel.: 1-602-406-3593; fax: 1-602-406-4104. E-mail address: [email protected] (N.R. Crawford)

lieve that reconstruction of the posterior tension band is important. A plethora of new and existing devices and surgical techniques are available to treat these fractures, and their use may be associated with significant complications and costs. Although anterior plating has been compared with posterior plating [1], the relative stability of a stand-alone anterior construct compared with a combined anterior and posterior approach has not been tested biomechanically. The purpose of this study was to compare the relative stability of a burst fracture with posterior element destruction treated with anterior plating or combined anterior and posterior plating through in vitro testing of human cadaveric

1529-9430/01/$ – see front matter © 2001 Elsevier Science Inc. All rights reserved. PII: S1529-9430(01)00 0 4 9 - 3

M.S. Adams et al. / The Spine Journal 1 (2001) 166–170

spines. Determining the relative stability of these two systems may help guide treatment of these injuries. Methods Six human cadaveric cervical spine specimens (mean age, 59.7 years) were studied (Table 1). Specimens were obtained fresh frozen, thawed in a bath of normal saline at 30 °C, and carefully cleaned of muscle tissue without damaging the ligaments, disks, or joint capsules. For testing, the caudalmost vertebral body was potted in an aluminum fixture and attached to the base of the testing apparatus. The rostralmost vertebral body was potted in a cylindrical aluminum fixture for application of loads. Nonconstraining, nondestructive torque loading was applied to each specimen through a system of cables and pulleys in conjunction with a standard servohydraulic test system (MTS, Minneapolis, MN), as described previously [2]. Loads were applied about the appropriate anatomical axes to induce flexion, extension, bidirectional lateral bending, and bidirectional axial rotation. Before data were collected, three preconditioning cycles were applied at 1.5 Nm, and a 60-second resting period was allowed for creep. In test runs, loads were applied quasistatically (held 45 seconds) in increments of 0.25 Nm to a maximum of 1.5 Nm. Three-dimensional specimen motion in response to the loads was determined using the Optotrak 3020 system (Northern Digital, Waterloo, Ontario, Canada). This system measured stereophotogrammetrically the three-dimensional displacement of infrared-emitting markers rigidly attached in a noncollinear arrangement to each vertebra. Custom software converted the marker coordinates to angular components about each of the anatomical axes using a method that models the spinal joints as overlapping cylinders [3]. After the nondestructive intact testing was completed, an injury intended to mimic a compressive flexion injury was created in each specimen. This injury was modeled by performing a C5 corpectomy with transection of the interspinous, supraspinous, yellow, and facet capsular ligaments at C5–C6 and transection of the facet capsular ligaments at C4–C5. A cadaveric fibular strut was fashioned and inserted into the defect. The appropriately sized two-level Atlantis plate (Medtronic Sofamor Danek, Memphis, TN) was placed anteriorly from C4 to C6 using four locking unicortical 3.5  14 mm screws (Table 1, Figure 1A). Posteriorly, appropriately sized three- or four-hole lateral mass plates

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(Medtronic Sofamor Danek) were secured bilaterally at C4, C5, and C6 with nonlocking 3.5  14 mm screws using the Roy-Camille technique (Table 1, Figure 1B). Testing was performed in the intact, destabilized (bone graft wedged in place), anterior plated, and combined anterior-posterior plated conditions. In half the specimens, the condition of anterior plating alone was tested before the combined anteriorposterior plated condition; in the other half, the testing order in the two plated conditions was reversed. By varying the order of testing, any advantage to being tested early in the order before any possible tissue degradation was eliminated. Specimens were wrapped with gauze and kept moist with saline during testing. Specimens were wrapped in moist gauze and frozen at the end of each test day to preserve their biomechanical properties. In any given day of experimentation, specimens were tested in only one or two of the four conditions described. Therefore, with preparation and surgical procedures, each specimen required four to six freeze-thaw cycles to complete testing of all conditions. Radiographs of the specimens were obtained to ensure proper placement of the instrumentation (Figure 2). The parameters studied were range of motion (ROM) and neutral zone (NZ). The ROM represents the amount of movement allowed in any given direction of loading, and the NZ represents the amount of laxity present near the neutral upright position before ligaments, hardware, or both are stressed substantially. The ROM and the NZ were determined for C4–C5 and C5–C6 and summed for presentation. The boundary of the NZ was defined as the position where the specimen rested after three preconditioning cycles and subsequent 60-second rest period [4]. The unidirectional ROM was defined as the angular displacement from the center of the bidirectional NZ to the angle at which the specimen experienced maximum load. Statistical analysis was conducted on motion parameters using one-tailed paired Student’s t tests to test the hypotheses that hardware decreased the mobility of a specimen relative to normal and that mobility was further decreased as each piece of hardware was attached. Data sets were tested for adequate normality to ensure the validity of a t test. In nonnormally distributed sets, Wilcoxon signed rank tests were used. Results In most loading modes, both the NZ and the ROM increased significantly with injury (Table 2, Figure 3).] The

