Sharp-Purser Test: Purpose, Procedure, Interpretation, and Clinical Significance

What is the Sharp-Purser Test?

The Sharp-Purser test is a clinical orthopedic assessment used to evaluate the stability of the atlantoaxial joint (the joint between the first and second cervical vertebrae, C1 and C2) and, specifically, the integrity of the transverse ligament.

Understanding the Cervical Spine and Its Vulnerabilities

The cervical spine, particularly its upper segments (C1, the atlas, and C2, the axis), is a marvel of anatomical design, allowing for the wide range of head movements we experience daily. However, this mobility comes with inherent vulnerabilities. The atlantoaxial joint is unique, featuring the dens (odontoid process) of C2 articulating with the anterior arch of C1. This articulation is held in place by a crucial structure: the transverse ligament of the atlas. This strong ligament prevents excessive anterior displacement of C1 on C2, thereby protecting the spinal cord from compression by the dens. Instability in this region, whether due to trauma, inflammatory conditions like rheumatoid arthritis, or congenital anomalies, can lead to serious neurological compromise.

What is the Sharp-Purser Test?

The Sharp-Purser test is a widely recognized and utilized clinical examination technique designed to assess for atlantoaxial instability (AAI). Developed by Sharp and Purser in the 1960s, its primary purpose is to identify excessive anterior subluxation of the atlas (C1) on the axis (C2). Detecting such instability is critical because it indicates a potential compromise of the transverse ligament and a heightened risk of the dens encroaching upon the spinal cord, which can result in myelopathy (spinal cord dysfunction) or even catastrophic neurological injury.

Anatomy and Biomechanics Behind the Test

The integrity of the transverse ligament is paramount for atlantoaxial stability. This ligament acts as a strong band spanning the C1 ring, holding the dens of C2 firmly against the anterior arch of C1. When the transverse ligament is compromised (e.g., stretched, torn, or lax), the atlas can translate excessively forward on the axis, particularly during cervical flexion.

The Sharp-Purser test is biomechanically designed to provoke this anterior subluxation and then assess if it can be reduced. By applying a posterior force to the head while stabilizing C2, the test attempts to reduce any existing anterior displacement of C1 on C2. If instability is present, this reduction will often be palpable or observable, and crucially, it may alleviate the patient's neurological symptoms.

How to Perform the Sharp-Purser Test

The Sharp-Purser test is typically performed with the patient seated, though it can also be done in supine. It requires precision and an understanding of the underlying anatomy.

• Patient Positioning: The patient is seated comfortably with their head in a neutral to slightly flexed position.
• Examiner Positioning: The examiner stands directly behind or to the side of the patient.
• Hand Placement:
  • One hand firmly stabilizes the spinous process of the axis (C2) using a pincer grasp or thumb and index finger. This ensures C2 remains stationary.
  • The other hand is placed on the patient's forehead, cradling the occiput.
• Action:
  • Gently ask the patient to perform a slight amount of cervical flexion (e.g., nodding the head down slightly). This position may provoke or exacerbate symptoms if atlantoaxial instability is present, as it can cause anterior subluxation of C1.
  • While maintaining stabilization of C2, the examiner applies a sustained, gentle, yet firm, posterior translational force through the hand on the forehead, pushing the head posteriorly relative to the stabilized C2.
• Key Observations: Observe for any palpable movement, patient symptoms, and changes in neurological signs.

Interpreting the Results

The interpretation of the Sharp-Purser test relies on both subjective patient feedback and objective clinical signs.

• Positive Test: A positive test is indicated by:
  • Reduction of Symptoms: As the posterior force is applied, the patient reports a reduction or disappearance of symptoms such as pain, paresthesia (numbness or tingling in the extremities), dizziness, or nystagmus (involuntary eye movements). This suggests that the anterior subluxation of C1 was causing these symptoms, and the test successfully reduced the subluxation.
  • Palpable "Clunk" or Excessive Movement: The examiner may feel a distinct "clunk" or excessive posterior glide of the head/occiput relative to the stabilized C2 as the subluxation is reduced. This indicates abnormal translation at the atlantoaxial joint.
  • Increased Range of Motion: Following reduction, the patient may report an increased range of motion or a feeling of relief.
• Negative Test: A negative test occurs when there is no reduction of symptoms and no palpable "clunk" or excessive movement during the application of posterior force. This suggests that significant atlantoaxial instability, as assessed by this test, is not present.
• Important Safety Note: If the patient experiences an increase in pain, neurological symptoms, or expresses significant discomfort during the initial flexion or during the test, the test should be immediately stopped.

Clinical Significance and Limitations

The Sharp-Purser test holds significant clinical value as a screening tool for atlantoaxial instability. Its ability to identify potential spinal cord compression risk makes it an indispensable part of a comprehensive cervical spine assessment, particularly in high-risk populations.

• Significance:

  • Early Detection: It can help identify AAI before more severe neurological deficits occur.
  • Guiding Management: A positive test necessitates further diagnostic imaging (e.g., X-rays in flexion/extension, CT, MRI) to confirm the diagnosis and guide appropriate management, which may range from bracing to surgical stabilization.
  • Preventing Injury: For individuals involved in contact sports, or those with conditions predisposing them to AAI, a positive test can prevent participation in activities that could lead to catastrophic spinal cord injury.

• Limitations:

  • Subjectivity: The "clunk" or reduction of symptoms can be subjective and may vary between examiners.
  • Sensitivity and Specificity: While generally considered a good screening test, its sensitivity and specificity can vary depending on the population studied and the diagnostic criteria used. It is not 100% accurate and false negatives or positives can occur.
  • Requires Skill: Proper execution requires a skilled and experienced examiner who understands the delicate nature of the cervical spine.
  • Not Definitive: A positive Sharp-Purser test is an indicator of potential instability but is not a definitive diagnosis on its own. It must always be correlated with the patient's history, other clinical findings, and confirmatory imaging studies.

Who Should Be Tested?

The Sharp-Purser test is particularly relevant for individuals presenting with:

• Suspected Cervical Trauma: Especially hyperextension injuries or high-impact trauma to the head and neck.
• Rheumatoid Arthritis: A significant percentage of individuals with long-standing rheumatoid arthritis can develop atlantoaxial instability due to inflammatory erosion of ligaments and bone.
• Down Syndrome: Individuals with Down syndrome have a higher incidence of congenital ligamentous laxity, predisposing them to AAI.
• Persistent Neck Pain and Neurological Symptoms: Patients presenting with unexplained chronic neck pain, occipital headaches, dizziness, vertigo, nystagmus, paresthesia in the extremities, gait disturbances, or signs of myelopathy should be screened.
• Congenital Anomalies: Certain congenital conditions affecting the craniovertebral junction.

Conclusion: A Vital Diagnostic Tool

The Sharp-Purser test remains a vital clinical tool in the assessment of upper cervical spine stability. As an "Expert Fitness Educator," I emphasize that while it is a powerful indicator, it is a clinical test that must be performed by a qualified healthcare professional. Its proper application and interpretation, always in conjunction with a thorough patient history, physical examination, and advanced imaging, are crucial for accurate diagnosis, appropriate management, and ultimately, the prevention of potentially life-altering neurological injury. Understanding this test is fundamental for anyone involved in the care and assessment of musculoskeletal health.