Stanford Synchrotron Radiation Lightsource An Office of Science User Facility

Line profiles on a reduced elastic modulus illustrating significant gradients at bone-PDL and cementum-PDL attachment sites of normal (a) and narrowed (c) wet bone-PDL-tooth complex. b, d) Insets illustrate linear elastic modulus profiles of normal (b) and narrowed (d) bone-tooth complex. Insets in the left hand corner illustrate representative tomographies of a normal and narrowed bone-tooth complex. PDL: periodontal ligament, ES: endosteal space, Er: reduced elastic modulus (GPa).

A common problem in orthodontics is the narrowing of the space taken up by tissue connecting the root of a tooth to its bony socket ("periodontal ligament space") around teeth that are forced to change position with braces or other orthopedic fixation devices.  Healthy periodontal ligament space is generally considered to be between 150 and 380 µm; yet as teeth are forced to move (and as patients age), this space can be narrowed too far, potentially causing a failure of the fibrous joint between the bone and tooth.

A team of researchers led by University of California-San Francisco, School of Dentistry's Dr. Sunita Ho recently conducted studies to determine the causes of 5-50 µm narrowed periodontal ligament space in humans. Using atomic force and scanning electron microscopy techniques and micro x-ray computed tomography, the team found microscopic scalloped regions in the periodontal ligament attachment sites with bone, and macro-scale stratified layers of bone, as well as macroscopic bony protrusions.

The team then conducted micro x-ray fluorescence imaging studies at SSRL Beam Line 2-3 to correlate these structural properties to calcium and phosphorus composition, revealing heterogeneous distribution of both elements in the mineralized tissues.  These inhomogeneities, the team concluded, can be used as indicators of adaptation to force placed on the periodontal ligament; over time, the force alters the interface, making it discontinuous.

Based on these results, the research team proposes that strain amplification is minimized when the periodontal ligament space is between 150 and 380 µm wide; 5-50 µm wide periodontal ligament space can lead to discontinuities, causing compromised function.