Uncinate Process

Original Editor - Rachael Lowe Top Contributors -

Description[edit | edit source]

The uncinate process of the vertebrae are associated with the uncovertebral joints, also known as the joint of Luschka, and are a unique feature of the cervical spine [1].

The uncinate processes refer to a hook-shaped process, or bony protuberance, located on the lateral or posterolateral margins of the superior endplates of the cervical vertebral bodies, most commonly found at the levels of C3-C7 [2]. These bony protuberances are positioned on each side of the associated intervertebral discs, lateral and anterior to the intervertebral foramina (IVF) [3].

In the upper cervical spine, the uncinate processes are positioned in a more anterior position in relation to the lower cervical spine, where they are positioned slightly more posterior [2].

Articulations[edit | edit source]

There are a total of four articulations that make up the uncovertebral joints, which are formed between the superolateral margins of the uncinate processes and the inferolateral portion of the vertebral body above [2].

Function[edit | edit source]

Generally speaking, the uncovertebral joints follow the movements of the cervical spine, contributing to both the mobility and stability of the spinal motion segments. Specifically, the uncovervebral joints play a role in the following:

  • Permit cervical flexion and extension.
  • Limit lateral flexion, which plays a role in preserving the integrity of the ipsilateral IVF [1].
  • Maintains the position of the intervertebral disc during axial rotation [1].

Anatomical Variation[edit | edit source]

Uncinate processes are subject to variation and may be absent on the vertebral body of C7. On occasion, they may extend down to the level of T1 and T2 [4].

Clinical Significance[edit | edit source]

The uncovertebral joints are a common site for osteoarthritic changes [1]. These hypertrophic and arthritic changes typically have no specific cause but rather are believed to be related to the dehydration and shrinkage of the intervertebral disc, leading to increased load and contact between the vertebra above and the uncinate process below. Additionally, it had been reported that the chondrocytes ability to regenerate decreases which overtime leads to thinning of the articular cartilage of the joint. This deformation of the articular cartilage is the earliest change in osteoarthritis. This thinning of the articular cartilage results in increased exposure of the subchondral bone, making the bone susceptible to chronic mechanical injuries, commonly resulting in the growth of bone spurs which are also known as osteophytes.

These osteoarthritic changes often begin in the fourth decade of life and progress to involve more uncovertebral joints and become more distorted with age [1].

The osteoarthritc changes to these uncovertebral joints preferentially appear in the lower cervical spine secondary to the relatively higher loads and stress experienced at these levels [1].

The growth of these osteophytes from the uncinaete processes project laterally and can therefore impinge on anatomical structures in the area. Some structures that have been reported to be affected include the spinal nerve root, vertebral arter, radicular (medullary( artery, cervical spinal cord, and cervical sympathetic trunk [1].

Intervertebral Foramen Stenosis[edit | edit source]

Uncovertebral osteophytes that emerge from the posterior aspect of the uncinate process may encroach the intervertebral foramen and mechanically irritate/compress the anatomical structures in the area, one of which are the cervical nerve roots[1]. In fact, uncovertebral osteophytes have been reported to be the most common cause of nerve root compression in cases of cervical spondylosis [1].

Narrowing of the IVF may also be caused by other anatomical structures encroaching the area anteriorly or posteriorly.

Structures that may narrow the IVF via posterior encroachment:

  • Zygapophysial joint degeneration,
  • Ligamenta flava and,
  • Periradicular fibrous tissue thickening

Structures that may cause narrow the IVF via anterior encroachment:

  • Protruded discs
  • "Bulging" posterior longitudinal ligament

In some cases, degenerative changes cause a decreased intervertebral disc height, which may cause the IVF size to be narrowed in superior-inferior height as well as in anterior-posterior width [5].

Signs and symptoms of a patient with cervical nerve room compression may include pain and paresthesia and well as neurological deficits including diminished pinprick sensation, diminished reflexes and muscle weakness. Wasting in the neck and upper extremity is rarely reported [1].

Nerve root compression secondary to osteophytes formation of the uncovertebral joints has been reported most commonly at the levels of C4-C6. Intervertebral stenosis is reported less commonly in the upper cervical spine.

Another structure in the intervertebral foramen that may be compressed by the formation of osteophytes on the uncovertebral joints is the radicular artery, which leads to reduced blood flow through this artery. This has been reported to be a factor that contributes to the development of cervical spondylotic myelopathy [1].

