Cervical Lateral Mass Advances

Dorn Spinal Therapy

Spine Healing Therapy

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Praveen V. Mummanenia, Vincent C. Traynelish, Rick C. Sassoc a Department of Neurosurgery, Emory University, Atlanta, Ga., b Division of Neurosurgery, University of Iowa, Iowa City, Iowa, and c Department of Orthopaedic Surgery, Indiana University School of Medicine, Indianapolis Neurosurgical Group, Indianapolis, Ind., USA

Posterior cervical fixation utilizing lateral mass plates has been shown to be a safe and efficacious method to achieve cervical fusion [1-3]. Lateral mass plating is biomechanically superior to laminar wiring or clamping in limiting cervical motion [4-7]. In addition, unlike posterior laminar wiring or clamping, lateral mass plating does not require the presence of the posterior elements.

However, lateral mass plates have numerous drawbacks. They are difficult to contour, and the screw positions are dictated by the fixed plate entry holes. In addition, the screw trajectories are divergent from the plate entry holes, and the connection of the screw to the plate is not rigid. There is no space to pack autograft bone under the screw-plate connection. Screws placed medially or laterally cannot be captured by the plate. Successive screws cannot be compressed or distracted because of the fixed plate hole distances. Moreover, if the plate needs to be revised, the screws must be removed. Finally, most of the systems currently available do not easily allow for extension of fusion up to the occiput or down to the thoracic spine [8].

The ideal posterior cervical instrumentation system will address these shortfalls from the lateral mass plate systems. Currently, there are three commercially available systems that address the problems of lateral mass plates. These systems do allow for initial screw placement with subsequent rod contouring. The three systems are Starlock/Cervifix (Synthes USA, Paoli, Pa., USA), SUMMIT (DePuy Acromed, Raynham, Mass., USA), and VERTEX (Medtronic Sofamor Danek, Memphis, Tenn., USA).

Instrumentation

The three systems that allow for initial screw placement with subsequent rod attachment differ from each other in several ways.

Starlock/Cervifix Systems (Synthes)

Unlike the VERTEX and SUMMIT systems, the Starlock/Cervifix systems do not have polyaxial screw heads. These systems require the use of an intervening closed-loop eyebolt to connect the screw to the rod and, as a result, the connection of the rod to the screw is not a locked, rigid attachment.

Starlock and Cervifix are two instrumentation sets from Synthes that have interchangeable hardware. Cervifix is more restrictive than Starlock because it requires the surgeon to place the lateral mass screws through closed loop eye-bolts, which are already mounted on a rod. The closed-loop eyebolts have the capability of sliding up and down the rod, thus enabling the screws to be placed variably in the sagittal plane [9]. Starlock is an improved version of this system that requires attaching closed-loop eye bolts to standard lateral mass screws and then subsequently threading the rod through the closed-loop eyebolts. However, the threading of a contoured rod through closed-loop eyebolts can be tedious and sometimes impossible [9].

Both the Starlock and Cervifix systems have features that distinguish them from standard lateral mass plate systems. They both have occipital plate extensions to accommodate occipitocervical fusions and laminar hooks for use when lateral mass screw attachment is not possible. It is possible to accommodate thoracic pedicle screws into the systems with appropriate rod contouring and attachment of the closed-loop eyebolts to upper thoracic pedicle screws. Starlock/Cervifix are also compatible with the thoracic hook and rod system marketed by Synthes, and can be attached to a separate thoracic instrumentation system via an adaptive connector.

VERTEX System (Medtronic Sofamor Danek)

The VERTEX system allows for initial polyaxial screw placement with subsequent multiplanar rod contouring and attachment with or without offset connectors. The capability of direct connection of the polyaxial screw to the rod allows for a locked, rigid attachment of the rod to the screw. This is biome-chanically a sounder construct than the closed-loop eyebolt connection in the Starlock/Cervifix system.

The novel VERTEX polyaxial screw heads are especially useful for facilitating rod attachments in patients with severely abnormal cervical curvatures. The polyaxial screw heads lock to the rod via top loading locking cap screws that fit inside the screw head. The VERTEX system's polyaxial screw heads are very low profile as a result of this attachment scheme [10].

The VERTEX system is easily adaptable for occipital and thoracic extensions. It consists of 6- to 10-mm titanium occipital screws (which are not polyaxial) and 14- to 18-mm titanium polyaxial cervicothoracic screws. In addition, noncannulated 40-50 mm transarticular titanium polyaxial screws are available for C1/C2 fusion. The system's titanium rods are malleable in three dimensions, and there is an available rod with an occipital plate on one end to allow for occipitocervical fusions.

