The current management of atlantoaxial dislocation emphasizes on C1-2 reduction and fusion. The fusion of C1-2, however, hampers the neck movements significantly. The author has designed and developed artificial C1-2 joints that can mimic the natural joints to some extent. The concept and nuances of the design has been described. The joints have been fixed in dry cadaveric bones to demonstrate the possible movements.
The face of management of atlanto-axial dislocation (AAD) has transformed over the last few decades.,,,,,,, With varied techniques, the final goal has been reduction of the dislocation and bony fusion to achieve stability with least morbidity.,,,,,,, Although the neurological deficits improve, fusion hampers the neck movements significantly. It also adversely affects the quality of life. Not surprisingly, the next logical step would be to develop an implant that provides stability and aids in movement.,
The author has designed and developed a novel pair of artificial joints for the C1–C2 facets that not only stabilizes the C1–C2 vertebrae but also preserves most of the movements. The joint has been fixed on the dry cadaveric bones and movements have been demonstrated in Video 1.
» Concept and Ergonomics of C1–c2
Designing of the artificial joint was the biggest challenge. It could not be a simple modification of the existing artificial knee or hip joint. The concept has been drawn from the natural C1–C2 joints. Studying the natural geometry and working of the natural joint was an efficient guide for the designing of the artificial joint. The dens of the axis vertebra (C2) acts as pivot around which the atlas (C1) rotates. Rotation in the axial plane is the most important movement in the normal C1–C2 kinematics. The C1-2 level accounts for almost 50% (45°) of the rotational movement of the neck. The dens is flanked by two lateral facets. The C1 has two lateral masses that articulate with the C2 facets. They lie along the sectors of an imaginary circle, the center of which is the odontoid. These two lateral joints have a unique anatomy. They are relatively flat and provide support while the C1 rotates around the dens of C2. In normal individuals, these joints are flat in the sagittal plane. However, there is a coronal inclination of around 25°. With rotation, the facetal movements integrate into the design of a 'cone resting on another cone.' This explains the slight concave surface of the C1 inferior facet and the convex superior surface of the C2 facet. The surfaces of facets with the cartilages touching each other are convex. This reduces the contact point to one on each side. This kind of anatomy allows for some vertical movement of the dens during rotation, known as coupling. The joints also provide a few degrees of lateral bending (10–11°) and flexion–extension (5–10°). There is translation or linear movement in all the three axes. Therefore, the C1–C2 joints have six degrees of freedom of movement, the most important being axial rotation. The strong alar ligaments, the capsule, and the transverse ligaments check excessive movements.
» the Need for C1–c2 Artificial Joints
The atlanto-axial joint is relatively less stable as compared to other joints. The capsular avulsion, ligamental tears, or fracture of the dens following trauma often gives rise to dislocation. The presence of congenitally deformed lateral C1–C2 joints or lax ligaments or inflammatory diseases destroying them can also cause dislocation. The odontoid fracture (noncomminuted) can be treated with an odontoid screw that preserves the motion. However, the latter group forms only a small subset of patients with AAD and the remaining patients need C1–C2 reduction and fusion. The C1-2 fusion, however, significantly hampers neck rotation and restricts simple daily activities such as driving a car and playing sports. An artificial joint is likely to stabilize the C1–C2 joint without hampering the normal movements. This is also likely to prevent the adjacent segment disease secondary to fusion.
» Objective of the Present Invention
The purpose of this invention was not only to design an artificial C1–C2 joint that could stabilize the natural joint preventing its dislocation in any plane but also to provide mobility that closely approximates the movements of the natural joint, as far as possible. The insertion of the prosthesis should be through a noninfected route. It should be possible to treat the AAD irrespective of the pathology (congenital, degenerative, traumatic, or inflammatory) that is responsible for its genesis. The design needed to be universal so that it could be fixed onto a series of patients with varying individual anatomy at the craniovertebral junction.
An artificial atlanto-dental joint (ADJ) has been described in the recent past. However, its insertion is through the potentially infected oral cavity. Besides, the ADJ cannot be used in pathologies that involve the facets or lateral C1–C2 joints. The only motion provided by the artificial ADJ is rotational in nature.
