Influence of muscle activity on the degree of bone mineralisation

Objectives: Bone remodeling is thought to be influenced by stress and strain in the bone tissue. At sites experiencing a high amount of strains (expressed as number and/or amplitude), a high rate of remodeling is found. This is associated with a decreased degree of mineralization of bone (DMB), because newly formed bone tissue is less mineralized than older bone tissue. Stresses and strains in mandibular bone are, amongst others, produced by jaw muscle activity. In this study the direct effect of muscle activity on the DMB was investigated. We hypothesized that at muscle attachment sites remodeling would be high and, accordingly, DMB low.

Methods: We used radio-telemetry to record daily muscle activity in New Zealand White rabbits (n=7). Daily muscle activity of the superficial masseter, temporalis and digastric muscles was expressed in burst numbers exceeding a certain percentage of daily peak-EMG. We used micro-computed tomography to determine the DMB, measured at various mandibular sites, including muscle attachments and locations without direct muscle influence.

Results: Comparison showed significant (p<0.01) differences in DMB between the various bone regions. Regions without direct muscle loading were highly mineralized (peak at 1081 ± 61 mg hydroxyapatite/cm3). The masseter and temporalis muscle attachment sites showed a similar distribution of their DMB (peaks at 593±103 and 620±132 mg/cm3), much lower than the site without direct muscle influence. The digastric attachment site showed an intermediate DMB (915±114 mg/cm3). Comparison of the inter-individual variations in DMB with the accompanying burst numbers showed no significant correlations.

Conclusion: Bone loading, expressed as number of muscle activity bursts, has a positive relationship with the rate of remodeling as indicated by the lower DMB at muscle attachment sites, compared to regions without direct applied muscular loading. Inter-individual variations in activity burst number cannot directly be linked to variations in the degree of mineralization.