Vertebral Artery Compression[edit | edit source]

Osteophyte formation at the uncovertebral joints may develop from the anterior aspect of the uncinate process. When these osteophytes project laterally, and there is associated fibroligamentous thickening, the anteromedial wall of the vertebral artery (second part) may become compressed. Osteophytes that lead to compression of the vertebral artery occurs most often in the mid-cervical spine (verse nerve root compression that is most commonly seen in the lower cervical spine) [1].

There are a number of other reported structures that have been associated with external compression of the vertebral artery. Some examples include zygapophysial joint osteophytes (less common than osteophytes arising from the uncinate process), cervical disc herniation, muscle compression of anterior scalene and longus colli etc. [1].

Clinical symptoms of vertebral artery compression from uncovertebral osteophytes can in some cases, be similar to those of vertebrobasilar insufficiency [1].

Trauma[edit | edit source]

Severe injuries to the head and neck commonly effect the uncovertebral joints of the cervical spine.

  • One study demonstrated that out of 22 participants that had been in a motor vehicle collision causing skull fractures, 77 uncovertebral hematomas were identified. Injuries to the uncovertebral joints were commonly isolated findings in this population, however disc ruptures were occasionally present [6].
  • Another study used computed tomography (CT) scans to demonstrate that all of the participants who had a fracture or dislocation of the articular cervical pillars also had subluxated uncovertebral joints. Therefore, the authors concluded that patients presenting with evidence of uncovertebral subluxation should be further evaluated for associated fracture or dislocation of the articular pillars [7].

Torticollis[edit | edit source]

One study evaluated a total of 10 children who presented with acute stiff necks (that were tilted and rotated towards the contralateral side of pain). All participants had an MRI taken (within 12-hours of symptoms). High-intensity lesions in the region of the C2–C3 or C3–C4 uncovertebral joints on the side opposite of head rotation and tilting, was found in all participants. These authors concluded that "rapid or gradual strangulation of vascular tissue in the uncovertebral region creates a wedge of hydropic tissue that irritates the posterior longitudinal ligament causing an antalgic position" [1].

Another study that was a case study of a 15-year-old male who presented with acute torticollis. The patient was unable to perform right rotation or lateral flexion, and was followed-up with magnetic resonance imaging (MRI), A lesion containing fluid was demonstrated via MRI at the level of the right C2-C3 uncovertebral articulation. It was hypothesized that the uncovertebral cleft acutely fissured, leading to swelling, tension, and reflex muscle spasm, which was attributed to the atlantoaxial rotary fixation [8].

References[edit | edit source]

  1. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 Hartman J. Anatomy and clinical significance of the uncinate process and uncovertebral joint: a comprehensive review. Clinical Anatomy. 2014 Apr;27(3):431-40.
  2. 2.0 2.1 2.2 Moore KL, Dalley AF. Clinically oriented anatomy. Wolters kluwer india Pvt Ltd; 2018 Jul 12.
  3. Vaskovis J. Uncovertebral joints [internet]. Anatomy, spine and back, spine. Kenhub; 2020 [cited 2020Sep26]. Available from: https://www.kenhub.com/en/library/anatomy/uncovertebral-joints
  4. Tubbs RS, Rompala OJ, Verma K, Mortazavi MM, Benninger B, Loukas M, Chambers MR. Analysis of the uncinate processes of the cervical spine: an anatomical study. Journal of Neurosurgery: Spine. 2012 Apr 1;16(4):402-7.
  5. Shen FH, Samartzis D, Khanna N, Goldberg EJ, An HS. Comparison of clinical and radiographic outcome in instrumented anterior cervical discectomy and fusion with or without direct uncovertebral joint decompression. The Spine Journal. 2004 Nov 1;4(6):629-35.
  6. Jonsson Jr H, Bring G, Rauschning W, Sahlstedt B. Hidden cervical spine injuries in traffic accident victims with skull fractures. Journal of spinal disorders. 1991 Sep 1;4(3):251-63.
  7. Yetkin Z, Osborn AG, Giles DS, Haughton VM. Uncovertebral and facet joint dislocations in cervical articular pillar fractures: CT evaluation. American journal of neuroradiology. 1985 Jul 1;6(4):633-7.
  8. Maigne JY, Mutschler C, Doursounian L. Acute torticollis in an adolescent: case report and MRI study. Spine. 2003 Jan 1;28(1):E13-5.