Human evolution and the development of spondylolysis

Area of Science:

• Orthopedics
• Biomechanics
• Anatomy

Background:

• Spondylolysis is a prevalent condition with an unclear etiology, often attributed to fatigue fractures of the pars interarticularis.
• The precise causes leading to spondylolysis remain unidentified in current literature.

Purpose of the Study:

• To investigate the hypothesis that chronic spondylolytic defects at L5 are linked to insufficient mediolateral distances between L4 and S1 articular facets.
• To compare transverse interfacet dimensions in the lumbar spine of individuals with and without spondylolysis.

Main Methods:

• Analysis of lumbar vertebrae from the Hamann-Todd osteological collection.
• Comparison of 30 individuals with bilateral spondylolysis at L5 against 30 age- and sex-matched controls.
• Statistical analysis (two-tailed t tests) of transverse interfacet distances and vertebral body breadths.

Main Results:

• Individuals without spondylolysis exhibited a significantly greater increase in interfacet dimensions from L4 to S1 compared to those with spondylolysis.
• This finding remained significant even after standardizing for vertebral body breadth, indicating it's not due to overall larger vertebrae.
• No systematic differences in vertebral body size were observed between the two groups.

Conclusions:

• Spondylolysis appears to result from direct contact pressures on the pars interarticularis due to inadequate separation of L4 inferior articular processes and S1 superior articular facets.
• Insufficient increase in transverse interfacet dimensions predisposes individuals to developing and maintaining spondylolytic defects.