Magnetic resonance spectroscopy of the masseter muscle in different facialmorphological patterns

The alms of this study were (1) to develop a reliable noninvasive method to evaluate the masseter muscle metabolism, by using P-31-magnetic resonance spectroscopy, and (2) to evaluate the metabolic profile of the masseter mus cle in subjects with various facial patterns. The maxillary-mandibular rela tionship, which varied from hypodivergent to hyperdivergent, was measured o n lateral cephalograms of 20 adults, 22 to 35 years of age. P-31-spectra we re acquired from the masseter muscle at rest with a custom-made, single-tur n, double-tuned, 3 x 5-cm oblong surface coil. The inorganic phosphate to p hosphocreatine (Pi/PCr) ratios were measured and compared in relation to ve rtical and sagittal cephalometric measurements. A statistically significant (R-2 = 0.65, r = 0.81, P =.001) relationship was found between Pi/PCr rati o and the palatal-to-mandibular plane angle. As the maxillary-to-mandibular divergence increased, the Pi/PCr ratio decreased. This correlation suggest s that muscles with a higher Pi/PCr ratio have a higher resting metabolic a ctivity than those with a lower Pi/PCr ratio. Consequently, these muscles m ay keep bone under more tension and influence its growth in a more horizont al direction. Another, possible explanation of the results is that the fibe r type composition of the masseter muscle varies with facial morphology.