The implant described by the author provides movements similar to the naturally occurring joints. The implant consists of two joints that work in unison. Each joint is made up of two interdigitating parts: One for the C1 facet, and the other for the C2 facet. The two joints are aligned along the circumference of an imaginary circle, the center of which is the midpoint of odontoid base. The outer edge of the C2 implant has a channel that corresponds to the circumference of the imaginary circle. The channel itself has a graduated width, narrowest as the center, and broadest at the two ends [Figure 1]. The anteroposterior length is the same as that of the facet. The outer edge of the C1 implant has a ridge or rail that houses the channel of C2 implant. The rail of C1 slides easily within the channel making circular movements possible [Figure 2]. There are locking pins on the edges of the implant that prevent rotation in excess of 22° on each side. The channel wall itself prevents the slipping of C1 laterally. The clockwise movement of one of the implants results in the counterclockwise movement of the other implant. The rail and channel has some play; this permits movements other than the circular movement also.
Figure 1: C2 components of the right and left joints as seen from the posterior aspect. Note the circular outer edge along with the channel in the lateral wall. This channel houses the circular rail from C1 component. The surface is convex
Figure 2: C1 components. The right one is upside down and left is turned 90° clockwise to show the surfaces clearly. There is rail connected to the foot plate that moves within the C2 channel. The surface is concave conforming to the convex surface of C2. There is an additional convexity on this concave surface for the gyroscopic action
The surfaces of the implants in contact with the bones are flat for an easy insertion. There are spiked ridges on each of these surfaces so that it anchors to the osseous tissue. The ridges prevent any play or movement between the bony–metal interface. The basic apposing surfaces of the implant are convex for C2 and concave for C1. There is an additional convex patch fixed to each of these apposing surfaces [Figure 1] and [Figure 2]. This makes the implant gyroscopic and reduces the contact point to one. It facilitates the coupling movements making rotation more efficient. The circumference of the imaginary circle changes with the joint anatomy and would vary from individual to individual. The implant's gyroscopic surfaces and the play provided in the rail and channel makes it universally adaptable and it can, therefore, be fitted in individuals in whom the coronal angulation of C1–C2 joints varies from 10° to 40°. The gyroscopic design coupled with a play in the 'rail and channel' also makes a small degree (8–10°) of lateral bending and flexion–extension possible. The play in the 'rail and channel' also allows for 1mm of translational movement in all the axes. Excessive movements are checked by the channel's wall.
The surfaces of the C1–C2 joints need to be drilled flat after denuding the cartilage. The insertion of the pair of implants has to done simultaneously. This requires a horse-shoe shaped holder to hold each implant on its arm. If the insertion is not proper, the implants fail to lie along the circumference of the imaginary circle (the center of which is the odontoid) and movements are restricted. Plates with holes have been provided on the posterior aspect of the C1 and C2 component. The facetal screws fasten the implants to the bone; these screws along with the ridges ensure that there is hardly any play between the bone and the implant [Figure 3]a and [Figure 3]b.
Figure 3: (a) Assembly to be inserted in bilateral C1–C2 joints simultaneously. The ridges on the surfaces along with plates secure the components of the implants to the bone. The locking pins limit excess rotation. (b) The artificial joints fixed into bilateral C1–C2 joints of dry bones
The implant does not require the presence of an intact dens, so it can be used in the presence of comminuted fracture of the dens, os odontoideum, or even in cases with an aplastic dens. Furthermore, the joints are drilled flat in the anteroposterior plane. This facilitates its use even in congenitally deformed joints or those involved by degeneration or rheumatoid arthritis.
The rapid prototyping was done with polyvinyl acrylate followed by steel using a three dimensional print and tested on cadaveric bones. Finally, titanium implants were made using computer numerical control cutting. Further testing on fresh cadavers is being carried out for the evaluation of the exact range of movement. A biomechanical testing of the device is also being carried out.
A bifid atlas is a contraindication for this implant. The splay in a bifid C1 would distort the circumference restricting the movements of the artificial joint. The implant has a metal-on-metal surface that increases the wear and tear. Implants with appropriate material within the metallic interface are under development. The placement of the artificial joint requires precision. Its use in extremely oblique or vertically oriented facets may not be feasible. Its use in cases with an anomalous vertebral artery or pseudofacets may be difficult.,,, Finally, it is yet to be used clinically and requires long-term clinical studies to assess its suitability in human subjects.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
The designing has been done by the author using solidedge software (Seimens). The implants have been manufactured at the facility at GESCO and are a proprietary item of GESCO-India. Patent has been applied for and the International search report is published online.
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