Table 1 Clinical summary of six cadaveric specimens Specimen

Age/sex

Cause of death

Levels used

Anterior plate length (mm)

Posterior plate length (mm)

1 2 3 4 5 6

60/M 67/F 54/M 66/F 47/M 64/M

Surgical complication Coronary artery disease Cerebrovascular accident Intracerebral hemorrhage Cirrhosis Lung cancer

C3–T1 C2–C7 C3–C6 C3–C7 C3–C7 C3–C7

45 47.5 47.5 45 45 45

13 13 15 13 13 13

F  female; M  male.

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Fig. 1. Photographs of a specimen (C3–C7) instrumented with the plating systems studied. (Left) The Atlantis plate was attached over the corpectomy and graft using four unicortical screws that were locked down by turning the central set screws. (Right) Lateral mass plates were attached to C4, C5, and C6 using nonlocking

mode least affected by injury was lateral bending. Addition of an anterior plate significantly reduced the ROM relative to normal in all modes and less often significantly reduced the NZ. The largest reductions in the NZ and the ROM were

associated with flexion and extension. Addition of posterior plates further significantly reduced the ROM in all modes of loading. Reductions in the NZ also occurred, but they were less often significant (Table 2).

Fig. 2. Radiographs of one of the six specimens studied demonstrating attachment of hardware. (Left) Anteroposterior view, (Right) lateral view.

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Table 2 Changes in unidirectional C4–C6 neutral zone and range of motion Change in condition (condition 1 → condition 2) Normal → injured

Normal → anterior plate

Anterior plate → anterior  posterior plates

Neutral zone

Range of motion

Load

Change in angle (degrees)

Percent change in angle

P value

Change in angle (degrees)

Percent change in angle

P value

Flexion Extension Axial rotation Lateral bending Flexion Extension Axial rotation Lateral bending Flexion Extension Axial rotation Lateral bending

23.3  5.6 23.3  5.6 2.8  2.3 0.4  2.7 4.7  4.6 4.7  4.6 1.4  3.2 1.7  2.4 1.2  1.6 1.2  1.6 1.5  1.9 2.3  1.9

853.8  771.7 853.8  771.7 106.7  90.1 31.5  87.3 72.8  26.8 72.8  26.8 18.5  131.0 35.7  43.0 85.1  24.6 85.1  24.6 74.0  37.7 94.9  4.9

.0001* .0001* .0254* .3696 .0259* .0259* .1660 .0766 .0250*† .0250*† .0568 .0154*

18.1  3.4 23.6  5.3 7.9  2.7 2.1  1.0 13.6  4.7 8.9  4.7 6.1  4.5 5.6  3.0 3.5  3.7 3.2  2.6 4.0  3.1 4.4  3.0

110.0  31.4 222.4  89.4 76.2  16.0 22.6  11.8 79.6  16.4 71.6  14.9 56.0  32.7 54.1  24.8 89.6  14.7 92.2  5.7 84.5  9.6 92.7  5.5

.0000* .0001* .0004* .0019* .0004* .0028* .0104* .0030* .0354* .0250*† .0133* .0080*

All values other than p values are presented as mean  standard deviation. p values are from one-tailed paired Student’s t tests except †, which are from Wilcoxon signed rank tests. *Represents significant reduction or increase. Values for axial rotation and lateral bending are left or right (averaged unidirectional) values.