With the VERTEX system, occipitocervical fusion can be performed, but the nonpolyaxial occipital screws must be placed through the apertures in the occipital plates. The polyaxial cervicothoracic screws, however, are placed independently of the rod system. These polyaxial 14- to 18-mm screws are ideal for placement in the C1 lateral mass, C2 pars, C3-7 lateral masses, and upper thoracic pedicles [10, 11]. The contoured rods are then linked either directly to the polyaxial screw heads with a locking cap screw or are linked via an offset connector.

SUMMIT System (DePuy Acromed)

The SUMMIT system shares many of the features of the VERTEX system. SUMMIT polyaxial screw heads are placed independently of the rod and are then connected directly to the rod through a rigid locked connection. Recently, an adaptive occipital extension has been made available, and an extension into the thoracic spine is also possible.

However, the SUMMIT polyaxial screw heads are higher profile than are the VERTEX screws, and the locking cap fits around the polyaxial screw head instead of within it, thus further increasing the profile of the system. Consequently, thin patients who have the SUMMIT system implanted in the posterior cervical spine may experience discomfort from the higher profile instrumentation. In addition, in our experience, the higher profile of the polyaxial screw heads of the SUMMIT system makes it more difficult to fit the contoured rod into the screw heads in severely spondylotic patients whose lateral masses are very close together as a result of their hyperlordosis.

Surgical Technique for Placement of Posterior Cervical Instrumentation

Exposure

A standard midline posterior cervical exposure is performed to reveal the lateral aspects of the cervical facets. The exposure is extended for one to two levels below the inferior end of the planned arthrodesis to allow for optimal screw placement. In patients with marked degenerative changes, the osteo-phytes on the dorsal facets are removed to provide better visualization, to help define the anatomy of the facets, and to provide a suitable surface to allow the polyaxial screw heads to rotate. However, care should be taken to preserve, where possible, the posterior cortex of the articular mass in order to provide for better screw purchase.

In cases where posterior cervical decompression is necessary, we drill and tap pilot holes for the cervical lateral mass screws prior to performing the full laminectomies in order to preserve the normal anatomic landmarks for the screw trajectories. In addition, the lamina serve to protect the neural elements during screw hole preparation.

For occipitocervical fusions, we expose the suboccipital area up to the inion.

For cervicothoracic fusions, we expose the thoracic transverse processes. At C7 and T1, when canal decompression is not necessary, at least minimal laminoforaminotomies are performed to expose the medial walls of the C7 and T1 pedicles to help guide C7 and T1 pedicle screw placement.

Screw Placement (VERTEX System)

We prefer to use the VERTEX system for posterior occipitocervicothoracic arthrodesis. This system has the advantage of having low profile polyaxial screw heads that can attach either directly or via an offset connector to the rod.

After the exposure is completed, we turn our attention to cervical polyaxial screw placement. Initially, we perforate the posterior cortices of the lateral masses with a high-speed drill. Our screw trajectories for C3-7 are based on the guidelines set by Haid, Papadopoulos, and Sonntag (unpubl. data, presented at the American Association of Neurological Surgeons Annual Meeting, 1991) and further elucidated by McCafferty et al. [12]. Entry points are 1 mm medial to the center of the lateral mass and trajectories are 20-30° cephalad and 20-30° lateral (fig. 1, 2). We 'normalize' the entry point and screw trajectory at each lateral mass to allow for changes in the orientation of the lateral masses secondary to accentuated cervical lordosis or kyphosis and to allow for each patient's unique pathoanatomy.

Some surgeons elect to place screws with the assistance of fluoroscopy or image guidance. However, we do not routinely use fluoroscopy or image guidance (except for screw placement into C1 and C2). Consequently, attention to the patient's unique cervical anatomy is of paramount importance for us.

For screw placement into the lateral mass of C1, we utilize the technique described by Harms and Melcher [13] and refined by Fiore et al. [11]. The screw entry point is at the junction of the posterior arch of C1 and the center of the posterior, inferior C1 lateral mass. The C2 nerve root is gently retracted inferiorly with a Penfield 4 to expose the screw entry point. The screw trajectory is parallel to the plane of the C1 lamina and is aimed straight anterior from

Root Fluroscopy

Fig. 1. Illustration of the lateral mass screw trajectory in the axial plane. a Appropriate angle. b The screw trajectory puts the nerve root at risk.

Root Fluroscopy

Fig. 2. Lateral mass screw trajectory in the sagittal plane.

the entry point. Screw depth is guided by lateral fluoroscopy and can be estimated by preoperative Stealth CT planning.

For screw placement into the C2 pars, we pay close attention to the preoperative CT to assess the course of the vertebral artery. In addition, we palpate the medial pars with a blunt probe to help guide our screw trajectory. We utilize a screw entry point 3 mm superior and lateral ('3 mm up and out') to the medial aspect of the C2/3 facet joint. The screw trajectory is 10-15° medial and 35° cephalad. We typically use 4-mm width and 16-mm length screws for the C2 pars [10, 11].

