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Clinical Case: Degenerative Sacrolisthesis

This article presents a clinical case about a man suffering from chronic and progressive lower back pain, urine retention, absent ankle tendon reflexes and decreased anal sensations. What differential diagnoses cross your mind? Is it a slipped vertebra or intervertebral disk, trauma, or inflammation? Continue reading to find out more about the cause, including radiological findings, management, and related anatomy. 

Key facts
Spondylolisthesis It is a condition involving the anterior shifting of a vertebral body in relation to the vertebra located immediately inferior to it. It can be degenerative or spondylolytic.
Pars interarticularis It is the part of vertebra located between the inferior and superior articular processes of the facet joint. Radiologically, it is represented by the neck of an imaginary 'Scottie dog' and a break at this point can lead to spondylolisthesis.
Muscle strength scale 0: No visible muscle contraction
1: Visible muscle contraction with no or trace movement
2: Limb movement, but not against gravity
3: Movement against gravity but not resistance
4: Movement against at least some resistance supplied by the examiner
5: Full strength
Tendon reflex response 0+ No response or absent reflex 
1+ Trace or Decreased response 
2+ Normal response
3+ Exaggerated or brisk response 
4+ Sustained response (clonus)
Anal wink (reflex) It is the reflexive contraction (puckering) of the external anal sphincter upon stroking of the skin around the anus. It is mediated by the inferior rectal nerves and the S2-S4 spinal cord segments.

After reviewing this case you should be able to describe the following:

  • What is meant by the pars interarticularis? How it relates to spondylolisthesis.
  • How muscle strength testing is done and measured; how tendon reflex testing is done and measured. 
  • Why bladder control was affected in this patient.
  • What is meant by the anal wink response; and what that meant for this patient.

This article is based on a case report published in the Journal "Case Reports in Surgery" in 2015, by Caroline C. Jadlowiec, Beata E. Lobel, Namita Akolkar, Michael D. Bourque, Thomas J. Devers, and David W. McFadden.

  1. Case description
    1. History
    2. Physical exam
    3. Imaging
    4. Management
    5. Recovery and evolution
  2. Anatomical and surgical considerations
  3. Explanations to objectives
    1. Objectives
    2. Pars interarticularis and spondylotisthesis relation
    3. Muscle strength and tendon reflex tests
    4. Bladder control in spondylotisthesis
    5. Anal wink response
  4. Sources
+ Show all

Case description


A 66-year-old male farmer presented to the clinic with complaints of lower back pain that had persisted for two years and that was insidious in onset, gradually progressive and non-radiating. He also complained of urine retention for the last 15 days for which he was catheterized. The patient also had a history of neurogenic claudication upon walking about 100 m. There was no history of any trauma.

Physical exam

A detailed systemic and neurological examination revealed strength of flexor hallucis longus (FHL) and flexor digitalis longus (FDL) as 4/5, absent ankle tendon reflexes, and sensory deficits in S1-S2 dermatome; rectal examination revealed decreased perianal sensations and anal tone, with an absent anal wink.


Plain radiographs of the lumbosacral spine showed spondylolisthesis of S1-S2 (Figures 1--3).

Figure 1. Lateral lower spinal radiograph showing spondylolisthesis at the S1/S2 junction. Note change in slope of curvature of the anterior margins of the vertebrae (blue arrow).

MRI and CT scans revealed lumbarization of S1 with spondylolisthesis of S1 over S2, facetal hypertrophy at L5-S1, and canal stenosis at S1-S2 and disk herniation at that level (Figures 2&3). Hematological analysis was negative.

Figure 2. Sagittal MRI showing separation of S1 and S2, with ligamentum flavum hypertrophy (blue arrow; typically associated with facet joint hypertrophy), S1/S2 disk herniation and S1/S2 spondylolisthesis.


The patient underwent posterior decompressive spinal surgery with laminotomy of S1 bilaterally and bilateral pedicular screw fixation at L5, S1, and S2 (Figure 4). S1 and S2 fixation were difficult because of the difficulty of adequate exposure of the sacrum (the trajectory was inferior). Nerve roots were decompressed and autologous bone grafting was done as part of the fusion procedure.

Figure 3. Sagittal CT showing lumbarization of S1 (note it is not part of sacrum) and the sharp reduction in the size of the sacral canal due to the sacrolisthesis (blue arrows).

Recovery and evolution

The surgery relieved the patient’s symptoms after a few weeks; bladder symptoms disappeared after three weeks and the strength of FHL/FDL improved from 4/5 to 5/5; the ankle tendon reflexes became normal after five weeks. At one year, the patient maintained this recovery.

Figure 4. Lateral postoperative radiograph showing implanted hardware to stabilize sacral vertebrae.

Anatomical and surgical considerations

Spondylolisthesis is the anterior shifting of a vertebral body in relation to the vertebra located immediately inferior to it. Degenerative spondylolisthesis (DS), as opposed to spondylolytic spondylolisthesis (see explanation 1) results from age-related degeneration of facet joints and intervertebral discs. DS usually occurs in patients older than 40 years with the deformity typically occurring at L4/5. DS of sacral vertebrae has not been previously reported and could only occur with lumbarization of S1. Sacralization of L5 and lumbarization of S1 (as in the case here, see Figures 6-8) are congenital anomalies of these vertebral segments. The anomalous segments are frequently referred to as lumbosacral transitional vertebra (LSTV). Lumbarization occurs due to non-fusion of the first two sacral segments causing the lumbar spine to appear to have six segments whereas the sacrum appears to have only four segments, rather than its normal five. 

Figure 6. Normal lumbar spine (right, B) contrasted with a person with lumbarization of S1 (blue circle) (left, A).

Sacralization occurs when one or both transverse processes of L5 fuse with the first sacral segment (or rarely, the ilium). This typically occurs bilaterally but may only occur on one side. There is no consensus as to whether these anomalies cause pain. Possibly the anomalies are associated with abnormal biomechanics, which can lead to pain.

Figure 7. Main image: Photograph of sacrum from person with lumbarization of S1. Note that there are only three sacral foramina (Main image courtesy of Dr. Shailesh Nagar). Compare with inset, which is a normal sacrum (including coccyx).

Many people who are shown to have LTSV are asymptomatic. The prevalence of complete lumbarization of S1 is under 2% and its combination with spondylolisthesis in this case is unique. How lumbarization contributed to the spondylolisthesis in the patient in this case is currently unknown.

Figure 8.: Photograph of a patient with sacralization of L5. SINGH, R. Classification and analyses of fifth pair of sacral foramina in Indian dry sacra. Int. J. Morphol., 32(1):125-130, 2014.

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