Discussion Previous attempts to study cervical instrumentation focused on injuries not requiring decompression and thus concentrated on posterior techniques. In an early study using first-generation anterior cervical plates, intact spines resisted axial compression, flexion-extension, and torsion load patterns better than injured spines instrumented with anterior plates [5]. Another early study using the same type of plate found that “anterior cervical plate instrumentation proved inadequate” in restoring flexural and axial compressive stability [6]. In a more recent study [7], an injury model with anterior and posterior disruption demonstrated that the stability of the normal spine was comparable to that of an injured spine stabilized by an anterior locking plate with unicortical screws or by a nonlocking plate with bicortical screws. In this study, the biomechanics of both anterior and posterior reconstruction with the latest surgical instrumentation were compared, and the results were clearly different from previous studies even though similar injury models were used. The anterior construct alone using the newer plating system provided significantly greater stability than that of the intact normal spine. This finding may reflect advances in plate design and its intrinsic properties. It could also reflect superior bone quality in the specimens studied. Regardless of the source of the different findings, we have shown that anterior plating alone is capable of significant stabilization in all rotational loading modes for the injury modeled. An anterior surgical approach offers a significant practical advantage. It allows direct decompression and stabilization through a single procedure. In this study, the anterior locking plate and graft returned the stability of the spine to well within its preinjury levels. As noted by Grubb et al. [7] “Any internal plating system is intended as a temporary adjunct to optimize bone healing. All will fail if healing does not occur.” Immobilization appears to improve the rate of fusion, and the combined approach immobilized the spine

more completely than anterior plating alone. Therefore, the use of a combined approach may be indicated when aggressive mobilization of the patient is needed or when the patient has poor bone quality or significant deformity. The extent of the clinical advantage that the added immobilization from combined hardware creates is unknown and requires further study.

Fig. 3. Mean combined C4–C6 angular motion for each mode of loading in each condition studied. The horizontal lines appearing on each bar represent the division of the range of motion (entire bar) into the neutral zone (portion nearer to zero) and the elastic zone (portion farther from zero). Error bars show standard deviation of the range of motion. Flexion, left axial rotation, and left lateral bending are positive, whereas extension, right axial rotation, and right lateral bending are negative.

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Our model of burst fracture focused mainly on extensive anterior bony injury while only representing moderate posterior soft tissue injury through transection of ligaments and joint capsules. However, some posterior damage to the bony structure of the facets may also occur in burst fractures, which was not modeled. Also, dislocation, which is often present in burst fractures, was not modeled. Dislocation can cause more extensive soft tissue tearing than distraction. Hence, our model represents only a particular division of burst fractures with mild distraction injuries to the posterior elements and without dislocation. The subset of patients that this model represents should be kept in mind when applying the study’s findings to a clinical setting. This study has some other limitations in terms of direct clinical applications. The in vivo state involves the biological process of graft incorporation, which was not modeled. Nor was the effect of cyclic fatigue on the stability of the construct investigated; this variable would indicate the extent that the hardware would loosen soon after surgery. The bone quality of the specimens used in this study was not measured and may have been better than is present in an average patient. Nevertheless, the cadaveric specimens studied probably had more underlying diseases and were older than patients who most commonly have burst fractures. The external bracing routinely used in clinical settings may also disallow the ROMs measured in this study to be reached in vivo. The added stability of the combined approach, however, may help eliminate the need for restrictive and dangerous forms of external orthoses, such as halo vests.

Conclusions In summary, this study cannot support the universal use of combined reconstruction for this type of injury. It does, however, demonstrate the added immediate stability conferred by the combined approach. Although the anteriorly inserted locking plate with graft is capable of returning the stability of the cadaver spine to well within its preinjury level, outcome studies are needed to study the clinical effects and risks associated with anterior versus combined reconstruction. References [1] Richman JD, Daniel TE, Anderson DD, Miller PL, Douglas RA. Biomechanical evaluation of cervical spine stabilization methods using a porcine model. Spine 1995;20:2192–7. [2] Crawford NR, Brantley AG, Dickman CA, Koeneman EJ. An apparatus for applying pure nonconstraining moments to spine segments in vitro. Spine 1995;20:2097–100. [3] Crawford NR, Yamaguchi GT, Dickman CA. A new technique for determining 3-D joint angles: the tilt/twist method. Clin Biomech (Bristol, Avon) 1999;14:153–65. [4] Panjabi MM. The stabilizing system of the spine. Part II. Neutral zone and instability hypothesis. J Spinal Disord 1992;5:390–7. [5] Coe JD, Warden KE, Sutterlin CE, McAfee PC. Biomechanical evaluation of cervical spinal stabilization methods in a human cadaveric model. Spine 1989;14:1122–31. [6] Sutterlin CE, McAfee PC, Warden KE, Rey RM, Farey ID. A biomechanical evaluation of cervical spinal stabilization methods in a bovine model. Static and cyclical loading. Spine 1988;13:795–802. [7] Grubb MR, Currier BL, Shih JS, Bonin V, Grabowski JJ, Chao EY. Biomechanical evaluation of anterior cervical spine stabilization. Spine 1998;23:886–92.