For C1/2 transarticular screw placement, the entry point and trajectory are the same as for C2 pars screws; the screw length, however, is longer (up to 50 mm in some cases) [14, 15]. We prefer to use lateral fluoroscopy for guidance when placing C1/2 transarticular screws.

Screw placement at C7 is dependent on the bony anatomy. We scrutinize the preoperative CT scan to assess if the patient's C7 lateral mass has a typical cervical anatomy or has a transitional thoracic anatomy with a well-formed pedicle. When the C7 anatomy is transitional, we prefer to place a C7 pedicle screw.

For pedicle screw placement at C7 or in the upper thoracic spine, we expose and palpate the medial walls of the pedicles and utilize an entry point 1 mm below the center of the facet joint. Our screw trajectory is typically 25-30° medial while maintaining a perpendicular angle in the sagittal plane [10]. We have found it useful to use the drill to create a small pilot hole for entry of the tap into the pedicle. We then use the 4.0-mm tap from the VERTEX set to create the screw path and simultaneously tap the pedicle. This instrument is delicate and particularly useful to find an appropriate trajectory into the C7 and thoracic pedicles. It has a tendency to be 'sucked down' the pedicle with gentle manipulation in experienced hands.

In the thoracic spine, pedicle screws also can be placed laterally into the costotransverse joint to achieve greater cortical purchase [16].

After polyaxial screw placement, the appropriate posterior decompressions are performed based on the patient's symptoms. In addition, the facet joints to be fused are stripped of cartilage and decorticated with a high-speed drill and packed with autograft.

The final step is rod contouring and attachment. A rod template is used to estimate the rod length and rod contour required. A small endotracheal tube stylet is particularly good for use as a rod template. The titanium rods are then measured, cut, contoured, and directly attached to the polyaxial screw heads with locking cap screws. In cases where the patient's pathoanatomy requires significantly different lateral or medial screw positions at successive levels, we utilize small offset connectors in order to facilitate rod attachment.

When occipitocervical fusions are planned, we utilize a specialized rod with an occipital plate on the cephalad end. The rod and occipital plate are contoured based on the rod trial. We position the occipital plate near the midline occipital keel to provide for the most bony purchase for the occipital screws. The occipital screws are not polyaxial and must be placed through the apertures in the occipital plates. Ideally six occipital screws (three on each side) should be placed near the midline of the suboccipital bone. This has been shown to be the most biomechanically favorable position for suboccipital fixation [17] (fig. 3).

After the instrumentation construct is placed, but before final tightening of the construct, we compress, distract, or laterally rotate each successive segment as needed to restore normal cervical lordosis and sagittal balance. Autograft is then packed over the tops of the fusion sites.

Suboccipital Cervical Fusion

Discussion and Conclusion

Lateral mass plating has been shown to be an effective method of achieving posterior cervical arthrodesis and stabilization [3]. Reported complication rates are low. Injuries to the cervical spinal cord or vertebral artery have not been reported in recent large published series on lateral mass plating [3, 5, 18-20]. The reported rate of radiculopathy from malpositioned screws has ranged from 0 to 6% of patients [3, 5, 18-20].

However, lateral mass plates are of limited use when fusing from the occiput to the thoracic spine in patients with abnormal cervical anatomy because of their lack of malleability and their predetermined screw hole trajectories. The Starlock/Cervifix systems overcome some of these problems but are suboptimal because they require threading of a contoured rod through closed loop eyebolts, which is not a rigid connection and which can be tedious and sometimes impossible [9].

The ideal posterior cervical instrumentation system will allow for initial screw placement with subsequent multiplanar rod contouring and attachment via a rigid connection with a locking cap screw. In addition, offset connectors are needed to allow for screw capture at any distance laterally away from the rod (as is the case with C7 and T1 pedicle screws (fig. 4). Finally, the system should be low profile so that the polyaxial screw heads will not be crowded in cases with cervical hyperlordosis.

The new VERTEX and SUMMIT posterior polyaxial cervical screw and rod systems satisfy many of the criteria for the ideal posterior cervical instrumentation system. They have the versatility to accommodate occipitocervical fusions with C1 lateral mass screws and C2 pars screws. In addition, they allow

Laterol Mass Screws

Fig. 4. Different screw trajectory and entry point needed for C7 and T1 pedicle screws as compared to cervical lateral mass screws.

Fig. 5. Photograph of the VERTEX system. Note the polyaxial screw heads, the offset connector, and the laminar hook.

for lateral mass fixation from C3-7 as well as pedicle fixation and laminar hook placement in the lower cervical and upper thoracic spine without the limitations inherent in placing screws through holes in lateral mass plates (fig. 5-8). Finally, the rods are easily contoured and attached directly and rigidly to the polyaxial screw heads.

Fig. 6. Photograph of occipitocervicothoracic fixation in a saw bone model utilizing the VERTEX system.

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  • kiros
    Where is the cervical lateral mass?
    1 year ago

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